Simulated Climate Change Enhances Microbial Drought Resilience in Ethiopian Croplands but Not Forests

Climate change and land-use change represent a dual threat to terrestrial ecosystem functioning. In the tropics, forest conversion to agriculture is occurring alongside warming and more pronounced periods of drought. Rainfall after drought induces enormous dynamics in microbial growth (potential soil carbon storage) and respiration (determining carbon loss), affecting the ecosystem carbon budget. We investigated how legacies of drought and warming affected microbial functional (growth and respiration) and structural (16S and ITS amplicon) responses after drought. Rain shelters and open-top chambers (OTCs) were used to simulate drought and warming in tropical cropland and forest sites in Ethiopia. Rain shelters reduced soil moisture by up to 25 vol%, with a bigger effect in the forest, while OTCs increased soil temperature by up to 6°C in the cropland and also reduced soil moisture but had no clear effect in the forest. Soils from these field treatments were then exposed to a standardized drought cycle to test how microbial community traits had been shaped by the different climate legacies. Microbial growth started increasing immediately after rewetting in all soils, reflecting a resilient response and indicating that microbial communities perceived the perturbation as relatively mild. Fungi recovered faster than bacteria, and the recovery of fungal growth was generally accelerated in soils with a legacy of drought. Microbial community functions and structures were both more responsive in the cropland than in forest soils, and a legacy of drought particularly enhanced microbial growth and respiration responses in the cropland but not the forest. Microbial communities in cropland soils also used carbon with a higher efficiency after rewetting. Together, these results suggest contrasting feedbacks to climate change determined by land use, where croplands will be associated with mitigated losses of soil carbon by microorganisms in response to future cycles of drought, compared to forests where soil carbon reservoirs remain more sensitive.