Document: TAM-3-66-12

Functional response of rhizosphere microbial communities at the Duke and Oak Ridge FACE experiments.

LONG, T.M.* and R.L.SINSABAUGH

University of Toledo, Toledo, OH 43606, USA. ¹

Abstract:
As the primary interface linking producers and decomposers, changes in the structure and/or function of the rhizosphere are key in predicting the response of intact ecosystems to increased atmospheric CO₂. In order to assess the impact of a rising CO₂ fraction on rhizosphere function, we have been monitoring changes in the activities of extracellular enzyme systems that mediate transformations of detrital C, N, and P at two different FACE experimental sites: A loblolly pine (Pinus taeda) plantation at Duke forest (3 years), and a sweetgum (Liquidamber styraciflua) plantation at Oak Ridge National Lab (ORNL, 1 year). At Duke, CO₂ enrichment increases the activities of - and -glucosidase (G, G), cellobiohydrolase (CBH), -xylosidase (X), N-acetyl-glucosaminidase (NAG), and leucine-aminopeptidase (LAP), indicating an increased demand for nitrogen and stimulation in the turnover of labile polysaccharides. This may be due to the effect of "C priming" increasing the availability of labile carbon due to fine root turnover. However, reduced activities of phenol oxidase (PO) and peroxidase (Px) suggest that CO₂ enrichment may inhibit the decomposition of more recalcitrant compounds such as lignin and humus. Taken together with substrate utilization profiles, the Duke data indicate a functional shift in the rhizosphere community toward more opportunistic bacterial activity in response to CO₂ enrichment. At ORNL, enzyme activities were markedly different from those observed at Duke. Our first year's data at ORNL showed that CO₂ enrichment increased the activities of PO and Px while reducing the activities of N-acquiring (NAG, LAP) and cellulolytic (G, G, X, CBH) enzymes. These results suggest that the direction and magnitude of microbial response to CO₂ enrichment will vary regionally and across forest types and may depend, in part, on initial nutrient status. The observed trends suggest that the Oak Ridge system may be less responsive to C priming, perhaps due to higher initial nutrient availability, and may respond over the long term by decreasing soil C storage. However, in nutrient limited, low C quality systems such as Duke forest, soil C storage may increase.

Keywords: global change, FACE, microbial community, rhizosphere, microbial enzymes, Duke forest, Oak Ridge

This abstract is being presented at: 10:45 AM in session:
Oral Session #40: Elevated CO₂ In Forest Systems.