"Soils contain the largest pool of carbon on Earth. Cellulose is a major constituent of this carbon since it is a key component of plant structural carbon. Members of the bacteria and fungi are essential for degrading cellulose and thus are essential for cycling carbon. However, the majority of our knowledge on cellulose degradation has been restricted to organisms that we can grow in the laboratory. The reliance on these growth-based methods to understand soil function can be misleading. My previous molecular-based studies, DNA-stable isotope probing (SIP), identified active cellulolytic bacteria and fungi in a collection of different soils, some of which were previously known to degrade cellulose along with other, putatively novel groups. However these studies did neither delaminate the specific bacterial and fungal contributions to cellulose degradation in these soils nor did they reveal systematic differences in the time course of cellulose degradation among these active groups. Furthermore, the putative novel cellulolytic fungal and bacterial groups warrant follow-up work to ascertain if these groups were responsible for the primary breakdown of cellulose and were not only detected as the result of cross feeding. In order to overcome these issues and expand on my previous SIP work, I propose to use SIP with 13C-cellulose in combination with Halogen In Situ Hybridization-Secondary Secondary Ion Mass Spectrometry (HISH-SIMS) to determine and to quantify the in situ contributions of active cellulolytic microorganisms in the soil environment in a previously unachievable manner. The expected scientific outcome include: (1) system-level understanding of the metabolic networks of cellulose degradation; (2) basic understanding of the degree and efficiency cellulose is degraded by microorganisms; (3) improved understanding of primary and secondary responding populations to cellulose; and (4) better characterization of the seemingly unknown groups of cellulose degraders."