A review of the development and implementation of the critical source area concept: A reflection of Andrew Sharpley's role in improving 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.

The importance of consensus science to managing phosphorus in the environment: SERA-17 and the legacy of Andrew Sharpley.

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.

A short history of the phosphorus index and Andrew Sharpley's contributions from inception through development and implementation.

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.

Distribution of livestock sectors in Canada: Implications for manureshed management.

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.

Evaluating water quality benefits of manureshed management in the Susquehanna River Basin.

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.

Envisioning the manureshed: Toward comprehensive integration of modern crop and animal production.

The specialization and intensification of agriculture have produced incredible gains in productivity, quality, and availability of agricultural commodities but have resulted in the separation of crop and animal production. A by-product of this separation has been the accumulation of manure regions where animal production is concentrated. Enter the "manureshed," an organizing framework for integrating animal and crop production where budgeting of manure nutrients is used to strategically guide their recycling and reuse in agricultural production systems where manure resources are of highest value. To move beyond regional nutrient balance analyses into the transformational realm required to mitigate "wicked" manure problems, manureshed management requires recognition of the challenges to systematically reorganizing resource flows. In better integrating crop and livestock systems, manureshed management must account for the unique nature of managing manure nutrients within individual livestock industries, anticipate trade-offs in substituting manure for commercial fertilizer, promote technologies to refine manure, and engage extensive social networks across scales that range from the farmgate to nation and beyond.

Manuresheds offer a system-level strategy for recovering manure's fertilizer value. Manuresheds address nutrient imbalances and environmental and socioeconomic outcomes. Manuresheds scale from single operations to a "mega-manureshed" transecting the southeastern United States. Manureshed management supports the strategic alignment of technologies, markets, and networks.

Challenges and opportunities for manureshed management across U.S. dairy systems: Case studies from four regions.

The manureshed represents cropland needed to safely assimilate manure nutrients from an animal feeding operation. Dairy manuresheds can be contained on-farm but may need to involve additional farms that can assimilate excess nutrients. We present case studies reviewing challenges and opportunities to manureshed management in four major dairy-producing states using available information on local manuresheds. Additionally, geographic information system software was used with data from regulated Minnesota dairies to assess cropland assimilative capacities and transport needs surrounding large dairies. Manureshed requirements vary across regions, but increased import of feed and soil phosphorus accumulation constrain on-farm manure utilization across the United States. In Minnesota, a growing proportion of Jersey cattle and differences in continuous corn (Zea mays L.) vs. corn-alfafa (Medicago sativa L.) rotations contribute to the amount of land needed to absorb dairy manure nutrients. Farm-gate budgets reveal that N-based manuresheds can be contained within Idaho dairies, but P-based manuresheds extend beyond the farm. In New Mexico, relocation of surplus manure nutrients off the farm is common via informal networks, but incentives to strengthen these networks could ensure sustainable manureshed management. Evaluation of manureshed requirements in Pennsylvania is often complicated by the need for additional nutrient management planning and greater understanding of nutrient balances on the preponderance of small dairies. Nutrient imbalances with highly concentrated dairy production often lead to the need for manure transport off-farm. However, advances in herd and cropland management offer opportunities to improve on-farm nutrient efficiencies, and emerging networks and technologies promise to facilitate manure export when needed.

Opportunities to implement manureshed management in the Iowa, North Carolina, and Pennsylvania swine industry.

The U.S. swine industry is diverse, but opportunities exist to strategically improve manure management, especially given much of the industry's vertical integration. We investigate opportunities for improving manureshed management, using swine production examples in Iowa, North Carolina, and Pennsylvania as a lens into historical trends and the current range of management conditions. Manure management reflects regional differences and the specialized nature of hog farms, resulting in a large range of land bases required to assimilate manure generated by these operations. Selected representative farm scenarios were evaluated on an annual basis; farm-level manuresheds were largest for Pennsylvania sow farms and smallest for North Carolina nursery farms. Compared with nitrogen-based manuresheds, phosphorus-based manuresheds were up to 12.5 times larger. Technology advancements are needed to promote export of concentrated nutrients, especially phosphorus, from existing "source" manuresheds to suitable croplands. The industry is dynamic, as revealed by historical analysis of the siting of hog barns in Pennsylvania, which are currently trending toward the north and west where there is greater isolation to prevent the spread of disease and a larger land base to assimilate manure. Industry expansion should focus on locating animals in nutrient "sink" areas.

Culturable antibiotic-resistant fecal coliform bacteria in soil and surface runoff after liquid dairy manure surface application and subsurface injection.

Land application of manure, while beneficial to soil health and plant growth, can lead to an overabundance of nutrients and introduction of emerging contaminants into agricultural fields. Compared with surface application of manure, subsurface injection has been shown to reduce nutrients and antibiotics in surface runoff. However, less is known about the influence of subsurface injection on the transport and persistence of antibiotic-resistant microorganisms. We simulated rainfall to field plots at two sites (one in Virginia and one in Pennsylvania) 1 or 7 d after liquid dairy manure surface and subsurface application (56 Mg ha-1 ) and monitored the abundance of culturable antibiotic-resistant fecal coliform bacteria (ARFCB) in surface runoff and soils for 45 d. We performed these tests at both sites in spring 2018 and repeated the test at the Virginia site in fall 2019. Manure subsurface injection, compared with surface application, resulted in less ARFCB in surface runoff, and this reduction was greater at Day 1 after application compared with Day 7. The reductions of ARFCB in surface runoff because of manure subsurface injection were 2.5-593 times at the Virginia site in spring 2018 and fall 2019 and 4-5 times at the Pennsylvania site in spring 2018. The ARFCB were only detectable in the 0-to-5-cm soil depth within 14 d of manure surface application but remained detectable in the injection slits of manure subsurface-injected plots even at Day 45. This study demonstrated that subsurface injection can significantly reduce surface runoff of ARFCB from manure-applied fields.

The social networks of manureshed management.

Manureshed management-the strategic use of manure nutrients that prioritizes recycling between livestock systems and cropping systems-provides a comprehensive framework for sustainable nutrient management that necessitates the collaboration of many actors. Understanding the social dimensions of collaboration is critical to implement the strategic and technological requirements of functional manuresheds. To improve this understanding, we identified aspirational networks of actors involved in manureshed management across local, regional, and national scales, principally in the United States, elucidating key relationships and highlighting the breadth of interactions essential to successful manureshed management. We concluded that, although the social networks vary with scale, the involvement of a common core set of actors and relationships appears to be universal to the successful integration of modern livestock and crop production systems necessary for functional manuresheds. Our analysis also reveals that, in addition to agricultural producers, local actors in extension and advisory services and private and public sectors ensure optimal outcomes at all scales. For manureshed management to successfully integrate crop and livestock production and sustainably manage manure nutrient resources at each scale, the full complement of actors identified in these social networks is critical to generate innovation and ensure collaboration continuity.

Poultry manureshed management: Opportunities and challenges for a vertically integrated industry.

Manureshed management seeks to address systemic imbalances in nutrient distributions at scales beyond the farmgate and potentially across county and state boundaries. The U.S. poultry industry, which includes broilers, layers, pullets, and turkeys, has many characteristics that are compatible with achieving a vision of manureshed management, including a history of engaging in local and regional programs to better distribute manure resources. Despite widespread vertical integration that supports large-scale strategic decision making and dry manures that favor off-farm transport, there are still many challenges to poultry manureshed management that require engaging stakeholders other than just the poultry industry. Analysis of county-level nutrient budgets highlights the industry's "mega-manureshed," extending from the Mid-Atlantic, across the southeast, and into northwest Arkansas, Oklahoma, and Texas. The analysis also identifies areas with legacy nutrient build-up that are still present today. Implementing manureshed management in the U.S. poultry industry requires comprehensive consideration of manure treatment technologies, alternative uses such as bioenergy production, market development for treated manure products, transport of manure nutrients from source to sink areas, and manure brokering programs that promote manure nutrient distribution. Fortunately, past and present evolution and innovation within the industry places it as a likely leader of the manureshed vision.

Land use change and collaborative manureshed management in New Mexico.

Agricultural communities of New Mexico regularly redistribute manure nutrients from dairies to nearby croplands to fulfill agronomic nutrient needs and protect water quality. Yet competition for water resources can result in land use change that affects these cooperative manure transfers. Focusing on three clusters of New Mexico dairy farms and their surrounding lands (three manuresheds), we calculated the magnitude of land use changes in 2008-2019 and the balance between manure nutrient supply and crop demand in 2019 to assess how past change may predict future prospects for sustainable management. The overall magnitude of change was small, with each manureshed experiencing a different complement: an exchange of cropland and rangeland in the Roosevelt manureshed (7,975 ha rangeland to cropland; 7,624 ha cropland to rangeland), a 464-ha gain in cropland but a 1,187-ha loss of "spreadable" land (cropland, rangeland, fallow) to developed land in the Doña Ana manureshed, and relatively minor changes in the Chaves manureshed. Nutrient supply and demand were mainly in balance, but a surplus of manure phosphorus (P) in the Chaves manureshed and a thin margin of P assimilation by croplands in the Roosevelt manureshed point to the need for preserving existing croplands and understanding of effects of dairy manure on shortgrass rangeland. Our assessment suggests that an ideal scenario would entail manure being generated in landscapes with portfolios of productive lands that can sustainably use the manure nutrients to minimize environmental quality concerns and agronomic tradeoffs. Coordinated, participatory, and interdisciplinary research and planning are needed.

Estimating dissolved phosphorus losses from legacy sources in pastures: The limits of soil tests and small-scale rainfall simulators.

A legacy of using P fertilizers on grazed pastures has been enhanced soil fertility and an associated increased risk of P loss in runoff. Rainfall simulation has been extensively used to develop relationships between soil test P (STP) and dissolved P (DP) in runoff as part of modeling efforts scrutinizing the impact of legacy P. This review examines the applicability of rainfall simulation to draw inferences related to legacy P. Using available literature, we propose a mixing layer model with chemical transfer to describe DP mobilization from pasture soils where readily available P in the mixing layer is rapidly exhausted and contact time controls DP concentrations responsible for subsequent DP mobilization. That conceptual model was shown to be consistent with field monitoring data and then used to assess the likely effect of rainfall simulation protocols on DP mobilization, highlighting the influence of soil preparation, scale and measurement duration, and, most important, hydrology that can facilitate the physical transport of P into and out of surface flow. We conclude that rainfall simulation experimental protocols can have severe limitations for developing relationships between DP in runoff and STP that are subsequently used to estimate legacy P contributions to downstream water resources.

One size does not fit all: Toward regional conservation practice guidance to reduce phosphorus loss risk in the Lake Erie watershed.

Agricultural phosphorus (P) losses to surface water bodies remain a global eutrophication concern, despite the application of conservation practices on farm fields. Although it is generally agreed upon that the use of multiple conservation practices ("stacking") will lead to greater improvements to water quality, this may not be cost effective to farmers, reducing the likelihood of adoption. At present, wholesale recommendations of conservation practices are given; however, the application of specific conservation practices in certain environments (e.g., no-till with surface application, cover crops) may not be effective and can even lead to unintended consequences. In this paper, we present the Lake Erie watershed as a case study. The Lake Erie watershed contains regions with unique physical geographies that include differences in climate, soil, topography, and land use, which have implications for both P transport from agricultural fields and the efficacy of conservation practices in mitigating P losses. We define major regions within the Lake Erie watershed where common strategies for conservation practice implementation are appropriate, and we propose a five-step plan for bringing regionally tailored, adaptive, and cost-conscious conservation practice into watershed planning. Although this paper is specific to the Lake Erie watershed, our framework can be transferred across broader geographic regions to provide guidance for watershed planning.

Addressing the spatial disconnect between national-scale total maximum daily loads and localized land management decisions.

Regulatory watershed mitigation programs typically emphasize widespread adoption of best management practices (BMPs) to meet total maximum daily load (TMDL) goals. To comply with the Chesapeake Bay TMDL, jurisdictions must develop watershed implementation plans (WIPs) to determine the number and type of BMPs to implement. However, the spatial resolution of the bay-level model used to determine these load reduction goals is so coarse that the regulatory plan cannot consider heterogeneity in local conditions, which affects BMP effectiveness. Using the Topo-SWAT modification of the Soil and Water Assessment Tool (SWAT), we simulated two BMP adoption scenarios in the Spring Creek watershed in central Pennsylvania to determine if leveraging fine-scale spatial heterogeneity to place BMPs could achieve the same (or better) nutrient and sediment reduction at a lower cost than the state-level WIP BMP adoption recommendations. Topo-SWAT was initialized with detailed land use and management practice information, systematically calibrated, and validated against 12 yr of observed data. After determining individual BMP cost effectiveness, results were ranked to design a cost-effective BMP adoption scenario that achieved equal or greater load reduction as the WIP scenario for 74% of the cost using eight management-based BMPs: no-till, manure injection, cover cropping, riparian buffers, land retirement, manure application timing, wetland restoration, and nitrogen management (15% less N input). Because watersheds of this size typically represent the smallest modeling unit in the Chesapeake Bay Model, results demonstrate the potential to use watershed models with finer inference scales to improve recommendations for BMP implementation under the Chesapeake Bay TMDL.

Regional environmental assessment of dairy farms.

A comprehensive, yet in depth, assessment is needed of the environmental impacts of dairy farms at regional and national scales to better track improvements made by the industry. With Pennsylvania as an example, a method using process-level simulation and cradle-to-farm gate life cycle assessment was developed and used to assess important environmental footprints of dairy farms within a state. Representative dairy farms of various sizes and management practices throughout 7 regions of the state were simulated with the Integrated Farm System Model. Environmental footprints varied widely among farms, with this variation influenced primarily by soil characteristics and climate and secondarily by farm management. Therefore, prescriptive mitigation strategies for individual farms are more effective than uniform enforcement of specific strategies across the state. Footprints for the whole state were determined by totaling values among farms and regions based on the amounts of milk produced by each. Pennsylvania dairy farms were determined to emit 4,555 with an uncertainty of ±415 Gg of CO2 equivalent of greenhouse gas with an intensity of 0.99 ± 0.09 kg of CO2 equivalent/kg of fat- and protein-corrected milk (FPCM) produced. Fossil energy consumption was 12,324 ± 1,946 TJ or 2.69 ± 0.42 MJ/kg of FPCM. Blue (nonprecipitation) water consumption was 64.1 ± 13.5 Tg with an intensity of 14.0 ± 3.0 kg/kg of FPCM. A total of all forms of reactive N loss was 43.2 ± 5.0 Gg with an intensity of 9.4 ± 1.1 g/kg of FPCM. These metrics were equivalent to 1.6% of the greenhouse gas emissions, 0.4% of fossil energy use, and 0.8% of fresh water consumption reported for the state. Thus, greenhouse gas emissions, fossil energy use, and blue water use associated with dairy farm production are relatively small compared with total estimates for the state. Perhaps the greatest environmental concern is that of ammonia emission, where dairy farms accounted for about half the estimated emissions of the state. This method can be applied to assessments of the dairy industry at larger regional and national scales.

Impacts of Cover Crops and Crop Residues on Phosphorus Losses in Cold Climates: A Review.

The use of cover crops and crop residues is a common strategy to mitigate sediment and nutrient losses from land to water. In cold climates, elevated dissolved P losses can occur associated with freeze-thaw of plant materials. Here, we review the impacts of cover crops and crop residues on dissolved P and total P loss in cold climates across ∼41 studies, exploring linkages between water-extractable P (WEP) in plant materials and P loss in surface runoff and subsurface drainage. Water-extractable P concentrations are influenced by plant type and freezing regimes. For example, WEP was greater in brassica cover crops than in non-brassicas, and increased with repeated freeze-thaw cycles. However, total P losses in surface runoff and subsurface drainage from cropped fields under cold climates were much lower than plant WEP, owing to retention of 45 to >99% of released P by soil. In cold climatic regions, cover crops and crop residues generally prevented soil erosion and loss of particle-bound P during nongrowing seasons in erodible landscapes but tended to elevate dissolved P loss in nonerodible soils. Their impact on total P loss was inconsistent across studies and complicated by soil, climate, and management factors. More research is needed to understand interactions between these factors and plant type that influence P loss, and to improve the assessment of crop contributions to P loss in field settings in cold climates. Further, tradeoffs between P loss and the control of sediment loss and N leaching by plants should be acknowledged.

Phosphorus mirabilis: Illuminating the Past and Future of Phosphorus Stewardship.

After its discovery in 1669, phosphorus (P) was named ("the miraculous bearer of light"), arising from the chemoluminescence when white P is exposed to the atmosphere. The metaphoric association between P and light resonates through history: from the discovery of P at the start of the Enlightenment period to the vital role of P in photosynthetic capture of light in crop and food production through to new technologies, which seek to capitalize on the interactions between novel ultrathin P allotropes and light, including photocatalysis, solar energy production, and storage. In this introduction to the special section "Celebrating the 350th Anniversary of Discovering Phosphorus-For Better or Worse," which brings together 22 paper contributions, we shine a spotlight on the historical and emerging challenges and opportunities in research and understanding of the agricultural, environmental, and societal significance of this vital element. We highlight the role of P in water quality impairment and the variable successes of P mitigation measures. We reflect on the need to improve P use efficiency and on the kaleidoscope of challenges facing efficient use of P. We discuss the requirement to focus on place-based solutions for developing effective and lasting P management. Finally, we consider how cross-disciplinary collaborations in P stewardship offer a guiding light for the future, and we explore the glimmers of hope for reconnecting our broken P cycle and the bright new horizons needed to ensure future food, water, and bioresource security for growing global populations.

Future Phosphorus: Advancing New 2D Phosphorus Allotropes and Growing a Sustainable Bioeconomy.

With more than 40 countries currently proposing to boost their national bioeconomies, there is no better time for a clarion call for a "new" bioeconomy, which, at its core, tackles the current disparities and inequalities in phosphorus (P) availability. Existing biofuel production systems have widened P inequalities and contributed to a linear P economy, impairing water quality and accelerating dependence on P fertilizers manufactured from finite nonrenewable phosphate rock reserves. Here, we explore how the emerging bioeconomy in novel, value-added, bio-based products offers opportunities to rethink our stewardship of P. Development of integrated value chains of new bio-based products offers opportunities for codevelopment of "P refineries" to recover P fertilizer products from organic wastes. Advances in material sciences are exploiting unique semiconductor and opto-electrical properties of new "two-dimensional" (2D) P allotropes (2D black phosphorus and blue phosphorus). These novel P materials offer the tantalizing prospect of step-change innovations in renewable energy production and storage, in biomedical applications, and in biomimetic processes, including artificial photosynthesis. They also offer a possible antidote to the P paradox that our agricultural production systems have engineered us into, as well as the potential to expand the future role of P in securing sustainability across both agroecological and technological domains of the bioeconomy. However, a myriad of social, technological, and commercialization hurdles remains to be crossed before such an advanced circular P bioeconomy can be realized. The emerging bioeconomy is just one piece of a much larger puzzle of how to achieve more sustainable and circular horizons in our future use of P.

A Global Perspective on Phosphorus Management Decision Support in Agriculture: Lessons Learned and Future Directions.

The evolution of phosphorus (P) management decision support tools (DSTs) and systems (DSS), in support of food and environmental security has been most strongly affected in developed regions by national strategies (i) to optimize levels of plant available P in agricultural soils, and (ii) to mitigate P runoff to water bodies. In the United States, Western Europe, and New Zealand, combinations of regulatory and voluntary strategies, sometimes backed by economic incentives, have often been driven by reactive legislation to protect water bodies. Farmer-specific DSSs, either based on modeling of P transfer source and transport mechanisms, or when coupled with farm-specific information or local knowledge, have typically guided best practices, education, and implementation, yet applying DSSs in data poor catchments and/or where user adoption is poor hampers the effectiveness of these systems. Recent developments focused on integrated digital mapping of hydrologically sensitive areas and critical source areas, sometimes using real-time data and weather forecasting, have rapidly advanced runoff modeling and education. Advances in technology related to monitoring, imaging, sensors, remote sensing, and analytical instrumentation will facilitate the development of DSSs that can predict heterogeneity over wider geographical areas. However, significant challenges remain in developing DSSs that incorporate "big data" in a format that is acceptable to users, and that adequately accounts for catchment variability, farming systems, and farmer behavior. Future efforts will undoubtedly focus on improving efficiency and conserving phosphate rock reserves in the face of future scarcity or prohibitive cost. Most importantly, the principles reviewed here are critical for sustainable agriculture.