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

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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.