Processes regulating progressive nitrogen limitation under elevated carbon dioxide: a meta-analysis.

Nitrogen (N) cycle has the potential to regulate climate change through its influence on carbon (C) sequestration. Although extensive researches have been done to explore whether or not progressive N limitation (PNL) occurs under CO₂ enrichment, a comprehensive assessment of the processes that regulate PNL is still lacking. Here, we quantitatively synthesized the responses of all major processes and pools in terrestrial N cycle with meta-analysis of CO₂ experimental data available in the literature. The results showed that CO₂ enrichment significantly increased N sequestration in plant and litter pools but not in soil pool. Thus, the basis of PNL occurrence partially exists. However, CO₂ enrichment also significantly increased the N influx via biological N fixation, but decreased the N efflux via leaching. In addition, no general diminished CO₂ fertilization effect on plant growth over time was observed. Overall, our analyses suggest that the extra N supply by the increased biological N fixation and decreased leaching may potentially alleviate PNL under elevated CO₂ conditions. Moreover, our synthesis showed that CO₂ enrichment increased soil ammonium (NH₄⁺) but decreased nitrate (NO₃^-). The different responses of NH₄⁺ and NO₃^-, and the consequent biological processes, may result in changes in soil microenvironment, community structures and above-belowground interactions, which could potentially affect the terrestrial biogeochemical cycles and the feedback to climate change. [ABSTRACT FROM AUTHOR]