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SOIL BIOLOGY & BIOCHEMISTRY

Denitrification and Nitrate Consumption in an Herbaceous Riparian Area and Perennial Ryegrass Seed Cropping System

^a USDA-ARS, NFSPRC, Corvallis, OR 97331
^b Univ. of California, Plant and Environmental Sci., Davis, CA 95616-8627
^c USDA-ARS National Program Staff, Beltsville, MD 20705
^d Oregon State Univ., Crop and Soil Sci., Corvallis, OR 97331

^* Corresponding author (jennifer.davis{at}ars.usda.gov).

Riparian ecosystems have the capacity to lower NO₃^– concentrations in groundwater entering from nonpoint agricultural sources. The processes responsible for decreases in riparian groundwater NO₃^– concentrations in the Willamette Valley of Oregon are not well understood. Our objective was to determine if denitrification and/or dissimilatory NO₃^– reduction to NH₄⁺ (DNRA) could explain decreases in groundwater NO₃^– 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 N₂O-N kg^–1 d^–1 for surface soils (0–15 cm) and 0.7 to 1.7 µg N₂O-N kg^–1 d^–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 NH₄⁺. Nitrate additions to surface soils increased DEA rates, indicating a potential to denitrify additional NO₃^–. Based on groundwater velocity estimates, NO₃^– (3.8 mg NO₃^–-N L^–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 NO₃^–. However, with the potentially slow movement of water in the subsoil, denitrification and DNRA (0 to 264 µg N kg^–1 d^–1) together could have completely consumed NO₃^– 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 • _g, gravimetric soil moisture • MBC, microbial biomass carbon • MinC, mineralizable carbon • Rip-CS, riparian area near the cropping system • Rip-Str, riparian area near the stream