Seasonal variations and cycling of nitrous oxide using nitrogen isotopes and concentrations from an unsaturated zone of a floodplain

Abstract:

Nitrous oxide (N₂O) concentrations and isotope ratios of ¹⁵N to ¹⁴N of N₂O in the vadose zone mainly depend on atmospheric deposition, symbiotic or non-symbiotic N₂ fixation, and nitrification/denitrification processes in underlying groundwater. In an effort to quantify N₂O seasonal variations, cycling and N budgets in an alluvial aquifer in western Colorado (Rifle, CO), the concentrations and nitrogen stable isotopes of N₂O within the pore space of partially saturated sediments have been monitored over the 2013-2014 years. Vertically resolved profiles spanning from 0m to 3m depth were sampled at 0.5m increments at a periodicity of one month. At each of the profile locations, N₂O concentrations decreased from 3m depth to the surface. The maximum concentrations were observed at the interface between the unsaturated zone and groundwater, with minimum values observed in the near surface samples. The d¹⁵N values tend to increase from the unsaturated zone/groundwater interface to the surface. Both variation of N₂O concentrations and d¹⁵N values suggest that denitrification is the main contribution to N₂O production and both parameters exhibited a strong seasonal variation. The maximum concentrations (~10ppmv) were observed at the beginning of summer, during the annual maximum in water table elevation. The minimum N₂O concentrations were observed in the period from January to May and coincided with low water table elevations. Additionally, nitrogen concentrations and d¹⁵N values of the shallowest sediments within the vertical profiles do not show variation, suggesting that the main source of N₂O is associated with groundwater denitrification, with the shallower, partially saturated sediments acting as a sink for N₂O.