Advancing sustainable agriculture with legumes

Legumes are a win-win for food security and the environment – two major problems facing the planet. Packed with protein, fibre, antioxidants, vitamins and minerals, they emit small amounts of greenhouse gases, sequester carbon and enrich the soil with nitrogen.

This makes them ideal plants for crop rotation, yet studies until now have been fragmented and only considered isolated facets of nutrition and sustainability, according to a European team of researchers.

They now present integrated life-cycle evidence across entire systems, published in the journal Frontiers in Sustainable Food Systems, that adding legumes to conventional cereal and oilseed crop rotations can produce nutritious human food with a lower environmental impact.

“This strategy can contribute significantly to the specific European Union Green Deal Farm to Fork objectives to reduce greenhouse gas emissions, chemical pesticide use and synthetic fertiliser use,” says first author Marcela Porto Costa, from Bangor University, UK.

“For example, in Scotland, we’ve shown that the introduction of a legume crop into the typical rotation reduced external nitrogen requirements by almost half whilst maintaining the same output of food measured in terms of potential human nutrition.”

The Farm to Fork strategy seeks to create a “fair, healthy, and environmentally-friendly food system”, aiming to reduce greenhouse gas emissions and chemical pesticide use by 50% and use of synthetic fertilisers by 20% over the next decade.

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Fast facts: legumes

• Legumes belong to the Fabaceae – the legume, pea, or bean family
• With more than 750 genera and about 19,000 known species, legumes make up about 7% of flowering plant species
• Edible legumes include beans, peas, chickpeas, lentils, soybeans and peanuts
• Most legumes have nitrogen-fixing bacteria in root nodules
• When used as a dry grain, a legume seed is also called a pulse

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Current monocrops rely heavily on chemicals, pesticides and nitrogen fertilisers, which have destructive environmental impacts. Growing beans and lentils, on the other hand, can tick myriad boxes for sustainable agriculture. 

“Legume cultivation has been associated with other benefits,” write Costa and team, “including diversification of crop rotations, which can break pest and disease cycles, improved soil quality and drought resistance through deep root systems, and support for pollinating insects.”

Primarily grown for human food and animal feed, legumes also deliver value chains for alcoholic drinks, biorefineries and green manures. Their nutritional content is richer than cereals or meat, offering additional environmental benefits if used as an alternative.

Costa and colleagues compared 10 different simulated crop sequences across 16 different impact categories. They did this over 3–5 years in contrasting climatic regions of Italy, Romania and Scotland, comparing cereal-cereal, cereal-oilseed and cereal-oilseed-legume rotation systems and exploring nutrient outputs for humans and livestock.

“Our innovative approach goes beyond simple food footprints by looking at the footprint of delivering a specific quantity of human, or livestock, nutrition from all crops produced within representative crop rotations,” says senior author David Styles, from Ireland’s University of Limerick. 

“This provides a clearer picture of inter-crop effects and the overall efficiency of different cropping sequences in delivering nutritious food (or livestock feed).”

Results showed the primary benefits of legume rotations were reduced need for synthetic nitrogen fertilisers, improved yields in ensuing crops and enhanced nutrition profile of produce. 

They also needed less external inputs, which the authors say is an important aspect of food-system sustainability: “Thus, legumes could play a crucial role in improving the sustainability of cropping systems at farm level.”

Nutritional benefits will ultimately depend on how the foods are processed and sold, and more research is needed to develop better calculations for livestock feed. The researchers aim to extend their approach to different crop rotations, locations and climates.