Profiling Rhizosphere Chemistry: Evidence from Carbon and Nitrogen K-Edge XANES and Pyrolysis-FIMS

The rhizosphere is a region of complex interactions among plants, sod, 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 sod showed increases in heterocyclic N compounds, proteins, and carboxylates. Pea rhizospheres developed on a sandy clay loam sod 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 sods offers useful, corroborating information regarding the organic chemistry of the rhizosphere when compared with established pyrolytic techniques.