Tradeoffs between long-term SOC storage and overall environmental impacts of supplying crop residues to the bioeconomy: where, when, how and what does it depend on?

Crop residues play a key role in supplying renewable carbon for the envisioned future greenhouse gas-neutral economy. However, their harvest is often limited below their technical potential to avoid soil organic carbon (SOC) stock losses. Nevertheless, the use of crop residues in the bioeconomy often involves the generation of a coproduct rich in recalcitrant carbon that can be reintegrated into soils to maintain SOC stocks. Yet, the environmental implications of such a strategy beyond those on climate change remain uncertain and contingent on specific contexts. Here, we present a novel framework that integrates a recalcitrance-adapted SOC model with a consequential life cycle assessment (LCA) to comprehensively evaluate the spatially explicit long-term SOC evolution and environmental impacts associated with returning various bioeconomy coproducts to croplands. The study spans diverse contexts, including temperate (France) and tropical (Ecuador) regions and five environmental impact categories, assessing the conversion of crop residues into maritime fuels, here hydrodeoxigenated pyrolysis oil (HPO) and cryogenic liquefied biomethane (bio-LNG), generating biochar and digestate as coproducts, respectively. The simulations were performed for >60,000 and >15,000 simulation units in France and Ecuador, employing adapted versions of AMG and RothC, respectively, under the RCP4.5 climate pathway in both regions. Results revealed that after 100 years, compared to a reference scenario where crop residues are directly incorporated into soils, biochar allows harvesting 100% of crop residues without any SOC losses in both French and Ecuadorian contexts. In contrast, digestate demonstrates a more limited potential, reaching 50% in France and 0% in Ecuador. This is referred to as the C-neutral harvest rate, which allows a surplus potential of 71-125 PJ in France and 113 PJ in Ecuador. The LCA revealed environmental benefits for all five impact categories for HPO and three categories for bio-LNG, per tonne of crop residues. Despite bio-LNG representing higher net avoided emissions (946 MgCO_2e) than HPO (563 MgCO_2e) per tonne of crop residues, the scaled results for the national C-neutral harvest rate showed the opposite trend with net avoided emissions of 11,912 MgCO_2e for HPO and 10,707 MgCO_2e for bio-LNG. Further, the scaled results revealed increased eutrophication impacts in marine water in the bio-LNG case, reflecting the nitrogen emissions associated with digestate, which is responsible for N₂O, NH₃, and nitrate losses to water. Yet, these can be mitigated with simple solutions such as treating digestate with nitrification inhibitors, microbial enrichment, or acidification at spreading. Moreover, biochar could be applied in tandem with digestate to create synergies from the nutrient-fertilizer effect of digestate and reduced C and N mineralization properties of biochar. In conclusion, defining a C-neutral harvest rate emerges as a pivotal strategy, ensuring a harmonious balance between SOC maintenance and net environmental impacts across diverse scenarios, emphasizing the potential of integrating bioeconomy practices with soil carbon management.