Improved method for removing siderite by in situ acidification before elemental and isotope analysis of soil organic carbon

Siderite (FeCO₃) is an iron carbonate mineral commonly found in sediments and soils. Similarly to other carbonates, such as calcite or dolomite, it may substantially affect the quantification of organic carbon (C) as well as determination of C isotope ratio of soil organic matter. Both analyses require effective removal of siderite by pretreatment with acid. However, little is known about the siderite removal efficiency of the acid pretreatment methods which have been previously proposed in the literature. In our study, we tested three previously proposed carbonate removal methods for siderite removal in siderite‐containing soils. Furthermore, we tested whether siderite C content in a soil sample can be detected as CO₂ evolved after H₃PO₄ addition which would allow organic C determination from the difference between total and inorganic C. None of the three tested pretreatment methods led to sufficient removal of siderite C when applied on siderite alone. Therefore, we developed a new protocol for a 4‐day treatment with 10% HCl at 25°C. At siderite content of up to 10 wt.%, the removal efficiency of our method (99–100%) was sufficient both for organic C as well as for C isotope analyses. This was further confirmed with tests on siderite‐containing soils. These showed that the method of Larson et al. ( 2008 ) developed for sediments is also suitable. However, the new protocol provides advantage in terms of less microplate manipulation, capsule overflow and oven use. We found that CO₂ is not evolved 2 minutes after H₃PO₄ addition from siderite in contrast to calcite and dolomite. This fact can be used for separate quantification of inorganic C from calcite/dolomite and siderite, e.g., in studies of their different role in soil development. We showed that siderite‐containing soils require special pretreatment procedure before organic C and ¹³C/¹²C analyses. We recommend using our protocol if techniques such as XRD or SEM‐EDS indicate the presence of siderite in soil.