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 (CO) and methane (CH) fluxes. However, the relative importance of microorganisms in the regulation of CH emissions under elevated salinity remains unclear. Here, we report the composition of CH 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 CH 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 SO concentrations showed significantly negative correlations with the CO and CH emission rates, while the NO-N concentration and NO/NO ratio showed significantly positive correlations with the CO and CH emission rates. Metabolic pathway analysis showed that the mcrA gene for CH production was highest in low-salinity soils. By contrast, the relative abundances of the fwdA, ftr, mch, and mer genes related to the CO pathway increased significantly with rising salinity. Regarding CH oxidation processes, the relative abundances of the pmoA, mmoB, and mdh1 genes transferred from CH 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 CH oxidation and reduced CH production in this study. Only CO emissions positively affected CH emissions from low- to medium-salinity soils, while the diversities of CH production and oxidation jointly influenced CH emissions from medium- to extreme-salinity plots. Hence, future investigations will also explore more metabolic pathways for CH emissions from different types of saline-alkali lands and combine the key soil enzymes and regulated biotic or abiotic factors to enrich the CH metabolism pathway in saline-alkali soils.