Contribution de l'agriculture à l'effet de serre. Importance de l'azote et interactions avec l'ozone
Résumé
Agriculture contributes to almost 20% of greenhouse gas emissions in France, and the same applies for most EU countries. This is due to a large extent to N2O emission after N fertilizer application, but the emissions are very variable in space and time, due mainly to the variability in environmental conditions. However, the figure of the relationship between agriculture and greenhouse effect is more complex. First a significant fraction of emitted N2O does not occur where N has been applied, but is due to indirect emissions in wetlands or forest where N is transferred by natural pathways. This implies the need to make an assessment at larger scale than the field where N is applied. Many other GHG emissions are more or less directly linked to nitrogen use. Carbon storage in ecosystems in enhanced by nitrogen; ozone (also a greenhouse gas) is produced by the reaction of N oxides and volatile organic compounds; fine particles formed by the reaction of ammonia (mainly from agricultural sources) and atmospheric acids can reflect solar radiation. Moreover the negative effect of ozone on photosynthesis could reduce significantly the biospheric CO2 sink in the future. It is thus very complex to make a complete balance of the influence of agriculture on the greenhouse effect and the impact of an agricultural practice or farming system on the greenhouse effect.