Large latitudinal difference in soil nitrogen limitation on global vegetation response to elevated CO2

This study is the first to evaluate latitudinal characteristics of the coupled impacts of atmospheric-CO2 fertilization and soil nitrogen limitation on global plant photosynthesis for 2001-2019 by using a remote sensing-based light use efficiency model (DTECM-l-CN). It predicted that the global GPP should have increased by 0.40 Pg C a (-) (2) (P-MK < 0.001) from 2001 to 2019. Leaf area index change contributed 55.0% to this predicted increase in the global GPP trend, followed by coupled CO2 and soil nitrogen effects (32.5%) and the effects from the climate forcing (12.5%). Without CO2 fertilization effects, the global GPP trend was predicted lower by 32.5%. With CO2 fertilization effects included but without a nitrogen constraint, the model would predict global GPP trend higher by 67.5%. These contrasting changes strongly imply the need to consider all these effects simultaneously to estimate future GPP. Comparing the annual mean CO2 fertilization factor with and without soil nitrogen stress showed that the fertilization factor under soil nitrogen stress was only 62.5% of that with no soil nitrogen stress.As a result of soil nitrogen stress, annual mean CO2 response metric (R-CO2), indicating the CO2 fertilization effect, declined from 2002 to 2019. Soil nitrogen stress was found to attenuate the positive effect of CO2 fertilization on GPP trend with a large latitudinal difference, i.e., the tropics had a weaker CO2 fertilization effect than the extratropics. This study highlights that soil nitrogen has progressively become limiting on plant response to CO2 fertilization with a large latitudinal divergence.