Soil aggregate and microbial traits mediate soil organic carbon accumulation in a paddy field under long-term...

Climate change is increasingly impacting ecosystem functions and soil microorganisms, altering the accumulation and stability of soil organic carbon (SOC). However, the effect of climate change on SOC dynamics at the aggregate scale is poorly understood. Here, we conducted a 13-year experiment to investigate how elevated CO₂ (+200 ppm) and warming (+2°C) affect microbial community and necromass, and potential enzyme activities within soil aggregates in a paddy field. Results showed that long-term elevated CO₂ and warming significantly increased SOC and total nitrogen contents but reduced soil pH and total phosphorus (TP) within all three aggregate fractions. The SOC and TP contents were the most closely related to changes in microbial communities in aggregates. Elevated CO₂ and warming significantly increased microbial activities, specially increasing both fungal and bacterial abundances, leading to increases in C- and N-potential enzyme activities in large macroaggregate and microaggregates. In contrast, P-acquiring enzyme activities and the stoichiometry for P:N enzyme ratio significantly decreased with elevated CO₂ and warming. Furthermore, climate change effects on the microbial necromass varied across aggregate sizes. Elevated CO₂ significantly increased fungal necromass C and its contribution to SOC in large macroaggregates, while warming increased bacterial necromass C and its contribution to SOC in microaggregates. The SOC was positively correlated with microbial necromass and fungal necromass C in macroaggregates. The highest microbial necromass C in large macroaggregates may be attributed to higher fungal abundance and lower microbial enzyme activities. These results suggest that climate change may regulate SOC dynamics by affecting microbial growth and activity in soil aggregates. Our study highlights that soil aggregates stratify microbial communities and necromass, which improve our understanding the responses of agricultural ecosystem C cycling to future climate change.