Minor carbon sequestration under nitrogen deposition due to downregulated nitrogen uptake and use efficiency

Global nitrogen (N) deposition substantially enhances ecosystem carbon cycling but usually results in minor carbon sequestration. The mechanisms underlying the minor stimulation of N deposition on carbon sequestration are not fully understood. Here, we used 22 sets of observations from a gradient N addition experiment with rates at 0, 2, 4, 8, 16, to 32 g N·m^-2·year^-1 in an alpine meadow ecosystem to constrain parameterization of the process-oriented Grassland ECOsystem (GECO) model. Our results indicate that the parameters related to plant N uptake and photosynthetic N use efficiency are proportionally downregulated with the rate of N addition. This is, the higher the rate of N addition, the larger the downward adjustment is in plant N uptake and use efficiency. GECO with parameter values not being adjusted to N treatments simulated higher annual GPP by 16.7 ± 7.1 %, 20.7 ± 6.7 %, 25.2 ± 8.2 %, 23.1 ± 7.0 %, and 49.5 ± 9.1 % under addition rates of 2, 4, 8, 16, and 32 g N·m^-2·year^-1, respectively, in comparison to these with parameter adjustment. Similarly, the ecosystem C storage simulated by GECO model without parameter adjustment was higher by 4.4 ± 2.5 % to 12.0 ± 3.0 % under these with parameter adjustment. Without adjustment of ecosystem physiological processes, such as the plant N uptake rate and use efficiency, Earth system models (ESMs) generally overestimate C uptake and storage under N deposition. Therefore, it is essential to incorporate these adjustments into ESMs to realistically predict global C dynamics under future N enrichment and its feedback to climate change.