Strengthening the bioeconomy in tropical countries while preserving soil organic carbon stocks by recycling recalcitrant coproducts: A case study for Ecuador.

Crop residues are a key supply of renewable carbon for the bioeconomy. However, the mobilization of crop residues from agricultural fields to the bioeconomy is commonly limited to 15-60% [1] of their technical potential to avoid depleting soil organic carbon (SOC) stocks [2]. Nevertheless, the coproducts remaining from several bioeconomy pathways are rich in recalcitrant carbon that could be returned to soils to maintain the SOC levels. In tropical countries, the great diversity of pedoclimatic conditions promotes the production of a vast variety of crops, which produce large amounts of residues that are commonly left unharvested or burned on the fields. This study investigates, for the specific context of tropical countries, the interaction between crop residue harvesting and long-term SOC dynamics when coproducts obtained from pyrolysis (biochar), gasification (char), hydrothermal liquefaction (hydrochar), and anaerobic digestion (digestate) are returned to soils. Besides the focus on tropical conditions, important novelties are that the soil carbon model RothC [3] was adapted to include extra pools representing the labile and recalcitrant (%C_L and %C_R) fractions of each coproduct, as well as their mineralization rates (k_L and k_R). In addition, the potential effect of coproducts in SOC mineralization, commonly referred to as priming effect (PE) was also included in the adapted model. The model was applied to the entire croplands in mainland Ecuador, used as a representative of tropical context given the availability of high spatial resolution data. Ecuador’s croplands were clustered in >105,000 simulation units, including 10 crops representing 95% of the total national production (i.e., banana, plantain, cocoa, coffee, oil palm, sugarcane, rice, maize, wheat, and barley). Simulations were carried out with our adapted RothC-bioeconomy model to compare, over 100 y (2020-2120), the four bioeconomy pathways studied herein to a reference scenario where crop residues are not supplied to the bioeconomy (unharvested). The climate variables used followed the RCP4.5 trajectory [4]. The spatially-explicit results, which are still being processed, describe the conditions and amount of crop residues that can be harvested for bioeconomy in order to maintain or increase long-term SOC stocks.

 

[1] C. Andrade Díaz, et al., 2022. doi: 10.1016/j.apenergy.2022.120192. [2] H. Blanco-Canqui and R. Lal, 2009. doi: 10.1080/07352680902776507. [3] K. Coleman and D. S. Jenkinson, 1996. doi: 10.1007/978-3-642-61094-3_17. [4] IPCC, 2018.