Soil organic C speciation and transformations following long-term anthropogenic perturbations in tropical ecosystems: evidence from 13C NMR and synchrotron-based C (1s) NEXAFS and FTIR-ATR spectroscopy.
Dawit Solomon1, Johannes Lehmann1, James Kinyangi1, Biqing Liang1, Ingo Lobe2, Wulf Amelung3, and Thorsten Schäfer4. (1) Cornell University, Bradfield Hall, Ithaca, NY 14853, (2) UFZ Centre for Environmental Research, Leipzig-Halle, Brueckstrasse 3a, Magdeburg, 39114, Germany, (3) University of Bonn, Nussallee 13, Bonn, 53115, Germany, (4) Institute for Nuclear Waste Management, P.O.Box 3640, Karlsruhe, 76021, Germany
Soil organic matter (SOM) is a physically and chemically heterogeneous conglomerate ranging in size and complexity from simple monomers to mixtures of complex macromolecular humic substances or biopolymers aggregated together in the form of cellular debris that differ in stability. The amount, chemical composition and polyelectrolytic characteristics of these biomolecules vary along a continuum of decomposition and humification. Such variations create significant analytical problems to characterize SOM and thus made studies investigating the effects of anthropogenic perturbations on the reactivity, fate and chemical speciation of soil organic C (SOC) as well as the mechanisms and processes that determine the potentials of soils to sequester C in tropical and subtropical ecosystems very challenging. In the present investigation, we used synchrotron-based C (1s) near-edge X-ray absorption fine structure (NEXAFS) and Fourier transform infrared-attenuated total reflectance (Sr-FTIR-ATR) spectroscopy to identify and finger print the structural composition of SOC macromolecules and to evaluate the long-term impact (up to 100 years) of anthropogenic land-use and land-cover changes on chemical speciation of SOC in humic substances extracted from soils collected from tropical forest (Ethiopia and Kenya) and subtropical grassland (South Africa) agro-ecosystems. The results from NEXAFS and Sr-FTIR-ATR spectroscopy were evaluated against the results obtained from 13C nuclear magnetic resonance (NMR) spectroscopy, which is a more established SOC characterization technique. Carbon K-edge NEXAFS spectra showed that carboxylic-C and O-alkyl-C functional groups were the dominant forms of SOC, followed by moderate amounts of aromatic-C and phenolic-C groups. The aliphatic-C forms contributed only to a small portion of the total SOM associated with the humic substances extracted from the soils under investigation. NEXAFS spectroscopy, for the most part exhibited good selectivity, where specific energy regions tended to correspond to C in discrete organic functional groups. However, regions of slight overlap between the bands associated especially with 1s-3p/ ó* transition of aliphatic-C and 1s-g* transition of carboxylic-C may not be excluded. Sr-FTIR-ATR spectroscopy, clearly demonstrated the long-term effects of anthropogenic land-use and land-cover changes on chemical speciation and structural stability of the different SOC functional forms present in the humic substances. Easily degradable SOM constituents such as polysaccharide-C and some labile components of aliphatic-C moieties were more prominent in the humic substance extracted from the native forest and grassland soils, while aromatic-C and some recalcitrant forms of aliphatic-C dominated the SOM associate with the humic substances extracted from the continuously cultivated fields. Despite the wide range of origin and chemical hetrogenity of the humic substance, the results from the deconvolution procedure used for the semi-quantitative analysis of the C (1s) NEXAFS spectra, compared very well with the results from 13C NMR spectroscopy.