Effects of land use, short-term drought and warming on microbial responses to drying and rewetting

Climate change predicts an increase in temperature and an intensification of the hydrological cycles resulting in more extreme drought and rainfall events. When dry soils experience a rainfall event, there is a big CO₂ release from soil to the atmosphere which is regulated by soil microorganisms. In the present study, we set out to investigate how drought and warming affects the soil microbial responses to drying and rewetting (DRW); and how those responses are affected by differences in land use. Previous work has shown that exposure DRW cycles in the laboratory and in the field can induce faster recovery (more ‘resilient’) of the microbial responses after a DRW cycle. In addition, a history of drought has been suggested to result in microbial communities with higher carbon use efficiency (CUE) during DRW in a wet heathland in Northern Europe and in semi-arid grasslands in Texas. We wanted to extend these observations to subtropical environments.

 

With the aim of simulating drought and warming, rain shelters and open top chambers (OTC) were installed in Northern Ethiopia in 2 contrasting land-uses (a degraded cropland and a pristine forest) for 1.5 years. Soils were then sampled and exposed to a DRW cycle in the laboratory. Microbial growth and respiration responses were followed with high temporal resolution over 3 weeks, as well as, changes in microbial community structure.  

 

Microbial functions universally showed a resilient response after a DRW cycle, with bacterial growth and fungal growth increasing immediately upon rewetting linked with the expected respiration response. The field treatments and land-use differences, therefore, did not have an effect on the resilience of soil microbial communities to DRW cycles. There were differences between the two main decomposer groups: fungi were more resilient than bacteria, as they showed a faster recovery rate. Microbial CUE upon rewetting responded differently in the different field treatments and land-uses. CUE was generally higher in croplands than in forests. Besides, while simulated drought reduced CUE, simulated drought increased CUE. These differences might be explained by either (i) the selection or more efficient microbial communities due to a higher exposure to DRW events or (ii) differences in resource availability (i.e. plant input).