Determination of organic and inorganic carbon, delta C-13, and nitrogen in soils containing carbonates after acid fumigation with HCI

In carbonate-containing soils a reliable determination of organic C requires a method that effectively separates organic and inorganic C without altering the organic matter. This study was conducted to determine whether HCI vapor completely removes carbonates even in dolomite-rich soils and to what extent a widely used acid-fumigation method has to be modified for humus-rich soils. Furthermore, it was tested whether HCI fumigation alters organic-C content. Since C and N parameters are often analyzed simultaneously we also tested the influence of acid-vapor treatment on N content and on delta C-13 of soil organic matter. We applied fumigation with 37% HCI for 8 and 32 h using 9 carbonate-containing soil samples. Inorganic C ranged from 7 to 124 and organic C from 9 to 267 g kg(-1). The maximum contents of dolomite and calcite were 940 and 640 g kg(-1), respectively. A time of 8 h was enough to completely remove all carbonates. Neither the content nor the delta C-13 of organic C were significantly affected by fumigation. In contrast, N contents were altered by acid treatment. Based on these results and on our experience in analyzing more than 1000 soil samples, a recommended procedure for acid fumigation of carbonate-containing soils with a wide range of organic- and inorganic-C contents was derived. Samples pretreated in this way can be analyzed reliably for their organic-C content and delta C-13. Furthermore, N and inorganic-C contents can be determined with a quality sufficient for many purposes.