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Life cycle assessment of perennial cultivation systems : advancing applicability and comprehensiveness
(2021) Lask, Jan; Lewandowski, Iris
Resource-efficient perennial cultivation systems are considered promising sources of sustainably produced biomass to meet the growing demand of a future European bioeconomy. They require fewer agricultural procedures than annual systems, contribute to an increase in soil carbon sequestration and can be productive on marginal land. In Europe, the C4 grass miscanthus is the most prominent and best researched perennial crop for lignocellulosic biomass production. Recently, wild plant mixtures (WPM) have been suggested as a more diverse alternative system. Perennial cultivation systems have already been the subject of multiple sustainability assessments, with life cycle assessment (LCA) being the method most commonly used. This method aims to provide a holistic depiction of the environmental performance of a system. However, two challenges are usually encountered. First, results of agricultural LCAs very much depend on site- and management-specific characteristics. Parameters such as biomass yield, quantity of fertiliser applied and carbon sequestered can vary considerably, impairing the applicability of the method. Second, most of these studies focus on greenhouse gas emissions only. Land use impacts on biodiversity are commonly neglected, casting doubt on the comprehensiveness that LCA is trying to achieve. This thesis aims to advance the applicability and comprehensiveness of LCA of perennial cultivation systems. For this purpose, it focuses on three aspects relevant to the assessment of such systems, each of which was addressed by a dedicated research question: 1) How can the conducting and application of LCAs of perennial cultivations systems be simplified? 2) Which methodological approaches are best suited for the consideration of carbon sequestration and storage in LCAs of perennial cultivation systems? 3) How can land use impacts of perennial cultivation systems on biodiversity best be incorporated into the LCA framework? These questions were answered by applying the LCA method to perennial cultivation systems in three case studies, using specific approaches for the inclusion of sensitivity analysis and the evaluation of carbon sequestration and storage. In addition, information on the biodiversity impacts of perennial crop cultivation was collated by means of a meta-analysis which compared species richness and abundance in annual and perennial crop cultivation systems. Due to the variability of agricultural systems, the life cycle inventory phase can be quite intricate. Thus, the conducting of an LCA can be substantially simplified by focusing on a few relevant inputs and outputs only. In this thesis a global sensitivity analysis was used to identify the most important inventory parameters in the greenhouse gas assessment of miscanthus cultivation: carbon sequestration, biomass yield, length of the cultivation period, nitrogen and potassium fertiliser application, and the distance over which the harvested biomass is transported. Focusing on these inventory parameters, a simplified model was developed. It allows farmers and SME active in miscanthus-based value chains easy access to customised LCA results. This thesis includes a detailed analysis of the relevance of carbon sequestration and storage in the sustainability assessment of perennial cultivation systems. It was found that the quantity and in particular the permanence of carbon sequestered through the cultivation of perennial crops are critical for their favourability in terms of global warming impacts. Two alternative methodological approaches for the quantification of carbon sequestered were tested within two of the case studies – a simple carbon model and an allometric approach. In addition, the handling of the uncertain permanence of the carbon storage was reflected upon. The approaches were compared with regard to their suitability for use by typical LCA practitioners. It was concluded that allometric models should be used for the quantification of carbon sequestered and the corresponding amount accounted for as delayed emissions. This combination provides a manageable approach for the accounting of benefits from carbon sequestration and storage, and also prevents their overestimation. Established impact assessment methods such as ReCiPe2016 suggest characterisation factors for the incorporation of land use impacts on biodiversity into LCA. These factors use relative species richness as an indicator and assume a higher species richness in perennial than annual cultivation systems. This thesis includes a critical review of these characterisation factors, drawing on the results of the meta-analysis comparison of species richness in annual arable crops and perennial rhizomatous grasses. The meta-study did not confirm a higher number of species in perennial rhizomatous grasses than in annual arable crops. It was concluded that LCA studies on perennial cultivation systems need to be cautious in their application of the land use characterisation factors suggested in present-day impact assessment methods. Criticisms of the approach include the application of one single characterisation factor for diverse perennial cultivation systems such as WPM and miscanthus and the sole focus on species richness. In future, LCA research should focus on context-specific adjustment options for land use characterisation factors to ensure an adequate representation of biodiversity impacts in agricultural LCAs. Finally, the current focus on species richness in biodiversity impact assessment needs to be reassessed.