Organic Matter Composition and Soil Mineral Interactions across a Substrate Age Gradient in Hawai'i

We examined the composition of organic matter and its interaction with soil minerals across a substrate age gradient in Hawai'i. Our objectives were to (1) determine the type of soil organic matter (SOM) that accumulates with depth (2) assess interactions between soil minerals and organic matter and (3) assess the possible mechanisms for SOM accumulation. Sequentially deeper soil samples were collected from 6 sites across a substrate age gradient (300y - 4.1My) of mantle derived lava. The sampling sites all have similar rainfall (2500 mm), vegetation and topographic position. Samples from major diagnostic soil horizons were analyzed for 15N stable isotope ratios and carbon composition using 13C NMR. The abundance of soil minerals (primary, Al-humus, noncrystalline, crystalline, and kaolin) was determined using a sequential oxalate and citrate dithionate extraction procedure. N,C isotope, C/N and 13C NMR trends in depth profiles were examined in relation to mineral composition and extractable soil metals (Alp, Fep). Across intermediate aged sites (20Ky-350Ky), we found the most isotopically enriched SOM pools accumulated in surface mineral horizons. The carbon was high in alkyl and carboxyl-carbonyl abundance, where humus-bound Al was found to be abundant. Intermediate age sites (20Ky-350Ky) contained increasing amounts of noncrystalline minerals (allophane, immogilite, ferrihydrite) with depth. A corresponding shift of decreasing 15N values, increasing C/N ratios and 13C NMR composition toward a carboxyl-carbonyl form was observed in association with increasing noncrystalline mineral abundance. We conclude that alkyl-rich SOM in surface mineral soil horizons formed largely as the result of soil microbial transformation mechanisms and the formation of Al-humus complexes. At greater depth, an increasingly labile, hydrophilic (carboxyl-carbonyl rich) SOM form which may have originated from dissolved organic carbon (DOC) was found in association with noncrystalline minerals. The soil environment at depth favored the formation of allophane.