Nitrate and Nitrous Oxide Dynamics Under Urine Application in an Alluvial Gravel Vadose Zone

Understanding nitrogen (N) processes within the vadose zone is important to estimate N losses to groundwater systems. A field trial was undertaken to examine the dynamics of nitrate (NO 3 ) and nitrous oxide (N 2 O) in an alluvial gravel vadose zone underlain by shallow groundwater. Synthetic urine (980 kg N ha −1 ) with a bromide (400 kg Br ha −1 ) tracer was applied to the surface of a 10- by 20-m plot, and changes in subsurface NO 3 , Br, and N 2 O concentrations were compared with those from an adjacent plot, with just the Br tracer applied. Soil solution and air were monitored at multiple depths (from 0.2 to 5 m) over an 18-mo period. Transport of solution was rapid in the gravel material with some Br transported to 3-m depth immediately after the urine application. N 2 O was produced within the soil after urine application. NO 3 to Br ratios indicated denitrification in the soil above the gravels, but none within the alluvial gravel vadose zone. In the “–urine” plot N 2 O concentrations increased with depth, with upward fluxes above the water table to the soil, suggesting N 2 O production near the water table. A different pattern was observed in the “+urine” plot where N 2 O was produced both at the soil zone above the gravels and near the water table leading to both upward and downward N 2 O fluxes. Overall N 2 O fluxes in the soil zone were greater than at the water table.