Effects of residual monensin in livestock manure on nitrogen transformation and microbial community during “crop straw feeding-substrate fermentation-mushroom cultivation” recycling system

Although crop-livestock integration recycling systems improve nitrogen (N) utilization in agroecosystems, there are limited studies regarding impacts of residual antibiotics in livestock manure on N transformation in entire recycling system. The objective was to evaluate effects of feeding monensin on N recycling during "straw feeding-substrate fermentation-mushroom cultivation". This experiment contained 3 steps. During straw feeding, beef cattle were allocated into 2 groups and fed diets with or without monensin, respectively. During fermentation, beef cattle manure (with or without monensin) and straw (corn or wheat) and were co-fermented for 35 d to produce substrates. During cultivation, Agaricus bisporus was cultivated on 4 substrates to recycle N in the form of mushrooms. Rates of N retention during fermentation were significant higher for monensin and corn straw treatments and there was an significant interaction between straw and antibiotic on N retention rate during cultivation. However, residual monensin significantly reduced amount of recycled N during entire recycling system, due to changes in N transformation-associated enzyme activity, ammonification and denitrification plus microbial community structure and succession. Specifically, residual monensin inhibited growth of dominant bacterial phylum Bacteroidetes and fungal phylum Neocallimastigomycota, and increased bacterial phylla Actinobacteriota and Firmicutes. These alterations in functional microbes increased N retention rates but reduced mushroom yields in antibiotic treatments during cultivation. In conclusion, monensin decreased the N recycling rate in recycling system, but also reduced N losses during fermentation by inhibiting ammonification and denitrification, so, avoiding antibiotics usage is an effective strategy to improve the efficiency of recycling systems.