New Tools for Managing Agricultural P

Best management practices (BMPs) generally focus on retaining nutrients (especially P) after they enter the watershed. This approach is expensive, unsustainable, and has not led to reductions of P pollution at large scales (e.g., Mississippi River). Although source reduction, which results in reducing inputs of nutrients to a watershed, has long been cited as a preferred approach, we have not had tools to guide source reduction efforts at the watershed level. To augment conventional TMDL tools, we developed an “actionable” watershed P balance approach, based largely on watershed–specific information, yet simple enough to be utilized as a practical tool. Interviews with farmers were used to obtain detailed farm management data, data from livestock permits were adjusted based on site visits, stream P fluxes were calculated from 3 years of monitoring data, and expert knowledge was used to model P fluxes through animal operations. The overall P use efficiency. Puse was calculated as the sum of deliberate exports (P in animals, milk, eggs, and crops) divided by deliberate inputs (P inputs of fertilizer, feed, and nursery animals x 100. The crop P use efficiency was 1.7, meaning that more P was exported as products that was deliberately imported; we estimate that this mining would have resulted in a loss of 6 mg P/kg across the watershed. Despite the negative P balance, the equivalent of 5% of watershed input was lost via stream export. Tile drainage, the presence of buffer strips, and relatively flat topography result in dominance of P loads by ortho-P (66%) and low particulate P. This, together with geochemical analysis (ongoing) suggest that biological processes may be at least as important as sediment transport in controlling P loads. We have developed a P balance calculator tool to enable watershed management organizations to develop watershed P balances and identify opportunities for improving the efficiency of P utilization.