Laura W. Good

Estimating Phosphorus Loss in Runoff from Manure and Fertilizer for a Phosphorus Loss Quantification Tool

Nonpoint-source pollution of fresh waters by P is a concern because it contributes to accelerated eutrophication. Given the state of the science concerning agricultural P transport, a simple tool to quantify annual, field-scale P loss is a realistic goal. We developed new methods to predict annual dissolved P loss in runoff from surface-applied manures and fertilizers and validated the methods with data from 21 published field studies. We incorporated these manure and fertilizer P runoff loss methods into an annual, field-scale P loss quantification tool that estimates dissolved and particulate P loss in runoff from soil, manure, fertilizer, and eroded sediment. We validated the P loss tool using independent data from 28 studies that monitored P loss in runoff from a variety of agricultural land uses for at least 1 yr. Results demonstrated (i) that our new methods to estimate P loss from surface manure and fertilizer are an improvement over methods used in existing Indexes, and (ii) that it was possible to reliably quantify annual dissolved, sediment, and total P loss in runoff using relatively simple methods and readily available inputs. Thus, a P loss quantification tool that does not require greater degrees of complexity or input data than existing P Indexes could accurately predict P loss across a variety of management and fertilization practices, soil types, climates, and geographic locations. However, estimates of runoff and erosion are still needed that are accurate to a level appropriate for the intended use of the quantification tool.

Phosphorus indices: why we need to take stock of how we are doing.

Many states have invested significant resources to identify components of their Phosphorus (P) Index that reliably estimate the relative risk of P loss and incentivize conservation management. However, differences in management recommendations and manure application guidelines for similar field conditions among state P Indices, coupled with minimal reductions in the extent of P-impaired surface waters and soil test P (STP) levels, led the U.S. Natural Resources Conservation Service (NRCS) to revise the 590 Nutrient Management Standard. In preparation for this revision, NRCS requested that a review of the scientific underpinnings and accuracy of current P Indices be undertaken. They also sought to standardize the interpretation and management implications of P Indices, including establishment of ratings above which P applications should be curtailed. Although some states have initiated STP thresholds above which no application of P is allowed, STP alone cannot define a sites risk of P loss. Phosphorus Indices are intended to account for all of the major factors leading to P loss. A rigorous evaluation of P Indices is needed to determine if they are directionally and magnitudinally correct. Although use of observed P loss data under various management scenarios is ideal, such data are spatially and temporally limited. Alternatively, the use of a locally validated water quality model that has been shown to provide accurate estimates of P loss may be the most expedient option to conduct Index assessments in the short time required by the newly revised 590 Standard.

Testing the Wisconsin Phosphorus Index with Year-Round, Field-Scale Runoff Monitoring

The Wisconsin Phosphorus Index (WPI) is one of several P indices in the United States that use equations to describe actual P loss processes. Although for nutrient management planning the WPI is reported as a dimensionless whole number, it is calculated as average annual dissolved P (DP) and particulate P (PP) mass delivered per unit area. The WPI calculations use soil P concentration, applied manure and fertilizer P, and estimates of average annual erosion and average annual runoff. We compared WPI estimated P losses to annual P loads measured in surface runoff from 86 field-years on crop fields and pastures. As the erosion and runoff generated by the weather in the monitoring years varied substantially from the average annual estimates used in the WPI, the WPI and measured loads were not well correlated. However, when measured runoff and erosion were used in the WPI field loss calculations, the WPI accurately estimated annual total P loads with a Nash-Sutcliffe Model Efficiency (NSE) of 0.87. The DP loss estimates were not as close to measured values (NSE = 0.40) as the PP loss estimates (NSE = 0.89). Some errors in estimating DP losses may be unavoidable due to uncertainties in estimating on-farm manure P application rates. The WPI is sensitive to field management that affects its erosion and runoff estimates. Provided that the WPI methods for estimating average annual erosion and runoff are accurately reflecting the effects of management, the WPI is an accurate field-level assessment tool for managing runoff P losses.

Environmental concerns of phosphorus management in potato production.

Phosphorus (P) losses from agricultural systems are a cause of degraded surface water quality of lakes and streams. In freshwater systems, P is often the most limiting nutrient for algae growth and an increase in P additions to these systems can cause a shift in ecology. These shifts can result in a degradation of the water resource as habitat or for recreation. In an effort to combat the negative effects of agriculture management practices on surface water quality, federal and state regulations require some level of assessment to guide P applications. Areas with large amounts of potato production are of particular concern with respect to P loss since potatoes are a high P demanding crop and are inefficient users of applied P. In many cases, soils in potato production are managed with a higher soil test P concentration compared to other crops and P applications for optimum production exceed P removal. When potato production fields are maintained at high soil test P levels, this may increase the risk of P loss in runoff. However, based on soils and landscape positions where potatoes are grown, there may be little risk of transport. While there appears to be little risk of P loss on low-sloping, sandy soils, output from the Wisconsin Phosphorus Index suggests that more steeply sloping fields can pose some risk, especially when soil test P concentrations exist at above optimum levels. At high soil test P levels, no P may be required for optimum yield in rotated crops, but production practices of these crops may need to be altered to reduce P losses. Furrow-irrigated and tile-drained fields may also pose risks of P loss to the environment. While the P demands of potato are greater than those for most crops, it is likely that most of this P will not be exported via surface runoff. Careful management considerations must be made when producing potatoes on high sloping soils, especially those close to surface water bodies. Future considerations of P management and water quality will focus on assessing leaching risk of P and this contribution to surface waters.ResumenLas pérdidas de fósforo (P) de sistemas agrícolas son una causa de degradación en la calidad del agua superficial de lagos y corrientes. En los sistemas de agua dulce, el P es a menudo el nutriente más limitante para el crecimiento de algas y un aumento en la adición de P a estos sistemas puede causar un cambio en la ecología. Estos cambios pueden resultar en degradación del recurso hídrico como hábitat o para recreación. En un esfuerzo para combatir los efectos negativos de prácticas de manejo en agricultura en la calidad del agua superficial, las regulaciones federales y estatales requieren algún nivel de análisis para guiar las aplicaciones de P. Las áreas con grandes cantidades de producción de papa son de preocupación particular con respecto a pérdida de P, ya que las papas son un cultivo de alta demanda de P y usan ineficientemente el P aplicado. En muchos casos, los suelos en la producción de papa se manejan con una concentración más alta de P en suelos probados en comparación con otros cultivos, y las aplicaciones de P para producción óptima exceden a su remoción. Cuando los campos de producción de papa se mantienen a altos niveles de P en el suelo, esto pudiera aumentar el riesgo de pérdida de P por lixiviación. No obstante, con base en los suelos y posiciones en el paisaje donde se cultivan las papas, pudiera haber poco riesgo de transporte. Mientras que aparentemente pudiera haber poco riesgo en pérdida de P en suelos arenosos, de laderas suaves, la información del Índice de Fósforo de Wisconsin sugiere que campos con mayor inclinación pudieran representar algún riesgo, especialmente cuando las concentraciones probadas de P existen por encima de los niveles óptimos. A altos niveles de P, pudiera no requerirse para rendimiento óptimo en cultivos en rotación, pero las prácticas de producción de estos cultivos pudieran necesitar alteración para reducir pérdidas de P. Campos de riego por surcos y con drenaje con losas pudieran también representar riesgos de pérdida de P al ambiente. Mientras que las demandas de P en papa son mayores que las de la mayoría de los cultivos, es probable que la mayor parte de este P no se exportará por vía de lixiviación superficial. Deben de hacerse consideraciones cuidadosas de manejo al producir papas en suelos de grandes inclinaciones, especialmente aquellos cercanos a los cuerpos de agua superficial. Futuras consideraciones en el manejo de P y calidad del agua se enfocarán en el análisis de riesgo de lixiviación de P y su contribución a aguas superficiales.

Measuring Water-Extractable Phosphorus in Manures to Predict Phosphorus Concentrations in Runoff

Water-extractable phosphorus (WEP) in manures can influence the risk of phosphorus (P) losses in runoff when manures are land applied. We evaluated several manure handling and extraction variables to develop an extraction procedure for WEP that will minimize pre-analysis manure-sample-handling effects on WEP measurements. We also related manure WEP determinations to runoff dissolved reactive phosphorus (DRP) concentrations found in previously conducted field simulated rainfall experiments using the same manures to evaluate WEP as a predictor of P runoff losses. Dairy and poultry manure WEP concentrations increased with manure-to-water extraction ratio and shaking time. Relative to fresh manures, drying and grinding dairy manures before analysis usually decreased WEP concentrations, while WEP in poultry manures was often increased. Pre-analysis handling effects on WEP were minimized at the 1:1000 extraction ratio with a 1-h shaking time. Relationships between manure WEP and runoff DRP concentrations were strongly influenced by season of year and WEP extraction procedure. The best prediction of DRP concentration in spring runoff experiments was with manure WEP concentration at the 1:1000 extraction ratio. With fall runoff studies, DRP concentrations were best predicted with WEP application rate rather than concentration. These seasonal differences can be explained by the greater percentage of rainfall that ran off in the fall compared to the spring. For all studies, runoff DRP concentrations were strongly related (r2 = 0.82) to the ratio of runoff to rainfall volumes, confirming that models need to take runoff hydrology into account as well as manure WEP in P-loss risk assessments.

Quantifying the Impact of Seasonal and Short-term Manure Application Decisions on Phosphorus Loss in Surface Runoff

Agricultural phosphorus (P) management is a research and policy issue due to P loss from fields and water quality degradation. Better information is needed on the risk of P loss from dairy manure applied in winter or when runoff is imminent. We used the SurPhos computer model and 108 site-years of weather and runoff data to assess the impact of these two practices on dissolved P loss. Model results showed that winter manure application can increase P loss by 2.5 to 3.6 times compared with non-winter applications, with the amount increasing as the average runoff from a field increases. Increased P loss is true for manure applied any time from late November through early March, with a maximum P loss from application in late January and early February. Shifting manure application to fields with less runoff can reduce P loss by 3.4 to 7.5 times. Delaying manure application when runoff is imminent can reduce P loss any time of the year, and sometimes quite significantly, but the number of times that application delays will reduce P loss is limited to only 3 to 9% of possible spreading days, and average P loss may be reduced by only 15% for winter-applied manure and 6% for non-winter-applied manure. Overall, long-term strategies of shifting manure applications to low runoff seasons and fields can potentially reduce dissolved P loss in runoff much more compared with near-term, tactical application decisions of avoiding manure application when runoff is imminent.

Scale-of-measurement effects on phosphorus in runoff from cropland

Phosphorus (P) risk loss assessment tools such as P indices are usually developed from small-plot scale data showing the relationships between various site and management variables and runoff P losses. Little information is available on how small-plot runoff composition compares with field-scale measurements. This study was conducted to compare runoff volume and composition measurements at the field scale with those obtained from natural runoff at the small-plot (1 m2 [10.8 ft2]) scale. Sediment, dissolved P, and total P in natural runoff from small plots located in two fields (7.2 and 12 ha [17.8 and 29.6 ac]) were compared with similar measurements from the fields over an 18-month period. Runoff from small-plot rainfall simulations in both fields was also analyzed for P and sediment. The fields, cropped with either corn (Zea mays L.) or alfalfa (Medicago sativa L) interseeded with bromegrass (Bromus inermis), were located on a Tama silt loam soil (fine-silty, mixed, superactive, mesic Typic Argiudoll) in southwest Wisconsin (42°42′ N, 90°22′W). Statistical analysis using repeated measures showed no significant differences between the two scales of measurements for dissolved P concentrations in runoff. Total P concentrations in small-plot runoff were greater than those in field runoff. Runoff volume and dissolved P concentrations were greater in winter than in summer, but summer runoff had higher sediment concentrations. Small plots had greater cumulative runoff volumes per unit area in both seasons compared to the fields. The dissolved P concentration relationships between the two scales of measurement for individual runoff events were very good in the corn field (r2 = 0.90) but not in the alfalfa field (r2 = 0.09). Sediment P enrichment ratios varied by crop and were similar in the small-plot and field runoff. Cumulative runoff-dissolved P concentrations were strongly related (r2 = 0.95) to average soil-test P at both scales. The agreement of small-plot and field runoff-dissolved P concentrations during the 18-month measurement period supports use of small-plot data in P loss risk assessment tools.

Spatially explicit methodology for coordinated manure management in shared watersheds

Increased clustering and consolidation of livestock production systems has been linked to adverse impacts on water quality. This study presents a methodology to optimize manure management within a hydrologic region to minimize agricultural phosphorus (P) loss associated with winter manure application. Spatial and non-spatial data representing livestock, crop, soil, terrain and hydrography were compiled to determine manure P production rates, crop P uptake, existing manure storage capabilities, and transportation distances. Field slope, hydrologic soil group (HSG), and proximity to waterbodies were used to classify crop fields according to their runoff risk for winter-applied manure. We use these data to construct a comprehensive optimization model that identifies optimal location, size, and transportation strategy to achieve environmental and economic goals. The environmental goal was the minimization of daily hauling of manure to environmentally sensitive crop fields, i.e., those classified as high P-loss fields, whereas the economic goal was the minimization of the transportation costs across the entire study area. A case study encompassing two contiguous 10-digit hydrologic unit subwatersheds (HUC-10) in South Central Wisconsin, USA was developed to demonstrate the proposed methodology. Additionally, scenarios representing different management decisions (storage facility maximum volume, and project capital) and production conditions (increased milk production and 20-year future projection) were analyzed to determine their impact on optimal decisions.

Testing a two-scale focused conservation strategy for reducing phosphorus and sediment loads from agricultural watersheds

This study tested a focused strategy for reducing phosphorus (P) and sediment loads in agricultural streams. The strategy involved selecting small watersheds identified as likely to respond relatively quickly, and then focusing conservation practices on high-contributing fields within those watersheds. Two 5,000 ha (12,360 ac) watersheds in the Driftless Area of south central Wisconsin, previously ranked in the top 6% of similarly sized Wisconsin watersheds for expected responsiveness to conservation efforts to reduce high P and sediment loads, were chosen for the study. The stream outlets from both watersheds were monitored from October of 2006 through September of 2016 for streamflow and concentrations of sediment, total P, and, beginning in October of 2009, total dissolved P. Fields and pastures having the highest potential P delivery to the streams in each watershed were identified using the Wisconsin P Index (Good et al. 2012). After three years of baseline monitoring (2006 to 2009), farmers implemented both field- and farm-based conservation practices in one watershed (treatment) as a means to reduce sediment and P inputs to the stream from the highest contributing areas, whereas there were no out-of-the-ordinary conservation efforts in the second watershed (control). Implementation occurred primarily in 2011 and 2012. In the four years following implementation of conservation practices (2013 through 2016), there was a statistically significant reduction in storm-event suspended sediment loads in the treatment watershed compared to the control watershed when the ground was not frozen (p = 0.047). While there was an apparent reduction in year-round suspended sediment event loads, it was not statistically significant at the 95% confidence level (p = 0.15). Total P loads were significantly reduced for runoff events (p < 0.01) with a median reduction of 50%. Total P and total dissolved P concentrations for low-flow conditions were also significantly reduced (p < 0.01) compared to the control watershed. This study demonstrated that a strategy that first identifies watersheds likely to respond to conservation efforts and then focuses implementation on relatively high-contributing fields within those watersheds can be successful in reducing stream P concentrations and loads.

Temperature and Manure Placement in a Snowpack Affect Nutrient Release from Dairy Manure during Snowmelt

Agricultural nutrient management is an issue due to N and P losses from fields and water quality degradation. Better information is needed on the risk of nutrient loss in runoff from dairy manure applied in winter. We investigated the effect of temperature on nutrient release from liquid and semisolid manure to water, and of manure quantity and placement within a snowpack on nutrient release to melting snow. Temperature did not affect manure P and NH-N release during water extraction. Manure P release, but not NH-N release, was significantly influenced by the water/manure solids extraction ratio. During snowmelt, manure P release was not significantly affected by manure placement in the snowpack, and the rate of P release decreased as application rate increased. Water extraction data can reliably estimate P release from manure during snowmelt; however, snowmelt water interaction with manure of greater solids content and subsequent P release appears incomplete compared with liquid manures. Manure NH-N released during snowmelt was statistically the same regardless of application rate. For the semisolid manure, NH-N released during snowmelt increased with the depth of snow covering it, most likely due to reduced NH volatilization. For the liquid manure, there was no effect of manure placement within the snowpack on NH-N released during snowmelt. Water extraction data can also reliably estimate manure NH-N release during snowmelt as long as NH volatilization is accounted for with liquid manures for all placements in a snowpack and semisolid manures applied on top of snow.