Bridging the Physics and Chemistry of Soil Phosphorus Reactivity to Further Environmental Risk Assessment: A Legacy of Laj Ahuja.

At the beginnings of my research career in 1979, I was fortunate enough to have a 7-year post doctorate at the USDA-ARS, National Water Quality Laboratory in Durant, Oklahoma.  Although it was a small lab, there was a core group of scientists who realized the benefits of team work; which included myself a soil chemist, Sam Smith an agronomist, and Laj Ahuja a soil physicist and himself newly appointed to USDA-ARS.  Thus, began one of the most productive and impactful stages of my research career.  Laj was the consummate professional who was always challenging us to think outside the box.  By integrating his knowledge and vision, we were able to better describe the kinetics of soil phosphorus (P) desorption, develop some simple mathematical derivations to explain the desorption, and then apply it to expand the power of nonpoint source models that were being developed at the time.  To this day, this research still frames the release of soil P as a function of soil type, management, and weathering.  From this basic understanding, Laj was the inspiration to quantifying the depth of interaction between runoff water and surface soil using P³².  The findings that about the top 2 to 5 cm of soil interact with surface runoff to control the transfer of P from soil to runoff water, laid the foundations for environmental soil testing and improved P transport modeling.  This paper describes the role of Laj Ahuja in defining team research for the common good and describes the major findings in the context of current soil research priorities.