Shallow Groundwater Denitrification in Riparian Zones of a Headwater Agricultural Landscape

Riparian zones adjacent to cropped lands are effective at reducing nitrate (NO3-) loads to receiving water bodies primarily through plant assimilation and denitrification. Denitrification represents a permanent removal pathway and a greenhouse gas source, converting NO3- to inert N-2 gas or nitrous oxide (N2O), and has been the subject of many studies in agricultural landscapes. Despite the prevailing notion that riparian zones can be areas of enhanced denitrification, there is a lack of in situ denitrification measurements from these areas that buffer streams and rivers from NO3- originating in upland cropped soils, especially over time scales that capture seasonal dynamics. We measured in situ groundwater denitrification rates in two riparian zones of an intensive dairy farm located in the headwaters of the Susquehanna River. Denitrification rates determined monthly over a 1-yr period with the N-15-NO3 push-pull method ranged from 0 to 4177 mu g N kg soil(-1) d(-1) (mean, 830 +/- 193 mu g N kg soil(-1) d(-1)). Denitrification showed a distinct seasonal pattern, with highest rates observed in the spring and summer, concomitant with warmer temperatures and decreasing dissolved oxygen. We estimate an annual N loss of 470 +/- 116 kg yr(-1) ha(-1) of riparian zone via denitrification in the shallow saturated zone, with the potential for >20% of this amount occurring as N2O. Total denitrification from shallow groundwater in the riparian zone was equivalent to 32% of manure N spread on the adjacent upland field, confirming the importance of riparian zones in agricultural landscapes at controlling N loads entering downstream waters.