Stable isotope analyses of NO2−, NO3−, and N2O in the hypersaline ponds and soils of the McMurdo Dry Valleys, Antarctica

Abstract

Nitrous oxide (N2O) is produced in significant quantities in the soils and lakes of the McMurdo Dry Valleys, Antarctica. Unraveling the mechanisms of N2O production in these soils and ponds is of great interest due to the extreme arid and cold conditions, which are hostile to life. Recent studies have shown production of N2O having unique stable isotopic signatures in certain Dry Valley soils through abiotic reduction of nitrate (NO3−) and nitrite (NO2−) on active surfaces of Fe(II)-containing minerals, a process known as ‘chemodenitrification’. In this study, δ15N and δ18O of N2O, NO2−, and NO3−, as well as the N2O site preference (SP), were measured at three sites to evaluate the role of chemodenitrification in N2O production. The δ15N and δ18O values in NO3− indicated an atmospheric source, while δ15N values in NO2− (−150‰) were indicative of abiotic reactions. Instead of finding unique SP values for N2O at Dry Valley sites, SP values mostly fell within the range associated with microbial N2O production mechansims. The δ15N and δ18O of N2O were also within a range of values expected for various biological N2O production mechanisms. However, efforts to detect biological activity in Don Juan Pond (DJP), a hypersaline pond in the Wright Valley, have been largely unsuccessful. We consider two possible scenarios for N2O production at DJP: (1) abiotic production in the pond, or (2) biological production in nearby freshwater and transport to the pond. Although little is known about the isotopic systematics of abiotic N2O production, these results indicate that if the observed N2O was produced by an abiotic mechanism, its isotopic signature is indistinguishable from that expected from a mixture of several microbial processes and thus, the formation pathway cannot be determined from isotopic composition alone.