Grain legumes and the environment: how to assess benefits and impacts. European Association for Grain legumes Research AEP, Paris, 179-180.
LCA of grain legumes: interpreting decreases in the use of N fertilisers and comparison to other factors of the production system
Raphaël CHARLES, Agroscope Changins- Wädenswil ACW, 1260 Nyon 1 - Switzerland
Introduction
From an agronomic and environmental point of view the main interest of grain legumes in the crop rotation is based on the absence of nitrogen fertilisation and on the residual N effect on the subsequent crops. However, it is questionable whether these decreases in the use of mineral fertilisers are sufficient to justify an effective environmental benefit? There is a need to quantify the environmental impacts of grain legumes cultivation. Results of an environmental analysis are interpreted here with the objective of providing an overview of the effective benefits of grain legumes and of the need for additional environmental improvements.
Material and methods
According to field trials (1), the residual nitrogen (N) effect of a grain legume on the following crop was quantified as equivalent to 20-60 kg N/ha in the form of mineral fertiliser. The absence of N fertilisation for the cultivation of grain legumes contributes to a supplementary saving ranging from 100 kg N/ha to more than 180 kg N/ha as a function of the substituted crop. These results are used for an environmental analysis of the pea crop system according to the Life Cycle Assessment methodology (2, 3).
The system includes the cultivation techniques between the harvest of a preceding cereal crop and the harvest of the spring pea crop. A cover crop is grown before the pea and a winter wheat crop is evaluated for comparison. The evaluations made are per hectare and per tonne.
Results and discussion
Environmental impacts per ha of spring pea are lower for pea than for winter wheat.
The saving of N fertiliser in pea is the key factor. However, the evaluation per tonne of grain shows that the yield of pea may be a limiting factor in the comparison with cereal crops. To be favourable, pea yield should reach at Ieast 30, 55, 70 or 95% of cereal yield according to the impact category. The environmental benefit per hectare may be counterbalanced by additional burdens due to the necessity to increase cultivated surfaces.
The analysis of emissions and impacts linked with N fertilisation in wheat make it possible to quantify from an environmental point of view the way in which grain legumes are different from other crops. The following emissions are responsible for the environmental impact of N mineral fertilisation in wheat (Table 1): nitrous oxide (N20), ammonia emissions (NH3), nitrogen oxides (NOx, as NO2) and nitrates (NO3-
).
Grain legumes and the environment: how to assess benetits and impacts? 179
Table 1. Contribution of N fertilisers to the impact of wheat: percentages during fertiliser production and during field use.
Impact - determinant N emission Fertiliser production Field emissions
Global warming - Nitrous oxide 20% 30%
Oxidant formation - Nitrogen oxides 20% 5%
Eutrophication - Nitrate - 90%
Acidification - Ammonia 5% 30%
Human toxicity - Nitrate - 10%
These emissions originate from fertiliser production or from fertiliser use in the field. The interval between the preceding crop and pea represents a key period for soit emissions. In the absence of a cover crop before pea, the reduced mechanisation leads to minor decreases of impacts (5%), but the eutrophication is largely increased (340%) by field
would be a potential alternative to a cover crop.
Mechanisation is generally an important source of impacts in field crops. It constitutes a potential for
optimisation, which should not be neglected in grain legumes. The PK mineral fertilisers are major sources of impacts mainly on ecotoxicity. This reduces the differences between grain legumes and other crops.
The environmental impacts due to pesticide use are limited in this study, with two herbicides, one insecticide and one fungicide. However, the expansion of grain legumes in new areas could lead to the intensification of crop protection.
Conclusions
Reductions in N fertilisers contribute to an effective benefit with respect to the environmental impacts per ha of pea. Additional optimisation potential concerns mainly the level of mechanisation and the use of PK fertilisers. In the absence of a cover crop before pea, minor decreases of impacts are observed related to decreased mechanisation, but the eutrophication is largely increased due to field emissions. The evaluation per tonne of grain shows that the yield of pea may be a limiting factor in the comparison with cereal crops.
(1) CHARLES, R. and VULLIOUD P. (2001). Pois protéagineux et azote dans la rotation. Revue Suisse Agric. 33, 265-270.
(2) CHARLES R. and NEMECEK Th. (2000). A standardised method to assess the environmental impact of grain legumes. Grain Legumes 36, 18-19.
(3) CHARLES R. and GOSSE G. (2000). Environmental impact related to nitrogenous émissions from crops of peas and wheat. Grain Legumes 36, 20.
Grain legumes and the environment: how to assess benetits and impacts? 180
Figure 1. Environmental impact assessment per ha of spring pea relative to the impact of winter wheat.
