『Abstract
We examined the hydrologic controls on nitrogen biogeochemistry
in the hyporheic zone of the Tanana River, a glacially-fed river,
in interior Alaska. We measured hyporheic solute concentrations,
gas partial pressures, water table height, and flow rates along
subsurface flowpaths on two islands for three summers. Denitrification
was quantified using an in situ 15NO3-
push-pull technique. Hyporheic water level responded rapidly to
change in river stage, with the sites flooding periodically in
mid-July to early-August. Nitrate concentration was nearly 3-fold
greater in river (ca. 100μg NO3--N
l-1) than hyporheic water (ca. 38μg NO3--N
l-1), but approximately 60-80％ of river nitrate was
removed during the first 50 m of hyporheic flowpath. Denitrification
during high river stage ranged from 1.9 to 29.4 mg N kg sediment-1
day-1. Hotspots of methane partial pressure, averaging
50,000 ppmv, occurred in densely vegetated sites in conjunction
with mean oxygen concentration below 0.5 mg O2
l-1. Hyporheic flow was an important mechanism of nitrogen
supply to microbes and plant roots, transporting on average 0.41
g NO3--N m-2 day-1,
0.22 g NH4+-N m-2 day-1,
and 3.6 g DON m-2 day-1 through surface
sediment (top 2m). Our results suggest that denitrification can
be a major sink for river nitrate in boreal forest floodplain
soils, particularly at the river-sediment interface. The stability
of the river hydrograph and the resulting duration of soil saturation
are key factors regulating the redox environment and anaerobic
metabolism in the hyporheic zone.
Keywords: Denitrification; Hyporheic; Methane; Nitrogen; River;
Taiga』
Introduction
Methods
Study site
Study design
Sampling and analytical techniques
In situ denitrification
Push-pull calculations
Data analysis
Analysis of subsurface hydrology and capillary rise
Long term patterns in climate and river hydrology
Results
Climate and river hydrology
Groundwater flowpaths
Spatial patterns in hyporheic chemistry
Subsurface hydrology and nitrogen losses
Temporal variation in hyporheic chemistry
Discussion
Hyporheic zone hydrology and nitrogen transformation
Subsurface methane and carbon dioxide
Climate, river hydrology and hyporheic chemistry
Acknowledgements
References