δ13C and δ15N profiles in 14C-dated Oxisol and Vertisols as a function of soil chemistry and mineralogy

The analyses of stable isotope ratios of carbon (delta(13)C) and nitrogen (delta(15)N) of soil organic matter (SOM) is an increasingly used tool to estimate soil carbon turnover, to assess degree of soil development, and to study historical C3/C4 vegetation changes. However, the exact processes that control C-13- and N-15-enrichment of SOM within a soil profile are still not clearly identified. To better understand the isotopic processes associated with decomposition: of SOM, we studied two Vertisol profiles and one Oxisol profile from southern Queensland by radibgenic (C-14), stable isotopic (delta(13)C, delta(15)N), and spectroscopic (C-13-NMR and FTIR) methods. The findings of this study demonstrate that fundamental differences exist in delta(13)C and delta(15)N fractionation dynamics in different soil types and that isotopic fractionation is highly influenced by soil chemistry, mineralogy, and type of organic matter input. Stable isotopic analyses of the Oxisol show the typically observed increase in delta(13)C and delta(15)N in the subsurface horizon whereas the Vertisols show consistently decreasing values with depth. The high degree of C-13-enrichment in the Oxisol compared with the Vertisols cannot be simply explained with increased fractionation due to soil age, as the C-14 age of the Vertisols is greater and increases more rapidly with depth, compared with that of the Oxisol. Data from C-13-NMR, XRF and IR data together with data on pH and clay content reveal a more complex picture of isotopic fractionation in soils. The Oxisol is dominated by O-alkyl carbon and aromatic material whereas the Vertisols contain higher amounts of alkyl carbon. Smectite and kaolinite are the dominant clay minerals in the Vertisols while the Oxisol is dominated by gibbsite, kaolinite, and Fe and Al-oxides. We suggest that the C-13- and N-15-depletion in the Vertisols is associated with low pH, which inhibits nitrification and promotes stabilization of C-13-depleted alkyl material by smectitic clays. The C-13-enrichment in the Oxisol correlates with a high abundance of relatively C-13-enriched O-alkylcarbon, which is a mix of primary materials (plant carbohydrates) as well as secondary (microbially synthesized) carbon. The abundance of relatively labile O-alkyl carbon even at depth is likely due to physico-chemical protection through complexation with Fe- and Al-oxides. (C) 2002 Elsevier Science B.V. All rights reserved.