wAbstract
@We reviewed published studies from primarily glaciated regions
in the United States, Canada, and Europe of the (1) transport
of nitrate from terrestrial ecosystems to aquatic ecosystems,
(2) attenuation of nitrate in the riparian zone of undisturbed
and agricultural watersheds, (3) processes contributing to nitrate
attenuation in riparian zones, (4) variation in the attenuation
of nitrate in the riparian zone, and (5) importance of in-stream
and hyporheic processes for nitrate attenuation in the stream
channel. Our objectives were to synthesize the results of these
studies and suggest methodologies to (1) monitor regional trends
in nitrate concentration in undisturbed 1st order watersheds and
(2) reduce nitrate loads in streams draining agricultural watersheds.
@Our review reveals that undisturbed headwater watersheds have
been shown to be very retentive of nitrogen, but the importance
of biogeochemical and hydrological riparian zone processes in
retaining nitrogen in these watersheds has not been demonstrated
as it has for agricultural watersheds. An understanding of the
role of the riparian zone in nitrate attenuation in undisturbed
watersheds is crucial because these watersheds are increasingly
subject to stressors, such as changes in land use and climate,
wildfire, and increases in atmospheric nitrogen deposition.
@In general, understanding processes controlling the concentration
and flux of nitrate is crucial to identifying and mapping the
vulnerability of watersheds to water quality changes due to a
variety of stressors. In undisturbed and agricultural watersheds
we propose that understanding the importance of riparian zone
processes in 2nd order and larger watersheds is critical. Research
is needed that addresses the relative importance of how the following
sources of nitrate along any given stream reach might change as
watersheds increase in size and with flow: (1) inputs upstream
from the reach, (2) tributary inflow, (3) water derived from the
riparian zone, (4) groundwater from outside the riparian zone
(intermediate or regional sources), and (5) in-stream (hyporheic)
processes.
Keywords: Nitrate; Riparian zone; Nitrate attenuation; Denitrificationx
Contents
1. Introduction
2. Transport of nitrate from terrestrial ecosystems to aquatic
ecosystems
3. Attenuation of nitrate in the riparian zone
4. Processes contributing to nitrate attenuation in riparian zones
5. Variation in the attenuation of nitrate in the riparian zone
@5.1. Variation in nitrate attenuation capacities of riparian
zones due to differences in the location of the riparian zone
with respect to local, intermediate, and regional groundwater
sources
@5.2.Variation in nitrate attenuation capacities of riparian zones
due to differences in hydrogeologic properties
@5.3. Effect of vegetation on the nitrate attenuation capacities
of riparian zones
6. Attenuation of nitrate in the stream channel
@6.1. Denitrification in the stream channel
7. Research needs
References