Impacts of land use change on carbon storage in termite mounds of South Africa

Abstract

Subterranean termite mounds are key contributors to soil carbon storage in the semi-arid landscapes of the Greater Cape Floristic Region of South Africa. This study assessed soil organic carbon (SOC) and soil inorganic carbon (SIC) stocks in cultivated and uncultivated mounds in the Swartland region of the Western Cape. Radiocarbon activities of organic matter and soil-respired CO₂ were measured to evaluate vertical carbon mixing rates within uncultivated mound soil profiles and to gain insight into microbial SOC utilization in topsoils of cultivated and uncultivated mounds. Cultivation significantly decreased total C stocks in mound topsoils. Spatial distribution of SOC, allocation of SOC to different stability fractions and SOC radiocarbon signatures in uncultivated mound soils showed that termite bioturbation facilitated the downward vertical movement of organic material and subsequent stabilization by organo-mineral interactions in subsoils. Spatial association of SOC with SIC provided evidence of biogenic carbonate formation in both cultivated and uncultivated mounds, which may serve as an additional C stabilization mechanism. Cultivation altered SOC stability, as soil-respired CO₂ with lower radiocarbon activity (Δ¹⁴C = −18.6 ‰) in cultivated mound topsoils compared to uncultivated mounds (Δ¹⁴C = 15.7–100.45 ‰) indicates microbial exhaustion of labile SOC and degradation of recalcitrant pools in cultivated mound soils. The findings highlight termite mounds as critical reservoirs of subsoil SOC and SIC in a semi-arid region of South Africa. Cultivation destabilizes SOC stores in mound topsoils, therefore sustainable management of termite-affected landscapes is essential to maintain their carbon storage potential.