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Title: Development of a process-based soil evolution model and its application for understanding biogeochemical cycles
Author: Johnson, Michelle
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2012
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The position of soils at the interface between other geochemical reservoirs such as the atmosphere and lithosphere mean that they play a central role in several global biogeochemical cycles. A model which is able to simulate processes occurring within soils over the course of their development will enable further understanding and quantification of such cycles. This thesis describes the development of a process-based soil evolution model and presents first comparisons with observations from soil chronosequences. The mechanistic, soil evolution model developed incorporates the major processes of pedogenesis, including i) mineral weathering, ii) perco- . lation of rainfall, iii) leaching of solutes, iv) surface erosion, v} bioturbation and vi) vegetation-soil interactions. The specific properties the model simulates over timescales of tens to hundreds of thousand years are, soil depth, vertical profiles of elemental composition, soil solution pH, organic carbon distribution and C02 production and concentration. The model is compared with soil properties from a soil chronosequence .in Hawaii. A good agreement is observed between measured and modelled Na (which is not a plant nutrient) and Mg and Ca which are less strongly cycled. The agreement is observed across both an age and rainfall gradient, suggesting a coherent representation of modelled soil processes. Differences between measured and modelled K and P profiles are however, substantial. This suggests that for the current, simple nutrient cycling framework, the model is not capturing the active role of vegetation in obtaining nutrients. This model result therefore, indirectly indicates the important role that vegetation and mycorrhiza may play in accelerating the release of specific nutrients from minerals. Geochemical tracers from soils developed on basalts in Queensland are exploited to constrain the processes of bioturbation and erosion in the model. This study demonstrated how these tracers can be successfully used for such a model framework. With improved parameterisation of processes the model predicts an exponential soil production function for the Queensland soils and quantifies maximum erosion rates acceptable for sustaining a cover of soil. The model is applied to the long-term carbon cycle specifically to understand the relationships between erosion, vegetation and silicate mineral weathering. Results indicate the complex relationship between these factors and atmospheric C02 consumption. A particularly important result to arise from this study is that before vascular plant colonisation onto land (~360-400Ma) the sensitivity of silicate mineral weathering to C02 concentrations may have been sufficient to regulate atmospheric C02 concentrations
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available