Saturday, 15 July 2006

Problem of Gley Diagnostics: Color and Iron Chemistry in Cryosols of Kolyma Lowland.

Yuriy N. Vodyanitskii, V.V. Dokuchaev Soil Science Institute, Pyzhevskii lane, 7, Moscow, 119017, Russia and Nikita S. Mergelov, Institute of Geography, RAS, Staromonetniy lane, 29, Moscow, 119017, Russia.

The color of both gleyic and non-gleyic loamy cryogenic soils of the Kolyma lowland expressed in highly informative optical system CIE-L*a*b* does not show significant difference. In order to raise effectiveness in distinguishing gleyic and non-gleyic soils/horizons all samples were treated with 30-40% H2O2. Two parameters have been used: 1) increase in redness after H2O2 treating; 2) the degree of lighting related to organic carbon content “Light” = (LH2O2 – Linitial) / (1 + lgCorg). The iron in oxalate (Feo) and dithionite (Fed) extracts have been measured. Mössbauer spectroscopy also was used to estimate possible mineral composition and the Fe3+/(Fe3++ Fe2+) ratio for initial and H2O2 treated samples.

Non-treated samples of Cryosols have low reddishness: a* < 2. Most of samples from B, Bg and G horizons showed slight decrease in redness after H2O2 treating, thus the greenish elements of color increase: 0 > Δa* > -1. The humus of soils being investigated is raw and hardly oxidized: the “Light” value is low (2-9). Hydromorphism development leads to higher degree of organic matter oxidation: in B horizon “Light” value=2-5 and for Bg and G horizons “Light”=5-9. The whole data show inverse relation between Δa* and “Light”.

The composition of Fe combinations in soils of various degree of hydromorphism was estimated using two chemical criteria: Feo:Fed (Shwertmann criteria) and content of biologically reducible iron: Fe=0,19 Feo-0,028 (Fed - Feo) (empirical criteria by Van Bodegom obtained in experiments on biological reduction of various soils from East Asia, East Africa, Europe and USA - Van Bodegom et al, 2003). Both criteria are able to distinguish B horizons from Bg and G, but Van Bodegom's criteria is more effective. The most effective differentiation of gleyic and non-gleyic soils was estimated in the following coordinates:  “Light” – reducible Fe.

Mössbauer spectroscopy of selected samples showed that most of total iron in gleyic and non-gleyic soils is contained in silicates. The amount of hydroxides free iron is rather low.

Samples from Kolyma Cryosols showed low degree of oxidation Fe3+/(Fe3++ Fe2+)=0,48-0,60 in comparison to samples of loamy soils from European territory of Russia where oxidation degree reached 0,85-0,95. This fact alongside with estimated absence of hematite (αFe2O3) and only some amount of dispersed hydroxides is evident for weakly expressed Fe oxidogenesis (oxides formation).

It has been found that H2O2 treatment of Kolyma soils with high content of silicate's Fe2+ results in partial Fe oxidation unlikely to European loamy soils with high content of iron hydroxides where H2O2 acts as a Fe reducer.

Gleyic and non-gleyic soils are clearly divided using simultaneously trans-Fe3+ /( trans-Fe3+ + cis-Fe3+) ratio and Feo:Fed ratio. Gleyic samples have much higher Fe3+ saturation of trans positions and higher Shwertmann criteria in comparison to non-gleyic B horizons. The trans-Fe3+ /( trans-Fe3+ + cis-Fe3+) ratio in gleyic horizons reached 0,72 and in non-gleyic (B) horizons was equal to 0,55. We propose the following mechanism for this phenomenon: Fe3+ mobilization and introduction in trans positions in silicate's lattice which results in shift to the green part of spectra. Thus Fe3+ is not completely removed from the horizon or just reduced in situ to Fe(II)-minerals but partially introduced in silicates.

It is evident that new data on Mössbauer spectroscopy demands new interpretation of oxalate- and dithionite-extractable Fe in Cryosols of the Kolyma lowland. The estimated high content of free Fe2O3d >1% in Bg and G horizons contradicts with Mössbauer spectroscopy data which is evident for small amount of Fe2O3d. It seems that CBD and oxalate extract iron of silicates. Thus Feo:Fed ratio in Kolyma's loamy soils is much more about Fe-silicates composition than of amorphous to free iron relation. Nevertheless de facto Feo:Fed ratio effectively works for distinguishing of gleyic – non-gleyic soils. 


Back to CR Soils of Northern, Southern Polar Region and Soils of High Elevations and Their Relationship to Global Climate Change - Poster
Back to WCSS

Back to The 18th World Congress of Soil Science (July 9-15, 2006)