Depth sequence distribution and reactivity of phosphorus in intensively managed agricultural soils in East-Central Illinois

Subsurface loss of phosphorus (P) in agricultural soils in the Midwestern U.S. has been receiving attention in the last few decades because of its negative environmental impacts to aquatic ecosystems such as eutrophication and hypoxia. Thus, there is a great interest in understanding the distribution and reactivity of P in the soils influencing the subsurface P loss. In this study, the depth sequence distribution, speciation, reactivity of P, and soil physicochemical properties were investigated in intensively managed agricultural soils in East-Central Illinois using chemical extraction/digestion, batch desorption experiments, infiltrometer measurements, Nuclear Magnetic Resonance spectroscopy, and X-ray diffraction analysis. The results suggested several sources of P contributing to subsurface P loss. During the growing season when the water table is low, calcium P (Ca-P: P adsorbed by calcite/dolomite and/or apatites) in subsoils is likely to be the main source of labile P (>70% of total inorganic phosphorus (IP) in subsoils). Both P desorption from calcite/dolomite and dissolution of Ca-P precipitates contribute to the concentration of labile P in subsoils. Although the amount of other IP species such as Fe-occluded P and non-occluded P was low, P desorption from amorphous Fe oxyhydroxides should not be excluded. During the wet spring season, surface soils becomes an additional source of P. In surface soils, Ca-P was a major IP species (~40-70% of total IP in topsoils) followed by non-occluded P (9-40% of total IP in topsoils) and Fe-occluded P (6-30% of total IP in topsoils). The labile P via dissolution and/or desorption of these IP pools and mineralized organic P in the surface soils can be translocated to tile lines through preferential flow paths. All of these findings should help the Illinois Nutrient Reduction Strategy Plan to develop the strategy to reduce the agricultural nutrient loss to Illinois waters and the Gulf of Mexico