The maximum rate of carboxylation (V_cmax) is a key photosynthetic parameter for gross primary production (GPP) estimation in terrestrial biosphere models. A set of observation-based V_cmax values, which take the nitrogen limitation on photosynthetic rates into consideration, are used in version 4.5 of the Community Land Model (CLM4.5). However, CLM4.5 with carbon-nitrogen (CN) biogeochemistry (CLM4.5-CN) still uses an independent decay coefficient for nitrogen after the photosynthesis calculation. This means that the nitrogen limitation on the carbon cycle is accounted for twice when CN biogeochemistry is active. Therefore, to avoid this double nitrogen down-regulation in CLM4.5-CN, the original V_cmax scheme is revised with a new one that only accounts for the transition between V_cmax and its potential value (without nitrogen limitation). Compared to flux tower-based observations, the new V_cmax scheme reduces the root-mean-square error (RMSE) in GPP for mainland China by 13.7 g C m⁻² yr⁻¹, with a larger decrease over humid areas (39.2 g C m⁻² yr⁻¹). Moreover, net primary production and leaf area index are also improved, with reductions in RMSE by 0.8% and 11.5%, respectively.

Citation: Wang, Y.-Y., Z.-H. Xie, B.-H. Jia, et al., 2015: Improving simulation of the terrestrial carbon cycle of China in version 4.5 of the Community Land Model using a revised V_cmax scheme, Atmos. Oceanic Sci. Lett., 8, doi:10.3878/AOSL20140090.

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