Mechanism of microbial regulation on methane metabolism in saline–alkali soils based on metagenomics analysis

Saline–alkali soils constitute a globally important carbon pool that plays a critical role in soil carbon dioxide (CO2) and methane (CH4) fluxes. However, the relative importance of microorganisms in the regulation of CH4 emissions under elevated salinity remains unclear. Here, we report the composition of CH4 production and oxidation microbial communities under five different salinity levels in the Yellow River Delta, China. This study also obtained the gene number of microbial CH4 metabolism via testing the soil metagenomes, and further investigated the key soil factors to determine the regulation mechanism. Spearman correlation analysis showed that the soil electrical conductivity, salt content, and Na+, and SO42− concentrations showed significantly negative correlations with the CO2 and CH4 emission rates, while the NO2−-N concentration and NO2−/NO3− ratio showed significantly positive correlations with the CO2 and CH4 emission rates. Metabolic pathway analysis showed that the mcrA gene for CH4 production was highest in low–salinity soils. By contrast, the relative abundances of the fwdA, ftr, mch, and mer genes related to the CO2 pathway increased significantly with rising salinity. Regarding CH4 oxidation processes, the relative abundances of the pmoA, mmoB, and mdh1 genes transferred from CH4 to formaldehyde decreased significantly from the control to the extreme–salinity plot. The greater abundance and rapid increase of methanotrophic bacteria compared with the lower abundance and slow increase in methanogenic archaea communities in saline–alkali soils may have increased CH4 oxidation and reduced CH4 production in this study. Only CO2 emissions positively affected CH4 emissions from low– to medium–salinity soils, while the diversities of CH4 production and oxidation jointly influenced CH4 emissions from medium– to extreme–salinity plots. Hence, future investigations will also explore more metabolic pathways for CH4 emissions from different types of saline–alkali lands and combine the key soil enzymes and regulated biotic or abiotic factors to enrich the CH4 metabolism pathway in saline–alkali soils.