METAGROFORESTRY - Influence of the management and spatial arrangement of agroforestry systems on nitrogen recycling: a meta-ecosystem modeling approach

This project was based on the hypothesis that the spatial arrangement of the sub-units of an agroforestry system, combined with management practices, is a major lever for influencing nitrogen flows, distribution and conservation within the system, via mechanisms of complementarity and transfer between sub-units. The main objective was to develop a modelling framework to jointly analyse the effects of spatial organisation and agricultural practices on nitrogen dynamics and to assess their consequences on plant production and system fertility.

Approaches

To achieve this objective, the project employed a modelling approach based on the concept of a meta-ecosystem, explicitly representing the various sub-units that make up an agroforestry system (trees, crops, herbaceous cover) and the nitrogen flows that connect them. The SAF is thus described as a set of ecosystems interconnected by material flows, the intensity of which depends on both spatial configuration and management practices.

A dynamic mathematical model, based on a system of differential equations, was constructed to describe nitrogen stocks and flows within each sub-unit and between sub-units. The spatial parameters of the model correspond to the relative areas of the subunits and their functional overlap zones, defined by the spatial distribution of tree litterfall and the horizontal and vertical distribution of tree root systems. These overlaps determine the intensity of nitrogen transfers between subunits, particularly via litter inputs and competition for soil resources.

The model also incorporates the presence of deep tree roots, representing a ‘safety net’ effect whereby trees can capture nitrogen that has leached into deeper horizons and contribute to its redistribution to surface layers via litter. Agricultural practices (sowing, harvesting, fertilisation, residue management, tree pruning) are incorporated in the form of disturbances to stocks and flows, in order to represent the managed and temporally structured nature of SAFs.

The model was applied to a Mediterranean agroforestry system studied at the DIAMs (Mauguio) experimental site. Its parameterisation was based on existing data, supplemented by new field measurements of the nitrogen content of different plant biomasses in the system's subunits. These measurements provided information on the nitrogen content of the system's various plant components and enabled the orders of magnitude to be integrated into the model

Results

The results show that the spatial organisation of the SAF, in interaction with agricultural practices, strongly structures the distribution of nitrogen stocks and flows between sub-units, with consequences for plant production and the system's ability to limit nitrogen losses through leaching. Beyond the case study, the project provides a generic and transferable modelling framework for exploring scenarios for the management and spatialisation of SAFs, contributing to a better understanding of the biogeochemical functioning of these complex agroecosystems

Participants

INRAE units involved

• UMR Eco&Sols - Écologie fonctionnelle & biogeochimie des sols & des agro-ecosystemes - IRD / Institut Agro Montpellier / Cirad
• UMR MISTEA - Mathématiques, Informatique et STatistique pour l'Environnement et l'Agronomie
• UMR ABSYS - Agrosystèmes biodiversifiés

Partners

• IEES Paris - Institute of ecology and environmental sciences of Paris