The interplay of soil spatial heterogeneity and water in stabilizing and destabilizing soil organic matter

Soil moisture is a main driver of soil organic matter dynamics of soil organic matter and is an important environmental variable in all models predicting changes in soil carbon stocks from site to global scales. Despite this, the mechanisms determining the response of heterotrophic soil respiration to soil moisture remain poorly quantified, being represented in most current carbon cycle models as simple empirical functions. Soils are extremely complex and heterogeneous environments and many properties observed at the profile or at the plot the scale are, in fact, determined by microscale conditions and processes.

The spatial heterogeneity of soil constituents and assemblages defines a myriad of contrasted micro-habitats, hosting diverse microorganisms, with contrasted moisture related characteristics and presumably contrasted levels of microbial activity. In addition, the respective spatial distributions of organic resources and microbial decomposers and the transfer rates between them, that depend on soil moisture, explain that similar organic compounds may have contrasted residence times in soil, being stabilized or not.

We consider how do soil moisture and soil spatial heterogeneity may explain the stabilisation of organic matter. Different pore size classes exhibit different rates of microbial decomposition, when considering different soils, across published studies and in a single laboratory experiment. Soil moisture affects the dynamics of biogeochemical hotspots, such as the detritusphere.

Regarding destabilization of soil organic matter, priming effect was revealed to be soil moisture dependent, that can be explained by varying access of newly produced enzymes to native soil organic matter. The Birch effect, a well-described flush of mineralization observed after rewetting dry soils, may also be partly explained at the microscale by changes in the local architecture of soils.

Overall, considering the interplay between soil spatial heterogeneity, soil moisture and the activity of microbial decomposers offers insights in understanding the stabilisation and de-stabilisation of soil organic matter. Incorporating this process level understanding into soil organic matter dynamics models is a challenge, as it requires the identification of relevant soil structure descriptors of these processes, at different time scales.