Bacteria contribute more than fungi to SOC decomposition in a paddy field under long-term free-air CO₂ enrichment

Microbial responses to future climate change are important in determining soil organic carbon cycling and evaluating carbon-climate feedback. Paddy soils from a 15-year free-air CO2 enrichment (FACE) experiment were incubated and analyzed to reveal the responses of soil microbial activity, community diversity and composition to the soil depth and elevated CO2. Network topology analysis was conducted to determine microbial complexity and stability, and Mantel tests were used to analyze the correlation between bacteria and fungi and soil respiration. Elevated CO2 stimulated cumulative soil respiration (topsoil 6.2 %, subsoil 21.8 %), which was positively correlated with bacterial diversity. The elevated CO2 effects on the microbial community were greater in the topsoil than in the subsoil, namely, bacterial diversity was increased by 2.1 % in the topsoil (0–15 cm). Elevated CO2 also increased the abundance of Nitrospirota in the top- but not in the subsoil. Fungal diversity and phyla were not affected by elevated CO2, but fungal diversity was significantly correlated with the contents of soil DOC, total dissolved N, and total P in the subsoil. Compared to the subsoil, bacterial richness was higher in topsoil, and more Ascomycota was found but fewer Mortierellomycota; the microbial network had a greater number of nodes and edges. These results suggested that 1) depth was a major factor affecting soil properties that determine microbial community and function; 2) bacterial taxa were more sensitive to elevated CO2 than fungal taxa; 3) elevated CO2 increased SOC decomposition directly via enhanced soil C availability and altered bacterial diversity and microbial complexity and stability.