Particle-facilitated transport is a key mechanism of phosphorus (P) loss in agroecosystems. We assessed contributions of colloid- and nanoparticle-bound P (nPP; 1-415 nm) to total P released from grassland soils receiving biannual poultry litter applications since 1995. In laboratory incubations, soils were subjected to 7 days of anoxic conditions or equilibrated at pH 6 and 8 under oxic conditions and then the extract was size fractionated by differential centrifugation/ultrafiltration for analysis of P, Al, Fe, Si, Ti, and Ca. Selected samples were characterized by transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) and field flow fractionation (FFF-ICP-MS). Particles released were present as nanoaggregates with a mean diameter of 200-250 nm, composed of ~50-nm aluminosilicate flakes studded with Fe and Ti-rich clusters (<10 nm) that contained most of the P detected by EDS. Anoxic incubation of stimulated nPP release with seasonally saturated soils released more nPP and Fe(2+)(aq) than well-drained soils; whereas, nonreductive particle dispersion, accomplished by raising the pH, yielded no increase in nPP release. This suggests Fe acts as a cementing agent, binding to the bulk soil P-bearing colloids that can be released during reducing conditions. Furthermore, it suggests prior periodic exposure to anoxic conditions increases susceptibility to redox-induced P mobilization.