Soil structure can be defined as the spatial organization of solid mineral and organic particles, and pore space. It is of great importance for soil functioning as it drives ecosystem functions (carbon sequestration, emission of greenhouse gases, nutrient cycling, primary productivity, etc.). Soil structure results from biotic and abiotic factors. Among biotic factors, numerous studies have shown the importance of organic matter, microorganisms, roots and invertebrates. Earthworms are known to play a key role in soil structure formation and maintenance through a continuous production of biogenic structures (casts and burrows). As far as we know, no models describe or quantify the effect of soil invertebrates on soil aggregation and porosity. It is a challenge to describe the physical soil environment for purposes of modelling because a soil is a multi-scale heterogeneous, three-dimensional and dynamic environment. An approach based on fractal theory (often used in soil sciences) was chosen to model such a real complex environment; it was integrated into a multi-agent system (MAS), which allows us to simulate agents (e.g. earthworms) situated in a virtual world (e.g. soil). It is a bottom-up approach that allows us to describe a system at a micro level (e.g. earthworms and their local soil environment) in order to observe, during simulations, macroscopic changes (e.g. soil structure evolution, organic matter dynamics, and microbial functions). In this paper we describe the SWORM (for 'Simulated Worms') model and the simulator, and present the results of the simulation applied to a case study. The effect of compacting and decompacting earthworm species on the structure of humid savanna soil at Lamto in Coˆte d'Ivoire has been widely studied. Quantitative and graphical outputs (e.g. thin sections of the virtual soil) indicate that the simulator was able to reproduce the effects of both compacting and decompacting species. Different ways to improve the model are discussed.
An individual-based model, called MIOR, was designed to assess hypotheses on the effect of the spatial distribution of organic matter and microbial decomposers on soil carbon and nitrogen dynamics. Two main types of object were defined to represent the decomposers and the soil organic substrates. All these objects were positioned in a 3D space. The exchange of carbon and nitrogen between these various entities was simulated. Two scenarios were tested according to the degrees of clustering of organic matter and of microorganisms. The results of simulations highlighted the effect of the ratio of accessible organic carbon to microbial carbon on the dynamics of microbial biomass and CO 2 release. This ratio was determined by the number of contacts between one object representing the microbial decomposers and the surrounding objects representing the organic substrates.MIOR: modèle individu-centré de simulation de la distribution spatiale des processus microbiens de la décomposition des matières organiques dans les sols Résumé Un mode`le individu-centre´, appele´MIOR, a e´te´conc xu pour tester les hypothe`ses concernant les effets de la distribution spatiale des matie`res organiques et des microde´composeurs dans les sols sur la dynamique du carbone et de l'azote mine´ral. Deux principaux types d'objets ont e´te´de´finis repre´sentant les microorganismes de´composeurs et les substrats organiques. Ces objets sont positionne´s dans une espace a`trois dimensions. Les e´changes de carbone et d'azote entre ces deux entite´s sont simule´s. Deux sce´narios sont teste´s selon des niveaux d'agre´gation des microorganismes et celui des matie`res organiques. Les re´sultats des simulations mettent en avant l'importance de la quantite´de carbone organique accessible par unite´de carbone microbien sur la dynamique de la biomasse microbienne et du CO2 de´gage´. Cette quantite´est de´termine´e par le nombre de contacts entre un objet repre´sentant des microde´composeurs et des objets qui l'entourent repre´sentant les substrats organiques.
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