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Abstract
Nitrogen (N) fertilisation has positive and negative effects on the environmental impact of crop cultivation. The mechanisms governing these effects are highly site-dependent, a factor often ignored in assessments of the environmental impact of crop cultivation. By assessing outputs of crop rotations using a life cycle approach, this study explored how greenhouse gas emissions and marine eutrophication caused by crop cultivation (including upstream processes such as production of farm inputs) depend on fertiliser rate and the site at which the cultivation occurs. Cereal unit (CU) was used as the functional unit. The calculations were based on data from multi-site long-term field experiments in Sweden and site-dependent data and models for non-measured processes. Cultivation at three N levels was evaluated, where the highest N rate was close to current average practices and the lowest level corresponded to one-third of that. Site characteristics had a stronger influence on both greenhouse gas emissions and marine eutrophication (variations of up to 330% and 490%, respectively, within N levels) than N level (variations of up to 74% and 59%, respectively, within sites). Main sources of variation in greenhouse gas emissions were soil nitrous oxide emissions (58–810 g CO2eq CU−1) and soil organic carbon changes (14–720 g CO2eq CU−1), while variations in marine eutrophication were mainly explained by field-level waterborne N losses (0.9–8.2 g Neq CU−1). The large variation between sites highlights the importance of considering site characteristics when assessing the environmental impact of crop cultivation and evaluating the environmental consequences of crop management practices.
Highlights
The introduction of mineral nitrogen (N) fertilisers has enabled yield increases and has been a crucial factor for increasing global food security during the past century (Sutton et al 2013)
The combined effect of soil organic carbon (SOC) changes and other emission sources was that the optimum N level in terms of minimising total greenhouse gas (GHG) emissions was low, medium or high depending on site
This study explored the influence of site and N fertiliser rate on the GHG emissions and marine eutrophication impact from crop cultivation in a life cycle perspective
Summary
The introduction of mineral nitrogen (N) fertilisers has enabled yield increases and has been a crucial factor for increasing global food security during the past century (Sutton et al 2013). N fertiliser use itself causes negative environmental impacts through fossil fuel use and losses of reactive N both during the production phase and after field application. The relationship between N fertiliser rate and environmental impacts is complex, since both yield and field emissions of reactive N exhibit non-linear responses to increased N fertiliser rate (Delin and Stenberg 2014). Since both yield and emissions are affected by local cultivation conditions such as climate and soil characteristics (Delin et al 2005; Rochette et al 2018), it is possible that the environmental impact of cultivation under different fertiliser rates will depend on the site. Multi-site long-term field experiments are a valuable source of data
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