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 (NO3β) ratio. The changed NH4+/NO3β
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
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