Bilans de nutriments à travers l'activité agricole d'un petit bassin versant

Nutrients balance in the agricultural activity of a small catchment

CEMAGREF - Département Gestion des milieux aquatiques 17, avenue de Cucillé , 35044 Rennes Cedex (Manuscrit reçu le 28 juin 1994, accepté le 20 novembre 1994)

Résumé

Le bassin versant du Coët-Dan (Naizin - 56) est un bassin hydrographique de 12 km² instrumenté depuis 1971 qui appartient au réseau européen des Bassins Versants de Référence Expérimentaux (BVRE).

L'activité y est exclusivement d'origine agricole, elle est très intensive : Le bassin est à cet égard un observatoire de terrain idéal pour appréhender les effets prévisibles de l'intensification de l'agriculture sur l'environnement.

Ce bassin est géré depuis son origine par le CEMAGREF qui étudie plus particulièrement les mécanismes de transfert de nutriments depuis la parcelle jusqu 'aux eaux de surface.

Des enquêtes sur les pratiques culturales ont été menées successivement en 1988 et 1991 auprès des agriculteurs du bassin. En outre, un relevé cultural annuel permet de suivre l'évolution de l'agriculture sur la zone étudiée depuis 1984.

Les quantités de nutriments transitant dans le bassin par le biais de l'activité agricole ont donc pu être évaluées.

Cet article explicite plus en détail les bilans en azote et phosphore imputables à l'activité agricole sur le bassin.

Abstract

Water quality is worsened by the excess of nutrients, mainly nitrate and phosphorus, which concentrations increase. That makes problem for water supply and allows the eutrophication to occur more and more. Most of these nutrients come from agriculture as non point sources.

In order to quantify and to understand the transfer of nitrogen and phosphorus from agriculture to water, the CEMAGREF carries several research on the catchment of the Coët-Dan in central Brittany. It is a 12 square kilometers catchment entirely devoted to very intensive agriculture. Crops surveys and two exhaustive enquiries has allowed to quantify the nutrients transfer through agricultural activity.

An original way of computing inputs and outputs, different from the methods used previously by INRA or the CORPEN in France, has been used. It allows especially to take into account the real content of nitrogen and phosphorus in manure and slurry in each farm instead of using mean values. That is more efficient because of the very large variability of nutrients contents in manure and slurry according to agricultural practices and building designs.

Results show that the main input for nitrogen as well as for phosphorus is animals feeds imported on the catchment. It leads to a very large excess of fertilizers which explains the increase of nutrients in the stream at the outlet of the catchment.

Nevertheless, the leakage of phosphorus by the stream is very weak in front of the large excess. It varies widely according to the quantity of rainfalls from 0.7% to 4.8%. The remaining part is stored in the top layer of soil and leads to a large enrichment of soil. However this weak leakage is widely large enough to give eutrophication in water.

The leakage of nitrogen by the stream, which occurs mainly in the nitrate form, is more important but it represents only between 8.6% and 14.6% of the yearly excess according to the measures made during six years. A part of the remaining excess can be stored also in the soil but, for nitrogen, several kinds of gazeous losses occur also. Ammonia volatilization, denitrification in wetlands as well as in subsoil are efficient althrough we are unable to quantify them because of their large variability in time and space.

Through the enquiries, it is also possible to observe the large difference between exploitations and the various practices of fertilization. Maïze and beans are the most overfertilized cultures.

It shows also the difficulty to equilibrate the fertilization of crops at the same time for nitrogen and phosphorus with organic fertilizers because the ratio of these two nutrients contents in manure and slurry does not fit with the ratio of needs of the crops.

The balance shows, at the same time, the possibility to reduce quite easily some unusefull inputs of nutrients and the difficulty to reduce them enough to equilibrate them with ouputs by crops and animals products. Therefore it is necessary to think to export some excess as fertilizers in regions where there is not excess of them or to built treatment plants for them.