Assessing soluble and precipitated phosphorus in heavily manured Oxisols

Context Heavily manured soils often exhibit P accumulation to levels that can pose significant environmental risks. Aim. This study aimed to assess the effect of incubating high doses of dairy liquid manure (DLM) for up to 365 days on the partial and total saturation of soil P adsorption sites and the speciation and dynamics of phosphate compounds in Oxisol solutions, having soluble KH2PO4 as control mineral fertilizer (MF). Methods. The concentration values of dissolved organic carbon (DOC), pH, and major and trace elements in soil solutions were input into the Visual MINTEQ chemical speciation software. Key Results. The highest dose of DLM exhibited higher pH values than all MF treatments, leading to increased acidity over the incubation period. Soluble P concentrations varied depending on the texture and buffering capacity of the Oxisols. DLM was more effective at increasing P solubility in clayey Oxisols, while MF was more effective in sandy Oxisol. The predominant species in the soil solution were protonated and deprotonated forms of DOC (HDOC/DOC-) as the negatively charged phosphates species as well (H2PO4-/HPO42-). The dynamics of P solubility, even with high doses of DLM, showed that: i) the formation of ternary P-cation-DOC soluble complexes depended on the simultaneous occurrence of the binary pairs of Mg-P and Mg-DOC in the soil solution; ii) overall, DLM application favored the formation of variscite and tricalcium phosphate, which act as phosphate sinks in the soils. Conclusions and implications. These two conditions (i and ii) are crucial for reducing P solubility and the risk of groundwater contamination when high doses of DLM are applied.