title: Combined effects of atmospheric CO2and N availability on the belowground carbon and nitrogen dynamics of aspen mesocosms.
abstract: It is uncertain whether elevated atmospheric CO2will increase C storage in terrestrial ecosystems without concomitant increases in plant access to N. Elevated CO2may alter microbial activities that regulate soil N availability by changing the amount or composition of organic substrates produced by roots. Our objective was to determine the potential for elevated CO2to change N availability in an experimental plant-soil system by affecting the acquisition of root-derived C by soil microbes. We grew Populus tremuloides (trembling aspen) cuttings for 2 years under two levels of atmospheric CO2(36.7 and 71.5 Pa) and at two levels of soil N (210 and 970 µg N g-1). Ambient and twice-ambient CO2concentrations were applied using open-top chambers, and soil N availability was manipulated by mixing soils differing in organic N content. From June to October of the second growing season, we measured midday rates of soil respiration. In August, we pulse-labeled plants with14CO2and measured soil14CO2respiration and the14C contents of plants, soils, and microorganisms after a 6-day chase period. In conjunction with the August radio-labeling and again in October, we used15N pool dilution techniques to measure in situ rates of gross N mineralization, N immobilization by microbes, and plant N uptake. At both levels of soil N availability, elevated CO2significantly increased whole-plant and root biomass, and marginally increased whole-plant N capital. Significant increases in soil respiration were closely linked to increases in root biomass under elevated CO2. CO2enrichment had no significant effect on the allometric distribution of biomass or14C among plant components, total14C allocation belowground, or cumulative (6-day)14CO2soil respiration. Elevated CO2significantly increased microbial14C contents, indicating greater availability of microbial substrates derived from roots. The near doubling of microbial14C contents at elevated CO2was a relatively small quantitative change in the belowground C cycle of our experimental system, but represents an ecologically significant effect on the dynamics of microbial growth. Rates of plant N uptake during both 6-day periods in August and October were significantly greater at elevated CO2, and were closely related to fine-root biomass. Gross N mineralization was not affected by elevated CO2. Despite significantly greater rates of N immobilization under elevated CO2, standing pools of microbial N were not affected by elevated CO2, suggesting that N was cycling through microbes more rapidly. Our results contained elements of both positive and negative feedback hypotheses, and may be most relevant to young, aggrading ecosystems, where soil resources are not yet fully exploited by plant roots. If the turnover of microbial N increases, higher rates of N immobilization may not decrease N availability to plants under elevated CO2.
authors.affiliation: School of Natural Resources and Environment, The University of Michigan, Ann Arbor, MI 48109, USA, , , , , , US.
authors.forename: C J
authors.initials: CJ
authors.lastname: Mikan
authors.affiliation: School of Natural Resources and Environment, The University of Michigan, Ann Arbor, MI 48109, USA, , , , , , US.
authors.forename: D R
authors.initials: DR
authors.lastname: Zak
authors.affiliation: School of Forestry and Wood Products, Michigan Technological University, Houghton, MI 49931, USA, , , , , , US.
authors.forename: M E
authors.initials: ME
authors.lastname: Kubiske
authors.affiliation: School of Forestry and Wood Products, Michigan Technological University, Houghton, MI 49931, USA, , , , , , US.
authors.forename: K S
authors.initials: KS
authors.lastname: Pregitzer
keywords: C cycle; Key words Atmospheric CO2; N cycle; Populus tremuloides Michx.; Rhizodeposition