ASSESSING THE IMPACT OF AGRICULTURAL PRACTICES ON PHOSPHOROUS AVAILABILITY AND LOSS USING OXYGEN ISOTOPES OF PHOSPHATE IN SOIL

Phosphorus (P), an essential element for growth and development, is taken up by plants as phosphate (Pi), however Pi is unevenly distributed and relatively immobile in soils. As a result, more than 30% of the world's arable land requires the application of P fertilizers for cropping and many natural ecosystems are P limited. Unfortunately, part of the applied P in intensive cropping systems can enter the waterways through runoff and erosion, contributing to pollution and eutrophication of surrounding groundwater, lakes and coastal environments. Tracking P cycling, mobility in soils, and determining soils' P availability to plants is challenging because adsorption/desorption, immobilization (occlusion by or precipitation as minerals), mineralization (conversion of organic P compounds to Pi), and uptake (by organisms) all occur simultaneously in the soil. Determining soil P availability and mobility and how these characteristics vary with soil type and agriculture practices will help reduce P loss from agriculture systems and contribute fundamental understanding to inform science based management plans.A clear understanding of all aspects of soil P biogeochemistry is required to determine P availability and mobility in soils, but analytical techniques have not been available to fully characterize P cycling in soils. We developed a natural isotopic tracing procedure of oxygen in phosphate to (d18Op) to track P sources and biological cycling in soils. We propose to apply this procedure to study P cycling in various agriculture soils representing an array of farm management practices. Work will take place at the Russell Ranch Experimental LTRAS, a long-term comparison of 10 conventional, organic and alternative cropping systems, both irrigated and non-irrigated is taking place.We expect that different processes will dominate P cycling in different soil types (soil pH, grain size, mineralogy, organic matter content, etc.) and under different management and farming systems (no tillage, and use of organic--plant residue and/or animal waste--vs. inorganic fertilizers). These processes will ultimately determine the availability of P to plants and the potential for P loss from the system.