Effect of elevated CO2 on ecosystem nitrogen fluxes and pools: Meta-analysis

Background/Question/Methods: It’s well known that increasing carbon dioxide concentration in the atmosphere could be dampened through carbon sequestration in terrestrial ecosystems. However, it is not well quantified how much terrestrial carbon sequestration would be limited by nutrients, especially nitrogen. The conventional nitrogen limitation theory predicts that as available nitrogen in the soil can’t meet the demand of plant growth, nitrogen limitation happens to ecosystem carbon sequestration.  Thus, it is critical to evaluate how soil nitrogen availability and related processes are affected by elevated CO₂. Here we used the meta-analysis method to synthesize 471 case studies to evaluate effects of elevated CO₂ on nitrogen pools in plant, litter and soil and 320 case studies for nitrogen fluxes, including mineralization, fixation, leaching, and denitrification.

Results/Conclusions: Our results showed that elevated CO₂ did not significantly affect soil available nitrogen pool but influenced soil nitrogen fluxes, plant and litter nitrogen pools to varied degrees.  For example, nitrogen leaching decreased by 32.3% and nitrogen fixation increased by 40.2% under elevated CO₂ in comparison with those under control. Thus, the soil available nitrogen pool gained more nitrogen input from fixation but lost less nitrogen through leaching at elevated than ambient CO₂. Moreover, more nitrogen was extracted from soil to plant as the aboveground plant nitrogen content increased by 7% and belowground plant nitrogen content increased by 18% under elevated CO₂ in comparison with those under ambient CO₂. Our data on various aspects of the whole soil nitrogen cycle all indicated that nitrogen cycle is probably much more open and dynamic than we thought and nitrogen limitation is unlikely to fully eliminate CO₂ stimulation of carbon sequestration in terrestrial ecosystems.