Contrasting Patterns of Nitrate Export From Two Hydrologically Similar Catchments

1
Contrasting Patterns of Nitrate Export From Two
Hydrologically Similar Catchments Sheila F.
Christopher1, Myron J. Mitchell1, Shreeram
Inamdar2, Blair D. Page1, and John
Campbell13 1SUNY-ESF, Syracuse, NY 2SUNY College
at Buffalo, Buffalo, NY 3Northeastern Research
Station, USDA Forest Service, Durham, NH
Introduction Variation in surface water NO3-
concentration within and across catchments has
been the focus of recent research. Variability
in stream water NO3- has been explained by
differences in sources, soil processes and
hydrology. Sources of N such as atmospheric
deposition can vary locally and regionally. Ito
et al. (2002) predicted wet deposition generally
decreased from the southwest to northeast in the
Adirondack Mountains of NY. Differences in
vegetation also play a role in controlling stream
water NO3-. In an assemblage of 39 catchments
near the Catskill Mountains, NY, the catchments
with the lowest NO3- concentrations all had
forests with red oak. Red oak has poor litter
quality with high lignin N ratios and low rates
of nitrification (Lovett et al. 2000).
Differences in denitrification can also explain
the variability of stream water NO3- (Hill 1990).
The variation of stream water NO3- concentration
can also be attributed to differences in
hydrologic flowpaths. Creed and Band (1998)
suggested that near-surface flow was responsible
for peak NO3- concentration while others have
found ground water flowpaths to be an important
mechanism by which NO3- reaches the stream.

• TOPMODEL Results
• Subsurface flow contribution along deep flow
  paths (50 cm) were very similar in S14 and S15
  across all seasons (Fig 3). For both catchments,
  deep flow contributed more than 15 of the
  subsurface flow with the higher values observed
  during drier summer periods. The similarity in
  deep flow path contributions for S14 and S15
  implies that the disparity in NO3- concentrations
  for these two subcatchments can not be explained
  in terms of hydrologic flowpaths alone. This
  suggests there may be another NO3- source in S14
  that is responsible for the high NO3-
  concentration.

Vegetation and Soil Results Results suggest that
S14 and S15 have different vegetative
characteristics with S14 having a significantly
greater amount of sugar maple (p 0.04) and S15
having significantly greater beech (p 0.006)
(Fig 4). Lovett et al. (2000) suggested that
potential nitrification is significantly greater
in maple stands than in beech stands in the
Catskill Mts. due to lower CN ratios contained
in maple verses beech litter. Exchangeable Ca
and Mg is also much higher in S14 verses S15,
providing a better environment for nitrification
(Fig 5).
Fig 4 Basal Area of 7 tree species in S14 and S15
Fig 1 The Archer Creek Catchment (defined by
yellow)and Subcatchments 14 and 15 (outlined in
black). Frequency distributions of the
topographic index for each subcatchment are
provided.
Hypotheses We recently conducted a synoptic
survey of stream water chemistry in the Archer
Creek Catchment of the Adirondacks, NY (Fig 1)
revealing two nearly adjacent subcatchments (S14
and S15) with very different mean annual stream
NO3- concentration. S14 had a mean annual NO3-
concentration of 56.05 ueq/l while S15 had a mean
annual NO3- concentration of 21.79 ueq/l.
However, the temporal pattern in stream water
chemistry was similar (Fig 2). We hypothesized
that the variability in stream water chemistry
between the 2 subcatchments could be explained by
differences in vegetation and soil
characteristics rather than differences in
hydrologic flowpaths.

• Conclusions
• Differences in hydrologic flowpaths could not
  explain the differences in NO3- concentration
  between S14 and 15
• New data suggest that the variability in stream
  water N may be explained by differences in soil
  and vegetation characteristics.
• Future research will explore these differences in
  solute export in more detail and will include
  hydrometric studies.

Fig 2 Temporal pattern in stream water NO3-
concentration at the outlets of S14 and S15.

• TOPMODEL
• We combined TOPMODEL predictions of flow and
  knowledge of NO3- concentration in different
  hydrologic reservoirs to test the hypothesis that
  hydrologic flowpaths were regulating the surface
  water chemistry in S14 and 15.
• During the growing season, groundwater is the
  controlling end-member of stream water NO3-
  whereas during the dormant season near-surface
  soil is the controlling end-member.
• The model was run for both dormant (Feb. 26th
  2000 event) and growing (July 10th 2000 event)
  seasons.

• References
• Creed, I. F. and Band, L. E., 1998. Export of
  nitrogen from catchments within a temperate
  forest evidence for a unifying mechanism
  regulated by variable source area dynamics.
  Water resources Research, 34 3105-3120.
• Hill, A. R., 1990. Groundwater flowpaths in
  relation to nitrogen chemistry in the near-stream
  zone. Hydrobiologia, 206 29-52.
• Ito, M., Mitchell, M. J. and Driscoll, C. T.,
  2002. Spatial patterns of precipitation quantity
  and chemistry and air temperature in the
  Adirondack region of New York. Atmospheric
  Environment, 36 1051-1062.
• Lovett, G. M., Weathers, K. C., and Sobczak, W.
  V., 2000. Nitrogen saturation and retention in
  forested watersheds of the Catskill Mountains,
  New York. Ecological Applications, 10 73-84.

Vegetation and Soil sampling We conducted a
stratified random sampling of the vegetation and
performed soil digests to determine if any
significant differences in soil and/or vegetation
could explain the relatively much greater NO3-
concentration in S14 verses S15.
Acknowledgments This research was sponsored by
NYSERDA, NSF and McIntire-Stennis (USDA).