Efficiency of plant biomass processing pathways for long-term soil carbon storage

The potential for soil carbon (C) sequestration strongly depends on the availability of plant biomass inputs, making its efficient use critical for designing net zero strategies. Here, we compared different biomass processing pathways and quantified the long-term effect of the resulting exogenous organic materials (EOMs) on soil organic carbon (SOC) storage. We estimated C losses during feed digestion of plant material, storage of manure, composting and anaerobic digestion of plant material and manure, and pyrolysis of plant material based on literature values. Then we applied the widely used SOC model RothC with newly developed parameters to quantify SOC storage efficiency, i.e., accounting for both processing losses and decomposition losses, of the different EOMs. Based on simulations for a 39-year long cropland trial in Switzerland, we found that the SOC storage efficiency is higher for plant material directly added to the soil (16 %) compared to digestate and manure (3 % and 5 % respectively). For compost, the effect was less clear (2 % ̶ 18 %; mean: 10 %) due to a high uncertainty in C-losses during composting. In the case of biochar, 43 % of the initial plant C remained in the soil, due to its high intrinsic stability despite C-losses of 54 % during pyrolysis. To provide robust recommendations for optimal biomass use, additional considerations such as nutrient availability of EOMs, environmental impacts of soil application, and life cycle assessments for the entire production processes should be included.