Alternative Simulation of Soil Phosphorus for Agricultural Land Uses in the Chesapeake Bay Watershed Model.

Current restoration efforts for the Chesapeake Bay watershed mandate a pollution cap and implementation timeline for reducing the load of nutrients and sediment to receiving waters. The multi-component Chesapeake Bay Watershed Model (Bay Model) estimates nutrient and sediment loading; however, some model components have not been updated to reflect current research findings. The objective of this research was to improve the simulation of soil phosphorus dynamics and factors contributing to field losses by using an independent phosphorus loss modeling tool (APLE) as an alternative to the current Bay Model’s soil phosphorus approach.  Identical assumptions of land use acreage, soil properties, nutrient management practices, and transport factors from the Bay Model were used as inputs to the APLE model to simulate management conditions from 1992-2005.  Spatial scales were simulated at the county level.  Outcomes represent revised estimates of phosphorus edge-of-field losses that can be compared to estimates from the Bay Model. Findings from APLE exhibit greater variability and higher mean estimates of total phosphorus loss compared to the Bay Model, 10.1±0.2 kg ha^-1 as compared to 2.4±0.05 kg ha^-1 for high-till land uses, 4.3±0.1 kg ha^-1 as compared to 2.4±0.04 kg ha^-1 for low-till land uses, and 3.8±0.1 kg ha^-1 as compared to 0.93±0.03 kg ha^-1 for pasture land uses. Results also indicate the strongest contributor to field total phosphorus losses are associated with field sediment loss. Findings support APLE as an appropriate alternative model tool to inform field phosphorus loss estimates.