Mitigation options to reduce phosphorus losses from the agricultural sector and improve surface water quality: a review

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.

An integrated approach through controlled experiment and LCIA to evaluate water quality and ecological impacts of irrigated paddy rice

This study used an integrated approach to mainly assess the water quality of paddy field during cultivation and quantify its equivalent ecological damages. Accordingly, an isolated pilot area with 0.6 ha and subsurface drainage pipes was prepared for flow measurement and multiple pollutant examination (DO, EC, pH, COD, TKN, TN, TP, NO3, butachlor) under controlled condition during 94 days of rice cultivation. Based on life cycle impact assessment (LCIA) database, the indices of ReCiPe (2016) were used to convert the examined nutrient and herbicide pollution. Results showed that TKN and TP were significant pollutants and reached the maximum concentrations of 7.2 and 4.9 mg/L in pilot outflow, respectively. Here, their average discharged loads were 56.2 gN/day and 45.3 gP/day. These loads equal leaching 8.5% and 9.4% of applied urea and phosphate fertilizers, respectively. The nutrient export coefficients were 8.4 kgN/ha and 6.8 kgP/ha. Nevertheless, the majority of this pollution was transferred by inflow. The net export coefficients were 0.3 kgN/ha and 2.6 kgP/ha while net leaching rates were 0.3%TN and 3.3%TP. The trend of combined ecological damages also showed that the 11-17th day of cultivation imposed the highest ecological risks. The state-of-the-art index of ecological footprint per food production estimates the equivalent ratio of lost lives by impaired ecosystem against lives saved from starvation. This index showed that 7% of the potential of produced paddy rice in this area for saving lives would be spoiled by releasing pollution to the terrestrial ecosystem in the long term. Yet, it can be enhanced as a matter of direct discharge to the freshwater. Therefore, using suitable agricultural operations or improving farm management practices for pollution abatement or assimilation potential is highly recommended.

Biodegradable Cassava Starch/Phosphorite/Citric Acid Based Hydrogel for Slow Release of Phosphorus: In Vitro Study

Phosphorous (P) is one the most important elements in several biological cycles, and is a fundamental component of soil, plants and living organisms. P has a low mobility and is quickly adsorbed on clayey soils, limiting its availability and absorption by plants. Here, biodegradable hydrogels based on Cassava starch crosslinked with citric acid (CA) were made and loaded with KH2PO4 and phosphorite to promote the slow release of phosphorus, the storing of water, and the reduction in P requirements during fertilization operations. Crosslinking as a function of CA concentrations was investigated by ATR-FTIR and TGA. The water absorption capacity (WAC) and P release, under different humic acid concentration regimens, were studied by in vitro tests. It is concluded that hydrogel formed from 10% w/w of CA showed the lowest WAC because of a high crosslinking degree. Hydrogel containing 10% w/w of phosphorite was shown to be useful to encouraging the slow release of P, its release behavior being fitted to the Higuchi kinetics model. In addition, P release increased as humic acid contents were increased. These findings suggest that these hydrogels could be used for encouraging P slow release during crop production.

Formation of reactive intermediates in paddy water from different temperature zones for the promotion of abiotic ammonification

The paddy field is a hot area of biogeochemical process. The paddy water has a large capacity in photo-generation of reactive intermediates (RIs) due to abundant photosensitive dissolved organic matter (DOM), which is influenced by the spatial heterogeneity of paddy soils but rarely been explored. Our work presents the first investigation of the role of soil properties on photochemistry in paddy water. Soil organic matter (SOM), determined by the temperature, was the dominant factor for the photo-generation of RIs in paddy water of main rice producing areas. The RI concentrations generated with abundant SOM from cool regions are 0.05-8.71 times higher than those for the warm regions in China. The humic-like substance and aromatic-like compounds of DOM plays an essential role in RIs generation, which is abundant in paddy soils rich in SOM from Chinese cool regions. In addition, RIs can efficiently accelerate the photo-ammonification of urea and free amino acids by 15.2 %-164 %, leading to 0.13-0.17 mmol/L/d photo-produced ammonium after fertilization, which is preferentially absorbed by rice. The findings of this study will extend our knowledge of the geochemistry of global paddy field ecosystem. The potential role of RIs in nitrogen cycle should be highlighted in the agroecosystem.

Greening the grid: A comprehensive review of renewable energy in Bangladesh

The escalating global demand for energy has coincided with economic development, while Bangladesh's reliance on renewable energy remains modest at 4.59%. Investigating economically viable solutions such as solar, biomass, and other renewable sources, the research underscores the pivotal role of sound policies and a strategic plan in transforming the current energy landscape. Despite facing various challenges, particularly in technology, the implementation of sound policies and a strategic plan can substantially alter the current landscape. By reviewing the Renewable Energy Policy of 2008 and incorporating recommendations from United States Agency for International Development (USAID) in 2023, this paper not only delves into challenges and future prospects but also aligns with the Sustainable Development Goal (SDG) aimed at achieving affordable and clean energy. This study contributes valuable insights by proposing methodologies to generate renewable energy by offering a comprehensive overview of the present energy scenario in Bangladesh, with a focus on strategic policy recommendations, thus surpassing previous efforts in the literature. The paper, in its entirety, strives to foster the adoption of renewable energy while concurrently mitigating reliance on conventional fossil fuels.

Effective microorganism water treatment method for rapid eutrophic reservoir restoration

Since reservoirs perform many important functions, they are exposed to various types of unfavorable phenomena, e.g., eutrophication which leads to a rapid growth of algae (blooms) that degrade water quality. One of the solutions to combat phytoplankton blooms are effective microorganisms (EM). The study aims to evaluate the potential of EM in improving the water quality of the Turawa reservoir on the Mała Panew River in Poland. It is one of the first studies providing insights into the effectiveness of using EM in the bioremediation of water in a eutrophic reservoir. Samples for the study were collected in 2019-2021. The analysis showed that EM could be one of the most effective methods for cleaning water from unfavorable microorganisms (HBN22, HBN36, CBN, FCBN, FEN) - after the application of EM, a reduction in their concentration was observed (from 46.44 to 58.38% on average). The duration of their effect ranged from 17.6 to 34.1 days. The application of EM improved the trophic status of the Turawa reservoir, expressed by the Carlson index, by 7.78%. As shown in the literature review, the use of other methods of water purification (e.g., constructed wetlands, floating beds, or intermittent aeration) leads to an increase in the effectiveness and a prolongation of the duration of the EM action. The findings of the study might serve as a guide for the restoration of eutrophic reservoirs by supporting sustainable management of water resources. Nevertheless, further research should be conducted on the effectiveness of EM and their application in the remediation of eutrophic water reservoirs.

Mapping and assessment of river water quality under varying hydro-climatic and pollution scenarios by integrating QUAL2K, GEFC, and GIS

Water quality modelling has proved to be effective method for managing river water quality. But the most effective and comprehensive approach involving integration of river water quality simulation and pollution visualization with the objective of pollution reduction and maintenance of environmental flow strategies has gained less attention. Thus, the objective of this study was to employ an integrated approach for mapping and analysing river water quality under various hydro-climatic and pollution scenarios. Specifically, this approach involved the integration of a river water quality simulation model, QUAL2K, Global Environmental Flow Calculator (GEFC), and Geographical Information System (GIS) to develop water quality index (WQI) based map charts of water quality. The calibrated QUAL2K model was utilized to simulate WQI parameters including water temperature, pH, electrical conductivity, dissolved oxygen (DO), biological oxygen demand (BOD), nitrates (NO3), ammonia (NH4), and alkalinity. To analyse the WQI, the Weighted Arithmetic-Water Quality Index (WA-WQI) method was employed for various individual and combined pollution scenarios, environmental flow (Eflow), and climate change scenarios. The developed integrated approach was applied to the Bhadravati segment of Bhadra River, India. The findings revealed that the prevailing WQI status of the study stretch ranged from poor to unsuitable for drinking purposes. This deterioration can be attributed to the impact of both industrial and municipal effluents. By maintaining the effective Environmental Management Class (EMC) flow rates (class C flowrate of EMC (40.32 m3/s)) in conjunction with appropriate Pollution Reduction (PR) level (10% PR) at headwater and incoming drains, the stream self-purification capacity was enhanced resulting in the Bhadravati River stretch water quality transitioning to favourable water quality condition.

A three-dimensional perspective of phosphorus retention across a field-buffer strip transition

Vegetated filter strips (VFS) act as buffer zones between fields and water bodies that are supposed to retain incoming runoff, sediment, and nutrients. The factors that govern nutrient retention and cycling in VFS are complex and act in all three dimensions. A key element that determines VFS effectivity is flow type, e.g., sheet vs. concentrated flow. These aspects are, however, often insufficiently accounted for in VFS research and design recommendations. In this study, we attempt to tackle these shortcomings by examining the nutrient distribution in detail at two field-VFS transitions, applying a three-dimensional sampling array together with extensive laboratory analyses. Concentrated runoff was the dominant type we found and we argue that flow convergence is the norm rather than the exception. Further complicating this issue is that entry locations of runoff may vary, calling for more sophisticated sampling designs. Overall trends were similar across the analyzed nutrient fractions (different K- and P-pools) and there were distinct trends of decreasing nutrients along the longitudinal (from the field to the VFS) and vertical planes. The horizontal plane (from outside to inside the area of concentrated flow) showed mostly inconclusive or U-shaped gradients. Both sites were similar and close to each other, nevertheless, there were significant differences that affected nutrient retention in the VFS which were linked to site-specific factors. The spatial extent (i.e., width) is often considered the main variable in VFS designs. However, other VFS traits such as vegetation type and structure, as well as external factors such as field topography and the severity of erosive events are equally important and should be attributed more significance.

Exploring the trend effects of diffuse anthropogenic pollution in a large river passing through a densely populated area

The detection of non-point pollution in large rivers requires high-frequency sampling over a longer period of time, which, however presumably provides data with large spatial and temporal variance. Variability may mean that data sets recorded upstream and downstream from a densely populated area overlap, suggesting at first glance that the urban area did not affect water quality. This study presents a simple way to explore trend-like effects of non-point pollution in the Danube based on data that varied strongly in space and time. For one year, biweekly sampling was carried out upstream and downstream from a large city with negligible emission of untreated wastewater and the surrounding settlements, industrial and agricultural areas. Although most of the values of the 34 examined physicochemical characteristics fell within the range of data previously published for the Danube, and the mean values of all parameters indicated unpolluted surface water, different water quality was revealed upstream and downstream from the metropolitan area at each sampling time. Since the physicochemical characteristics causing the separation also differed from time to time, univariate tests and consensus ordination were used to determine which variables changed similarly during most of the examined period. With this evaluation method, several diffuse pollutants of anthropogenic origin contaminating the Danube in the long term were identified, such as nitrogen, phosphorus, sulphate, chloride, potassium and vanadium. The results demonstrated that trend-like effects of non-point pollution can be detected even in a large river, where physicochemical measurements can vary strongly in space and time.

Concurrent datasets on land cover and river monitoring in Fukushima decontaminated catchment during 2013-2018

After the Fukushima nuclear accident, the Japanese government implemented extensive decontamination work in 137Cs contaminated catchments for residents' health and local revitalization. Whether dramatic land use changes in the upstream decontaminated regions affected river suspended sediment (SS) and particulate 137Cs discharge downstream remain unknown because of the poor quantification on land cover changes and long-term river SS dynamics. We here introduce a 6-year concurrent database of the Niida River Basin, a decontaminated catchment, including the first available vector decontamination maps, satellite images in decontaminated regions with a spatial resolution of 10 m, and long-term river monitoring datasets spanning decontamination (2013-2016) and subsequent natural restoration stages (2017-2018). These datasets allow us, for the first time, to directly link the transport dynamics of river SS (particulate 137Cs) to land use changes caused by humans in real-time, which provide fundamental data for better understanding the river response of sediment to land use change. Moreover, the data obtained by interdisciplinary methods offer a template for land use change impact assessment in other river basins.

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.

The impacts of spatio-temporal variation of natural and agricultural influences on the environmental water quality in a fluvial-lacustrine watershed in China

Despite the significant impacts of natural factors such as rainfall, topography, soil type, and river network as well as agricultural activities on the environmental water quality, little is known about the influence of their temporal and spatial variations in a fluvial-lacustrine watershed. In this study, a whole process accounting method based the export coefficient model (WP-ECM) was first developed to quantify how natural factors and agricultural activities distribution influenced water quality. A case study was performed in a typical fluvial-lacustrine area - Dongting basin, China. The simulated results indicated that the natural factors can promote and inhibit the migration and transformation of agricultural pollutants generated from the watershed and the spatial distribution of the natural factors displayed high variability. It should be priority to monitor the areas with greater natural impact in the basin. Moreover, the cultivated land area and the number of pig-breeding were positively correlated with the pollutant discharge. From the perspective of the spatial distribution of comprehensive influence, the comprehensive high-impact areas are mainly distributed in the Dongting Lake district in 2005-2010 and in Xiang River watershed in 2010-2020. A key strategy for controlling or reducing the cultivated land area and the intensity of livestock breeding in these high-impacts areas is recommended to reduce the impact of the environmental water quality for the entire basin.

Chemical and biological tracers to identify source and transport pathways of septic system contamination to streams in areas with low permeability soils

Septic systems are widely used in rural areas that lack centralized sewage treatment systems. Incomplete removal of domestic wastewater contaminants in septic systems can lead to leaching of nutrients (P and N), bacteria/viruses, and trace contaminants to surrounding groundwater and surface water. This study focuses on delineating the fate of wastewater contaminants in localities where septic systems are installed in moderate to fine-grained overburden materials to assess potential impacts on groundwater and surface water quality in these settings. Nutrients and a suite of anthropogenic tracers, including host-specific fecal indicator bacteria (bovine- and human-specific Bacteroides), pharmaceutical compounds (caffeine, carbamazepine, gemfibrozil, ibuprofen, naproxen, and sulfamethoxazole), and an artificial sweetener (acesulfame-K), were selected to evaluate differences in transport properties. Surface water samples (n = 103) were collected from streams upstream (US) and downstream (DS) of three rural hamlets up to two times monthly over one year. Results indicate the presence of wastewater indicators in the streams, with DS locations showing significantly elevated concentrations of both chemical and biological anthropogenic tracers. Human-specific Bacteroides, caffeine, and acesulfame-K were consistently observed at elevated concentrations at all DS sites. Nutrients exhibited varied concentrations between US and DS locations at three study sites. The occurrence of human-specific Bacteroides in the surface water samples suggests the presence of preferential flow pathways within the silt/clay overburden. These results demonstrate the advantages of using a combined tracer approach, involving a conservative tracer such as acesulfame-K coupled with the human-specific biological indicator Bacteroides (BacHum), to understand not only impacting sources but also potential transport pathways of septic system contamination to nearby streams. Septic systems may be an underappreciated contaminant source in rural hamlets located in fine-grained overburden materials; although, a distinction of specific nutrient sources (septic systems vs. agriculture) remains challenging.

Effect of external and internal loading on source-sink phosphorus dynamics of river sediment amended with iron-rich glauconite sand

Glauconite sands (GS) are abundantly available iron (Fe)-rich minerals that are efficient in lowering the release of phosphorus (P) from sediments to the overlying water. Many river sediments are, however, net sinks for P rather than sources and it is unclear if these GS minerals also enhance the P uptake from water. This is because the concentration of Fe(III) minerals at the sediment-water interface (SWI) depends on the redox potential that is affected by physicochemical processes. This study was set-up to investigate if a sediment amendment with GS can both lower P release from the sediment and enhance P uptake from the overlying water. The P fluxes across the SWI were compared between GS-amended (added at 10% weight fraction) and non-amended river sediment in static (incubation) and dynamic (flume) systems. The net P uptake was measured in response to a pulse external P loading (0.5-5 mg P L^-1). Sodium glutamate was added to all treatments to simulate water with a high oxygen demand. Before the P pulse, the GS-amended sediments released significantly less P to the overlying water than the non-amended sediments in both static as dynamic systems. Spiking the water reverted the net P flux over the SWI only in the dynamic system, and the net P uptake in the sediment was factor two larger in GS-amended sediment compared to the non-amended sediment. This study showed that GS addition not only reduced internal P release, but also enhanced P uptake from the overlying water. However, the long-term efficiency in streams likely decreases over time due to saturation processes.

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.

Sustainable Low-Cost Phosphorus Recovery Using Nanostructured Materials with Reusability Potential

A new low-cost material with a polymeric base formed from sodium silicate was developed. The material presents a nanostructured, highly rich iron surface with a large phosphorus retention capacity and potential reuse as a crop fertilizer. In the present study, we demonstrate that iron is the element that acts as an adsorbent for phosphate, while the polymeric base functions exclusively as a support for iron. The iron is uniformly adsorbed on the surface of the material, forming nanostructures, which ensure that iron works similarly to nanoparticles in solution but avoid other problems, such as particle agglomeration or the difficulty of separating them after the removal process. Materials were characterised by SEM, EDS, N₂ sorption, and image processing, and the effect of pH, ionic strength, and temperature was studied. Sorption kinetics were analysed using Boyd's diffusion model, and adsorption equilibria were studied using several adsorption models. A maximum iron adsorption on the polymeric base of 23.9 ± 0.3 mg Fe∙g^-1 was found, while maximum phosphorus adsorption was 366 ± 21 mg P∙g^-1 Fe. Thus, phosphorus is recovered from the aqueous medium with an inexpensive material that has the potential to be used directly as a fertilizer.

Regional Analysis of Nitrogen Flow within the Chesapeake Bay Watershed Food Production Chain Inclusive of Trade

In the Chesapeake Bay Watershed, excess nitrogen has contributed to poor water quality, leading to nitrogen mitigation efforts to restore and protect the watershed. The food production system is a top contributor to this nitrogen pollution. While the food trade plays a vital role in distancing the environmental impacts of nitrogen use from the consumer, previous work on nitrogen pollution and management in the Bay is yet to carefully consider the effect of embedded nitrogen found in products (nitrogen mass within the product) imported and exported throughout the Bay. Our work advances understanding across this area by creating a mass flow model of nitrogen embedded in the food production chain throughout the Chesapeake Bay Watershed that separates phases of the production and consumption processes for crops, live animals, and animal products and considers commodity trade at each phase by combining aspects of both nitrogen footprint and nitrogen budget models. Also, by tracking nitrogen embedded in products imported and exported in these processes, we distinguished between direct nitrogen pollution and nitrogen pollution externalities (displaced N pollution from other regions) from outside of the Bay. We developed the model for the watershed and all its counties for major agricultural commodities and food products for 4 years 2002, 2007, 2012, and 2017 with a specific focus on 2012. Using the developed model, we determined the spatiotemporal drivers of nitrogen loss to the environment from the food chain within the watershed. Recent literature leveraging mass balance approaches has suggested that previous long-term declines in nitrogen surplus and improvements in nutrient use efficiency have stagnated or begun to reverse. Our results suggest that within the Chesapeake Bay, increased corn and wheat acreage and steadily increasing livestock/poultry production may have led to the stagnation in decreasing N loss trends from agricultural production observed over the past two decades. We also show that at the watershed scale, trade has reduced the food chain nitrogen loss by about 40 million metric tons. This model has the potential to quantify the effect of various decision scenarios, including trade, dietary choices, production patterns, and agricultural practices, on the food production chain nitrogen loss at multiple scales. In addition, the model's ability to distinguish between nitrogen loss from local and nonlocal (due to trade) sources makes it a potential tool to optimize regional domestic production and trade to meet local watershed's needs while minimizing the resulting nitrogen loss.

Phosphorus immobilisation in sediment by using iron rich by-product as affected by water pH and sulphate concentrations

Iron (Fe) rich by-product from drinking water treatment plants can be added to rivers and lakes to immobilise phosphorus (P) in sediment and lower eutrophication risks. This study was set up to investigate the P immobilisation efficiency of an Fe rich by-product as affected by the pH and sulphate (SO₄) concentration in the overlying water. Both factors are known to inhibit long-term P immobilisation under anoxic conditions. A static sediment-water incubation was conducted at varying buffered water pH values (6, 7 and 8) and different initial SO₄ concentrations (0-170 mg SO₄ L^-1) with or without Fe rich by-product amendment to the sediment. In the unamended sediment, the P release to the overlying water was highest, and up to 6 mg P L^-1, at lowest water pH due to higher reductive dissolution of Fe(III) oxyhydroxides. The Fe rich by-product amendment to the sediment largely reduced P release from sediment by factors 50-160 depending on pH, with slightly lowest immobilisation at highest pH 8, likely because of pH dependent P sorption. The total sulphur (S) concentrations in the overlying water reduced during incubation. The P release in unamended sediments increased from 2.7 mg L^-1 to 4.2 mg L^-1 with higher initial SO₄ concentrations, suggesting sulphide formation during incubation and FeS precipitation that facilitates release of P. However, no such SO₄ effects were found where Fe rich by-product was applied that lowered P release to <0.1 mg L^-1 illustrating high stability of immobilised P in amended sediments. This study suggests that Fe rich by-product is efficient for P immobilisation but that loss of Fe in low pH water may lower its long-term effect.

Phosphorus sorption and desorption as affected by long-term cover cropping at two soil surface depths

Phosphorus (P) loss from agricultural land is a persistent environmental challenge, and a better understanding of the impact of continuous cover crops (CCs) growth on soil P sorption and desorption characteristics is needed to inform mitigation strategies. This study investigated the impact of CC species on soil P pools, sorption characteristics, and dissolved reactive P (DRP) after 9 yr. Soil samples were collected at 0-to-2- and 2-to-4-cm soil depths from a silty clay loam Mollisol. Treatments included cereal rye (Secale cereal L.; CR), annual ryegrass (Lolium multiflorum, AR), oats/radish (Avena sativa L./Raphanus sativus L.; OR), and no CC (CN). A sorption experiment was done with varying P concentrations for 24 h equilibration, and sorption parameters were estimated using the Langmuir model. The DRP was estimated using sequential soil extraction by 0.01 M CaCl₂ for 5 h. Long-term CC significantly decreased P sorption maximum but increased binding energy relative to CN. Annual ryegrass significantly decreased soil water extractable P, Mehlich 3 P, and degree of P saturation relative to OR and CN at the 0-to-2-cm depth. Annual ryegrass and CR significantly decreased desorbed DRP by an average of 42 and 45% relative to CN and OR, respectively, at the 0-to-2-cm depth. These results demonstrated that long-term grass species decreased the concentrations of labile P pools and desorbed DRP at the soil runoff interaction zone. Therefore, planting of AR and CR should be promoted in fields susceptible to runoff DRP losses.

Phosphorus adsorption on iron-coated sand under reducing conditions

Mitigation measures are needed to prevent large loads of phosphate originating in agriculture from reaching surface waters. Iron-coated sand (ICS) is a residual product from drinking water production. It has a high phosphate adsorption capacity and can be placed around tile drains, taking no extra space, which increases the farmers' acceptance. The main concern regarding the use of ICS filters below groundwater level is that limited oxygen supply and high organic matter concentrations may lead to the reduction and dissolution of iron (hydr)oxides present and the release of previously adsorbed phosphate. This study aimed to investigate phosphate adsorption on ICS at the onset of iron reduction. First, we investigated whether simultaneous metal reduction and phosphate adsorption were relevant at two field sites in the Netherlands that use ICS filters around tile drains. Second, the onset of microbially mediated reduction of ICS in drainage water was mimicked in complementary laboratory microcosm experiments by varying the intensity of reduction through controlling the oxygen availability and the concentration of degradable organic matter. After 3 yr, ICS filters in the field removed phosphorus under low redox conditions. Over 45 d, the microbial reduction of manganese and iron oxides did not lead to phosphate release, confirming field observations. Electron microscopy and X-ray absorption spectroscopy did not evince systematic structural or compositional changes; only under strongly reducing conditions did iron sulfides form in small percentages in the outer layer of the iron coating. Our results suggest that detrimental effects only become relevant after long periods of operation.

Phosphorus control and dredging decrease methane emissions from shallow lakes

Freshwater ecosystems are an important source of the greenhouse gas methane (CH₄), and their emissions are expected to increase due to eutrophication. Two commonly applied management techniques to reduce eutrophication are the addition of phosphate-binding lanthanum modified bentonite (LMB, trademark Phoslock©) and dredging, but their effect on CH₄ emissions is still poorly understood. Here, this study researched how LMB and dredging affected CH₄ emissions using a full-factorial mesocosm design monitored for 18 months. The effect was tested by measuring diffusive and ebullitive CH₄ fluxes, plant community composition, methanogen and methanotroph activity and community composition, and a range of physicochemical water and sediment variables. LMB addition decreased total CH₄ emissions, while dredging showed a trend towards decreasing CH₄ emissions. Total CH₄ emissions in all mesocosms were much higher in the summer of the second year, likely because of higher algal decomposition and organic matter availability. First, LMB addition lowered CH₄ emissions by decreasing P-availability, which reduced coverage of the floating fern Azolla filiculoides, and thereby prevented anoxia and decreased surface water NH₄⁺ concentrations, lowering CH₄ production rates. Second, dredging decreased CH₄ emissions in the first summer, possibly it removed the methanogenic community, and in the second year by preventing autumn and winter die-off of the rooted macrophyte Potamogeton cripsus. Finally, methanogen community composition was related to surface water NH₄⁺ and O₂, and porewater total phosphorus, while methanotroph community composition was related to organic matter content. To conclude, LMB addition and dredging not only improve water quality, but also decrease CH₄ emissions, mitigating climate change.

Phosphorus retention and agronomic efficiency of refined manure-based digestate—A review

Digestate, a by-product from anaerobic digestion of organic materials such as animal manure, is considered a suitable plant fertilizer. However, due to its bulkiness and low economic value, it is costly to transport over long distances and store for long periods. Refinement processes to valorize digestate and facilitate its handling as a fertilizer include precipitation of phosphorus-rich mineral compounds, such as struvite and calcium phosphates, membrane filtration methods that concentrate plant nutrients in organic products, and carbonization processes. However, phosphorus retention efficiency in output products from these processes can vary considerably depending on technological settings and characteristics of the digestate feedstock. The effects of phosphorus in plant fertilizers (including those analogous or comparable to refined digestate products) on agronomic productivity have been evaluated in multiple experiments. In this review, we synthesized knowledge about different refinement methods for manure-based digestate as a means to produce phosphorus fertilizers, thereby providing the potential to increase phosphorus retention in the food production chain, by combining information about phosphorus flows in digestate refinement studies and agronomic fertilizer studies. It was also sought to identify the range, uncertainty, and potential retention efficiency by agricultural crops of the original phosphorus amount in manure-based digestate. Refinement chains with solid/wet phase separation followed by struvite or calcium phosphate precipitation or membrane filtration of the wet phase and carbonization treatments of the solid phase were included. Several methods with high potential to extract phosphorus from manure-based wet phase digestate in such a way that it could be used as an efficient plant fertilizer were identified, with struvite precipitation being the most promising method. Synthesis of results from digestate refinement studies and agronomic fertilizer experiments did not support the hypothesis that solid/wet separation followed by struvite precipitation, or any other refinement combination, results in higher phosphorus retention than found for unrefined digestate. Further studies are needed on the use of the phosphorus in the solid phase digestate, primarily on phosphorus-rich soils representative of animal-dense regions, to increase understanding of the role of digestate refinement (particularly struvite precipitation) in phosphorus recycling in agricultural systems.

Configuration of urban water eutrophication abatement systems using a novel regression-based optimization scheme

A novel regression-based target-oriented robust optimization (R-TORO) framework is introduced to obtain robust designs of surface water eutrophication abatement systems during the early stages of urban water planning. The proposed framework can process a manifold of data for estimating the relationships of system elements and the uncertainty distribution of imprecise parameters, a novel extension of the canonical TORO framework. A demonstrative case study about an early-stage urban water infrastructure planning for abating Phosphorous (P) accumulation in urban waters is employed to demonstrate the R-TORO framework. The following system targets are considered: target cost and target water cycle sustainability index (WCSI). The findings show that the abatement of P accumulation in soil contributes the largest to WCSI. Consequently, the R-TORO-generated system design suggests the implementation of impervious local roads connected to sewers with improved P removal. The simulated/estimated performance of the eutrophication abatement system design obtained using the R-TORO are as follows: import of water = 2.88 × 10⁶ m³/y; export of P = 4.96 × 10⁶ g P/y; P concentration in recreational lake = 0.04 mg P/l; P concentration in canal system = 0.30 mg P/l; P accumulation in lake sediments = 0.08 g P/m²,y; P accumulation in canal sediments = 1.18 g P/m²,y; P accumulation in soil = 0.01 g P/m²,y.

Phosphorus Sorption in Soils and Clay Fractions Developed from Different Parent Rocks in Limpopo Province, South Africa

Phosphorus (P) sorption dynamics in soils have implications for the environment and soil fertility. Soils and clay fractions that were developed from basalt, granite, arkosic sandstone, and gneiss in Limpopo Province, South Africa were analysed for their P adsorption characteristics and external phosphorus requirements (EPR). The relationship between the P adsorption parameters and EPR of the soils and clay fractions were also assessed. The Langmuir adsorption isotherms for the soils and clay fractions gave a better fit with slightly higher R-square values relative to the Freundlich adsorption isotherms. The Langmuir P sorption maxima were between 285.71 and 833.33 mg/kg and 238.09 and 625.0 mg/kg for the soils and clay fractions, respectively, and the EPR values ranged from 7.78 to 92.91 mgP/kg and 5.13 to 65.85 mgP/kg for the soils and clay fractions, respectively. The variations in the EPR suggest a single, uniform P fertiliser application to the soils could cause under-fertilisation and over-fertilisation problems. The soils that were developed from basalt, relative to the others, showed no risk to the water quality in the region at the current rate of P fertiliser application. The P sorption parameters of the soils and clay fractions showed no statistically significant differences. Hence, the P sorption parameters of the clay fractions could be reliable predictors of the P sorption and buffering in their respective soils.

A loss function to evaluate agricultural decision-making under uncertainty: a case study of soil spectroscopy

Modern sensor technologies can provide detailed information about soil variation which allows for more precise application of fertiliser to minimise environmental harm imposed by agriculture. However, growers should lose neither income nor yield from associated uncertainties of predicted nutrient concentrations and thus one must acknowledge and account for uncertainties. A framework is presented that accounts for the uncertainty and determines the cost-benefit of data on available phosphorus (P) and potassium (K) in the soil determined from sensors. For four fields, the uncertainty associated with variation in soil P and K predicted from sensors was determined. Using published fertiliser dose-yield response curves for a horticultural crop the effect of estimation errors from sensor data on expected financial losses was quantified. The expected losses from optimal precise application were compared with the losses expected from uniform fertiliser application (equivalent to little or no knowledge on soil variation). The asymmetry of the loss function meant that underestimation of P and K generally led to greater losses than the losses from overestimation. This study shows that substantial financial gains can be obtained from sensor-based precise application of P and K fertiliser, with savings of up to £121 ha-1 for P and up to £81 ha-1 for K, with concurrent environmental benefits due to a reduction of 4-17 kg ha-1 applied P fertiliser when compared with uniform application.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11119-022-09887-2.

Assessing the impact of water use in conventional and organic carrot production in Poland

As global water resources are decreasing and the demand for it is constantly increasing, the problem of proper water management is becoming more pressing. Poland is one of the largest producers of vegetables in Europe, including carrots, with significant exports. However its freshwater resources are relatively small. The paper presents the results of research on the water footprint (WF) life cycle assessment (LCA) in conventional and organic carrot production. The methodology of calculating WF was used in accordance with PN-EN ISO 14046. It was found, e.g., that WF for organic production of carrot (WF = 1.9 m³ ha⁻¹) is over five times lower, as compared to conventional production (WF = 10.4 m³ ha⁻¹). In the case of conventional production, the fertilization process (67.0–67.7%) has the greatest impact on the shaping of WF in the individual impact categories, i.e. Human Health, Ecosystem Quality and resources. In organic production, the WF-shaping factor is carrot harvesting (41.9–43.1%). The research can be used to develop pro-ecological carrot production technologies, as well as to shape sustainable development plans in agricultural areas. It can also be used to outline policy directions regarding foreign trade in water-consuming agricultural products.

Evaluation of the Potential Release Risk of Internal N and P from Sediments—A Preliminary Study in Two Freshwater Reservoirs in South China

Growing evidence has demonstrated the influence of internal nitrogen (N) and phosphorus (P) on harmful algae blooms in eutrophic freshwater ecosystems. However, the main controlling factors for internal N and P release risks, and whether these factors vary as environmental conditions change, remains poorly understood. We evaluated potential release risks of N and P from sediments in two freshwater reservoirs in Beihai City, southern China, by evaluating apparent nutrient fluxes during simulated static incubation experiments at two temperatures (15 °C and 25 °C). Sediments were analyzed to determine their basic properties as well as N and P fractions. Results showed that the main controlling factors of the apparent fluxes in dissolved total P, soluble reactive P, total N, and ammonium were related to sediment adsorption properties, redox properties, and microbial-mediated properties (e.g., water-extractable P, total inorganic N, redox-sensitive P, total organic carbon, organic P). The primary controlling factors for apparent N and P fluxes were dependent on the form of N and P and changed with temperature. The results suggest that care should be taken when simply using total N and P contents in sediments to evaluate their internal nutrient release risks.

Mechanisms for improving phosphorus utilization efficiency in plants

BACKGROUND

Limitation of plant productivity by phosphorus (P) supply is widespread and will probably increase in the future. Relatively large amounts of P fertilizer are applied to sustain crop growth and development and to achieve high yields. However, with increasing P application, plant P efficiency generally declines, which results in greater losses of P to the environment with detrimental consequences for ecosystems.

SCOPE

A strategy for reducing P input and environmental losses while maintaining or increasing plant performance is the development of crops that take up P effectively from the soil (P acquisition efficiency) or promote productivity per unit of P taken up (P utilization efficiency). In this review, we describe current research on P metabolism and transport and its relevance for improving P utilization efficiency.

CONCLUSIONS

Enhanced P utilization efficiency can be achieved by optimal partitioning of cellular P and distributing P effectively between tissues, allowing maximum growth and biomass of harvestable plant parts. Knowledge of the mechanisms involved could help design and breed crops with greater P utilization efficiency.

An evaluation of the Chesapeake Bay management strategy to improve water quality in small agricultural watersheds

Chesapeake Bay water quality has been a concern since 1970. In rural areas, agriculture is the dominant N and P source, and the voluntary application of best management practices (BMPs) is the primary management tool. Here we test the hypothesis that the current management approach of primarily voluntary, untargeted BMP implementation is insufficient to create detectable, widespread reductions in N, P, and total suspended solid (TSS) concentrations in agricultural watersheds of the Choptank basin, a tributary of Chesapeake Bay. To test this hypothesis, we assessed BMP implementation and sampled water quality on participating farms, at intermediate streams within each watershed, and at watershed outlets of four watersheds from 2013 to 2014. We also present water quality data from 2003 to 2014 at the outlets of 12 additional agricultural and one forested watershed and survey-directed interviews of farmers. By the end of 2014, large numbers of BMPs, both structural and cultural, had been implemented. Of the 16 agricultural watersheds, 50% showed significant decreases in baseflow N, 37.5% showed no changes, and 12.5% showed increasing TN. Baseflow P significantly decreased at just one watershed, increased at one, and remained stable at 14. Stormflow N was similar to baseflow, but stormflow P was 5 times higher than baseflow. These data partially support our hypothesis. Surveys suggested farmers considered themselves responsible for the quality of water leaving their farms, but out-of-pocket cost was the major impediment to further BMP adoption. We suggest that greater outreach and more financial support for farmers to implement BMPs is required to increase the types and densities of BMPs needed to achieve regional water quality goals.

Excessive application of chemical fertilizer and organophosphorus pesticides induced total phosphorus loss from planting causing surface water eutrophication

Total phosphorus (TP) loss from planting was one of the resources causing agricultural non-point source pollution. It is significant to clarify the factors influencing TP loss, as well as explore the relationship between TP loss from planting and surface water eutrophication for making recommendations on the reduction of environmental pollution. In this study, the minimum and maximum of average TP loss was appeared in Qinghai and Shandong province with the TP loss of 7.7 × 10² t and 7.5 × 10³ t from 2012 to 2014, respectively. The results of structural equation model (SEM) indicating that the effect of anthropogenic drivers on TP loss was more important than natural conditions due to the higher path coefficient of anthropogenic drivers (0.814) than that of natural conditions (0.130). For anthropogenic drivers, the path coefficients of usage of fertilizer and pesticides, which was often excessively applied in China, were 0.921 and 0.909, respectively causing they the two dominant factors affecting TP loss. Annual precipitation and relative humidity, which were belongs to natural conditions, increased TP loss by enhancing leaching and surface runoff. However, light duration could reduce TP loss by promoting crop growth and increasing TP absorption of crops, with a path coefficient of - 0.920. TP loss of each province in per unit area from planting was significantly correlated with TP concentration of its surface water (p < 0.05), suggesting that TP loss from planting was the main factor causing surface water eutrophication. This study targeted presented three proposals to reduce the TP loss from planting, including promotion of scientific fertilization technologies, restriction of organophosphorus pesticides, and popularization of water saving irrigation technologies. These findings as well as suggestions herein would provide direction for the reduction of TP loss from planting.

Applying the nutrient transfer continuum framework to phosphorus and nitrogen losses from livestock farmyards to watercourses

Farmyards are commonly conceptualized as point sources of nutrient pollution nested within the wider agricultural landscape. However, within farmyards there are individual sources and delivery pathways, each of which is affected by a range of management practices and infrastructure. Rainfall mobilizes these nutrients, which may then be delivered to a receptor or to the wider drainage network. As such, the nutrient transfer continuum (NTC), which has been established as a framework to understand and mitigate nutrient loss at a landscape scale, can be similarly applied to disentangle the stages of nutrient transfer from farmyards. The NTC differentiates nutrient transfer into source, mobilization, delivery, and impact stages. This differentiation allows targeting of mitigation measures and evaluation of costs and benefits. This review paper applies the NTC template to farmyard nitrogen and phosphorus transport to conceptualize causative factors and to identify mitigation options.

Management of phosphorus nutrient amid climate change for sustainable agriculture

Nutrients are essential for plant growth and development and influence overall agricultural production. Phosphorus (P) is a major nutrient required for many physiological and biochemical functions of a plant. Phosphate rock is the major source of phosphate fertilizer but is becoming increasingly limited in both developing and developed countries. The resources of phosphate rock need to be conserved, and import dependency on phosphate fertilizer needs to be minimized; this will help increase the availability of phosphate fertilizer over the next 300 yr. Climate change creates new challenges in the management of nutrients including P, affecting the overall production of crops. The availability, acquisition, and translocation of P are influenced by the fluctuation of temperatures, pH, drought, and elevated CO₂ . Both lower and higher soil temperatures reduce uptake and translocation of P. High soil pH affects P concentration and decreases the rate of plant P uptake. Low soil pH decreases the activity of soil microorganisms, the rate of transpiration, and P uptake and utilization. Elevated CO₂ decreases P uptake from soil by the plants. Future research is needed on chemical, molecular, microbiological, and physiological aspects to improve the understanding on how temperature, pH, drought, and elevated CO₂ affect the availability, acquisition, and transport of P by plants. Better P management strategies are required to secure the P supply to ensure long-term protection of soil fertility and to avoid environmental impacts such as eutrophication and water pollution, ensuring sustainable food production.

Characterization of soil phosphate status, sorption and saturation in paddy wetlands in usangu basin-Tanzania

Phosphorus (P) is a vital plant macronutrient required for plant growth which usually available in limited amount. P availability for plant uptake in highly weathered soil is controlled by soil erosion and high fixation. The availability of P applied from fertilizers depend on the soil pH, soil sorption capacity (PSC) and P saturation status (PSD), which determines P storage, losses, fixation, and additional P to be added with minimal loss to the environment. PSC and PSD are agro-environmental indicators used to estimate P availability and P loss to the environment. However, PSC and PSD of agricultural soils had been never studied in Tanzanian soils. This study was conducted to assess and estimate P availability, PSC and PSD and the risks of P losses in tropical soils from Usangu basin popular for paddy farming. In total, 198 soil samples from 10 paddy irrigation schemes were collected (November-December 2019) and analyzed for inherent P (P_M3), metal oxides of Aluminium (Al _M3), iron (Fe _M3), and calcium (Ca _M3) as main PSC and PSD determinant. The determined concentrations were in range of; P _M3 014.9-974.69 mg/kg, Al _M3 234.56-3789.36 mg/kg, Fe _M3 456.78-2980.23 mg/kg, and Ca _M3 234.67-973.34 mg/kg. Estimated PSC_M3 ranged 5.62-34.85 mmol/kg with a mean value of 14.14 mmol/kg corresponding to high status, ensuring high P holding capacity for plant uptake. However, some soils had very low PSC_M3 creating a risk of P loss to environment. Among soils, the estimated PSD _M3 ranged from 0.01 to 17.57% and was below (<24%), indicating low P loss risks to surface and groundwater, however, some soils were observed to have PSD_M3 above 15% which correspond to a critical degree of phosphate saturation of 25% in a watershed using oxalate extraction method. Therefore some sites were associated with high P loss to the environment, immediate and precautionary actions for sustainable P management to increase productivity, environmental safety and sustainability are needed to be in place.

Anthropogenic influences on Zambian water quality: hydropower and land-use change

The Zambezi River Basin in Southern Africa is undergoing rapid development and population growth. Agricultural intensification, urbanization and future development of hydropower dams will likely lead to a degradation of surface water quality, but there have been few formal assessments of where, how and why these changes impact specific water quality parameters based on in situ data spanning a large region. We sampled a large suite of biogeochemical water quality parameters at 14 locations in four field campaigns in central and southern Zambia in 2018 and 2019 to characterize seasonal changes in water quality in response to large hydropower dams and human landscape transformations. We find that the major rivers (Zambezi and Kafue) are very clean with extremely low concentrations of solutes, but suffer from thermal changes, hypoxia and loss of suspended sediment below dams. Smaller tributaries with a relatively large anthropogenic landcover footprint in their catchments show signs of pollution in the form of higher concentrations of nutrients and dissolved ions. We find significant relationships between crop and urban land cover metrics and selected water quality metrics (i.e. conductivity, phosphorus and nitrogen) across our data set. These results reflect a very high-quality waterscape exhibiting some hotspots of degradation associated with specific human activities. We anticipate that as agricultural intensification, urbanization and future hydropower development continue to accelerate in the basin, the number and extent of these hotspots of water quality degradation will grow in response. There is an opportunity for governments, managers and industry to mitigate water quality degradation via investment in sustainable infrastructure and practice, such as wastewater treatment, environmental dam operations, or riparian protection zones.

A comprehensive review on magnetic carbon nanotubes and carbon nanotube-based buckypaper for removal of heavy metals and dyes

Industrial effluents contain several organic and inorganic contaminants. Among others, dyes and heavy metals introduce a serious threat to drinking waterbodies. These pollutants can be noxious or carcinogenic in nature, and harmful to humans and different aquatic species. Therefore, it is of high importance to remove heavy metals and dyes to reduce their environmental toxicity. This has led to an extensive research for the development of novel materials and techniques for the removal of heavy metals and dyes. One route to the removal of these pollutants is the utilization of magnetic carbon nanotubes (CNT) as adsorbents. Magnetic carbon nanotubes hold remarkable properties such as surface-volume ratio, higher surface area, convenient separation methods, etc. The suitable characteristics of magnetic carbon nanotubes have led them to an extensive search for their utilization in water purification. Along with magnetic carbon nanotubes, the buckypaper (BP) membranes are also favorable due to their unique strength, high porosity, and adsorption capability. However, BP membranes are mostly used for salt removal from the aqueous phase and limited literature shows their applications for removal of heavy metals and dyes. This study focuses on the existence of heavy metal ions and dyes in the aquatic environment, and methods for their removal. Various fabrication approaches for the development of magnetic-CNTs and CNT-based BP membranes are also discussed. With the remarkable separation performance and ultra-high-water flux, magnetic-CNTs, and CNT-based BP membranes have a great potential to be the leading technologies for water treatment in future.

Land use and landscape position influence soil organic phosphorus speciation in a mixed land use watershed

Land use can significantly alter soil P forms, which will influence P loss in runoff. Organic P (P_o ) compounds are an important component of soil P, but their forms and cycling in soils with different land uses are still poorly understood. In addition, streambanks are potential sources of P loss; P forms and concentrations in streambank soils may vary with land use, affecting potential P loss to water. This study used solution ³¹ P nuclear magnetic resonance spectroscopy to characterize and quantify P in interior and streambank soils (0-10 cm) under duplicate sites from four different land uses along streams in the Missisquoi River basin (VT, USA): silage corn, hay meadow, emergent wetlands, and forest. Orthophosphate monoesters were the dominant P compound class regardless of land use or landscape position. Forest soils had the lowest P_o concentrations, less labile P forms than other soils, and significantly lower concentrations of total inositol hexakisphosphates and total orthophosphate monoesters compared with corn soils. Riparian buffer zones for agricultural soils lowered P concentrations in streambank soils for many soil P pools relative to interior soils. The wetland soils of this study had P concentrations and P forms that were similar to those for interior agricultural soils and generally showed no reduction in P concentrations in streambank soils relative to interior soils. This is consistent with the role of wetlands as P sinks in the landscape but also suggests these wetlands should be carefully monitored to minimize P accumulation, especially in streambank soils.

Internal loading of phosphate in rivers reduces at higher flow velocity and is reduced by iron rich sand application: an experimental study in flumes

Many lowland regions are afflicted with high phosphorus (P) peaks in rivers during the summer months. Static incubations of sediments have shown that reductive dissolution of ferric iron (Fe(III)) minerals in the sediment explain these P peaks. This study was set up to identify if that mechanism also dominates in a dynamic system, thereby testing the roles of water flow velocity and sediment Fe/P ratio. Decreasing flow velocity was suspected to lower the flux of dissolved oxygen (DO) towards the sediment. The role of the Fe(III)/P ratio was tested by amending iron-rich glauconite sand (GS) to the sediment, in this manner testing possible remediation techniques. Eight flumes (1.80 m long) were constructed with duplicates of four treatments of two laminar flow velocities over the sediment (0.05 m s^-1 or 0.15 m s^-1) that was either or not amended with GS (10% w/w). In all flumes a daily dose of sodium glutamate was added as a carbon source to mimic wastewater with high BOD, the flumes were operated for 28 days. A decreased velocity lowered the steady-state DO concentration and enhanced the sediment-water release of P by a factor 3. Sediment amendment with GS reduced solution P by factors 3 (low flow velocity) and 2 (high flow velocity). This effect is related to a combination of increasing binding sites for P and of lowering the DO consumption. These experimental data suggest that previously unexplained summer peaks of P in lowland rivers are related to low flow events that limit the DO flux. The internal loading of P requires management of DO in water and can be mitigated by enhancing sediment Fe.

Digital mapping of structural conservation practices in the Midwest U.S. croplands: Implementation and preliminary analysis

The application of best management practices is a long-term conservation effort in Midwest U.S. croplands, and many farmers have adopted structural conservation practices (SCPs) to reduce soil erosion and surface water runoff, such as terraces and grassed waterways. Despite that, the geographic distribution of these practices is barely known in the region, and mapping initiatives are required to develop timely and spatially explicit inventories of SCP areas. This study presents the first mapping of SCPs in the agricultural areas over 12 Midwest U.S. states. Semantic segmentation model (adapted U-Net) and National Agriculture Imagery Program 2018-2019 data were used to map the SCP areas at 2-m spatial resolution (490.2 billion pixels). In general, mapping results achieved 78.2% overall accuracy across 20 counties. Our results indicate that 52% of SCP areas are distributed over Iowa (26%), Illinois (15%), and Nebraska (11%). In contrast, the states with the lowest SCP areas are Michigan and North Dakota, with less than 4% of SCP areas. Since the SCP extent is also dependent on the number of cropland areas per state, the percentage of SCP per cropland area was calculated. Specifically, the average percentage of SCP area per cropland is ~1.19%, ranging from 0.8 (e.g., North Dakota and south Minnesota) to 5.5% (e.g., northeast Kansas and southwest Iowa). Interestingly, results also illustrate that regions with high soil erosion rates present the largest percentage of SCP areas in croplands, indicating conservation efforts by farmers. While this preliminary analysis shows some limitations in the mapping quality (mislabel, non-accurate location or discontinuity of SCP areas), the framework has a potential for operational conservation monitoring. The development of such mapping has positive implications for conservation programs, and this geospatial inventory is easily accessible information for the large-area evaluation of conservation practices across Midwest U.S. croplands.

Human disturbance on phosphorus sources, processes and riverine export in a subtropical watershed

Phosphorus (P) is a key nutrient in freshwater systems, often acting as the limiting nutrient. The dominant sources of P in the Jiulong River watershed (S.E. China) are anthropogenic. Dissolved and particulate P species were measured in the West (WJR) and North (NJR) rivers during the wet and dry seasons of 2018 and at their river outlets during a storm (June 2019). Sources of P pollution were characterized from mainly single source subcatchments (dry season). The Agriculture source (WJR) had a total P of 114.7 ± 13.1 μg P L^-1, which was mainly dissolved inorganic P (DIP) from excess fertilizer washed from the fields. By contrast, the West Urban source (sewage effluent) was mainly particulate (POP) and dissolved organic P (DOP). The effect of reservoirs in the main NJR was to decrease total particulate P (TPP) and DIP and increase POP, due to increased sedimentation of particles and biological uptake. An increase in all P species was observed at the beginning of the storm, followed by a decrease on the rising hydrograph due to dilution. The final concentration of all P species was higher than baseflow, confirming that storms increase the P flux out of the watershed. P was initially washed off the fields during the storm, and during the falling hydrograph P increased due to interflow and other longer-term sources. The high DIN:DIP ratio confirmed the key importance of P inputs from human activities in substantially altering P sources and cycling, and hence the importance of science-based management to alleviate the eutrophication problem.

Shifts in precipitation and agricultural intensity increase phosphorus concentrations and loads in an agricultural watershed

Fertilizers and manure applied to cropland to increase yields are often lost via surface erosion, soil leaching, and runoff, increasing nutrient loads in surface and sub-surface waters, degrading water quality, and worsening the 'dead zone' in the Gulf of Mexico. We leverage spatial and temporal variation in agricultural practices and precipitation events to examine how these factors affect stream total phosphorus (TP) concentrations and loads in the Sugar River (Wisconsin), recently listed as impaired. To perform our analysis, we first collected water quality data from 1995 to 2017 from 40 sites along the Sugar River and its tributaries. Starting in 2004, three dairy farms expanded to become concentrated animal feeding operations (CAFOs) in this watershed. We then estimated how time of year, stream position, discharge volume, and proximity to the newly expanded CAFOs affected TP concentrations and loads. Total P concentrations, which ranged from 0.02 to 1.4 mg/L and often exceeded the EPA surface water standard of 0.1 mg/L, increased with increases in stream discharge and proximity to dairy operations, peaking in early spring to mid-summer coincident with extreme precipitation events. Our empirical analysis also shows that TP concentrations downstream from the newly permitted CAFOs increased by 19% relative to upstream concentrations. When examining total daily phosphorus loads (concentration × discharge) from this 780 km² watershed, we found that loads ranged from 5.88 to 4801 kg. Compared to upstream TP loads, those downstream from the CAFOs increased by 91% after the expansions - over four times that of concentration increases - implying that the rate of downstream phosphorus transfer has increased due to CAFO expansion. Our results argue for standards that focus on loads rather than concentrations and monitoring that includes peak events. As agriculture intensifies and extreme rainfall events become more frequent, it becomes increasingly important to limit soil and TP runoff from manure and fertilizer. Siting CAFOs carefully, limiting their size, and improving farming practices in proximity to CAFOs in spring and early summer could considerably reduce nutrient loads.

Influence of Anthropogenic Loads on Surface Water Status: A Case Study in Lithuania

Twenty-six water bodies and 10 ponds were selected for this research. Anthropogenic loads were assessed according to pollution sources in individual water catchment basins. It was determined that 50% of the tested water bodies had Ntotal values that did not correspond to the good and very good ecological status classes, and 20% of the tested water bodies had Ptotal values that did not correspond to the good and very good ecological status classes. The lake basins and ponds received the largest amounts of pollution from agricultural sources with total nitrogen at 1554.13 t/year and phosphorus at 1.94 t/year, and from meadows and pastures with total nitrogen at 9.50 t/year and phosphorus at 0.20 t/year. The highest annual load of total nitrogen for lake basins on average per year was from agricultural pollution from arable land (98.85%), and the highest total phosphorus load was also from agricultural pollution from arable land (60%).

Fertilizers and nitrate pollution of surface and ground water: an increasingly pervasive global problem

Nitrate pollution of ground and surface water bodies all over the world is generally linked with continually increasing global fertilizer nitrogen (N) use. But after 1990, with more fertilizer N consumption in developing countries especially in East and South Asia than in the industrialized nations in North America and Europe, nitrate pollution of freshwaters is now increasingly becoming a pervasive global problem. In this review it has been attempted to review the research information generated during the last two decades from all over the world on different aspects of nitrate pollution of natural water bodies. It is now evident that not more than 50% of the fertilizer N is directly used by the crops to which it is applied. While a small portion may directly leach down and may reach ground and surface water bodies, a large proportion ends up in the soil organic N pool from where N is mineralized and is taken up by plants and/or lost via leaching during several decades. Present trends of nitrate pollution of freshwaters, therefore, reflect legacies of current and past applications of fertilizers and manures. Tools such as simulation models and the natural variation in the stable isotopes of N and oxygen are now being extensively used to study the contribution of fertilizers and other sources to nitrate enrichment of freshwaters. Impacts of agricultural stewardship measures are being assessed and nitrate enrichment of water bodies is being managed using modern digital models and frameworks. Improved water and fertilizer management in agroecosystems can reduce the contribution of fertilizers to nitrate pollution of water bodies but a host of factors determine the magnitude. Future research needs are also considered.

Full-scale thermophilic aerobic co-composting of blue-green algae sludge with livestock faeces and straw

A high incidence of harmful algal bloom in eutrophic surface waters causes many environmental problems. Thermophilic aerobic composting enables effective treatment and disposal of algal sludge that remains after the dewatering of algae slurries, and provides a value-added organic fertiliser. Previous studies have either only dealt with the composting of a single waste component or were conducted at a lab-/pilot-scale; however, this work is a comprehensive assessment of full-scale mechanized thermophilic aerobic co-composting of algal sludge and other typical biomass-based wastes, including chicken faeces and rice straw, in a water-rich rural area in the Tai lake basin, China. With the optimised feedstock material mass ratio (6.0:1.8:1.0 for straw:algae:faeces; initial C/N ratio of 20; and initial moisture of 60 wt%), the co-composting process effectively achieved the reduction, harmlessness, and reuse of waste. The moisture content (28.36 wt% of wet weight), organic matter content (57.91 wt% of dried weight), total nutrient content (6.59 wt% for TN + TP + TK of dried weight), and heavy metal contents as well as the pH of the final product fully met the Chinese National Agricultural Organic Fertiliser Standard requirements. The reduction rates of microcystin and toxic volatile fatty acid contents were higher than 99.5%, and the seed germination index of the product was 114.5%. A notable economic benefit with a gross profit margin of 167-434% of the process was highlighted. Investigation of the associated mechanisms, including statistical analysis, spectral characterisation, micro-morphological observation, and microbial community analysis, revealed that a decreased particle sizes with a looser structure and an efficient humification effect, resulting from the work of several identified dominant microbial species, contributed to the high product quality. The current study provided a demonstration of the promising full-scale co-composting technology for comprehensive management of the environment in water-rich rural areas and the construction of a sustainable watershed.

Phosphorus fractionation related to environmental risks resulting from intensive vegetable cropping and fertilization in a subtropical region

Overuse of phosphorus (P) fertilizer and the resulting soil P accumulation in vegetable production increases the risk of P runoff and leaching. However, P transformations under continuous fertilization and their effects on environmental risk are unclear. The current study examined the effects of long-term P fertilizer application on P fractions in different soil layers, and assessed the correlations between P fractions and environmental risks in intensive vegetable production in a subtropical region. A total of 32 fields were studied, including 8 uncultivated fields and 24 fields continuously used for vegetable production for 1-3, 4-9, or 10-15 years. The results showed that excessive P fertilizer input caused soil P surpluses ranging from 204.6 to 252.4 kg ha^-1 yr^-1. Compared to uncultivated fields, vegetable fields contained higher levels of labile P, moderately labile P, sparingly labile P, and non-labile P. The combined percentage of labile P and moderately labile P increased from 55.2% in fields cultivated for 0-3 year to 65.5% in fields cultivated for 10-15 years. The concentrations of soil P fractions were higher at 0-20 cm soil depth than at 20-40 and 40-60 cm soil depth. Soil available P was positively correlated with all soil P fractions except diluted HCl-P_i or concentrated HCl-P_o. Long-term vegetable production increased CaCl₂-P downward movement, which was positively correlated with levels of labile and moderately labile P. The P index indicated a high risk of P losses from the vegetable fields. The P index was on average 3.27-fold higher in the vegetable fields than in uncultivated fields, and was significantly correlated with soil available P and organic and inorganic P fertilizer input. The environmental risk caused by P in vegetable production should be reduced by reducing P fertilizer input so as to maintain soil available P within an optimal range for vegetable production.

Soil Phosphorus Pools, Bioavailability and Environmental Risk in Response to the Phosphorus Supply in the Red Soil of Southern China

Excess phosphorus (P) accumulation in the soil can change the bioavailability of P and increase the leaching risks, but the quantitative evaluation of these responses in acidic red soil is lacking. This study aimed to investigate the composition of soil P fractions under different phosphorus apparent balances (PAB) in acidic red soil and the bioavailability and the leaching change-points of different P fractions. Five phosphorus (P) fertilization rates were applied (0, 16.38, 32.75, 65.50, 131.00 kg P·ha^-1) in every sweet corn cultivation from the field experiment, and the treatments were marked as P0, P1, P2, P3, and P4, respectively. The PAB showed negative values in P0 and P1 which were -49.0 and -15.0 kg P·ha^-1 in two years, respectively. In contrast, PAB in P2 as well as in P3 and P4 were positive, the content ranging from 40.2 to 424.3 kg P·ha^-1 in two years. Per 100 kg ha^-1 P accumulate in the soil, the total P increased by 44.36 and 10.41 mg kg^-1 in the surface (0-20 cm) and subsurface (20-40 cm) soil, respectively. The content of inorganic P fractions, including solution phosphate (Sol-P), aluminum phosphate (Al-P), iron phosphate (Fe-P), reduction phosphate (Red-P), and calcium phosphate (Ca-P), significantly increased by 0.25, 16.22, 22.08, 2.04, and 5.08 mg kg^-1, respectively, in surface soil per 100 kg ha^-1 P accumulated in the soil. Path analysis showed that the most important soil P fractions contributing to Olsen-P were Sol-P and Al-P, which can directly affect Olsen-P, and their coefficients were 0.24 and 0.73, respectively. Furthermore, the incubation experiments were conducted in the laboratory to investigate the leaching risk of different P fractions, and they showed Sol-P was a potential source of leaching, and the leaching change-points of Al-P and Fe-P were 74.70 and 78.34 mg·kg^-1, respectively. Continuous P that accumulated in soil changed the composition of P fractions, and the bioavailability as well as the leaching risks increased. This is important in optimizing soil P fertilization management in agricultural ecosystems based on the bioavailability and critical levels for leaching of P fractions.

Analysis of standard accounting method of economic compensation for ecological pollution in watershed

With rapid development of social industry and agriculture, there is an increasing demand for water resources in a watershed, which leads to a series of serious problems in the watershed such as water resource shortage, water environment deterioration, water resource pollution, etc. This paper builds an econometric model of economic loss due to water pollution in a watershed based on the basic process of standard water pollution compensation calculation of economic loss. The econometric model is used to calculate the amount of compensation for economic loss due to water pollution; a mathematic model method of water environment is applied to calculate the degree of effect on the lower reaches; thus the proportion of the impact on the areas in the lower reaches to overall impact is obtained. The amount of compensation that should be borne by all areas is obtained in combination of amount of compensation for water pollution. Empirical analysis is carried out by taking the Taihu Basin for example, and the standard compensation and the amount of compensation for the economic loss caused by water pollution between different areas along the Jiangnan Canal in the Taihu Basin are defined, which provides scientific and theoretical bases for standard calculation of the water pollution compensation. This provides a theoretical basis for solving the problem of water pollution in the watershed and the contradiction in the development of the watershed, and realizing social equity and harmonious development.

Land Use and Water Quality

The interaction between land use and water quality is of great importance worldwide as agriculture has been proven to exert a huge pressure on the quality of groundwater and surface waters due to excess losses of nutrients (nitrogen and phosphorous) through leaching and erosion processes. These losses result in, inter alia, high nitrate concentrations in groundwater and eutrophication of rivers, lakes and coastal waters. Combatting especially non-point losses of nutrients has been a hot topic for river basin managers worldwide, and new important mitigation measures to reduce the input of nutrients into groundwater and surface waters at the pollution source have been developed and implemented in many countries. This Special Issue of the Land use and Water Quality conference series (LuWQ) includes a total of 11 papers covering topics such as: (i) nitrogen surplus; (ii) protection of groundwater from pollution; (iii) nutrient sources of pollution and dynamics in catchments and (iv) new technologies for monitoring, mapping and analysing water quality.

Best Management Practices for Diffuse Nutrient Pollution: Wicked Problems Across Urban and Agricultural Watersheds

Extensive time and financial resources have been dedicated to address nonpoint sources of nitrogen and phosphorus in watersheds. Despite these efforts, many watersheds have not seen substantial improvement in water quality. The objective of this study is to review the literature and investigate key factors affecting the lack of improvement in nutrient levels in waterways in urban and agricultural regions. From 94 studies identified in the academic literature, we found that, although 60% of studies found improvements in water quality after implementation of Best Management Practices (BMPs) within the watershed, these studies were mostly modeling studies rather than field monitoring studies. For studies that were unable to find improvements in water quality after the implementation of BMPs, the lack of improvement was attributed to lack of knowledge about BMP functioning, lag times, nonoptimal placement and distribution of BMPs in the watershed, postimplementation BMP failure, and socio-political and economic challenges. We refer to these limiting factors as known unknowns. We also acknowledge the existence of unknown unknowns that hinder further improvement in BMP effectiveness and suggest that machine learning, approaches from the field of business and operations management, and long-term convergent studies could be used to resolve these unknown unknowns.