Abiotic processes dominate soil organic matter mineralization: Investigating the regulatory gate hypothesis by inoculating a previously fumigated soil with increasing fresh soil inocula

Abstract

The Regulatory Gate Hypothesis proposes that the rate of soil organic C mineralization is largely controlled by abiotic processes rather than biological ones. To further test this hypothesis, a repeatedly fumigated soil was inoculated with increasing weights of the initial fresh soil (0, 2, 10 and 25%) and then incubated for 30 days. The CO2-C evolved, soil microbial biomass C and adenosine 5′-triphosphate were measured at days 1, 5, and 30. The fresh soil was incubated as a control. Our aim was to further test the Regulatory Gate hypothesis that the rate of mineralization of stabilized soil organic matter is not regulated by the size, activity or community structure of the microbial biomass.Cumulative CO2 evolved changed little, irrespective of weight of inoculum, in the inoculated soils during the incubation. Soil respiration rates of inoculated soils (21 µg CO2-C g−1 soil) were significantly higher than those of non-inoculated-fumigated soil (18 µg CO2-C g−1 soil) and fresh soil (18.5 µg CO2-C g−1 soil) at the beginning of the incubation (day 0–1). However, they were not significantly different (6.5 µg CO2-C g−1 soil in inoculated soils) from those in non-inoculated-fumigated and fresh soil (5.5 and 7.1 µg CO2-C g−1 soil respectively) at the end of the incubation. Soil adenosine 5′-triphosphate (ATP) proportionally increased in the inoculated soils, but remained much lower (0.49–0.78 nmol g−1 soil) than that in fresh soil (2.28 nmol g−1 soil) until the end of the incubation. The recoveries of the microbial communities and diversity in the inoculated soils were proportional to the inoculation rates. Proteobacteria, mainly Gammaproteobacteria, had the highest recovery in inoculated soils after incubation, and Nitrospira the lowest. We conclude that soil organic matter mineralization is not regulated by the size or composition of the soil microbial population, but mainly depends on the conversion of non-bioavailable SOC to bioavailable substrate C. The precise processes involved now require further research to improve our understanding of the mechanisms driving the mineralization of soil organic matter.