Impact factors and mechanisms of dissolved reactive phosphorus (DRP) losses from agricultural fields: A review and synthesis study in the Lake Erie basin

Dissolved Reactive Phosphorus (DRP) losses from agricultural fields promote algae growth in water bodies, and may increase the risk of Harmful Algal Blooms (HABs). Using existing data from the Lake Erie Basin, we applied multiple regression analysis to better understand the impacts of both site-specific conditions (e.g., soil types/properties) and management practices (e.g., Agricultural Conservation Practices [ACP]) on annual DRP losses in subsurface and surface runoff. Results showed that soil properties significantly impact DRP losses. Greater DRP losses were associated with increased soil pH and Soil Test Phosphorus (STP). By contrast, soil organic matter (SOM) was inversely correlated with DRP losses. Soil clay content was also inversely correlated with DRP subsurface losses, but had no impact on DRP surface losses. The ACPs evaluated had varied effectiveness on DRP loss reduction. Cropping systems involving soybean could reduce DRP subsurface losses, whereas winter cover crops could cause unintended DRP subsurface losses. Cropping systems involving soybean and cover crops, however, had no impact on DRP surface losses. In addition, no-till and conservation tillage also enhanced DRP losses compared to conventional tillage, particularly for soils with high SOM and/or high clay content. Precipitation amount and fertilizer application rate significantly increased DRP surface losses as expected. Fertilizer application rate, however, had no impact on DRP subsurface losses. The impact of precipitation amount on DRP subsurface losses depends on STP levels. Precipitation amount significantly increases DRP subsurface losses when STP is higher (>41 mg kg^-1 in this analysis). The optimal STP level for crop growth is 30 to 50 mg kg^-1. Results from this study help us to better understand DRP losses and the effectiveness of ACPs for controlling them. We suggest taking soil surveys and soil tests into consideration when designing and/or implementing ACPs to manage DRP losses. Furthermore, the method we used for this study could be applied to other agricultural regions to investigate impacts of site-specific conditions and management practices on water quality.