Contrasting Patterns of Nitrate Export From Two Hydrologically Similar Catchments

1. S14-Total Basal Area S15-Total Basal Area S15 (2.4ha) (per Ha) (per Ha) S14 (3.0ha) 0.25 s14 - ln(a/tanB) s15 - ln(a/tanB) 50 0.2 50 Bir Bir 0.18 mean = 5.5 0.15 0.16 40 0.14 40 Pine 0.1 Pine 0.12 mean = 5.8 0.1 0.05 Hop 0.08 Hop 30 30 0.06 0 0.04 m2 4 5 7 8 9 10 11 12 m2 Bass 0.02 Bass 0 20 20 3 5 6 7 8 10 11 12 Ash Ash 10 10 SugM SugM 0 0 Beech Beech FAGR FAGR a b WS14 WS15 Growing Season-July 0-15cm 1% 0-15cm 0% 86% 85% 15-20cm 15-20cm 14% 13% >50cm >50cm 0 20 40 60 80 100 0 20 40 60 80 100 Fig 5 Mean Ca (a) and Mg (b) concentration at 3 different soil depths within S14 and S15. % Flow % Flow WS14 WS15 Dormant Season-February Fig 3 Percent flow contribution from surface and subsurface water for the Feb. 26th and July 10th events. 0-15cm 0% 1% 0-15cm 91% 15-20cm 90% 15-20cm 9% >50cm >50cm 8% Fig 4 Percent flow contribution from surface and subsurface water for the Feb. 26th and July 10th events. 0 20 40 60 80 100 0 20 40 60 80 100 % Flow % Flow 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 C:N 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 of56.05 ueq/lwhile S15had a mean annual NO3-concentration of21.79 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).