Moisture as a regulator of microbial life in soil

Abstract: Climate change models predict an increase in the intensity and frequency of drought periods as well as precipitation events. Moisture and its fluctuations have a large impact on soil microorganisms, which are key drivers of the terrestrial carbon (C) cycle. When there is a drought period followed by a rainfall event there is a big CO2 release from soil to the atmosphere, which can dominate the C budget of some ecosystems. During this period, respiration and microbial growth have been shown to be transiently uncoupled. Earlier studies showed that microbial growth and respiration can respond in two different ways upon rewetting, resulting in differences in microbial carbon use efficiency (i.e., the fraction of used C allocated to growth) and resilience (i.e., the ability of microbial growth to recover to levels before the soils were disturbed). An understanding of how moisture and its fluctuations impact soil microbial communities is thus key to predict terrestrial ecosystem responses to ongoing global change. The aim of this thesis was to understand how soil microbial communities, and the processes they regulate, are affected by moisture and moisture fluctuations. Specifically, the objectives were to understand (1) what determines the two different microbial response patterns upon rewetting, (2) how the historical conditions microbial communities have been exposed shape their responses to drought and drying and rewetting (DRW) events, and (3) the differences in responses to drought and DRW events between the two major microbial groups, bacteria and fungi. It was found that (1) the conditions of the DRW disturbance as well as the microbial community’s ability to cope with DRW could affect microbial responses to DRW. In addition, individual studies did not show that historical conditions could shape microbial drought tolerance and responses to DRW. However, when taking all the results together with other preliminary results that cover a wider climate range, (2) historical conditions that microbial communities had been exposed to were important. A history of drier condition, as well as a history of higher soil disturbance resulted in more efficient and resilient responses upon rewetting. These results might be due to either (i) microbial adjustment to better cope with disturbances or (ii) differences in resource availability and quality due to differences in climate history or aboveground community. Finally, (3) fungi tolerated drought better than bacteria, and could be equally or more resilient than bacteria after a DRW event. In summary, to better predict how terrestrial ecosystems will respond to the increase of drought periods and precipitation events, ecosystem models should take into account that bacteria and fungi are differently affected by moisture. In addition, the harshness of the DRW disturbance as well as the previous conditions that microbial communities have been exposed to are important to determine their response to drought and DRW events as well as their carbon use efficiency.