Low elevation inland habitats of the Willamette River floodplain support enhanced denitrification

Background/Question/Methods

Floodplain nitrate removal via denitrification in sediment provides an important ecosystem service that may be a valuable sink for nitrate pollution. At this time, much floodplain restoration is taking place with little consideration for in-situ nutrient processing, necessitating research to identify habitats for restoration and preservation that enhance denitrification and nitrogen pollution removal. The Green Island floodplain of the Willamette River near Coburg, Oregon is under active restoration, led by the McKenzie River Trust, to hydrologically re-connect the Willamette to its historic floodplain and restore native vegetation. In this study, we measured the relative rates and limiting nutrients (carbon and/or nitrate) of denitrification using the acetylene-block technique in the summer and fall of 2008 on sediments of several representative floodplain habitats to include a range of inundation frequency from low elevation permanently wet alcoves, ephemerally wet fringe, re-vegetated former agriculture (within the last 3 years), and upland grass seed fields. These denitrification rates, combined with quarterly sampled nutrient concentration and isotopes of N and O in nitrate from 50 shallow groundwater monitoring wells in the floodplain, will help us identify habitats that support enhanced denitrification and aid in future restoration activities in the region.

Results/Conclusions

Preliminary results show that  permanently wet alcoves and ephemerally wet habitats in low lying areas of a floodplain support significantly greater denitrification potential than restored former ag and current ag habitats in higher elevation areas (697.38±144.91 nG g^--1hr^-1 vs. 22.35±3.83, p<0.01). We found that in the higher elevation sites, organic carbon limits denitrification potential (post hoc p<0.01) while permanently wetted sites are limited by the availability of nitrate. At Green Island, the more inland sites of a historic channel away from the river possess elevated nitrate in shallow groundwater (Max NO₃^--N=2.33 mg L^-1 vs 0.14±0.45,n=50) and support significantly greater potential denitrification rates (658.02±147.50 nG g^-1hr^-1 vs. 61.72±17.92 p<0.01) than those sites nearest the river. Based on these preliminary results, lower elevation wet regions that are positioned to intercept nitrate from upland agricultural areas possess elevated organic carbon and greater potential to retain and remove nitrate in the Green Island floodplain. Thus, low elevation habitats located near nitrate sources away from the river are an important and potentially valuable nitrate sink for restoration activities that desire enhanced nitrogen-removing ecosystem services.

This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.