Biochar combined with ferrous sulfate reduces nitrogen and carbon losses during agricultural waste composting and enhances microbial diversity

This study investigated the effects of adding biochar, ferrous sulfate, and their combination during composting of pig manure mixed with straw to reduce the loss of carbon and nitrogen during agricultural waste composting. Four 100 L fermentation tank-scale experimental composting treatments were established: the additive-free control (CK), addition of 5% biochar (BC), addition of 5% ferrous sulfate (SF), and addition of 2.5% BC + 2.5% SF (BS). The transformation of carbon and nitrogen and the change characteristics of bacterial communities during composting of the four treatments were explored. BS significantly reduced NH3, N2O, CO2, and CH4 emissions by 59.57%, 48.41%, 35.80%, and 32.25% compared with CK, respectively. The BS treatment showed lower carbon and nitrogen losses than the single additive treatment. BS treatment enhanced the diversity and richness of bacterial communities during composting. The redundancy and Pearson correlation analyses revealed that the composition of dominant species between treatments had significant differences in different compost periods (mesophile, thermophilic, and cooling periods). BS treatment significantly increased the abundance ratio of carbon sequestration and nitrogen retention communities, and it enhanced nutrient preservation by microbial metabolism. Therefore, the composite addition of biochar and ferrous sulfate is an effective method to reduce the loss of carbon and nitrogen during agricultural waste composting.