Termite-affected soils in the Western Cape – a toolkit for the assessment of oxalotrophy and carbon storage potential
Date
2024-12
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Stellenbosch : Stellenbosch University
Abstract
Soil carbon (C) sequestration is a nature-based strategy to reduce atmospheric CO2 levels. Soil inorganic carbon (SIC) represents a store of durable C in (semi-)arid soils. Earthen mounds (heuweltjies) occupied by Microhodotermes viator termites are ubiquitous in the Greater Cape Floristic Region of South Africa and may be active sites for the biogenic formation of calcium carbonate (CaCO3) via the oxalate-carbonate pathway (OCP). However, a lack of techniques to monitor the OCP has hindered its investigation. The impact of land use change on C storage capacity of heuweltjies demands research efforts to inform land-use decisions. No studies have confirmed whether OCP is active in heuweltjie soils or assessed the quantity and distribution of soil organic C (SOC) and SIC in heuweltjies of mesic climatic regions. The aims of this study were to develop an analytical toolkit to identify and monitor the OCP and apply these techniques to evaluate biotic and abiotic drivers of C dynamics in termite-affected soils. This toolkit included novel methods of quantifying oxalate concentrations in environmental samples by mid-infrared (MIR) spectroscopy and monitoring soil pore gas composition in incubation experiments. Soils, vegetation and termite frass (excrement) from semi-arid and mesic regions (Koringberg and Stellenbosch) and from cultivated and uncultivated heuweltjies were compared to evaluate climatic controls as well as the impact of land-use change on C dynamics in the mounds. The heuweltjie in Stellenbosch contributed to 51% of landscape SOC stocks (predominantly in the topsoil) and subsoil alkalinization suggested that bicarbonates may be produced in these heuweltjies and leached to aquifers for long-term C storage. In Koringberg, heuweltjie subsoils were enriched in SOC and SIC relative to off-mound soils. The SOC enrichment factor of uncultivated heuweltjie topsoils relative to off-mound soils was up to 5.6 times greater than that of cultivated mounds due to higher organic C content of frass derived from renosterveld vegetation compared to crops. The deeper, cultivated mound (up to 140 cm depth) stored more C than the shallower mounds in undisturbed soil (mean maximum depth of 85 cm) and contributed 53% toward total landscape C stocks. MIR spectroscopic models accurately quantified CaOx in compound clay mineral mixtures, frass samples and vegetation extracts (normalized root mean squared error of prediction ≤ 0.07). Higher soluble oxalate content in vegetation from drier climatic zones may reflect drought stress-induced oxalate synthesis in plant tissues of these regions. CaOx in frass samples may have degraded more rapidly in soils with greater moisture content. Increases in pH, calcite saturation index and apparent respiratory quotient values provided evidence of oxalotrophy in heuweltjie soils treated with CaOx or frass. Soils treated with frass sequestered more C (18.5% increase) than CaOx treatments, which may be attributed to CO2-fixation. Decreased apparent respiratory quotient (ARQ) values in frass treatments (ca. 0.5 units lower than the control) suggested preferential metabolism of lignin. This study provides an analytical toolkit that enables more comprehensive assessment of C dynamics in oxalate-rich ecosystems to inform land use decisions based on evaluation of climate-regulating ecosystem services.
Description
Thesis (PhDAgric)--Stellenbosch University, 2024.