Microbial activities during composting of spent pig-manure sawdust litter at different moisture contents

The changes in microbial properties, including total aerobic heterotrophs, O2-consumption rate, ATP content, dehydrogenase activity and microbial biomass C and N of the spent pig-manure sawdust litter were examined during further composting. The effects of three moisture levels, 50% (pile A), 60% (pile B) and 70% (pile C), on the composting process were also evaluated. Piles A and B had very similar trends of change in temperature and microbial properties during the composting period but pile C was significantly different. Temperatures in the first two piles increased to a peak of 64-69 degrees C by day 4, while that of pile C rose to a lower peak (58 degrees C) on day 7. The high moisture content (about 70%) of pile C led to early cooling of the pile and decreased the production of microbial activity and biomass. Although water was added frequently to maintain the moisture content of each pile, it was difficult in practice to maintain the moisture content of pile C at 70%, since water leaked out from the pile. Therefore, a moisture content of between 50 and 60% can be considered as the optimal moisture level for further composting of the spent litter. In general, the total aerobic heterotrophs, O2 consumption rate and ATP content of all piles increased dramatically during the thermophilic stage of composting, but then decreased slowly and were maintained at lower levels at the end of the composting process. Stability of microbial properties was observed at day 60, indicating that two months is enough to convert spent litter to a mature compost. Temperature was found to be correlated with ATP content, dehydrogenase activity and oxygen consumption rate, and so these parameters could be used to indicate microbial activity and degradation of the spent pig-manure sawdust litter.