Application of numerical modelling to the long-term assessment of the effects of agricultural practices on nitrogen fluxes and concentrations in water and in the atmosphere at the scale of three breeding watersheds

Abstract

The aim of this paper is to estimate the level of nitrogen emissions (into the air and water compart¬ments) that can result from the adoption of alternative farming practices and manure management. The modelling has been tested on three breeding watersheds in the Western part of France. The framework that we developed includes a GIS connected with, on the one side, softwares which design scenarios for farming practices and on the other side a continuously distributed agro-hydrological model. The results on these three watersheds emphasize that the total annual nitrogen load that is emitted into the air (N2 and N2O) and into the water (nitrate) has the same magnitude than the annual mineral N surplus. The model has estimated the mean emission index for N2O at a 5 % of the inputs level, which is significantly higher than the coefficients that are usually used for these emissions. The total loads into the atmosphere are as important from cultivated soils on the unsaturated area in the watersheds than from riparian areas. Management strategies that are the simplest and the more likely to be accepted by the farmers have proven to decrease the total N loads at the outlet of the watershed by 25 % after 21 simulated years. For the studied watershed, this decrease is high enough to attain nitrate concentrations below 50 mg/l, but not below 25 mg/l. The optimised management strategies reduce the emissions into the air from 12 to 20 % when compared to the current farming practices. The model predicts that these results cannot be attained before at least 10 years of improved management. As a consequence, the environmental evaluation of better management practices requests at least some ten years. Evaluating the sustainability of the newly designed systems also request to consider at the same time the use of soil and the agronomic and hydrodynamic characteristics of the system. In other words, the risks of pollution displacements from one compartment to the other have to be evaluated too.