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

Abstract

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 <italic>Populus tremuloides</italic> 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 <super>14</super>CO₂ to trace the flow of C into soil and its metabolism by microorganisms. <super>15</super>Nitrogen pool-dilution techniques were simultaneously used to quantify <italic>in situ</italic> rates of N cycling. Elevated CO₂ increased the flow of C belowground, as evidenced by greater rates of sod respiration. Microbial <super>14</super>C contents (elevated 0.30 +/- 0.06 vs. ambient 0.18 0.03 MBq chamber<super>--1 </super>) and rates of N immobilization (elevated 450 +/- 64 vs. ambient 123 15 mg N chamber<super>--1</super> d<super>--1</super>) 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 <italic>in situ</italic> 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.