Estimating Legacy Soil Phosphorus Impacts on Phosphorus Loss in the Chesapeake Bay Watershed.

Agricultural nutrient management is an issue due to P loss from fields and water quality degradation. This is especially true in watersheds where a history of P application in excess of crop needs has resulted in elevated soil P (legacy P). As practices and policy are implemented in such watersheds to reduce P loss, information is needed on time required to draw down soil P and how much P loss can be reduced by drawdown. We used the Annual P Loss Estimator (APLE) model to simulate soil P drawdown in Maryland, and to estimate P loss at a statewide scale associated with different combinations of soil P and P transport. Simulated APLE soil P drawdown compared well with measured rates from three field sites, showing that APLE can reliably simulate P dynamics for Maryland soils. Statewide APLE simulations of average annual P loss from cropland (0.84 kg ha) also compared well with estimates from the Chesapeake Bay Model (0.87 kg ha). The APLE results suggest that it is realistic to expect that a concerted effort to reduce high P soils throughout the state can reduce P loss to the Chesapeake Bay by 40%. However, P loss reduction would be achieved gradually over several decades, since soil P drawdown is very slow. Combining soil P drawdown with aggressive conservation efforts to reduce P transport in erosion could achieve a 62% reduction in state-level P loss. This 62% reduction could be considered a maximum amount possible that is still compatible with modern agriculture.