Macromolecular Speciation of Organic Matter in Black C rich Anthrosols: Insight from 13C CP-MAS NMR and Synchrotron Based C (1s) NEXAFS and FTIR-ATR Spectroscopy.
Dawit Solomon1, Johannes Lehmann1, Janice Thies1, Biqing Liang1, James Kinyangi1, Flavio Luizão2, and Jan Skjemstad3. (1) Cornell Univ, Bradfield Hall, Ithaca, NY 14853, (2) Instituto Nacional de Pesquisa da Amazonia INPA, Manaus, Brazil, (3) CSIRO Land and Water, PMB 2,, Glen Osmond, SA., Glen Osmond, 5064, Australia
Biomass-derived black carbon (BC) results from incomplete combustion during vegetation burning (forest and grassland fires) or for energy production, and exists ubiquitously in soils. It includes a suite of compounds occurring along a “combustion continuum,” ranging from partially charred biomass, through charcoal, to sub-micron highly graphitized soot particles. Black C is highly resistant to microbial and chemical oxidation, and it is considered to be an important sink in the global C cycle. Further impetus for studying the soils rich in BC results from its key role in nutrient retention and it influence on a wide range of biogeochemical processes in soils. In the present investigation, we employed solid-state 13C CP-MAS NMR coupled with UV-oxidation and synchrotron-based C (1s) near-edge X-ray absorption fine structure (NEXAFS) and Fourier transform infrared-attenuated total reflectance (Sr-FTIR-ATR) spectroscopy techniques to identify and fingerprint the structural composition of soil organic matter (SOM) macromolecules from Anthrosols (ranging in age between 500 to 6700 years BP) rich in of biomass-derived BC and adjacent soils developed from the same parent materials under the same climatic conditions in central Amazon, Brazil. The results from 13C CP-MAS NMR spectroscopy showed that the SOM in BC rich Anthrosols contains higher proportions of H- and C-substituted aryl-C structures, followed by carboxylic-C and highly heterogeneous alkyl-C functionalities. Relatively lower proportions of O-alkyl-, di-O-alkyl-, methoxyl-, N-alkyl- and O-substituted aryl-C species were found in these BC rich soils. Our results also indicated that BC rich Antrosols have generally higher degree of aromaticity (0.48 to 1.17) and aliphaticity expressed by the ratio of aliphatic-C to O-alkyl-C (0.69 to 1.69) compared to the corresponding adjacent soils (0.20 to 0.40, aromaticity and 0.86 to 1.28). Compared to the Anthrosols (0.15 to 0.40), however, the ratio of O-alkyl-C to aromatic-C was generally higher in the adjacent soils (0.89 to 2.08). The results from 13C NMR and synchrotron-based spectroscopic techniques provided unequivocal evidence that the functional group chemistry of the SOM from BC rich Anthrosols was considerably different from the structural composition of organic matter from adjacent soils. These macromolecular level compositional changes could mainly be attributed to the high proportion of BC in the Anthrosols.