The soil carbon saturation controls phosphate sorption and mobility in organically amended Ferralsols.

Phosphorus (P) fertilizer efficiency in highly weathered soils is constrained by phosphate sorption on iron (Fe) and aluminum (Al) oxides. Integrated soil fertility management promotes the combined use of P fertilizers and organic amendments (OAs), but the mechanisms underlying OA-P interactions remain unclear. This study evaluated how OA dose, OA quality, and time affect P mobility, assessed through phosphate sorption and diffusion from a fertilizer granule. Three Ferralsols were incubated with compost, farmyard manure (FYM), or straw at four doses (0-4 % w/w) in a full factorial design. Phosphate sorption strength (KD) was quantified after 1 and 49 days of incubation and related to the soil's degree of carbon saturation (DCS), an operational metric that is proportional to the molar ratio of organic C to the reactive Fe and Al oxides. Across all soils and treatments, KD decreased with increasing DCS (R2 = 0.66) at 49 days after amendment, consistent with increasing competition of organic anions with phosphate for sorption sites. In contrast, the short-term (1-day) effects of amendments on KD were primarily related to soil pH. A diffusion experiment using Ferralsol samples from a field trial with three years of FYM application showed wider diffusion zones around fertilizer granules in FYM-amended than in control soils. Together, these findings suggest that OA-induced improvements in P fertilizer use efficiency observed in field trials are related to competition effects and that the effects of OA dose and quality can be predicted with a mechanistic model.