Differentiated mechanisms of biochar- and straw-induced greenhouse gas emissions in tobacco fields

Addition of biochar and straw has constituted an effective way to improve soil fertility in tobacco fields. However, little is known about the effects of these amendments on CH4 and N2O emissions and their potential microbial mechanisms. Here, we conducted an experiment in tobacco-planted soil using four treatments: mineral fertilizers only (F), biochar with mineral fertilizers (BF), wheat straw with mineral fertilizers (SF), and biochar + wheat straw with mineral fertilizers (BSF). Soil physical and chemical parameters, nitrous oxide (N2O) and methane (CH4) emissions, and the related microorganisms and functional genes (pmoA, mcrA, amoA, nirS, nirK, and nosZ) were analyzed. The results showed that biochar and straw addition significantly increased the uptake of CH4 by 34.2%, 9.3%, and 22.1%, respectively, and significantly increased the relative abundance of pmoA. Compared to the F treatment, the SF and BSF treatments significantly increased cumulative N2O emissions by 1.2- and 1.1-fold, respectively, while the BF treatment significantly decreased N2O emissions from the soil by almost 30%. Correspondingly, decreased relative abundance of amoA-ammonia-oxidizing archaea (AOA), nirS, nirK, and nosZ genes in response to biochar amendment were observed, whereas the relative abundance of amoAammonia-oxidizing bacteria (AOB) increased. Conversely, straw addition increased the relative abundance of amoA-AOA and nirS, nirK, and nosZ. Compared to SF, BSF significantly increased the relative abundance of nosZ and decreased the number of denitrifying bacteria. A structural equation model indicated that the important factors affecting N2O emissions were NH4+-N, NO3--N, pH, and amoA-AOA and nirK genes, while the important factors affecting CH4 emissions were bulk density, pH, SMBC, and pmoA and mcrA genes. Biochar amendment could significantly reduce CH4 and N2O emissions by regulating functional microorganisms and soil physicochemical properties. By contrast, returning straw to the field would increase N2O emissions by increasing the relative abundance of denitrification-related genes.