Microbial pathways of N2O production vary among N-polluted soils as revealed by 15N-labeled substrate additions coupled with N2O isotopomer analyses

It is well understood that ecosystems receiving excess nitrogen (N), hereafter N-polluted ecosystems, contribute to enhanced nitrous oxide (N₂O) emissions, but it is not clear which microbial transformations are enhanced to drive these emissions. Measuring stable isotopomers of microbially-emitted N₂O from soils is a potentially powerful technique that could better our understanding of the microbial processes that drive soil N₂O emissions. In this study, we took advantage of a new analytical instrument for quantifying N₂O isotopomers. This instrument uses laser absorption spectrometry, which is more time and cost efficient than the traditional isotope ratio mass spectroscopy.

Our work evaluates how N₂O production pathways vary with N loading and soil moisture by contrasting the response of soils from N-polluted alpine and agricultural sites. The alpine soils were collected from Rocky Mountain National Park in Colorado, USA and the agricultural soils from the USDA Limited Irrigation Research Farm in Greeley, Colorado, USA. All soils were amended with ¹⁵N isotopically enriched substrate (i.e., ¹⁵NH₄⁺ or ¹⁵NO₃^-) and then incubated for 28 hours. The laser-based infrared absorption spectroscopy allowed us to partition among microbial N₂O-generating processes by characterizing which atoms in the emitted N₂O became isotopically enriched during incubation. We found strong isotopic differences in N₂O-generating processes both within and between ecosystems. N-polluted alpine soils were primarily dominated by denitrification whereas N-polluted agricultural soils exhibited more variable processes. Thus far, we have found that soil moisture was the primary control on N₂O production rate. We hypothesize that broader application of N₂O isotopomer analyses can both further constrain the global N₂O budget, and also help to diagnose early stages of N saturation in vulnerable ecosystems.