Drying intensity and acidity slow down microbial growth recovery after rewetting dry soils

Soil microbes perceive drying and rewetting (DRW) events as more or less harsh depending on the previous soil moisture history. If a DRW event is not perceived as harsh, microbial growth recovers rapidly after rewetting (referred to as ‘type 1’ response), while a harsh DRW will be followed by a delayed growth recovery (‘type 2’ response). Predicting these responses based on pedoclimatic factors is important because they can determine how carbon is partitioned between growth (soil C stabilization) and respiration (C loss to the atmosphere). To characterize the microbially perceived harshness between the two extreme types 1 and 2, and its pedoclimatic drivers, we described microbial growth with a single logistic function and respiration with a rescaled gamma distribution using ∼100 growth and respiration datasets. These functions captured microbial growth and respiration rates well during the recovery phase after rewetting. Therefore, the fitted parameters from these functions could help us to capture the continuum of microbial recovery between type 1 and 2 and characterize harshness levels. The product of growth parameters τ (delay time) and b (the slope of the growth curve at time τ) was an effective index that could capture and quantify perceived harshness because it allowed separating type 1 and 2 responses better than τ or b alone or than any other parameter describing the growth or respiration response. The drier the soil before rewetting and the lower the pH, the higher was the perceived harshness ($\tau \times b$), the longer the delay of growth recovery, and the larger the CO₂ loss at rewetting. Overall, this study places soil microbial responses to DRW along a continuous gradient from fast to slow recovery, where the faster the recovery, the better adapted the microbial community is to the DRW event.