Nitrogen availability does not decrease progressively under carbon dioxide enrichment

Background/Question/Methods

Elevated CO₂ concentration in atmosphere can stimulate plant growth and ecosystem carbon (C) sequestration, and consequently dampen further increase in atmospheric CO₂ concentration and global warming. However progressive nitrogen limitation (PNL) hypothesis posits that soil nitrogen (N) availability will gradually decrease due to more N sequestered in plant, litter and soil organic matter. Therefore, at the long-term time scales, the stimulations of plant growth and C sequestration would be limited. However, terrestrial N cycle includes many processes, and the PNL would probably not occur if increased N fixation and decreased N losses under elevated CO₂ result in long-term ecosystem N gains. In this study, a meta-analysis of 1018 lines of data from 175 published papers was conducted to test the hypothesis.

Results/Conclusions

Overall, although elevated CO₂ concentration stimulates available N efflux from soil to vegetation (uptake) and atmosphere (denitrification), it also simultaneously increases N fixation and reduces leaching. Together, neither total inorganic N, ammonium nor nitrate is reduced by atmospheric CO₂ enrichment regardless of the treatment duration. Our results suggest that CO₂ enrichment will not aggravate, even alleviate in some cases, the N limitation on plant growth and C assimilation, which means generally the PNL would not occur. In addition, the quantitative estimations of the responses of N processes to elevated CO₂ can also facilitate parameterization of Earth system models and improve their simulations and projections.