Long-term throughfall exclusion reduced soil organic carbon towards higher soil microbial carbon use efficiency and lower microbial enzyme activities in Phyllostachys edulis plantations

Abstract. Soil microbial carbon utilization efficiency (CUE) serves as a crucial metric for evaluating the effectiveness with which microbes assimilate organic carbon, acting as a vital benchmark for assessing the potential of soil carbon sequestration. Previous studies have shown that drought can significantly affect microbial CUE, which is crucial for the carbon cycle in forest ecosystems. However, the mechanisms of microbial CUE on soil carbon stability under drought conditions are not well understood. In this study, a throughfall exclusion experiment was conducted in subtropical Phyllostachys edulis plantations (The control, CK; throughfall exclusion, T). The results showed that drought increased microbial CUE by 9.12% (p<0.05) and 2.56% in 0–10 cm and 10–20 cm soil layers, respectively. Soil organic carbon (SOC), soil particulate organic carbon (POC), soluble organic carbon (DOC), and easily oxidized organic carbon (EOC) all significantly decreased, while the proportion of mineral-associated organic carbon (MAOC) in SOC increased by 4.1% and 6.3% (p<0.05) at two layers, respectively. Microbial CUE was positively correlated with POC/SOC ratio and MAOC/SOC ratio, indicating that variation of SOC components substrate quality was an important factor driving microbial physiological changes. Structural equation model (SEM) further showed that soil polyphenol oxidase (PPO) and cellobiohydrolase (CBH) enzymes were the main factors driving changes in microbial CUE. Our results suggested that drought indirectly regulates the storage and transformation of SOC by affecting microbial community structure and function, which would have a profound impact on the carbon cycle of forest ecosystems.