This document is a synthesis coordinated by B. Carrouée in a LINK working group on this item, with, among others, amendements from E.S. Jensen, H. Pahl, A. Schneider - 2002.

In the past years, a lot of scientific references have been which support the positive environment-friendly impact of grain legumes on the European cropping and animal feeding systems. Since it was the most studied crop, peas are the reference of this synthesis; however the results can often be transferred to other pulses such as faba beans, lentils, chickpeas and in a lesser extend lupins.

Five characteristics having consequences on the environment :
> No nitrogen fertilisation: like other plants of the legumes• family, and unlike other crops, pulses are able to fix atmospheric nitrogen and do not require nitrogen fertilisation. In normal conditions in Europe, approximately 2/3 to 3/4 of the total nitrogen requirements are provided by the symbiotic fixation of the atmospheric nitrogen, and 1/3 to 1/4 come from the soil nitrate assimilation. When the soil is very rich in nitrogen (e.g. after slurry spreading) those ratio can be inverted. [1, 9, 15]
> A very high nitrogen harvest index: for instance at harvest,• near 80% of the nitrogen of aerial parts and near 70% of the total nitrogen of a pea crop (including roots, nodules and rhizodeposits) ) are exported through the seeds.
> A short duration cycle: in Europe, for instance spring peas are• generally sown in February–march and harvested in July–August; winter peas are generally sown in November–December and harvested in June–July.
> A protein very rich in lysine, one of the essential amino acids:• 7,4 % compared to 6,5 % for soya bean and faba bean and to 2,8 % for wheat. [14]
> Beneficial impact on the following crops of the crop rotation:• pulses often enables an easier control of weeds and diseases and a better efficiency of mineral nutrition for the following cereals [12], resulting into both an increased yield and reduced protection costs for the following crop. In particular the N fertilisation of a cereal following a legume crop is reduced by 20 to 40 kg N/ha.

The consequences concerning environmental questions can be developed as follows:
 
1. Reduction of the energy consumption
The manufacturing, transport and application of nitrogen fertilisers is extremely energy consuming. At the world scale it represents 6,6 1015J per year of fossil energy consumption [10].
Consequently, the amount of fossil energy consumption (including fertilisers, mechanisation and pesticides) of a pea crop or other grain legumes crops is half of a wheat crop or most other non fixing crops using between 150 kg to 200 kg N/ha (approximately 8 to 10 GJ/ha vs 16 to 20 GJ/ha) [5, 8]

2. Reduction of the greenhouse effect
The impact of the pulse share in the cropping systems on the greenhouse effect is highly significant when considering the global agricultural activities: the mainly CO2 emissions but also N2O, CH4, NO and NO2 emissions at the different steps of manufacturing and application of N fertilisers required by the non fixing crops [10] induce a greenhouse effect 2 to 3 times higher per hectare than with a legume crop [16].

3. Reduction of the punctual pollution risks
Punctual pollution events are mainly due the direct run off of fertilisers or pesticides in the surface water. It happens for instance when a high rainfall occurs just after spreading. Concerning phosphorus and pesticides the pea crop is not much different from most other crops.
Concerning nitrogen, it is obviously different : the risks with pulses doesn’t exist since they do not require N fertiliser.

4. Reduction of the acidification risks
Another consequence of the saving of nitrogen fertilisers is the reduction of the risk of ammonia volatilisation : synthetic fertilisers applied in the form of urea or ammonium bicarbonate are susceptible to significant NH3 volatilisation when applied on the soil surface. This ammonia will then contribute to the ecosystem acidification when it is deposited and nitrified [10].

5. No reduction of the soil nitrate leaching in water, except with catch-crops
The risk of nitrate leaching in underground water mainly occurs when the soil nitrate content is high at the beginning of the winter. Due to the rather short duration cycle of most pulses, and also to their relatively shallow root system, the risk of nitrate leaching in the water can be high in the soils which tend to mineralise high amounts of nitrogen after harvest.

At the beginning of the winter after a pea crop , the amount of nitrate in the soil is similar to what is measured after most other crops (rapeseed, potatoes, maïze…), a bit higher than after winter cereals and significantly higher than after sugar beat [3,11]
Contrary to what is often said, the net N mineralization in the soil during autumn is not higher after dry pea than after most other crops : the amount of N in the dry pea residues is limited and the net mineralization of those residues occurs relatively slowly. The major part of the nitrogen content of the pea residues (including roots and nodules) is released during the following years [6,9,13]

The main difference between pea and winter cereals is explained by the difference in the root systems, which is deeper and denser for cereals [9,1]. That explains why the mineral N residue in the soil is generally 20 kg/ha higher (for 1 meter depth) after a pea crop than after a wheat crop [4].

Consequently, the introduction of a pea crop or of another grain legume in a crop rotation doesn’t enable to reduce in itself the risk of nitrate leaching.
However, the short duration pulse crops such as a pea crop are favourable to the implementation of “nitrate catch crops” which are very efficient to reduce nitrate leaching [2]. Due to the late autumn or spring sowing of most pulses, it is easy to settle a catch crop in the autumn before sowing if there is a specific risk. It is also possible to implement an efficient short duration catch crop between a dry pea crop and a winter wheat crop (since pea is harvested early during summer and leaves few crop residues on the soil), at least in the European regions where winter cereal sowing occurs in October or after : indeed, this technique requires at least 1,5 to 2 months to enable the catch crop to reach the objective of 2 t DM/ha (this threshold is generally considered as a minimum to reduce significantly the leaching of nitrogen during the following winter). [2,5,12].

6. Reduction of nitrate wastage in animal husbandry
Dry pea seed are mainly used in Europe for pig feeding, generally at rather high inclusion rates (20-30 %) [14]. The very high lysin concentration of its protein enables to balance the diet with a minimum amount of total protein. The consequence, when compared to other raw materials rich in protein such as soybean meal is a significant reduction of N losses in the animal urine an feces [7]. That can be an asset in the regions where the amount of animal manure is too high according to the land available

7. Reduction of the use of pesticides and fertilisers in the crop rotations
Rotations including peas or other grain legumes are well known to enable significant reduction in pesticide applications as compared to continuous cereal rotations, mainly for herbicides and fongicides. Sometimes, this benefit is perceptible immediately on the following wheat, for example in reducing some diseases (eg : take-all disease). In other cases, the advantage appears only after several years. This is the case for instance of foxtail grass (Alopecurus arvensis) which may become a weed difficult to control even with constant applications of herbicides in continuous cereal rotation, especially with the new techniques of reduced soil tillage, whereas in rotations with spring pea it possible to have a good control without applying herbicides every years [12].
In the same way, the uptake of nitrogen and phosphorus by the cereal after a grain legume is generally more efficient than after another cereal. That explains why the farmers use to apply reduced amounts of fertilisers for the same or even a higher yield target

8. Low water consumption
Dry pea have a low water requirement because the growth period is short and occurs mainly during spring. For example, the global water requirement of a pea crop is estimated to approximately 300 l/m2 (including rain and soil reserves) as compared to 450 for soybean and 500 for maize [12]. When irrigated in southern parts of Europe, it generally requires about 50 l/m2 only as compared to 150 to 250 for summer crops. This characteristic is common to other Mediterranean grain legumes like lentils or chickpeas.

9. Impact on soil fertility
The global N balance of a dry pea crop (N entering the soil-plant system through symbiotic fixation minus N exported at harvest) is close to zero (+10 to +20 kg/ha when only seeds are harvested, or -20 to -30 when the pea straw is also used). [1,9]. Consequently the pea crop maintain the nitrogen soil fertility : in the absence of N fertilisers it does not increase nor decrease significantly the stock of organic nitrogen in the soil.
The exports of others nutrients (P,K,S…) through the seeds are relatively limited and can be compensated with low amounts of mineral or organic fertilisers.

10. Impact on soil organic matter
Concerning organic matter, grain legumes and especially pea do not bring back large amounts of carbon to the soil. However, their favourable impact on the soil structure and the limited amount of crop residues frequently enable to avoid ploughing and thus the risk of dilution of organic mater.

Conclusion
The introduction of grain legumes such as dry peas in crop rotations and in animal diets has a series of beneficial impacts on the environment. Some of them, such as the reduction of pesticides uses, are common to all other break crops such as rapeseed or sunflower for instance and are linked to the good balance of crop rotations. Yet, the most significant beneficial impacts are linked to the possibility to avoid any nitrogen fertilisation on grain legumes, with positive consequences on energy consumption, greenhouse effect, punctual pollution and acidification risks.

References
[1] Aveline A., Crozat Y., Carrouée B., Gillet J.P., 1998. Rotation pois/blé. Comparaison de bilans azotés. Perspectives Agricoles. 239 : 36-40.
[2] Briffaux G. et Aubrion G. 1998. Cultures intermédiaires : les meilleurs pièges à nitrates. Perspectives Agricoles n°239 : 36-42.
[3] Carrouée B., Combes A., Royer L., 1999. Grille de risques et évaluation des pratiques. Comité scientifique Fertimieux 24-11-99.
[4] Carrouée B., Jannot B., Justes E., Kouassi A.S., 1999. Fertilisation azotée de trois légumineuses : le haricot, le pois protéagineux et la luzerne. CORPEN (Ed.)
[5] Combes A. 2000. Pois protéagineux : un bilan énergétique favorable . Perspectives Agricoles 254.22-25
[6] Corre N., Simon J.C., Boucaud J., 1997. Enfouissement des légumineuses : quel avenir pour la culture suivante ? Perspectives Agricoles.230 : 62-69.
[7] Gatel F., Jondreville C., Buron G ; Peyronnet C. ; Grosjean F., 1993. Effet de l’introduction de pois dans les régimes sur les rejets azotés du porc charcutier. JRP. 301-306.
[8] ITCF-ADEME, 1999. Référentiel pour le calcul des bilans énergétiques, Production agricole. ITCF (Ed).
[9] Jensen E.S., 1997. The role of grain legume N2 fixation in the „wnitrogen cycling of temperate cropping systems. D.Sc. Thesis RIS National Laboratory. R-885 (EN).86 p.
[10] Jensen E.S., 2001. How can increased use of biological N2 fixation in agriculture benefit the environment. Submitted for FAO technical expert meeting.
[11] Laurent F. et al. Le pois dans la rotation : comment maîtriser le risque nitrates ? Perspectives Agricoles n°240, novembre 1998.
[12] Munier-Jolain N.G., Carrouée B., 2001. Quelle place pour le pois dans une agriculture respectueuse de l’environnement. To be published in Cahiers Agriculture.
[13] Nicolardot B., Duthion C. et Cheneby D., 1996. Decomposition of crop residues : a rather slow mineralisation. Special report nitrogen-environment.
[14] UNIP-ITCF, 1995. Peas, utilisation in animal feeding. 99p.
[15] Voisin A.S., Munier-Jolain N.G., Ney B., Salon C., 2001. I. Symbiotic nitrogen fixation along the growth cycle of pea (Pisum sativum L.) is affected by mineral soil nitrogen availability, growth potential, and phenological stage. Submitted at Plant and Soil.
[16] Charles R., 2001. L’écobilan en agriculture : présentation d’une méthode d’analyse environnementale. Proceedings of the 2nd “Rencontres annuelles des protéagineux”, Paris, déc. 2001. UNIP (Ed).

More:
- about environmental references : Link to a pdf file of a more complete document (French version for the moment)
- about LINK releases related to the contribution of grain legumes to a sustainable agriculture