Feedback effects of elevated atmospheric carbon dioxide on the belowground cycling of carbon and nitrogen

Given that N availability limits plant growth in many temperate environments, it is uncertain whether elevated atmospheric CO2 will stimulate ecosystem productivity without concomitant changes in soil N availability. Elevated CO2 may alter microbial activities that regulate N availability by changing the amount or composition of organic substrates entering sod in plant fitter. The objective of my study was to describe effects of elevated CO2 on C and N cycling in sods of low and high fertility. I grew Populus tremuloides cuttings for two years in open-bottom root boxes containing sods of low and high N availability. Ambient and twice-ambient CO2 concentrations were applied using open-top chambers. Rates of sod respiration were measured repeatedly. Plants were labeled with 14CO2 to trace the flow of C into soil and its metabolism by microorganisms. 15Nitrogen pool-dilution techniques were simultaneously used to quantify in situ rates of N cycling.; Elevated CO2 increased the flow of C belowground, as evidenced by greater rates of sod respiration. Microbial 14C contents (elevated 0.30 +/- 0.06 vs. ambient 0.18 0.03 MBq chamber--1 ) and rates of N immobilization (elevated 450 +/- 64 vs. ambient 123 15 mg N chamber--1 d--1) indicated that elevated CO2 increased the supply and/or quality of organic substrates used by microorganisms. However, standing pools of microbial N were unaffected, suggesting that microbial N may be turning over more rapidly under elevated CO2.; There was no evidence to suggest that increased rates of microbial activity under elevated CO2 changed N availability to plants. Plants grown under elevated CO2 contained 17 % more N at harvest than ambient-grown plants, and in situ rates of N uptake by plants were stimulated by elevated CO2. The capacity of plants to forage for soil N was enhanced under elevated CO2 by greater fine-root biomass. Taken together, my results illustrate that N immobilization is only one factor controlling N availability under elevated CO2. This may be especially relevant to rapidly aggrading ecosystems, in which roots have not yet fully exploited soil resources, and microbial dynamics are not yet in equilibrium with inputs of plant fitter.