Stability of organic carbon in soil particle-size fractions at different depths: insight on C dynamics in two Australian soils

Soil organic carbon comprises a large and highly dynamic reservoir of the global carbon cycle. Due to its origins as atmospheric CO2, its numerous benefits to soil fertility, and the implications this has for feeding a growing world population, soil carbon sequestration, that is the transfer of atmospheric CO2 to soils, has been trumpeted as a winning solution for humans and the environment. Several issues surrounding our knowledge of soil carbon dynamics must be resolved to ensure the successful implementation of soil carbon sequestration schemes, specifically the factors influencing soil organic carbon storage and stability. Identifying soils with the highest capacity to store organic carbon provides the greatest potential to meet greenhouse gas reduction targets, whereas targeting highly stable mechanisms of soil organic carbon storage is advocated as the approach with the greatest likelihood for long-term success. This thesis combines physical and chemical techniques to investigate soil organic carbon storage and dynamics throughout the profile. Two soils of different mineralogy and texture (a 'clayey' soil and a 'sandy' soil) were sampled to bedrock and fractionated into differing particle-sizes, which were then chemically investigated. Analysis techniques included elemental analysis for organic carbon content, diffuse-reflectance infrared spectroscopy to investigate the chemical components of soil organic carbon, and radiocarbon analysis to determine the age of soil organic carbon. The sampling sites were chosen to minimise differences in external factors influencing soil organic carbon dynamics, such as climate and topography, enabling assessment of soil organic carbon storage and dynamics as a function of mineralogy, texture and depth. Two main concepts are probed within this work. Firstly, that soil mineralogy and texture are driving factors determining soil organic carbon storage and depth distribution. Organic carbon was preferentially enriched in the finest fraction in both soils, showing that increased specific surface area enhances soil organic carbon storage. The depth distribution of soil organic carbon differed between the soils: the sandy soil had a stronger gradient of organic carbon from topsoil to bedrock than the clayey soil, highlighting the influence of mineralogy and texture on soil organic carbon retention. The second focus of this thesis are the stabilisation mechanisms of soil organic carbon, that is mechanisms which lead to a reduction in the biodegradation of soil organic carbon and hence increase in the retention time of organic carbon in soils. The results of the radiocarbon analysis and DRIFT spectroscopy indicate different stabilisation mechanisms in the soils. In the clayey soil, the oldest, most stable carbon is physically protected within aggregates, which are highly stable due to soil mineralogy providing effective binding agents for aggregates. No link was found between soil organic carbon chemistry and stability in the clayey soil. In the sandy soil, mineralogy and texture do not physically protect soil organic carbon, and the greatest soil organic carbon stability arises from pyrogenesis, which alters the chemical composition of soil organic carbon, leading to stable carbon stored as charcoal. Soil organic carbon stability increased with greater soil depth and lower organic carbon content, implying that organic carbon stability result from low carbon availability inhibiting microbial activity in subsoils. Several conclusions are drawn from the results of this thesis. Firstly, that enhanced soil carbon storage is not linked with enhanced soil carbon stability, as the greatest carbon storage occurs within the finest soil fractions, but this is not the most stable carbon. This implies that trade-offs between amount and longevity of soil carbon storage will have to be made within the framework of soil carbon sequestration schemes. Secondly, that the stabilisation mechanisms of soil organic carbon are texture and mineralogy dependent. The independence of soil organic carbon chemistry and stability in the clayey soil leads to the conclusion that the input quality of soil organic matter in heavy-textured soils may not be relevant to its long-term retention: sequestering carbon in such soils may be as simple as increasing carbon inputs and allowing aggregation to occur. In contrast, in sandy soils the quality of the soil organic matter appears highly relevant to its stability in the soil, and addition of highly stable carbon forms, namely charcoal, to sandy soils is required to increase stable carbon forms in such soils. Lastly, the high stability of organic carbon in subsoils may result from substrate limitation. If confirmed, this has far-reaching implications for soil carbon sequestration schemes, as targeting subsoils will not lead to a long-term increase in soil organic carbon stocks if carbon availability is increased. Future studies should, therefore, investigate the links between radiocarbon age and organic carbon content in subsoils.