TY - JOUR

T1 - Separation of light and heavy organic matter fractions in soil - Testing for proper density cut-off and dispersion level

AU - Cerli, C.

AU - Celi, L.

AU - Kalbitz, K.

AU - Guggenberger, G.

AU - Kaiser, K.

N1 - Funding information: This work was funded by the EU-INCO project no. 013388 : OMRISK . We are grateful to Gerlinde Hardt for assistance in density fractionation and elemental analyses. We thank the COST Action FP0803 “Belowground carbon turnover in European forests” for the opportunity of helpful discussions and feedbacks. We also thank the two anonymous reviewers whose comments helped to improve the manuscript.

PY - 2012/1/15

Y1 - 2012/1/15

N2 - 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. We tested 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 mineral material. Under the test conditions, a density of 1.6gcm -3 gave best results for all test soils, allowing for separation of maximums amounts of almost pure organic material. The density cut-off at 1.6gcm -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 pre-experiments 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.

AB - 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. We tested 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 mineral material. Under the test conditions, a density of 1.6gcm -3 gave best results for all test soils, allowing for separation of maximums amounts of almost pure organic material. The density cut-off at 1.6gcm -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 pre-experiments 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.

KW - Aggregate disruption

KW - Density fractionation

KW - Lignin

KW - Sodium polytungstate

KW - Soil aggregates

KW - Sonication

UR - http://www.scopus.com/inward/record.url?scp=84855170122&partnerID=8YFLogxK

U2 - 10.1016/j.geoderma.2011.10.009

DO - 10.1016/j.geoderma.2011.10.009

M3 - Article

AN - SCOPUS:84855170122

VL - 170

SP - 403

EP - 416

JO - GEODERMA

JF - GEODERMA

SN - 0016-7061

ER -