Comments on 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 & Biochemistry 40, 61–73]: The biology

Kemmitt et al. (Kemmitt, S.J., Lanyon, C.V., Waite, I.S., Wen, Q., Addiscott, T.M., Bird, N.R.A., OʹDonnell, A.G., Brookes, P.C., 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 & Biochemistry 40, 61–73) recently proposed the “Regulatory Gate” hypothesis, which states that decomposition of soil organic matter (SOM) is regulated solely by abiotic factors. Without studying the mechanisms of such regulation, Kemmitt with coauthors challenged the classical Winogradsky theory of soil microbiology and questioned the concept of autochtonous and zymogenous microbial populations. In this letter, we revive the significance of microbial activity for SOM decomposition especially for the short-term (hours to weeks) processes and show that the “Regulatory Gate” is (micro)biologically driven. lain the results of the three experiments in Kemmitt et al. (2008) from a microbiological point of view and suggest that SOM decomposition is mainly regulated by exoenzymes. We criticize the abiotic Regulatory Gate hypothesis based on bottleneck processes and pools limiting the SOM decomposition rate, comparison of constant and changing environmental conditions, as well as the connection between community structure and functions. We explain the results of Kemmitt et al. (2008) according to the properties of soil microbial community: functional redundancy and inconsistency between the excessive (but largely inactive) pool of total microbial biomass and the real mineralization activity. Finally, we suggest that to gain new perspectives on SOM decomposition and many other biochemical processes, future studies should focus on hot spots of (micro)biological activity (i.e., the rhizosphere, drillosphere, detritosphere, biopores, etc.) rather than on the bulk soil.