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) to that of direct plant residue input 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, using values reported in the literature. We then applied an extended version of the widely used SOC model RothC with newly developed parameters to quantify the SOC storage efficiency, that is, accounting for both processing losses off-site and decomposition losses of the different EOMs in the soil. 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, it is essential to consider additional factors such as nutrient availability of EOMs, environmental impacts of soil application, and life cycle assessments for the entire production processes.
