Phosphorus losses from agricultural areas in river basins: effects and uncertainties of targeted mitigation measures

Ironstone and red mud barriers to reduce subsurface movement of soil phosphorus

Loss of phosphorus in seepage may contribute to eutrophication of downstream water bodies. This study examined the potential use of pedogenic ironstone and untreated red mud (bauxite refining residue) as P sorbents in a permeable reactive barrier (PRB) to mitigate such loss. Effects of ironstone and red mud on P sorption (batch), transport (columns), saturated hydraulic conductivity (KS), and growth of common bermudagrass (Cynodon dactylon; greenhouse) were examined. Both materials had sorption maxima of ∼30 mmol P kg-1 or about five times that of a P-enriched sandy soil; however, sorption by red mud greatly increased with decreasing pH. Transport of P through columns of ironstone and red mud (diluted with nonreactive sand) was similar and slower compared to soil + sand. However, when red mud was mixed with soil, increased sorption at lower pH resulted in greater P retention compared to ironstone + soil (76% vs. 13%). Although addition of ironstone to soil up to 20% did not reduce KS, red mud at even 5% did. Soil amendment with red mud increased bermudagrass growth and P uptake. Given long-term neutralization of red mud in an acidic soil and increased P sorption, it may be suitable in a PRB if incorporated at a low rate and/or co-incorporated with a coarser material.

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.

Phosphorus and iron-oxide transport from a hydrologically isolated grassland hillslope

Dissolved reactive phosphorus (DRP) loss from agricultural soils can negatively affect water quality. Shallow subsurface pathways can dominate P losses in grassland soils, especially in wetter months when waterlogging is common. This study investigated the processes controlling intra- and inter-event and seasonal DRP losses from poorly drained permanent grassland hillslope plots. Temporal flow related water samples were taken from surface runoff and subsurface (in-field pipe) discharge, analysed, and related to the likelihood of anaerobic conditions and redoximorphic species including nitrate (NO₃^-) over time. Subsurface drainage accounted for 89% of total losses. Simple linear regression and correlation matrices showed positive relationships between DRP and iron and soil moisture deficit; and negative relationships between these three factors and NO₃^- concentrations in drainage. These data indicate that waterlogging and low NO₃^- concentrations control the release of P in drainage, potentially via reductive dissolution. The relationship between DRP and metal release was less obvious in surface runoff, as nutrients gathered from P-rich topsoil camoflaged redox reactions. The data suggest a threshold in NO₃^- concentrations that could exacerbate P losses, even in low P soils. Knowledge of how nutrients interact with soil drainage throughout the year can be used to better time soil N and P inputs via, for example, fertiliser or grazing to avoid to excessive P loss that could harm water quality.

Sewage sludge-derived biochar for the adsorptive removal of wastewater pollutants: A critical review

The production of biochar from sewage sludge pyrolysis is a promising approach to transform the waste resultant from wastewater treatment plants (WWTPs) to a potential adsorbent. The current review provides an up-to-date review regarding important aspects of sewage sludge pyrolysis, highlighting the process that results major solid fraction (biochar), as high-value product. Further, the physio-chemical characteristics of sewage-sludge derived biochar such as the elemental composition, specific surface area, pore size and volume, the functional groups, surface morphology and heavy metal content are discussed. Recent progress on adsorption of metals, emerging pollutants, dyes, nutrients and oil are discussed and the results are examined. The sewage sludge-derived biochar is a promising material that can make significant contributions on pollutants removal from water by adsorption and additional benefit of the management of huge volume of sewage. Considering all these aspects, this field of research still needs more attention from the researchers in the direction of the technological features and sustainability aspects.

Phosphorus Transport along the Cropland–Riparian–Stream Continuum in Cold Climate Agroecosystems: A Review

Phosphorus (P) loss from cropland to ground and surface waters is a global concern. In cold climates (CCs), freeze–thaw cycles, snowmelt runoff events, and seasonally wet soils increase P loss potential while limiting P removal effectiveness of riparian buffer zones (RBZs) and other practices. While RBZs can help reduce particulate P transfer to streams, attenuation of dissolved P forms is more challenging. Moreover, P transport studies often focus on either cropland or RBZs exclusively rather than spanning the natural cropland–RBZ–stream gradient, defined here as the cropland–RBZ–stream continuum. Watershed P transport models and agronomic P site indices are commonly used to identify critical source areas; however, RBZ effects on P transport are usually not included. In addition, the coarse resolution of watershed P models may not capture finer-scale soil factors affecting P mobilization. It is clear that site microtopography and hydrology are closely linked and important drivers of P release and transport in overland flow. Combining light detection and ranging (LiDAR) based digital elevation models with P site indices and process-based models show promise for mapping and modeling P transport risk in cropland-RBZ areas; however, a better mechanistic understanding of processes controlling mobile P species across regions is needed. Broader predictive approaches integrating soil hydro-biogeochemical processes with real-time hydroclimatic data and risk assessment tools also hold promise for improving P transport risk assessment in CCs.

The impact of incineration phase-out on municipal solid waste landfilling and life cycle environmental performance: Case study of Madrid, Spain

Reducing the amount of municipal solid waste (MSW) fed to incineration while enhancing source separation and biological treatments is being considered a mean to protect the environment and human health and promote recycling. However, such a strategy can compromises the landfill reduction targets while the associated environmental benefits remain so far unexplored and, in any case, any potential benefit should be evaluated for specific situations. In this study we applied material flow analysis (MFA) and life cycle assessment (LCA) to quantitatively evaluate the potential impact of phasing-out incineration in Madrid, Spain. The current MSW management system was assessed against future scenarios that describe the elimination of incineration as well as the increase of source separation, recycling, composting, and anaerobic digestion. The results revealed that incineration phase-out jeopardizes landfill reduction. However, phasing-out incineration can reduce the impact on acidification, terrestrial and marine eutrophication, photochemical ozone formation, human toxicity cancer effects, and ecotoxicity. The climate impact ranges from irrelevant to largely beneficial depending on how the biogenic carbon is considered. The transition towards a renewable electricity mix and the increase in source separation of biodegradable waste seriously compromise the climate benefits of incineration over landfilling. Overall, actions are required in order to align incineration phase-out with the landfill reduction objective, namely upgrading material recovery facilities to reduce rejects and seeking alternative pathways for the rejects that will always exist.

Particulate phosphorus and suspended solids losses from small agricultural catchments: Links to stream and catchment characteristics

Excessive phosphorus (P) inputs from agriculture are well established as a contributor to freshwater eutrophication. Decreasing these inputs is an important step in improving the ecological state of impaired waters. Particulate P (PP) is a significant contributor to diffuse P inputs in agricultural catchments. Identifying the main correlates for PP losses is an important step in reducing these inputs. However, there are few studies of long term temporal and spatial dynamics of PP in agricultural streams. Here, we investigate the relative importance of hydrology, catchment characteristics and geochemistry on PP concentrations and fluxes in agricultural headwaters. We evaluate long-term monitoring data from eleven small (<35 km²) Swedish catchments with at least seven years of measured flow and flow proportional water quality sampling. Using parametric and non-parametric regression together with principal components analysis (PCA), we identify in-stream and catchment variables relevant for predicting PP concentrations, e.g., suspended solids concentrations (SS), soil texture and average catchment soil P content, measured as ammonium lactate/acetic acid extractable P (P-AL). We show that PP is primarily correlated to SS concentrations, which in turn are correlated to average clay content and land use. However, the SS:PP relationships differ between catchments. No correlation between PP concentrations in the stream and soil P content was found. An increasing clay content decreases the slope of the relationship between SS and PP, i.e., in catchments with higher clay content, less PP is transported per unit SS. The PP/SS ratio increased significantly (p < 0.05) over time in four catchments, despite limited changes in SS or PP concentrations. Our study highlights the importance of long time series since the enrichment of P on SS in the streams is only detected when using long term monitoring data.

Edge-of-Field Technologies for Phosphorus Retention from Agricultural Drainage Discharge

Agriculture is often responsible for the eutrophication of surface waters due to the loss of phosphorus—a normally limiting nutrient in freshwater ecosystems. Tile-drained agricultural catchments tend to increase this problem by accelerating the transport of phosphorus through subsurface drains both in dissolved (reactive and organic phosphorus) and particulate (particle-bound phosphorus) forms. The reduction of excess phosphorus loads from agricultural catchments prior to reaching downstream surface waters is therefore necessary. Edge-of-field technologies have been investigated, developed and implemented in areas with excess phosphorus losses to receive and treat the drainage discharge, when measures at the farm-scale are not able to sufficiently reduce the loads. The implementation of these technologies shall base on the phosphorus dynamics of specific catchments (e.g., phosphorus load and dominant phosphorus form) in order to ensure that local retention goals are met. Widely accepted technologies include constructed wetlands, restored wetlands, vegetated buffer strips and filter materials. These have demonstrated a large variability in the retention of phosphorus, and results from the literature can help targeting specific catchment conditions with suitable technologies. This review provides a comprehensive analysis of the currently used edge-of-field technologies for phosphorus retention in tile-drained catchments, with great focus on performance, application and limitations.

A new approach to generalizing riparian water and air quality function across regions

There is growing interest in generalizing the impact of hydrogeomorphology and weather variables on riparian functions. Here, we used RZ-TRADEOFF to estimate nitrogen, phosphorus, water table (WT) depth, and greenhouse gas (GHG: N₂O, CO₂, CH₄) functions for 80 riparian zones typical of the North American Midwest, Northeast (including Southern Ontario, Canada), and Mid-Atlantic. Sensitivity to weather perturbations was calculated for temperature and precipitation-dependent functions (CO₂, phosphate concentration, and water table), and multivariate statistical analysis on model outputs was conducted to determine trade-offs between riparian functions. Mean model estimates were 93.10 cm for WT depth, 8.45 mg N L^-1 for field edge nitrate concentration, 51.57% for nitrate removal, 0.45 mg PO₄^3- L^-1 for field edge phosphate concentration, 1.5% for subsurface phosphate removal, 91.24% for total overland phosphorus removal, 0.51 mg N m^-2 day^-1 for N₂O flux, 5.5 g C m^-2 day^-1 for CO₂ fluxes, and - 0.41 mg C m^-2 day^-1 and 621.51 mg C m^-2 day^-1 for CH₄ fluxes in non-peat sites and peat sites, respectively. Sites in colder climates were most sensitive to weather perturbations for CO₂, sites with deep water tables estimates had the highest sensitivity for WT, and sites in warm climates and/or with deep confining layers had the lowest sensitivity for phosphate concentration. Slope, confining layer depth, and temperature were the primary characteristics influencing similarities and trade-offs between sites. This research contributes to understanding how to optimize riparian restoration and protection in watersheds based on both water (nitrogen, phosphorus) and air quality (GHG) goals.

RZ-TRADEOFF: A New Model to Estimate Riparian Water and Air Quality Functions

Riparian zones are often used as best management practices due to their ability to remove nitrate (NO3−) from subsurface flow. Research suggests that beyond local biogeochemical controls, the impact of riparian zones on nitrogen removal and other functions, such as phosphorus dynamics and greenhouse gas emissions, largely depends on land-use/land-cover, hydrogeomorphology, and weather. In this study, we therefore present RZ-TRADEOFF, a novel and easily applicable model that connects multiple riparian functions and characteristics (NO3− and phosphate (PO43−), concentration and removal in subsurface flow, total phosphorus (TP) removal in overland flow, nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) emissions, water table) to landscape hydrogeomorphic characteristics, weather, and land-cover/land-use. RZ-TRADEOFF was developed with data from past studies and digital databases, and validated with data collected from the literature. Three functions (water table, PO43− and CO2) were observed to be significantly influenced by climate/weather, while the others were primarily influenced by hydrogeomorphology and land use. The percent bias and normalized root mean square error respectively were −3.35% and 0.28 for water table, 16.00% and 0.34 for NO3− concentration, −7.83% and 20.82 for NO3− removal, 6.64% and 0.35 for PO43− concentration, 2.55% and 0.17 for TP removal, 40.33% and 0.23 for N2O, 72.68% and 0.18 for CH4, and −34.98% and 0.91 for CO2. From a management standpoint, RZ-TRADEOFF significantly advances our ability to predict multiple water and air quality riparian functions using easily accessible data over large areas of the landscape due to its scalability.

Management Options to Reduce Phosphorus Leaching from Vegetated Buffer Strips

Vegetated buffer strips (VBS) between agricultural areas and surface waters are important retention areas for eroded particulate P through which they may obtain critically high degrees of P saturation imposing high risk of soluble P leaching. We tested topsoil removal and three harvesting frequencies (once, twice, or four times per year) of natural buffer vegetation to reduce P leaching with the aim to offset erosional P accumulation and high degrees of P saturation. We used a simple numerical time-step model to estimate changes in VBS soil P levels with and without harvest. Harvesting offset erosional deposition as it resulted in an annual ammonium oxalate-extractable P reduction of 0.3 to 2.8% (25-cm topsoil content) in soils of the VBS and thus, with time, reduced potential P leaching below a baseline of 50 μg L. Topsoil removal only marginally reduced potential leaching at two sites and not anywhere near this baseline. The harvest frequency only marginally affected the annual P removal, making single annual harvests the most economical. We estimate 50 to 300 yr to reach the P leaching baseline, due to substantial amounts of P accumulated in the soils. Even in high-erosion-risk situations in our study, harvesting reduced soil P content and the P leaching risk. We suggest harvesting as a practical and efficient management to combat P leaching from agricultural VBS, not just for short-term reductions of dissolved P, but also for reductions of the total soil P pool and for possible multiple benefits for VBS.

Phosphorus sorption capacity of biochars varies with biochar type and salinity level

Biochar is recognized as an effective material for recovering excess nutrients, including phosphorus (P), from aqueous solutions. Practically, that benefits the environment through reducing P losses from biochar-amended soils; however, how salinity influences P sorption by biochar is poorly understood and there has been no direct comparison on P sorption capacity between biochars derived from different feedstock types under non-saline and saline conditions. In this study, biochars derived from wheat straw, hardwood, and willow wood were used to compare P sorption at three levels of electrical conductivity (EC) (0, 4, and 8 dS m^-1) to represent a wide range of salinity conditions. Phosphorus sorption by wheat straw and hardwood biochars increased as aqueous solution P concentration increased, with willow wood biochar exhibiting an opposite trend for P sorption. However, the pattern for P sorption became the same as the other biochars after the willow wood biochar was de-ashed with 1 M HCl and 0.05 M HF. Willow wood biochar had the highest P sorption (1.93 mg g^-1) followed by hardwood (1.20 mg g^-1) and wheat straw biochars (1.06 mg g^-1) in a 25 mg L^-1 P solution. Although the pH in the equilibrium solution was higher with willow wood biochar (~ 9.5) than with the other two biochars (~ 6.5), solution pH had no or minor effects on P sorption by willow wood biochar. The high sorption rate of P by willow wood biochar could be attributed to the higher concentrations of salt and other elements (i.e., Ca and Mg) in the biochar in comparison to that in wheat straw and hardwood biochars; the EC values were 2.27, 0.53, and 0.27 dS m^-1 for willow wood, wheat straw, and hardwood biochars, respectively. A portion of P desorbed from the willow wood biochar; and that desorption increased with the decreasing P concentration in the aqueous solution. Salinity in the aqueous solution influenced P sorption by hardwood and willow wood but not by wheat straw biochar. We conclude that the P sorption capacity of the studied biochars is dependent on the concentration of the soluble element in the biochar, which is dependent on the biochar type, as well as the salinity level in the aqueous solution.

Evaluating riparian solutions to multiple stressor problems in river ecosystems - A conceptual study

Rivers are among the most sensitive of all ecosystems to the effects of global change, but options to prevent, mitigate or restore ecosystem damage are still inadequately understood. Riparian buffers are widely advocated as a cost-effective option to manage impacts, but empirical evidence is yet to identify ideal riparian features (e.g. width, length and density) which enhance ecological integrity and protect ecosystem services in the face of catchment-scale stressors. Here, we use an extensive literature review to synthesise evidence on riparian buffer and catchment management effects on instream environmental conditions (e.g. nutrients, fine sediments, organic matter), river organisms and ecosystem functions. We offer a conceptual model of the mechanisms through which catchment or riparian management might impact streams either positively or negatively. The model distinguishes scale-independent benefits (shade, thermal damping, organic matter and large wood inputs) that arise from riparian buffer management at any scale from scale-dependent benefits (nutrient or fine sediment retention) that reflect stressor conditions at broader (sub-catchment to catchment) scales. The latter require concerted management efforts over equally large domains of scale (e.g. riparian buffers combined with nutrient restrictions). The evidence of the relationships between riparian configuration (width, length, zonation, density) and scale-independent benefits is consistent, suggesting a high certainty of the effects. In contrast, scale-dependent effects as well as the biological responses to riparian management are more uncertain, suggesting that ongoing diffuse pollution (nutrients, sediments), but also sources of variability (e.g. hydrology, climate) at broader scales may interfere with the effects of local riparian management. Without concerted management across relevant scales, full biological recovery of damaged lotic ecosystems is unlikely. There is, nevertheless, sufficient evidence that the benefits of riparian buffers outweigh potential adverse effects, in particular if located in the upstream part of the stream network. This supports the use of riparian restoration as a no-regrets management option to improve and sustain lotic ecosystem functioning and biodiversity.

Detecting and analyzing soil phosphorus loss associated with critical source areas using a remote sensing approach

The detection of critical source areas (CSAs) is a key step in managing soil phosphorus (P) loss and preventing the long-term eutrophication of water bodies at regional scale. Most related studies, however, focus on a local scale, which prevents a clear understanding of the spatial distribution of CSAs for soil P loss at regional scale. Moreover, the continual, long-term variation in CSAs was scarcely reported. It is impossible to identify the factors driving the variation in CSAs, or to collect land surface information essential for CSAs detection, by merely using the conventional methodologies at regional scale. This study proposes a new regional-scale approach, based on three satellite sensors (ASTER, TM/ETM and MODIS), that were implemented successfully to detect CSAs at regional scale over 15years (2000-2014). The approach incorporated five factors (precipitation, slope, soil erosion, land use, soil total phosphorus) that drive soil P loss from CSAs. Results show that the average area of critical phosphorus source areas (CPSAs) was 15,056km² over the 15-year period, and it occupied 13.8% of the total area, with a range varying from 1.2% to 23.0%, in a representative, intensive agricultural area of China. In contrast to previous studies, we found that the locations of CSAs with P loss are spatially variable, and are more dispersed in their distribution over the long term. We also found that precipitation acts as a key driving factor in the variation of CSAs at regional scale. The regional-scale method can provide scientific guidance for managing soil phosphorus loss and preventing the long-term eutrophication of water bodies at regional scale, and shows great potential for exploring factors that drive the variation in CSAs at global scale.

Nutrient load concept-reservoir vs. bay impacts: a case study from a semi-arid watershed

Large flow-through reservoirs and lakes possess environmental gradients and monitoring programs are mostly adapted for cost and time effectiveness. Bay areas are often more isolated from the main water body and are likely to have unobserved different environmental processes and impacts. This study was performed at the Itaparica Reservoir, São Francisco River, located in semi-arid Northeast Brazil, with dendritic form. Water residence time in the Icó-Mandantes Bay was estimated by hydrodynamic flow and transport simulations. The P-chlorophyll a relationship was used to develop the P use efficiency coefficient for critical P load estimation of 25 μg P L^-1. Phosphorus sources and input rates into a bay and the respective reservoir were calculated and compared regarding their different origins for the period after flooding (1988) and for 2013. After impoundment, the P load highly exceeded the carrying capacity because of leaching and mineralization processes. In 2013, P inputs were still above this threshold, whereas inflow and sub-basin P export during the rainy season were crucial. But eutrophication processes have increased in the bay relative to the main water body. Hence, water in hydraulic isolated parts is prone to eutrophication processes, thus, bays have to be specially considered in water resource management.

Sediment cores from kettle holes in NE Germany reveal recent impacts of agriculture

Glacial kettle holes in young moraine regions receive abundant terrigenous material from their closed catchments. Core chronology and sediment accumulation were determined for two semi-permanent kettle holes, designated RG and KR, on arable land close to the villages of Rittgarten and Kraatz, respectively, in Uckermark, NE Germany. Core dating ((210)Pb, (137)Cs) revealed variable sediment accretion rates through time (RG 0.4-23.1 mm a(-1); KR 0.2-35.5 mm a(-1)), with periods of high accumulation corresponding to periods of intensive agricultural activity and consequent erosional inputs from catchments. Sediment composition (C, N, P, S, K, Ca, Fe, Mn, Zn, Cu, Mo, Pb, Cd, Zr) was used to determine sediment source and input processes. At RG, annual P input increased from 0.65 kg ha(-1) in the early nineteenth century to 1.67 kg ha(-1) by 2013. At KR, P input increased from 0.6 to 4.1 kg ha(-1) over the last century. There was a concurrent increase in Fe input in both water bodies. Thus, Fe/P ratios showed no temporal trend and did not differ between RG (18.5) and KR (18.4), indicating similar P mobility. At RG, the S/Fe ratio increased from 0.4 to 2.3, indicating more iron sulphides and thus higher P availability, coinciding with high coverage of duckweed (Spirodela polyrhiza (L.)) and soft hornwort (Ceratophyllum submersum L.). At KR, however, this ratio remained low and relatively unchanged (0.3 ± 0.4), indicating more efficient Fe-P binding and lower hydrophyte productivity. Trends in sediment composition indicate a shift towards eutrophication in both kettle holes, but with differences in timing and magnitude. Other morphologically similar kettle holes in NE Germany that are prone to erosion could have been similarly impacted but may differ in the extent of sediment infilling and degradation of their ecological functions.

Greenhouse Gas Fluxes from Salt Marshes Exposed to Chronic Nutrient Enrichment

We assessed the impact of nutrient additions on greenhouse gas fluxes using dark static chambers in a microtidal and a macrotidal marsh along the coast of New Brunswick, Canada approximately monthly over a year. Both were experimentally fertilized for six years with varying levels of N and P. For unfertilized, N and NPK treatments, average yearly CO₂ emissions (which represent only respiration) at the microtidal marsh (13, 19, and 28 mmoles CO₂ m^-2 hr^-1, respectively) were higher than at the macrotidal marsh (12, 15, and 19 mmoles m^-2 hr^-1, respectively, with a flux under the additional high N/low P treatment of 21 mmoles m^-2 hr^-1). Response of CH₄ to fertilization was more variable. At the macrotidal marsh average yearly fluxes were 1.29, 1.26, and 0.77 μmol CH₄ m^-2 hr^-1 with control, N, and NPK treatments, respectively and 1.21 μmol m^-2 hr^-1 under high N/low P treatment. At the microtidal marsh CH₄ fluxes were 0.23, 0.16, and -0.24 μmol CH₄ m^-2 hr^-1 in control, N, and NPK and treatments, respectively. Fertilization changed soils from sinks to sources of N₂O. Average yearly N₂O fluxes at the macrotidal marsh were -0.07, 0.08, and 1.70, μmol N₂O m^-2 hr^-1 in control, N, NPK and treatments, respectively and 0.35 μmol m^-2 hr^-1 under high N/low P treatment. For the control, N, and NPK treatments at the microtidal marsh N₂O fluxes were -0.05, 0.30, and 0.52 μmol N₂O m^-2 hr^-1, respectively. Our results indicate that N₂O fluxes are likely to vary with the source of pollutant nutrients but emissions will be lower if N is not accompanied by an adequate supply of P (e.g., atmospheric deposition vs sewage or agricultural runoff). With chronic fertilization the global warming potential of the increased N₂O emissions may be enough to offset the global cooling potential of the C sequestered by salt marshes.

Total and Labile Phosphorus Concentrations as Influenced by Riparian Buffer Soil Properties

Riparian buffers can act as a phosphorus (P) source under active stream bank erosion. Using soil and landscape variables (soil series, drainage class, organic matter, and pH) to index P concentrations could improve P loss risk tools for buffers. The objectives of this study were (i) to determine if soil properties could predict total and labile P concentrations within a 10-ha riparian buffer and (ii) to quantify the degree of spatial dependence of P and related properties. Soil samples were taken in 15-cm increments to a depth of 60 cm using a grid ( = 71) from an established riparian buffer along the Rock River in Vermont. Total soil P (TP), plant-available P determined by Modified Morgan extraction (MM-P), pH, soil organic matter (SOM), soil texture, and select cations were measured. We found that TP (152-1536 mg P kg) and MM-P (0.4-14.6 mg kg) ranged widely, with distinct differences between soil series. Mean TP and MM-P were greater in alluvial and glaciolacustrine soils compared with glacial till. Across all samples, MM-P was weakly related to soil properties; however, total labile P (orthophosphate + organic P measured by ICP) and unreactive labile P (ICP-P - colorimetric-P) could both be predicted by SOM ( = 0.59 and 0.73, respectively). Strong spatial dependence was found for P and related properties as revealed by geospatial analyses. Results show that P availability in the buffer was strongly related to soil genesis and support site-specific approaches for P loss risk evaluation in buffers.

Phosphorus and nitrogen fluxes carried by 21 Finnish agricultural rivers in 1985-2006

The Finnish Agri-Environmental Programme aims to reduce nutrient load to waters. Using national monitoring data, we estimated the agricultural load (incl. natural background) of total phosphorus (TP) and total nitrogen (TN) transported by 21 Finnish rivers to the northern Baltic Sea and analysed the flow-adjusted trends in the loads and concentrations from 1985 to 2006. We also related the loads to spatial and temporal patterns in catchment and agricultural characteristics. Agricultural load of TN increased, especially in the rivers discharging into the Bothnian Bay, while the load of TP decreased in most of the rivers, except those discharging into the Archipelago Sea. The trends may partly be related to a decrease in grassed area (TP, TN) and increased mineralisation (TN), but the available data on catchment and agricultural characteristics did not fully explain the observed pattern. Our study showed that data arising from relatively infrequent monitoring may prove useful for analysing long-term trend. The mutual correlation among the explaining variables hampered the analysis of the load generating factors.

Stewardship to tackle global phosphorus inefficiency: The case of Europe

The inefficient use of phosphorus (P) in the food chain is a threat to the global aquatic environment and the health and well-being of citizens, and it is depleting an essential finite natural resource critical for future food security and ecosystem function. We outline a strategic framework of 5R stewardship (Re-align P inputs, Reduce P losses, Recycle P in bioresources, Recover P in wastes, and Redefine P in food systems) to help identify and deliver a range of integrated, cost-effective, and feasible technological innovations to improve P use efficiency in society and reduce Europe's dependence on P imports. Their combined adoption facilitated by interactive policies, co-operation between upstream and downstream stakeholders (researchers, investors, producers, distributors, and consumers), and more harmonized approaches to P accounting would maximize the resource and environmental benefits and help deliver a more competitive, circular, and sustainable European economy. The case of Europe provides a blueprint for global P stewardship.

Synthetic apatite nanoparticles as a phosphorus fertilizer for soybean (Glycine max)

Some soluble phosphate salts, heavily used in agriculture as highly effective phosphorus (P) fertilizers, cause surface water eutrophication, while solid phosphates are less effective in supplying the nutrient P. In contrast, synthetic apatite nanoparticles could hypothetically supply sufficient P nutrients to crops but with less mobility in the environment and with less bioavailable P to algae in comparison to the soluble counterparts. Thus, a greenhouse experiment was conducted to assess the fertilizing effect of synthetic apatite nanoparticles on soybean (Glycine max). The particles, prepared using one-step wet chemical method, were spherical in shape with diameters of 15.8 ± 7.4 nm and the chemical composition was pure hydroxyapatite. The data show that application of the nanoparticles increased the growth rate and seed yield by 32.6% and 20.4%, respectively, compared to those of soybeans treated with a regular P fertilizer (Ca(H2PO4)2). Biomass productions were enhanced by 18.2% (above-ground) and 41.2% (below-ground). Using apatite nanoparticles as a new class of P fertilizer can potentially enhance agronomical yield and reduce risks of water eutrophication.

Modelling the cost-effectiveness of mitigation methods for multiple pollutants at farm scale

Reductions in agricultural pollution are essential for meeting nationally and internationally agreed policy targets for losses to both air and water. Numerous studies quantify the impact of relevant mitigation methods by field experimentation or computer modelling. The majority of these studies have addressed individual methods and frequently also individual pollutants. This paper presents a conceptual model for the synthesis of the evidence base to calculate the impact of multiple methods addressing multiple pollutants in order to identify least cost solutions for multiple policy objectives. The model is implemented as a farm scale decision support tool that quantifies baseline pollutant losses for identifiable sources, areas and pathways and incorporates a genetic algorithm based multi-objective procedure for determining optimal suites of mitigation methods. The tool is generic as baseline losses can be replaced with measured data and the default library of mitigation methods can be edited and expanded. The tool is demonstrated through application to two contrasting farm systems, using survey data on agricultural practices typical of England and Wales. These examples show how the tool could be used to help target the adoption of mitigation options for the control of diffuse pollution from agriculture. The feedback from workshops where Farmscoper was demonstrated is included to highlight the potential role of Farmscoper as part of the farm advisory process.

Phosphorus mitigation to control river eutrophication: murky waters, inconvenient truths, and "postnormal" science

This commentary examines an "inconvenient truth" that phosphorus (P)-based nutrient mitigation, long regarded as the key tool in eutrophication management, in many cases has not yet yielded the desired reductions in water quality and nuisance algal growth in rivers and their associated downstream ecosystems. We examine why the water quality and aquatic ecology have not recovered, in some case after two decades or more of reduced P inputs, including (i) legacies of past land-use management, (ii) decoupling of algal growth responses to river P loading in eutrophically impaired rivers; and (iii) recovery trajectories, which may be nonlinear and characterized by thresholds and alternative stable states. It is possible that baselines have shifted and that some disturbed river environments may never return to predisturbance conditions or may require P reductions below those that originally triggered ecological degradation. We discuss the practical implications of setting P-based nutrient criteria to protect and improve river water quality and ecology, drawing on a case study from the Red River Basin in the United States. We conclude that the challenges facing nutrient management and eutrophication control bear the hallmarks of "postnormal" science, where uncertainties are large, management intervention is urgently required, and decision stakes are high. We argue a case for a more holistic approach to eutrophication management that includes more sophisticated regime-based nutrient criteria and considers other nutrient and pollutant controls and river restoration (e.g., physical habitat and functional food web interactions) to promote more resilient water quality and ecosystem functioning along the land-freshwater continuum.

Reducing phosphorus loading of surface water using iron-coated sand

Phosphorus losses from agricultural soils is an important source of P in surface waters leading to surface water quality impairment. In addition to reducing P inputs, mitigation measures are needed to reduce P enrichment of surface waters. Because drainage of agricultural land by pipe drainage is an important pathway of P to surface waters, removing P from drainage water has a large potential to reduce P losses. In a field trial, we tested the performance of a pipe drain enveloped with Fe-coated sand, a side product of the drinking water industry with a high ability to bind P, to remove P from the drainage water. The results of this trial, encompassing more than one hydrological season, are very encouraging because the efficiency of this mitigation measure to remove P amounted to 94%. During the trial, the pipe drains were below the groundwater level for a prolonged time. Nevertheless, no reduction of Fe(III) in the Fe-coated sand occurred, which was most likely prevented by reduction of Mn oxides present in this material. The enveloped pipe drain was estimated to be able to lower the P concentration in the effluent to the desired water quality criterion for about 14 yr. Manganese oxides are expected to be depleted after 5 to 10 yr. The performance of the enveloped pipe drain, both in terms of its ability to remove P to a sufficiently low level and the stability of the Fe-coated sand under submerged conditions in the long term, needs prolonged experimental research.

Watershed land use effects on lake water quality in Denmark

Mitigating nutrient losses from anthropogenic nonpoint sources is today of particular importance for improving the water quality of numerous freshwater lakes worldwide. Several empirical relationships between land use and in-lake water quality variables have been developed, but they are often weak, which can in part be attributed to lack of detailed information about land use activities or point sources. We examined a comprehensive data set comprising land use data, point-source information, and in-lake water quality for 414 Danish lakes. By excluding point-source-influenced lakes (n = 210), the strength in relationship (R2) between in-lake total nitrogen (TN) and total phosphorus (TP) concentrations and the proportion of agricultural land use in the watershed increased markedly, from 10-12% to 39-42% for deep lakes and from 10-12% to 21-23% for shallow lakes, with the highest increase for TN. Relationships between TP and agricultural land use were even stronger for lakes with rivers in their watershed (55%) compared to lakes without (28%), indicating that rivers mediate a stronger linkage between landscape activity and lake water quality by providing a "delivery" mechanism for excess nutrients in the watershed. When examining the effect of different near-freshwater land zones in contrast to the entire watershed, relationships generally improved with size of zone (25, 50, 100, 200, and 400 m from the edge of lake and streams) but were by far strongest using the entire watershed. The proportion of agricultural land use in the entire watershed was best in explaining lake water quality, both relative to estimated nutrient surplus at agricultural field level and near-lake land use, which somewhat contrasts typical strategies of management policies that mainly target agricultural nutrient applications and implementation of near-water buffer zones. This study suggests that transport mechanisms within the whole catchment are important for the nutrient export to lakes. Hence, the whole watershed should be considered when managing nutrient loadings to lakes, and future policies should ideally target measures that reduce the proportion of cultivated land in the watershed to successfully improve lake water quality.

Use of reactive materials to bind phosphorus

Phosphorus (P) losses from agricultural soils have caused surface water quality impairment in many regions of the world, including The Netherlands. Due to the large amounts of P accumulated in Dutch soils, the generic fertilizer and manure policy will not be sufficient to reach in time the surface water quality standards of the European Water Framework Directive. Additional measures must be considered to further reduce P enrichment of surface waters. One option is to immobilize P in soils or manure or to trap P when it moves through the landscape by using reactive materials with a large capacity to retain P. We characterized and tested two byproducts of the process of purification of deep groundwater for drinking water that could be used as reactive materials: iron sludge and iron-coated sand. Both materials contain low amounts of inorganic contaminants, which also have a low (bio)availability, and bound a large amount of P. We could describe sorption of P to the iron sludge in batch experiments well with the kinetic Freundlich equation (Q = × t (m) × C(n)). Kinetics had a large influence on P sorption in batch and column experiments and should be taken into account when iron-containing materials are tested for their capability to immobilize or trap P. A negative aspect of the iron sludge is its low hydraulic conductivity; even when mixed with pure sand to a mixture containing 20% sludge, the conductivity was very low, and only 10% sludge may be needed before application is possible in filters or barriers for removing P from groundwater. Due to its much higher hydraulic conductivity, iron-coated sand has greater potential for use under field conditions. Immobilizing P could be an option for using iron sludge as a reactive material.

Emerging technologies for removing nonpoint phosphorus from surface water and groundwater: introduction

Coastal and freshwater eutrophication continues to accelerate at sites around the world despite intense efforts to control agricultural P loss using traditional conservation and nutrient management strategies. To achieve required reductions in nonpoint P over the next decade, new tools will be needed to address P transfers from soils and applied P sources. Innovative remediation practices are being developed to remove nonpoint P sources from surface water and groundwater using P sorbing materials (PSMs) derived from natural, synthetic, and industrial sources. A wide array of technologies has been conceived, ranging from amendments that immobilize P in soils and manures to filters that remove P from agricultural drainage waters. This collection of papers summarizes theoretical modeling, laboratory, field, and economic assessments of P removal technologies. Modeling and laboratory studies demonstrate the importance of evaluating P removal technologies under controlled conditions before field deployment, and field studies highlight several challenges to P removal that may be unanticipated in the laboratory, including limited P retention by filters during storms, as well as clogging of filters due to sedimentation. Despite the potential of P removal technologies to improve water quality, gaps in our knowledge remain, and additional studies are needed to characterize the long-term performance of these technologies, as well as to more fully understand their costs and benefits in the context of whole-farm- and watershed-scale P management.

Factors controlling phosphate interaction with iron oxides

Factors such as pH, solution ion composition, and the presence of natural organic matter (NOM) play a crucial role in the effectiveness of phosphorous adsorption by iron oxides. The interplay between these factors shows a complicated pattern and can sometimes lead to controversial results. With the help of mechanistic modeling and adsorption experiments, the net macroscopic effect of single and combined factors can be better understood and predicted. In the present work, the relative importance of the above-mentioned factors in the adsorption of phosphate was analyzed using modeling and comparison between the model prediction and experimental data. The results show that, under normal soil conditions, pH, concentration of Ca, and the presence of NOM are the most important factors that control adsorption of phosphate to iron oxides. The presence of Ca not only enhances the amount of phosphate adsorbed but also changes the pH dependency of the adsorption. An increase of dissolved organic carbon from 0.5 to 50 mg L can lead to a >50% decrease in the amount of phosphate adsorbed. Silicic acid may decrease phosphate adsorption, but this effect is only important at a very low phosphate concentration, in particular at high pH.

Phosphorus load to surface water from bank erosion in a Danish lowland river basin

Phosphorus loss from bank erosion was studied in the catchment of River Odense, a lowland Danish river basin, with the aim of testing the hypothesis of whether stream banks act as major diffuse phosphorus (P) sources at catchment scale. Furthermore, the study aimed at analyzing the impact of different factors influencing bank erosion and P loss such as stream order, anthropogenic disturbances, width of uncultivated buffer strips, and the vegetation of buffer strips. A random stratified procedure in geographical information system (GIS) was used to select two replicate stream reaches covering different stream orders, channelized vs. naturally meandering channels, width of uncultivated buffer strips (≤ 2 m and ≥ 10 m), and buffer strips with different vegetation types. Thirty-six 100-m stream reaches with 180 bank plots and a total of 3000 erosion pins were established in autumn 2006, and readings were conducted during a 3-yr period (2006-2009). The results show that neither stream size nor stream disturbance measured as channelization of channel or the width of uncultivated buffer strip had any significant ( < 0.05) influence on bank erosion and P losses during each of the 3 yr studied. In buffer strips with natural trees bank erosion was significantly ( < 0.05) lower than in buffer strips dominated by grass and herbs. Gross and net P input from bank erosion amounted to 13.8 to 16.5 and 2.4 to 6.3 t P, respectively, in the River Odense catchment during the three study years. The net P input from bank erosion equaled 17 to 29% of the annual total P export and 21 to 62% of the annual export of P from diffuse sources from the River Odense catchment. Most of the exported total P was found to be bioavailable (71.7%) based on a P speciation of monthly suspended sediment samples collected at the outlet of the river basin. The results found in this study have a great importance for managers working with P mitigation and modeling at catchment scale.

Integrating economic and biophysical data in assessing cost-effectiveness of buffer strip placement

The European Union Water Framework Directive (WFD) requires Member States to set water quality objectives and identify cost-effective mitigation measures to achieve "good status" in all waters. However, costs and effectiveness of measures vary both within and between catchments, depending on factors such as land use and topography. The aim of this study was to develop a cost-effectiveness analysis framework for integrating estimates of phosphorus (P) losses from land-based sources, potential abatement using riparian buffers, and the economic implications of buffers. Estimates of field-by-field P exports and routing were based on crop risk and field slope classes. Buffer P trapping efficiencies were based on literature metadata analysis. Costs of placing buffers were based on foregone farm gross margins. An integrated optimization model of cost minimization was developed and solved for different P reduction targets to the Rescobie Loch catchment in eastern Scotland. A target mean annual P load reduction of 376 kg to the loch to achieve good status was identified. Assuming all the riparian fields initially have the 2-m buffer strip required by the General Binding Rules (part of the WFD in Scotland), the model gave good predictions of P loads (345-481 kg P). The modeling results show that riparian buffers alone cannot achieve the required P load reduction (up to 54% P can be removed). In the medium P input scenario, average costs vary from £38 to £176 kg P at 10% and 54% P reduction, respectively. The framework demonstrates a useful tool for exploring cost-effective targeting of environmental measures.

Competitive and synergistic effects in pH dependent phosphate adsorption in soils: LCD modeling

The pH dependency of soluble phosphate in soil was measured for six agricultural soils over a pH range of 3-10. A mechanistic model, the LCD (ligand charge distribution) model, was used to simulate this change, which considers phosphate adsorption to metal (hydr)oxides in soils under the influence of natural organic matter (NOM) and polyvalent cations (Ca(2+), Al(3+), and Fe(3+)). For all soils except one, the description in the normal pH range 5-8 is good. For some soils at more extreme pH values (for low P-loading soils at low pH and for high P-loading soils at high pH), the model over predicts soluble P. The calculation shows that adsorption is the major mechanism controlling phosphate solubility in soils, except at high pH in high P-loading soils where precipitation of calcium phosphate may take place. NOM and polyvalent cations have a very strong effect on the concentration level of P. The pattern of pH dependency of soluble P in soils differs greatly from the pH effects on phosphate adsorption to synthetic metal (hydr)oxides in a monocomponent system. According to the LCD model, the pH dependency in soil is mainly caused by the synergistic effects of Ca(2+) adsorption to oxides. Adsorption of Al(3+) to NOM adsorbed plays an important role only at a pH < 4.5. Presence of NOM coating strongly competes with phosphate for the adsorption and is an important factor to consider in modeling phosphate adsorption in natural samples.

Multiyear nutrient removal performance of three constructed wetlands intercepting tile drain flows from grazed pastures

Subsurface tile drain flows can be a major s ource of nurient loss from agricultural landscapes. This study quantifies flows and nitrogen and phosphorus yields from tile drains at three intensively grazed dairy pasture sites over 3- to 5-yr periods and evaluates the capacity of constructed wetlands occupying 0.66 to 1.6% of the drained catchments too reduce nutrient loads. Continuous flow records are combined with automated flow-proportional sampling of nutrient concentrations to calculate tile drain nutrient yields and wetland mass removal rates. Annual drainage water yields rangedfrom 193 to 564 mm (16-51% of rainfall) at two rain-fed sites and from 827 to 853 mm (43-51% of rainfall + irrigation) at an irrigated site. Annually, the tile drains exported 14 to 109 kg ha(-1) of total N (TN), of which 58 to 90% was nitrate-N. Constructed wetlands intercepting these flows removed 30 to 369 gTN m(-2) (7-63%) of influent loadings annually. Seasonal percentage nitrate-N and TN removal were negatively associated with wetland N mass loadings. Wetland P removal was poor in all wetlands, with 12 to 115% more total P exported annually overall than received. Annually, the tile drains exported 0.12 to 1.38 kg ha of total P, of which 15 to 93% was dissolved reactive P. Additional measures are required to reduce these losses or provide supplementary P removal. Wetland N removal performance could be improved by modifying drainage systems to release flows more gradually and improving irrigation practices to reduce drainage losses.

Evaluation of the ecological status of an impaired watershed by using a multi-index approach

The objective of this study was to use an integrative approach to assess the ecological status of a small river impacted by multiple sources of disturbance. The River Febros (NW Portugal) is a small and highly impacted non-regulated river; approximately 44% of the watershed area is dedicated to agriculture, but there is also some urbanization. Environmental status was evaluated using a new multi-index approach, combining quality indices for water (Water Quality Index (WQI)), benthic macroinvertebrates (Iberian Biological Monitoring Working Party Index (IBMWP)), and human modification (Riparian Forest Quality Index (QBR)); Ecological Status River Mediterranean Index (ECOSTRIMED); River Habitat Survey (RHS)). Surveys were carried out between October 2002 and 2003, along four stretches of the main course of the river for physicochemical characteristics, as well as water microbiology and macroinvertebrates. Water quality was poor at all sites (WQI averaged 30.4%) and decreased downstream. The RHS (assessment of instream habitats and the river corridor) and the QBR quantification displayed the same trend. Macroinvertebrates diversity was permanently low with only the most tolerant insects taxa present; oligochaetes dominated. The ecological status of River Febros was well represented using the experimental assessment system, where a downstream decrease in condition was noticeable (in spite of a lower water quality upstream). Our results emphasize the importance of assessing several components of disturbed ecosystems in order to ascertain overall quality and the importance of promoting improvement of the riparian gallery as a buffer against multiple agents of disturbance.

Interactions between soil texture and placement of dairy slurry application: I. Flow characteristics and leaching of nonreactive components

Land application of manure can exacerbate nutrient and contaminant transfers to the aquatic environment. This study examined the effect of injecting a dairy cattle (Bostaurus L.) manure slurry on mobilization and leaching of dissolved, nonreactive slurry components across a range of agricultural soils. We compared leaching of slurry-applied bromide through intact soil columns (20 cm diam., 20 cm high) of differing textures following surface application or injection of slurry. The volumetric fraction of soil pores >30 microm ranged from 43% in a loamy sand to 28% in a sandy loam and 15% in a loam-textured soil. Smaller active flow volumes and higher proportions of preferential flow were observed with increasing soil clay content. Injection of slurry in the loam soil significantly enhanced diffusion of applied bromide into the large fraction of small pores compared with surface application. The resulting physical protection against leaching of bromide was reflected by 60.2% of the bromide tracer was recovered in the effluent after injection, compared with 80.6% recovery after surface application. No effect of slurry injection was observed in the loamy sand and sandy loam soils. Our findings point to soil texture as an important factor influencing leaching of dissolved, nonreactive slurry components in soils amended with manure slurry.

Mitigation options for sediment and phosphorus loss from winter-sown Arable Crops

Sediment and P inputs to freshwaters from agriculture are a major problem in the United Kingdom (UK). This study investigated mitigation options for diffuse pollution losses from arable land. Field trials were undertaken at the hillslope scale over three winters at three UK sites with silt (Oxyaquic Hapludalf), sand (Udic Haplustept), and clay (Typic Haplaquept) soils. None of the mitigation treatments was effective in every year trialled, but each showed overall average reductions in losses. Over five site years, breaking up the compaction in tramlines (tractor wheel tracks) using a tine reduced losses of sediment and P to losses similar to those observed from areas without tramlines, with an average reduction in P loss of 1.06 kg TP ha(-1). Compared to traditional plowing, TP losses under minimum tillage were reduced by 0.30 kg TP ha(-1) over five site years, TP losses under contour cultivation were reduced by 0.30 kg TP ha(-1) over two site years, and TP losses using in-field barriers were reduced by 0.24 kg TP ha(-1) over two site years. In one site year, reductions in losses due to crop residue incorporation were not significant. Each of the mitigation options trialled is associated with a small cost at the farm-scale of up to pound5 ha(-1), or with cost savings. The results indicate that each of the treatments has the potential to be a cost-effective mitigation option, but that tramline management is the most promising treatment, because tramlines dominate sediment and P transfer in surface runoff from arable hillslopes.

Phosphorus retention in riparian buffers: review of their efficiency

Ground water and surface water interactions are of fundamental importance for the biogeochemical processes governing phosphorus (P) dynamics in riparian buffers. The four most important conceptual hydrological pathways for P losses from and P retention in riparian buffers are reviewed in this paper: (i) The diffuse flow path with ground water flow through the riparian aquifer, (ii) the overland flow path across the riparian buffer with water coming from adjacent agricultural fields, (iii) irrigation of the riparian buffer with tile drainage water from agricultural fields where disconnected tile drains irrigate the riparian buffer, and (iv) inundation of the riparian buffer (floodplain) with river water during short or longer periods. We have examined how the different flow paths in the riparian buffer influence P retention mechanisms theoretically and from empirical evidence. The different hydrological flow paths determine where and how water-borne P compounds meet and interact with iron and aluminum oxides or other minerals in the geochemical cycling of P in the complex and dynamic environment that constitutes a riparian buffer. The main physical process in the riparian buffer-sedimentation-is active along several flow paths and may account for P retention rates of up to 128 kg P ha(-1) yr(-1), while plant uptake may temporarily immobilize up to 15 kg P ha(-1) yr(-1). Retention of dissolved P in riparian buffers is not as pronounced as retention of particulate P and is often below 0.5 kg P ha(-1) yr(-1). Several studies show significant release of dissolved P (i.e., up to 8 kg P ha(-1) yr(-1)).

Mitigating diffuse phosphorus transfer from agriculture according to cost and efficiency

Potential options for mitigating phosphorus (P) transfer from agriculture to water in England and Wales (E&W) were collated across a range of farm systems to assess their potential effectiveness in reducing mass of P transferred and potential cost (pounds sterling [ pound]) to the farming industry. A simple model framework (called PEASE) incorporating a number of assumptions was used to identify 15 methods for mitigating inputs of P to agricultural systems, 19 methods for preventing mobilization of P, and six methods for controlling the transport of P to streams. The scope for largest reductions in P inputs was to grassland and horticulture. Potential reductions in P mobilization were up to 1.2 kg P ha(-1). Reductions in P transfer associated with transport mitigation were larger than those associated with input and mobilization methods (up to 2.2 kg P ha(-1)). The largest estimated reductions were achieved by installing buffer zones and constructed wetlands, the former being very cost effective ( pound3-5 kg(-1) P saved). Plots of cost curves helped identify where the combined and cumulative P transfer reductions were attainable; these were approximately 0.2 kg ha(-1) for uplands, 0.6 kg ha(-1) for outdoor pigs, 0.9 kg ha(-1) for intensive dairy, and 2.2 kg ha(-1) for arable examples. We concluded that established catchment-scale evidence for mitigation is sparse, especially for specific farm systems in E&W. Sensitivities and uncertainties in the approach, especially associated with expert coefficients, are noted. This approach is nonetheless considered useful for prioritizing where and how best options might be most effectively targeted for least cost but greatest benefit.

Evaluating the success of phosphorus management from field to watershed

Studies have demonstrated some P loss reduction following implementation of remedial strategies at field scales. However, there has been little coordinated evaluation of best management practices (BMPs) on a watershed scale to show where, when, and which work most effectively. Thus, it is still difficult to answer with a degree of certainty, critical questions such as, how long before we see a response and where would we expect to observe the greatest or least response? In cases where field and watershed scales are monitored, it is not uncommon for trends in P loss to be disconnected. We review case studies demonstrating that potential causes of the disconnect varies, from competing sources of P at watershed scales that are not reflected in field monitoring to an abundance of sinks at watershed scales that buffer field sources. To be successful, P-based mitigation strategies need to occur iteratively, involve stakeholder driven programs, and address the inherent complexity of all P sources within watersheds.

Critical evaluation of the implementation of mitigation options for phosphorus from field to catchment scales

Nutrient regulations have been developed over the past decades to limit anthropogenic inputs of phosphorus (P) to surface waters. All of the regulations were promulgated in response to decreased water quality, which was at least partially associated with agricultural non-point source pollution. Improvements in water quality can take years, so the impacts of these regulations on water quality can not always be seen. Denmark has had nutrient management regulations aimed at achieving mass balance of P for 20 yr, and although great progress has been made, an average surplus of 11 kg P ha(-1) remains. Northern Ireland is also trying to move toward mass balance, but decreases in inorganic P fertilizer use have been undermined by an increase in the use of feed concentrates. In the Chesapeake Bay watershed, which covers several states in the USA, a variety of best management practices are starting to have an effect on P losses from agriculture, but water quality has only improved slightly. Impairment to the supply of drinking water to the City of Tulsa Oklahoma led to a lawsuit that has greatly affected the management of poultry litter in the supplying watershed. This paper discusses the different regulations that have developed in these four regions, evaluates the strategies used to prevent non-point source pollution of P, reports impacts on water quality, and looks for lessons that can be learned as we move forward.

International phosphorus workshop: diffuse phosphorus loss to surface water bodies--risk assessment, mitigation options, and ecological effects in river basins

Agriculture is a major source of P to the aquatic environment in many countries. Although efforts have been made to improve the P utilization in agricultural production, which is reflected in modestly declining P surpluses in many countries, increasing agricultural P surpluses are still observed in some countries. The IPW5 Special Submission included in this issue addresses and discusses four key topics that emerged from the workshop: (i) managing agricultural P losses-effectiveness, uncertainties, and costs; (ii) P modeling at different scales; (iii) functioning of riparian buffers; (iv) ecological responses to P loadings and impacts of climate change. Each of these four topics interacts with each other as well as with the four tiers of the P Transfer Continuum (Source, Mobilization, Transport, and Ecological Effects). In this review paper we highlight the main outcomes of the workshop and the special collection of eight papers. Moreover, we identify the main gaps in our knowledge and future research directions on P, which are linked to important issues such as addressing scale effects, improved P models with the ability to quantify uncertainty, the linking of P losses with ecological effects, and climate change.

Vegetated buffer strips can lead to increased release of phosphorus to waters: a biogeochemical assessment of the mechanisms

Establishing vegetated buffer strips (VBS) between cropland and watercourses is currently promoted as a principal control of diffuse pollution transport. However, we lackthe mechanistic understanding to evaluate P retention in VBS and predict risks of P transport to aquatic ecosystems. We observed that VBS establishment led to enhanced rates of soil P cycling, increasing soil P solubility and the potential amount leached to watercourses. Soil in VBS, relative to adjacentfields, had increased inorganic P solubility indices, dissolved organic P, phosphatase enzyme activity, microbial diversity, and biomass P. Small relative increases in the pool of soil P rendered labile had disproportionate effects on the P available for leaching. We propose a mechanism whereby the establishment of VBS on previous agricultural land causes a diversifying plant-microbial system which can access previous immobilized soil P from past fertilization or trapped sediment P. Laboratory experiments suggested that sediment-P inputs to VBS were insufficient alone to increase P solubility without biological cycling. Results showthat VBS management may require strategies, for example, harvesting vegetation, to offset biochemical processes that can increase the susceptibility of VBS soil P to move to adjoining streams.

Phytoextraction of phosphorus-enriched grassland soils

High soil P contents in agricultural soils in the Netherlands cause excessive losses of P to surface waters. The reductions in P application rates in the present manure policy are not sufficient to reach surface water quality standards resulting from the European Water Framework Directive in 2015. Accordingly, additional measures are necessary to reduce P loading to surface water. Greenhouse experiments showed that a rapid reduction in soluble P and readily available soil P can be obtained by zero P application. However, field data confirming these findings are scarce. In 2002 a phytoextraction experiment started on four grasslands sites on sand, peat, and clay soils. The phytoextraction (mining) plots receive no P and 300 kg N ha(-1) yr(-1) and the grass is removed by mowing. The experiment showed that zero P application, over a period of 5 yr, led to a strong (30-90%) reduction in P concentrations in soil solution in the upper soil layer (0-0.05 m). The reduction in concentrations declined with depth. Mining also resulted in a decline in P pools in the soil solid phase. The largest decline (10-60%) was found in weakly bound P pools (water extractable P; P(w), and ammonium lactate extractable P; P-AL), whereas reductions in more strongly bound P forms were relatively small. It may be concluded that phytoextraction appears an effective method of reducing soil P concentrations in the uppermost soil layers in a couple of years and prolonged mining may thus be effective in reducing leaching and runoff of P.

The sources of phosphorus in the waters of Great Britain

Total phosphorus (TP) and soluble reactive phosphorus (SRP) loads to watercourses of the River Basin Districts (RBDs) of Great Britain (GB) were estimated using inventories of industrial P loads and estimates of P loads from sewage treatment works and diffuse P loads calculated using region-specific export coefficients for particular land cover classes combined with census data for agricultural stocking densities and human populations. The TP load to GB waters was estimated to be 60 kt yr(-1), of which households contributed 73%, agriculture contributed 20%, industry contributed 3%, and 4% came from background sources. The SRP load to GB waters was estimated to be 47 kt yr(-1), of which households contributed 78%, agriculture contributed 13%, industry contributed 4%, and 6% came from background sources. The 'average' area-normalized TP and SRP loads to GB waters approximated 2.4 kg ha(-1) yr(-1) and 1.8 kg ha(-1) yr(-1), respectively. A consideration of uncertainties in the data contributing to these estimates suggested that the TP load to GB waters might lie between 33 and 68 kt yr(-1), with agriculture contributing between 10 and 28% of the TP load. These estimates are consistent with recent appraisals of annual TP and SRP loads to GB coastal waters and area-normalized TP loads from their catchments. Estimates of the contributions of RBDs to these P loads were consistent with the geographical distribution of P concentrations in GB rivers and recent assessments of surface waters at risk from P pollution.

Assessing the effectiveness of actions to mitigate nutrient loss from agriculture: a review of methods

Diffuse nutrient loss from agriculture is degrading surface and groundwater quality throughout Europe, leaving water bodies at risk of not reaching targets set by the Water Framework Directive (WFD). Mitigation methods to reduce diffuse agricultural nutrient loss need to be implemented where water bodies have been identified as at risk of not reaching good status by 2015. Though the effectiveness of individual mitigation methods has usually been assessed in controlled experiments, it is necessary to quantify impact under a wider range of environmental and agricultural conditions and at the catchment scale to ensure that action taken now will be sufficient to meet WFD targets. Due to catchment buffering and long transit times (>50 years), it is unlikely that responses to intervention will be observed by 2015 in many water bodies. In this review, we compare the attributes and usefulness of different approaches (direct measurement, nutrient budgeting, risk assessment and modelling) to assess the efficacy of actions to mitigate sources and transport of nitrogen (N) and phosphorus (P) from agricultural land to water. Modelling and 'measured changes in farm activity' through budgeting avoid the time lags associated with direct measurement and enable rapid evaluations of different options before implementation. Budgeting approaches using on-farm data also provide a simpler, more communicable means of assessment but currently fail to consider the timing and transport aspects of mitigation and assume a direct causal relationship between potential and actual nutrient loss. Risk assessment and modelling applications are potentially more comprehensive and able to better reflect choice of mitigation at a range of scales, but assessments demand increased availability of data, and there is a large degree of uncertainty associated with their spatial and temporal dimensions which is difficult to validate adequately. The failings of individual approaches suggest that these assessment methods should be integrated to maximise their potential usefulness and positive attributes. This will enable nutrient inputs to be utilised most efficiently at broad scales and site specific actions to reduce nutrient transport and delivery can be targeted most cost-effectively at smaller scales. Such an integrated approach will also more effectively engage and involve the farmer in what must be an iterative process.