Jordy Salmon-Monviola

Modelling denitrification at the catchment scale

The objective of this work was to evaluate the importance of heterotrophic denitrification in the fate of nitrogen surpluses at the catchment scale. For that purpose we modified the denitrification module of TNT2 model and calibrated the model on a small catchment where denitrification measurements had been performed in different locations. The main interest of the TNT2 model is its ability to simulate the dynamics of the zones where soil and shallow water table interact, making it possible to spatialize the denitrification process. Daily water and nitrogen flux at the outlet were relatively well simulated (Nash of 0.85 and 0.77). In average, the model correctly simulates the denitrification measurements (R=0.68). Nitrogen flux towards the atmosphere, at the catchment scale (4.70 g N m(-2) year(-1)), is of the same order of magnitude as the soluble N flux in the stream. The model was able to reproduce the distribution of denitrification in the riparian (mean of 9.26 g N m(-2) year(-1)) and hillslope (mean of 3.45 g N m(-2) year(-1)) domains of the catchment. The results confirm the importance of riparian denitrification, but show also that hillslope soils contribute significantly (60%) to the whole catchment denitrification. The variations of denitrification rates, and also of nitrate concentrations in stream were not very well simulated by the model, highlighting the complexity of the spatial and temporal controls of nitrogen dynamics in areas with high inputs of nitrogen fertilizers, especially under organic forms.

NitroScape: A model to integrate nitrogen transfers and transformations in rural landscapes

Modelling nitrogen transfer and transformation at the landscape scale is relevant to estimate the mobility of the reactive forms of nitrogen (N(r)) and the associated threats to the environment. Here we describe the development of a spatially and temporally explicit model to integrate N(r) transfer and transformation at the landscape scale. The model couples four existing models, to simulate atmospheric, farm, agro-ecosystem and hydrological N(r) fluxes and transformations within a landscape. Simulations were carried out on a theoretical landscape consisting of pig-crop farms interspersed with unmanaged ecosystems. Simulation results illustrated the effect of spatial interactions between landscape elements on N(r) fluxes and losses to the environment. More than 10% of the total N(2)O emissions were due to indirect emissions. The nitrogen budgets and transformations of the unmanaged ecosystems varied considerably, depending on their location within the landscape. The model represents a new tool for assessing the effect of changes in landscape structure on N(r) fluxes.

Modeling the potential benefits of catch-crop introduction in fodder crop rotations in a Western Europe landscape

Among possible mitigation options to reduce agricultural-borne nitrate fluxes to water bodies, introduction of catch crop before spring crops is acknowledged as a cost-efficient solution at the plot scale, but it was rarely assessed at the catchment level. This study aims to evaluate a set of catch crop implantation scenarios and their consequences in a coastal catchment prone to eutrophication. The objectives are (i) to discuss the potential benefits of catch crop introduction taking into account the limitations due to the physiographic and agricultural context of the area (ii) to propose a multicriteria classification of these scenarios as a basis for discussion with stakeholders. We used the distributed agro-hydrological model TNT2 to simulate 25 scenarios of catch crop management, differing in length of catch crop growing period, place in the crop rotation and residue management. The scenarios were classified considering the variations in main crop yields and either nitrogen fluxes in stream or the global nitrogen mass balance at the catchment level. The simulations showed that in the catchment studied, little improvement can be expected from increasing the catch crop surface. Catch crop cultivation was always beneficial to reduce nitrogen losses, but led to adverse effects on main crop yields in some cases. Among the scenarios involving additional catch crop surface, introducing catch crop between two winter cereals appeared as the most promising. The classification of scenarios depended on the chosen criteria: when considering only the reduction of nitrogen fluxes in streams, exporting catch crop residues was the most efficient while when considering the global nitrogen mass balance, soil incorporation of catch crop residues was the most beneficial. This work highlights the interest, while using integrated models, of assessing simulated scenarios with multicriteria approach to provide stakeholder with a picture as complete as possible of the consequences of prospective policies.