Modeling of phosphorus loss from field to watershed: A review

Study on load allocation of land-based total nitrogen in the Bohai Sea

Total nitrogen (TN) is one of the terrigenous pollutants in the Bohai Sea. A reasonable releasing scheme can effectively improve the seawater quality. Based on the observed TN concentrations and the national pollution control requirements, the TN load ratios of four major rivers around the Bohai Sea are adjusted in multiple levels within the range of 50 %-200 % in the frame of total control. Then, a pollutant transport model is used to evaluate each load allocation scheme by calculating the area of different seawater qualities. The optimum scheme is loads of the Yellow River and Luan River are set to 50 % and 200 % of the levels in 2018, and thus the area of heavily polluted seawater can be decreased by 33.14 %. Additionally, the reasonable TN reduction amounts of four major rivers in 2018-2020 are calculated according to the optimum scheme, which can be provided as a decision-making basis of marine managements.

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

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

Long-term variations in external phosphorus inputs and riverine phosphorus export in a typical arid and semiarid irrigation watershed

Excessive phosphorus (P) along with drained water from farmland in the arid and semiarid watersheds when entering into water bodies brings about serious environmental problems in the aquatic ecosystem. It is critical to explore variations in watershed P balance and the relationship between anthropogenic P input and riverine total phosphorus (TP) export in a typical irrigation watersheds. In this study, long-term anthropogenic P variations in Ulansuhai Nur watershed (UNW), a typical irrigation watershed in Yellow River basin, was investigated using a quantitative Net Anthropogenic Phosphorus Input (NAPI) budget model. The results showed that annual NAPI exhibited a significant upward trend with a multi-year average of 2541.6 kg P km^-2 yr^-1 in the UNW. Hotspots for watershed NAPI were discovered in Linhe and Hangjin Houqi counties. Chemical P fertilizers and livestock breeding were two dominated sources of NAPI. Annual riverine TP export showed a significantly declined trend with a net decrease of 80.6%. The export ratio of watershed NAPI was 0.6%, lower than those reported for other watersheds worldwide. There was a significant positive linear correlation between NAPI and riverine TP export from 2005 to 2009. However, after 2009, riverine TP export exhibited a decreased trend with increasing watershed NAPI, which was attributed to environmental treatment measures. By reconstructing riverine TP export without the impact of pollution treatment measures, annual average reduction amount of riverine TP export from 2009 to 2019 was estimated to be 237.2 ton, 47.2% and 52.8% of which were attributed to the point and nonpoint sources measures. This study not only widens the application scope of NAPI budget method, but also provides useful information of nutrient management and control in the arid and semiarid irrigation watershed.

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

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

Particulate PhozzyLogic Index for policy makers-an index for a more accurate and transparent identification of critical source areas

The identification of critical source areas (CSAs) is a key element in a cost-effective mitigation of diffuse emissions of phosphorus from agricultural soils into surface waters. One of the challenges related to CSAs is how to couple complex, data-intensive fate and transport models with easy-to-use information on field level for management purposes at the scale of large watersheds. To fill such a gap and create a bridge between the two tasks, this study puts forward the new Particulate PhozzyLogic Index (PPLI) based on the innovative combination of the results of a complex watershed model (in this case the PhosFate model) with fuzzy logic. Its main feature is the ability to transform the results of diverse scenarios or even models into a final map showing a catchment-wide ranking of the possibility of high PP emissions reaching surface waters for all agricultural fields. Further, this study enhances the PhosFate model with a new algorithm for the allocation of particulate phosphorus (PP) loads entering surface waters to their sources of origin. This is a basic requirement for the identification of critical PP source areas and in consequence for a cost-effective implementation of mitigation measures. By means of a sensitivity analysis, this study investigates the impacts of storm drains, discharge frequencies and flow directions on the designation of CSAs with the help of present-day scenarios for a case study catchment with an area of several hundred square kilometres. The upfront model calibration exhibits a Nash-Sutcliffe efficiency (NSE) of about 0.95 and a modified Nash-Sutcliffe efficiency (mNSE) of around 0.83. A core result of the sensitivity analysis is that the scenarios at least partially disagree on the identified CSAs and suggest that especially open furrows at field borders have the potential to lead to deviating outcomes. All scenario results nevertheless support the 80:20 rule, which states that about 80% of the phosphorus inputs into the surface waters of a catchment originate from only about 20% of its area.

SWAT model application for evaluating agricultural conservation practice effectiveness in reducing phosphorous loss from the Western Lake Erie Basin

Lake Erie is threatened by eutrophication and harmful algal blooms due to excess nutrient loading from agricultural sources. Agricultural conservation practices (ACPs) have been developed and implemented to reduce nutrient losses but estimating ACP effectiveness is challenging. The Soil and Water Assessment Tool (SWAT) has been used to investigate ACP effectiveness for water quality improvement. Many SWAT applications have been developed by different investigators to evaluate ACP effectiveness for reducing nutrient, particularly phosphorus (P), loading in the agriculturally-dominated Western Lake Erie Basin (WLEB). Our objective is to establish what has been achieved by past modeling research and make suggestions for future applications and improvements. We synthesized the findings of 28 SWAT modeling studies within the WLEB. Models generally performed satisfactorily against accepted criteria for streamflow and sediment, but performance for P loads, like soluble reactive P, was mostly "unsatisfactory". The "unsatisfactory" performance maybe due to imperfections and idealizations in model formulations and/or parameterization. Thus, simulations of P transport and transformation processes need improvement. In addition, model parameter selection is the key part of model set-up. Most SWAT modeling studies used default values during initial set-up, then performed calibration and validation. It was found that the calibrated P related parameter values varied widely across different studies, even within the same watershed with some values unrealistic for the study areas. The phenomena of different combinations of model parameters producing similar outputs indicates equifinality. Equifinality in the baseline model may impact results when ACPs are incorporated. Furthermore, the unrealistic values used in ACP assessment undermine the credibility of ACP effectiveness. Future model applications should try to re-examine the calibrated P parameters and make sure they are realistic for the study area as well as reduce equifinality by constraining the model with characterization of watershed conditions, better understanding of hydrologic processes, and parameter values based on real-world observations. In summary, future model applications should focus on improving P transport and transformation processes, using measured watershed characteristics for parameterization, and improving reflections of climate change, which could result in more accurate assessments of ACP effectiveness to meet targeted goals.

Untangling the Governance of Public Health Aspects of Manure in The Netherlands

The Netherlands is one of the most densely populated countries in terms of people and livestock and is the second largest exporter of agricultural products worldwide. As a result, the Netherlands has a manure surplus. Excess application of manure can lead to environmental problems; therefore, manure needs to be treated and discharged. Manure can contain zoonotic pathogens, but whether exposure to manure and manure treatment also poses a risk to public health is still unknown. This study analysed the regulations, relevant actors, and responsibilities in the complex system of manure and public health in the Netherlands. Interviews and system mapping have demonstrated interlinkages between environmental, economic, and health aspects. Constraints and opportunities for public health protection have been identified. This study reveals the complexity of the Dutch manure policy, its scattered responsibilities, the challenge to deal with uncertainties, and, most importantly, the need for a microbial risk assessment in order to adequately communicate and manage possible risks to protect the health of animals, the environment, and people.

Screening fertilizers for their phosphorus runoff risk using laboratory methods

Losses of phosphorus (P) from fertilized fields may result in degradation of water quality. Various initiatives are under evaluation to minimize water contamination, including the adoption of less soluble or coated P fertilizer formulations aiming to mitigate losses of P in runoff. Field-based rainfall simulators are traditionally used to evaluate P runoff, but using these is time consuming, labor intensive, and costly given the complex apparatus and analyses involved. We hypothesized that laboratory-based methods could be useful to evaluate the risk of P runoff from fertilizers. In order to identify a rapid, inexpensive, and efficient screening process, we compared two laboratory-scale measurements, one in water (based on electrical conductivity measurements) and one in soil (based on visualization of P diffusion in soil), with runoff results from field-, glasshouse-, and laboratory-based rainfall simulators, using coated soluble phosphate fertilizers. The laboratory-based methods assessing the P release rate in water and in soil correlated closely (r ≥ .96) with the losses of P obtained in the three rainfall simulators regardless of the type of coating or solubility of the fertilizer. The faster and inexpensive electrical conductivity and diffusion visualization methodologies were useful to rank the fertilizers by P release to runoff. Hence, these tools may be useful for screening fertilizer formulations with respect to their runoff risk.