Oxalate content of vegetation and termite frass in western South Africa

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.