Does the accelerated soil N cycling sustain N demand of Quercus mongolica after decade-long elevated CO2 treatment?

The stimulation of plant growth and biomass accumulation by elevated CO2 may be limited by soil nitrogen (N) availability. However, our understanding of the response of soil N cycling to elevated CO2 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 CO2 on ecosystem carbon (C) and N dynamics in a Quercus mongolica (oak) dominated system in northeastern China. Elevated CO2 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 CO2. Thus, the supply of soil available N can sustain the tree growth stimulated by elevated CO2, 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 CO2 fertilization effect has weakened with increasing treatment duration. [ABSTRACT FROM AUTHOR]