Transformation and Transport of P Influenced By Organic Amendments in a Long Term Wheat Fallow Rotation in the Pacific Northwest.

Understanding the transformation and transport of P in soils is very important for developing innovative strategies to increase P availability and reduce P loss. Changes in soil properties as a result of long term applications of manure or straw and N management may influence the forms of P and its distribution at various depths in the soil profile.  Long-term fertility and soil management trials began in 1931 for a wheat-fallow cropping system at the Columbia Basin Agricultural Research Center, Pendleton, OR, was used for this evaluation.  Soil samples taken in 2014 and historical archived soil samples were used for P fractionation to evaluate the influence of annual applications of 3 Mg/ha straw either incorporated into soil or burned in the fall or in the spring or various combinations of straw plus N (45 or 90 Kg/ha) or straw plus manure (22.4 Mg/ha).   Long term changes in soil properties (i.e. pH, C/P, microbial P etc.) as a result of the above treatments as well as their influence on P fractionation at various depths in the soil profile will be presented. Changes in P availability in this wheat- fallow cropping system will be compared to those in an adjacent long-term grass pasture.