The crop residue conundrum: maintaining long-term soil organic carbon stocks while reinforcing the bioeconomy, compatible endeavors?

Abstract

Crop residues are key for supplying carbon to the bioeconomy without interfering with food security. However, it is often suggested to export no more than half of this potential to ensure the maintenance of soil organic carbon (SOC) stocks. In this study, we challenge this idea by assessing how the residues usage for the bioeconomy is intertwined with the maintenance of long-term SOC stocks and thus the amount that can safely be harvested. We considered the coproduct return to the soil from five bioeconomy scenarios: i) pyrolysis biochar, ii) gasification biochar, iii) hydrothermal liquefaction hydrochar, iv) anaerobic digestion digestate, and v) lignocellulosic ethanol molasses. To compare the long-term SOC changes from these scenarios against a business-as-usual (BAU) scenario, in which crop residues are unharvested, we developed a framework coupling a SOC model with a bioeconomy module, that we applied at high spatial resolution to cover over 60,000 combinations of crop rotations and pedoclimatic units over France, for 2020 – 2120. The adapted SOC model considers the recalcitrance to degradation of each coproduct, while the bioeconomy module determines the share of carbon from the crop residues allocated to the coproducts. Our results show that the available harvestable crop residues could be completely harvested if pyrolysis or gasification biochar returns to soils, with SOC expected to double compared to the BAU scenario. Replacing crop residues with hydrochar showed increased SOC stocks in 88% of the areas (max +8%), while the digestate scenario predicted minor SOC increases in 50% (max +0.76%) and decreases in 40% of the areas (min -4%). The molasses scenario yielded SOC losses in all the areas and is thus not recommended. Excluding these, an additional amount of 60.4 – 191 PJ of crop residues (use-dependent) could be available for the French bioeconomy compared to applying a universal removal rate of 31.5%.