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

Abstract. The nitrogen (N) cycle has the potential to regulate climate change through its influence on carbon (C) sequestration. Although extensive research has explored 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 the 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 the plant and litter pools but not in the soil pool, partially supporting one of the basic assumptions in the PNL hypothesis that elevated CO₂ results in more N sequestered in organic pools. However, CO₂ enrichment significantly increased the N influx via biological N fixation and the loss via N₂O emission, but decreased the N efflux via leaching. In addition, no general diminished CO₂ fertilization effect on plant growth was observed over time up to the longest experiment of 13 years. 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 in spite of the increases in plant N sequestration and N₂O emission. Moreover, our syntheses indicate that CO₂ enrichment increases soil ammonium (NH₄⁺) to nitrate (NO₃⁻) ratio. The changed NH₄⁺/NO₃⁻ ratio and subsequent biological processes may result in changes in soil microenvironments, above-belowground community structures and associated interactions, which could potentially affect the terrestrial biogeochemical cycles. In addition, our data synthesis suggests that more long-term studies, especially in regions other than temperate ones, are needed for comprehensive assessments of the PNL hypothesis.