Calcium carbonate in subterranean termite foraging galleries in the northern Chihuahuan Desert

In the northern Chihuahuan Desert, calcium carbonate occurs in many subterranean termite foraging galleries—structures constructed with soil particles that encase aboveground plant debris. Two hypotheses for the origin of carbonate were tested: (1) termites are mining (translocating) calcium carbonate from subsoil carbonate horizons; (2) termites are biomineralizing (making) carbonate. This study was undertaken because the origin of calcium carbonate in termite foraging galleries may have important implications for the carbon cycle and carbon sequestration in desert environments.; The study was conducted at four sites on New Mexico State University Chihuahuan Desert Rangeland Research Center and USDA Jornada Experimental Range. Two sites (B_w and M_w) had soils that contained calcic diagnostic subsurface horizons. The other two sites (B_w/o and M_w/o) had soils without calcic diagnostic subsurface horizons. Stable isotope δ13C values, petrographic thin sections, scanning electron microscopy (SEM), x-ray diffraction (XRD), and field observations were used to test the hypotheses.; Findings that provide evidence for the exclusion of the biomineralization hypothesis include the following: (1) calcareous termite galleries only occur in the study sites where soils have calcium carbonate accumulation at shallow depths (10–25 cm); (2) thin sections revealed crystals with optical properties similar to calcite distributed within termite bodies. However, SEM spot chemical analysis and x-ray chemical maps indicated that these crystals are composed of potassium and phosphorus, but not calcium; (3) calcium is mainly distributed in termite guts, which is coincident with silica distribution and other ingested particles.; Evidence that supports the hypothesis that carbonate particles in termite galleries on the soil surface were transported from carbonate horizons in subsoil include the following: (1) after minimizing the spatial variation that exists within a study site, δ13C values of carbonate in termite galleries were similar to the range of subsoil carbonate horizons; (2) the XRD results indicated that carbonate mineralogy of termite galleries is similar to carbonate mineralogy of subsoil carbonate horizons.