Dynamic spatially explicit mass-balance modeling for targeted watershed phosphorus management: II. Model application

Cost-effective nonpoint source phosphorus (P) control should target the land areas at greatest risk for P loss. We combined mass-balance modeling and geographic analysis to identify and map high-risk areas for P export by integrating long-term P input/output accounting with spatially variable physiographic, land use, and agronomic factors. The dynamic interactive simulation of phosphorus loss areas (DISPLA) model evaluates changes over time and space in soil P concentration and P export in response to management interventions targeted specifically to critical P source areas. Five scenarios were simulated in a test watershed dominated by dairy agriculture in Vermont's Champlain Valley: (1) baseline; (2) nutrient management applied to corn and hay land and to urban lawns; (3) erosion control applied to silage corn land; (4) conversion of critically eroding cropland to permanent grass; and (5) all management changes combined. If present-day conditions continue, soil test P and P export will inevitably increase as P inputs continue to exceed outputs. Soil test P levels on corn land are projected to increase more than fourfold over 80 years if present management continues; estimated P export is expected to more than double over the same period. Increases in soil test P over time in the watershed are not uniform, but varied spatially in response to variability in initial conditions and input/output P balance. Targeted nutrient management was effective in reducing soil test P concentrations (50–90%) and appeared to hold the line on P export for the test watershed over the 80-year simulation. Simulated P export in the test watershed at the end of the nutrient management simulation was reduced by 64% compared to baseline. Implementation of erosion control on row cropland had little effect on soil test P and achieved only a transitory reduction in P export. Exclusive reliance on cropland erosion control to manage nonpoint source P is unlikely to succeed over the long term. Conversion of critical row cropland to permanent grass reduced P export by 54%, but did not affect soil P levels. Because row cropland converted to grassland retains its soil test P concentration, management of converted grassland to reduce runoff and P export is very important; row cropland with elevated soil test P converted to riparian buffer may still serve as a source of dissolved P to runoff. Application of all management measures combined yielded a 74% reduction in P export. Implications to watershed P management are discussed.