Heat waves, defined as events associating high temperatures with severe drought, are expected to become increasingly recurrent. Research has focused heavily on the impacts of drought and temperature increase on soil functioning and microbial diversity, but little attention has been paid to soil microbial community responses to combined heat−drought stresses. Heat waves, which combine heat and drought stresses, may induce different microbial responses to those observed in studies focusing on heat or drought alone. Microbial recovery strategies to withstand heat−drought conditions, along with patterns of microbial functional redundancy and complex interactions with the soil physical−chemical−biological interface may have marked effects on soil ecosystem functioning, particularly in agroecosystems through the rhizosphere. To better understand how heat waves affect soil ecosystem functioning, we advocate the development of mechanistic approaches integrating individual to community level and biophysicochemical studies on the indirect effects of combined heat−drought stresses in microbial communities, observed through soil environment parameters in experimental and field studies. The challenge will be to define trait-based functional indicators of the microbial community response to heat waves, particularly the potential interrelatedness between the traits responsible for tolerance to drought and heat.

International audiencePurpose The purposes of this study were to identify the influence of a severe drought period on the impact of a subsequent heat–drought disturbance on the microbial community of a Mediterranean agricultural soil and particularly to highlight the long-term effects on the microbial catabolic profiles. Materials and methods We performed an experiment in microcosms and applied the MicroResp™ method on soil microbial communities. Results and discussion A 21-day combined heat–drought disturbance had less impact on soil microbial communities pre-exposed to a 73-day severe drought than on those that were not pre-exposed. These differences were observed not only for biomass and physiological traits (basal respiration, qCO 2), but also for catabolic microbial structure evolution during the recovery time. Conclusions These observations suggest that the physiological stress imposed by the initial severe drought changed the microbial catabolic structure or physiological state and favoured a portion of the microbial community best adapted to cope with the final heat–drought disturbance. Consequently, the initial severe drought may have induced a community tolerance to the subsequent heat wave. In this study, we also note that resilience was, more than resistance, an indicator of pre-exposure to stress. In the context of assessing the effects of extreme climatic events on soil microbial processes, these results suggest that future studies should take into account the historic stress of habitats and resilience parameters

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