Effects of C/N ratio and bulking agent on speciation of Zn and Cu and enzymatic activity during pig manure composting

- Effects of C/N ratio and bulking agent on speciation of Zn and Cu and enzymatic activity were studied.
- Maize straw as bulking agent significantly enhanced degradation of organic matter.
- The composting mixture with initial C/N ratio of 25 led to low mobility of Zn and Cu.
- C/N ratio could affect urease activity by influencing the concentration of metal ions.

The aim of this study was to investigate the effects of initial C/N ratio (15, 20, 25) and bulking agent on speciation of Zn and Cu and enzymatic activities (dehydrogenase, urease) during pig manure composting. Rice straw and maize straw were separately chosen as bulking agent. Results showed that maize straw addition greatly enhanced organic matter (OM) degradation. The final NH4+-N/NO3−-N ratio in composting mixture with initial C/N ratio of 25 did not exceed the limit of 0.16 to be considered the index of mature compost. The composting mixture with initial C/N ratio of 25 could reduce the mobility of Cu and Zn, and 90% of Cu belonged to residual fraction in the final compost, while total contents for Zn and Cu increased by 112.8-192.7% and 115.5-132.6%, respectively. In addition, the composting mixture with initial C/N ratio of 25 accelerated the decline of urease activity than those in composting mixtures of initial C/N ratio of 15 and 20, and C/N ratio could affect urease activity by influencing the content of metal ions. Therefore, the final compost with initial C/N ratio of 25 was of low risk and high quality for land application.