Effect of K2SO4 concentration on extractability and isotope signature (δ13C and δ15N) of soil C and N fractions

Determination of the labile soil carbon (C) and nitrogen (N) fractions and measurement of their isotopic signatures (δ¹³C and δ¹⁵N) 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 δ¹³C and δ¹⁵N of salt-extractable organic carbon (SEOC), inorganic (N-NH<inf>4</inf>⁺ and N-NO<inf>3</inf>^-) and organic nitrogen (SEON) and salt-extractable microbial C (SEMC) and N (SEMN) in 0.05 and 0.5mK<inf>2</inf>SO<inf>4</inf> 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<inf>2</inf>SO<inf>4</inf> concentration on C and N extractability. Organic C and N were extracted 1.6-5.5 times more effectively with 0.5mK<inf>2</inf>SO<inf>4</inf> than with 0.05mK<inf>2</inf>SO<inf>4</inf>. Extra SEOC extractability with greater K<inf>2</inf>SO<inf>4</inf> 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<inf>2</inf>SO<inf>4</inf> concentrations, while SEMN was greater when extracted with 0.5mK<inf>2</inf>SO<inf>4</inf>. We demonstrate that the δ¹³C and δ¹⁵N values of extractable non-microbial and microbial C and N are not affected by K<inf>2</inf>SO<inf>4</inf> concentrations, but use of a small concentration of extract (0.05mK<inf>2</inf>SO<inf>4</inf>) gives more consistent isotopic results than a larger concentration (0.5m).