Abstract

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 (NO3) and nitrous oxide (N2O) 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 NO3, Br, and N2O 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. N2O was produced within the soil after urine application. NO3 to Br ratios indicated denitrification in the soil above the gravels, but none within the alluvial gravel vadose zone. In the “–urine” plot N2O concentrations increased with depth, with upward fluxes above the water table to the soil, suggesting N2O production near the water table. A different pattern was observed in the “+urine” plot where N2O was produced both at the soil zone above the gravels and near the water table leading to both upward and downward N2O fluxes. Overall N2O fluxes in the soil zone were greater than at the water table.

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