The pursuit of sustainable phosphorus (P) management represents a long-standing challenge in agricultural arenas, with far-reaching implications for the environment and societal development. Few scholars are as synonymous with P science as Andrew Sharpley. Renowned over his 44-year professional career for providing foundational insights into the fate and management of agricultural P, Sharpley also became the central figure in organizing responses to concerns over the contribution of non-point source P pollution to eutrophication. As a global authority, Sharpley led teams that crafted scientific consensus for use by managers and policymakers in the face of complexity and uncertainty. His leadership offers lessons to those continuing in his footsteps and a model for all who desire to achieve scientific impact. This special collection of papers celebrates the perspectives, contributions, and legacy of Andrew Sharpley.

Excessive phosphorus (P) concentrations can lead to conditions that limit the amenity of freshwater resources. This problem is particularly acute in agricultural catchments, where P fertilizer and manure amendments have been used to increase soil fertility and productivity. In these catchments, P indices are often used to help target critical source areas in order to reduce P exports. However, the overall impact of agricultural mitigation efforts on receiving waters has not always been consistent with declines in total P exports from catchments. In this paper we propose a model of dissolved P mobilization (i.e., entrainment) in surface runoff that accounts for this outcome and examine modifications to P indices that better accommodate dissolved P mobilization. We suggest that dissolved P mobilization commences near the soil surface and has two phases. When water is first applied, labile P is mostly mobilized by dissolution and advection. Subsequently, as the supply of readily accessible P is exhausted, diffusion and hydrodynamic dispersion mobilize P from other sources at a near constant rate for the remainder of the event. As most P exports occur in larger (i.e., longer) events, the second phase appears responsible for most dissolved P exports. Such a model of dissolved P mobilization is consistent with runoff monitoring data under natural and simulated rainfall, suggesting that on low (shallow) slopes where the interaction between surface soil and water may be prolonged, dissolved P concentrations are likely to be higher. Dissolved P mobilization from low-slope areas is not well represented in P indices at present. We suggest that there needs to be a more complex, mechanistic structure to P indices that involves additional compartmentalization. Further, we suggest that this can be achieved without losing the simplicity of P indices or flexibility to integrate research data and experiential knowledge into tools that are relevant to specific regions.

Context: The manureshed concept minimizes nutrient imbalance in livestock-intensive agricultural systems by transporting surplus manure to agricultural fields with nutrient demands. The impacts of manureshed-based manure management across the contiguous United States (CONUS) and its potential to improve soil nutrient dynamics and water quality are not well known. Objective: This study developed a framework to evaluate the impacts of manureshed-based manure nutrient management at the CONUS scale. Methods: Across CONUS, county-scale manure imports and exports were balanced by delineating manuresheds according to historic agronomic nitrogen (N) and phosphorus (P) demands and the transportation potentials of the nearest manure types (wet vs. dry). The water quality impacts of manureshed-based nutrient management

The pace of global change complicates the assessment of the outcomes of agricultural management, hindering decision-making by producers, researchers, and consumers. The Long-Term Agroecosystem Research Network (LTAR) is in a unique position to advance monitoring to inform decision-making. Here, we describe how the network selected performance indicators designed to measure the trade-offs from various farming and ranching approaches. Indicator selection was motivated by the need for common indicators that apply to the diversity of LTAR sites, but they are intended for widespread use by producers and other managers via the Agricultural Performance Indicator and Context Knowledge System (AgPICKS). An initial set of domains, attributes, and indicators was developed via synthesis of structured conversations at national LTAR meetings. Early use revealed the need for a systematically inclusive process toward improvement. We designed and implemented an iterative decision-making protocol to reach a consensus for a new version. The indicator framework differs from others in its attention to production and social outcomes and its grounding in networked agricultural science. Next steps entail developing web tools and personnel for AgPICKS that use LTAR's data and knowledge ecosystem to guide users in setting benchmarks of the desired conditions for their prioritized indicators, collect data, and visualize data to assess how well their management meets their benchmarks, toward the accurate measurement of management outcomes in a changing world.

Legacy phosphorus (P) in agricultural soils (i.e., P that derives from historical human activities) can resist conventional nutrient management strategies to improve water quality (e.g., placement, rate, source, and timing of application). Further, soil conservation practices such as reduced tillage, while potentially beneficial for improving soil health and minimizing erosion, can promote dissolved P loss. Comprehensive legacy P management requires targeted mitigation strategies that consider the sources and processes involved in P mobilization and transport. We modeled trade-offs and interactions of nutrient management and soil conservation strategies in legacy P mitigation efforts at three key sites in the northern United States where legacy P contributions to water quality are a concern. The Annual Phosphorus Loss Estimator (APLE) model was used to simulate generalized management scenarios at each site: current site-specific practices, conventional conservation practices (no-till and manure injection), and P drawdown (curtailing fertilizer P additions and extracting P from soils via crop uptake and harvest). Modeled results highlight that the effects of legacy P are not always obvious; even at sites near the range of agronomic optimum, losses of legacy P in runoff can be significant. Phosphorus drawdown via crop uptake and removal offers the potential to deplete legacy P stores but requires dedication and time. In model simulations, no-till reduced total P losses due to reductions in sediment transport. Coupling drawdown strategies with appropriate conservation management to avoid inadvertent P losses can reduce both dissolved and particulate P losses. Focusing on either soil conservation or soil P drawdown alone is insufficient to meet water quality goals. Phosphorus drawdown strategies must be accompanied by practices supporting soil conservation to ensure that legacy P management benefits water quality in the short and long term.

In the 1980s, growing recognition of agricultural phosphorus (P) sources to surface water eutrophication led to scrutiny of animal feeding operations. In 1990, the USDA-Natural Resources Conservation Service (NRCS) invited prominent scientists to find a solution. It was at an initial meeting that Dr. Andrew Sharpley suggested that P assessment could be modeled after the Universal Soil Loss Equation, where a matrix of factors influencing P loss would be associated with farm nutrient management recommendations. After codifying the P assessment into the USDA-NRCS 590 Nutrient Management Standard some 10 years later, 48 states chose to develop their own P Index. Sharpley, working with many others, helped develop several state P Indices. In 2000, Sharpley secured funding from the USDA-Agricultural Research Service to support the National P Research Project, which conducted in-field P runoff assessments using standardized rainfall simulated studies across 20 states; this allowed individual trials to be aggregated for agroecological regions that were then incorporated into specific state P Indices. Eventually, comparison of P Indices across state boundaries led to a white paper at the behest of USDA-NRCS that resulted in three regional projects evaluating modeling approaches to support or replace P Indices. Sharpley's national umbrella project pointed to shortcomings in water quality models, such as APEX or TBET, as a replacement for state P Indices, which remain a key part of the USDA-590 standard. As a selfless leader, capable of attracting and assembling diverse, productive interdisciplinary teams, Sharpley was essential to the inception, development, and implementation of the P Index.

Adopting the right agricultural conservation practices (CPs) at the right place is critical to maximizing water quality benefits. The Agricultural Conservation Planning Framework (ACPF) tool identifies all potential CPs and their locations within a target watershed based on the landscape characteristics. The ACPF tool suggests hundreds of CP locations in a watershed, making it challenging to prioritize the CP implementation. We develop and demonstrate an efficient approach using a multicriteria ranking technique for prioritizing the CPs suggested by ACPF, considering pollution hotspots and CP cost and effectiveness to support decision-makers. The pollution hotspots are estimated with simulations from an ecohydrological model, the Soil and Water Assessment Tool (SWAT). The CP cost and effectiveness were estimated from CP installation costs and pollutant reduction efficiencies from the literature. The methodology was demonstrated in the Conewago and Mahantango watersheds in Pennsylvania, US, for grassed waterways (GWWs) and water and sediment control basins (WASCOBs) for sediment load reduction. Multiple CP prioritization scenarios were evaluated with individual and combined criteria for reductions in total sediment load, yield (loading per area), and cost. In single criterion-based prioritization, the cost-based and load-based prioritization indicated cost efficiency and fewer CPs, respectively. The yield-based approach correctly prioritized CPs in sediment loss hotspots in the case study watersheds; however, it needed more CPs to meet the target reductions. The multicriteria approach efficiently prioritized CPs in sediment hotspots to meet target reductions. Although this approach was demonstrated in two case study watersheds and for sediment loss reduction, it is applicable for any location or pollutant for which similar input variables can be provided, thereby providing a means for prioritizing the results of ACPF for implementation in the Mid-Atlantic region of the US.

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

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

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

Legacy phosphorus (P) is a concept advanced by Dr. Andrew Sharpley and colleagues that was originally applied to the persistence of anthropogenic signatures in watersheds, and it has since been adopted in a diversity of settings to help guide the science and management of P. Following Sharpley's example to develop consensus-based science, we considered contrasting perspectives on legacy P and defined legacy P as those stores within the environment that arise from historic human activity excluding "natural" or "background" geogenic sources. Legacy P is not restricted to one system or setting; it may reside in soils, sediments, biota, and water bodies. Legacy P has been estimated by fluxes (inputs minus outputs of P to a system) or, equivalently, by mass stocks (total minus geogenic). Because the origin of P in the environment cannot currently be directly quantified, we recommend that researchers report "total P" to track wider watershed P stocks and fluxes of P that include legacy P. We recognize that the definition of legacy P will continue to evolve as we continue to work toward consensus. Ultimately, the final definition of legacy P has consequences for the implementation and success of regulatory and voluntary strategies for legacy P management in agricultural systems. We support continued progress toward a consensus-backed, research-grounded definition for legacy P that is widely applicable yet useful for guiding management and policy.

The Long-Term Agroecosystem Research (LTAR) network is a collaborative initiative funded by the U.S. Department of Agriculture, Agricultural Research Service, aimed at advancing sustainable, resilient agriculture through coordinated research conducted on croplands, grazing lands, and integrated crop/livestock systems. Here we provide an overview of the LTAR network, highlighting its vision, mission, initiatives, recent network activities, and future directions. Network-level research is critical for developing contextually relevant solutions to the challenges faced by agricultural producers. Long-term data collection, stakeholder engagement, and the integration of scientific knowledge are needed to enhance agricultural productivity and its resiliency, environmental quality, profitability, and social well-being into the future.

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

Canada's livestock production and human populations are concentrated in southern regions. Understanding spatial and temporal distributions of animals and excreted nutrients is key to optimizing manure resources and minimizing impact of livestock. Here, we identify manureshed concerns and opportunities by reconciling nitrogen supply and demand on a regional and national scale. Data based on national statistics and farm surveys were allocated to homogeneous soil polygons (Soil Landscapes of Canada [SLC]) to quantify changes in nutrient distribution and ammonia (NH3 ) emissions across Canada (1981-2018). Livestock sectors tied to domestic consumption, dairy and poultry, were stable over time and well dispersed. Export driven beef production has moved west since 1981, whereas pig production was prominent in Manitoba, Quebec, and Ontario. Per ha manure N excretion across livestock sectors in 2018 was generally low with 58% and 6% of the SLCs averaging <25 and >100 kg N ha-1 , respectively. Although only 3% of SLCs had average NH3 emissions reaching 16-200 kg ha-1 , most of these were located near cities and emissions spiked in spring when more people might be exposed. The greatest concentrations of nutrients and livestock occurred around the three largest metropolitan areas: Toronto, Montreal-Quebec City, and Vancouver, posing challenges for nutrient recycling and public health. This study shows that as Canadian cities and livestock agriculture grow in southern Canada, so will challenges around food production, human health, and managing nutrients. Livestock and land use strategies are needed to reconcile changing animal sectors and growing populations.

Injecting manure and commercial fertilizer beneath the soil surface is an important nutrient management practice that conserves ammonia-nitrogen (N) but creates distinct bands of N below the soil surface. To date, no widely accepted soil nitrate sampling protocol has been developed to account for the extreme heterogeneity created by injection. To develop sampling recommendations for Pre-Sidedress Nitrate Test (PSNT), we quantified patterns of NO3--N concentrations in soil of corn (Zea mays L.) plots injected with liquid dairy cattle (Bos taurus L.) manure at 76 cm spacing over 2 years. Soil monoliths were collected to allow precise sampling of 30 cm deep by 2.5 cm soil cores from which a mid-season PSNT was determined. Monte Carlo simulation was conducted to simulate the effects of alternative soil sampling protocols on bias and error. Results from the simulation support the following equispaced sampling protocol: five, 30-cm deep soil cores are spaced 15 cm apart and oriented in a line perpendicular to the injected manure bands, collected at four locations in the field, to produce a single composite of 20 samples for NO3- analysis. It is not necessary to know manure band location. As spatially discrete manure application patterns become more prevalent with the expansion of manure injection, we believe this PSNT sampling protocol balances risk of error with practical concerns needed to promote adoption.

Inputs of fertilizer nutrients in agriculture are estimated to have contributed to > 40% increase in crop production over the past century, resulting in widespread benefits to food security and prosperity. However, fertilizer nutrient redistribution has fundamentally altered global and local nutrient cycles alike, yielding trade-offs in socioeconomic and environmental outcomes. David Pimentel’s body of work on the management of energy, water, and soil resources in agriculture, along with his perspectives on agronomy and sustainable resource management, resonates with a critical understanding of the consequences of nutrient redistribution in agriculture. With Pimentel's legacy in mind, we consider trade-offs of global nutrient redistribution, improved recycling of nutrients in agricultural systems, as well as the challenges of, and opportunities for, transformations that seek to adjust nutrient cycles in modern agriculture. Pimentel’s legacy and contributions provide valuable insight into agriculture’s wicked nutrient challenge, as he framed the costs and opportunities of production systems across different scales of food production, developed foundational understanding of global resource challenges, promoted often marginalized or underemployed management strategies to enhance agriculture’s ecosystem services, confronted conventional wisdom and popular trends, and appropriately, attacked the use of “silver bullets” as singular solutions to ecological challenges and instead promoted systems-level analyses.

Highlights The MAPHEX System removes and concentrates most of the P and other nutrients from liquid dairy manures. Most of the N and K are left in the liquid fraction for the beneficial use of the farmer. Design and expected performance of a simplified but much larger System is discussed. Abstract. The relatively recent concept of the manureshed highlights the problem of the broken nutrient cycle in modern animal agriculture and the low nitrogen:phosphorus ratio in manure relative to crop requirement that results in P accumulation in soils near source areas. One solution to avoid P accumulation is to transport the manure to soils with a deficit of P, but liquid manure’s bulkiness and low nutrient density present challenges for transport over great distances. While the full MAnure PHosphorus EXtraction (MAPHEX) System has shown to be capable of removing greater than 90% of the P from liquid manures while leaving much of the N in the liquid fraction for use on the farm, other nutrients present in manures in lesser amounts than N and P have not been reported on. This study indicates that both the full MAPHEX System and a newly designed MAPHEX Lite System, that not only conserves more N but is more efficient and less costly, are highly efficient at extracting and concentrating most nutrients in solid form while leaving most of the N and K in the liquid phase for beneficial use by the farmer near the manure source. Therefore, it seems clear that both Systems, and the components they include have the potential to play a significant role in manureshed management. Keywords: Chemical treatment, Liquid-solid separation, Manure, Nitrogen, Phosphorus, Potassium, Treatment systems.

The uncoupling of animal and crop production has resulted in long-term accumulation of manure nutrients in many areas, contributing to nutrient pollution. Prudent recycling of manure's nutrient resources requires reconnecting operations that produce manure with agricultural lands in need of nutrients. Thus the need to frame manure management via “manuresheds": the land (i.e. cropland, rangeland) surrounding livestock production operations where nutrients can be recycled on agricultural lands while balancing goals for production, environmental quality, and quality of life. We explore manureshed management as an evolution of national, regional, and local trends in nutrient sources and sinks. Results of our temporal assessments highlight not only system inertia, but the dynamic nature of nutrient flows and the potential for manureshed management to reverse nutrient imbalances at various scales. As a tool for a circular economy, manureshed management requires coordination beyond the farmgate, engaging specialists, industries, and other stakeholders.

More than 40 leading US agricultural and water scientists developed a water research vision designed to address the most critical water and agricultural challenges in a changing climate to sustain agricultural production and natural systems. Water sustainability can only be realized by balancing the

Manureshed management guides the sustainable use of manure resources by matching areas of crop demand (nutrient sinks) with areas generating livestock manure (nutrient sources). A better understanding of the impacts of manureshed management on water quality within sensitive watersheds is needed. We quantified the potential water quality benefits of manureshed-oriented management through scenario-based analyses in the Susquehanna River Basin (SRB) using the Soil and Water Assessment Tool. Five manureshed management scenarios were developed and compared with a baseline "business-as-usual" scenario. The baseline assumes manure is less transportable, which means some locations have manure application in excess of crop demand. The "watershed nutrient balance" scenarios assume excess manure from surplus locations is transportable and that manure is applied around the SRB based on crop nutrient demand. The "watershed nutrient balance avoiding runoff prone areas" scenarios assume manure is transportable but not applied in vulnerable landscapes of the SRB. Each scenario was evaluated under two application rates considering crop nitrogen demand (N-based) and phosphorus demand (P-based). Phosphorus-based manureshed management was more effective in water quality improvements than N-based management. Phosphorus-based nutrient balance scenarios simulated 3 and 25% reduction in total N (TN) and total P (TP), respectively, from the baseline scenario at the watershed outlet. The N- and P-based scenarios avoiding runoff prone areas simulated 3 and 6% reduction in TN loss and 4 and 25.2% reduction in TP loss, respectively, from the baseline. Overall, the manureshed management scenarios were more effective in improving the quality of local streams in livestock-intensive regions than at the watershed outlet.