Environmental impacts, energy efficiency and economic effectiveness of feedstock production in the logistic chain depend on the input of chemicals and in particular on the use of mineral nitrogen fertiliser. Even though annual and multi-annual crops are interesting energy crops, they have major drawbacks compared to perennial crops due to higher needs in nitrogen fertilizer, leading to higher N losses and energy costs.
Intercrop is defined as the cultivation of two (or more) species in the same space and for a significant time. As legumes are able to fix nitrogen from the atmosphere, they could play a major role to reduce N fertilization of annual and multiannual energy crops if they are grown with them. As a result, grass-legumes intercrop, already used for various purposes such as grain production and forage production, could be interesting for bioenergy production.
Two experiments were set in the North of France to assess the performances of various grass-legumes intercrops. They included annual winter crops (triticale with pea, vetch or red clover) and multi-annual crops (e.g. tall fescue with alfalfa). Results showed that intercrops, even without N fertilizer application present higher yields and lower environmental impacts than sole crops. They generally showed higher economic and energetic efficiency compared to sole crops. Indirect and direct CO2 emissions are also in favour of intercrops. Lastly, intercropping grass with legumes did not affect the yield of the following crop (winter wheat in the experiment) compared to sole crops. An SRC intercropping experiment was also established, by growing Eucalyptus with a legume tree (black locust). At the end of the first year, the survival rate reached 87% and 81% for Eucalyptus and black locust respectively. Eucalyptus showed a smaller height than black locust the first year of growth but this relative difference decrease during the second year. Lastly, the SRC intercropping did not affect the height of both species.
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