Composition and turnover time of organic matter in soil fractions with different magnetic susceptibility

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 (0.5-2.0 mm 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, C-14 concentration, and spectroscopic properties of SOM (C-13 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 C-14 concentration revealed shortest TT of SOM in the < 0.5 mm MA fraction and longest TT in the < 0.5 mm NM fraction, while the 0.5-2.0 mm 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.