Drying and rewetting (DRW) events in soils cause the release of molybdate-unreactive dissolved phosphorus (MUP) into soil solutions, which has been historically considered biologically-derived phosphorus (P) from microbial cell lysis. This unreactive P, however, could also represent P bound to soil colloids/nanoparticles, whose releases are also known to be physically stimulated during DRW events. To explore this possibility, two riparian wetland soils (A and B) with contrasting soil P speciation were subjected to three successive DRW cycles in soil columns. Leachates were successively filtrated/ultrafiltrated with 0.45 μm, 30 kDa and 5 kDa pore size membranes to separate the different colloidal/nanoparticulate fractions, in which molybdate-reactive P (MRP), total P (TP), MUP (defined as TP minus MRP), Fe, Al and dissolved organic carbon (DOC) concentrations were measured. For both soils, TP concentrations peaked at the beginning of each rewetting event then decreased rapidly upon leaching. MUP concentrations appeared similar variations as TP, but MRP concentrations remained relatively constant during leaching. Soil B showed larger TP release peaks than soil A and contained different P forms (more MUP and MRP, respectively). In both soils, colloidal/nanoparticulate P was an important fraction (up to 70%) of TP (<0.45 μm) in leachates immediately after rewetting, then this proportion decreased markedly at the end of each DRW cycle. On average, the proportion of colloidal/nanoparticulate P in TP was much higher in soil B than in soil A (45% vs. 17%). Results suggested that the colloidal/nanoparticulate carriers likely consisted of a mixture of Fe/Al oxides and organic matter, and highlighted a higher colloidal/nanoparticulate P release capacity in soil B with higher soil organic matter content and porosity. In addition, colloidal/nanoparticulate P and truly dissolved MUP had a greater response to DRW events and were exhausted more rapidly than truly dissolved MRP and smaller nanoparticulate P, suggesting potential differences in their sources and production mechanisms. Thus, this study demonstrates that colloidal/nanoparticulate P can represent an important part of the unreactive P fraction released during DRW events in soils and further highlights the importance of the colloidal/nanoparticulate P fraction in the transport and cycling of P in soils and waters.