Calcium carbonate in termite galleries – biomineralization or upward transport?

Abstract

Termites and soil calcium carbonate are major factors in the global carbon cycle: termites by their role in decomposition of organic matter and methane production, and soil calcium carbonate by its storage of atmospheric carbon dioxide. In arid and semiarid soils, these two factors potentially come together by means of biomineralization of calcium carbonate by ter- mites. In this study, we evaluated this possibility by testing two hypotheses. Hypothesis 1 states that termites biomineralize calcium carbonate internally and use it as a cementing agent for building aboveground galleries. Hypothesis 2 states that termites transport calcium carbonate particles from subsoil horizons to aboveground termite galleries where the carbonate detritus becomes part of the gallery construction. These hypotheses were tested by using (1) field docu- mentation that determined if carbonate-contain- ing galleries only occurred on soils containing calcic horizons, (2) 13 C/ 12 C ratios, (3) X-ray diffraction, (4) petrographic thin sections, (5) scanning electron microscopy, and (6) X-ray mapping. Four study sites were evaluated: a C4- grassland site with no calcic horizons in the underlying soil, a C4-grassland site with calcic horizons, a C3-shrubland site with no calcic hori- zons, and a C3-shrubland site with calcic horizons. The results revealed that carbonate is not ubiq- uitously present in termite galleries. It only occurs in galleries if subsoil carbonate exists within a depth of 100 cm. 13 C/ 12 C ratios of carbonate in termite galleries typically matched 13 C/ 12 C ratios of subsoil carbonate. X-ray diffraction revealed that the carbonate mineralogy is calcite in all galleries, in all soils, and in the termites them- selves. Thin sections, scanning electron micros- copy, and X-ray mapping revealed that carbonate exists in the termite gut along with other soil particles and plant opal. Each test argued against the biomineralization hypothesis and for the upward-transport hypothesis. We conclude, therefore, that the gallery carbonate originated from upward transport and that this CaCO3 plays a less active role in short-term carbon sequestra- tion than it would have otherwise played if it had been biomineralized directly by the termites.