Ecological approaches to characterizing soil organic matter stability
The ecosystem functions of soil organic matter (SOM) are a result of not only SOM quantity, but also SOM stability. SOM stability has important implications for the permanence and vulnerability of soil C stocks in response to disturbances such as climate change. It generally refers to how easily C and nutrients in SOM can be mineralized by the microbial population and its enzymes, and is a function of a variety of stabilization and destabilization mechanisms. SOM stability is implicitly embedded in current ecosystem models through the use of multiple compartments to reflect the heterogeneous nature of SOM with varying rates of decomposition. The problem, however, is that these are largely mathematical and statistical constructs, rather than mechanistic ones. The evolving challenge set by Schmidt et al. (2011, Nature) is the development of new quantitative, measurable and mechanistic methods for characterizing SOM stability. This presentation will provide an overview of several approaches for characterization of SOM stability, and will evaluate the degree to which these approaches can be associated with ecological processes.
The demand for high-throughput, cost-effective techniques to characterize SOM stability has generated a plethora of physical and chemical SOM fractionation approaches that quantify or isolate labile from stable pools of SOM. The shortcomings of these approaches, however, is that they are empirical and only weakly associated with ecological processes and that they frequently do not demonstrate patterns or outcomes that are consistent with our conceptual expectations. In spite of the artificial environment created by laboratory incubations, this approach does more closely simulate ecological processes of microbial mineralization. The shortcoming of laboratory incubations is that they need to be extended over long time periods to include a significant proportion of total SOM, making high sample throughput challenging. An evolving approach is to return to ecosystem energetics. In many cases, carbon has become the de facto proxy for energy in studies of ecosystem processes, but returning to an explicit focus on energy may provide new insights into SOM stabilization. In this approach: soil organic matter represents a substantial energy stock in soil ecosystems, microbial decomposition is an energy seeking/consuming process, and SOM stabilization processes represent energy barriers to decomposition.