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

Abstract

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.