Does the accelerated soil N cycling sustain N demand of Quercus mongolica after decade-long elevated CO 2 treatment?

The stimulation of plant growth and biomass accumulation by elevated CO 2 may be limited by soil nitrogen (N) availability. However, our understanding of the response of soil N cycling to elevated CO 2 and when progressive N limitation occurs remains limited. Here, we used an open top chamber experiment to examine the effects of 10 years of elevated CO 2 on ecosystem carbon (C) and N dynamics in a Quercus mongolica (oak) dominated system in northeastern China. Elevated CO 2 increased oak biomass, C and N stocks and C/N by 26.4, 26.2, 16.5 and 8.6% respectively, which suggests increased plant N demand. Soil gross N mineralization, re-mineralization of microbial N and nitrification were accelerated likely due to increased photosynthesis (by 34.9%) and microbial biomass (by 24.2%) under elevated CO 2 . Thus, the supply of soil available N can sustain the tree growth stimulated by elevated CO 2 , and to date progressive N limitation has not happened. Nevertheless, both the annual increase of oak biomass, C and N stocks and C/N ratio and the seasonal variations of soil available N and microbial N concentrations, and net N transformation rates indicated that gradual N deficiency may be occurring and the CO 2 fertilization effect has weakened with increasing treatment duration.