Influence of straw pretreatment on phosphorus forms during the process of pig manure thermophilic composting

Objectives To clarify the efficiency of straw pretreatment in accelerating the composting process of straw with manure, the transformation of phosphorus fractions during the composting were investigated.

Methods The rice straw was pretreated via static composting by straw decomposing agent (B) and calcium hydroxide (C) for 10 days (B1, C1) or 20 days (B2, C2), respectively. The control (CK) was the rice straw without pretreatment. The rice straw was mixed with pig manure for the thermophilic composting. The measurement methods were performed by infrared spectroscopy (IR), scanning electron microscopy (SEM) and phase analysis of X-ray diffraction (XRD) to observe the changes of straw structure. The phosphorus fractions were determined by sequential extraction procedure with conventional analyses.

Results Compared with control, the pretreatment with B shortened the requirement of high temperature period, while that with C did not. When the composting was stopped, the total phosphorus contents were 22.51, 26.47 and 23.68 g/kg in CK, C1 and C2, while those were 29.84 g/kg in B1 and 32.88 g/kg in B2, respectively, significantly higher than CK. Although the contents of phosphorus fractions showed significant differences among the treatments, the content of each phosphorus fraction was in all in order of HCl-P > NaHCO₃-P > H₂O-P > NaOH-P. According to the P availability, the composting straw and manure pretreated with B accumulated more available P (H₂O-P+NaHCO₃-P) than that with C by 14.44 g/kg for B1 and 13.74 g/kg for B2, but only 12.27 g/kg for C1 and 12.02 g/kg for C2, respectively. Straw pretreatment time had no significant effect on the transformation of phosphorus fractions during the composting.

Conclusions Compared with calcium hydroxide, straw decomposing agent could promote the transformation of NaOH-Po to NaOH-Pi, so lead to more phosphorous availability. The straw decomposing agent shows stronger destructivity to rice straw than calcium hydroxide, and demonstrate prospective in compost manufacture.