Changes in Phosphorus Fractions and Nitrogen Forms During Composting of Pig Manure with Rice Straw

The study was conducted to reveal P fractions and N forms changing characters during composting of pig manure with rice straw. During composting, the NH4+-N concentration decreased and reached at a relatively low value (<400 mg kg−1) in the final compost, while the NO3−-N concentration increased. Total N losses mainly occurred during thermophilic phase due to the high temperature, the high NH4+-N concentration and the increase of pH value. Labile inorganic P was dominated in the pig manure and initial compost mixture. During composting, the proportion of labile inorganic P of total extracted P decreased, while the proportion of Fe+Al-bound P, Ca+Mg-bound P and residual P increased. The evolutions of the proportion of labile inorganic P, Fe+Al-bound P and Ca+Mg-bound P were well correlated with the changes of pH value, organic matter and C/N ratio. Therefore, composting could increase the concentration of N and P and decrease the presence of NH4+-N and labile P fractions which might cause environmental issues following land application.