Denitrification in riparian forests receiving agricultural discharges

Riparian forests can remove large amounts of nitrate from the groundwater discharges of adjacent uphill croplands. We review the mechanisms that could account for NO3 removal. Denitrification is hypothesized to be important, but measurement problems limit the relevant field data. We used large chambers (1-20 mZ) and tunable-diode laser infrared spectrophotometry to measure nitrous oxide (NzO) emissions in a riparian forest and adjacent cropland. Forest NzO emissions were very spatially heterogeneous but had clear seasonal and diurnal cycles that paralleled temperature changes. Springtime N20 emission rates were higher in the cropland (18 pg-N m-' hr-I) than in the forest (6 pg-N m" hr-I), but fall emission rates in both systems were similar (2 pg-N m-' hr-'). We also measured NzO concentration in riparian forest groundwater. Mean groundwater NzO was low (17 pg-N I-') and spatially heterogeneous, with no apparent relationship to groundwater No3 or to distance from the forest-field boundary. Estimated annual N20 loss in soil emission (0.35 kg-N ha?) and groundwater (0.04 kg-N ha-') together accounted for <1% of the intercepted nitrogen not incorporated into wood (45-60 kg-N ha?). However, denitrification may produce mostly NZ or NO rather than NzO. Future experiments will measure NO emission and use the acetylene block technique to examine the balance between N and N20 in the gaseous nitrogen emissions from the cropland and forest.