XANES and Pyrolysis-FIMS Evidence of Organic Matter Composition in a Hummocky Landscape

Abstract

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.