Denitrification and nitrate consumption in an herbaceous riparian area and perennial ryegrass seed cropping system

Riparian ecosystems have the capacity to lower [NO.sub.3.sup.-] concentrations in groundwater entering from nonpoint agricultural sources. The processes responsible for decreases in riparian groundwater N[O.sub.3.sup.-] concentrations in the Willamette Valley of Oregon are not well understood. Our objective was to determine if denitrification and/or dissimilatory N[O.sub.3.sup.-] reduction to N[H.sub.4.sup.+] (DNRA) could explain decreases in groundwater N[O.sub.3.sup.-] moving from a perennial ryegrass cropping system into a mixed-herbaceous riparian area. In situ denitrification rates (DN) were not different between the riparian area (near-stream or near-cropping system) and cropping system the first year. In the second year, during the transition to a clover planting, DN was highest just inside of the riparian/cropping system border. Median denitrification enzyme activity (DEA) rates ranged from 29.5 to 44.6 mg [N.sub.2]O-N [kg.sup.-1] [d.sup.-1] for surface soils (0-15 cm) and 0.7 to 1.7 [micro]g [N.sub.2]O-N [kg.sup.-1] [d.sup.-1] in the subsoil (135-150 cm). Denitrification enzyme activity rates were not different among the zones and were most often correlated to soil moisture and N[H.sub.4.sup.+]. Nitrate additions to surface soils increased DEA rates, indicating a potential to denitrify additional N[O.sub.3.sup.-]. Based on groundwater velocity estimates, N[O.sub.3.sup.-] (3.8 mg N[O.sub.3.sup.-]-N [L.sup.-1]) entering the riparian surface soil could have been consumed in 0.2 to 7 m by denitrification and 0.03 to 1.0 m by DNRA. Denitrification rates measured in the subsoil could not explain the spatial decrease in [NO.sub.3.sup.-]. However, with the potentially slow movement of water in the subsoil, denitrification and DNRA (0 to 264 [micro]g N [kg.sup.-1] [d.sup.-1]) together could have completely consumed N[O.sub.3.sup.-] within 0.5 m of entering the riparian zone. Abbreviations: CS, cropping system; DEA, denitrification enzyme activity; DN, in situ denitrification rates; DNRA, dissimilatory nitrate reduction to ammonium; DOC, dissolved organic carbon; ECD, electron capture detector; EOC, extractable organic carbon; [[theta].sub.g], gravimetric soil moisture; MBC, microbial biomass carbon; MinC, mineralizable carbon; Rip-CS, riparian area near the cropping system; Pdp-Str, riparian area near the stream.