Innov’Azote: Understanding nitrogen dynamics in different catchment areas of the Bourbre watershed

Prélèvement de sol dans un champ de maïs, photo de Marie BENOIT, ISARA 2024.

La Bourbre et ses affluents au quotidien, photo de Marie BENOIT, ISARA 2024.

The Bourbre watershed, located east of Lyon, has abundant drinking water resources, but these are highly sensitive to pollution. Of the 11 water catchments in this watershed, classified as priority areas by the Water Agency, eight are classified as such based on the parameters “nitrates” or “nitrates + phytosanitary products.” Stakeholders in the Bourbre watershed have been concerned about the issue of diffuse pollution for many years. In order to limit nitrogen pollution in the environment, nitrogen losses are studied in the area using indicators (in particular, residual nitrogen at the start of drainage). To this end, the scientific teams at ISARA were approached to assist in the implementation of a study and monitoring methodology in the designated area, with a particular focus on the six priority drinking water catchments that were identified as being of concern with regard to nitrates.

The present project is designed to assess the impact of agriculture on the quality of groundwater suitable for human consumption. The work to be conducted is expected to facilitate a more profound comprehension of the nitrogen dynamics within the specified soil and climate context. This enhanced understanding will serve as a foundation for the estimation of losses from hydrosystems (nitrate leaching) and the atmosphere (ammonia volatilisation and nitrous oxide denitrification).

As a follow-up to these monitoring activities, analysis of nitrogen residues in the soil are carried out in different cropping systems. The aim of this study is twofold: firstly, to validate the relevance of these measurements as an environmental indicator, and secondly, to provide analysis tools for farmers and other stakeholders in the region. The dissemination of solutions and results is to be achieved through workshops and communications during technical and steering committee meetings on water catchments.

In each water catchment area, a specific action program has been developed with farmers and agricultural partners to implement agricultural practices aimed at reducing inputs (fertilizers and plant protection products). Each action encompasses a wide range of activities that the stakeholders concerned can implement, including: the establishment of diversified cover crops, legume-based mixed crops, the use of decision support tools (DSTs), etc.

etapes du projet

Study area

A network of reference farms, distributed across six catchment areas, will be subject to a three-year monitoring program. The objective of this program is to establish threshold values for residual levels at the initiation of drainage, with the overarching aim of ensuring the preservation of the quality of raw water in the catchments.

For this project, at least three cropping systems will be monitored annually in each catchment, for a total of 18 CS among volunteer farmers. The purpose of this reference network is to provide a better understanding of multi-year nitrogen dynamics within the BV.

Monitoring methods

The project will be rolled out through the following four actions:

• Action 1: Review of farmers’ practices
• Action 2: Measurement of nitrogen residues at the start of drainage
• Action 3: System-scale modeling
• Action 4: Communication & coordination

Regarding action 2: the experimental plots must implement “good agricultural practices” (i.e., compliance with regulations and agronomic advice). For this reference network, 87 plots are being monitored for the six catchment areas concerned.

Indicators selected

At this stage, the team has defined the indicators are to be monitored during the measurement phase:

• Agricultural practices: mineral and/or organic fertilization, intercropping management
• Groundwater and surface water quality: nitrate concentrations.
• Nitrogen dynamics: mineral nitrogen residues in the soil at the start of the drainage period (RDD), residues at the end of winter (RSH), apparent nitrogen balance at farm level (an apparent balance corresponds to the difference between inputs and outputs). Modeling will be used to estimate nitrogen losses to the environment (volatilization, denitrification, leaching).
• Adherence to the approach: proportion of farmers and UAA (Utilized Agricultural Area) involved in the experiment, number of farmers participating in training/awareness days.

Project schedule

Starting in early September 2023, the project is planned to run for a period of five years (ending in late 2027): During this period, three years of measurements and two years of coordination and communication are planned.

Performance objectives

The potential leached nitrogen measurements obtained via the reference network will enable estimates to be made of the quantities of potentially leachable nitrogen according to the type of crop planted and the type of soil.

Subsequently, drainage water depths (corresponding to the amount of water that will infiltrate beyond the root zone into the water table during the drainage period: rainy winter period) will be estimated according to soil types and climatic years using modeling. Finally, all of these elements will make it possible to estimate whether the agricultural practices implemented are in line with the objectives of preserving raw water from catchments.

In addition, mapping of nitrogen pressures from catchment areas will make it possible to identify whether the crop rotations in place present a risk in terms of nitrogen pressure on water resources.

ISARA project team

Marie Benoit, Adeline Cadiergues, Mathieu Rouge, Benoit Sarrazin etJean-François Vian.

Partners

Établissement Public d’Aménagement et de Gestion des Eaux (EPAGE) de la Bourbre, Agricultural Chamber of Isère (CA38) and farmers.