Composition and turnover time of organic matter in soil fractions with different magnetic susceptibility
Author(s)
Date Issued
2019
Type
article
Volume
349
Start Page
88
End Page
96
Journal
Abstract
The composition and turnover time (TT) of organic matter in soil fractions with different magnetic susceptibility
were studied in a tropical primary forest in Ghana. The starting hypothesis was that soil organic matter (SOM)
composition and properties depend on the magnetic susceptibility of the minerals SOM is associated with. Soil
samples from 0 to 5, 5–15, 15–30, and 30–50 cm depth intervals were sieved to remove rock fragments
(> 2.0 mm) and then separated into two size fractions (0.5–2.0mm and<0.5 mm) that were processed by a High Gradient Magnetic Separator (HGMS) to finally obtain four fractions with different size and/or magnetic
susceptibility. All fractions were analysed for their mineral composition, 14C concentration, and spectroscopic
properties of SOM (13C CPMAS NMR).
From a mineralogical point of view, the magnetic (MA) fractions essentially differed from the non-magnetic
(NM) ones for a higher presence of oxides and kaolinite, which per se is non-magnetic. In terms of chemical
composition of SOM, the MA fractions showed higher contribution of labile compounds than the NM ones. At all
depths, the 14C concentration revealed shortest TT of SOM in the<0.5mm MA fraction and longest TT in
the<0.5mm NM fraction, while the 0.5–2.0mm fractions showed intermediate and similar TT's. At
depths<5 cm, the fine NM fraction was not significantly influenced by the carbon fixed after the 1950s (“bomb
carbon”), having TT of almost 1000 years and suggesting that in this tropical soil some SOM can be stabilized
also in relatively superficial horizons.
Our findings support the hypothesis that minerals are driving factors of the fate of SOM. As a consequence,
soil fractionation based on magnetic susceptibility might be a meaningful procedure for having insight into SOM
dynamics.