An approach to provide maps of the N2O emission risks by soils at the regional scale: A case-study at the Haut-Loir watershed, France

International audience

anglais. Nitrous oxide (N2O) is often emitted by soils after nitrogen fertilization when the reduction of nitrate into N2 is incomplete and the soil is in hydromorphic condition. To take action to reduce N2O emissions, it is necessary to identify and locate areas that present a risk of N2O emissions. In this study, an approach to map N2O emission risk by soils was therefore developed based on soil properties. The risk of N2O emission was assessed through two components linked to static properties, independent on climate and agricultural practices; the Vulnerability: the ability of the soil to reduce N2O and the Hazard: the probability of soil water-logging. This approach was tested in the Haut-Loir watershed (3600 km2), a highly cropped area in the French Center Region. Vulnerability and hazard were estimated using French soil databases. The databases contain the drainage class information which allowed inferring the hazard. They also have measurements of pH, CEC and clay content which allowed estimating vulnerability through a pedotransfer function. In this watershed, contrasting risks were highlighted between different soil types and agricultural regions. High risk soils (∼2% of the studied area) were generally found in valleys and were not under crop because of their hydromorphy and acidity. However, attention should be given to medium risk soils (∼32% of the area) which were mainly found in the western region. Oppositely, soils of the eastern region present generally no risk of N2O emissions. Some former field studies have been reported in the studied watershed: they generally supported this soil risk classification. For medium-risk soils, different actions of mitigation depending on the degree of risk were suggested: liming or adjusting nitrogen input periods. This risk mapping approach could be applied in other cropland regions to help mitigation strategy.