Combined effects of atmospheric CO2 and N availability on the belowground carbon and nitrogen dynamics of aspen mesocosms

It is uncertain whether elevated atmospheric CO2 will increase C storage in terrestrial ecosystems without concomitant increases in plant access to N. Elevated CO2 may alter microbial activities that regulate soil N availabil ity by changing the amount or composition of organic substrates produced by roots. Our objective was to determine the potential for elevated CO2 to ch ange N availability in an experimental plant-soil system by affecting the a cquisition 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 mu g N gl). Amb ient and twice-ambient CO2 concentrations were applied using open-top chamb ers, and soil N availability was manipulated by mixing soils differing in o rganic N content. From June to October of the second growing season, Lye me asured midday rates of soil respiration. In August, we pulse-labeled plants with (CO2)-C-14 and measured soil (CO2)-C-14 respiration and the C-14 cont ents of plants, soils, and microorganisms after a 6-day chase period. In co njunction with the August radio-labeling and again in October, we used N-15 pool dilution techniques to measure in situ rates of gross N mineralizatio n, N immobilization by microbes, and plant N uptake. At both levels of soil N availability, elevated CO2 significantly increased whole-plant and root biomass, and marginally increased whole-plant N capital. Significant increa ses in soil respiration were closely linked to increases in root biomass un der elevated CO2. CO2 enrichment had no significant effect on the allometri c distribution of biomass or C-14 among plant components, total C-14 alloca tion belowground, or cumulative (6-day) (CO2)-C-14 soil respiration. Elevat ed CO2 significantly increased microbial C-14 contents, indicating greater availability of microbial substrates derived from roots. The near doubling of microbial C-14 contents at elevated CO2 was a relatively small quantitat ive change in the belowground C cycle of our experimental system, but repre sents an ecologically significant effect on the dynamics of microbial growt h. 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 fin e-root biomass. Gross N mineralization was not affected by elevated CO2. De spite significantly greater rates of N immobilization under elevated CO2, s tanding pools of microbial N were not affected by elevated CO2, suggesting that N was cycling through microbes more rapidly. Our results contained ele ments of both positive and negative feedback hypotheses, and may be most re levant to young, aggrading ecosystems, where soil resources are not yet ful ly exploited by plant roots. If the turnover of microbial N increases, high er rates of N immobilization may not decrease N availability to plants unde r elevated CO2.