Rotational grass/clover for biogas integrated with grain production – A life cycle perspective

Rotational perennial grass/clover has multiple effects in cropping systems dominated by cereals. This study evaluated the environmental impact of rotational grass/clover ley for anaerobic digestion in a cer- eal-dominated grain production system in Sweden. Life cycle assessment (LCA) methodology was used to compare two scenarios: (i) a cropping system including only spring barley and winter wheat; and (ii) a cropping system including 2-year grass/clover ley in combination with spring barley and winter wheat. The functional unit was one tonne of grain. The two main functions of the grass/clover crop were to pro- vide feedstock for biogas production and to act as an organic fertiliser for allocation among the cereal crops in the rotation. Special consideration was given to nitrogen (N) management and the rotational effects of the grass/clover ley. In total, 73% of the N requirement of cereals in the ley scenario was met through symbiotic N fixation. Replacing diesel with biogas and mineral fertiliser with digested grass/clo- ver biomass (digestate) reduced the use of fossil fuels substantially, from 1480 MJ per tonne in the ref- erence scenario to 2900 MJ per tonne in the ley scenario. Potential eutrophication per tonne grain increased in the ley scenario, mainly owing to significantly higher ammonia emissions from spreading digestate and the larger area required for producing the same amount of grain. Potential acidification also increased when N mineral fertiliser was replaced by digestate. Crops relying on symbiotic N fixation are a promising feedstock for reducing the use of non-renewable energy in the production chain of farm-based bioenergy, but careful handling of the N-rich digestate is required. Replacing cereals intended for feed or food with bioenergy crops leads to indirect land use changes (iLUC) when the displaced crops must be produced elsewhere and the benefits obtained when biofuels replace fossil fuels may thereby be out- weighed. In this study, the iLUC factor assumed had a critical effect on global warming potential in the ley scenario. However, carbon sequestration and the higher yield potential of subsequent cereal crops can mitigate greenhouse gas emissions from iLUC to a varying extent. We recommend that crop sequences rather than single crops be considered when evaluating the environmental impact of production systems that include perennial legumes for food, feed and bioenergy.

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DOI:

https://doi.org/10.1016/j.agsy.2014.05.015

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