Nitrogen K-Edge XANES Spectroscopy of Speciation of Soil Organic Nitrogen During 4000-Year of Soil and Ecosystem Development.

Organic nitrogen compounds in soils are both a source and sink for N in the terrestrial N cycle. The latest terrestrial N cycle paradigm considers soluble organic nitrogen (SON) as a rate limiting step for regulating the overall N availability in ecosystems. Despite its importance little is known about the chemical nature of organic N associated with soil minerals. Different from destructive wet chemical analysis, synchrotron-based nitrogen K-edge x-ray absorption near-edge structure (XANES) spectroscopy has been shown to be a selective, sensitive, and nondestructive method capable of providing molecular-scale information on N speciation. In this study, soils with developmental stages ranging from 0 to 4000 years were collected in a soil chronosequence. Approximately 5000 years of deposition have formed this chronosequence of sediments originating from glacial deposits and Paleozoic bedrock underlying the lake basin. Clear patterns of vegetation succession are found where grasses tend to dominate early, while conifers and the occasional hardwood dominate at later stages of soil development. The soil samples were first rinsed with 0.01 M KCl solution to remove nitrate, ammonium, and other soluble N. The organic debris was removed by flotation and manual removal. Nitrogen speciation in soils with and without organic debris removal were determined using N K-edge XANES. The N K-edge XANES spectra were collected at the U4B beamline of the National Synchrotron Light Source in Brookhaven National Laboratory. Aromatic-N, amide-N, nitro aromatic-N, and alkyl-N were detected in the samples. The XANES results suggested that more amide-N, indicative of proteinaceous organic N, was determined in soils with increasing soil development. Significant amount of proteinaceious organic N seemed to bound with soil minerals.