Comparing phosphorus management strategies at a watershed scale

The persistence of water quality problems has directed attention toward reduction of agricultural non-point sources of phosphorus (P). We assessed the practical impact of three management scenarios of the USDA-EPA Strategy for Animal Feeding Operations to reduce P losses from a watershed. Using an agronomic threshold of 50 mg Mehlich-3 P kg^-1 soil, 55% of our watershed would receive no P as fertilizer or manure. An environmental threshold of 190 mg Mehlich-3 P kg^-1 soil, above which P loss in runoff increases, restricts future P inputs to less titan crop removal on 32% of the watershed. Finally, a site assessment P index, which accounts for likely source and transport risks was used. This showed none of the watershed was at high risk of P loss and that areas of medium risk (where remedial measures should be considered) were near the stream channel. In the watershed studied, the P index was the best method to target remedial management to minimize P export and impacted less land area than the other strategies.