Bioavailability of carbon in microaggregates formed from clay minerals and metal oxyhydroxides

Jessie Horton (2011). Bioavailability of carbon in microaggregates formed from clay minerals and metal oxyhydroxides PhD Thesis, School of Agriculture and Food Sciences, The University of Queensland.

       
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Author Jessie Horton
Thesis Title Bioavailability of carbon in microaggregates formed from clay minerals and metal oxyhydroxides
School, Centre or Institute School of Agriculture and Food Sciences
Institution The University of Queensland
Publication date 2011-07
Thesis type PhD Thesis
Language eng
Subjects 050301 Carbon Sequestration Science
050399 Soil Sciences not elsewhere classified
Abstract/Summary The sequestration of organic matter (OM) in soil is an important issue in relation to soil health and productivity and the cycling of carbon on a global scale. Commonly up to 80% of OM in soil is associated within the microstructure, defined here as the microaggregates within the soil fraction <100 ìm. Research of this fraction within a soil sample has generally focussed on the extraction of the OM under varying conditions and the nature of the fractions using microscopy. However, little is known about the soil processes that cause the OM to become sequestered in this fraction. A fundamental approach has been used in this research to study the factors that affect the processes of aggregation and the subsequent carbon sequestration in the microstructure of soils, by creating soil microaggregates under controlled laboratory conditions using well characterised minerals. The components used in the preparation of microaggregates included various types of OM (extracted from soil and leaf litter and commercial organic compounds), cations (iron, aluminium and calcium) and soil mineral particles (bentonite and kaolinite) under a range of conditions. A review of the literature suggests that the factors that affect the aggregation processes in these microaggregates might include typical soil conditions (wetting and drying cycles, salts, pH, temperature and time), nature of the soil components, and the sequence of interactions between these components. The impact of these factors on the extent of aggregation and the stability of the aggregates were determined by particle size analysis and sonication respectively. Microaggregates formed from kaolinite were not able to withstand more than one wetting and drying cycle before being dispersed. OM type, sequence of addition, pH, cation type and concentration had minimal impact on microaggregate size, but this may be mostly due to the unstable nature of the aggregates formed from kaolinite. Microaggregates formed from bentonite increased in size and in stability with an increase in the ratio of cation to clay and cation to organic matter. The solution pH did not impact on the size of the aggregates after sonication. Aluminium produced larger aggregates than iron, and iron produced larger aggregates than calcium. Whereas wetting and drying cycle had an impact on aggregate size, OM type did not. The second stage of this research was to quantify the bioavailability of the OM associated within these microaggregates, and hence the quantity of sequestered carbon. This involved the inoculation of the microaggregates with bacteria isolated from natural soil, and monitoring their subsequent respiration using gas chromatography. Carbon sequestration is the removal and long-term storage of carbon dioxide from the atmosphere. Carbon that is sequestered in soil is not bioavailable and hence bioavailability was chosen here as an indirect measure of sequestration. It is also known that the majority of carbon lost from the natural soil environment is due to microbial action. The bentonite aggregates were used in the bioavailability studies because they were larger and more stable than the kaolinite aggregates. Increasing size and stability was assumed to be associated with decreasing bioavailability of the incorporated carbon. Lysine was chosen as the organic substrate because it had a low molecular weight and was readily degradable by soil bacteria. The rate and amount of lysine bioavailable to the bacteria was dependent on the ‘factors’ used to form the bentonite aggregates. For example, microaggregates formed in solution from iron oxyhydroxides (in the absence of bentonite) resulted in the least amount of bioavailable carbon. Microaggregates formed in solution from bentonite, and bentonite/aluminium, and bentonite/iron mixtures, were also found to have significantly less bioavailable carbon than the control. Microaggregates formed in solution from aluminium oxyhydroxides alone did not impact on the amount of bioavailable carbon within the aggregates. This research has improved our understanding of the interaction between clay minerals, metal oxyhydroxides, and organic matter in soil, and how this interaction influences aggregation and carbon sequestration in soil. The knowledge gained and the new methods developed in this research can be further developed and used to improve our fundamental knowledge aggregation and carbon sequestration in soil.

 
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Created: Mon, 16 Apr 2012, 16:19:06 EST by Miss Jessie Horton on behalf of Library - Information Access Service