The interactive effect of land-use and soil depth on microbial activity during drying and rewetting – an experimental and computational investigation

The alternation of drought periods and rainfall events, intensified by climate change, has huge impacts on carbon cycle dynamics. Changes in soil moisture induce significant releases of CO₂ from soils to the atmosphere. This phenomenon, known as the Birch effect, is accompanied by drastic changes in the microbiology as well. Based on the response patterns of microbial growth and respiration to the rewetting of dry soil, two different types have been identified. Microbial communities either respond immediately after rewetting and start increasing growth in a linear way (so-called “type 1” response), or they recover growth after a lag phase preceding an exponential increase (“type 2” response). The reasons behind the different responses, including how harsh the drought is perceived by the communities and what history of moisture conditions they were subjected to, are not yet fully resolved. Moreover, most studies focus on the top few centimeters of soil and the effect of depth and the contribution of deeper soils to the overall dynamics have been largely overlooked.

In order to investigate the influence of depth on microbial dynamics during drought and rainfall events, taking into account land-use, we performed a set of laboratory experiments that were also used to parameterize and validate numerical modelling-based analysis of the ecology driving soil biogeochemistry. We collected soil samples from permanent pasture and tilled and cropped arable fields at two different depths (0-5 cm and 20-30 cm). We then subjected them to a week of air drying followed by rewetting to optimal moisture, and measured respiration, bacterial growth and fungal growth at high temporal resolution.

The patterns were significantly different between soil types, showing type 1 responses in arable soils and type 2 responses in pasture soils. The type 1 responses in arable soils were also characterized by a higher carbon use efficiency after the rewetting perturbation. Moreover, the deeper microbial communities were relatively more affected by the drying and rewetting experiment than the respective shallow ones. Taken together, these results suggested that the drying and rewetting event was perceived by soil microbial communities as a stronger disturbance in the pasture soils, and at deeper depths, as illustrated by more sensitive microbial responses.

We then incorporated these laboratory data into a soil microbial model (EcoSMMARTS) and identified the depth- and community-specific differences in osmolyte regulation, necromass turnover, and cell residue activity as the microbial mechanisms potentially explaining the observed patterns. These findings provide insights into soil-climate feedback from different ecosystems, where intensively used arable soils were more resilient than permanent pasture soils and stored larger amounts of carbon due to a higher fraction of microbial growth to respiration under climate change scenarios. The capacity of microbial communities to adapt and regulate soil carbon dynamics is not uniform through the soil profile nor across management practices, therefore indicating a need for future studies incorporating depth and especially land-use which has the strongest effect on microbial activity during soil drying and rewetting.