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The quantity and quality of soil organic N and C are strongly influenced by variations in landscape position and management practices such as cultivation. In this study, we used synchrotron-based C and N K-edge x-ray absorption near-edge structure (XANES) spectroscopy coupled with pyrolysis-field ionization mass spectrometry (Py-FIMS) to explore C and N chemistry among soils obtained from different landscape positions along a hummocky transect and from locations under different management practices. A distinct landscape pattern in soil organic matter (SOM) functionality was observed, but only a small effect of management (cultivated vs. uncultivated) was detected. We observed that both carbohydrates and low-molecular-weight aromatics, which are usually considered readily degradable, were apparently stabilized in divergent (i.e., water-shedding) slope positions, while lipids and high-molecular-weight lignins were enriched in depressional areas. We posit that tillage incorporation was probably responsible for the enrichment of older, microbially altered carbohydrates and lignins at divergent positions. Using novel application of nonmetric multidimensional scaling ordination to XANES data, we observed more heterocyclic N compounds in cultivated vs. uncultivated soils. The XANES data 'also showed the presence of unique, oxidized N-bonded aromatics, which predominated at calcareous divergent slope positions. These types of organic N compounds are rarely reported in the literature except for samples obtained in acidic or anaerobic environments. This study provides analytical evidence that C and N K-edge XANES and Py-FIMS differentiate SOM composition at the molecular level from soils obtained from diverse topographic positions and management regimes in a hummocky prairie landscape. Results provide complementary evidence that SOM comprises more stabilized compounds at divergent slope positions and more active compounds at depressional positions. [ABSTRACT FROM AUTHOR]

The rhizosphere is a region of complex interactions among plants, soil, and microbiota and by its very nature, it presents many technical challenges to the analyst. Whereas previous studies have generally focused on root exudation in artificial systems, we compared whole-soil samples from bulk and rhizosphere soils developed under field pea (Pisum sativa L.). Synchrotron-based C and N K-edge x-ray absorption near-edge structure (XANES) spectroscopy and pyrolysis field-ionization mass-spectrometry (Py-FIMS) were used to investigate plant effects on the organic chemistry of the rhizosphere in a growth chamber experiment. Soil type appeared to play a significant role in the development of the rhizosphere. Indeed, the C and N K-edge XANES analyses revealed patterns of rhizosphere development that were strongly influenced by soil type. Relative to the bulk soil, pea rhizospheres developed on a clay soil showed increases in hetcrocyclic N compounds, proteins, and carboxylates. Pea rhizospheres developed on a sandy clay loam soil showed increased nitroaromatic compounds and reduced aromatics, while proteins, carbohydrates, and carboxylic compounds remained unchanged relative to the bulk soils. Pyrolysis mass spectrometry results showed that rhizosphere processes promote selective enrichment of lipid compounds (alkanes, alkenes, alkylamides, alkylaromatics, and alkyl, benzoic, and phthalic esters) with a concomitant depletion of carbohydrates, proteins, phenols, and lignins. These results suggest enhanced decomposition of labile organic matter at a molecular level in the rhizosphere, presumably as a result of enhanced microbial activity. The presence of nitroaromatic and alkylamide compounds in the rhizosphere indicates a unique cycling of N in this region. Synchrotron-based analysis of whole soils offers useful, corroborating information regarding the organic chemistry of the rhizosphere when compared with established pyrolytic techniques. [ABSTRACT FROM AUTHOR]