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.

Characterizing the phosphorus forms extracted from soil by the Mehlich III soil test.

Phosphorus (P) can limit crop production in many soils, and soil testing is used to guide fertilizer recommendations. The Mehlich III (M3) soil test is widely used in North America, followed by colorimetric analysis for P, or by inductively coupled plasma-based spectrometry (ICP) for P and cations. However, differences have been observed in M3 P concentrations measured by these methods. Using 31P nuclear magnetic resonance (P-NMR) and mass spectrometry (MS), we characterized P forms in M3 extracts. In addition to the orthophosphate that would be detected during colorimetric analysis, several organic P forms were present in M3 extracts that would be unreactive colorimetrically but measured by ICP (molybdate unreactive P, MUP). Extraction of these P forms by M3 was confirmed by P-NMR and MS in NaOH-ethylenediaminetetraacetic acid extracts of whole soils and residues after M3 extraction. The most abundant P form in M3 extracts was myo-inositol hexaphosphate (myo-IHP, phytate), a compound that may not contribute to plant-available P if tightly sorbed in soil. Concentrations of myo-IHP and other organic P forms varied among soils, and even among treatment plots on the same soil. Extraction of myo-IHP in M3 appeared to be linked to cations, with substantially more myo-IHP extracted from soils fertilized with alum-treated poultry litter than untreated litter. These results suggest that ICP analysis may substantially over-estimate plant-available P in samples with high MUP concentrations, but there is no way at present to determine MUP concentrations without analysis by both colorimetry and ICP. This study also tested procedures that will improve future soil P-NMR studies, such as treatment of acid extracts, and demonstrated that techniques such as P-NMR and MS are complimentary, each yielding additional information that analysis by a single technique may not provide.

Water-Extractable Phosphorus in Animal Manure and Manure Compost: Quantities, Characteristics, and Temporal Changes.

Water-extractable P (WEP) in manure and manure compost is widely used as an indicator of P release to runoff from manure and compost that are land applied. A survey of 600 manures and composts was conducted to assess trends in WEP (dry weight equivalent) related to manure and compost types from sources in the Mid-Atlantic region. Manure and compost WEP ranged from 0.2 to 20.8 g kg. Mean WEP was highest in turkey and swine manures (manure: 4.1-5.6 g kg; no composts tested), followed by layer and broiler chicken manures (manure: 3.0-3.5 g kg; compost: 4.6-5.1 g kg), cattle manure (dairy and beef manure: 2.1-2.8 g kg; compost: 1.1-2.7 g kg), and horse manure (manure: 2.7 g kg; compost: 1.9 g kg). Across all manures and composts, WEP was negatively correlated with manure dry matter content ( = 0.42, < 0.001). Moreover, WEP was strongly correlated ( = 0.66, < 0.001) to degree of P saturation expressed as a molar ratio of total P to total metals (Ca, Mg, Fe, Al, and Mn). Although WEP levels of beef, broiler chicken, and turkey manures from this survey are similar to those from a decade ago, WEP is now significantly lower for dairy (30%, < 0.001), swine (46%, < 0.001), and layer chickens (39%, < 0.05). Lower WEP resulted from decreasing total P and/or increasing P sorption capacity, combined with increasing dry matter content. Results highlight the potential to use degree of P saturation to predict WEP and suggest an opportunity to reduce WEP by managing manure handling, storage, and chemistry.

Phosphorus Leaching from Soil Cores from a Twenty-Year Study Evaluating Alum Treatment of Poultry Litter.

Adding alum to poultry litter is a best management practice used to stabilize P in less soluble forms, reducing nonpoint-source P runoff. However, little research has been conducted on how alum additions to litter affect subsequent leaching of P from soil. The objective of this study was to evaluate the effects of alum-treated versus untreated poultry litter on P leaching from soil cores receiving long-term poultry litter applications. Two intact soil cores were taken from each of 52 plots in a long-term study with 13 treatments: a control, four rates each of untreated and alum-treated litter (2.24, 4.49, 6.72, and 8.96 Mg ha), and four rates of ammonium nitrate (65, 130, 195, and 260 kg N ha). One core from each plot received the same fertilizer as for the previous 20 yr, whereas the other was unfertilized in the study year, resulting in a total of 25 treatments. Cores were exposed to natural rainfall, and P leaching was measured for 1 yr. The average soluble reactive P concentrations in the leachate varied from 0.16 to 0.44 mg P L in fertilized alum-treated cores, whereas leachate from cores fertilized with untreated litter ranged from 0.40 to 2.64 mg P L. At the highest litter rate (8.96 Mg ha), alum reduced total dissolved P and total P concentrations in leachate by 83 and 80%, respectively, compared with untreated litter. These results indicate that alum additions to poultry litter significantly reduced soluble and total P fractions in leachate.

Temperature and Nitrogen Effects on Phosphorus Uptake by Agricultural Stream-Bed Sediments.

Climate change will likely increase the growing season, temperatures, and ratio of nitrogen (N) to phosphorus (P) loss from land to water. However, it is unknown how these factors influence P concentrations in streams. We sought to evaluate differences in biotic and abiotic processes affecting stream sediment P dynamics under different temperature and N-enrichment regimes. Three sediments of varying P composition and sorption characteristics were placed into a fluvarium. Synthetic runoff water, with or without added N, was added to the flume's reservoir, and the solution was maintained at 19 or 26°C. Water and sediment samples were taken with time since runoff was introduced. The rate and magnitude of P uptake by sediment was greater at 19°C compared with 26°C, and also when N was added compared with no N added. Analysis of sediment samples indicated that P uptake via abiotic processes was greater at 19 than at 26°C. The addition of N stimulated P uptake by the microbial biomass at 19°C, but microbial uptake was potentially inhibited at 26°C. Because microbial biomass is a temporary store of P, these data suggest that more P may be available with increasing temperatures during the growing season, especially under baseflow, implying that strategies to mitigate P losses from land to water should be strengthened to prevent potential water quality impairment.

Hydrology and Soil Manipulations of Iron-Rich Ditch Mesocosms Provide Little Evidence of Phosphorus Capture within the Profile.

Agricultural drainage ditches function as first-order streams and affect nutrient management. Soil mesocosms from a ditch featuring a vertical (increasing upward) gradient in iron (Fe) and phosphorus (P) were subjected to hydraulic and soil treatments. These manipulations mimicked aspects of dredging and controlled drainage and inspected the soil release and retention of P. Treatments did not remove P from simulated groundwater. Throughput water either gained in P (lack of dredging, especially under Fe-reducing conditions) or had P concentrations indistinguishable from input water (dredging). Undredged mesocosms, when Fe-reducing, released Fe and P simultaneously. Simultaneous release of P and Fe from our Fe-reducing mesocosms indicates a mechanism whereby P capture occurs by Fe precipitation upon emergence to aerated surficial waters. Upwelling and surficial phases of ditch hydrology and the lowering of the ditch surface on dredging complicate interpretation of traditional means of describing ditch P retention and release.

Managing Surface Water Inputs to Reduce Phosphorus Loss from Cranberry Farms.

Cranberry ( Ait.) production in Massachusetts represents one-fourth of the US cranberry supply, but water quality concerns, water use, and wetland protection laws challenge the future viability of the state's cranberry industry. Pond water used for harvest and winter flooding accounts for up to two-thirds of phosphorus (P) losses in drainage waters. Consequently, use of P sorbing salts to treat pond water holds promise in the mitigation of P losses from cranberry farms. Laboratory evaluation of aluminum (Al)-, iron (Fe)-, and calcium (Ca)-based salts was conducted to determine the application rate required for reducing P in shallow (0.4 m) and deep (3.2 m) water ponds used for cranberry production. Limited P removal (<22%) with calcium carbonate and calcium sulfate was consistent with their relatively low solubility in water. Calcium hydroxide reduced total P up to 49%, but increases in pond water pH (>8) could be detrimental to cranberry production. Ferric sulfate and aluminum sulfate applications of 15 mg L (ppm) resulted in near-complete removal of total P, which decreased from 49 ± 3 to <10 µg P L (ppb). However, ferric sulfate application lowered pH below the recommend range for cranberry soils. Field testing of aluminum sulfate demonstrated that at a dose of 15 mg L (∼1.4 Al mg L), total P in pond water was reduced by 78 to 94%. Laboratory and field experiments support the recommendation of aluminum sulfate as a cost-effective remedial strategy for reducing elevated P in surface water used for cranberry production.

Analyzing Within-County Hydrogeomorphological Characteristics as a Precursor to Phosphorus Index Modifications.

Phosphorus (P) site assessment is used nationally and internationally to assess the vulnerability of agricultural fields to P loss and identify high-risk areas controlling watershed P export. Current efforts to update P site assessment tools must ensure that these tools are representative of the range of conditions to which they will be applied. We sought to identify key parameters available in public GIS data that are descriptive of potential source areas in Pennsylvania and that ensure that modifications of the P Index span all feasible parameter combinations. Relevant soil and topographic variables were compiled for Pennsylvania at 30-m resolution, and areas within 90 m of permanent streams were extracted. Within each county, -means and classification trees were used to identify and create classification rules for topoedaphic groups. Within counties, two to five groups adequately represented near-stream complexity, with available water capacity, hydraulic conductivity, and organic matter being the most important environmental variables. Discontinuities across soil survey boundaries made it impossible to develop clusterings beyond the county level. For county-scale research and management efforts, these groupings provide a manageable approach to identifying representative sites for near-stream agricultural lands. The full set of representative sites across the state enables evaluation of the P Index throughout the full hydrogeomorphic diversity of Pennsylvania. In future work, we can then combine a set of reasonable management practices with each of the main hydrogeomorphological regions resulting from this study and verify the revised P Index against expert knowledge and simulation results.

Seasonal Manure Application Timing and Storage Effects on Field- and Watershed-Level Phosphorus Losses.

Timing of manure application to agricultural soils remains a contentious topic in nutrient management planning, particularly with regard to impacts on nutrient loss in runoff and downstream water quality. We evaluated the effects of seasonal manure application and associated manure storage capacity on phosphorus (P) losses at both field and watershed scales over an 11-yr period, using long-term observed data and an upgraded, variable-source water quality model called Topo-SWAT. At the field level, despite variation in location and crop management, manure applications throughout fall and winter increased annual total P losses by 12 to 16% and dissolved P by 19 to 40% as compared with spring. Among all field-level scenarios, total P loss was substantially reduced through better site targeting (by 48-64%), improving winter soil cover (by 25-46%), and reducing manure application rates (by 1-23%). At the watershed level, a scenario simulating 12 mo of manure storage (all watershed manure applied in spring) reduced dissolved P loss by 5% and total P loss by 2% but resulted in greater P concentrations peaks compared with scenarios simulating 6 mo (fall-spring application) or 3 mo storage (four-season application). Watershed-level impacts are complicated by aggregate effects, both spatial and temporal, of manure storage capacity on variables such as manure application rate and timing, and complexities of field and management. This comparison of the consequences of different manure storage capacities demonstrated a tradeoff between reducing annual P loss through a few high-concentration runoff events and increasing the frequency of low peaks but also increasing the annual loss.

Urea Release by Intermittently Saturated Sediments from a Coastal Agricultural Landscape.

Urea-N is linked to harmful algal blooms in lakes and estuaries, and urea-N-based fertilizers have been implicated as a source. However, the export of urea-N-based fertilizers appears unlikely, as high concentrations of urea-N are most commonly found in surface waters outside periods of fertilization. To evaluate possible autochthonous production of urea-N, we monitored urea-N released from drainage ditch sediments using mesocosms. Sediments from a cleaned (recently dredged) drainage ditch, uncleaned ditch, forested ditch, riparian wetland, and an autoclaved sand control were isolated in mesocosms and flooded for 72 h to quantify urea-N, NH-N, and NO-N in the floodwater. Sediments were flooded with different N-amended solutions (distilled HO, 1.5 mg L NH-N, 3.0 mg L NH-N, 2.6 mg L NO-N, or 5.1 mg L NO-N) and incubated at three water temperatures (16, 21, and 27°C). Urea-N concentrations in mesocosms representing uncleaned and cleaned drainage ditches were significantly greater than nonagricultural sediments and controls. While flooding sediments with N-enriched solution had no clear effect on urea-N, warmer (27°C) temperatures resulted in significantly higher urea-N. Data collected from field ditches that were flooded by a summer rainstorm showed increases in urea-N that mirrored the mesocosm experiment. We postulate that concentrations of urea-N in ditches that greatly exceed environmental thresholds are mediated by biological production in sediments and release to stagnant surface water. Storm-driven urea-N export from ditches could elevate the risk of harmful algal blooms downstream in receiving waters despite the dilution effect.

Short-term Forecasting Tools for Agricultural Nutrient Management.

The advent of real-time, short-term farm management tools is motivated by the need to protect water quality above and beyond the general guidance offered by existing nutrient management plans. Advances in high-performance computing and hydrologic or climate modeling have enabled rapid dissemination of real-time information that can assist landowners and conservation personnel with short-term management planning. This paper reviews short-term decision support tools for agriculture that are under various stages of development and implementation in the United States: (i) Wisconsin's Runoff Risk Advisory Forecast (RRAF) System, (ii) New York's Hydrologically Sensitive Area Prediction Tool, (iii) Virginia's Saturated Area Forecast Model, (iv) Pennsylvania's Fertilizer Forecaster, (v) Washington's Application Risk Management (ARM) System, and (vi) Missouri's Design Storm Notification System. Although these decision support tools differ in their underlying model structure, the resolution at which they are applied, and the hydroclimates to which they are relevant, all provide forecasts (range 24-120 h) of runoff risk or soil moisture saturation derived from National Weather Service Forecast models. Although this review highlights the need for further development of robust and well-supported short-term nutrient management tools, their potential for adoption and ultimate utility requires an understanding of the appropriate context of application, the strategic and operational needs of managers, access to weather forecasts, scales of application (e.g., regional vs. field level), data requirements, and outreach communication structure.

Spatial Scale and Field Management Affect Patterns of Phosphorus Loss in Cranberry Floodwaters.

Although cranberries ( Ait.) are indigenous to the northeastern United States, phosphorus (P) fertilizer additions and periodic flooding make commercial cranberry a potential source of P to the region's lakes and streams. In this study, we report values of P export in cranberry floodwaters that range from <0.8 to 4.7 kg P ha, generally reflecting differences in the hydrological, edaphic, and management factors underlying soil P transfer to floodwater. The relatively high P loading rate (4.7 P kg P ha) was associated with harvest flooding of organic-rich soils. Periods of winter flooding and the discharge of harvest floodwater from mineral soils resulted in relatively low P loss (<0.8 kg P ha). Increases in concentrations of total dissolved P (DP) and total particulate P (PP) in floodwater as stage decreased below the surface of the cranberry bed were consistent with the transport of dissolved P in soil porewater and mobilization of particulate P in ditches. Variations in floodwater DP, as well as conservative and reactive tracer concentrations, suggested that the processes by which soil P is released to porewater included desorption of near-surface soil P and anaerobic dissolution of iron-P compounds deeper in the soil profile. At the farm scale, concentrations of DP and PP steadily increased over time, presumably because drainage waters from beds farther upgradient had longer contact times with P-rich sources, such as soil porewater and ditch sediments. Overall, the study illustrates the role that scale-dependent processes impart on patterns of P loss in agricultural production systems.

Estrogen Transport in Surface Runoff from Agricultural Fields Treated with Two Application Methods of Dairy Manure.

This study compares two methods of dairy manure application-surface broadcast and shallow disk injection-on the fate and transport of natural estrogens in surface runoff from 12 field plots in central Pennsylvania. Ten natural surface runoff events were sampled over a 9-mo period after fall manure application. Results show that the range of estrogen concentrations observed in surface runoff from the broadcast plots was several orders of magnitude higher (>5000 ng L) than the concentrations in runoff from the shallow disk injection plots (<10 ng L). Additionally, the transport dynamics differed, with the majority of the estrogen loads from the surface broadcast plots occurring during the first rainfall event after application, whereas the majority of the loads from the shallow disk injection plots occurred more than 6 mo later during a hail storm event. Total estrogen loads were, on average, two orders of magnitude lower for shallow disk injection compared with surface broadcast. Independent of the method of manure application, 17α-estradiol and estrone were preserved in the field for as long as 9 mo after application. Overall, injection of manure shows promise in reducing the potential for off-site losses of hormones from manure-amended soils.

Persistence and Surface Transport of Urea-Nitrogen: A Rainfall Simulation Study.

Studies of harmful algal blooms and associated urea concentrations in the Chesapeake Bay and in coastal areas around the globe strongly suggest that elevated urea concentrations are associated with harmful algal blooms. The observed increased frequency and toxicity of these blooms in recent decades has been correlated with increased agricultural use of N inputs and increased use of urea as a preferred form of commercial N. This rainfall simulation study sought to assess the potential for different N fertilizers and manures to contribute to urea in runoff from a Coastal Plain soil on the Eastern Shore of Maryland. Under worst-case conditions, ~1% of urea-N applied as commercial fertilizer and surface-applied poultry litter was lost in runoff in a simulated rainfall event, roughly equivalent to a 1-yr return period rain storm in the study area, 12 h after application. Cumulative urea-N losses, including four subsequent weekly rainfall events, approached 1.7% from urea-N fertilizer containing a urease inhibitor. Urea-N loss from incorporated poultry litter was negligible, and losses from dairy manure were intermediate. These losses are likely confined to hydrological contributing areas that extend several meters from a drainage ditch or stream for storms with frequent recurrence intervals. Cumulative dissolved N losses in runoff (urea-N + ammonium-N + nitrate-N) as a proportion of total applied plant-available N were <5%, suggesting that most of the applied N was lost by other pathways or was immobilized in soil. Results also highlight the potential for simple management options, such as shallow incorporation or timing, to greatly reduce urea runoff losses.

Reducing Phosphorus Runoff and Leaching from Poultry Litter with Alum: Twenty-Year Small Plot and Paired-Watershed Studies.

Treating poultry litter with alum has been shown to lower ammonia (NH) emissions and phosphorus (P) runoff losses. Two long-term studies were conducted to assess the effects of alum-treated poultry litter on P availability, leaching, and runoff under pasture conditions. From 1995 to 2015, litter was applied annually in a paired watershed study comparing alum-treated and untreated litter and in a small plot study comparing 13 treatments (an unfertilized control, four rates of alum-treated litter, four rates of untreated litter, and four rates of NHNO). In the paired watershed study, total P loads in runoff were 231% higher from pasture receiving untreated litter (1.96 kg P ha) than from that receiving alum-treated litter (0.85 kg P ha). In both studies, alum-treated litter resulted in significantly higher Mehlich III P (M3-P) and lower water-extractable P at the soil surface, reflecting greater retention of applied P and lesser availability of that P to runoff or leaching. In soils fertilized with alum-treated litter, M3-P was much higher when analyzed by inductively coupled argon plasma emission spectrometry than by colorimetry, possibly due to the formation of aluminum phytate. Indeed, alum-treated poultry litter leached less P over the 20-yr study: M3-P at 10 to 50 cm was 266% greater in plots fertilized with untreated litter (331 kg M3-P ha) than with alum-treated litter (124 kg M3-P ha). This research provides compelling evidence that treating poultry litter with alum provides short-term and long-term benefits to P conservation and water quality.

Improved Simulation of Edaphic and Manure Phosphorus Loss in SWAT.

Watershed models such as the Soil Water Assessment Tool (SWAT) and the Agricultural Policy Environmental EXtender (APEX) are widely used to assess the fate and transport of agricultural nutrient management practices on soluble and particulate phosphorus (P) loss in runoff. Soil P-cycling routines used in SWAT2012 revision 586, however, do not simulate the short-term effects of applying a concentrated source of soluble P, such as manure, to the soil surface where it is most vulnerable to runoff. We added a new set of soil P routines to SWAT2012 revision 586 to simulate surface-applied manure at field and subwatershed scales within Mahantango Creek watershed in south-central Pennsylvania. We corroborated the new P routines and standard P routines in two versions of SWAT (conventional SWAT, and a topographically driven variation called TopoSWAT) for a total of four modeling "treatments". All modeling treatments included 5 yr of measured data under field-specific, historical management information. Short-term "wash off" processes resulting from precipitation immediately following surface application of manures were captured with the new P routine whereas the standard routines resulted in losses regardless of manure application. The new routines improved sensitivity to key factors in nutrient management (i.e., timing, rate, method, and form of P application). Only the new P routines indicated decreases in soluble P losses for dairy manure applications at 1, 5, and 10 d before a storm event. The new P routines also resulted in more variable P losses when applying manure versus commercial fertilizer and represented increases in total P losses, as compared with standard P routines, with rate increases in dairy manure application (56,000 to 84,000 L ha). The new P routines exhibited greater than 50% variation among proportions of organic, particulate, and soluble P corresponding to spreading method. In contrast, proportions of P forms under the standard P routines varied less than 20%. Results suggest similar revisions to other agroecosystem watershed models would be appropriate.

Chemical and Isotopic Tracers Illustrate Pathways of Nitrogen Loss in Cranberry Floodwaters.

Limited research exists on the sources of nitrogen (N) in cranberry floodwaters, which have been identified as a prominent cause of concern to watershed N loading in the cranberry-producing region of southeastern Massachusetts. In this study, we used naturally occurring chemical and isotopic tracers to infer the sources of N transported by harvest floodwaters. In 2012, the cranberry bed was a net source of total N (TN), exporting 0.8 kg N ha (primarily as organic N) to a nearby lake. Systematic increases in TN concentration were associated with increasing fractions of pre-event soil water and groundwater ("porewater") in discharge. Results showed that N concentrations in porewater generally derive from the natural mixing of soil water and perched groundwater within the cranberry bed but locally display a connection to deep groundwater where the underlying peat is absent. These findings illustrate the environmental significance of stored pools of porewater in cranberry beds and the ability to focus on moments of disproportionate N transfer to most efficiently curtail floodwater N losses (i.e., 58% of N export occurred in only 22% of floodwater discharge).

Phosphorus fate, management, and modeling in artificially drained systems.

Phosphorus (P) losses in agricultural drainage waters, both surface and subsurface, are among the most difficult form of nonpoint source pollution to mitigate. This special collection of papers on P in drainage waters documents the range of field conditions leading to P loss in drainage water, the potential for drainage and nutrient management practices to control drainage losses of P, and the ability of models to represent P loss to drainage systems. A review of P in tile drainage and case studies from North America, Europe, and New Zealand highlight the potential for artificial drainage to exacerbate watershed loads of dissolved and particulate P via rapid, bypass flow and shorter flow path distances. Trade-offs are identified in association with drainage intensification, tillage, cover crops, and manure management. While P in drainage waters tends to be tied to surface sources of P (soil, amendments or vegetation) that are in highest concentration, legacy sources of P may occur at deeper depths or other points along drainage flow paths. Most startling, none of the major fate-and-transport models used to predict management impacts on watershed P losses simulate the dominant processes of P loss to drainage waters. Because P losses to drainage waters can be so difficult to manage and to model, major investment are needed (i) in systems that can provide necessary drainage for agronomic production while detaining peak flows and promoting P retention and (ii) in models that can adequately describe P loss to drainage waters.

The Pivotal Role of Phosphorus in a Resilient Water-Energy-Food Security Nexus.

We make the case that phosphorus (P) is inextricably linked to an increasingly fragile, interconnected, and interdependent nexus of water, energy, and food security and should be managed accordingly. Although there are many other drivers that influence water, energy, and food security, P plays a unique and under-recognized role within the nexus. The P paradox derives from fundamental challenges in meeting water, energy, and food security for a growing global population. We face simultaneous dilemmas of overcoming scarcity of P to sustain terrestrial food and biofuel production and addressing overabundance of P entering aquatic systems, which impairs water quality and aquatic ecosystems and threatens water security. Historical success in redistributing rock phosphate as fertilizer to enable modern feed and food production systems is a grand societal achievement in overcoming inequality. However, using the United States as the main example, we demonstrate how successes in redistribution of P and reorganization of farming systems have broken local P cycles and have inadvertently created instability that threatens resilience within the nexus. Furthermore, recent expansion of the biofuels sector is placing further pressure on P distribution and availability. Despite these challenges, opportunities exist to intensify and expand food and biofuel production through recycling and better management of land and water resources. Ultimately, a strategic approach to sustainable P management can help address the P paradox, minimize tradeoffs, and catalyze synergies to improve resilience among components of the water, energy, and food security nexus.

Linking nitrogen management, seep chemistry, and stream water quality in two agricultural headwater watersheds.

Riparian seepage zones in headwater agricultural watersheds represent important sources of nitrate-nitrogen (NO-N) to surface waters, often connecting N-rich groundwater systems to streams. In this study, we examined how NO-N concentrations in seep and stream water were affected by NO-N processing along seep surface flow paths and by upslope applications of N from fertilizers and manures. The research was conducted in two headwater agricultural watersheds, FD36 (40 ha) and RS (45 ha), which are fed, in part, by a shallow fractured aquifer system possessing high (3-16 mg L) NO-N concentrations. Data from in-seep monitoring showed that NO-N concentrations generally decreased downseep (top to bottom), indicating that most seeps retained or removed a fraction of delivered NO-N (16% in FD36 and 1% in RS). Annual mean N applications in upslope fields (as determined by yearly farmer surveys) were highly correlated with seep NO-N concentrations in both watersheds (slope: 0.06; = 0.79; < 0.001). Strong positive relationships also existed between seep and stream NO-N concentrations in FD36 (slope: 1.01; = 0.79; < 0.001) and in RS (slope: 0.64; = 0.80; < 0.001), further indicating that N applications control NO-N concentrations at the watershed scale. Our findings clearly point to NO-N leaching from upslope agricultural fields as the primary driver of NO-N losses from seeps to streams in these watersheds and therefore suggest that appropriate management strategies (cover crops, limiting fall/winter nutrient applications, decision support tools) be targeted in these zones.