Acid fumigation of soils to remove carbonates prior to total organic carbon or carbon-13 isotopic analysis

The use of C-13 natural abundance (delta C-13) to follow C input to soil has gained widespread acceptance. However, inorganic C present in the soil as carbonates will interfere with the measurement of soil organic C-13 unless removed or excluded from measurement. We report a simple and convenient HCl-fumigation method to remove inorganic C from soil. Soil samples are weighed in Ag-foil capsules, arranged on a microliter plate, wetted with water to approximately field capacity, and placed in a desiccator containing a beaker with concentrated (12 M) HCl. The carbonates are released as CO2 by the acid treatment in 6 to 8 It. The soil samples are then dried at 60 degreesC prior to isotope determination. The advantages of the HCl-fumigation method to remove inorganic C from the soil are that: (i) no water soluble C will be lost from the soil; (ii) a large number of samples can be processed simultaneously; (iii) the removal of inorganic C is rapid and complete; and (iv) the method could also be used to determine both organic and inorganic C content in the soil. A potential disadvantage, however, is that the HCl fumigation changed the N-15 natural abundance of soil N.