Carbon-nitrogen interactions in idealized simulations with JSBACH (version 3.10)

Recent advances in the representation of soil carbon decomposition and carbonnitrogen interactions implemented previously into separate versions of the land surface scheme JSBACH are here combined in a single version, which is set to be used in the upcoming 6th phase of coupled model intercomparison project (CMIP6). Here we demonstrate that the new version of JSBACH is able to reproduce the spatial variability in the reactive nitrogen-loss pathways as derived from a compilation of delta N-15 data (R = 0. 76, root mean square error (RMSE) = 0. 2, Taylor score = 0. 83). The inclusion of carbonnitrogen interactions leads to a moderate reduction (-10 %) of the carbon-concentration feedback (beta L) and has a negligible effect on the sensitivity of the land carbon cycle to warming (gamma L) compared to the same version of the model without carbonnitrogen interactions in idealized simulations (1 % increase in atmospheric carbon dioxide per year). In line with evidence from elevated carbon dioxide manipulation experiments, pronounced nitrogen scarcity is alleviated by (1) the accumulation of nitrogen due to enhanced nitrogen inputs by biological nitrogen fixation and reduced losses by leaching and volatilization. Warming stimulated turnover of organic nitrogen further counteracts scarcity. The strengths of the land carbon feedbacks of the recent version of JSBACH, with beta L = 0. 61 Pg ppm(-1) and gamma L = -27. 5 Pg C-1, are 34 and 53 % less than the averages of CMIP5 models, although the CMIP5 version of JSBACH simulated beta L and gamma L, which are 59 and 42 % higher than multi-model average. These changes are primarily due to the new decomposition model, indicating the importance of soil organic matter decomposition for land carbon feedbacks.