Abstract
Removal of excess nitrogen (N) can best be achieved through denitrification processes that transform N in water and terrestrial ecosystems to di-nitrogen (N2) gas. The greenhouse gas nitrous oxide (N2O) is considered an intermediate or end-product in denitrification pathways. Both abiotic and biotic denitrification processes use a single N source to form N2O. However, N2 can be formed from two distinct N sources (known as hybrid N2) through biologically mediated processes of anammox and codenitrification. We questioned if hybrid N2 produced during fungal incubation at neutral pH could be attributed to abiotic nitrosation and if N2O was consumed during N2 formation. Experiments with gas chromatography indicated N2 was formed in the presence of live and dead fungi and in the absence of fungi, while N2O steadily increased. We used isotope pairing techniques and confirmed abiotic production of hybrid N2 under both anoxic and 20% O2 atmosphere conditions. Our findings question the assumptions that (1) N2O is an intermediate required for N2 formation, (2) production of N2 and N2O requires anaerobiosis, and (3) hybrid N2 is evidence of codenitrification and/or anammox. The N cycle framework should include abiotic production of N2.
Highlights
Has been suggested as a plausible codenitrification pathway[8], reports of N2O consumption during fungal production of N2[21] are lacking
Abiotic nitrosation of organic matter by NO2− in soil was first suggested by Nelson and Bremner[26] when they recovered over 20% of added N (5 mmols NO2− g−1 soil) as N2 for sterile soil at neutral pH in helium (He) and heliox (20% O2, 80% He) atmospheres
We questioned whether N2 reportedly due to fungal codenitrification in pure culture experiments was formed abiotically in the presence and absence of O2, given diverse inorganic and organic sources of N
Summary
Has been suggested as a plausible codenitrification pathway[8], reports of N2O consumption during fungal production of N2 (commonly observed during bacterial denitrification)[21] are lacking. Trimmer and Prudy[27] showed that deep seawater samples amended with 15NO2− and 14NH4 produced more 29N2 when organic N (allylthiourea) was added They suggested an alternative metabolic pathway to anammox but did not address the possibility of abiotic 29N2 formation. We questioned whether N2 reportedly due to fungal codenitrification in pure culture experiments was formed abiotically in the presence and absence of O2, given diverse inorganic and organic sources of N. Necromass would provide more surface area for chemical decomposition of NO2− to N2O24 (Fig. 1) This would serve as a test for abiotic formation of N2O and N2 in the presence of decomposing organic material
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