Phosphate, as an additive to composting, could significantly reduce ammonia emission and nitrogen loss but may also cause adverse effects on the degradation of organic matter. However, there is little information about the influence of pH change, salt content, and phosphate on different organic fraction degradation during composting with the addition of phosphate at a higher level. In this study, the equimolar phosphoric acid (H3PO4), sulfuric acid (H2SO4), and dipotassium phosphate (K2HPO4) were added into pig manure composting with 0.25 mol mass per kilogram of dry matter basis addition amount to evaluate the effect of H+, PO43−, and salinity on carbon component transformation and organic matter degradation. The results showed that both H3PO4 and K2HPO4 additives could lead to shorter duration in the thermophilic phase, lower degradation of lignocellulose, and lesser carbon loss compared to CK, even though had different pH, i.e., acidic and alkaline conditions, respectively. Besides, the addition of H3PO4, H2SO4, and K2HPO4 could increase the degradation of soluble protein and lipid during composting. Redundancy analysis demonstrated that the variation in different organic carbon fractions was significantly correlated with the changes of pH and the presence of PO43−, but not with SO42− and electrical conductivity, suggesting that pH and phosphate were the more predominant factors than salinity for the inhibition of organic matter degradation. Taken together, as acidic phosphate addition produces a true advantage of controlling nitrogen loss and lower inhibition of organics transformation during composting, the expected effects may result in more efficient composting products.
