Combined effects of atmospheric CO 2 and N availability on the belowground carbon and nitrogen dynamics of aspen mesocosms

Abstract

It is uncertain whether elevated atmospheric CO 2 will increase C storage in terrestrial ecosystems without concomitant increases in plant access to N. Elevated CO 2 may 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 CO 2 to 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 CO 2 (36.7 and 71.5 Pa) and at two levels of soil N (210 and 970 µg N g –1 ). Ambient and twice-ambient CO 2 concentrations 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 with 14 CO 2 and measured soil 14 CO 2 respiration and the 14 C contents of plants, soils, and microorganisms after a 6-day chase period. In conjunction with the August radio-labeling and again in October, we used 15 N 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 CO 2 significantly 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 CO 2 . CO 2 enrichment had no significant effect on the allometric distribution of biomass or 14 C among plant components, total 14 C allocation belowground, or cumulative (6-day) 14 CO 2 soil respiration. Elevated CO 2 significantly increased microbial 14 C contents, indicating greater availability of microbial substrates derived from roots. The near doubling of microbial 14 C contents at elevated CO 2 was 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 CO 2 , and were closely related to fine-root biomass. Gross N mineralization was not affected by elevated CO 2 . Despite significantly greater rates of N immobilization under elevated CO 2 , standing pools of microbial N were not affected by elevated CO 2 , 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 CO 2 .