Arctic Soil C and N Cycling Are Linked With Microbial Adaptations During Drought

Climate change increases the frequency and intensity of drought events, yet the mechanisms of microbe-mediated soil carbon (C) and nitrogen (N) cycling under drought are poorly understood. We conducted a microcosm experiment with a Greenlandic soil subjected to five levels of drought, reducing water content from 180% to 15% over the course of 3 weeks followed by rewetting, mimicking a natural drought event. We linked changes in microbial gene expression related to stress response as well as C and N cycling with greenhouse gas (GHG) emissions, extracellular enzyme activities, and soil C and N status. Maximum changes in gene expression occurred at intermediate levels of drought (80% water content), characterized by acclimation of microbial physiology to drought conditions, including production of osmolytes as well as cell wall and membrane modifications. This peak in gene expression changes marked a tipping point associated with a pronounced decline in microbial respiration as well as extracellular enzyme activities under more intense drought conditions. Interestingly, C-cycling gene expression correlated with soil dissolved organic nitrogen (DON), NH₄ ⁺, NO₃ ^- and PO₄ ^3- contents. Moreover, N-cycling gene expression correlated with PO₄ ^3- contents and with the activity of laccases. These findings highlight linkages between microbial C, N, and P cycling because of stoichiometric constraints under drought. 24 h after rewetting, we found a shift in microbial expression of C usage genes towards more labile compounds, and an increase in genes related to anabolic activity and signaling, but no signatures of stress responses, suggesting that the microbial community had overcome rewetting-induced changes in osmotic pressure and allocated metabolic activity to growth. Overall, we show that microbial physiological drought responses and microbial resource usage related to C:N:P stoichiometry are key mechanisms of C and N cycling in the Arctic soil under drying and rewetting.