The formation of soil organic matter (SOM), humification, encompasses a complex suite of biochemical processes. While microorganisms are known to be involved in humification, little information exists on how microbes regulate the conversion of plant material into humic substances. An often-overlooked influence of soil microorganisms on humification is that their bodies, or biomass, are primary building blocks of humic substances. Since microbial communities can differ substantially in their biochemical composition and activity across ecosystems, we hypothesize that soil microbial community composition influences rates of humification and the amount and stability of humic substances formed. Defining the role of microbial community composition will enhance our conceptual understanding of SOM formation, an important step in reliably predicting C dynamics in forest ecosystems. The fate of C in microbial bodies will be followed from four distinct fungal and bacterial groups isolated from a temperate and a tropical forest soil. The microbial groups (whole, dead cells) will be biochemically characterized, then reciprocally applied to each of the two soils to compare the SOM formation processes in two dissimilar forest ecosystems. The fate of microbial litter will be tracked for three years to determine which microbial group(s) forms the greatest amount and most stable SOM. This research will define the microbial controls on humification that are critical to sustainable forest production and environmental issues including global warming. The results will identify key characteristics of the microbial community that influence humification processes and its products, thereby substantially increasing our understanding of the mechanisms involved in C storage.
