ELEVATED ATMOSPHERIC CO2 AND FEEDBACK BETWEEN CARBON AND NITROGEN CYCLES

We tested a conceptual model describing the influence of elevated atmo spheric CO2 on plant production, soil microorganisms, and the cycling of C and N in the plant-soil system. Our model is based on the observa tion that in nutrient-poor soils. plants (C3) grown in an elevated CO2 atmosphere often increase production and allocation to belowground st ructures. We predicted that greater belowground C inputs at elevated C O, should elicit an increase in soil microbial biomass and increased r ates of organic matter turnover and nitrogen availability. We measured photosynthesis, biomass production, and C allocation of Populus grand identata Michx. grown in nutrient-poor soil for one field season at am bient and twice-ambient (i.e., elevated) atmospheric CO2 concentration s. Plants were grown in a sandy subsurface soil i) at ambient CO2 with no open top chamber, ii) at ambient CO2 in an open top chamber, and i ii) at twice-ambient CO2 in an open top chamber. Plants were fertilize d with 4.5 g N m 2 over a 47 d period midway through the growing seaso n. Following 152 d of growth, we quantified microbial biomass and the availabilities of C and N in rhizosphere and bulk soil. We tested for a significant CO2 effect on plant growth and soil C and N dynamics by comparing the means of the chambered ambient and chambered elevated CO 2 treatments. Rates of photosynthesis in plants grown at elevated CO2 were significantly greater than those measured under ambient condition s. The number of roots, root length, and root length increment were al so substantially greater at elevated CO2. Total and belowground biomas s were significantly greater at elevated CO2. Under N-limited conditio ns, plants allocated 50-70% of their biomass to roots. Labile C in the rhizosphere of elevated-grown plants was significantly greater than t hat measured in the ambient treatments; there were no significant diff erences between labile C pools in the bulk soil of ambient and elevate d-grown plants. Microbial biomass C was significantly greater in the r hizosphere and bulk soil of plants grown at elevated CO2 compared to t hat in the ambient treatment. Moreover, a short-term laboratory assay of N mineralization indicated that N availability was significantly gr eater in the bulk soil of the elevated-grown plants. Our results sugge st that elevated atmospheric CO2 concentrations can have a positive fe edback effect on soil C and N dynamics producing greater N availabilit y. Experiments conducted for longer periods of time will be necessary to test the potential for negative feedback due to altered leaf litter chemistry.