Effects of air availability and straw amendment on swine manure phosphorus runoff potential

Manure is a significant source of phosphorus (P) fertilizer, but its P solubility may influence both environmental impacts and fertilizer effectiveness. The transformation of P fractions in manure during long-term storage remains unclear. This study evaluated P speciation dynamics during extended swine manure storage under both unsealed and sealed conditions, and examined corn straw as an amendment to enhance P stabilization. Results showed that during unsealed conditions, Olsen-P (H₂O-P and NaHCO₃-P) concentration decreased by 16%, while Fix-P (NaOH-P + HCl-P) concentration increased by 150%, indicating the conversion of P to more stable forms. In contrast, sealed conditions had smaller effects on P concentration and solubility, with Fix-P increased by 3%. The incorporation of corn straw in unsealed system resulted in a 45% increase in total phosphorus (TP) concentration and a 12% reduction in Olsen-P, suggesting that the addition of corn straw enhanced chemical precipitation and organic complexation of P. Sealed conditions with straw amendment exhibited similar trends to those in sealed and unamended ones. Under long-term unsealed storage, soluble P in swine manure converted to more stable minerals including MgNH₄PO₄·6H₂O, Fe₃(PO₄)₂·H₂O, and AlPO₄·2H₂O, and the manure particles aggregated more. This study may provide valuable insights for developing cost-effective swine manure management practice to reduce P runoff potential.