Particulate organic matter in artificial soils

Abstract

It is estimated that around 2 ×109 hectares of land (15% of global land area) has been degraded by human activity, and rates of soil degradation are increasing. Soil is a precious resource, it sustains almost all terrestrial life and provides 90% of the food that feeds humanity. New methods are needed to improve degraded soils and recover desertified soils, particularly in higher risk areas such as deforested areas in the tropics. Waste products of industrial bio-refining may provide a convenient and abundant material for use as a soil amendment, with the added benefit of increasing the carbon sequestration potential of bio-based products. Unfortunately, knowledge of how soils can be restored in a targeted manner are missing. This thesis aims to determine the key inter-particulate interactions which promote the formation of stable aggregates, with a focus on bio-refinery products as soil amendments. New methodologies are established in order to explore particulate interactions in the context of soils, and tested on artificial soils and aggregates with defined compositions. Abiotic artificial soils are studied, to identify the physical and chemical aggregate forming processes separately of biological processes. Particulate - clay interactions are found to be complex, and the mechanical properties of the aggregate are found not only to be dependent on surface chemistry but also on the fabric morphology, pore space and particle shape. Organosolv lignin, a biorefinery waste product, was shown to bind to the silica face of kaolinite, driven by hydrophobic interactions in suspension, and were found to strengthen kaolinite aggregates up to a percolation threshold. Video image analysis is used to determine slaking kinetics of aggregates submerged in a flow cell. The role of these interactions for soil development is also investigated, in order to determine if aggregates form by accumulating stable organo-mineral interactions over repeated wet/dry cycles. The nature of lignin-kaolinite interactions are investigated using solid-state magic angle spinning NMR and T1 relaxation NMR. Finally, a soil microcosm experiment was carried out, in order to determine the microbial response to the addition of lignin, which present a challenging substrate for ecosystems of degraded soils. These experiments illustrate the importance of particulate interactions for forming stable soil aggregates and soil structure, and the results provide insights into how particulates may be engineered to improve soils with a targeted approach.