Review of scenario analyses to reduce agricultural nitrogen and phosphorus loading to the aquatic environment

Simulating agroecosystem soil inorganic nitrogen dynamics under long-term management with an improved SWAT-C model

Despite the extensive application of the Soil and Water Assessment Tool (SWAT) for water quality modeling, its ability to simulate soil inorganic nitrogen (SIN) dynamics in agricultural landscapes has not been directly verified. Here, we improved and evaluated the SWAT-Carbon (SWAT-C) model for simulating long-term (1984-2020) dynamics of SIN for 40 cropping system treatments in the U.S. Midwest. We added one new nitrification and two new denitrification algorithms to the default SWAT version, resulting in six combinations of nitrification and denitrification options with varying performance in simulating SIN. The combination of the existing nitrification method in SWAT and the second newly added denitrification method performed the best, achieving R, NSE, PBIAS, and RMSE of 0.63, 0.29, -4.7 %, and 16.0 kg N ha^-1, respectively. This represents a significant improvement compared to the existing methods. In general, the revised SWAT-C model's performance was comparable to or better than other agroecosystem models tested in previous studies for assessing the availability of SIN for plant growth in different cropping systems. Sensitivity analysis showed that parameters controlling soil organic matter decomposition, nitrification, and denitrification were most sensitive for SIN simulation. Using SWAT-C for improved prediction of plant-available SIN is expected to better inform agroecosystem management decisions to ensure crop productivity while minimizing the negative environmental impacts caused by fertilizer application.

Integrated effects of co-evolutions among climate, land use and vegetation growing dynamics to changes of runoff quantity and quality

Climates, Land use/land cover (LULC) and vegetation growing dynamics have been regarded as the main factors affecting terrestrial hydrological process. However, the mechanisms underlying their integrated effects on terrestrial runoff and nutrient dynamics are not understood well. Here, we constructed a framework to disentangle and quantify the independent and coupled contributions of climate, LULC and vegetation leaf area index (LAI) changes to watershed runoff and nutrient yields changes. Long series of changing meteorological, LULC and LAI data between 1990 and 2020 were integrated into a factor-controlled simulation protocol in a distributed hydrological model, to quantify their comprehensive contributions (individual contribution of single factor change and coupling contribution of multiple factor synchronous changes) to runoff and nutrient changes. The results showed that changes of runoff and nutrient yields are more induced by climate change, rather than LULC and LAI transformations. Increase in annual precipitation significantly elevated runoff and nutrient yields. TP yield was more sensitive to climate change than runoff and TN yields. LULC transformation and climate change have synergistic effects on runoff and nutrient yields. Shift of vegetation areas to construction lands will amplify the effect of climate change on runoff and nutrient yields. Single LAI change has weak effect on runoff and nutrient yields, but it can significantly alter the hydrological effects derived from climate change and the synergistic effects between climate change and LULC transformation. This study considered the coupling and potential synergistic effects among climate change, LULC conversion and LAI variation, which elucidated the comprehensive effects of changing environment on runoff and nutrients evolutions in a more systematic and integrated perspective.

Farmland nutrient pollution and its evolutionary relationship with plantation economic development in China

Contradiction between growing plantation economic demand and agro-ecological degradation has always restricted sustainable development of agricultural countries. This study applied the unit inventory analysis to evaluate the productions and discharges of farmland non-point source (FNPS) nitrogen (TN) and phosphorus (TP) among China's nine national-level agricultural districts over 1999-2019. On this basis, we quantified the evolutionary relationship between plantation economic output and FNPS pollution based on optimal regression fitting. The results showed that over 1999-2019, farmland cumulative TN and TP discharges for the whole China were approximately 15807 × 10⁴ t and 1312 × 10⁴ t, with prominent district heterogeneity. According to FNPS discharge magnitudes, China's agricultural districts can be classified into three categories: high, moderate and slight discharge zones. Huang-Huai-Hai Plain and Middle-lower Yangtze Plain were identified as the main severely-polluted districts. Mineral fertilizer is the primary contributor to FNPS pollution. Annual FNPS load showed a trend of increasing followed by decreasing, and the peak interval was recorded in 2014-2016. Spatiotemporal dynamics in FNPS discharge intensities were disparate from that in discharge magnitudes. SC has the highest TN discharge intensity, with an annual average intensity of 0.068 t/ha, followed by MLYP (0.044 t/ha) and HHHP (0.041 t/ha). HHHP has the highest TP discharge intensity, with an annual average intensity of 0.0051 t/ha, followed by SC (0.0038 t/ha) and MLYP (0.0031 t/ha). District-based agro-ecological restoration strategies were accordingly proposed considering FNPS discharge magnitude and intensity concurrently. In most agricultural districts, with the growing economic output in plantation, the FNPS load showed an increase followed by a decrease or to leveling off. Furthermore, with the increasing TN/TP economic partial productivity, the FNPS TN/TP discharge intensities reached the climax, then declined or tended to be flattening out.

Decoding river pollution trends and their landscape determinants in an ecologically fragile karst basin using a machine learning model

Karst watersheds accommodate high landscape complexity and are influenced by both human-induced and natural activity, which affects the formation and process of runoff, sediment connectivity and contaminant transport and alters natural hydrological and nutrient cycling. However, physical monitoring stations are costly and labor-intensive, which has confined the assessment of water quality impairments on spatial scale. The geographical characteristics of catchments are potential influencing factors of water quality, often overlooked in previous studies of highly heterogeneous karst landscape. To solve this problem, we developed a machining learning method and applied Extreme Gradient Boosting (XGBoost) to predict the spatial distribution of water quality in the world's most ecologically fragile karst watershed. We used the Shapley Addition interpretation (SHAP) to explain the potential determinants. Before this process, we first used the water quality damage index (WQI-DET) to evaluate the water quality impairment status and determined that COD_Mn, TN and TP were causing river water quality impairments in the WRB. Second, we selected 46 watershed features based on the three key processes (sources-mobilization-transport) which affect the temporal and spatial variation of river pollutants to predict water quality in unmonitored reaches and decipher the potential determinants of river impairments. The predicting range of COD_Mn spanned from 1.39 mg/L to 17.40 mg/L. The predictions of TP and TN ranged from 0.02 to 1.31 mg/L and 0.25-5.72 mg/L, respectively. In general, the XGBoost model performs well in predicting the concentration of water quality in the WRB. SHAP explained that pollutant levels may be driven by three factors: anthropogenic sources (agricultural pollution inputs), fragile soils (low organic carbon content and high soil permeability to water flow), and pollutant transport mechanisms (TWI, carbonate rocks). Our study provides key data to support decision-making for water quality restoration projects in the WRB and information to help bridge the science:policy gap.

Impact of Plant-Based Amendments on Water-Soluble Nitrogen Release Dynamics in Cultivated Peatlands

Drained cultivated peatlands have been an essential agricultural resource for many years. To slow and reduce the degradation of these soils, which increases with drainage, the use of plant-based amendments (straw, wood chips, and biochar) has been proposed. Literature on the effects of such amendments in cultivated peatlands is scarce, and questions have been raised regarding the impact of this practice on nutrient cycling, particularly nitrogen (N) dynamics. By means of a six-month incubation experiment, this study assessed the effects of four plant-based amendments (biochar, a forest mix, willow, and miscanthus) on the release kinetics of water-soluble N pools (mineral and organic) in two histosols of differing degrees of decomposition (Haplosaprist and Haplohemist). The amendment rate was set at 15 Mg ha−1 on a dry weight basis. The N release kinetics were significantly impacted by soil type and amendment. Miscanthus and willow were the amendments that most reduced the release of soluble organic N (SON) and mineral N (minN). The addition of plant-based amendments reduced the total amount of released N pools during the incubation (cumulative N pools) by 50.3 to 355.2 mg kg−1, depending on the soil type, the N pool, and the type of amendment. A significant relationship was found between microbial biomass N, urease activity, and the cumulative N at the end of the incubation. The results showed that the input of plant-based amendments in cultivated peatland decreases N release, which could have a beneficial impact by decreasing N leaching; however, it could also restrict crop growth. Further research is needed to fully assess the impact of such amendments used in cultivated peatlands on N and on C fluxes at the soil–plant and soil–atmosphere interfaces to determine if they constitute a long-term solution for more sustainable agriculture.

Phosphorus Absorption and Excretion in Hybrid Sturgeon (Huso dauricus♀ X Acipenser schrenckii♂) Intubated with Different Ca/P Ratios

To study the effect of Ca/P ratio on the P and Ca absorption and excretion in hybrid sturgeon (Huso dauricus♀ X Acipenser schrenckii♂), five groups of fish were intubated with 100 mg P·kg−1 BW with the Ca/P ratios of 0:1, 0.25:1, 0.5:1, 1:1, and 2:1. Plasma P concentrations were significantly elevated at Ca/P ratios below 2:1, and the highest value was obtained at Ca/P ratio of 0.5:1. Plasma Ca content was significantly increased at the highest Ca/P ratio. Urine P excretion rate in the fish intubated with Ca/P ratio of 0.5:1 was significantly higher than that of the groups with Ca/P ratios of 0:1 and 2:1. The highest urea excretion rates were observed at Ca/P ratio of 0.5:1 and 1:1. The total P excretion at 48 h post intubation reached about 30 mg·kg−1 BW, which was recorded for the group with Ca/P ratio of 0.5:1. The present study showed that P absorption efficiency was improved in hybrid sturgeon at Ca/P ratio of 0.5:1, indicating that P inclusion level in sturgeon feed can be further optimized to reduce dietary P input and lower the excessive undigested P discharge into the rearing water.

A review of alternative climate products for SWAT modelling: Sources, assessment and future directions

Alternative climate products, such as gauge-based gridded data, ground-based weather radar, satellite precipitation and climate reanalysis products, are being increasingly applied for hydrological modelling. This review aims to summarize the studies that have evaluated alternative climate products within Soil and Water Assessment Tool (SWAT) applications and to propose future research directions, primarily for modelers who wish to study limited gauge, ungauged or transnational river basins. A total of 126 articles have been identified since 2004, the majority of which have been published within the last five years. About 58% of the studies were conducted in Asia, mostly in China and India, while another 14% were reported for United States studies. CFSR and TRMM are the most popular applied products in SWAT modelling, followed by PERSIANN, CMADS, APHRODITE, CHIRPS and NEXRAD. Generally, the performance of climate products is region-dependent; e.g., CFSR typically performs well in the United States and South America, but performs more poorly for Asia, Africa and mountainous basin conditions, as compared to other products. In contrast, the CMADS, TRMM, APRHODITE and NEXRAD have shown the strongest capability for supporting SWAT modelling in these regions. However, most of the evaluated products contain only precipitation input; therefore, merging reliable precipitation with CFSR-temperature is recommended for hydro-climatic modelling. Future research directions include: (1) examination of optimal combinations; e.g. CHIRPS-precipitation and CFSR-temperature, for simulating streamflow in different types of river basins; (2) development of a standardized validation scheme which incorporates the commonly accepted products, statistical approaches and temperature variables; (3) further evaluation of existing climate data products to accurately capture extreme events, pattern and indices as well as WGEN statistics; (4) improvement of climate data in terms of averaging approach, bias correction and additional factors or indices integration; and (5) bias correction of CMIP6 climate projections using the optimal climate data combinations.

Development of a groundwater contamination index based on the agricultural hazard and aquifer vulnerability: Application to Portugal

Reducing nitrate leaching may not result in a significant improvement of groundwater quality. The amount of nitrate reaching groundwater depends not only on the hazard related to agricultural activities but also on-site specific groundwater vulnerability. Using national databases and other compiled datasets, the agricultural hazard was calculated as the ratio of (i) the nitrate leached estimated from the N surplus, and (ii) the water surplus, a proxy of the percolating water below the root zone. By combining the hazard with a multi-parameter groundwater vulnerability, a spatially explicit groundwater contamination risk, developed for mainland Portugal, was computed for 1999 and 2009. Results show an increase from 8,800 to 82,679 ha of the territory rated with a very high contamination risk. The priority areas were successfully screened by the Index, coinciding with the current Vulnerable Zones, although additional hotspots were detected in southern Portugal. Percolation, including both irrigation activity and precipitation, was found to be a key driver for the groundwater contamination risk due to its opposite effects in the hazard and in the vulnerability. Reducing nitrogen leaching may be insufficient to reduce the risk of nitrate contamination if there is a relatively larger reduction in precipitation. This index is particularly useful when applied to contrasting situations of vulnerability and hazard, which require distinct mitigation measures to mitigate groundwater contamination.

ESPRES: A web application for interactive analysis of multiple pressures in aquatic ecosystems

ESPRES (Efficient Strategies for anthropogenic Pressure Reduction in European waterSheds) is a web-based Decision Support System (DSS) designed to explore management options for achieving environmental targets in European freshwaters. The tool integrates multi-objective optimization (MOO) algorithms for selecting the best management options in a river basin and models assessing the consequent changes in the water quantity (water flow) and quality (nutrient concentration). The MOO engine identifies Pareto front strategies that are trade-offs between environmental objectives for water bodies and the effort required for reducing the pressures. The web interface provides tools to set the effort perceived by different river basin stakeholders considering technical feasibility, political difficulty, and social acceptability of the alternative options. The environmental impact of management options (scenarios) is assessed with models developed at the European scale. ESPRES enables comparison of management solutions and allows quantifying environmental and socio-economic trade-offs inherent to the decision making process.

Impacts of water temperature on phosphorus release of sediments under flowing overlying water

River sediment typically acts a 'sink' or 'source' of phosphorus for the overlying water owing to the adsorption of phosphorous by the sediment or release of phosphorous to the water, respectively. In this study, a method is proposed for the determination of the equilibrium concentration of the total phosphorus (EC_TP) and total phosphorus release rate (R_r). Phosphorus release experiments were conducted on natural river sediments in flowing water with a low velocity under different water temperatures. An empirical equation was established for the correlation between EC_TP and R_r at an arbitrary temperature and at 20 °C. The results indicate that an increase in the water temperature significantly enhances the phosphorus release. In comparison with total phosphorus, dissolved phosphorus and particulate phosphorus in the overlying water do not change with the water temperature. Thus, the results of this study aid in understanding the contaminant exchange between sediments and water in flowing rivers.

Multi-functional benefits from targeted set-aside land in a Danish catchment

In this study, we explored how a targeted land use change in a Danish catchment (River Odense) may provide multi-functional benefits through nitrogen (N)-load reductions to obtain good ecological quality in Odense estuary, protection of N-vulnerable groundwater aquifers, protection of Natura2000 sites and carbon sequestration. An N-load model linked to GIS thematic layers of known protected areas (Natura2000 sites and N-vulnerable groundwater aquifers) was utilised targeting high N-load areas to locate set-aside land. The achieved multi-functional benefits within the catchment and estuary were assessed and cost-benefit assessment was performed by dividing the total welfare costs of the set-aside by the total multi-functional benefits gained from each strategy. The results show that obtaining multi-functional benefits at the lowest cost requires a targeted shift of set-aside from the traditional hot-spot N-load areas to designated protected areas.

Potential impacts of a future Nordic bioeconomy on surface water quality

Nordic water bodies face multiple stressors due to human activities, generating diffuse loading and climate change. The 'green shift' towards a bio-based economy poses new demands and increased pressure on the environment. Bioeconomy-related pressures consist primarily of more intensive land management to maximise production of biomass. These activities can add considerable nutrient and sediment loads to receiving waters, posing a threat to ecosystem services and good ecological status of surface waters. The potential threats of climate change and the 'green shift' highlight the need for improved understanding of catchment-scale water and element fluxes. Here, we assess possible bioeconomy-induced pressures on Nordic catchments and associated impacts on water quality. We suggest measures to protect water quality under the 'green shift' and propose 'road maps' towards sustainable catchment management. We also identify knowledge gaps and highlight the importance of long-term monitoring data and good models to evaluate changes in water quality, improve understanding of bioeconomy-related impacts, support mitigation measures and maintain ecosystem services.

A Heuristic Method for Determining Changes of Source Loads to Comply with Water Quality Limits in Catchments

A common land and water management task is to determine where and by how much source loadings need to change to meet water quality limits in receiving environments. This paper addresses the problem of quantifying changes in loading when limits are specified in many locations in a large and spatially heterogeneous catchment, accounting for cumulative downstream impacts. Current approaches to this problem tend to use either scenario analysis or optimization, which suffer from difficulties of generating scenarios that meet the limits, or high complexity of optimization approaches. In contrast, we present a novel method in which simple catchment models, load limits, upstream/downstream spatial relationships and spatial allocation rules are combined to arrive at source load changes. The process iteratively establishes the critical location (river segment or lake) where the limits are most constraining, and then adjusts sources upstream of the critical location to meet the limit at that location. The method is demonstrated with application to New Zealand (268,000 km²) for nutrients and the microbial indicator E. coli, which was conducted to support policy development regarding water quality limits. The model provided useful insights, such as a source load excess (the need for source load reduction) even after mitigation measures are introduced in order to comply with E. coli limits. On the other hand, there was headroom (ability to increase source loading) for nutrients. The method enables assessment of the necessary source load reductions to achieve water quality limits over broad areas such as large catchments or whole regions.

Optimizing nitrogen management to achieve high yield, high nitrogen efficiency and low nitrogen emission in winter wheat

Achieving both high yield and high nitrogen (N) use efficiency (NUE) simultaneously is a current research hotspot in crop production. To investigate approaches for achieving high yield and NUE, field experiments using N fertilizer rates of 0, 120, 180, 240, 300 and 360 kg N ha^-1 were conducted to study relationships between yield, N uptake and N efficiency during three wheat growing seasons from 2013 to 2016 in three experimental sites (Shangshui, Kaifeng and Wenxian) in the Huang-Huai Plain. Yield, biomass and N concentrations of plants and soil were determined. The results indicated that increased N application would affect soil N residue and increase N₂O emission, suitable N application rate (N240-N268) contributed to maintaining soil fertility and reducing N₂O emission for achieving high yield, high NUE and low N₂O emission. High plant N accumulation (PNA) during jointing to anthesis had the best correlation coefficient with yield and NUE compared to other growth stages, which contributed to achieving high yield and NUE simultaneously. The dry matter produced by a unit of N was defined as N productivity, such as plant N net phase productivity (PNPn) and leaf N productivity (LNP). High PNPn during jointing to anthesis was significantly related to both yield and NUE. The LNP indicator (i.e. photosynthetic N use efficiency, PNUE) in the flag showed significant correlation with both yield and NUE after booting under high PNA levels. These results suggest that PNPn and PNUE could combine high yield and high NUE under high PNA conditions. Besides, to match soil N supply to plant N demand, optimum soil nitrate N accumulation and alkali-hydrolysable N (AHN) content ranges were determined. This study provides a theoretical basis to achieve high yield, high NUE and low N₂O emission for N management in wheat field production.

Nitrate leaching losses from two Baltic Sea catchments under scenarios of changes in land use, land management and climate

Pollution with excess nutrients deteriorate the water quality of the Baltic Sea. The effect of combined land use and climate scenarios on nitrate leaching and nitrogen (N) loads to surface waters from two Baltic Sea catchments (Norsminde in Denmark and Kocinka in Poland) was explored using different models; the NLES and Daisy models for nitrate leaching, and MIKE SHE or MODFLOW/MT3DMS for N transport. Three Shared Socioeconomic Pathways (SSP1, SSP2 and SSP5) defined change in land use and agricultural activities. The climate change scenarios covered 2041-2060 compared with 1991-2010 under RCP8.5, applying four different climate models. Increases in predicted N-load from climate change vary from 20 to 60% depending on climate model. SSPs moderate these N-load changes with small changes for SSP1 to large increases for SSP5, with greater increases for Norsminde than Kocinka due to land use differences. This stresses needs for new measures and governing schemes to meet sustainability targets.

Spatially differentiated regulation: Can it save the Baltic Sea from excessive N-loads?

The Baltic Sea Action Plan and the EU Water Framework Directive both require substantial additional reductions of nutrient loads (N and P) to the marine environment. Focusing on nitrogen, we present a widely applicable concept for spatially differentiated regulation, exploiting the large spatial variations in the natural removal of nitrate in groundwater and surface water. By targeting mitigation measures towards areas where nature's own capacity for removal is low, spatially differentiated regulation can be more cost-effective than the traditional uniform regulation. We present a methodology for upscaling local modelling results on targeted measures at field scale to Baltic Sea drainage basin scale. The paper assesses the potential gain and discusses key challenges related to implementation of spatially differentiated regulation, including the need for more scientific knowledge, handling of uncertainties, practical constraints related to agricultural practice and introduction of co-governance regimes.

Best management practices from agricultural economics: Mitigating air, soil and water pollution

Often the several stakeholders involved in the agricultural sector place a greater emphasis on the negative externalities from farming production rather than on the solutions and approaches to mitigate, namely impacts from pollution. The scientific literature, in certain circumstances, follows this tendency leaving a vast chasm of enormous potential left to be explored. It is important to contribute towards the reduction of this gap, highlighting the best management practices implemented across the agricultural sector around the world, specifically to make them more visible and give incentive to the several agents in adopting and spreading their use. In this way, the main objective is to stress the best management practices presented by the global scientific literature from the farming sector. To achieve this objective methodology based on bibliometric analysis-factor-analysis-literature survey approach was considered, applied to 150 documents obtained from the Web of Science (core collection) related with the following topics: best management practice; agricultural economics; air, soil and water pollution. As main insights, it is worth referring the best management practices to deal with problems from agricultural production, such as, for example, the use of agricultural residues as feedstock for renewable energies. With regard to sustainable development in the agricultural sector, concepts such as "sustainable remediation" have their place. On the other hand, the agricultural and environmental policies and the agricultural costs associated with the several farming practices also play a determinant role here. Finally, only fraction of the scientific documents analysed (16 papers) belong to the group of studies related to policies, showing that there are potential subjects to be addressed here in future studies related with these topics. The same happens for cost-benefit analyses (24 documents).

Pursuing implementation solutions for targeted nitrogen management in agriculture- a novel approach to synthesize knowledge and facilitate sustainable decision making based on collaborative landscape modelling

Regulation of nitrate emission from agriculture to aquatic environments in Denmark currently depend on general rules for nutrient application and associated farm-level reporting schemes. Similar or comparable centralized regulation instruments dedicated to controlling and limiting rather than focusing and improving N application practices exist in large parts of the OECD. Recent policy debates have exposed an array of problems relating to this type of regulation. Problems include issues of appropriate scale, transparency and failures to adapt intervention and regulation to relevant geo-ecological variations in contexts where general rules are being imposed on varied agro-ecosystems. Therefore it has been proposed to rescale regulation to better fit relevant socio-political and agro-environmental processes including the scale of farmers' decision making, the scale of relevant hydrological systems and the scale of key agro-ecological conditions such as soil characteristics and drainage. However, the challenge of shifting the regulation to a more local scale raises a number of questions. These include (1) How information produced locally can be integrated with national scale data? (2) In what way integrated datasets can used to model environmental effects of current and possible land use patterns? (3) In what way data and estimates of consequences of land use changes are best made available in decision making processes? To address these questions this article reports on ongoing work in Denmark to develop a decision support tool for N-management at the scale of agricultural landscapes, which are areas where a similar pattern of land use is repeated across the land surface, reflecting a specific mode of adapting agriculture to natural conditions. The aim of the article is to evaluate the design of a decision support tool aiming at enabling strategic N-management at landscape scales by linking decision support at the scale of individual farms with decision support targeted at groups of farms where a coordinated effort to solve common problems may be more efficient. Design targets for the tool were established empirically based on evidence from exploratory workshops with farmers and other stakeholders in 6 case areas across Denmark. On this basis a prototype GIS-tool for capturing, storing, editing, displaying and modelling landscape scale farming practices and associated emission consequences was developed. The tool was designed to integrate locally held knowledge with national scale datasets in live scenario situations through the implementation of a flexible, uniform and editable data model for land use data - the dNmark landscape model. Based on input data that is corrected and co-authored by workshop participants, the tool estimates the effect of potential land use scenarios on nutrient emissions. The tool was tested in 5 scenario workshops in case areas in Denmark in 2016, on the basis of which its design is evaluated and discussed in this article.

Modeling the Ecological Impact of Phosphorus in Catchments with Multiple Environmental Stressors

The broken phosphorus (P) cycle has led to widespread eutrophication of freshwaters. Despite reductions in anthropogenic nutrient inputs that have led to improvement in the chemical status of running waters, corresponding improvements in their ecological status are often not observed. We tested a novel combination of complementary statistical modeling approaches, including random-effect regression trees and compositional and ordinary linear mixed models, to examine the potential reasons for this disparity, using low-frequency regulatory data available to catchment managers. A benthic Trophic Diatom Index (TDI) was linked to potential stressors, including nutrient concentrations, soluble reactive P (SRP) loads from different sources, land cover, and catchment hydrological characteristics. Modeling suggested that SRP, traditionally considered the bioavailable component, may not be the best indicator of ecological impacts of P, as shown by a stronger and spatially more variable negative relationship between total P (TP) concentrations and TDI. Nitrate-N ( < 0.001) and TP ( = 0.002) also showed negative relationship with TDI in models where land cover was not included. Land cover had the strongest influence on the ecological response. The positive effect of seminatural land cover ( < 0.001) and negative effect of urban land cover ( = 0.030) may be related to differentiated bioavailability of P fractions in catchments with different characteristics (e.g., P loads from point vs. diffuse sources) as well as resilience factors such as hydro-morphology and habitat condition, supporting the need for further research into factors affecting this stressor-response relationship in different catchment types. Advanced statistical modeling indicated that to achieve desired ecological status, future catchment-specific mitigation should target P impacts alongside multiple stressors.

An integrated approach to investigate the relationship of coupling coordination between social economy and water environment on urban scale - A case study of Kunming

With a rapid economic growth and social development in China, the associated problems of water pollution and shortage of water resources would limit the sustainable and coordinated development of socioeconomic and water environmental systems of urban cities. To investigate the relationship of coupling coordination between social economy and water environment on urban scale, we introduced an integrated approach that enables the dynamic evaluation of coupling coordination degree (CCD), which consists of a system dynamics model and a coupling coordination degree model; and applied it to a case study in Kunming in 2016-2025. The business-as-usual (BAU) scenario and five alternative regulating scenarios are simulated to evaluate the effectiveness exerted by various socioeconomic development patterns and water protection efforts in improving CCD. We found that the improvement of CCD could attribute to both the sufficient water protection efforts and the maintaining sustainable speed and scale of socioeconomic development patterns. Under BAU scenario, Kunming would maintain the current state of barely balanced development with CCD at 0.5-0.8, predominantly due to substantial water consumption and pollution. Through the comparison of dynamic evolutions of system indicators and CCD under five alternative regulating scenarios, it is realistic for Kunming to plan its future development in accordance to M-H scenario (Medium-speed socioeconomic development pattern; High-intensity water protection effort). Following this scenario, Kunming's CCD would conform to a steadily increasing trend in 2016-2025 and remain above 0.8 in 2022-2025, tracing a shift in the development stage of that coupling coordination from "barely balanced development" to "highly balanced development" despite the difficulty to cut NH₃-N emission significantly. The effective and feasible regulatory measures such as reducing productive or domestic water pollutants and consumptions; improving collection and reuse rates of wastewater, should be prioritized when adjusting coordination development during decision-making.

Using landscape scenarios to improve local nitrogen management and planning

Scenario-building is a widely used tool to initiate discussions on future land uses. In scenarios possible futures can be explored and peoples' ideas as well as societal trends can be visualized by the use of maps, pictures and figures. With focus on agricultural nitrogen management, and point of departure in the farmers' decisions-regarding fertilizer inputs, crop rotations, land use, and drainage, landscape scenarios are formulated based on local ideas for future nitrogen management and general prospects for local development. The key research question addressed in this paper is how landscape scenarios can guide farmers to improve nitrogen management in smaller catchments dominated by farming. Participatory modelling was used to develop landscape scenarios, depicting the change of nitrogen emission as a result of changes in landscape management and agricultural practices. In the development of the scenarios we used an ArcMap based tool combining statistical data, experimental knowledge, nitrate leaching modelling and input from local stakeholders on biophysical as well as land use and farm management issues. The scenarios presented are the result of a collaborative planning experiment within the frames of the dNmark research alliance on nitrogen. Three different types of scenarios are presented and discussed and their effects in terms of N reduction are estimated. The three scenarios were called: River valley set-aside, constructed wetlands, and land zonation. All the modelled scenarios are estimated to have a positive effect i.e. a reduction of the level of N leached to the root zone. Based on the experience gathered in the project, the feasibility of using scenarios for future environmental planning in the agricultural landscapes is discussed. Further, this is related to the current discussion in Denmark on geographically targeted nitrogen regulation. It is concluded that the co-creative approach to formulation of scenarios can be an effective way of increasing the knowledge and ownership of possible future solutions, however the cost associated with this planning approach is likely to substantially higher that more traditional planning approaches. Consequently, the estimated transactions costs should be weighed against the expected benefits in terms of more successful implementation.

The reduction effects of riparian reforestation on runoff and nutrient export based on AnnAGNPS model in a small typical watershed, China

The continuous deterioration of the aquatic environment in rivers and streams is increasingly causing social and political tensions. To alleviate aquatic environmental problems, especially for the nonpoint source pollution, establishment of riparian forest buffers has been demonstrated as an effective control measure. However, few comprehensive studies of the reduction effects of riparian reforestation on the aquatic environment have been performed, particularly in identifying the suitable widths of reforestation projects. In this paper, the Annualized Agricultural Non-Point Source (AnnAGNPS) model was used to simulate the reduction effects of riparian reforestation on runoff and nutrient loads in Wucun watershed, China. The results showed that 20-m, 40-m, and 60-m widths of riparian buffer reforestation had significant effects on the yearly loads of total nitrogen (TN) and total phosphorus (TP), with reduced rates of 23.21 to 56.2% and 18.16 to 52.14%, respectively. The reduction effect on annual runoff varied from 2.8 to 5.4%. Furthermore, the reduction effect of nutrients performed best during the transition period, while the best runoff reduction was found during the dry period. These distinct reductions indicated that the implementation of riparian forest buffers was capable of reducing the risk and frequency of flooding and eutrophication, especially during the wet and transition periods. Additionally, the 20-m width of riparian buffer reforestation achieved the highest reduction efficiency for runoff, and the 40-m width was the most suitable reforested riparian buffer width for TN and TP. Therefore, 40 m may be the optimum buffer width for the implementation of riparian reforestation in the Wucun watershed. These research results provided scientific information on selecting the optimum buffer width for aquatic environmental regulators and managers as the reduction effects of different widths of riparian buffers on runoff and nutrients were different when considering buffer reforestation.

Effect of poplar trees on nitrogen and water balance in outdoor pig production - A case study in Denmark

Nitrate leaching from outdoor pig production is a long-standing environmental problem for surface and groundwater pollution. In this study, the effects of inclusion of poplar trees in paddocks for lactating sows on nitrogen (N) balances were studied for an organic pig farm in Denmark. Vegetation conditions, soil water and nitrate dynamics were measured in poplar and grass zones of paddocks belonging to main treatments: access to trees (AT), no access to trees (NAT) and a control without trees (NT), during the hydrological year April 2015 to April 2016. Soil water drainage for each zone, simulated by two simulation models (CoupModel and Daisy), was used to estimate nitrate leaching from the zones in each paddock. N balances (input minus output) for the treatments were computed and compared. The results showed that, in terms of annual water balance and regardless of treatment, simulated evapotranspiration of poplar was 560-569 and 489-498 mm for CoupModel and Daisy, respectively, and corresponding evapotranspiration of grass-clover was 250 and 400 mm, against precipitation of 1076 mm. Simulated drainage below the root zone varied as 620-723 mm for Daisy and 568-958 mm for CoupModel, the higher end of the latter being probably overestimated. Annual nitrate leaching ranged from 32 kg N ha^-1 in the poplar zone of NAT up to 289 kg N ha^-1 in the control grass zone of NT. The poplar zone showed significantly lower nitrate leaching, by 75-80%, compared to the grass zone. For the control NT treatment, nitrate leaching was approximately 50% higher in the grass zone closest to the hut compared to the grass zone further away. NT treatment also had the largest surface N balance of 468 kg N ha^-1 compared to 436 and 397 kg N ha^-1 for AT and NAT, respectively. When N losses by leaching and volatilisation were included, soil N balances were 118, 157 and 113 kg N ha^-1 for AT, NAT and NT, respectively. Overall, the two simulation models were found useful tools for analyses of water balance for complex agroforestry systems. The findings collectively suggest that it is possible to decrease nitrate leaching from outdoor pig production on sandy soils by inclusion of poplar trees. Additional measures are nevertheless needed to reduce N losses on a mean area basis in paddocks with 20% tree cover.

Potential benefits of farm scale measures versus landscape measures for reducing nitrate loads in a Danish catchment

To comply with the EU Water Framework Directive, Denmark must further reduce the nitrate (N)-load to marine ecosystems from agricultural areas. Under the anticipated future spatially targeted regulation, the required N-load reductions will differ between catchments, and these are expected to be mitigated by a combination of land and water management measures. Here, we explored how the expected N-load reduction target of 38% for a Danish catchment (River Odense) could be achieved through a combination of farm and landscape measures. These include: (a) N-leaching reduction through changing the crop rotation and applying cover crops, (b) enhancing N-reduction through (re)establishment of wetlands, and (c) reducing N-leaching through spatially targeting of set-aside to high N-load areas. Changes in crop rotations were effective in reducing N-leaching by growing crops with a longer growing season and by allowing a higher use of cover crops. A combination of wetlands and changes in crop rotations were needed for reaching the N-load reduction target without use of set-aside. However, not all combinations of wetlands and crop rotation changes achieved the required N-load reduction, resulting in a need for targeted set-aside, implying a need for balancing measures at farm and landscape scale to maximize N load reduction while minimizing loss of productive land. The effectiveness of farm scale measures is affected by farm and soil types as well as by N-reduction in groundwater, while the possibilities for using wetlands for decreasing the N-load depends on landscape features, allowing the establishment of wetlands connected to streams and rivers.

Reducing uncertainty of estimated nitrogen load reductions to aquatic systems through spatially targeting agricultural mitigation measures using groundwater nitrogen reduction

The need to further abate agricultural nitrate (N)-loadings to coastal waters in Denmark represents the main driver for development of a new spatially targeted regulation that focus on locating N-mitigation measures in agricultural areas with high N-load. This targeting makes use of the spatial variation across the landscape in natural N-reduction (denitrification) of leached nitrate in the groundwater and surface water systems. A critical basis for including spatial targeting in regulation of N-load in Denmark is the uncertainty associated with the effect of spatially targeting measures, since the effect will be critically affected by uncertainty in the quantification of the spatial variation in N-reduction. In this study, we used 30 equally plausible N-reduction maps, at 100 m grid and sub-catchment resolutions, for the 85-km² groundwater dominated Norsminde catchment in Denmark, applying set-aside as the measure on high N-load areas to reach a N-load reduction target of 20%. The uncertainty on these N-reduction maps resulted in uncertainty on the estimated N-load and on the required set-aside area. We tested several methods for spatially targeting set-aside that took into account the uncertainty on set-aside area and developed methods to reduce uncertainty on the estimated N-load reductions. These methods includes application of set-aside based on each individual N-reduction map compared to a mean N-reduction map, using spatial frequency of high N-load and using spatial frequency of low N-reduction. The results revealed that increasing the ensemble size for averaging the N-reduction maps would decrease the uncertainty on the estimated set-aside area with a stable effect when using an ensemble of 15 or more maps. The spatial resolution of the groundwater N-reduction map is essential for the effectiveness of set-aside, but uncertainty of the finer spatial resolution of N-reduction is greater compared to sub-catchment scale, and application of a spatially targeted strategy with uncertain N-reduction maps will result in incorrect set-aside area and uncertain estimations of N-load reductions. To reduce the uncertainty on estimated N-load reductions, this study finds the method of set-aside application based on spatial frequency of high N-load to be more effective than other methods tested.

Paracoccus versutus KS293 adaptation to aerobic and anaerobic denitrification: Insights from nitrogen removal, functional gene abundance, and proteomic profiling analysis

A novel strain KS293 exhibiting excellent aerobic and anaerobic denitrification performance was isolated and identified as Paracoccus versutus KS293. Nitrate nitrogen and total organic carbon could be effectively removed by P. versutus KS293 without nitrite accumulation, whilst 82% and 85% of total nitrogen was converted into gaseous products under aerobic and anaerobic conditions (P > .05), respectively. Based on the ratio of anaerobic to aerobic, relative abundance values were increased 1.41, 1.45, and 2.31 folds for nirS, nosZ, and narG, respectively. A comparison of the two-dimensional gel electrophoresis and principal component analysis showed significant differences in proteomic profiles between aerobic and anaerobic conditions. In total, 78 proteins that displayed fluctuations in relative expression were observed. 10 proteins including nitrate reductase, maintenance of cell membrane (TolA), and RNA polymerase-binding transcription factor (DksA) were differentially expressed. These findings demonstrated that P. versutus KS293 was effective for nitrogen removal under aerobic or anaerobic conditions.

Spatially differentiated strategies for reducing nitrate loads from agriculture in two Danish catchments

Nutrient loss from agriculture is the largest source of diffuse water pollution in Denmark. To reduce nutrient loads a number of solutions have been implemented, but this has been insufficient to achieve the environmental objectives without unacceptable repercussions for agricultural production. This has substantiated the need to develop a new approach to achieve nitrogen (N) load reduction to the aquatic environments with lower costs to farmers. The new approach imply targeting N leaching mitigation to those parts of the landscape which contribute most to the N-loadings. This would involve either reducing the source loading or enhancing the natural reduction (denitrification) of N after it is leached from the root zone of agricultural crops. In this study, a new method of spatially differentiated analysis for two Danish catchments (Odense and Norsminde) was conducted that reach across the individual farms to achieve selected N-load reduction targets. It includes application of cover crops within current crop rotations, set-a-side application on high N-load areas, and changes in agricultural management based on maps of N-reduction available for two different spatial scales, considering soil type and farm boundaries as spatial constraints. In summary, the results revealed that considering spatial constraints for changes in agricultural management will affect the effectiveness of N-load reduction, and the highest N-load reduction was achieved where less constraints were considered. The results also showed that the range of variation in land use, soil types, and N-reduction potential influence the reduction of N-loadings that can originate from critical source areas. The greater the spatial variation the greater the potential for N load reduction through targeting of measures. Therefore, the effectiveness of spatially differentiated measures in term of set-a-side area in Odense catchment were relatively greater compared to Norsminde catchment. The results also showed that using a fine spatial N-reduction map provides greater potential for N load reductions compared to using sub-catchment scale N-reduction maps.

Spatiotemporal patterns and source attribution of nitrogen pollution in a typical headwater agricultural watershed in Southeastern China

Excessive nitrogen (N) discharge from agriculture causes widespread problems in aquatic ecosystems. Knowledge of spatiotemporal patterns and source attribution of N pollution is critical for nutrient management programs but is poorly studied in headwaters with various small water bodies and mini-point pollution sources. Taking a typical small watershed in the low mountains of Southeastern China as an example, N pollution and source attribution were studied for a multipond system around a village using the Hydrological Simulation Program-Fortran (HSPF) model. The results exhibited distinctive spatio-seasonal variations with an overall seriousness rank for the three indicators: total nitrogen (TN) > nitrate/nitrite nitrogen (NO_x^--N) > ammonia nitrogen (NH₃-N), according to the Chinese Surface Water Quality Standard. TN pollution was severe for the entire watershed, while NO_x^--N pollution was significant for ponds and ditches far from the village, and the NH₃-N concentrations were acceptable except for the ponds near the village in summer. Although food and cash crop production accounted for the largest source of N loads, we discovered that mini-point pollution sources, including animal feeding operations, rural residential sewage, and waste, together contributed as high as 47% of the TN and NH₃-N loads in ponds and ditches. So, apart from eco-fertilizer programs and concentrated animal feeding operations, the importance of environmental awareness building for resource management is highlighted for small farmers in headwater agricultural watersheds. As a first attempt to incorporate multipond systems into the process-based modeling of nonpoint source (NPS) pollution, this work can inform other hydro-environmental studies on scattered and small water bodies. The results are also useful to water quality improvement for entire river basins.

Simulation of targeted pollutant-mitigation-strategies to reduce nitrate and sediment hotspots in agricultural watershed

About 50% of U.S. water pollution problems are caused by non-point source (NPS) pollution, primarily sediment and nutrients from agricultural areas, despite the widespread implementation of agricultural Best Management Practices (BMPs). However, the effectiveness of implementation strategies and type of BMPs at watershed scale are still not well understood. In this study, the Soil and Water Assessment Tool (SWAT) ecohydrological model was used to assess the effectiveness of pollutant mitigation strategies in the Raccoon River watershed (RRW) in west-central Iowa, USA. We analyzed fourteen management scenarios based on systematic combinations of five strategies: fertilizer/manure management, changing row-crop land to perennial grass, vegetative filter strips, cover crops and shallower tile drainage systems, specifically aimed at reducing nitrate and total suspended sediment yields from hotspot areas in the RRW. Moreover, we assessed implications of climate change on management practices, and the impacts of management practices on water availability, row crop yield, and total agricultural production. Our results indicate that sufficient reduction of nitrate load may require either implementation of multiple management practices (38.5% with current setup) or conversion of extensive areas into perennial grass (up to 49.7%) to meet and maintain the drinking water standard. However, climate change may undermine the effectiveness of management practices, especially late in the 21st century, cutting the reduction by up to 65% for nitrate and more for sediment loads. Further, though our approach is targeted, it resulted in a slight decrease (~5%) in watershed average crop yield and hence an overall reduction in total crop production, mainly due to the conversion of row-crop lands to perennial grass. Such yield reductions could be quite spatially heterogeneously distributed (0 to 40%).

A review on effectiveness of best management practices in improving hydrology and water quality: Needs and opportunities

Best management practices (BMPs) have been widely used to address hydrology and water quality issues in both agricultural and urban areas. Increasing numbers of BMPs have been studied in research projects and implemented in watershed management projects, but a gap remains in quantifying their effectiveness through time. In this paper, we review the current knowledge about BMP efficiencies, which indicates that most empirical studies have focused on short-term efficiencies, while few have explored long-term efficiencies. Most simulation efforts that consider BMPs assume constant performance irrespective of ages of the practices, generally based on anticipated maintenance activities or the expected performance over the life of the BMP(s). However, efficiencies of BMPs likely change over time irrespective of maintenance due to factors such as degradation of structures and accumulation of pollutants. Generally, the impacts of BMPs implemented in water quality protection programs at watershed levels have not been as rapid or large as expected, possibly due to overly high expectations for practice long-term efficiency, with BMPs even being sources of pollutants under some conditions and during some time periods. The review of available datasets reveals that current data are limited regarding both short-term and long-term BMP efficiency. Based on this review, this paper provides suggestions regarding needs and opportunities. Existing practice efficiency data need to be compiled. New data on BMP efficiencies that consider important factors, such as maintenance activities, also need to be collected. Then, the existing and new data need to be analyzed. Further research is needed to create a framework, as well as modeling approaches built on the framework, to simulate changes in BMP efficiencies with time. The research community needs to work together in addressing these needs and opportunities, which will assist decision makers in formulating better decisions regarding BMP implementation in watershed management projects.

Sources, distribution and export coefficient of phosphorus in lowland polders of Lake Taihu Basin, China

Identifying phosphorus (P) sources, distribution and export from lowland polders is important for P pollution management, however, is challenging due to the high complexity of hydrological and P transport processes in lowland areas. In this study, the spatial pattern and temporal dynamics of P export coefficient (PEC) from all the 2539 polders in Lake Taihu Basin, China were estimated using a coupled P model for describing P dynamics in a polder system. The estimated amount of P export from polders in Lake Taihu Basin during 2013 was 1916.2 t/yr, with a spatially-averaged PEC of 1.8 kg/ha/yr. PEC had peak values (more than 4.0 kg/ha/yr) in the polders near/within the large cities, and was high during the rice-cropping season. Sensitivity analysis based on the coupled P model revealed that the sensitive factors controlling the PEC varied spatially and changed through time. Precipitation and air temperature were the most sensitive factors controlling PEC. Culvert controlling and fertilization were sensitive factors controlling PEC during some periods. This study demonstrated an estimation of PEC from 2539 polders in Lake Taihu Basin, and an identification of sensitive environmental factors affecting PEC. The investigation of polder P export in a watershed scale is helpful for water managers to learn the distribution of P sources, to identify key P sources, and thus to achieve best management practice in controlling P export from lowland areas.

Contrasting effects of biochar on N2O emission and N uptake at different N fertilizer levels on a temperate sandy loam

Biochar has been frequently suggested as an amendment to improve soil quality and mitigate climate change. To investigate the optimal management of nitrogen (N) fertilization, we examined the combined effect of biochar and N fertilizer on plant N uptake and N₂O emissions in a cereal rotation system in a randomized two-factorial field experiment on a sandy loam soil in Brandenburg, Germany. The biochar treatment received 10Mgha^-1 wood-derived biochar in September 2012. Four levels of N fertilizer, corresponding to 0, 50%, 100%, 130% of the recommended fertilizer level, were applied in winter wheat (Triticum aestivum L.)) and winter rye (Secale cereal L.) in 2013 and 2014 followed by the catch crop oil radish (Raphanus sativus L. var. oleiformis). Biomass and N uptake of winter wheat and winter rye were significantly affected by the level of N fertilizer but not by biochar. For N uptake of oil radish an interaction effect was observed for biochar and N fertilizer. Without applied fertilizer, 39% higher N uptake was found in the presence of biochar, accompanied by higher soil NH₄⁺ content and elevated cumulative CO₂ emissions. At 130% of the recommended fertilizer level, 16% lower N uptake and lower cumulative N₂O emissions were found in the biochar-mediated treatment. No significant change in abundance of microbial groups and nosZ gene were observed. Our results highlight that biochar can have a greenhouse gas mitigation effect at high levels of N supply and may stimulate nutrient uptake when no N is supplied.