A Global Perspective on Integrated Strategies to Manage Soil Phosphorus Status for Eutrophication Control without Limiting Land Productivity.

Unnecessary accumulation of phosphorus (P) in agricultural soils continues to degrade water quality and linked ecosystem services. Managing both soil loss and soil P fertility status is therefore crucial for eutrophication control, but the relative environmental benefits of these two mitigation measures, and the timescales over which they occur, remain unclear. To support policies toward reduced P loadings from agricultural soils, we examined the impact of soil conservation and lowering of soil test P (STP) in different regions with intensive farming (Europe, the United States, and Australia). Relationships between STP and soluble reactive P concentrations in land runoff suggested that eutrophication control targets would be more achievable if STP concentrations were kept at or below the current recommended threshold values for fertilizer response. Simulations using the Annual P Loss Estimator (APLE) model in three contrasting catchments predicted total P losses ranging from 0.52 to 0.88 kg ha depending on soil P buffering and erosion vulnerability. Drawing down STP in all catchment soils to the threshold optimum for productivity reduced catchment P loss by between 18 and 40%, but this would take between 30 and 40+ years. In one catchment, STP drawdown was more effective in reducing P loss than erosion control, but combining both strategies was always the most effective and more rapid than erosion control alone. By accounting for both soil P buffering interactions and erosion vulnerability, the APLE model quickly provided reliable information on the magnitude and time frame of P loss reduction that can be realistically expected from soil and STP management. Greater precision in the sampling, analysis, and interpretation of STP, and more technical innovation to lower agronomic optimum STP concentrations on farms, is needed to foster long-term sustainable management of soil P fertility in the future.

Modeling the risk of phosphorus runoff following single and split phosphorus fertilizer applications in two contrasting catchments.

The potential loss of P in runoff is a function of the combined effects of fertilizer-soil interactions and climatic characteristics. In this study, we applied a Bayesian approach to experimental data to model the annualized long-term risk of P runoff following single and split P fertilizer applications using two example catchments with contrasting rainfall/runoff patterns. Split P fertilizer strategies are commonly used in intensive pasture production in Australia and our results showed that three applications of 13.3 kg P ha(-1) resulted in a greater risk of P runoff compared with a single application of 40 kg P ha(-1) when long-term surface runoff data were incorporated into a Bayesian P risk model. Splitting P fertilizer applications increased the likelihood of a coincidence of fertilizer application and runoff occurring. We found that the overall risk of P runoff is also increased in catchments where the rainfall/runoff pattern is less predictable, compared with catchments where rainfall/runoff is winter dominant. The findings of our study also question the effectiveness of current recommendations to avoid applying fertilizer if runoff is likely to occur in the next few days, as we found that total P concentrations at the half-life were still very high (18.2 and 8.2 mg P L(-1)) following single and split P treatments, respectively. Data from the current study also highlight that omitting P fertilizer on soils that already have adequate soil test P concentrations is an effective method of reducing P loss in surface runoff. If P fertilizer must be applied, we recommend less frequent applications and only during periods of the year when the risk of surface P runoff is low.