Resistance of shallow soil carbon to both short-term isolated and recurrent droughts in two semi-arid grasslands

Increasing drought events may impair carbon (C) storage in terrestrial ecosystems, but the impacts of isolated vs. recurrent droughts on soil C and its drivers remain unclear. We studied how isolated drought (66% reduction in growing season precipitation over two years) and recurrent drought (the same reduction applied to plots that had previously experienced a 4-year drought followed by two recovery years) influence shallow soil C and its plant-related drivers in two semi-arid grasslands. Despite a one-third decline in soil moisture under both droughts, the responses of above- and belowground C pools and species richness were limited and inconsistent, and soil C remained stable across years, sites, and treatments. Soil C resistance to these droughts was unrelated to the resistance of above- and belowground C pools in the first year of this study. In the second year, the resistance of aboveground forb C pool and belowground C pool promoted soil C resistance under isolated and recurrent drought, respectively. Relationships were observed only at one site, where soil C resistance increased with greater resistance of aboveground biomass, forb, and belowground C pools under isolated drought but decreased with higher species richness under recurrent drought. Overall, short-term isolated or recurrent drought events may not alter shallow soil C when plant-related C drivers show limited negative responses. However, caution is warranted when extrapolating these results, as soil C changes slowly and its response to drought may differ over time and across grassland types.