Amount and composition of clay-associated soil organic matter in a range of kaolinitic and smectitic soils

In the global carbon cycle, soil organic matter (SOM) is a major source/sin k of atmospheric carbon. Clay minerals stabilize part of the SOM through mi neral-organic matter binding. Stabilization of organic matter is essential for tropical soils. Since the climatic conditions of the tropics favor deco mposition of organic matter, tropical soils would be very poor in organic m atter without this stabilization process. This research aims at determining the effect of clay mineralogy on the amount and composition of organic mat ter 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 C-13 NMR spectroscopy and Py-GC/MS. The content of carbon in the total clay-size fra ction showed no significant difference between kaolinitic and smectitic soi ls. 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 kaolinit e-associated SOM was extractable by NaOH. In the smectitic soils, pyrophosp hate extracted more organic carbon than did NaOH. The Py-GC/MS and NMR resu lts indicate that kaolinite-associated SOM is enriched in polysaccharide pr oducts, while smectite-associated organic matter contains many aromatic com pounds. We suggest that different clay minerals use different binding mecha nisms to complex SOM. As a result, the composition of clay-associated organ ic matter would be influenced by the type of clay that is dominantly presen t in the soil. (C) 2001 Elsevier Science B.V. All rights reserved.