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, 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 <0.5 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. © 2019 Elsevier B.V.

Chiti T.1, 2 , Certini G.3 , Marzaioli F.4 , D'Acqui L.P.5 , Forte C.6 , Castaldi S. 2, 7, 8 , Valentini R. 1, 2, 8
  • 1 Department for Innovation in Biological, Agro-food and Forest Systems (DIBAF), University of Tuscia, via San C. De Lellis snc, Viterbo, 01100, Italy
  • 2 Foundation Euro-Mediterranean Center on Climate Change (CMCC), Division on Impacts on Agriculture, Forests and Ecosystem Services (IAFES), viale Trieste 127, Viterbo, 01100, Italy
  • 3 Dipartimento di Scienze e Tecnologie Agrarie, Alimentari, Ambientali e Forestali (DAGRI), Università di Firenze, p.le delle Cascine 28, Firenze, 50144, Italy
  • 4 Dipartimento di Matematica e Fisica, Università degli Studi della Campania Luigi Vanvitelli, viale Lincoln 5, Caserta, 81100, Italy
  • 5 Istituto di Ricerca sugli Ecosistemi Terrestri (IRET), CNR, via Madonna del Piano 10, Sesto Fiorentino, 50019, Italy
  • 6 Istituto di Chimica dei Composti Organometallici (ICCOM), CNR, via G. Moruzzi 1, Pisa, 56124, Italy
  • 7 Dipartimento di Scienze e Tecnologie Ambientali, Biologiche e Farmaceutiche (DISTABIF), Università degli Studi della Campania Luigi Vanvitelli, via Vivaldi 43, Caserta, 81100, Italy
  • 8 RUDN University, Moscow, Russian Federation
Biogeochemistry; Biological materials; Carbon; Kaolinite; Magnetic susceptibility; Magnetism; Nuclear magnetic resonance spectroscopy; Organic minerals; Soils; Tropics; Chemical compositions; Different sizes; Driving factors; Labile compounds; Mineral composition; Soil fractionations; Soil organic matters; Spectroscopic property; Nuclear magnetic resonance; chemical composition; chemical compound; concentration (composition); magnetic susceptibility; soil horizon; soil organic matter; tropical forest; tropical soil; turnover; Ghana
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