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Abstract Oxalate salts in organic matter are potential substrates for the oxalate‐carbonate pathway, which can sequester carbon in drylands. We compared calcium oxalate (CaOx) and water‐soluble oxalate (SOx) concentrations of samples of vegetation and termite excrement (frass) collected from termite mounds in sites across a regional rainfall gradient in western South Africa. We developed mid‐infrared (MIR) spectroscopic models to quantify oxalate components in vegetation extracts (n = 30) and frass samples (n = 39). The MIR spectroscopic method was more efficient than chemical analytical techniques of oxalate measurement. The median CaOx concentration of plants (0.311 mmol g⁻¹) was four times greater than frass (0.081 mmol g⁻¹), which may be explained by degradation of oxalates by microorganisms or selective harvesting of low‐oxalate vegetation by termites. The mean CaOx content of frass from sites in mesic regions (0.042 mmol g⁻¹) was lower relative to frass from sites in more arid regions (0.156 mmol g⁻¹), and lower in termite mounds (0.048 mmol g⁻¹) compared with off‐mound samples (0.131 mmol g⁻¹). Frass collected from sites with higher rainfall had a lower mean SOx content (0.006 mmol g⁻¹, respectively) compared with frass from sites with lower rainfall (0.013 mmol g⁻¹, respectively). This may be attributed to faster degradation of CaOx in soils with greater moisture content. Estimated annual inputs of carbon (17.6 kg mound⁻¹) and calcium (2.55 kg mound⁻¹, 20% of which occurs as CaOx) due to termite frass deposition may be instrumental in the formation of calcite via the oxalate‐carbonate pathway in soils of earthen mounds occupied by termites. This work is relevant to modeling carbon storage in drylands where termites are significant consumers of vegetation. 

Abstract Oxalic acid is one of the most abundant organic acids produced by plants. Much of the global production of oxalic acid is deposited on soil surfaces in leaf litter to be oxidized by microorganisms, resulting in a pH increase and shifting the carbonate equilibria. In what is known as the oxalate-carbonate pathway, calcium oxalate metabolism results in CO2 being sequestered into soils as insoluble calcite (CaCO3). There is a growing appreciation that the global scale of this process is sufficiently large to be an important contribution to global carbon turnover budgets. The microbiomics, genetics, and enzymology of oxalotrophy are all soundly established, although a more detailed understanding of the landscape-scale kinetics of the process would be needed to incorporate oxalotrophy as an element of process models informing the relevant Sustainable Development Goals. Here, we review the current state of knowledge of oxalotrophs and oxalotrophy and the role they play in terrestrial ecosystem services and functions in terms of carbon sequestration and nutrient cycling. We emphasize the relevance of these to the Sustainability Development Goals (SDGs) and highlight the importance of recognizing oxalotrophy, when accounting for the natural capital value of an ecosystem.