Stable isotopic evidence for nitrification and denitrification in a High Arctic glacial ecosystem

Solute chemistry and stable isotope tracers of NO3 − were used to assess bacterial NO3 − production and denitrification in a High Arctic glacial ecosystem during 2009. Changes in the NO3 − concentration and the δ18O–NO3 in all the proglacial streams revealed that up to 95 % of total NO3 − was most likely bacterially-derived during low flow conditions towards the end of the summer (day of year 250). However, overlapping ranges of δ15N values for snow NH4 +, soil organic matter, cryoconite debris and geological nitrogen in host rocks mean that neither the preferred substrate(s), nor the pathway (i.e. nitrification or simple mineralisation) can be discerned. The most plausible explanation for the bacterial production of NO3 − is nitrification in snowmelt-fed flowpaths through avalanche fans that flank the glacier and along subglacial drainage pathways at the glacier bed. Interestingly, there was no evidence for denitrification in subglacial outflow, which is contrary to earlier research at this site. Instead, increases in the δ15N–NO3 of up to 20 ‰ downstream of the glacier margin, suggests that denitrification in the glacier forefield and/or the sediments that flank it was most discernable during 2009. Our observations therefore suggest that poorly understood temporal variations in the mixing ratio of nitrifying and denitrifying flowpaths occur in this glacial ecosystem.