Illuminating Microbial Contributions To Soil Carbon Cycling Dynamics Using High Resolution Stable Isotope Probing

Abstract

Terrestrial carbon (C) represents the largest active global C pool. Microbes are estimated to mediate ~80-90% of soil C-cycling, yet the complexities of the soil ecosystem have limited our ability to disentangle specific microbial contributions. We are still grappling with the importance of microbial community structure and function to ecosystem processes. Diversity in microbial communities may not be important to contemporary ecosystem processes, but maintaining microbial diversity may prove to be of the utmost importance to prevent disruption of ecosystem processes as the climate and environmental conditions change. Here, I use high resolution stable isotope probing (HR-SIP) to examine the variations of C substrate use in soil when composition and timing of amendment additions are varied. I found that decomposition of C substrates occurs in a successional pattern characterized by the use of low molecular weight compounds at an early stage, followed by the use of high molecular weight compounds later in time. Specifically, xylose (low molecular weight) was metabolized quickly (within 7 days) while cellulose was metabolized more slowly (2+ weeks). This succession of decomposition was accompanied by transitions in the microbial community: fast-growing spore formers responded quickly and assimilated xylose-C, while slow-growing microorganisms responded slowly and assimilated cellulose-C. I also found the amount of cellulose decomposed largely depended on the composition of accompanying amendments but not on the phylogenetic composition of cellulose utilizers. While the total amount of cellulose decomposed varies with differences in amendment composition, the pattern of cellulose decomposition over time (from beginning to end of incubation) is the same, albeit with varying magnitude, regardless of amendment composition. Together, these results demonstrate that cellulose decomposition is not disrupted by varying amendment conditions or phylogenetic affiliation of cellulose responders. This means that biodiversity maintains ecosystem function as environmental conditions fluctuate and a decrease in biodiversity could result in unforeseen suspension or loss of ecosystem functions.