Soil carbonate decomposition by acid has little effect on δ13C of organic matter

The δ13C of soil organic C is often utilized to document vegetation changes and to estimate rates of soil organic C turnover. Soil carbonate C is isotopically distinct from organic C, and must be removed by acidification prior to isotopic analysis of soil organic matter. We determined the effects of acid concentration (0.1, 0.5, 1, or 6 m HCl) and duration of treatment (1, 2, 3, or 8 d) on soil δ13C and the concentration of C and N in a carbonate-free entisol and a calcareous vertisol both obtained from Texas. δ13C values of organic C in the entisol were reduced significantly by treatment with 6 m HCl for more than 1 d; however, all other combinations of acid strength and treatment duration had no effect on δ13C of organic C in that soil. In the calcareous soil, δ13C of organic C was largely unaffected by acid concentration or duration of treatment; however, soils treated with 0.1 m HCl required 3 d to fully remove all carbonate and achieve δ13C values comparable to those in other treatment combinations. Treatment of calcareous soils with 0.5 m HCl for 24 h appears sufficient to remove all carbonate C prior to isotopic analysis, and this treatment has no discernible effect on the δ13C of the soil organic matter.