Determination of the labile soil carbon (C) and nitrogen (N) fractions and measurement of their isotopic signatures ( 13 C and 15 N) has been used widely for characterizing soil C and N transformations. However, methodological questions and comparison of results of different authors have not been fully solved. We studied concentrations and 13 C and 15 N of salt-extractable organic carbon (SEOC), inorganic (N-NH 4 + and N-NO 3 − ) and organic nitrogen (SEON) and salt-extractable microbial C (SEMC) and N (SEMN) in 0.05 and 0.5 m K 2 SO 4 extracts from a range of soils in Russia. Despite differences in acidity, organic matter and N content and C and N availability in the studied soils, we found consistent patterns of effects of K 2 SO 4 concentration on C and N extractability. Organic C and N were extracted 1.6-5.5 times more effectively with 0.5 m K 2 SO 4 than with 0.05 m K 2 SO 4 . Extra SEOC extractability with greater K 2 SO 4 concentrations did not depend on soil properties within a wide range of pH and organic matter concentrations, but the effect was more pronounced in the most acidic and organic-rich mountain Umbrisols. Extractable microbial C was not affected by K 2 SO 4 concentrations, while SEMN was greater when extracted with 0.5 m K 2 SO 4 . We demonstrate that the 13 C and 15 N values of extractable non-microbial and microbial C and N are not affected by K 2 SO 4 concentrations, but use of a small concentration of extract (0.05 m K 2 SO 4 ) gives more consistent isotopic results than a larger concentration (0.5 m).
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