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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.