Autochthonous and Zymogenous Pools of Soil Microbial Biomass: Continuation of Debate

Newly proposed Regulatory Gate Hypothesis described mineralization of soil organic matter (SOM) as a two step process, first abiological and second biological, and thus contradicted conventional view on SOM mineralization involving autochthonous/zymogenous populations. We assessed (i) growth rate after adding glucose, (ii) preferential utilization of glucose vis-a-vis soil organic carbon for energy, (iii) tendency for energy conservation, and (iv) ability to decompose complex compounds of microbial biomass in seven forest soils to experimentally support the conventional view. Three soils (Lappwald, Zierenberg and Goettingerwald) were mildly acidic (pH 5. 2 to 5. 5; Al³⁺ 2 to 11%; base saturation 87 to 97%) and more suitable for proliferation of zymogenous population and other four (Harste, Unterluess, Spanbeck and Soiling) were highly acidic (pH 3. 7 to 4. 4; Al³⁺ 66 to 83%; base saturation 7 to 21%) and more suitable for autochthonous population. Our results suggested that there were indeed two pools of soil microbial biomass, one autochthonous and the other zymogenous in nature and that both the pools existed together. However, their relative distributions varied with soil conditions. We have also revisited discussions and methodology used for development of Regulatory Gate Hypothesis and have argued that (i) unexplained trends of CO₂ evolution from fumigated soils that provided a basis of formulation of the hypothesis can be explained through conventional autochthonous/zymogenous approach. Additionally, the experimental evidences in support of this hypothesis could be the result of inadvertent chemical reactions. Therefore, for present we conclude that the conventional approach is adequate to explain mineralization of organic matter in soil.