Nitrogen cycling in forest soils under elevated CO2: response of a key soil nutrient to climate change

Forests under elevated atmospheric CO₂ concentration as a result of climate change are expected to require more available nitrogen (N) to sustain the enhanced CO₂ uptake for photosynthesis and C storage. Therefore, it is essential to evaluate how CO₂ fumigation of forests will affect availability of N to trees. Main pathways to sustain the high N demand are increasing biological N fixation (BNF), increasing N turn-over and reducing N losses. The purpose of this research is to explore the effects of elevated CO₂ on soil N cycling in a temperate forest under the Birmingham Institute of Forest Research (BIFoR) Free Air Carbon Dioxide Enrichment facility. We hypothesize that under CO₂ fertilization, trees will allocate more carbon belowground to enhance microbial activity for increasing N mineralization as well as enhancing BNF to meet N demands. We also hypothesize that the subsequent microbial activity will up-regulate N₂O and N₂ emissions due to denitrification. BNF by free-living organisms is investigated using the ¹⁵N assimilation method. Mineralization and N gas production rates is determined using the ¹⁵N pool dilution and ¹⁵N-Gas flux method at the same time. Early results are showing trends toward an enhancement of N mineralization and microbial N immobilization rates. However, BNF in the forest floor is hardly detectable more likely because of the high N deposition in the area; therefore, it doesn’t appear to be responsive to CO₂ fumigation. This research is expected to help us improve our understanding of the changes and magnitude of nutrient availability and gaseous losses under future climates.