Inverse analysis of coupled carbon-nitrogen cycles against multiple datasets at ambient and elevated CO₂

Aims Carbon (C) sequestration in terrestrial ecosystems is strongly regulated by nitrogen (N) processes. However, key parameters that determine the degree of N regulation on terrestrial C sequestration have not been well quantified. Methods Here, we used a Bayesian probabilistic inversion approach to estimate 14 target parameters related to ecosystem C and N interactions from 19 datasets obtained from Duke Forests under ambient and elevated carbon dioxide (CO₂). Important Findings Our results indicated that 8 of the 14 target parameters, such as C:N ratios in most ecosystem compartments, plant N uptake and external N input, were well constrained by available datasets whereas the others, such as N allocation coefficients, N loss and the initial value of mineral N pool were poorly constrained. Our analysis showed that elevated CO₂ led to the increases in C:N ratios in foliage, fine roots and litter. Moreover, elevated CO₂ stimulated plant N uptake and increased ecosystem N capital in Duke Forests by 25.2 and 8.5%, respectively. In addition, elevated CO₂ resulted in the decrease of C exit rates (i.e. increases in C residence times) in foliage, woody biomass, structural litter and passive soil organic matter, but the increase of C exit rate in fine roots. Our results demonstrated that CO₂ enrichment substantially altered key parameters in determining terrestrial C and N interactions, which have profound implications for model improvement and predictions of future C sequestration in terrestrial ecosystems in response to global change.