Modelling reactive nitrogen fluxes and mitigation scenarios on a landscape in Central France

Abstract

The intensification of agriculture has been made possible by increasing the supply of synthetic mineral nitrogen to crops. That has led to increased losses of reactive nitrogen (Nr e.g. ammonia NH3, nitrous oxide N2O, nitrogen oxides NOx, nitrate NO3−, ammonium NH4+) in the environment that may produce negative impacts on agroecosystems: soil, water or air pollution, greenhouse gas emission, biodiversity loss. The nitrogen losses result from a cascade of a large number of processes that interact spatially and temporally in agroecosystems. Integrated models are very useful to investigate such complex systems.We used the NitroScape model that couples an agroecosystem model, a cattle farm model, an atmospheric model of dispersion, transport and deposition, and a hydrological model. It made it possible to simulate processes of the nitrogen cascade at the landscape scale (i.e. a domain from a few square metres to a few tens of square kilometres). The model was applied on an agricultural site of 427 ha in Central France to simulate nitrogen flows for years 2014 and 2015. It used data from measurement campaigns and farm surveys that provided soil characteristics, meteorological data and crop management for the two years of simulation. The simulation results were compared to nitrogen fluxes and concentrations measured in the air, the soil and plants.The simulated Nr fluxes were consistent with the observed fluxes (i.e. low root mean square error, coefficients of regression significant at a 5% level). The coupled NitroScape model that integrates numerous related-nitrogen processes was therefore able to reproduce the main Nr fluxes. However, there were discrepancies between simulated and observed values for N2O emissions resulting from denitrification and for NH3 volatilisation. The model showed that the main Nr losses were due to NO3− leaching, which accounted for 11% of the nitrogen outflows (29 kg N ha−1 yr−1). Total losses of Nr (emissions of NH3, NO and N2O, and NO3− leaching) in the environment accounted for 13% of the nitrogen outflows.Two alternative scenarios aiming at enhancing nitrogen use efficiency and mitigating losses of Nr in the environment were built and assessed with the model. Simulations showed that changing nitrogen fertilisation and including catch crops and buffer strips led to a 18% decrease of NO3− losses. They also showed that including pea in crop rotation led to a 25% decrease of mineral fertilisation and a reduction of NO3− losses of 2 kg N ha−1 yr−1. The NitroScape model is a valuable tool to assess the effect of nitrogen management at the landscape scale on mitigation of nitrogen losses in the environment.