Physical vs. physiological controls on water-stress in soil microbial communities

Background/Question/Methods

Soil heterotrophic respiration and nutrient mineralization are strongly affected by environmental conditions, in particular by soil moisture. As soil moisture decreases, decomposers’ activity slows down, with micro-fauna generally undergoing stress sooner than bacteria and fungi. This environmental control is typically modeled using soil moisture-dependent ‘correction factors’ that decrease the potential microbial growth or mineralization rate. The shape of these factors varies widely across biogeochemical models and generally lacks a clear physical or biological interpretation. Particularly important is the value of soil moisture where activity (and the correction factors used to model it) becomes negligible. Based on laboratory observations, this ‘water-stress threshold’ has been hypothesized to depend upon microbial community composition, with fungal-dominated, drought-adapted communities exhibiting lower soil moisture at the stress thresholds (corresponding to more negative water potentials). Here we test this hypothesis using soil respiration vs. soil water potential curves obtained from nearly 30 published studies spanning humid to semi-arid conditions.

Results/Conclusions

Despite observed differences in the responses of individual decomposer groups to moisture availability, we show that responses of soil decomposers at the community level are similar across biomes and climates, and hence across a range of different microbial communities, resulting in a nearly constant water-stress threshold. In mineral soils, the stress threshold corresponds to a water potential of about -16 MPa, while in surface litter it nearly reaches -40 MPa. In mineral soils such a threshold is shown to be comparable to the soil moisture value where solute diffusion becomes strongly inhibited. We thus conclude that intrinsic physical constraint rather than biological adaptation control the respiration-soil moisture relationships in dry soils. Because of the lack of adaptation of microbial communities to different hydro-climatic regimes, changes in rainfall patterns (primary drivers of the soil moisture balance) may have dramatic impacts on soil carbon and nutrient cycling.