The Mechanism of Mineralisation of Soil Organic Matter by the Soil Microbial Biomass – Introducing the ‘Regulatory Gate' Hypothesis.

Soil organic matter (SOM) is formed from decomposition of organic residues entering soil by the soil microbial biomass (biomass).  SOM ranges from freshly produced microbial metabolites to carbon forms which, following repeated recycling, are essentially biologically inert (Jenkinson et al., 1992).  Understanding the behaviour of the biomass is essential if we are to understand SOM dynamics.  Unlike organic residues, SOM probably has no definable structure.  Instead it functions as a random product of microbial metabolism, existing in largely non-polymeric forms.  However, many functional groups can be determined, e.g. carboxyl, alkyl, aryl, hydroxyl, amino.  This lack of regular polymeric structure (highly unfavourable for enzymic degradation) largely gives soil organic matter its resistance to microbial degradation, coupled also with physical protection, e.g. being sorbed onto of soil minerals surface or held within soil aggregates.  Nevertheless, stabilised soil organic matter is slowly mineralised by the biomass.   Here we introduce the ‘Regulatory Gate’ hypothesis to account for the observation that the rate of mineralisation of non-biomass SOM is independent of the biomass size, community structure or activity.  This stems from the pioneering work of Jenkinson (1966) and Jenkinson and Powlson (1976) who developed the Fumigation-Incubation method to measure the biomass.  They showed that, following 24 h fumigation with CHCl₃ then fumigant removal, the fumigated soil mineralised SOM at the same rate as a non-fumigated control soil.  This is despite the biomass in the fumigated soil being only about 5 to 10 % of that in the non-fumigated soils and of very different diversity and activity.  To explain for this, we suggest that the rate-limiting step in SOM mineralisation is not, as previously considered,  controlled by endo- or exocellular enzymic activity but by abiotic processes e.g. diffusion, desorption and chemical oxidation.  This work has important implications in increasing our understanding of soil organic matter dynamics.