Drought changed soil organic carbon composition and bacterial carbon metabolizing patterns in a subtropical evergreen forest.

Subtropical forests are considerable carbon sinks in the northern hemisphere, yet are increasingly suffering from the impact of extreme drought. To better understand the dynamics and kinetics of forest soil carbon storage under long-term drought, a rainfall-reduction experiment was established in a subtropical evergreen forest of eastern China. Soil organic carbon (SOC) composition, microbial carbon metabolism and the interactions with soil microbial community structure were investigated across different soil aggregate size fractions. After five years' treatment of rainfall reduction, a significant loss of large macroaggregates, as well as an increase of microaggregates by over 100% was observed. Meanwhile, drought changed the composition of SOC, reducing the non-hydrolyzed carbon and humin contents in large- to medium-size macroaggregates. Microbial metabolizing capacity of polymeric compounds was also reduced especially in the above aggregate fractions, whereas the utilization of small-molecular compounds was more impacted in small macroaggregates and microaggregates. The changes in carbon metabolizing patterns were further associated with the abundance changes of specific microbial taxa, revealing the microbially mediated mechanism of soil carbon metabolism under long-term drought. In addition, carbon metabolism in microaggregates was particularly sensitive to the changes of soil moisture, suggesting long-term drought may continually influence the functional resistance of the microbial communities. Taken together, our results provide insights into how biotic and abiotic processes together influence the SOC metabolizing processes, continued monitoring and investigation of which shall contribute to better understanding of the dynamics and kinetics of SOC storage under the impact of long-term drought.