Role of natural nanoparticles in phosphorus transport processes in ultisols

Labile P is a well-recognized nonpoint source pollutant in agroecosystems. Predicting the fate and transport of P in watershed systems is critical in protecting water quality. In this case study, we investigated the role of soil nanoparticles in P release in South Carolina agricultural soils. Batch desorption experiments were coupled with scanning and transmission electron microscopy (EM) and x-ray absorption spectroscopy (XAS) to better assess the reactivity of soil nanoparticles. The surface soils contained a total P concentration of 260 to 940 mg kg-1, approximately 40 to 60% of which was ammonium oxalate extractable P. The 30-d desorption experiments showed that P desorption was initially rapid, followed by a slow continuous release at pH 5.5 and 7. It was generally not correlated with the release of soil nanoparticles (operationally defined as 10-200 nm). Phosphorus was not readily retained on these nanoparticles at pH 4 to 7. These nanoparticles were rich in Si and S, and coated with Fe and C and trace amounts of P, Ca, Na, Mg, and Al. The EM images of nanoparticle morphology showed C-associated globular formations as well as assembled platelets and spherical nanoparticles. The colloid- and nanoparticlefacilitated P release has often been suggested as one of the important pathways for P transport in the soil-water environment. This study presents a new aspect of nanoparticle reactivity in the soil environment, however: nanoparticles do not always contribute to P transport processes.