Manure Storage Capacity and Application Timing Influence Watershed-Level Nutrient Losses.

Current concerns over water quality requires best management practices for land-applied manure. One important strategy is to apply manure at right timing, which though is often greatly affected by manure generation and storage capacities in a given watershed. This study was to test the hypotheses: (1) different distributions of manure application timings within a year as determined by manure storage capacities result in similar amounts of annual nutrient losses, but they present different temporal dynamics; (2) winter manure applications increase risk of nutrient losses compared with applications in other seasons; and (3) effects of manure application timing on water quality vary with cropping systems. We use the Soil and Water Assessment Tool (SWAT) to assess effects of manure application timing on phosphorus (P) and nitrogen (N) losses from a 7.3 km² watershed located in the Ridge and Valley physiographic region of south central Pennsylvania. The study watershed has been intensively monitored by the USDA-ARS since 1968, and specific field management practices are recorded since 1999. Thus, long-term data (climate, hydrology, water quality, soil, land use, and nutrient management) are available and were used to create a detailed SWAT model of the watershed. Our simulation scenarios include four manure application timings (historically-reported applications by the farmer; spring application; spring and fall applications; and spring, summer, fall, and winter applications), representing existing, 12-month, 6-month, and 3-month manure storage capacities, respectively. Seasonal and annual nutrient losses are compared among scenarios and between cropping systems to assess the range of losses predicted and the degree of environmental benefit achieved by moving to more restrictive application. The results will provide scientific support for decision makers regarding the impacts of manure storage capacities and application timing on seasonal risk-levels of nutrient losses from the watershed.