Combining soil conservation with phosphorus drawdown can confront legacy phosphorus accumulation and transfer

Legacy phosphorus (P) in agricultural soils (i.e., P that derives from historical human activities) can resist conventional nutrient management strategies to improve water quality (e.g., placement, rate, source, and timing of application). Further, soil conservation practices such as reduced tillage, while potentially beneficial for improving soil health and minimizing erosion, can promote dissolved P loss. Comprehensive legacy P management requires targeted mitigation strategies that consider the sources and processes involved in P mobilization and transport. We modeled trade-offs and interactions of nutrient management and soil conservation strategies in legacy P mitigation efforts at three key sites in the northern United States where legacy P contributions to water quality are a concern. The Annual Phosphorus Loss Estimator (APLE) model was used to simulate generalized management scenarios at each site: current site-specific practices, conventional conservation practices (no-till and manure injection), and P drawdown (curtailing fertilizer P additions and extracting P from soils via crop uptake and harvest). Modeled results highlight that the effects of legacy P are not always obvious; even at sites near the range of agronomic optimum, losses of legacy P in runoff can be significant. Phosphorus drawdown via crop uptake and removal offers the potential to deplete legacy P stores but requires dedication and time. In model simulations, no-till reduced total P losses due to reductions in sediment transport. Coupling drawdown strategies with appropriate conservation management to avoid inadvertent P losses can reduce both dissolved and particulate P losses. Focusing on either soil conservation or soil P drawdown alone is insufficient to meet water quality goals. Phosphorus drawdown strategies must be accompanied by practices supporting soil conservation to ensure that legacy P management benefits water quality in the short and long term.