Modelling biomass fluxes and fertility transfers : Animal wastes management in Reunion Island

The development of agriculture in Reunion Island encounters two problems related to the management of organic material. These are the risk of pollution generated by livestock farming (principally located in the highlands) and the loss of soil fertility (mainly in the lowlands, where sugar cane and vegetable crops are grown). A solution being considered is better organisation of the transfer of organic material from livestock units to crops, either for the individual farm or at the district level. To this end, we first study farmers' practices and then devise and test new management strategies. An interdisciplinary research program enabled us to build models to simulate these strategies with the aim of analysing current practices and aiding decision-making by agricultural stakeholders. Here we describe the problem and summarise our results. These are: (i) farm survey data generalised as conceptual models in the form of a farm classification and sets of management rules; (ii) several simulation models based on various modelling approaches (hybrid dynamical systems, agent-based simulation models); (iii) experimental data on some critical biophysical processes (mineralisation, ammonia volatilisation) in order to assess the environmental impact of waste management strategies. Using these models, either coupled together or in isolation, allows one to address a great diversity of management situations concerning: the operation of liquid or solid manure transfers at the farm level and risk assessment in terms of nitrogen released into the environment; the assessment of mass balance, technical parameters, investment/operating costs and supply policies of pig slurry treatment units using various processes; the transfers of organic material surpluses from livestock farms to recipient crop farms in rural areas; the assessment of environmental measures taking into account the economic behaviour of the farmers. The article ends with a critical discussion and an outline of future research plans. (Résumé d'auteur)