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.