COS 124-9 - Long-term carbon exclusion alters soil microbial function but not community structure across forests of contrasting productivity

Thursday, August 10, 2017: 10:50 AM
D132, Oregon Convention Center
Stephen C. Hart, Life & Environmental Sciences and Sierra Nevada Research Institute, University of California, Merced, CA, Nicholas C. Dove, Environmental Systems Graduate Program, University of California Merced, Merced, CA and John M. Stark, Department of Biology, Utah State University, Logan, UT

While it is well-documented that distinct heterotrophic microbial communities emerge under different conditions of carbon (C) availability (i.e., copiotrophs vs. oligiotrophs), the response of soil microbial communities and their function to long-term (> 5 y) conditions of C exclusion in situ has yet to be investigated. We evaluated the role of C in controlling soil microbial communities and function by experimentally excluding plant C inputs for six years at four forest sites along a productivity gradient in Oregon, USA. Carbon exclusion treatments were implemented by preventing belowground inputs by root trenching to a depth of 30 cm using 25-cm diameter steel pipe, and minimizing aboveground inputs as plant litter by covering the pipe with a 1-mm mesh screen. After six years, we measured rates of gross and net nitrogen (N) transformations and microbial respiration in situin the upper 15-cm of mineral soil in both C excluded plots and undisturbed control soils. In the laboratory, we measured the soil total C and N concentration and potential extracellular enzyme activities. We used phospholipid fatty acid (PLFA) analysis to determine potential changes in the microbial community structure.


Six years of C exclusion reduced soil total C by about 20%, except at the highest productivity site where no statistically significant change was observed. Although PLFA community structure and microbial C were unchanged, microbial respiration was reduced by 15-45% at all sites. Similarly, specific extracellular enzyme activities (enzymatic activity/microbial biomass) for all enzymes increased at these sites with C exclusion, suggesting that the microbial communities were substrate-limited. Although gross N mineralization decreased under C exclusion, decreases in gross N immobilization were greater, resulting in increased net N mineralization rates in all but the lowest productivity site. Furthermore, C exclusion only increased net nitrification in the highest productivity site. Overall, our results suggest that C exclusion, while not significantly affecting the microbial community structure (measured by PLFA), impacts microbial function in higher productivity sites, resulting in increased extracellular enzyme activity and increased soil N availability. Although these field-based results are consistent with previous laboratory studies indicating a strong coupling between C and N biogeochemical cycles, they build upon this earlier research by suggesting that the “C connection” to the N cycle depends on the rate of C cycling within the ecosystem.