Jennifer L. Weld

Identifying critical sources of phosphorus export from agricultural watersheds

Surface runoff accounts for much of the phosphorus (P) input to and accelerated eutrophication of the fresh waters. Several states have tried to establish general threshold soil P levels above which the enrichment of surface runoff P becomes unacceptable. However, little information is available on the relationship between soil and surface runoff P, particularly for the northeastern United States. Further, threshold soil P criteria will be of limited value unless they are integrated with site potential for runoff and erosion. In response, the Natural Resource Conservation Service (NRCS) developed a P Index (PI), which ranks the vulnerability of fields as sources of P loss in runoff, based on soil P, hydrology, and land use. This study evaluated the relationship between soil and surface runoff P in a study watershed in central Pennsylvania. The relationship was then incorporated into the (PI), and its impact on the identification of critical source areas within the watershed was examined. Using simulated rainfall (6.5 cm h−1 for 30 min), the concentration of dissolved P in surface runoff (0.2–2.1 mg l−1) from soils was related (r2=0.67) to Mehlich-3 extractable soil P (30–750 mg kg−1). Using an environmentally based soil P threshold level of 450 mg kg−1 determined from the soil-runoff P relationship, the PI identified and ranked areas of the watershed vulnerable to P loss. The vulnerable areas were located along the stream channel, where areas of runoff generation and areas of high soil P coincide, and where careful management of P fertilizers and manure should be targeted.

Assessment of best management practices to minimise the runoff of manure‐borne phosphorus in the United States

Abstract Phosphorus (P), an essential nutrient in crop and livestock agriculture, can cause and accelerate freshwater eutrophication. Intensification of farming systems has resulted in local accumulations of P in some agricultural watersheds with related increases in P runoff. In most cases, continual land application of manure at rates exceeding crop P removal is the proximate cause of P runoff. To mitigate associated water quality impairments, P‐based agricultural best management practices (BMPs) are now becoming a part of farm nutrient planning. This planning involves the selection, timing, and implementation of source and transport BMPs at field, farm, and watershed scales. Source measures include balancing P imports and exports, improved livestock feed management, chemical and physical treatment of manures, appropriate rate, method, and timing of land application based upon regular soil and manure testing, adequate manure storage and transport infrastructure, and composting. Transport measures aim to reduce runoff and erosion via practices such as conservation tillage, contour ploughing, and vegetative filter strips. To be effective, these measures must be carefully selected and targeted to areas at greatest risk to P loss. This vulnerability can be identified and ranked by P indices, which account for source and transport factors controlling P loss. We demonstrate that the P Index can provide flexible yet reliable manure management and provide farmers with options to minimise the risk of P loss from several farms in Pennsylvania, United States. Overall, a comprehensive and holistic approach to manure management can decrease P transfers from land to water.

Developing an Environmental Manure Test for the Phosphorus Index

Abstract Widespread implementation of the phosphorus (P) index has focused attention on environmental manure tests that can be used to estimate the relative availability of P in manure to runoff water. This article describes the development and use of a water extractable P (WEP) test to assess the capacity of land‐applied manure to enrich P in runoff water. WEP of surface‐applied manure has been shown to be strongly correlated to dissolved P concentrations in runoff from agricultural soils. WEP tests that have a defined water‐to‐manure‐solids ratio and involve extraction times of 30 to 120 min provide the best prediction of dissolved P in runoff across a wide range of manures. Consistent measurement of manure WEP can be achieved with manure sample storage times of up to 22 days (4°C), acidified extract holding times of 18 days, and solid separation by either centrifugation or paper filtration. Reproducibility of WEP tests is comparable to that of other common manure tests (e.g., total P), as verified by within‐laboratory and inter laboratory evaluations. A survey of 140 livestock manures revealed significant differences in mean WEP among different livestock manures, with swine greater than poultry (turkey, broiler and layer chickens) and dairy cattle greater than beef cattle. Such results support the use of WEP‐based coefficients to modify the source component of the P index.

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.