Amount and composition of clay-associated soil organic matter in a range of kaolinitic and smectitic soils |
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| Institution: | 1. Laboratoire d''Hydrologie et de Géochimie de Strasbourg, EOST, Université de Strasbourg, CNRS, 1 rue Blessig, 67084 Strasbourg, Cedex, France;2. Laboratoire de Biogéochimie des Ecosystèmes Forestiers, INRA, 54280, Champenoux, France;1. Aarhus University, Department of Agroecology, Research Centre Foulum, Blichers Allé 20, P.O. Box 50, DK-8830 Tjele, Denmark;2. University of Guelph, School of Environmental Sciences, Guelph, Ontario N1G 2W1, Canada;3. Agroscope, Department of Natural Resources & Agriculture, Reckenholzstrasse 191, CH-8046 Zürich, Switzerland;4. Swedish University of Agricultural Sciences, Department of Soil & Environment, Box 7014, SE-75007 Uppsala, Sweden;1. Department of Earth & Environmental Science, University of Pennsylvania, Philadelphia, PA 19104-6316, USA;2. Department of Crop & Soil Science, University of Georgia, Athens, GA, USA;1. Agricultural Research Organization, Institute of Soil, Water and Environmental Sciences, The Volcani Center, Bet Dagan, Israel;2. Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA |
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| Abstract: | In the global carbon cycle, soil organic matter (SOM) is a major source/sink of atmospheric carbon. Clay minerals stabilize part of the SOM through mineral–organic matter binding. Stabilization of organic matter is essential for tropical soils. Since the climatic conditions of the tropics favor decomposition of organic matter, tropical soils would be very poor in organic matter without this stabilization process. This research aims at determining the effect of clay mineralogy on the amount and composition of organic matter that is bound to the mineral surface. We focused on organic matter that is associated with kaolinite and smectite. We characterized kaolinite- and smectite-associated SOM in soils from seven countries, employing 13C NMR spectroscopy and Py-GC/MS. The content of carbon in the total clay-size fraction showed no significant difference between kaolinitic and smectitic soils. This suggests that the total amount of organic carbon in the clay-size fraction is independent of the clay mineralogy. We first extracted the clay fraction with NaOH and thereafter with Na4P2O7. About half of the kaolinite-associated SOM was extractable by NaOH. In the smectitic soils, pyrophosphate extracted more organic carbon than did NaOH. The Py-GC/MS and NMR results indicate that kaolinite-associated SOM is enriched in polysaccharide products, while smectite-associated organic matter contains many aromatic compounds. We suggest that different clay minerals use different binding mechanisms to complex SOM. As a result, the composition of clay-associated organic matter would be influenced by the type of clay that is dominantly present in the soil. |
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