Dry–wet cycles induce the decoupling of carbon and nitrogen mineralization at high temperatures in semi-arid grassland soil

Soil carbon (C) and nitrogen (N) transformations are typically interlinked because of the conserved elemental stoichiometry of microbes, which can be remarkably altered by intensified temperature variations and dry-wet (DW) cycles. However, the interactive effects of temperature and DW cycles on soil C and N transformations and how microbial communities mediate these processes remain largely unknown. In this study, we subjected a semi-arid grassland soil from northern China to four successive DW cycles at 15, 25, and 35 °C. The soil respiratory rates during incubation, labile C and N fractions, activities of C and N acquisition enzymes, community-level physiological profiles, and microbial community composition at the end of the incubation were determined. Soil respiration rates decreased with drying but sharply increased with wetting, particularly at higher temperatures. Soil DW cycles did not significantly affect cumulative C mineralization and net N mineralization at 15 °C. However, they decreased cumulative C mineralization by 24.5% and 23.4% and increased net N mineralization by 48.6% and 41.7% at 25 and 35 °C, respectively, indicating a decoupling of C and N mineralization at higher temperatures. As the temperature increased, soil DW cycles decreased cellobiohydrolase and peroxidase activities and the microbial use of amino acids and lipids but increased N-acetyl-β-glucosaminidase activities and the microbial use of saccharide polymers and recalcitrant amines. Moreover, soil DW cycles increased the abundances of gram-positive bacteria and actinomycetes and gram-positive-to-gram-negative bacteria ratios with increasing temperature, particularly at 35 °C. Overall, our findings suggest that DW cycles at higher temperatures decelerated the mineralization of organic soil C by decreasing cellobiohydrolase and peroxidase activities. However, DW cycles at higher temperatures improved N mineralization by shaping the microbial communities to excrete more recalcitrant-organic-N acquisition enzymes, thereby meeting their high N requirements.