ISSN:
1365-2389
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
,
Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
Notes:
Thirteen acid soils were collected from typical volcanic regions in Japan (S content: 0.9–4.1, mean = 2.2g kg−1; pH (H2O): 2.81–3.93, mean = 3.33), as well as nine reference soils (S: 0.6–1.7, mean= 1.1 gkg−1; pH(H2O): 4.10–4.74, mean = 4.47). Humic acids were extracted from the soils separately with 0.1 M NaOH and precipitated by acidification (pH = 2, HCl). After purification, the humic acids were subjected to colorimetric analysis using a DTNB reagent [5,5′-dithiobis(2–nitrobenzoic acid] for the active -SH group. Since humic acids have significant absorption at 4I2 nm, the coloured compound (5–mercapto-2–nitrobenzoic acid) was separated from the humic acids by ultrafiltration or solvent extraction prior to the colorimetric measurement. Humic acids also caused discoloration of the coloured compound when they coexisted in the reaction solutions. Thus, the reproducible determination of -SH was accomplished by employing a standard addition technique (-SH standard: cysteine). Although -SH contents obtained by the ultrafiltration method were considerably higher than those by the solvent-extraction method, probably due to the denaturation of humic acids by the higher buffer concentration used in the ultrafiltration method, they correlated well with each other. The humic acids from acid soils contained apparently higher concentrations of -SH (120–510, mean = 270mg S kg−1 by the ultrafiltration method; 8–110, mean = 38mg S kg−1 by the solvent-extraction method) compared to those from reference soils [20–260, mean = 90mg S kg−1 by the former; not detectable (〈5)-34, mean = 11 mg S kg−1 by the latter]. This -SH enrichment in the humic acids from acid soils may result from the degradation and subsequent humification of S-rich debris of plants and micro-organisms and/or direct incorporation of volcanic acid gas (H2S) into the humic acids.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-2389.1994.tb00500.x
