Does Stripping Topsoil of Carbon Cause Irreversible Physical Degradation?

Severe soil degradation is prevalent across much of sub-Saharan Africa, driven primarily by the depletion of carbon from the soil. We found preliminary evidence in southern Ethiopia that stripping soils of carbon disaggregates clays that then move downwards in the soil profile, causing irreversible soil degradation. The movement of clay to subsoil may clog soil pores, so drainage is poorer, decrease bonding and aggregation in the topsoil, resulting in greater erosion and less water storage, and remove exchange sites important for nutrient storage. If clay is lost from topsoil, the degradation is irreversible and agriculture will be less resilient to challenges of drought, climate change and increased food demand.

This PhD project will explore the interaction between soil carbon loss, clay migration and the physical behaviour of soils. It can use sites from any region in sub-Saharan Africa, although we have strong links to the Hawassa Region in southern Ethiopia where our preliminary evidence was obtained. Here, farms often contain more carefully managed ‘Home Gardens’ that receive greater organic matter inputs, surrounded by a carbon gradient that decreases towards far fields. On a number of farms you will measure how soil physical properties (hydraulic conductivity, water retention, porosity), clay content and carbon varies with depth across different management regions spanning from ‘Home Gardens’ to far fields. This initial survey of farms will assess the extent of clay migration and its impact on soil physical behaviour, taking care to account for human drivers such as organic residue use, wealth, and the placement of ‘Home Gardens’ on soils with the best edaphic properties. On a subset of farms from this initial survey you will explore physical impacts in greater detail. X-Ray CT imaging will allow for pore clogging to be visualised in 3D, exploring impacts to pore connectivity and size distribution. Laboratory studies will characterise the underlying processes driving clay migration and pore clogging in relation to carbon depletion. These will assess clay dispersion and movement under cycles of accentuated wetting and drying. Modelling will allow for the longer term impacts of clay migration, particularly on carbon storage and cycling, to be assessed.

At the University of Aberdeen you will have access to state-of-the-art facilities to explore the physical properties of soil including unique approaches to measure hydromechanical properties and X-Ray CT. Co-supervision by the Environmental Modelling Team provides an opportunity to forecast impacts of carbon depletion and clay migration over wider areas and into the future. We provide training in research techniques, scientific writing and analysis and range of other skills valuable for your PhD and future career.

This PhD will be conducted in both sub-Saharan Africa and Aberdeen. Prospective students with links to institutions in this region are strongly encouraged to apply. We will be able to adapt the project to fit interests, providing it explores the links between soil carbon, clay migration and soil physical behaviour. In seeking funding for this project, students will need to factor in travel costs and shipment of soils.