Cultivate annual legume crops
System: Arable Crops
Mainly applicable for: In Europe, there are many contexts in which cropping systems can be enriched by annual legume crops, thanks to the wide diversity of candidate species. Technical expertise makes it possible to adapt cropping systems and crop management so as to minimise limiting factors and optimise the benefits of legume integration.
Not applicable or effective for: The soil must not be too compact in order to ensure proper crop establishment and the development of a functional root system with effective nodules. In addition to climatic characteristics, some soils may not be suitable for certain species due to their pH, water reserves, more or less hydromorphic profile, or potential risks associated with soil pathogens.
Description
Growing leguminous (nitrogen-fixing) crops (Fabaceae) enables the production of protein-rich grains (also containing starch and/or oil, depending on the species) or protein-rich biomass for forage or pasture. The major grain legume species are annual crops: pea, soya, faba bean, lentil, chickpea and lupin. They are grown either as monospecific crops or as mixtures of non-legume (mainly cereals) and legume crops, which are harvested at physiological maturity (grains with about 14% of humidity).
The mitigation effect linked to legume integration is immediate and certain (non-reversible). The increase in the legume area should be from 10 to 20% and the other crop management should be adapted in order to get a significant mitigation effect.
Fresh peas or beans are not considered here (since their harvest concerns immature pods). Non-harvested legumes are covered in the factsheet ‘Cover crops’. For pluriannual legumes, see the factsheet ‘Incorporate legumes in grassland’.
Mechanism of effect
Legumes are the only crops able to fix atmospheric nitrogen (N₂) through symbiosis with soil bacteria (e.g., Rhizobium) in root nodules, while also taking up soil nitrogen. This biological fixation allows them to synthesise proteins without relying on nitrogen fertilisers. Their high nitrogen self-sufficiency (except for Phaseolus beans) generally removes the need for N fertiliser during the crop cycle, avoiding upstream CO₂ and N₂O emissions from fertiliser production and on-farm N₂O emissions from nitrification and denitrification.
After harvest, nitrogen-rich residues can increase soil N₂O emissions during mineralisation compared with non-legumes, but the effect depends on residue amount—often lower for annual legumes—and on residue composition (N content, lignin). In annual grain legumes, more than half of the accumulated nitrogen is exported, leaving less N in the field than perennial forage legumes.
Legumes also improve soil N supply for the next crop, reducing fertiliser needs. Rhizodeposition, residues, enhanced mycorrhization and better root health increase nitrogen use efficiency, allowing subsequent crops to maintain or increase yields with lower N inputs and thus fewer GHG emissions.
Over time, legume-based rotations improve soil fertility and stimulate microbial activity, which enhances residue decomposition and humification, promoting the formation of stable organic matter and thereby supporting long-term soil carbon storage.
Reference situation
Non-legume crop based systems (or systems with low percentage of annual legumes)
Legend
| ● – Small effect (<5%) | ● – small unfavourable effect (<5%) | o – No effect |
| ●● – Medium effect (5-20%) | ●● – large unfavourable effect (>=5%) | N/A – effect unknown |
| ●●● – Large effect (>20%) | ● – ● – Variable effect (depending on farm characteristics or way/level of implementation) |
Effect on total greenhouse gas (GHG) emissions (kg CO2-eq)
| per ha | |||
| Mean | min-max | Level of evidence | |
| Include (more) annual legume crops in rotations | ●●● | ●–●●● | High |
Effect per emission source
| Mean effect on emission from | Soil (per ha) | Inputs | Energy use |
| N20 | CO2 | CO2 | |
| Lime soils | ●● | ●● | ● |
*risk of an adverse effect (see ’cause of variable or unfavourable effect’)
Effect on soil organic carbon (SOC) stocks
| Relative change (%) in SOC | |||
| Mean | min-max | Level of evidence | |
| Include (more) annual legume crops in rotations | ● | ●●–●● | High |
| ● – Small increase (<10%) | ● – small decrease (<5%) | o – No effect |
| ●● – Medium increase (10-25%) | ●● – large decrease (>=5%) | ? – effect unknown |
| ●●● – Large increase (>25%) | ● – ● – Variable effect (depending on farm characteristics or way/level of implementation) |
Explanation of variable effect
Include (more) legumes in rotations
Effects on GHG emissions:
The effect of annual legumes on GHG emissions is always favorable and non-reversible. The level effect depends on (i) the degree of increase in legume surface on the farm, (ii) the type of crop system: the less diversified the current cropping system, the more potential for climate mitigation, (iii) the legume species used (fixing more or less nitrogen from the air), and (iv) the degree of reduction in synthetic nitrogen application rate. Emissions are reduced to a greater extent when the N fertilizer rate of the succeeding crop is reduced, compared to non-adjusted N fertilizer rate. The N requirement depends on type of crop and the pre-crop effects varies depending on the type of combination of the legume crop and succeeding non-legume crop. Compared with the non-legume monospecific crop, less N is required for the association of legume-nonlegume crops (grown together in the same field, all harvested or some used as companion crops).
Effects on SOC stocks :
Because SOC outcomes depend on the balance between carbon added (via legume residues and stimulated productivity) and carbon lost (via faster decomposition or lower biomass return), legumes show highly variable SOC effects across systems and time of implementation. The introduction of annual legumes into crop rotations change both carbon inputs to the soil and nitrogen dynamics within the cropping system and in the soil. The effect depends upon soil type, legumes species and system management.
In some cases, annual legumes may reduce SOC because they typically produce less biomass—particularly below-ground biomass—than cereals or grasses. The legume residues often have a low C/N ratio and decompose rapidly, leading to limited carbon stabilisation and sometimes increased mineralisation of existing SOC at short term.
Conversely, annual legumes can also increase SOC when they add substantial biomass to the soil or when their nitrogen inputs stimulate overall system productivity (especially in low input systems). In addition the increase in the soil biological activities related to legume presence leads to have a higher biomass of microorganisms which increase the amount of SOC (microorganisms are a major proportion of carbon stocks in the soils).
In total, when combining this climate solution ‘include legume crops’ with the solution ‘cover crops’ (which fit easily after spring legumes), the potential for SOC accumulation and GHG mitigation becomes larger.
| Literature references | Include annual legumes in rotations |
|---|---|
| MacWilliam et al., 2018 | A meta-analysis approach to examining the greenhouse gas implications of including dry peas (Pisum sativum L.) and lentils (Lens culinaris M.) in crop rotations in western Canada |
| Sainju, 2016 | A Global Meta-Analysis on the Impact of Management Practices on Net Global Warming Potential and Greenhouse Gas Intensity from Cropland Soils |
| Gan et al., 2011 | Lowering carbon footprint of durum wheat by diversifying cropping systems |
| Jeuffroy et al., 2013. | Nitrous oxide emissions from crop rotations including wheat, rapeseed and dry peas, Biogeosciences 10, 1787-1797. |
| Rahman et al., 2022 | Potential of legume-based cropping systems for climate change adaptation and mitigation |
| Nemecek et al., 2008 | Environmental impacts of introducing grain legumes into European crop rotations |
| Nemecek et al., 2015 | Designing sustainable crop rotations using life cycle assessment of crop combinations. Eur. Journal of Agronomy, 65, 40-51 |
| Schneider et al., 2023 | Les légumineuses à graines pour assurer des réductions d’émissions, In : Dossier Label bas carbone – Grandes cultures, Perspectives agricoles 508, 50-52. |
| Véricel et al., 2018 | Impact de l’introduction des légumineuses dans les systèmes de culture sur les émissions de protoxyde d’azote |