Drought legacies on soil respiration and microbial community in a Mediterranean forest soil under different soil moisture and carbon inputs

Soil moisture can strongly affect the fate of soil organic carbon (C) during microbial decomposition, but the characterisation and prediction of the effects remain challenging, especially the long-term effects of drought history and its interaction with current levels of soil moisture. We investigated the legacy effects of drought on soil activity and microbial community composition and its interaction with actual soil moisture and C addition. Soils from a long-term drought field experiment in a forest were incubated for 80 days under two levels of soil moisture after the addition of 13C-labelled glucose or cellulose. The drought legacy manifested as significantly higher soil CO2 efflux in wet soils from the long-term drought plots than in soils from all other treatments, including the historical control plots, as well as by a higher respiration in the dry treatment, but not in the wet one when cellulose was added. The supply of glucose primed the decomposition of SOM during the whole incubation whereas the supply of cellulose caused a negative priming at the very early stage but had an overall positive priming effect. The composition of the bacterial community varied with soil moisture, but the fungal community was more resistant to water stress and acquired labile C more efficiently under low moisture levels. Fungi dominated cellulose decomposition and bacteria dominated glucose decomposition. These results suggest a key role of fungi in SOM decomposition in the often water-stressed Mediterranean ecosystems. The legacy effects of long-term drought may increase soil respiration during the periods when soils are wet, although the effect may vary with the type of the C inputs. © 2021 Elsevier B.V.