Straw-based bioenergy can be an effective climate change mitigation strategy, but only if soil emission debts due to straw removal are considered

Crop residues, particularly straw, are abundant sources of agricultural feedstock for energy conversion when removed from the field but also a crucial agroecological resource that should be kept in the field to improve soil quality. Reaching a trade-off is therefore necessary. How can soil fertility be reconciled with the production of renewable energy from straw and other biomass? A life-cycle approach is required in order to take a comprehensive view of the entire bioenergy system. A unique, deterministic equation was developed to calculate the optimal radius of the biomass supply area for a bioenergy plant, with the aim of maximizing greenhouse gas savings and achieving the most effective climate mitigation. This equation considers every component of the bioenergy system, from upstream field conditions to downstream energy plant operations aimed at power generation and possibly heat recovery. First, a general overview of the model and its mathematical properties was provided. Then, the equation was used to evaluate the GHG emissions associated with three different wheat cropping systems and straw management conditions in southern Italy. A sensitivity analysis of the equation parameters was also conducted. Although this work is built upon previously published studies, it significantly expands their scope of application. It was found that neglecting the GHG emission debt resulting from the long-term soil dynamic conditions caused by systematic straw removal could significantly affect the supply radius of a bioenergy plant and the climate mitigation capacity of the entire bioenergy project. A case study of an actual biomass plant designed to maximize economic profits revealed that it was oversized, in other words, not in line with the size required to deliver the greatest climate benefits, which tend to be smaller. Conversely, careful calculations and conservative estimates of soil GHG emission burdens must be made on a case-by-case basis in order to develop well-designed, fine-tuned bioenergy projects. These calculations must take into account the specific pedoclimatic conditions and cropping management practices actually being applied in the area, as well as the emission factor of the current electricity generation mix achieved by the national or European electricity sector, which should be used as a benchmark. This work demonstrates that emissions resulting from straw removal can significantly impact the GHG balance, potentially invalidating the climate mitigation objective of a bioenergy project. • Straw is an abundant energy feedstock, but also a crucial agroecological resource for soil quality. • European regulations assume that there are no emissions costs when straw is used for energy. • A deterministic equation was developed to calculate the optimal radius of the biomass supply area maximizing GHG savings. • The soil emission debt due to straw removal can invalidate the climate mitigation objective of a bioenergy project. • The soil emissions debt must be calculated taking specific pedoclimatic conditions and cropping management into account.