Additional carbon inputs to reach a 4 per 1000 objective in Europe: feasibility and projected impacts of climate change based on Century simulations of long-term arable experiments

Abstract. The 4 per 1000 initiative aims to promote better agricultural practices to maintain and increase soil organic carbon stocks for soil fertility, food security and climate change adaptation and mitigation. The most straightforward way to enhance soil organic carbon stocks is to increase carbon inputs to the soil. In this study, we assessed the amount of organic carbon inputs that are necessary to reach a target of soil organic carbon stocks increase by 4 ‰ per year on average, for 30 years. We used the Century model to simulate soil organic carbon stocks in 14 European long-term agricultural experiments and assessed the required level of carbon inputs increase to reach the 4 per 1000 target. Initial simulated stocks were computed analytically assuming steady state. We compared modelled carbon inputs to different treatments of additional carbon used on the experimental sites (exogenous organic matter addition and one treatment with different crop rotations). We then analyzed how this would change under future scenarios of temperature increase. The model was calibrated to fit the control plot, i.e. conventional management without additional carbon inputs, and was able to reproduce the SOC stocks dynamics. We found that, on average among the selected experimental sites, annual carbon inputs will have to increase by 43.15 ± 5.05 %, which is 0.66 ± 0.23 MgC ha−1 per year (mean ± standard error), with respect to the control situation. The simulated amount of carbon inputs required to reach the 4 ‰ SOC increase was lower or similar to the amount of carbon inputs actually used in the majority of the additional carbon input treatments of the long-term experiments. However, Century might be overestimating the effect of additional C inputs on the variation of SOC stocks in some sites, since we found that treatments with additional carbon inputs were increasing by 0.25 % on average among the experimental sites. We showed that the modeled carbon inputs required to reach the target depended linearly on the initial SOC stocks. We estimated that annual carbon inputs would have to increase further due to temperature increase effect on decomposition rates, that is 54 % for a 1 °C warming and 120 % for a 5 °C warming.