Friday, 14 July 2006

Modelling Soil Profile Evolution Considering Physical and Chemical Weathering, and Incorporating Bioturbation Processes.

Budiman Minasny1, Sebastien Salvador-Blanes2, and Alex McBratney1. (1) The University of Sydney, Faculty of Agriculture, Food & Natural Resorces, JRA McMillan Building A05, Sydney, NSW, 2006, Australia, (2) Laboratoire GeEAC, Faculté des Sciences et Techniques, Parc de Grandmont, Tours, 37200, France

Modelling soil genesis is of great importance in assessing the effects of global change on ecosystems. While many mechanistic models have been developed to simulate soil processes (eg. transport of water, solute, gas, and heat in soil), few attempts have been made to simulate the processes that lead to the development of a soil profile. Most models of pedogenetic processes account for chemical reactions and fluxes at the horizon scale and are difficult to extrapolate to the landscape scale. Soil formation at the catena and landscape scale, represented by the evolution of soil depth through time has been modelled (Minasny and McBratney, 2001). Improvements to this model require further development of the evolution of the soil materials weathered from the bedrock. The aim of this study is to develop a model at the profile scale by taking into account soil-forming processes, such as the physical and chemical weathering of primary minerals, as well as bioturbation. The model is formulated as follow: a given thickness of soil material (called layer) is released through physical disintegration of bedrock. For each time step, a layer of regolith is created. The regolith layer is then subjected to physical and chemical weathering, as well as bioturbation. We follow the evolution of each layer which enabled the calculation of the particle size distribution and chemical composition with time. The processes in the model are: 1. Bedrock lowering, release of soil material. The physical breakdown of bedrock resulting in production of soil materials is modelled as an exponential function with the rate of soil production declining with thickening of soil. 2. The evolution of the coarse fraction. The evolution of soil's coarse fraction (>2 mm) through time is considered only as a physical weathering process. This coarse fraction is assumed to be composed of an assemblage of rock fragments. 3. The evolution of the fine fraction. The soil's fine fraction (<2 mm) is considered to weather both physically and chemically in various proportions according to the nature of the primary mineral. For this purpose, we consider that the fraction <2mm of each horizon is represented to occupy 1000 boxes corresponding to particle radii ranging from 1000 to 1µm for each primary mineral particle, that is considered to be spherical in shape. This is similar to the model of Legros and Pedro (1985). The chemical weathering consists in calculating for each layer and each time step the quantity of a given primary mineral that is weathered and whenever it is the case the number of moles of secondary minerals formed according to known chemical weathering pathways. The primary mineral weathering rate in our model is calculated as a function of the reaction rate constant and the surface area of the mineral considered (White et al., 1996). 4. Bioturbation. As a consequence of macrofauna activities, such as earthworm, ant, and termite, bioturbation of soil materials is considered. This can result in translocation of fine fraction (<2 mm) from subsoil to topsoil (Müller-Lemans and van Dorp, 1996). The quantity of fine fraction translocated from a subsoil layer is modelled to be dependent on: biological productivity (related to soil thickness), and activity follows a negative exponential depth distribution. An example of the simulation on a soil profile will be demonstrated. This simple in-situ, soil-profile model with 3 major pedogenic processes: physical and chemical weathering, and bioturbation, is able to simulate the formation of soil horizons, and stone line. The evolution of soil properties at each layer, particle size, bulk density, mineralogy, elemental composition, strain, can be tracked through time. References: Legros J.P., Pedro, G., 1985. The causes of particle size distribution in soil profiles derived from crystalline rocks. France. Geoderma 36, 15-25. Minasny, B., McBratney, A.B., 2001. A rudimentary mechanistic model for soil formation and landscape development: II. A two-dimensional model incorporating chemical weathering. Geoderma 103, 161-179. Müller-Lemans, H., van Dorp, F., 1996. Bioturbation as a mechanism for radionuclide transport in soils: relevance of earthworms. Journal of Environmental Radioactivity 31, 7-20. White, A.F., Blum, A.E., Schulz, M.S., Bullen, T.D., Harden, J.W., Peterson, M.L., 1996. Chemical weathering rates of a soil chronosequence on granitic alluvium: I. Quantification of mineralogical and surface area changes and calculation of primary silicate reaction rates. Geochimica et Cosmochimica Acta 60, 2533-2550.

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