Biochar combined with gypsum reduces both nitrogen and carbon losses during agricultural waste composting and enhances overall compost quality by regulating microbial activities and functions

Composting is an efficient method for treating agricultural wastes. This study investigated the effects of the addition of biochar (B) and gypsum (G) to straw mixed with chicken manure (SC) (i.e. SC, SC + B, SC + G and SC + B + G) on composting performance at different initial C/N ratios (20, 25 and 30). In general, biochar combined with gypsum (BCG) efficiently shortened composting time and reduced N loss, C loss and potential ecological risk. It also enhanced lignocellulose decomposition, nutrient retention and the overall compost quality expressed by a compost quality index (CQI), and increased the biomass of four different test crops. The BCG-induced increase in CQI was closely associated with microbial enzyme activities and C catabolic profiles. These results indicated that the combination of biochar and gypsum is more effective than each single additive during composting, and emphasized that microbial activities and functions play pivotal roles in determining compost quality and thereby agronomic performance.