Microbial Guilds Track Resource Availability: Compositional Shifts with Litter Carbon Depletion and Reintroduction.

A chemically protected pool of soil organic matter contributes to soil carbon storage. It has been hypothesized that specific guilds of microbes grow on labile SOM, while others grow on recalcitrant SOM. However, previous studies have shown similar average levels of microbial respiration of fresh litter amended to fresh soil and long term incubated soil, which indicates that microbial guilds may not have shifted as expected based on the expected responses to long term incubation without labile carbon addition. The objective of our study is to elucidate the response of specific guilds of microbes to depletion and reintroduction of fresh SOM. Soil samples (0-10 cm depth) were collected and incubated for 707 days. Experimental treatments included no tillage, conventional tillage and native forest. After 707 days, samples were amended with fresh litter and harvested. Microbial diversity was measured using 454 FLX titanium pyrosequencing of rRNA gene. Quality trimmed data were processed on a bioinformatics pipeline and sequences were clustered at 97% similarity. Significant differences were found in microbial community composition after the long term incubations. Field treatments and incubation significantly affected (p<0.005**) both bacterial and fungal communities. Carbon depleted microbes were able to effectively capture fresh labile carbon, but only fungal community showed significant change in community structure (p<0.005**). This study demonstrated that soil fungal community composition was able to track resource availability over time. During incubation without a labile carbon source, the fungal guild capable of degrading labile carbon that was originally present appeared to shift to a guild capable of growth on recalcitrant carbon pools. With the addition of fresh litter, a presumably dormant surviving subset of the fungal guild adapted for growth on labile carbon proliferated. Thus, the responses of fungal guilds may be more important than bacteria in understanding microbial guilds responsible for degradation of different carbon pools.