Phosphorus legacy: overcoming the effects of past management practices to mitigate future water quality impairment

Linking sediment geochemistry with catchment processes, internal phosphorus loading and lake water quality

Research in the field of sediment geochemistry suggests potential linkages between catchment processes (land use), internal phosphorus (P) loading and lake water quality, but evidence is still poorly quantified due to a limited amount of data. Here we address the issues based on a comprehensive data set from 27 lakes in southern Finland. Specifically, we aimed at: 1) elucidating factors behind spatial variations in sediment geochemistry; 2) assessing the impact of diagenetic transformation on sediment P regeneration across lakes based on the changes in the vertical distribution of sediment components; 3) exploring the role of the sediment P forms in internal P loading (IL), and 4) determining the impact of IL on lake water quality. The relationship between sediment P concentration and field area percentage (FA%) was statistically significant in (mainly eutrophic) lakes with catchments that included more than 10 % of fields. We found that sediment iron-bound P (Fe-P) increased with increasing FA%, which agrees with the high expected losses from the cultivated areas. Additionally, populated areas increased the pool of sediment Fe-P. Internal P loading was significantly positively related to both sediment Fe-P and sediment organic P (Org-P). However, Org-P was not significant (as the third predictor) in models that had a trophic state variable as the first predictor and Fe-P as the second predictor. Further, the vertical profiles of sediment components indicated a role of diagenetic transformations in the long-term sediment P release, especially in lakes with deeper maximum depth and longer water residence time. Finally, IL was significantly positively correlated to water quality variables including phytoplankton biomass, its proportion of cyanobacteria, chlorophyll a concentration and trophic state index. Our findings suggest that reduction of P losses from the field and populated areas will decrease internal P loads and increase water quality through a reduced pool of Fe-P.

Phosphorus lability across diverse agricultural contexts with legacy sources

The buffering of phosphorus (P) in the landscape delays management outcomes for water quality. If stored in labile form (readily exchangeable and bioavailable), P may readily pollute waters. We studied labile P and its intensity for >600 soils and sediments across seven study locations in the United States. Stocks of labile P were large enough to sustain high P losses for decades, indicating the transport-limited regime typical of legacy P. Sediments were commonly more P-sorptive than nearby soils. Soils in the top 5 cm had 1.3-3.0 times more labile P than soils at 5-15 cm. Stratification in soil test P and total P was, however, less consistent. As P exchange via sorption processes follows the difference in intensities between soil/sediment surface and solution, we built a model for the equilibrium phosphate concentration at net zero sorption (EPC0) as a function of labile P (quantity) and buffer capacity. Despite widely varying properties across sites, the model generalized well for all soils and sediments: EPC0 increased sharply with more labile P and to greater degree when buffer capacity was low or sorption sites were likely more saturated. This quantity-intensity-capacity relationship is central to the P transport models we rely on today. Our data inform the improvement of such P models, which will be necessary to predict the impacts of legacy P. Further, this work reaffirms the position of labile P as a key focus for environmental P management-a view Dr. Sharpley developed in the 1980s with fewer data and resources.

Far-future hydrology will differentially change the phosphorus transfer continuum

Climate change is likely to exacerbate land to water phosphorus (P) transfers, causing a degradation of water quality in freshwater bodies in Northwestern Europe. Planning for mitigation measures requires an understanding of P loss processes under such conditions. This study assesses how climate induced changes to hydrology will likely influence the P transfer continuum in six contrasting river catchments using Irish national observatories as exemplars. Changes or stability of total P (TP) and total reactive P (TRP) transfer processes were estimated using far-future scenarios (RCP4.5 and RCP8.5) of modelled river discharge under climate change and observed links between hydrological regimes (baseflow and flashiness indices) and transfer processes (mobilisation and delivery indices). While there were no differences in P mobilisation between RCP4.5 and RCP8.5, both mobilisation and delivery were higher for TP. Comparing data from 2080 (2070-2099) with 2020 (2010-2039), suggests that P mobilisation is expected to be relatively stable for the different catchments. While P delivery is highest in hydrologically flashy catchments, the largest increases were in groundwater-fed catchments in RCP8.5 (+ 22% for TRP and + 24% for TP). The inter-annual variability of P delivery in the groundwater-fed catchments is also expected to increase. Since the magnitude of a P source may not fully define its mobility, and hydrological connections of mobilisation areas are expected to increase, we recommend identifying critical mobilisation areas to target future mitigation strategies. These are hydrologically connected areas where controls such as soil/bedrock chemistry, biological activity and hydrological processes are favourable for P mobilisation.

Relationships between soil test phosphorus and county-level agricultural surplus phosphorus

National nutrient inventories provide surplus phosphorus (P) estimates derived from county-scale mass balance calculations using P inputs from manure and fertilizer sales and P outputs from crop yield data. Although bioavailable P and surplus P are often correlated at the field scale, few studies have investigated the relationship between measured soil P concentrations of large-scale soil testing programs and inventory-based surplus P estimates. In this study, we assessed the relationship between national surplus P data from the NuGIS dataset and laboratory-measured soil test phosphorus (STP) at the county scale for Arkansas, North Carolina, and Oklahoma. For optimal periods of surplus P aggregation, surplus P was positively correlated with STP based on both Pearson (Arkansas: r = 0.65, North Carolina: r = 0.45, Oklahoma: r = 0.52) and Spearman correlation coefficients (Arkansas: ρ = 0.57, North Carolina: ρ = 0.28, and Oklahoma: ρ = 0.66). Based on Pearson correlations, the optimal surplus P aggregation periods were 10, 30, and 4 years for AR, NC, and OK, respectively. On average, STP was more strongly correlated with surplus P than with individual P inventory components (fertilizer, manure, and crop removal), except in North Carolina. In Arkansas and North Carolina, manure P was positively correlated with STP, and fertilizer P was negatively correlated with STP. Altogether, results suggest that surplus P moderately correlates with STP concentrations, but aggregation period and location-specific factors influence the strength of the relationship.

Phosphorus distributions in alluvial soils of the Lower Mississippi River Basin: A case of dual legacies

Legacies can become intertwined, none more so than the body of work of Dr. Andrew Sharpley examining agricultural nutrient delivery to waterbodies and the phosphorus (P) accumulation in agricultural soils, or "legacy P." Although Sharpley's work focused on the anthropogenic influence on soil P, our study suggests soils of the Lower Mississippi Alluvial Plain (MAP) represent a natural legacy with moderate levels of available P resulting from minimal anthropogenic input. In 2019, we collected surface (0-5 cm) soil samples from four regionally dominant soil series in either cropland or forested land uses, spanning 76 locations within the MAP. Soil chemical and physical properties were measured utilizing a suite of extractions and texture analysis to correlate properties with soil P values. Total soil P did not vary between land uses. Mehlich-3 extractable P was slightly higher in cropland soils due to higher concentrations in Forestdale and Sharkey soils. Dundee, Forestdale, and Sharkey cropland soils showed significant associations between Mehlich-3-extractable iron (Fe) and P. Ratios of total carbon (TC) to total nitrogen (C:N) and TC to P (C:P) were consistent across all sampled soil series but differed between forest and cropland soils. These ratios are critical for establishing baseline soil nutrient values in simulation models and can be used to improve water quality model simulations that help guide P management in the MAP. As Sharpley routinely demonstrated, understanding sources of P is critical for developing an appropriate management strategy. This study provides critical knowledge on soil P dynamics in the MAP region.

Examining contaminant transport hotspots and their predictability across contrasted watersheds

Hydrobiogeochemical processes governing water quantity and quality are highly variable in space and time. Focusing on thirty river locations in Québec, Canada, three water quality hotness indices were used to classify watersheds as contaminant transport hotspots. Concentration and load data for suspended solids (SS), total nitrogen (TN), and total phosphorous (TP) were used to identify transport hotspots, and results were compared across hotness indices with different data requirements. The role of hydroclimatic and physiographic characteristics on the occurrence and temporal persistence of transport hotspots was examined. Results show that the identification of transport hotspots was dependent on both the type of data and the hotness index used. Relationships between temporal and spatial predictors, however, were generally consistent. Annual transport hotspot occurrence was found to be related to temporal characteristics such as the number of dry days, potential evapotranspiration, and snow water equivalent, while hotspot temporal persistence was correlated to landcover characteristics. Stark differences in the identification of SS, TN, and TP transport hotspots were attributed to differences in mobilization processes and provided insights into dominant water and nutrient flowpaths in the studied watersheds. This study highlighted the importance of comparing contaminant dynamics across watersheds even when high-frequency water quality data or discharge data are not available. Characterizing hotspot occurrence and persistence, among hotness indices and water quality parameters, could be useful for watershed managers when identifying problematic watersheds, exploring legacy effects, and establishing a prioritization framework for areas that would benefit from enhanced routine monitoring or targeted mitigation strategies.

Unraveling spatial heterogeneity of soil legacy phosphorus in subtropical grasslands

Humans have profoundly altered phosphorus (P) cycling across scales. Agriculturally driven changes (e.g., excessive P-fertilization and manure addition), in particular, have resulted in pronounced P accumulations in soils, often known as "soil legacy P." These legacy P reserves serve as persistent and long-term nonpoint sources, inducing downstream eutrophication and ecosystem services degradation. While there is considerable scientific and policy interest in legacy P, its fine-scale spatial heterogeneity, underlying drivers, and scales of variance remain unclear. Here we present an extensive field sampling (150-m interval grid) and analysis of 1438 surface soils (0-15 cm) in 2020 for two typical subtropical grassland types managed for livestock production: Intensively managed (IM) and Semi-natural (SN) pastures. We ask the following questions: (1) What is the spatial variability, and are there hotspots of soil legacy P? (2) Does soil legacy P vary primarily within pastures, among pastures, or between pasture types? (3) How does soil legacy P relate to pasture management intensity, soil and geographic characteristics? and (4) What is the relationship between soil legacy P and aboveground plant tissue P concentration? Our results showed that three measurements of soil legacy P (total P, Mehlich-1, and Mehlich-3 extractable P representing labile P pools) varied substantially across the landscape. Spatial autoregressive models revealed that soil organic matter, pH, available Fe and Al, elevation, and pasture management intensity were crucial predictors for spatial patterns of soil P, although models were more reliable for predicting total P (68.9%) than labile P. Our analysis further demonstrated that total variance in soil legacy P was greater in IM than SN pastures, and intensified pasture management rescaled spatial patterns of soil legacy P. In particular, after controlling for sample size, soil P was extremely variable at small scales, with variance diminished as spatial scale increased. Our results suggest that broad pasture- or farm-level best management practices may be limited and less efficient, especially for more IM pastures. Rather, management to curtail soil legacy P and mitigate P loading and losses should be implemented at fine scales designed to target spatially distinct P hotspots across the landscape.

Response of hydrology and nutrient losses to different extreme rainfall conditions in a coastal watershed influenced by orchards

Global warming is altering the frequency of extreme rainfall events and introducing uncertainties for non-point source pollution (NPSP). This research centers on orchard-influenced planting areas (OIPA) in the Wulong River Watershed of Shandong Province, China, which are known for their heightened nitrogen (N) and phosphorus (P) pollution. Leveraging meteorological data from both historical (1989-2018) and projected future periods (2041-2100), this research identified five extreme rainfall indices (ERI): R10 (moderate rain), R20 (heavy rain), R50 (rainstorm), R95p (Daily rainfall between the 95th and 99th percentile of the rainfall), and R99p (>99th percentile). Utilizing an advanced watershed hydrological model, SWAT-CO2, this study carried out a comparison between ERI and average conditions and evaluated the effects of ERI on the hydrology and nutrient losses in this coastal watershed. The findings revealed that the growth multiples of precipitation in the OIPA for five ERI varied between 16 and 59 times for the historical period and 14 to 65 times for future climate scenarios compared to the average conditions. The most pronounced increases in surface runoff and total phosphorus (TP) loss were observed with R50, R95p, and R99p, showing growth multiples as high as 352 and 330 times, and total nitrogen (TN) growth multiples varied between 4.6 and 30.3 times. The contribution rates of R50 and R99p for surface runoff and TP loss in the OIPA during all periods exceeded 55%, however, TN exhibited the opposite trend, primarily due to the dominated NO3-N leaching in the sandy soil. This research revealed how the OIPA reacts to different ERI and pinpointed essential elements influencing water and nutrient losses.

A national-scale high-resolution runoff risk and channel network mapping workflow for diffuse pollution management

Managing diffuse pollution from agricultural land requires a spatially explicit risk assessment that can be applied over large areas. Major components of such assessments are the precise definition of both channel networks that often originate as small channels and streams, and Hydrologically Sensitive Areas (HSAs) of storm runoff that occur on land surfaces. Challenges relate to regions of complex topography and land use patterns, particularly those which have been heavily modified by arterial drainage. In this study, a national scale, transferrable workflow and analysis were developed using a specifically commissioned LiDAR survey. Research on the first half of Northern Ireland (6927 km2) is reported where field-edge drain to major river channels were mapped from 1 m (16 points per metre) digital terrain models, and in-field HSAs were defined across over 400,000 fields with a median field size of 0.86 ha. Manual drainage mapping supplemented with a novel automated drainage channel correction process resulted in an unparalleled high-resolution national drainage network with 37,320 km of channels, increasing mapped channel density from 0.9 km km-2 to 5.5 km km-2. The HSAs were based on a Soil Topographic Index (STI) system using hillslope and contributing area models combined with soil hydraulic characteristics. In all, 249 km2 of runoff risk HSAs were identified by extracting the top 95th percentile of the modelled STI as the areas with the highest propensity to generate in-field runoff. At field and individual farm scale these targeted risk maps of diffuse pollution were delivered to over 13,000 farmers and form part of the nationwide Soil Nutrient Health Scheme programme.

Soil legacy phosphorus and loss risk in subtropical grasslands

The accumulation of soil legacy phosphorus (P) due to past fertilization practices poses a persistent challenge for agroecosystem management and water quality conservation. This study investigates the spatial distribution and risk assessment of soil legacy P in subtropical grasslands managed for cow-calf operations in Florida, with two pasture types along the intensity gradient: improved vs semi-native pastures. Soil samples from 1438 locations revealed substantial spatial variation in soil legacy P, with total P concentrations ranging from 11.46 to 619.54 mg/kg and Mehlich-1 P concentrations spanning 0.2-187.27 mg/kg. Our analyses revealed that most of the sites in semi-native pastures may function as P sinks by exhibiting positive Soil P Storage Capacity (SPSC) values, despite having high levels of soil total P. These locales of higher SPSC values were associated with high levels of aluminum, iron, and organic matter that can adsorb P. In addition, our results from spatial random forest modelling demonstrated that factors including elevation, soil organic matter, available water storage, pasture type, soil pH, and soil order are important to explain and predict spatial variations in SPSC. Incorporating SPSC into the Phosphorus Index (PI) spatial assessment, we further determined that only 3% of the study area was considered as high or very high PI categories indicative of a significant risk for P loss. Our evaluation of SPSC and PI underscores the complexity inherent in P dynamics, emphasizing the need for a holistic approach to assessing P loss risk. Insights from this work not only help optimize agronomic practices but also promote sustainable land management, thus ensuring the long-term health and sustainability of grass-dominated agroecosystems.

Influence of climate and land use on watershed anthropogenic phosphorus inputs and riverine phosphorus export dynamics: A global analysis

Many studies have found predictive relationships between riverine phosphorus (P) export and net anthropogenic P inputs (NAPI) at the watershed scale, but the global or regional extent of these relationships has not been empirically quantified. Herein, we present a data-driven global assessment of the response of riverine total P (TP) fluxes to NAPI based on 358 watersheds. NAPI exhibited high spatial heterogeneity (2-12,085 kg P km-2 yr-1) and was well correlated with riverine TP fluxes. Riverine TP export fractions of NAPI were primarily regulated by NAPI components, hydroclimate factors, and land-use as determined through a random-forest meta-analysis. In watersheds dominated by disturbed land-use (e.g., agricultural and developed lands), runoff emerged as pivotal climate-related factors influencing riverine export fractions of NAPI. In watersheds dominated by natural land-use, runoff, precipitation and temperature were identified as the most critical factors. We developed a mixed-effects meta-regression model (R2 = 0.63-0.70, RMSE = 19-78 %, n = 87-202) to examine the quantitative relationship between riverine TP fluxes and NAPI, which avoids subjectivity in selecting influencing factors and regression forms. The model estimated that legacy P contributed 14-17 % of annual riverine TP fluxes in Chinese watersheds, 25 % in North American watersheds and 11-27 % in European watersheds. Annual NAPI contributions to annual riverine TP flux were 83-86 % in China, 75 % in North America and 73-89 % in Europe. The model forecasted 52-67 %, 69-71 % and 74-77 % reductions in riverine TP fluxes across Chinese, North American, and European watersheds by 2050 under five shared socio-economic pathway scenarios compared to 2010 baseline conditions, respectively. This study provides a straightforward and reliable method for quantifying anthropogenic P input and riverine P export dynamics within an acceptable error range. It provides guidance for developing phosphorus pollution control strategies to counter potential increases in phosphorus inputs due to expected changes in climate and land use.

Spatiotemporal investigation of geochemical and hydrological controls on release of soluble reactive phosphorus from the shallow aquifer of a riparian zone

Phosphorus (P) that accumulates in agricultural riparian zones can be released under certain hydrological and biogeochemical conditions, thereby limiting the effectiveness of these zones in reducing P loads from field to stream. The study objective was to explore factors that may be contributing to, or limiting, high soluble reactive phosphorus (SRP) concentrations in the shallow aquifer of an alluvial upland riparian zone located in a continental climate. Field investigations including porewater sampling from six vertical nests, soil sampling, and continuous soil moisture, groundwater table, and redox measurements were conducted over 19 months. Porewater SRP concentrations were generally low in the aquifer considering all sampling times (median = 14.7 µg/L; interquartile range [IQR] = 11.1 µg/L, 287 samples). The overall low SRP may be due to low reducible labile soil P (median = 21.1 µgP/g dw, IQR = 10.9 µgP/g dw, 21 samples). However, high SRP concentrations (>52 µg/L, 95% quartile) did occur intermittently in space and time with no clear spatial or temporal patterns. Analyses indicate that most high concentrations were likely not associated with factors previously reported to influence SRP release in riparian aquifers, including redox conditions, pH, and soil drying and wetting. Further, data indicate that internally released or externally supplied SRP may undergo rapid (re-) sequestration within the aquifer, limiting its vertical or horizontal transport. The study findings highlight the complexity of P behavior in riparian zones and the need for caution when assessing the effectiveness of conservation practices and in interpreting potential impacts of subsurface water quality on stream water quality when monitoring locations are distant from the stream.

Trade-offs between nitrogen and phosphorus removal with floodplain remediation in agricultural streams

To improve water quality and reduce instream erosion, floodplain remediation along agricultural streams can provide multiple ecosystem services through biogeochemical and fluvial processes. During floodplain inundation, longer water residence time and periodic anoxic conditions can lead to increased nitrogen (N) removal through denitrification but also mobilization of phosphorus (P), impeding overall water quality improvements. To investigate the capacity for N and P processing in remediated streams, we measured potential denitrification and nitrous oxide production and yields together with potential P desorption and P fractions in floodplain and stream sediments in ten catchments in Sweden. Sediment P desorption was measured as equilibrium P concentration, using P isotherm incubations. Denitrification rates were measured with the acetylene inhibition method. Sediment nutrient process rates were combined with hydrochemical monitoring along remediated streams and their paired upstream control reaches of trapezoidal shape to determine the impact of floodplains on water quality. The correlation between floodplain denitrification rates and P desorption (r = 0.53, p = 0.02) revealed a trade-off between soluble reactive P (SRP) and nitrate removal, driven by stream water connectivity to floodplains. Nitrous oxide production was not affected by differences in P processing, but nitrous oxide yields decreased with higher denitrification and P desorption. The release of SRP from floodplains (0.03 ± 0.41 mg P kg-1 day-1) was significantly lower than from trapezoidal stream banks (0.38 ± 0.37 mg P kg-1 day-1), predicted by long-term SRP concentrations in stream water and floodplain inundation frequency. The overall impact of SRP release from floodplains on stream SRP concentrations in remediated reaches was limited. However, the remediated reaches showing increased stream SRP concentrations were also frequently inundated and had higher labile P content and coarse soil texture in floodplain sediments. To fully realize the potential for water quality improvements with constructed floodplains in agricultural streams, the promotion of denitrification through increased inundation should be balanced against the risk of P release from sediments, particularly in streams with high SRP inputs.

Site matters: site-specific factors control phosphorus retention in buffer strip soils under concentrated field runoff

Soil erosion from agricultural fields is a persistent ecological problem, potentially leading to eutrophication of aquatic habitats in the catchment area. Often used and recommended mitigation measures are vegetated filter strips (VFS) as buffer zones between arable land and water bodies. However, if they are designed and managed poorly, nutrients - especially phosphorus (P) - may accumulate in the soil. Ultimately, VFS can switch from being a nutrient sink to a source. This problem is further aggravated if the field runoff does not occur as uniform sheet flow, but rather in concentrated form, as is usually the case. To assess the impact of concentrated flow on VFS performance, we have taken soil core samples from field-VFS transition zones at six sites in Lower Austria. We determined a multitude of physical and chemical soil parameters, focusing on P fractions and indices. Our results revealed that concentrated flow can lead to an accumulation of P in the VFS. P levels in the VFS inside the area of concentrated runoff can be equal to or higher than in the field, even though they receive no direct fertilization. However, the concentration and distribution of nutrients in the fields and VFSs were also site-specific and affected by local factors such as the age of the VFS, cropping, and fertilization. Accordingly, there is a need for more sophisticated, bespoke VFS designs that can cope with site-specific runoff volumes and movements of nutrients that occur.

Denitrifying bioreactors and dissolved phosphorus: Net source or sink?

Understanding the world through a lens of phosphorus (P), as Dr. Andrew Sharpley aimed to do, adds a deeper dimension for water quality work in the heavily tile-drained US Midwest where nitrate is often the nutrient of biggest concern. Denitrifying woodchip bioreactors reduce nitrate pollution in drainage water, but dissolved phosphorus leached from the organic fill is a possible pollution tradeoff. Recent work by Dr. Sharpley and others defined such tradeoffs as strategic decisions in which a negative outcome is accepted with prior knowledge of the risk. In this vein, we assessed 23 site-years from full-size bioreactors in Illinois to determine if bioreactors were a net dissolved reactive phosphorus (DRP) source and, if so, to determine flow-related correlation agents (1904 sample events; 10 bioreactors). DRP was removed across the bioreactors in 15 of 23 site-years. The 23 site-years provided a median annual DRP removal efficiency of 12% and a median annual DRP removal rate of 7.1 mg DRP/m3 bioreactor per day, but the ranges of all removal metrics overlapped zero. The highest daily bioreactor DRP removal rates occurred with high inflow concentrations and under low hydraulic retention times (i.e., under higher loading). Dr. Sharpley was one of the first to explore losses of DRP in subsurface drainage and performed decades of useful applied studies that inspired approaches to management of P loss on both drained and undrained land. We seek to honor this legacy with this practical study of the DRP benefits and tradeoffs of denitrifying bioreactors.

Phosphorus speciation in manure and fertilizer impacted Mid-Atlantic coastal plain soils

Historical applications of manures and fertilizers at rates exceeding crop P removal in the Mid-Atlantic region (United States) have resulted in decades of increased water quality degradation from P losses in agricultural runoff. As such, many growers in this region face restrictions on future P applications. An improved understanding of the fate, transformations, and availability of P is needed to manage P-enriched soils. We paired chemical extractions (i.e., Mehlich-3, water extractable P, and chemical fractionation) with nondestructive methods (i.e., x-ray absorption near edge structure [XANES] spectroscopy and x-ray fluorescence [XRF]) to investigate P dynamics in eight P-enriched Mid-Atlantic soils with various management histories. Chemical fractionation and XRF data were used to support XANES linear combination fits, allowing for identification of various Al, Ca, and Fe phosphates and P sorbed phases in soils amended with fertilizer, poultry litter, or dairy manure. Management history and P speciation were used to make qualitative comparisons between the eight legacy P soils; we also speculate about how P speciation may affect future management of these soils with and without additional P applications. With continued P applications, we expect an increase in semicrystalline Al and Fe-P, P sorbed to Al (hydro)oxides, and insoluble Ca-P species in these soils for all P sources. Under drawdown scenarios, we expect plant P uptake first from semicrystalline Al and Fe phosphates followed by P sorbed phases. Our results can help guide management decisions on coastal plain soils with a history of P application.

Long-term dairy manure amendment promotes legacy phosphorus buildup and mobility in calcareous soils

Continuous application of dairy manure to soils can lead to excessive phosphorus (P) accumulation (legacy P), which requires understanding for managing nutrient availability and leaching. This study was conducted in Kimberly, ID, where dairy manure or conventional fertilizer was applied to calcareous soil plots under continuous crop rotations for 8 years (2013-2020), followed by 2 years with no amendment. To understand legacy P behavior in the soils, total P, organic/inorganic P, and plant-available Olsen bicarbonate P and Truog extraction measurements were made from surface and subsurface samples. Additionally, P in soluble and less soluble calcium phosphate (Ca-P) minerals was estimated using selective extractions, and P desorption was measured in a flow-through reactor. Manure amendments resulted in increased total soil P and plant-available P, particularly in the initial 5 years. In the 0- to 30-cm depth, 54%-65% of the soil P added from manure amendments was readily soluble by the Truog P test. Phosphorus released from the 2022 manure-amended soil in the desorption experiments was about five times greater than the fertilizer-amended soil, suggesting high leaching potential. After 8 years of manure amendment, subsurface Olsen-P levels exceeded the 40 mg kg-1 management threshold, suggesting P adsorption potential of the surface had become saturated, allowing for P leaching. In the manure-amended surface soils, calcium phosphate minerals increased compared to the controls. Even after 2 years without manure amendment, soluble Ca-P mineral phases persisted in the soils, which can be a long-term source of P leaching.

Toward better targeting of mitigation measures for reducing phosphorus losses from land to water: Andrew Sharpley's legacy in Norway and Sweden

Nordic agriculture faces big challenges to reduce phosphorus (P) loss from land to water for improving surface water quality. While understanding the processes controlling P loss and seeking for P mitigation measures, Norwegian and Swedish researchers have substantially benefited from and been inspired by Dr. Andrew Sharpley's career-long, high-standard P research. Here, we demonstrate how Sharpley and his research have helped the Nordic researchers to understand the role of cover crops in cold environmental conditions, best manure P management practices, and ditch processes. His work on critical source area (CSA) identification and site assessment tool development have also greatly inspired our thinking on the targeting of mitigation measures and the contextualizing tools for Nordic climate, landscape, and soils. While reflecting on Sharpley's legacy, we identify several needs for Norwegian and Swedish P research and management. These include (1) tackling the challenges caused by local/regional unevenness in livestock density and related manure management and farm P surpluses, (2) identifying CSAs of P loss with high erosion risk and high P surplus, (3) obtaining more high-resolution mapping of soils with low P sorption capacity both in the topsoil and subsoil, (4) improving cross-scale understanding of processes and mitigation measures and proper follow-up of applied mitigation measures, and (5) increasing collaborations of researchers with farmers and farmers' advisory groups and watershed groups by developing high-quality educational courses and extension materials. The needs should be addressed in the context of the challenges and opportunities created by climate change.

Spatial and temporal variation in surface nitrate and phosphate in the Northern Gulf of Mexico over 35 years

Dissolved inorganic nutrient concentrations in the surface waters (0 to 5 m) of the Northern Gulf of Mexico (NGoM) were analyzed from 1985 to 2019 (> 10,000 observations) to determine spatiotemporal trends and their connection to nutrients supplied from the Mississippi/Atchafalaya River (MAR). In the NGoM, annual mean dissolved inorganic P (DIP) concentrations increased significantly over time, while dissolved inorganic N (DIN) concentrations showed no temporal trend. With greater salinity, mean DIN:DIP decreased from above the Redfield ratio of 16 to below it, reflecting DIN losses and the more conservative behavior of DIP with salinity. Over the same time period, annual mean P (total dissolved P, DIP, dissolved organic P) loading from the MAR to the NGoM significantly increased, annual mean DIN and total dissolved N loading showed no temporal trend, and dissolved organic N loading significantly decreased. Though DIP increased in the MAR, MAR DIP alone was insufficient to explain the surface distribution of DIP with salinity. Therefore, increases in surface DIP in the NGoM are not simply a reflection of increasing MAR DIP, pointing to temporal changes in other DIP sources. The increase in NGoM DIP suggests greater N limitation for phytoplankton, with implications for N fixation and nutrient management.

Improving water quality in a hypereutrophic lake and tributary through agricultural nutrient mitigation: A Multi-year monitoring analysis

Anthropogenic eutrophication remains a critical global issue, significantly impacting surface water quality. Numerous regions have implemented beneficial management practices to combat agricultural nonpoint pollution, often evaluating efficacy at the field scale, but not downstream. In this study, we conducted an extensive, 11-year (2010-2020), all-season, weekly monitoring program in a small, shallow, hypereutrophic lake and main tributary located in a cold climate, northern temperate zone, within a predominantly agricultural-forested mesoscale watershed. The monitoring took place before and after the implementation of field-scale agricultural nutrient mitigation measures in the catchment, allowing assessment of changes over time in the downstream tributary and lake. We analyzed long-term trends and temporal change points for nitrogen and phosphorus concentrations, aquatic trophic status, and nutrient stoichiometric ratios. The results revealed significant reductions in nitrogen and phosphorus concentrations, improved lake trophic status from hypereutrophic to eutrophic, and an increase in total nitrogen : total phosphorus ratios following the implementation of field-scale agricultural nutrient mitigation measures. Notably, both the lake and its main tributary exhibited significant temporal change points for these parameters. Our findings offer evidence of a relatively rapid, positive effect of the implementation of field-scale agricultural nutrient mitigation measures contributing to subsequent improvements in downstream water quality.

New phosphorus losses via tile drainage depend on fertilizer form, placement, and timing

Agricultural phosphorus (P) losses are harmful to water quality, but knowledge gaps about the importance of fertilizer management practices on new (recently applied) sources of P may limit P loss mitigation efforts. Weighted regression models applied to subsurface tile drainage water quality data enabled estimating the new P losses associated with 155 P applications in Ohio and Indiana, USA. Daily discharge and dissolved reactive P (DRP) and total P (TP) loads were used to detect increases in P loss following each application which was considered new P. The magnitude of new P losses was small relative to fertilizer application rates, averaging 79.3 g DRP ha-1 and 96.1 g TP ha-1 , or <3% of P applied. The eight largest new P losses surpassed 330 g DRP ha-1 or 575 g TP ha-1 . New P loss mitigation strategies should focus on broadcast liquid manure applications; on average, manure applications caused greater new P losses than inorganic fertilizers, and surface broadcast applications were associated with greater new P losses than injected or incorporated applications. Late fall applications risked having large new P losses applications. On an annual basis, new P contributed an average of 14% of DRP and 5% of TP losses from tile drains, which is much less than previous studies that included surface runoff, suggesting that tile drainage is relatively buffered with regard to new P losses. Therefore old (preexisting soil P) P sources dominated tile drain P losses, and P loss reduction efforts will need to address this source.

A review of the development and implementation of the critical source area concept: A reflection of Andrew Sharpley's role in improving water quality

Critical source areas (CSAs) are small areas of a field, farm, or catchment that account for most contaminant loss by having both a high contaminant availability and transport potential. Most work on CSAs has focused on phosphorus (P), largely through the work in the 1990s initiated by Dr. Sharpley and colleagues who recognized the value in targeting mitigation efforts. The CSA concept has been readily grasped by scientists, farmers, and policymakers across the globe. However, experiences and success have been mixed, often caused by the variation in where and how CSAs are defined. For instance, analysis of studies from 1990 to 2023 shows that the proportion of the annual contaminant load coming from a CSA decreases from field to farm to catchment scale. This finding is consistent with increased buffering of CSAs and greater contribution of other sources with scale, or variation in the definition of CSAs. We therefore argue that the best application of CSAs to target mitigation actions should be at small areas that truly account for most contaminant loss. This article sheds light on the development and utilization of CSAs, paying tribute to Dr. Sharpley's remarkable contributions to the improvement of water quality, and reflecting upon where the CSA concept has succeeded or not in reducing contaminant (largely P) loss.

Evaluating the longevity of in-stream phosphorus legacies: A downstream cascade of recovery following point source remediation

In-stream phosphorus (P) legacies cause lags between upstream remediation and downstream load reductions. However, the length of these lags is largely unknown, especially for long stream distances. As a result, lag time estimates at the large-watershed scale have been abstract and sometimes understated. Here, we leverage a large area watershed model with newly improved in-stream P simulation (SWAT+P.R&R) to evaluate the magnitude, longevity, and spatial cascade of legacy P remobilization in a U.S. corn belt watershed. Our results illustrate the "spiraling recovery" of P loads after a hypothetical point source remediation, where locations further downstream take longer to recover to baseline load levels. At the watershed outlet, in-stream legacy P contributions are equivalent to 30% of the baseline average annual P loads for three years after remediation. In-stream legacies do not approach exhaustion (95% remobilized) until at least 9 years after remediation. In hypothetical weather scenarios beginning with dry years, legacy contributions persist even longer. These findings (1) suggest that in-stream legacies could impact P loads for years to decades in large river basins, (2) support explicit accounting for spatial scale in future studies of in-stream legacies, and (3) provide concerning implications for water quality recovery in large river basins.

Legacy sediment as a potential source of orthophosphate: Preliminary conceptual and geochemical models for the Susquehanna River, Chesapeake Bay watershed, USA

Nutrient pollution from agriculture and urban areas plus acid mine drainage (AMD) from legacy coal mines are primary causes of water-quality impairment in the Susquehanna River, which is the predominant source of freshwater and nutrients entering the Chesapeake Bay. Recent increases in the delivery of dissolved orthophosphate (PO4) from the river to the bay may be linked to long-term increases in pH, decreased acidity of precipitation, and decreased acidity, iron, and aluminum loading from widespread AMD. Since the 1950s, baseline pH increased from ~6.5 to ~8 in the West Branch and "North Branch" of the Susquehanna River, which drain bituminous and anthracite coalfields of Pennsylvania. A current baseline pH of ~8 and daily maxima exceeding 9 have been documented along the lower Susquehanna River. In response to improved river quality, bioavailable PO4 now may be released into solution from legacy sediment that has filled major impoundments in lower reaches of the river. At typical pH (5-8) of natural water, aqueous PO4 species tend to be adsorbed by hydrous iron, aluminum, and manganese oxides that coat soil and sediment particles; however, PO4 may be substantially desorbed at pH >8. We created a geochemical model that simulates equilibrium aqueous/solid distributions of PO4 as pH and other solution characteristics change. Considering current conditions in the lower Susquehanna River, the model demonstrates potential for extensive release of adsorbed PO4 at pH >8. Empirical data from laboratory experiments corroborate model results. The transfer of PO4 into the water column may increase algae growth, which removes CO2 and drives pH to higher values, facilitating additional PO4 release and exacerbating the potential for harmful algal blooms. Thus, legacy sediment is a currently unquantified source of PO4 that warrants consideration by resource managers and programs collaborating to reduce phosphorus loads to the bay and similar settings worldwide.

The importance of consensus science to managing phosphorus in the environment: SERA-17 and the legacy of Andrew Sharpley

Phosphorus (P) loss from agricultural systems to surface waters, and ultimately, eutrophication, presents a wicked problem requiring transdisciplinary solutions. The mission of SERA-17 (Southern Extension and Research Advisory Information Exchange Group-17) has been to address this problem by developing "Innovative Solutions to Minimize Phosphorus Losses from Agriculture." Over the course of his career, Dr. Andrew Sharpley demonstrated a rare ability to collaboratively achieve consensus around issues related to the science and management of P. The SERA-17 organization served as the central community of experts and stakeholders where that consensus was built and applied. The consensus-based approach, demonstrated by Sharpley and at the core of the SERA-17 organization, was routinely applied to key areas of P science to produce applied outcomes that have been readily adopted: advance foundational science to resolve knowledge gaps and to promote innovation; promote consistency in methods to facilitate comprehensive investigations and conclusions across a diversity of systems; engage diverse stakeholders to prioritize research, and ultimately, ensure that outcomes reflect a plurality of perspectives; and deliver pragmatic solutions that reflect the best information available at a particular time. We review the history of SERA-17 in delivering new science and management recommendations for P, with an eye to elucidating Sharpley's role and legacy in this process.

Balancing agriculture and environment: Andrew Sharpley's nutrient, soil, and water management legacy

Managing agricultural phosphorus (P) to balance food security and water quality priorities is a massive challenge fraught with uncertainty and competing interests. Throughout his career, Andrew Sharpley addressed this challenge by building our understanding of the fundamental principles and processes that control P behavior in agricultural land, developing tools to assess P losses, and then evaluating and refining nutrient, soil, and water beneficial management practices (BMPs). Together with an exceptionally large and diverse group of collaborators, Sharpley developed, tested, refined, calibrated, and validated management practices and risk assessment tools to develop site-specific recommendations for the right practices, in the right places, and at the right times. This approach has resonated globally, with the strategic use of BMPs in "critical source areas" widely implemented in an effort to improve the effectiveness of BMPs while reducing implementation costs. Additional contributions to nutrient management include determining environmental thresholds for soil test P and measuring the risk of P loss from different sources of P (e.g., various manures and commercial fertilizers). Sharpley's work was also distinctly realistic, ensuring that strategies for mitigating P loss were critically evaluated so that not only were the benefits highlighted, but also that trade-offs were measured. Nowhere is this better illustrated than with trade-offs in particulate P loss and dissolved P loss with conservation tillage. This review summarizes Sharpley's enormous contributions to our knowledge of agricultural P stewardship as well as his model of collaborative, multi-disciplinary leadership, helping the world to maintain agricultural productivity and protect water quality.

Prioritizing river basins for nutrient studies

Increases in fluxes of nitrogen (N) and phosphorus (P) in the environment have led to negative impacts affecting drinking water, eutrophication, harmful algal blooms, climate change, and biodiversity loss. Because of the importance, scale, and complexity of these issues, it may be useful to consider methods for prioritizing nutrient research in representative drainage basins within a regional or national context. Two systematic, quantitative approaches were developed to (1) identify basins that geospatial data suggest are most impacted by nutrients and (2) identify basins that have the most variability in factors affecting nutrient sources and transport in order to prioritize basins for studies that seek to understand the key drivers of nutrient impacts. The "impact" approach relied on geospatial variables representing surface-water and groundwater nutrient concentrations, sources of N and P, and potential impacts on receptors (i.e., ecosystems and human health). The "variability" approach relied on geospatial variables representing surface-water nutrient concentrations, factors affecting sources and transport of nutrients, model accuracy, and potential receptor impacts. One hundred and sixty-three drainage basins throughout the contiguous United States were ranked nationally and within 18 hydrologic regions. Nationally, the top-ranked basins from the impact approach were concentrated in the Midwest, while those from the variability approach were dispersed across the nation. Regionally, the top-ranked basin selected by the two approaches differed in 15 of the 18 regions, with top-ranked basins selected by the variability approach having lower minimum concentrations and larger ranges in concentrations than top-ranked basins selected by the impact approach. The highest ranked basins identified using the variability approach may have advantages for exploring how landscape factors affect surface-water quality and how surface-water quality may affect ecosystems. In contrast, the impact approach prioritized basins in terms of human development and nutrient concentrations in both surface water and groundwater, thereby targeting areas where actions to reduce nutrient concentrations could have the largest effect on improving water availability and reducing ecosystem impacts.

Water quality in a large complex catchment: Significant effects of land use and soil type but limited ability to detect trends

Globally, significant societal resources are devoted to mitigating negative effects of eutrophication from excessive phosphorus (P) and nitrogen (N) loading. Potential effectiveness of mitigation measures and possible confounding factors are often assessed using studies conducted in headwater catchments. However, success is often evaluated based on trends in river mouth water chemistry. It is not clear how transferrable insights from headwater catchments are to larger rivers. Here, relationships between P and suspended solids (SS) identified in small agricultural headwater catchments were applied to 30 larger, mixed land use catchments draining into Mälaren, a Swedish great lake. Relationships identified in headwater streams between SS concentration, catchment agricultural land percentage and arable land clay content were corroborated for the larger catchments (R2 = 0.59, p-value<0.001. The same was true for connections between SS and particulate P (R2 = 0.74, p-value<0.001). This study highlights the importance of agricultural land, clay content and SS for P transport, on both smaller headwater as well as larger catchment scales, supporting the use of headwater findings on larger, management relevant scales. Consequently, these relationships should be used to target mitigation measures to reduce SS and P losses. To explore the effectiveness of mitigation measures on water quality, we assessed long-term (20 year) trends in tributary water quality and compared these trends to the amount of mitigation measures implemented in the catchment. Overall improving trends were detected using regional Mann Kendall tests, but few decreasing trends in nutrient concentrations were found for individual sites using Generalized Additive Models (GAM). The lack of significant trends and identifiable connections to amount of mitigation measures implemented could be due to several reasons, e.g. insufficient time for recently implemented measures to have an effect, ongoing release of legacy P as well as low areal coverage and poor spatial placement of implemented measures. In addition, trend detection requires large amounts of data and the results should be carefully interpreted and communicated.

Muddied Waters: The Use of "Residual" And "Legacy" Phosphorus

Phosphorus (P) inputs to the biosphere have quadrupled in less than a century due to intensification of rock phosphate mining and the use of P fertilizers for crop production. Accumulation of P in soils can increase P transfers across the soil-water continuum that impair aquatic ecosystem function and water resource quality for society. However, what this accumulated P is called, and subsequent connotations of magnitude versus mechanism at pedon versus watershed scale, varies in the literature. We argue that the two commonly used terms of "residual" and "legacy" P, though often used interchangeably, hold distinct meanings and connotations. Tracing the historical origins and trajectories of these terms reveals that "residual P" refers to the magnitude of fertilizer P that remains in the soil after crop harvest, whereas "legacy P" refers to the mechanism of P transfer across the watershed and its long-term impacts on water quality. The use of "legacy P" in many cases refers to the residuality of anthropogenic P inputs, and thus should be "residual P". We recommend that the term "residual P" be used when referring to the accumulation of P in soils under agricultural management from past inputs, and the term "legacy P" be used when referring to the transfer of P within watersheds. The intentional and thus consistent use of residual versus legacy P stands to provide important nuance in the environmental sciences and overlapping fields of agronomy and biogeochemistry.

Streambank erosion and phosphorus loading to surface waters: Knowns, unknowns, and implications for nutrient loss reduction research and policy

To monitor and meet water quality objectives, it is necessary to understand and quantify the contribution of nonpoint sources to total phosphorus (P) loading to surface waters. However, the contribution of streambank erosion to surface water P loads remains unclear and is typically unaccounted for in many nutrient loading assessments and policies. As a result, agricultural contributions of P are overestimated, and a potentially manageable nonpoint source of P is missed in strategies to reduce loads. In this perspective, we review and synthesize the results of a special symposium at the 2022 ASA-CSSA-SSSA annual meeting in Baltimore, MD, that focused on streambank erosion and its contributions to P loading of surface waters. Based on discussions among researchers and policy experts, we overview the knowns and unknowns, propose next steps to understand streambank erosion contribution to P export budgets, and discuss implications of the science of streambank erosion for policy and nutrient loss reduction strategies.

Revealing the hidden burden for lake management: the sediment phosphorus storage pools in Eastern Plain Lake Zone, China

As one of the essential components in ecosystems, lakes play a major role in the global phosphorus (P) cycle. It is helpful for further understanding of the inside lake P geochemical cycle to research P pollution and storage in lakes, which is of positive significance for lake eutrophication restoration. In this study, we investigated the total phosphorus concentrations (TPC) of water and sediments from 37 lakes in the Eastern Plain Lake Zone (EPL) of China, evaluated the P pollution degree of lakes, and estimated P storage in lake sediments with quantitative data of lake area and number. The results indicate that the total phosphorus concentrations of water (TPCW) and total phosphorus concentrations of the surface sediments (0-1 cm, TPCSS) in EPL were high, the mean values were 0.11 mg·L-1 and 869.85 mg·kg-1 respectively, with obvious differences between urban and rural areas, as well as between different river basins. Over half (56.76% and 70.27% respectively) of the lakes reached severe pollution levels in water and surface sediments. There were 16224 lakes (> 0.01 km2) with a total area of 21662.37 km2 in the EPL, and the P storage in the lake sediments (0-30 cm) was about 4.87 ± 2.08 Tg (1 Tg = 1 × 1012 g), accounting for about 2.74% of the basin soil. TPCW and TPCSS of lakes in the EPL were significantly positively correlated, may suggest a close nutrient cycling relationship between the lake water and the sediment. During periods of high winds and waves, the stored P in the top sediments in the EPL may continue to participate in the internal P geochemical cycle and migrate to the overlying water, posing a potential pollution hazard. Therefore, it is crucial to take into account the sediment P pools when formulating effective lake phosphorus management strategies.

Application of classification machine learning algorithms for characterizing nutrient transport in a clay plain agricultural watershed

Excess nutrients in surface water and groundwater can lead to water quality deterioration in available water resources. Thus, the classification of nutrient concentrations in water resources has gained significant attention during recent decades. Machine learning (ML) algorithms are considered an efficient tool to describe nutrient loss from agricultural land to surface water and groundwater. Previous studies have applied regression and classification ML algorithms to predict nutrient concentrations in surface water and/or groundwater, or to categorize an output variable using a limited number of input variables. However, there have been no studies that examined the application of different ML classification algorithms in agricultural settings to classify various output variables using a wide range of input variables. In this study, twenty-four ML classification algorithms were implemented on a dataset from three locations within the Upper Parkhill watershed, an agricultural watershed in southern Ontario, Canada. Nutrient concentrations in surface water were classified using geochemical and physical water parameters of surface water and groundwater (e.g., pH), climate and field conditions as the input variables. The performance of these algorithms was evaluated using four evaluation metrics (e.g., classification accuracy) to identify the optimal algorithm for classifying the output variables. Ensemble bagged trees was found to be the optimal ML algorithm for classifying nitrate concentration in surface water (accuracy of 90.9%), while the weighted KNN was the most appropriate algorithm for categorizing the total phosphorus concentration (accuracy of 87%). The ensemble subspace discriminant algorithm gave the highest overall classification accuracy for the concentration of soluble reactive phosphorus and total dissolved phosphorus in surface water with an accuracy of 79.2% and 77.9%, respectively. This study exemplifies that ML algorithms can be used to signify exceedance of recommended concentrations of nutrients in surface waters in agricultural watersheds. Results are useful for decision makers to develop nutrient management strategies.

A graph-based modeling framework for tracing hydrological pollutant transport in surface waters

Anthropogenic pollution of hydrological systems affects diverse communities and ecosystems around the world. Data analytics and modeling tools play a key role in fighting this challenge, as they can help identify key sources as well as trace transport and quantify impact within complex hydrological systems. Several tools exist for simulating and tracing pollutant transport throughout surface waters using detailed physical models; these tools are powerful, but can be computationally intensive, require significant amounts of data to be developed, and require expert knowledge for their use (ultimately limiting application scope). In this work, we present a graph modeling framework - which we call HydroGraphs - for understanding pollutant transport and fate across waterbodies, rivers, and watersheds. This framework uses a simplified representation of hydrological systems that can be constructed based purely on open-source data (National Hydrography Dataset and Watershed Boundary Dataset). The graph representation provides a flexible intuitive approach for capturing connectivity and for identifying upstream pollutant sources and for tracing downstream impacts within small and large hydrological systems. Moreover, the graph representation can facilitate the use of advanced algorithms and tools of graph theory, topology, optimization, and machine learning to aid data analytics and decision-making. We demonstrate the capabilities of our framework by using case studies in the State of Wisconsin; here, we aim to identify upstream nutrient pollutant sources that arise from agricultural practices and trace downstream impacts to waterbodies, rivers, and streams. Our tool ultimately seeks to help stakeholders design effective pollution prevention/mitigation practices and evaluate how surface waters respond to such practices.

Legacy phosphorus delays the accomplishment of expected phosphorus management object

Legacy phosphorus (P) in watersheds continuously affects the water quality. The time lag between anthropogenic P input and algal bloom has made P dynamics prediction in aquatic ecosystems more challenging. Whether the legacy P in the Yangtze River Watershed (YRW) exceeds its storage threshold remains unknown, and the continuous impact of legacy P on the water quality has not been analyzed. This study aimed to evaluate variation trends (1970-2018) and influencing factors for accumulated P in the YRW under different economic development periods, quantitatively identify the watershed P storage threshold based on the two split line models and estimate the time required for the return of legacy P to the baseline level using an exponential decay process. The results showed that the P storage threshold of the YRW was surpassed due to intense anthropogenic activities, and the residual P still had an impact on aquatic ecosystems for a long time. The dissolved total P loadings may become the top priority to achieve better P management goals. The time lags for the legacy P restoration would require for about 1000 years to be exhausted. The legacy P in the YRW would continuously undermine the restoration efforts of the water quality. The combined effects of watershed P surplus reductions and depletion of residual P may become essential to better manage P in the future. We still need to strengthen our efforts to make soil legacy P more absorbed by crops and improve sewage treatment capacity to achieve sustainable development of YRW.

The spatial and temporal dynamics of sediment phosphorus attenuation and release in impacted stream catchments

Sediments can attenuate phosphorus (P) from overlying water and reduce trophic status in zero and first order ditches and streams. These features can be considered as intermediate mitigation features between P mobilised from land, and onward delivery to river systems, if the risk of chemical P release from sediments is minimal. However, risk assessments are rarely based on temporal scale dynamics and especially at fine scale in both sediment and water column environments. In this study, in eutrophic stream catchments, bed sediments were tested fortnightly and spatially over one year for EPC0 (to derive phosphate exchange potential-PEP) and for P across a spectrum from labile to recalcitrant fractions. At the same time stream discharge and P concentrations were measured synchronously at high frequency and resolved to 1-hour intervals and indicated high water quality pressures at all flow rates. PEP indicated spatial and temporal changes most likely caused by periods of source disconnection/reconnection and sediment mobilisation during storm events, moving from periods of high attenuation potential to near saturation. Despite these spatial and temporal changes, PEP did not indicate much potential for chemical P release from the sediments (distributing mostly below or close to zero). However, this may be a misleading risk assessment by itself as physical P release, especially of the labile bicarbonate-dithionite (B-D) P fraction of sediments, was a more dominant process mobilised during storm events reducing by up to 84 % during a succession of summer storm events. The total P and total reactive P loads monitored leaving the catchments were coincident with these changes. The specific downstream trophic effects of this episodic P release will need to be assessed in terms of its bioavailability, in combination with other more noted diffuse and point P source processes.

Nonpoint source pollution measures in the Clean Water Act have no detectable impact on decadal trends in nutrient concentrations in U.S. inland waters

The Clean Water Act (CWA) of 1972 regulates water quality in U.S. inland waters under a system of cooperative federalism in which states are delegated implementation and enforcement authority of CWA provisions by the U.S. Environmental Protection Agency. We leveraged heterogeneity in state implementation of the CWA to evaluate the efficacy of its nonpoint source provisions in reducing nutrient pollution, the leading cause of water quality impairment in U.S. inland waters. We used national survey data to estimate changes in nutrient concentrations over a decade and evaluated the effect of state-level policy implementation. We found no evidence to support an effect of (i) grant spending on nonpoint source pollution remediation, (ii) nutrient criteria development, or (iii) water quality monitoring intensity on 10-year trends in nutrient concentrations. These results suggest that the current federal policy paradigm for improving water quality is not creating desired outcomes.

Effectiveness of flue gas desulfurization gypsum in reducing phosphorus solubility in poultry litter when applied as an in-house amendment

Phosphorus (P) runoff from agricultural lands receiving poultry litter (PL) poses a major environmental challenge. Application of flue-gas-desulfurization (FGD)-gypsum produced from coal power plants in agricultural lands has shown promise to reduce P losses. However, no information is available about the effectiveness of FGD-gypsum addition in reducing P solubility when applied as an in-house amendment. Hence, the objectives of this study were to understand a) effectiveness of FGD-gypsum as a litter amendment in reducing P loss risk; and b) how FGD-gypsum amendment in PL alters the distribution of P forms. Broiler chickens were raised for five flocks in seven individual litter treatments replicated four times in a randomized complete block design. Based on the FGD-gypsum addition, the PL treatments were broadly classified as FGD-gypsum treated and untreated. Toxic metal concentrations were analyzed in FGD-gypsum as well as the treatments. Sequential water extractions were performed to understand P solubility. Litter P fractionation was performed to identify bioavailable P (Water-P), labile P (NaHCO3-P), aluminum/iron chemisorbed P (NaOH-P), and mineral occluded P (HCl-P). Results indicated significantly higher soluble P in all untreated than in all FGD-gypsum treated litters in the initial water extraction. The FGD-gypsum treated litters reduced soluble P by 58 to 67% in the 1st water extraction compared to untreated litters. Fractionation study revealed lower proportion of Water-P and higher proportion of NaHCO3-P and HCl-P in all FGD-gypsum treated than in untreated litters. This study suggests reuse of FGD-gypsum in broiler houses can help reduce P mobility without any toxic metals concerns.

Value and limitations of machine learning in high-frequency nutrient data for gap-filling, forecasting, and transport process interpretation

High-frequency monitoring of water quality in catchments brings along the challenge of post-processing large amounts of data. Moreover, monitoring stations are often remote and technical issues resulting in data gaps are common. Machine learning algorithms can be applied to fill these gaps, and to a certain extent, for predictions and interpretation. The objectives of this study were (1) to evaluate six different machine learning models for gap-filling in a high-frequency nitrate and total phosphorus concentration time series, (2) to showcase the potential added value (and limitations) of machine learning to interpret underlying processes, and (3) to study the limits of machine learning algorithms for predictions outside the training period. We used a 4-year high-frequency dataset from a ditch draining one intensive dairy farm in the east of The Netherlands. Continuous time series of precipitation, evapotranspiration, groundwater levels, discharge, turbidity, and nitrate or total phosphorus were used as predictors for total phosphorus and nitrate concentrations respectively. Our results showed that the random forest algorithm had the best performance to fill in data-gaps, with R² higher than 0.92 and short computation times. The feature importance helped understanding the changes in transport processes linked to water conservation measures and rain variability. Applying the machine learning model outside the training period resulted in a low performance, largely due to system changes (manure surplus and water conservation) which were not included as predictors. This study offers a valuable and novel example of how to use and interpret machine learning models for post-processing high-frequency water quality data.

Legacy contributions to diffuse water pollution: Data-driven multi-catchment quantification for nutrients and carbon

Legacy pollutants are increasingly proposed as possible reasons for widespread failures to improve water quality, despite the implementation of stricter regulations and mitigation measures. This study investigates this possibility, using multi-catchment data and relatively simple, yet mechanistically-based, source distinction relationships between water discharges and chemical concentrations and loads. The relationships are tested and supported by the available catchment data. They show dominant legacy contributions for total nitrogen (TN), total phosphorus (TP) and total organic carbon (TOC) across catchment locations and scales, from local to country-wide around Sweden. Consistently across the study catchments, close relationships are found between the legacy concentrations of TN and TOC and the land shares of agriculture and of the sum of agriculture and forests, respectively. The legacy distinction and quantification capabilities provided by the data-driven approach of this study could guide more effective pollution mitigation and should be tested in further research for other chemicals and various sites around the world.

Streamflow duration curve to explain nutrient export in Midwestern USA watersheds: Implication for water quality achievements

As part of federal programs to reduce nutrient pollution, states across the Midwest have developed nutrient reduction strategies, which focus on implementation of agricultural conservation practices (ACPs) or best management practices (BMPs). Despite several decades of federal investment in implementing ACPs/BMPs for reducing nutrient pollution, nutrient pollution is a continuing and growing challenge with profound implications for water quality and public health as well as ecological functions. Pollutant transport depends on water and sediment fluxes, which are governed by local hydrology. Therefore, knowing how flow conditions affect nutrients export is critical to develop effective nutrient reduction strategies. The objective of this study was to investigate the role of streamflow duration curve in controlling nutrient export in the western Lake Erie Basin and the Mississippi River Basin. To achieve this goal, we used long-term monitoring data collected by the National Center for Water Quality Research. We focused on the percentage of the annual pollutant load (nitrate-NO3-N, dissolved reactive phosphorus-DRP, total phosphorus-TP, and total suspended solids-TSS) exported during five flow intervals that spanned the flow duration curve: High Flows (0-10th percentile), Moist Conditions (10-40th percentile), Mid-Range Flows (40-60th percentile), Dry Conditions (60-90th percentile), and Low Flows (90-100th percentile). The results show that the top 10% of flows (i.e., high flows) transported more than 50% of the annual nutrient loads in most of the studying watersheds. Meanwhile, the top 40% of flows transported 54-98% of the annual NO3-N loads, 55-99% of the annual DRP loads, 79-99% of the annual TP loads, and 86-100% of the annual TSS loads across the studying watersheds. The percentage of the annual loads released during high flows increased as the percentage of the agricultural land use in the watershed increased, but it decreased as the watershed area increased across different watersheds. Finally, flow condition/nutrient export relationships were consistent over studying period. Therefore, reducing nutrient loads during high flow condition is the key for effective nutrient reduction.

Effects of paddy irrigation-drainage system on water quality and productivity of small rivers in the Himi region of Toyama, Central Japan

The aim of this study was to clarify the impact of differences between historical and recently introduced irrigation and drainage management systems on water quality in the rivers around paddy fields. We investigated the seasonal variation in nutrients concentration and dissolved organic carbon (DOC) components in single- (used for intake only) and dual-purpose (used for both intake and drainage) channels during a 4-year period in the Himi region of Toyama, Central Japan. The system of dual-purpose channel has traditionally been used in the region of this study. A total of 197 three-dimensional excitation-emission matrix (3DEEM) fluorescence spectra of DOM in waters were applied for the parallel factor analysis (PARAFAC) modeling. Based on the 3DEEM and PARAFAC, the abundance of terrestrial humic-like components in the dual-purpose channel was significantly higher than that in the single-purpose channel. The even long-chain n-fatty acids derived associated with rice cropping in sediments of the dual-purpose channels were 22-30-fold higher than that of the single-purpose channel. In addition, the turbidity values of the river waters had significantly positive linear correlations with concentrations of K⁺, DOC, and humic-like components. These observations indicate that the dissolved nutrient concentrations in the river water were higher in the dual-purpose channel compared to those of the single-purpose channel, which may be supplied by leaching from the inflow of soil particles from the paddy fields. During the mid-irrigation period, the quantity of epiphytic chlorophyll a on artificial substrate tiles in the dual-purpose channel were 3.1-4.1-fold higher than that in the single-purpose channel. This study clear that the input of paddy drainage during the irrigation season significantly changes the DOC components in river waters and irrigation management is strongly linked to the primary production in agricultural channels. Therefore, it is important to consider the impact of the introduction of different irrigation and drainage management systems on water quality and productivity in order to maintain the riverine ecosystems around rice paddies, which are based on historical water use systems.

Amination-modified lignin recovery of aqueous phosphate for use as binary slow-release fertilizer

To address the global phosphorus crisis and solve the problem of eutrophication in water bodies, the recovery of phosphate from wastewater for use as a slow-release fertilizer and to improve the slow-release performance of fertilizers is considered an effective way. In this study, amine-modified lignin (AL) was prepared from industrial alkali lignin (L) for phosphate recovery from water bodies, and then the recovered phosphorus-rich aminated lignin (AL-P) was used as a slow-release N and P fertilizer. Batch adsorption experiments showed that the adsorption process was consistent with the Pseudo-second-order kinetics and Langmuir model. In addition, ion competition and actual aqueous adsorption experiments showed that AL had good adsorption selectivity and removal capacity. The adsorption mechanism included electrostatic adsorption, ionic ligand exchange and cross-linked addition reaction. In the aqueous release experiments, the rate of nitrogen release was constant and the release of phosphorus followed a Fickian diffusion mechanism. Soil column leaching experiments showed that the release of N and P from AL-P in soil followed the Fickian diffusion mechanism. Therefore, AL recovery of aqueous phosphate for use as a binary slow-release fertilizer has great potential to improve the environment of water bodies, enhance nutrient utilization and address the global phosphorus crisis.

Enhancing the Soil Health-Watershed Health Nexus: Introduction

Scientific concepts and measurements that relate soil and water resources are lacking in several areas, limiting our development of a framework or nexus to assess soil-watershed health. Current research designs rely on land management practices as a proxy for soil condition. Yet, conservation practices are often studied in isolation of each other, and adoption may be driven by state and federal farm programs that can incentivize a given management practice without accounting for current, novel farmer-driven adoption of conservation systems. Despite the value of conservation management, its ability to predict soil health is often limited if based solely on land management because chemical, physical, and biological processes vary across time, discipline, and terrain. Similarly, connections between soil health and water quality are constrained due to several "grand challenges" that include dissimilar scales and the number of metrics required to correlate soil and water systems. Equally important is soil sampling within the critical flow path(s) that determines sediment/contaminant loading. In some instances, most of the sediment/contaminant loading during a portion or entire year results from channel and bank erosion and not overland flow that may not be within conservation management hectares. Additional challenges include legacy effects of prior land management, climate variability, and varying turnover rates of soil and water systems. This special section aims to frame research issues that inspire new approaches and collaborations for tackling the challenge of leveraging soil health to strengthen water management across plot, field, and watershed scales, using models, statistics, and other novel methodologies.

Phosphorus sources, forms, and abundance as a function of streamflow and field conditions in a Maumee River tributary, 2016-2019

Total phosphorus (TP), dissolved P (DP), and suspended sediment (SS) were sampled in Black Creek, Indiana, monthly during base flow and for 100 storm events during water years 2016-2019, enabling analysis of how each of these varied as a function of streamflow and field conditions at nested edge-of-field sites. Particulate P was normalized for SS (P_SS  = [TP - DP]/SS). Streamflow events were differentiated by maximum TP concentrations co-occurring with maximum SS (SED) or DP (SOL). The combination of new precipitation and high antecedent soil-water storage during months when fields were exposed coincided with higher streamflow that drove SED events. These SED events carried more SS, including sediment eroded from streambanks that added sediment P but also may have provided for sorption of DP. During SOL events, DP was higher and contributed approximately half of TP; SS was lower. These SOL events had higher P_SS , more similar to that in base flow as well as composited samples of overland flow and tile-drain discharge from fields. Base-flow samples had significantly higher P_SS concentrations than most event samples, with ≤25 times enrichment relative to soil P concentrations in fine-grained source material. Combining base-flow and event samples showed that P_SS integrates SS, DP, and streamflow. Addition of new suspended sediment during events may provide for sorption of DP during and after events and storage in the system, delaying delivery of this P to Lake Erie relative to what would be expected for the dissolved form but adding to the legacy P stored in the stream system.

Transport-limited kinetics of phosphate retention on iron-coated sand and practical implications

Iron-coated sand (ICS) is a by-product from drinking water treatment made of sand coated with ferric iron (hydr)oxides. It is considered a suitable material for large-scale measures for phosphate removal from natural and agricultural waters to prevent eutrophication. Previous studies demonstrated that the residence time of water must be very long to reach equilibrium partitioning between phosphate and ICS but specifics for application are missing. First, SEM-EDX images were used to support the conceptual assumption that P adsorption inside the coating is a transport-limited process. Second, a conceptual model of phosphate adsorption was proposed considering two types of sites: one type with fast adsorption kinetics and reaching equilibrium with the percolating solution, and another type for which adsorption is also reversible but described by pseudo-first-order kinetics. The latter is conceived to account for transport-limited adsorption in the interior of the coating while the former fraction of sites is assumed to be easily accessible and located close to the grain surface. Third, the kinetics of phosphate adsorption on ICS were quantitatively determined to describe and predict phosphate retention in filters under various flow conditions. The model was calibrated and validated with long-term column experiments, which lasted for 3500 h to approach equilibrium on the slowly reacting sites. The model reproduced the outflowing phosphate concentrations: the pronounced increase after a few pore volumes and the slow increase over the remaining part of the experiment. The parameterized model was also able to predict the time evolution of phosphate concentrations in the outflow of column experiments with different flow velocities, flow interruption, and in desorption experiments. The equilibrium partition coefficient for the experimental conditions was identified as 28.1 L/g-Fe at pH 6.8 and a phosphate concentration of 1.7 mg-P / L. The optimized first-order mass transfer coefficient for the slow adsorption process was 1.56 10^-4 h^-1, implying that the slow adsorption process has a time scale of several months. However, based on the parameterized model, the slow adsorption process accounted for 95.5% of the equilibrium adsorption capacity, emphasizing the potential relevance of this process for practical applications. The implications for the design, operation, and lifespan of ICS filters are exemplarily illustrated for different scenarios.

Algal community structure prediction by machine learning

The algal community structure is vital for aquatic management. However, the complicated environmental and biological processes make modeling challenging. To cope with this difficulty, we investigated using random forests (RF) to predict phytoplankton community shifting based on multi-source environmental factors (including physicochemical, hydrological, and meteorological variables). The RF models robustly predicted the algal communities composed by 13 major classes (Bray-Curtis dissimilarity = 9.2 ± 7.0%, validation NRMSE mostly <10%), with accurate simulations to the total biomass (validation R² > 0.74) in Norway's largest lake, Lake Mjosa. The importance analysis showed that the hydro-meteorological variables (Standardized MSE and Node Purity mostly >0.5) were the most influential factors in regulating the phytoplankton. Furthermore, an in-depth ecological interpretation uncovered the interactive stress-response effect on the algal community learned by the RF models. The interpretation results disclosed that the environmental drivers (i.e., temperature, lake inflow, and nutrients) can jointly pose strong influence on the algal community shifts. This study highlighted the power of machine learning in predicting complex algal community structures and provided insights into the model interpretability.

Advances in Catchment Science, Hydrochemistry, and Aquatic Ecology Enabled by High-Frequency Water Quality Measurements

High-frequency water quality measurements in streams and rivers have expanded in scope and sophistication during the last two decades. Existing technology allows in situ automated measurements of water quality constituents, including both solutes and particulates, at unprecedented frequencies from seconds to subdaily sampling intervals. This detailed chemical information can be combined with measurements of hydrological and biogeochemical processes, bringing new insights into the sources, transport pathways, and transformation processes of solutes and particulates in complex catchments and along the aquatic continuum. Here, we summarize established and emerging high-frequency water quality technologies, outline key high-frequency hydrochemical data sets, and review scientific advances in key focus areas enabled by the rapid development of high-frequency water quality measurements in streams and rivers. Finally, we discuss future directions and challenges for using high-frequency water quality measurements to bridge scientific and management gaps by promoting a holistic understanding of freshwater systems and catchment status, health, and function.

Periphyton Phosphorus Uptake in Response to Dynamic Concentrations in Streams: Assimilation and Changes to Intracellular Speciation

Effective modeling and management of phosphorus (P) losses from landscapes to receiving waterbodies requires an adequate understanding of P retention and remobilization along the terrestrial-aquatic continuum. Within aquatic ecosystems, the stream periphyton can transiently store bioavailable P through uptake and incorporation into biomass during subscouring and baseflow conditions. However, the capacity of stream periphyton to respond to dynamic P concentrations, which are ubiquitous in streams, is largely unknown. Our study used artificial streams to impose short periods (48 h) of high SRP concentration on stream periphyton acclimated to P scarcity. We examined periphyton P content and speciation through nuclear magnetic resonance spectroscopy to elucidate the intracellular storage and transformation of P taken up across a gradient of transiently elevated SRP availabilities. Our study demonstrates that the stream periphyton not only takes up significant quantities of P following a 48-h high P pulse but also sustains supplemental growth over extended periods of time (10 days), following the reestablishment of P scarcity by efficiently assimilating P stored as polyphosphates into functional biomass (i.e., phospho-monoesters and phospho-diesters). Although P uptake and intracellular storage approached an upper limit across the experimentally imposed SRP pulse gradient, our findings demonstrate the previously underappreciated extent to which the periphyton can modulate the timing and magnitude of P delivery from streams. Further elucidating these intricacies in the transient storage potential of periphyton highlights opportunities to enhance the predictive capacity of watershed nutrient models and potentially improve watershed P management.

Investigating relationships between climate controls and nutrient flux in surface waters, sediments, and subsurface pathways in an agricultural clay catchment of the Great Lakes Basin

Water quality within agricultural catchments is governed by management practices and climate conditions that control the transport, storage, and exchange of nutrients between components of the hydrologic cycle. This study aims to improve knowledge of nitrogen (N) and phosphorus (P) transport in low permeability agricultural watersheds by considering spatial and temporal trends of surface water nutrient concentrations in relation to hydroclimatic drivers, sediment quality, shallow hyporheic exchange, groundwater quality, and tile drain discharge over a 14-month field study in a clay hydrosystem of the Lake Huron basin, one of the five Great Lakes. Results found that events of varying magnitude and intensity enhanced nutrient release from overland flow and subsurface pathways. Tile drain discharge was found to be a consistent and elevated source of P and N to surface waters when flowing, mobilizing both diffuse nutrients from fertilizer application and legacy stores in the vadose zone. Surface water quality was temporally variable at the seasonal and event scale. Targeted sampling following fertilization periods, snowmelt, and moderate precipitation events revealed catchment wide elevated nutrient concentrations, emphasizing the need for targeted sampling regimes. Controls other than discharge magnitude and overland flow were found to contribute to peak nutrient concentrations, including internal nitrate loading, soil-snowmelt interaction, catchment wetness, and freeze thaw cycles. Sediments were found to store P in calcium minerals and have a high P storage capacity. Instream mechanisms such as sediment P fixation and hyporheic exchange may play a role in mediating surface water quality, but currently have no discernable benefit to year-round surface water nutrient concentrations. Best management practices need to focus on reducing sources of agricultural nutrients (e.g., field phosphorus concentrations and tile drain discharge loading) at the watershed scale to reduce nutrient concentrations and export in flashy clay catchments.