Particle facilitated transport is a key mechanism of phosphorus (P) loss in agroecosystems. We assessed contributions of colloid and nanoparticle-bound P (nPP; 1 nm – 415 nm) to total P released from surface soils of grasslands receiving bi-annual poultry litter applications since 1995. In laboratory incubations, soils were equilibrated at pH 6 and 8 or subjected to 7 days of anoxic conditions and then 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). Across all treatments, 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. We found greater release of nPP and greater Fe²⁺(aq) production following anoxic incubation of seasonally saturated field soils, than from incubation of well-drained field soils. Non-reductive particle dispersion, stimulated by adding base, yielded no increased 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.