Study of nitrate transfer in the unsaturated zone of two agricultural plots (North of France)
Résumé
The impact of fertilization in water quality trough the unsaturated zone (i.e. soil and subsoil up to the groundwater) is always difficult to predict because several interrelated processes are occurring. These processes are not only linked to agriculture practices but also to climatic and geological environment. The forecasts are also made complex because these processes vary with time. Indeed, concentrations in the unsaturated zone and in groundwater are the result of the farming practices of several years or even more.
This study surveyed various plots in Picardy (North of France) on chalky substrate to better understand the impact of agricultural practices on the (possible) changes in water quality.
Surveys were conducted on an agricultural plots with a cultural marker (major change in farming practice). These surveys allowed establishing nitrogen profiles in soil at different depths. Studying these profiles, it is possible on the one hand, to evaluate the present stock of nitrate in the unsaturated zone, and on the other hand, to estimate the speed of transfer of these nitrates.
Soil profiles show that concentrations fluctuate in response to agricultural practices varying over the years and also in response to rainfalls. For the study plots, nitrate inputs to soil are less important today than in the past. However, they remain higher than those observed in the plot without fertilization used as a reference.
Thanks to the data provided by the farmer, values of nitrogen fertilization have been used to calculate the surpluses and deficits of fertilization, year by year and culture by culture on the plot. A good correlation between the farmer practices and the concentration of nitrogen in the soil was found on the period 1990-2013. This work shows the impact of the fertilization on soil over several years.
In the surveyed plots, the speeds of nitrate transfer could be estimated (around 0.6 m/year). Taking into account the thickness of the unsaturated zone (several tens of meters locally) and low estimated speed, the impacts of environmental actions on the water quality could be highly delayed . In the studied plots, where the transfer matrix is preponderant, the impact of current practices would be visible in only approximately 50 years.
The findings of the project are valid for the investigated plot only. The change of scale from the surveyed plot to the entire water catchment was not performed during the project.