Effect of phosphate additive on the nitrogen transformation during pig manure composting

Previous studies revealed that phosphate, as an additive to composting, could significantly reduce NH₃ emission and nitrogen loss through change of pH and nitrogen fixation to form ammonium phosphate. However, few studies have explored the influence of pH change and phosphate additive on NO _x^--N, NH₄⁺-N, NH₃, and N₂O, which are dominate forms of nitrogen in composting. In this study, the equimolar H₃PO₄, H₂SO₄, and K₂HPO₄ were added into pig manure composting to evaluate the effect of H⁺ and PO₄^3- on nitrogen transformation. As a result, we reached the conclusion that pH displays significant influence on adsorption from PO₄^3- to NH₄⁺. The NH₄⁺-N concentration in H₃PO₄ treatment kept over 3 g kg^-1DM (dry matter) which is obviously higher than that in H₂SO₄ treatment, and NH₄⁺-N concentration in K₂HPO₄ treatment (pH>8.5) is lower than 0.5 g kg^-1DM because adsorption capacity of PO₄^3- is greatly weakened and NH₄⁺-N rapidly transformed to NH₃-N influenced by high pH value. The N₂O emission of composting is significantly correlated with incomplete denitrification of NO _x^--N, and PO₄^3- addition could raise NO _x^--N contents to restrict denitrification and further to promote N₂O emission. The study reveals the influence mechanism of phosphate additive to nitrogen transformation during composting, presents theoretical basis for additive selection in nitrogen fixation, and lays foundation for study about nitrogen circulation mechanism during composting.