Over century long timescales, the climatic forcing of nitrous oxide (N2O) dwarfs that of carbon dioxide. Although increasing atmospheric levels are linked to excess nitrogen loading and consequent formation via microbially mediated pathways, the factors regulating the emission of N2O to the atmosphere remain difficult to predict and the global N2O budget remains poorly constrained. In large part, these challenges stem from the fact that a diverse number of N2O production pathways may be operative, especially in environments hosting dynamic redox conditions - and thus disentangling their relative roles in regulating N2O is challenging. As coastal ecosystems are especially subject to elevated nitrogen loading, we have been focusing investigations on better understanding the controls on N2O production mechanisms in intertidal sediments using a variety of novel isotopic approaches. Surprisingly, our initial findings have indicated that under elevated nitrate loading, increased emissions of N2O are not mediated by direct bacterial activity, but instead appear to be largely catalyzed by fungal denitrification and/or abiotic reaction with reduced iron (chemodenitrification). Expanding on these findings, results from lab experiments geared toward improving calibration of N2O isotope signatures of non-traditional production pathways demonstrate a high potential for cryptic cycling processes under dynamic redox oscillations and shed new light on the factors controlling kinetics, yields, and isotopic composition of N2O produced via less frequently considered pathways. As both fungal and chemodenitrification typically exhibit N2O yields far greater than bacterial production (whether nitrifying or denitrifying), even small levels of their activity could produce disproportionaetly large amounts of N2O, suggesting the possibility of their potentially substantial, yet widely overlooked, role especially in coastal ecosystem N2O fluxes. Finally, these findings may help to explain the notoriously high variability of environmental N2O fluxes, which may be in part driven by spatial and temporal heterogeneity in organic matter respiration by fungi and the redox cycling of iron.

Intricate coupling of carbon, nitrogen and iron redox cycling underlying the biogeochemical dynamics of N2O

253rd National Meeting (2017)

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