Comments on the regulatory gate hypothesis and implications for C-cycling in soil

The paper by Kemmitt et al. [2008. Mineralization of native soil organic matter is not regulated by the size, activity or composition of the soil microbial biomass - a new perspective. Soil Biology and Biochemistry 40, 61-73] proposing the existence of an abiotic regulatory gate that controls the rate-limiting step of stabilised soil organic matter (SOM) mineralization, has initiated a fundamental and far-reaching debate. In this contribution the implications of a functioning abiotic regulatory gate are considered in the context of microbial community diversity and soil carbon cycling. I argue that although the evidence presented in support of the regulatory gate is strong, abiotic routes for SOM-mineralization function in parallel with biologically mediated mechanisms. Evidence is now accumulating that, in the presence of plant-inputs to soil, enhanced microbial mobilisation of SOM into biomass is a quantitatively important and ubiquitous process. I argue that this mineralization of SOM is fuelled by energy-rich substrates and is driven by microbial nutrient-demand. This implies that the mineralization of stabilised SOM and the turnover of C-inputs from current vegetation are intimately linked through the functioning of microbial communities associated with plants. This suggests that the microbial ‘eye of the needle’ is a crucial control-point in determining the carbon balance of soils. Fortunately, there are now excellent methods that allow quantification of SOM- and plant-derived C-fluxes through the members of soil microbial communities, and will also allow quantification of the relative importance of the abiotic and biotic routes of SOM-mineralization.