Enhanced carbon storage in semi-arid soils through termite activity

Abstract

Termites are keystone species in natural ecosystems and their role in the C cycle is potentially substantial but poorly understood. Large (20–40 m) mounds (heuweltjies) of the harvester termite Microhodotermes viator occupy up to a quarter of the semi-arid west coast region of South Africa but their C storage potential is unknown. This study determined the organic and inorganic C fractions, C stocks, and their correlation with each other, depth, and biogenic features in these mounds. Trenches (30–60 m) were excavated through 3 mounds: Buffels River (m.a.p < 100 mm), Klawer (m.a.p 100–200 mm) and Piketberg (m.a.p 300–400 mm) and grid sampled. Mound soils had significantly higher soil organic carbon (SOC) and inorganic carbon (SIC) than surrounding soils. Total C was strongly correlated (ρ > 0.9; p < 0.001) with SIC in the arid mounds and SOC (ρ > 0.75; p < 0.001) in the higher rainfall mound. There was no consistent relationship between SOC and SIC distributions throughout the mounds, which is likely related to solubility-linked translocations of carbonates. For all mounds, SOC was highest in topsoils with a second clear peak in subsoils (>1 m) that was associated with biogenic features, termite channels and burrows. Subsoils contributed substantially (36–41 %) to the total C stock. Total C stocks for the intermediate rainfall mound (Klawer) were estimated at 14.6 tons per mound, with 1.1 tons SOC. In this region, mounds occupy 27 % of the total area but contribute 44 % of the total SOC stock to a depth of 80 cm. This highlights the disproportionate contribution termite mounds make to carbon stocks of these semi-arid environments and demonstrates the importance of deep (<1 m) soil carbon for C modelling. Termite activity needs to be recognized as a major contributor to C stock variability both laterally and at depth and accounted for in land-use change (CO2-LULUCF) models.