Abiotically Controlled Solubilization of Soil Organic Matter, the Hidden Perspective.

Dissolved organic matter (DOM) is a small fraction of the total soil OM that contributes considerably to many soil biochemichal processes due to its mobility and highly reactive nature. The flux of DOM is considered to be largely responsible for controlling microbial activity and turnover, especially at greater soil depth where DOM is an important substrate. DOM is important in controlling  biogeochemistry of nutrient cycling, metal availability and C stabilization in soil. Despite the growing body of information about the sources of DOM in soil, the processes involved in the production of DOM within the soil are poorly understood. Recently, Kemmitt et al (2008) challenged the well-established Winogrdsky’s theory, proposing the “regulatory gate” hypothesis. Based on this hypothesis, soil OM is solubilized during a two-step process. In the first step, non-bioavailable compounds are transformed to bioavailable OM as a result of abiotic processes. During the second step, the altered (bioavailable) humified compounds become metabolites for the microbial community. This hypothesis has been questioned mainly because of its emphasis on the role of abiological factors in controlling the bioavailability of OM. The difficulties in distinguishing biotic and abiotic sourced DOM have hindered our understanding of DOM processes in soil. We used C-13 isotope labeled plant material and DOM to evaluate the source of C for DOM production to address the premise of the “regulatory gate” hypothesis. We monitored the production of the DOM from added sources of OM and extant soil OM in the presence or absence of the microbial community. The objective of this study was to evaluate the role of microbial activity on the net production of DOM from fresh OM source to test the assumption of abiotic control of DOM release proposed by the regulatory gate hypothesis.