Effect of drought on soil microbial carbon utilization efficiency of rhizosphere in moso bamboo forests

Clarifying the impact of drought on soil microbial composition and carbon utilization efficiency (CUE) would help reveal the mechanisms underlying its effects on soil microbial structure and function in moso bamboo forest. We examined the chemical properties, enzyme activities, microbial community structure and diversity of soil rhizosphere of moso bamboo in response to simulated drought from 2019 to 2023, and calculated the CUE of rhizosphere soil microorganisms to clarify the impact of drought on rhizosphere soil microbial CUE. The results showed that drought significantly reduced soil pH by 4.8%, total nitrogen by 33.5%, available nitrogen by 38.2%, available phosphorus by 33.0%, and cation exchange capacity by 24.6% on 2-year-old moso bamboo. Under the drought treatment, soil organic carbon in 2-year-old and 4-year-old moso bamboo was significantly decreased by 38.6% and 28.4%, respectively, while easily oxidizable organic carbon in 3-year-old moso bamboo was increased by 21.6%. The response of rhizosphere soil enzyme activity to drought varied with the age of bamboo. β-glucosidase activity of 1- to 4-year-old moso bamboo significantly decreased by 54.0%-78.1%, whereas the leucine aminopeptidase activities of 1-year-old moso bamboo increased by 40.7%. The acid phosphatase activity decreased significantly by 24.2% and 35.6% in 1- and 3-year-old bamboos, respectively, while that of 2-year-old bamboo increased by 44.2%. Drought significantly reduced microbial biomass carbon in the rhizosphere soil of bamboo across all age groups, with the most pronounced decrease being observed in 1-year-old group (46.3%). Soil microbial biomass nitrogen decreased by 5.8% to 33.7% in 1- to 4-year-old groups, with significant reductions in 1- to 3-year-old groups. Drought significantly reduced the Shannon and Simpson indices of soil bacteria (by 11.3% and 38.7%, respectively) as well as the Chao1 and Ace indices of fungi (by 23.0% and 22.5%, respectively) in the 1-year-old group, but did not affect α-diversity of soil microorganisms in other age classes. At the phylum level, the abundance of Proteobacteria decreased while that of Actinobacteria increased across all bamboo age groups, and the abundance of Ascomycota fungi generally increased. Under drought conditions, the microbial carbon use efficiency (CUE) in the rhizosphere of bamboos of all ages increased, with an increase ranging from 4.9% to 23.1%, and the highest CUE was observed in 1-year-old group. Structural equation modeling showed that soil microbial CUE was directly influenced by soil nutrient content, nitrogen cycle-related enzyme activities, and changes in microbial community composition, and was indirectly regulated by soil pH. In conclusion, drought significantly altered microbial community composition by modifying soil chemical properties, enzyme activities and increased soil microbial CUE, and such effect diminished with increasing bamboo age.