DISSOLVED PHOSPHORUS DYNAMICS AND MANAGEMENT WITHIN THE AGRICULTURAL LANDSCAPE

Abstract

Agricultural phosphorus loss was identified as a water quality priority within the Minnesota Nutrient Reduction strategy identifying a 45% yield reduction goal. Implementation plans call for total phosphorus (TP) reduction with traditional management strategies designed for erosion control and particulate nutrient removal, leaving the dissolved, or bioavailable, forms of phosphorus un-accounted for. Substantial yield increases in agricultural tributaries over recent decades highlight the need for dissolved reactive phosphorus (DRP) management, with some sources documenting over 50% DRP contributions to TP loads in Minnesota. DRP, hydrology, management and site-specific factors were investigated at two field research sites in southern Minnesota, with additional data harnessed from the Minnesota Discovery Farms Program. Data was used to assess the impacts of various site and management factors including cover crops, riparian buffers, edge-of-field wetlands, tillage category, fertilizer application and soil properties on phosphorus loads from farm fields and edge-of-field best management practice (BMPs) uptake. Four project objectives were addressed; 1.) to quantify and characterize current and target DRP yields from Southern Minnesota agricultural fields, 2.) to quantify the influence of local field and management conditions on DRP yields, 3.) to assess the effectiveness or inefficacy of common management practices for phosphorus and nitrogen removal, and 4.) to explore novel management strategies for DRP yield reductions including treatment trains, microbial soil amendment and plant harvest. Methodology included hydrologic monitoring, soil assessment, edge of field nutrient concentration analysis and measurement of phosphorus in vegetation to track phosphorus movement through the soil, water and plant components of agroecosystems. Current DRP loss rates from agricultural fields were quantified at 0.49 kg ha-1, with a target DRP yield of 0.27 kg ha-1 to achieve a 45% phosphorus reduction. To meet target yields, project results demonstrated the importance of both surface and subsurface DRP loss pathways, legacy phosphorus monitoring and management and the need for coordinated edge of-field and in-field management strategies. Significant conditions driving drain tile DRP concentrations included manure application rate, number of tillage passes and soil test phosphorus (STP). Significant conditions driving surface DRP concentrations included cumulative manure and fertilizer application rate and STP. STP accumulation was driven most significantly by manure application rate, number of tillage passes, organic matter content, clay content, soil pH and cover crop implementation. Cover crops, which were placed into the context of an agricultural treatment train, were found to reduce subsurface DRP concentrations by 63% and annual yields by .07 kg ha-1 through reduced constituent mobilization at higher flows but also to contribute to increased STP. Crop use efficiency, fertilizer application and soil phosphorus draw down where also associated as part of a mass balance to further correlate management action to DRP yields. Research findings will help to inform agricultural management for DRP removal strategies necessary for setting and meeting realistic nutrient reduction and water quality goals.