Soil carbonate decomposition by acid has little effect on delta C-13 of organic matter
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abstract
The 13C 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 13C and the concentration of C and N in a carbonate-free entisol and a calcareous vertisol both obtained from Texas. 13C 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 13C of organic C in that soil. In the calcareous soil, 13C 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 13C 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 13C of the soil organic matter.
