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.
Results/Conclusions
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.