Drought intensity effects on grassland plant communities and soil microbial community function

Plant and soil communities are intimately connected. Plants shape soil microbial community composition through their resource acquisition strategies and via root carbon (C) inputs, which has cascading effects on biogeochemical cycles. Drought has been shown to disrupt the connection between plants and soil microorganisms. However, the effects of drought intensity on soil microbial community functioning, including the uptake of recent plant-derived C, are largely unknown. Here, we determined how two plant communities with contrasting resource acquisition strategies (acquisitive versus conservative) responded to a gradient of drought (control, and eight drought intensities). Using a ¹³C pulse labelling approach, we tracked C allocated from plants to soil and its uptake by the microbial community. We measured potential extracellular enzyme activity as a proxy of microbial community functioning. We hypothesized that (1) drought responses are non-linear, and (2) acquisitive plant communities have lower drought resistance but recover faster than conservative plant communities, which is reflected in lower ¹³C transfer and reduced microbial functioning during drought and increases after drought. In general, we found that the responses we measured were non-linearly related to drought intensity. After three weeks of drought, drought intensity decreased aboveground net primary productivity (ANPP) of both plant communities. Soil extractable organic ¹³C decreased with increasing drought intensity, indicating that less recently assimilated C was allocated to root exudation. Although microbial biomass remained stable over the drought intensity gradient, ¹³C uptake into microbial biomass decreased at peak drought, and was lower in the conservative vs. acquisitive plant community at mild drought levels. Potential enzyme activity of β-1,4-glucosidase, involved in cellulose breakdown, and  β-N-acetyl-glucosaminidase, involved in chitin breakdown, decreased with increasing drought intensity. Urease activity was higher in conservative than acquisitive plant communities exposed to drought. Seven days after re-wetting, we found that microbial uptake of ¹³C increased along the drought gradient and was higher than the control in communities previously subjected to high drought intensities. This fast microbial recovery could affect nutrient mobilisation, which could underlie longer-term plant community recovery. Two months after re-wetting, we indeed found that plant communities that had previously experienced high drought intensity (> 75% soil water deficit) had higher ANPP than the control. We conclude that drought intensity has significant non-linear effects on microbial uptake of recent plant C and on potential extracellular enzyme activities both during drought and recovery, with consequences for plant community recovery dynamics.