Is the rate of mineralization of soil organic carbon under microbiological control?

Abstract

A theory called the Regulatory Gate Hypothesis was previously proposed to considers that the rate limiting step in soil organic carbon (SOC) mineralization is independent of the size, community stricture or specific activity (mg CO2-C evolved g−1 biomass C) of the soil microbial biomass. Here we report new experiments to test this hypothesis. In the first experiment, six different soils were perfused with CHCl3-saturated water to model SOC release and to stop microbial activity. Apart from one highly organic soil, they all released SOC at low and roughly constant rates, over sixty three days. In the second experiment, when the freeze-dried perfusates were returned to the parent soils, their % mineralization ranged from 17 to 35% over ten days, in contrast to bulk SOC (range 0.46–0.77%). In the third experiment, two soils were given three consecutive fumigations, each followed by 10 days aerobic incubation. The microbial biomass was decreased by > 90%, yet SOC mineralization proceeded at the same rate as in nonfumigated soil. In the fourth experiment, the six soils were subjected to various perturbations, including non-perturbed controls, fumigation-incubation, air-drying rewetting, freeze-thaw (−20 °C and −80 °C) and sieving < 0.3 mm. After an initial flush due to the perturbations, the rates of mineralization became roughly equal in nearly all soil treatments and comparable to the control, despite significant declines in biomass. This shows that basal respiration was little affected by the perturbations. In Experiment five the effects of the perturbations on the microbial communities in the different soils and perturbations were determined. The bacterial community was significantly modified by both fumigation and air drying-rewetting, due mainly to increased Firmiculites and decreased Proteobacteria populations. Our findings suggest that mineralization of SOC is a two-stage process: firstly, non-bioavailable forms are converted abiologically to bioavailable forms (termed the Regulatory Gate), which, only then, undergo second process, biological mineralization. This finding has serious implications for theories of e.g. SOC dynamics, effects of global warming and soil nutrient cycling.