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

Abstract. Recent advances in the representation of soil carbon decomposition and carbon–nitrogen 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 δ¹⁵N data (R = 0. 76, root mean square error (RMSE)  = 0. 2, Taylor score  = 0. 83). The inclusion of carbon–nitrogen interactions leads to a moderate reduction (−10 %) of the carbon-concentration feedback (β_L) and has a negligible effect on the sensitivity of the land carbon cycle to warming (γ_L) compared to the same version of the model without carbon–nitrogen 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 β_L = 0. 61 Pg ppm⁻¹ and γ_L = −27. 5 Pg °C⁻¹, are 34 and 53 % less than the averages of CMIP5 models, although the CMIP5 version of JSBACH simulated β_L and γ_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.