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

Abstract

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).

15N Natural Abundance of Soil Microbial Biomass in Alpine and Tundra Ecosystems

“…The model is also confirmed in soils of the substrate age gradient in Hawaii [14]. These outstanding works were followed by a number of new evidences, confirming the 15 N-enrichment of soil microbial biomass [20,22,26,30,32], as well as of some species of microorganisms during their cultivation on different nitrogen-containing media [28,33]. Single studies confirmed the proposed model by a laboratory incubation experiment with the soil [20] and by the analysis of soils of different land-use systems [32].…”

“…There is a good agreement of δ 15 N micr in humus horizons with previously published results. There are evidence of the values in the range 6.3-7.2‰ for mountain-meadow alpine soils [22]; and within 4.7-5.9‰ for soils of coniferous and mixed forests in Austria [26], coniferous plantations in Central China [32], and grass ecosystems in Kansas prairies [31]. Higher values (7-11‰) were obtained for plowed Luvisol in France [20] and for soils of the elevation gradient in northern Arizona [14].…”

“…samples, using 0.05 M K 2 SO 4 instead of the standard 0.5 M K 2 SO 4 . It has been shown that this smaller salt concentration does not significantly affect the extractability of chloroform-labile nitrogen, but provides more reproducible results of the isotope analysis due to higher nitrogen concentration in the analyzed sample: salt after evaporation of the K 2 SO 4 extract [22].…”

Legacy Effects of Agricultural Practices Override Earthworm Control on C Dynamics in Kiwifruit Orchards

“…Soil microbial carbon was determined by the fumigationextraction method (Vance et al, 1987), which involves the fumigation of one sample with ethanol-free chloroform followed by extraction of the organic carbon from the fumigated and nonfumigated samples with 100 mL of 0.5 M K 2 SO 4 . The organic C content from all samples (non-fumigated and fumigated) was quantified using the continuous flow autoanalyzer (Innovox TOC Analyzer, Sievers), with the results from the non-fumigated sample representing the salt-extractable organic C (Makarov et al, 2015). Microbial biomass C was calculated according to the following formula:…”

Water extractable organic carbon and nitrogen and their stable isotopes from long-term experiment in a Japanese rice paddy

“…There are many studies on the extractable organic N and dissolved organic N in soils to predict N mineralization, N leaching, and to evaluate agricultural (nutrient) management practices (Curtin et al, 2006;Ros et al, 2009), but very few on determining δ 15 N values of WEN or WEON (Makarov et al, 2015). HWEN (or WEN) could be divided into two parts of organic N and inorganic N (NO 3 --N and NH 4 + -N).…”