Nitrogen cycling in forest soils under elevated CO2: response of a key soil nutrient to global 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 (FACE) facility. We hypothesize that under CO₂ fertilization, trees will allocate more carbon belowground to enhance microbial activity resulting in an increase of all nitrogen fluxes (i.e. N mineralisation, N fixation and N gas emissions). Net mineralisation rates were measured in-situ every month over 10 months and gross mineralisation rates were measured in-situ in spring, summer and autumn using the ¹⁵N pool dilution method. BNF by free-living organisms was investigated using the ¹⁵N assimilation method and N₂O production rates were measured using the ¹⁵N-Gas flux method. Net mineralisation was increased on average by 30% under elevated CO₂, delivering an extra 24 kgN.ha^-1.y^-1, and by 80% during the budburst period (April). Gross ammonification and NH₄⁺ immobilisation rates were also slightly higher under elevated CO₂ by respectively 33% and 19%. Yet, nitrification, NO₃ immobilisation and N₂O emission rates were not affected, demonstrating that, ammonium and nitrate cycling responded differently to elevated CO₂. In addition, under elevated CO₂, soils were more concentrated in DOC, DON and ammonium (p<0.05). Soil respiration and fine root biomass were also significantly higher by respectively 25% and 40%. Together, these findings suggest that trees allocate more C belowground through a higher root production and exudation enhancing N mineralisation under elevated CO₂. Yet, the effects of elevated CO₂ on N cycling are focused on increasing soil NH₄⁺ availability as NO₃^- availability, N₂O emission and N₂ fixation were not responsive to eCO₂. Collectively, our results suggest that trees are able to control and shape the nitrogen cycling communities to cope with N limitation. Nonetheless, it is difficult to predict if that will be sufficient to delay or alleviate progressive nitrogen limitation under future climate.