Effect of elevated CO2 and soil nitrogen availability on plant C allocation and soil C turnover from a whole-plant mesocosm experiment

Elevated atmospheric carbon dioxide (CO₂) concentrations can have a positive effect on plant growth, and are also expected to alter the plant belowground allocation of photosynthetically fixed carbon (C). The release of root-derived exudates can potentially stimulate soil microbial activity and the turnover of existing soil C. However, soil organic matter decomposition response to elevated CO₂ may vary depending on the level of available nutrients, particularly nitrogen (N), which is the most limiting nutrient. To investigate the combined effect of CO_2­ and available N on rhizodeposition and soil C turnover, a greenhouse mesocosm experiment was conducted. A total of 64 hornbeam (Carpinus betulus L.) trees were exposed to ambient (400 ppm, aCO₂) or elevated (580 ppm, eCO₂) concentrations of CO₂ at ¹³C-enrichment of 100 permil, in order to trace C in the system. Two levels of N were applied to the soils in the form of ¹⁵N-labelled ammonium nitrate (NH₄NO₃). Above and belowground C fluxes were continuously monitored for partitioning of soil heterotrophic and autotrophic respirations. After one growing season, the trees and soils were destructively harvested. The trees were separated into buds, leaves, branches, stem and roots, which were analysed for CN and their respective isotopic compositions. Soils were separated into rhizosphere and bulk soils, and were analysed for CN, isotopic compositions, microbial biomass and enzyme activities. We observed an increase in the belowground C allocation under eCO₂ and trees showed a higher active fine root growth response to eCO₂. Consequently, the overall microbial biomass in the rhizosphere as well as the fraction of microbial biomass C derived from roots increased under eCO₂. Enzyme activities, especially those of β-Glucosidase and N-acetyl-β-d-glucosaminidase, increased, while phosphatase and peroxidase activities decreased under eCO₂. Despite the stimulated microbial activities, changes in soil C were not observed. Furthermore, we did not find any interactive effect of available N with eCO₂, which suggested that N availability did not strongly influence belowground C allocation by trees.