Modelling site-dependent environmental impacts of nitrogen fertiliser use in life cycle assessments of crop cultivation

Use of mineral nitrogen fertilisers in crop cultivation has enabled substantial yield increases, strengthening global food security. High yields also allow better resource efficiency and result in higher organic matter inputs to soil, increasing the potential for soil carbon sequestration. However, nitrogen fertilisers cause substantial greenhouse gas emissions and nutrient losses to water bodies when the excess nitrogen leaves the field in reactive form. Thus nitrogen fertiliser can either increase or decrease the environmental impact of crop cultivation, depending on soil management, site characteristics and the aspects considered. Life cycle assessment (LCA) is a commonly used tool to assess the environmental impact of crop cultivation. In LCA, the impacts of all or part of the life cycle of a product, process or service are compiled. For crop cultivation, this generally includes production of inputs, machinery use and soil emissions. However, reactive nitrogen emissions, yield response and soil organic carbon dynamics are highly dependent on site conditions, relationships often poorly depicted in LCAs. This thesis examined the influence of nitrogen fertiliser rate and site on the climate impact and marine eutrophication of crop cultivation as determined by LCA. Methods for quantifying nitrogen emissions from crop cultivation and their impacts were compared, and new methods for assessing marine eutrophication impacts in Sweden and including soil fertility effects of yield increase were developed. The results showed that nitrogen fertiliser rate influenced the climate impact and marine eutrophication of crop cultivation, but that the effect of site was generally stronger. Site affected the two impact categories differently and also affected the nitrogen rate that gave the lowest impact. The level of impact and the effect of nitrogen rate and site also varied considerably with methodological choices, including: emissions models for soil nitrous oxide and nitrogen leaching, marine eutrophication characterisation model and accounting for the symbiotic relationship between yield and soil organic matter dynamics. These findings highlight the importance of careful model selection and interpretation of results when using LCA to assess the environmental impact of crop cultivation