Johannes Lehmann and Dawit Solomon. 909 Bradfield Hall, Cornell University, Dept of Crop and Soil Science, Ithaca, NY 14853
Near-edge x-ray fine structure (NEXAFS) spectroscopy provides unique capabilities to combine macro-molecular speciation of nutrients with spatial analyses on very fine scale using scanning transmission x-ray microscopy (STXM). Also chemical speciation of nitrogen, for which very few alternative approaches to spectroscopy exist, can be quantified by NEXAFS with a spatial resolution of less than 50nm. Heterocyclic N compounds can in principle be speciated which typically poses challenges to more established techniques such as NMR. However, peak assignments are still being developed and in the case of amide N versus pyridone N compounds are posing challenges for spectral separation. Relative peak intensities at 398.6-399.8 eV attributable to pyridine N, decreased from organo-mineral soil pools separated by sonication and a density above 1.8 g/cm3, to intraaggregate and free light fractions of a Kenyan Oxisol. Within a 200 micrometer-sized aggregate of the same soil, pyridine nitrogen was associated with black carbon identified by carbon NEXAFS, whereas nitrogen forms with a peak at 400.9 eV dominated organic nitrogen compounds. In different black carbon fractions separated by oxidation of different strengths, spectral features were the least specific in the black carbon fraction that had the oldest C-14 age and lowest resistance to oxidants, separated by UV oxidation. Black carbon fractions that were the most difficult to oxidize, the youngest, the most aromatic as revealed by carbon NEXAFS and were presumably the most resistant had a large proportion of pyridone N at 401.8 eV. The scarcity of standard spectra of nitrogen NEXAFS relevant to soil studies still limit interpretations but the improved analytical capabilities of STXM open exciting possibilities that need to be more fully explored in the future.
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