Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2007
Publication Date: 9/1/2008
Citation: Davis, J.H., Griffith, S.M., Horwath, W.R., Steiner, J.J., Myrold, D.D. 2008. Denitrification and Nitrate Consumption in an Herbaceous Riparian Area and Perennial Ryegrass Seed Field. Soil Science Society of America Journal. 72:1299-1310. Interpretive Summary: In agricultural landscapes, riparian ecosystems have numerous functions along river ways. One important function is improving the quality of water that drains from crop fields that are upslope. For example, nitrate leaching from a crop field through a riparian area can be processed by riparian vegetation and soils, thus lowering nitrate concentrations prior to its movement to the stream. The soil microbial processes responsible for decreases in riparian groundwater NO3 – concentrations in the poorly drained soils of the Willamette Valley of Oregon are not well understood. Our objective was to determine if two important soil nitrogen processes were important in lowering groundwater nitrate. We found that, based upon groundwater velocity estimates, nitrate entering the riparian surface soil was consumed completely within a couple of feet after entering the riparian zone from the crop field edge.
Technical Abstract: Riparian ecosystems have the capacity to lower NO3- concentrations in groundwater entering from non-point agricultural sources. The microbial 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 an established perennial ryegrass field 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). DEA rates were not different among the zones when dates were pooled 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 upon 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.