Cultivation Affects Soil Organic Nitrogen: Pyrolysis‐Mass Spectrometry and Nitrogen K‐edge XANES Spectroscopy Evidence

Abstract

Elucidating molecular–chemical changes that the soil organic N (SON) pool has undergone following long‐term cultivation remains a challenge. Our objective was to examine SON compounds in paired native and long‐term cultivated soils using three independent analytical methods. Curie‐point pyrolysis‐gas chromatography/mass spectrometry revealed approximately 60 different organic N‐containing molecules of which 11, 13, and 14 were less and 8, 16, and 11 more abundant in the cultivated than in the native soils from Lethbridge, Macklin, and St. Denis, respectively. Pyrolysis‐field ionization mass spectrometry (Py‐FIMS) revealed that heterocyclic N compounds such as substituted pyrroles and pyridines contributed to the overall cultivation‐induced changes. Furthermore, Py‐FIMS showed that cultivated sites preferentially lost thermally labile peptides and, to a lesser extent, other labile N‐containing compounds. The magnitude of losses decreased in the order Lethbridge (80 yr cultivation) > Macklin (85 yr cultivation) > St. Denis (57+ yr cultivation). Relative gains in thermally stable N‐containing compounds (all sites) and peptides (Macklin only) followed the same order. The weaker cultivation effect at St. Denis probably reflects a greater inherent stability of SON compounds in the native soil, as indicated by onsets and peaks in the thermal volatilization curves at 40 K higher pyrolysis temperature. Synchrotron‐based N K‐edge x‐ray near‐edge fine structure spectroscopy confirmed the enrichment of nitriles and N‐heterocycles in the cultivated Lethbridge soil at the expense of amide N. Our multimethodological approach provided evidence for the enrichment of a relatively stable nonproteinaceous pool of SON on cultivation.