Fractionation of Light Fraction Organic Matter In Soil: Test of Density.

For understanding and modelling of soil organic matter (OM) dynamics density fractionation is increasingly used for separation of meaningful OM fractions. The density in previous studies varies nevertheless greatly although it is crucial for amount and quality of obtained OM pools. To investigate influence of liquid density we separated the >50 mm sized non-complexed light fractions (nLF) and intra-aggregate light fraction (iLF) macro-OM from a French (Closeaux site) and a Dasnish soil (Hornum) at six liquid density ranging 1.0 to 2.2 g cm^-3 obtained by sodium polytungstate. At the two sites C4-type crops had replaced C3-type since respectively 8 and 19 years, allowing us to recognize recent and native OM. Bulk soil and aggregate-size classes (6-1 mm, 1-0.25 mm, <0.25 mm) were analysed for total-C and d¹³C by IRMS.

The French silty soil generally had large differences between bulk soil and aggregate-size classes while the two LFs did not behave in similar ways to increasing fractionation density. Here, the proportion of C4-derived C to total-C in both LFs was 56-82% at 1.0 g cm^-3 steadily decreasing to 2.2 g cm^-3 where 24-40% recent C remained.

At the Danish sandy soil nLF and iLF in bulk soil and aggregate-size classes generally behaved similar to increasing fractionation density. Here, the proportion of C in both LF fractions relative to total-C in the faction increased especially after 2.0 g cm^-3 (4-28 %), and the proportion of C4-cerived C to total-C in both LFs in this sandy soil decreased gradually from 40-75 % to 12-44 % with increasing liquid density.

Comparison the two very different soils show that OM characteristics in bulk soil and LFs gradually changed with liquid densities but in quit different ways. On this background one world-wide accepted density for isolation of functional LF macro-OM pools in soil may be questionable.

Keywords: d¹³C; Density fractionation; Light fraction macro-organic matter; Method evaluation; Soil organic matter;