Increased microbial carbon and nitrogen use efficiencies under drought stress in a poplar plantation

• Drought increased enzyme activity C:N and fungi:bacteria ratios. • Microbial C and N use efficiencies increased under drought. • The threshold elemental ratio values were larger than the soil C:N ratio. Drought has been observed to decrease the accumulation of carbon (C) and nitrogen (N) in forest ecosystems. The microbial C use efficiency (CUE) and N use efficiency (NUE) are critical to elemental cycling in terrestrial ecosystems, which can be calculated by the C:N stoichiometry of soils, microbial biomass, and extracellular enzyme activities. However, little is known about how drought affects microbial CUE and NUE. We extracted soil samples at different depths (0–15 cm, 15–30 cm, and 30–45 cm) in forest stands that were subjected to 30% and 50% throughfall reduction manipulation experiments, in contrast to those obtained from stands that were left undisturbed from July 2019 to April 2020 in a poplar plantation of Eastern China. Under drought conditions, we found that the soil moisture, soil C:N ratio, and C- and N-acquiring enzyme activities decreased; however, the soil pH, microbial biomass C:N ratio, ecoenzymatic activity C:N ratio, and fungi:bacteria ratio increased. Microbial CUE and NUE, which were calculated based on these C:N stoichiometric ratios, also increased. These responses were similar at different soil depths across sampling dates. Our structural equation models indicated that drought-induced increases in CUE and NUE were driven by changes in the soil moisture, fungi:bacteria ratio, and ecoenzymatic activity C:N ratio. Our results suggested that decreases in soil moisture and increases in the ecoenzymatic activity C:N and fungi:bacteria ratios could stimulate microbial CUE and NUE.