Effect of K ₂ SO ₄ concentration on extractability and isotope signature ( δ ¹³ C and δ ¹⁵ N ) of soil C and N fractions

Summary Determination of the labile soil carbon ( C ) and nitrogen ( N ) fractions and measurement of their isotopic signatures ( δ 13 C and δ 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 δ 13 C and δ 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 R ussia. 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 U mbrisols. 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 δ 13 C and δ 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 ).