Effect of K2SO4 concentration on extractability and isotope signature (13C and 15N) of soil C and N fractions

Determination of the labile soil carbon (C) and nitrogen (N) fractions and measurement of their isotopic signatures (C-13 and N-15) 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 C-13 and N-15 of salt-extractable organic carbon (SEOC), inorganic (N-NH4+ and N-NO3-) and organic nitrogen (SEON) and salt-extractable microbial C (SEMC) and N (SEMN) in 0.05 and 0.5mK(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 K2SO4 concentration on C and N extractability. Organic C and N were extracted 1.6-5.5 times more effectively with 0.5mK(2)SO(4) than with 0.05mK(2)SO(4). Extra SEOC extractability with greater K2SO4 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 K2SO4 concentrations, while SEMN was greater when extracted with 0.5mK(2)SO(4). We demonstrate that the C-13 and N-15 values of extractable non-microbial and microbial C and N are not affected by K2SO4 concentrations, but use of a small concentration of extract (0.05mK(2)SO(4)) gives more consistent isotopic results than a larger concentration (0.5m).