Nitrous oxide processing in carbonate karst aquifers

Abstract

The increased environmental abundance of anthropogenic reactive nitrogen species (Nr = ammonium [NH4+], nitrite [NO2−] and nitrate [NO3−]) may increase atmospheric nitrous oxide (N2O) concentrations, and thus global warming and stratospheric ozone depletion. Nitrogen cycling and N2O production, reduction, and emissions could be amplified in carbonate karst aquifers because of their extensive global range, susceptibility to nitrogen contamination, and groundwater-surface water mixing that varies redox conditions of the aquifer. The magnitude of N2O cycling in karst aquifers is poorly known, however, and thus we sampled thirteen springs discharging from the karstic Upper Floridan Aquifer (UFA) to evaluate N2O cycling. The springs can be separated into three groups based on variations in subsurface residence times, differences in surface–groundwater interactions, and variable dissolved organic carbon (DOC) and dissolved oxygen (DO) concentrations. These springs are oxic to sub-oxic and have NO3− concentrations that range from < 0.1 to 4.2 mg N-NO3−/L and DOC concentrations that range from < 0.1 to 50 mg C/L. Maximum spring water N2O concentrations are 3.85 µg N-N2O/L or ~ 12 times greater than water equilibrated with atmospheric N2O. The highest N2O concentrations correspond with the lowest NO3− concentrations. Where recharge water has residence times of a few days, partial denitrification to N2O occurs, while complete denitrification to N2 is more prominent in springs with longer subsurface residence times. Springs with short residence times have groundwater emission factors greater than the global average of 0.0060, reflecting N2O production, whereas springs with residence times of months to years have groundwater emission factors less than the global average. These findings imply that N2O cycling in karst aquifers depends on DOC and DO concentrations in recharged surface water and subsequent time available for N processing in the subsurface.