Separation of light and heavy organic matter fractions in soil — Testing for proper density cut-off and dispersion level
Articolo
Data di Pubblicazione:
2012
Abstract:
Density fractionation is frequently applied to separate soil organic matter according to the degree and the
mode of interaction with minerals. Density fractions are operationally defined by density cut-off and sonication
intensity, which determine the nature of the separated material. However, no tests or general agreements exist
on the most appropriate density cut-off as well as on method and intensity of dispersion. Numerous variants
have been proposed and applied, with results often contrasting each other and being hard to interpret. Here,
we aimed at separating two light fractions (free and occluded into aggregates) composed of almost pure organic
material, and one heavy fraction comprising the organic–mineral associations. Wetested effects of different density
cut-offs and sonication intensities, in combination and separately, on fraction yields, as well as on the fractions'
organic carbon, total nitrogen and lignin-derived phenols. We tried to find optimum density cut-offs and
sonication intensities, providing light fractions with maximum organic material and minimum contamination
by mineralmaterial. Under the test conditions, a density of 1.6 g cm−3 gave best results for all test soils, allowing
for separation of maximums amounts of almost pure organic material. The density cut-off at 1.6 g cm−3 is well
in line with previous studies and theoretical considerations, therefore we recommend the use of this density
as most suitable for separation of organic debris. Sonication levels for aggregate disruption to achieve complete
separation of occluded light organic matter varied amongst soils. The necessary intensity of dispersion relates
to the type of soil, depending on the stability of contained aggregates. The application of one single dispersion
energy level to different soils may result either in mineral contamination or in incomplete separation of light
and heavy fractions as well as in redistribution of organic material amongst fractions. This means there is no
single sonication level that can be applied to all soils. Thus, obtaining a meaningful light fraction residing within
aggregates (occluded light fraction) requires assessment of the dispersion energy necessary to disrupt the aggregate
system of a given soil without dispersion of organic–mineral associations. This can be done in preexperiments
where the soil is fractionated at different sonication levels. The appropriate dispersion is determined
by mass yields and OC content of the obtained occluded fractions.
Tipologia CRIS:
03A-Articolo su Rivista
Elenco autori:
Cerli, C.; Celi, L.; Kalbitz, K.; Guggemberger, G.; Kaiser, K
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