John Wiley & Sons, Ltd., surface waters, drainage waters, Ecological Applications, Nitrogen saturation, nitrogen sources, forested catchments, Journal of Environmental Quality, forested watersheds, American Water Resources Association, HYDROLOGICAL PROCESSES, riparian forest, Copyright, Mitchell MJ, Driscoll CT, forest ecosystem, Isotope Tracers in Catchment Hydrology, relative importance, interrelationships, biogeochemical controls, hydrologic processes, models, tracer techniques, anthropogenic influences, atmospheric inputs, MYRON J. MITCHELL, biogeochemical processes, dormant season, snowmelt periods, surface water, acidification, concentrations, Water Resources Research
HYDROLOGICAL PROCESSES Hydrol. Process. 15, 3305 3307 (2001) DOI: 10.1002/hyp.503
Linkages of nitrate losses in watersheds to hydrological processes
Myron J. Mitchell Faculty of Environmental and Forest Biology, College of Environmental Science
and Forestry, State University of New York
, Syracuse, NY 13210, USA Correspondence to: Myron J. Mitchell, Faculty of Environmental and Forest Biology, College of Environmental Science and Forestry, State University
of New York, Syracuse, NY 13210, USA. E-mail: [email protected]
Nitrogen Saturation in Watersheds A variety of factors can affect NO3- concentrations in surface waters, including atmospheric nitrogen (N) deposition, fertilizer N inputs, wastewater N contaminants, internal N cycling processes (e.g. plant uptake, mineralization, nitrification, denitrification) and hydrological flowpaths. Elevated levels of N inputs to ecosystems due to anthropogenic influences may have profound effects, including alterations of biotic diversity and N cycling (Vitousek et al., 1997). The dominant N solute in surface and ground waters for those systems experiencing `N saturation' is often NO3-, with NH4+ and dissolved organic N (DON) generally being of lesser importance. In contrast, DON often predominates in systems with lower total N (TN) concentrations. `Nitrogen saturation' can be defined as an increase in NO3- when N sources exceed the biotic demand for this element. Stages of `N saturation' in catchments have been described as a continuum with increasing NO3- concentrations and changes in the seasonal pattern
s of NO3- in surface waters (Stoddard, 1994). Elevated NO3- can contribute to freshwater acidification and coastal eutrophication.
Relationships between hydrological flowpaths and nitrate There are complex relationships among hydrology, chemical and biotic processes that affect N cycling. Hydrological flowpaths affect not only the sources of N, but also the relative influence of various biochemical processes on N transformations (Cirmo and McDonnell, 1997). In some catchments NO3- can be consumed in streams, riparian areas, groundwater (Groffman et al., 1996), hyporheic zones (Triska et al., 1993) and wetlands (mostly due to denitrification), whereas in other catchments some of these zones may be NO3- sources (Ohrui and Mitchell, 1998). The relative importance of these zones in affecting the temporal patterns and concentrations of NO3- is a function of not only the strength of various biotic transforma- tions, but also the ways these zones are hydrologically connected. Temporal patterns can have a major influence on NO3- export. For example, the importance of spring snowmelt episodes to NO3- loss in snow-dominated systems has been clearly demonstrated. Although some of the NO3- is derived directly from the snowpack
Copyright 2001 John Wiley
& Sons, Ltd.
Received 22 June 2001 Accepted 6 July 2001
MYRON J. MITCHELL
itself, most of the NO3- is lost by the water draining through the soil. During the dormant season, N is not taken up by the vegetation, allowing more N to be available for microbial N mineralization and subsequent nitrification. Not only can snowmelt periods dominate annual NO3- export from small, upland catchments, these episodes can also lead to acidification of surface waters with low acid-neutralizing capacity. Such areas are found in some regions of North America
and Europe. This `episodic acidification' can be deleterious to water quality
and the aquatic biota
. The influence of temporal patterns of hydrology on NO3- has also been shown for temperate forest catchments in Japan. In contrast to much of North America and Europe, NO3- exports and concentrations are highest during the summer on Honshu Island. During the summer, high temperatures
and elevated precipitation in Japan contribute to high rates of nitrification and elevated NO3- concentrations in drainage waters. Within Japan this elevated NO3- does not result in acidification of surface waters since most drainage waters have high acid-neutralizing capacities (Ohte et al., 2001). Catchments with high landscape heterogeneity (e.g. uplands, wetlands, streams and lakes) show different spatial and temporal patterns in the relative importance of biotic and abiotic processes affecting N retention and NO3- loss (Inamdar et al., 2000). The importance of hydrological pathways has been demonstrated in Ontario, Canada
, by a modelling approach that linked hydrological and biogeochemical processes and showed how topographic features can be used to explain much of the variability in NO3- export among small watersheds (Creed and Band, 1998). Burns et al. (1998) have suggested for Catskill Mountain watersheds in New York, USA that deep groundwaters with high concentrations of NO3- may be an important source of N in surface waters during periods of baseflow when groundwater would be the major contributor. However, Lovett et al. (2000) have argued that the differences in the spatial patterns in surface water NO3- concentrations in the Catskill Mountains are mostly attributed to differences in forest species composition
and forest history that affect N retention. Thus, there is some controversy on the relative importance of
biotic versus hydrologic processes affecting NO3- concentrations in drainage waters. Research needs Concomitant evaluation of the interrelationships between hydrology and biotic processes is clearly needed. Recent advances using isotopic tracer techniques have advanced our ability to identify sources and sinks of N and follow water pathways (Kendall, 1998). Various groups are currently using these techniques (e.g. Hill et al., 2000; Mitchell et al., 2001). However, biogeochemical and hydrological relationships need to be better integrated and tested in models that predict NO3- concentrations in catchment surface waters. Some of the models place more emphasis on biotic processes (e.g. PnETBGC; Gbondo-Tugbawa et al., 2001), whereas other models emphasize hydrological components (e.g. TOPMODEL; Creed and Band, 1998). These models need to be developed and tested with actual Field measurements
that include both hydrologic and biogeochemical processes. Moreover, anthropogenic influences such as agriculture, suburbanization, urbanization and industrialization, which can dominate N inputs, should be included in these models, especially across broad regions with significant human influences. Measuring hydrological and biogeochemical parameters within and across a range of catchments with different hydrological regimes, vegetation types, disturbance histories, and anthropogenic influences will be very useful in developing model predictions of how changes in atmospheric inputs, climatic regimes and land use will affect retention and/or loss of N to surface waters. References Burns DA, Murdoch PS, Lawrence GB, Michel RL. 1998. The effect of ground-water springs on NO3- concentrations during summer in Catskill Mountain streams. water resource
s Research 34: 1987 1996. Cirmo CP, McDonnell JJ. 1997. The hydrological and biogeochemical control
s of nitrogen transport in near-stream riparian zones of temperate forested watersheds. Journal of Hydrology 199: 88120. Creed IF, Band L. 1998. Export of nitrogen from catchments within a temperate forest: evidence for a unifying mechanism
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NITRATE LOSSES AND HYDROLOGICAL PROCESSES
regulated by variable Source area
dynamics. water resources
Research 34: 3105 3120. Gbondo-Tugbawa SS, Driscoll CT, Aber JD, Likens GE. 2001. Evaluation of an integrated biogeochemical model (PnETBGC) at a northern hardwood forest ecosystem. Water Resources Research 37: 1057 1070. Groffman PM, Howard G, Gold AJ, Nelson WM. 1996. Microbial nitrate processing in shallow groundwater in a riparian forest. Journal of Environmental Quality 25: 1309 1316. Hill AR, Devito KJ, Campagnolo S, Sanmugadas K. 2000. Subsurface denitrification in a forest riparian zone: interactions between hydrology and supplies of nitrate and organic carbon. Biogeochemsitry 51: 193 223. Inamdar SP, Mitchell MJ, McDonnell JJ. 2000. Topographic and riparian controls on hydrologic and biogeochemical response of forested catchments. In Riparian Ecology and Management in Multi-Land Use Watersheds, Wigington PJ, Beschta RL (eds). American Water Resources Association: Middleburg, VA; 137 142, (TPS 00 2). Kendall C. 1998. Tracing nitrogen sources and cycles in catchments. In Isotope Tracers in Catchment Hydrology, Kendall C, McDonnell J (eds). Elsevier: Netherlands; 519576. Lovett GM, Weathers KC, Sobczak WV. 2000. Nitrogen saturation and retention in forested watersheds of the Catskill Mountains, New York. Ecological Applications 10: 7384.
Mitchell MJ, Driscoll CT, Owen J, Schaefer D, Michener R, Raynal DJ. 2001. Nitrogen biogeochemistry of three hardwood forest ecosystems in the Adirondack Mountains. Biogeochemistry in press. Ohrui K, Mitchell MJ. 1999. Hydrological flow paths controlling stream chemistry in Japanese forested watersheds. Hydrological Processes 13: 877 888. Ohte N, Mitchell MJ, Shibata H, Tokuchi N, Toda H, Iwatsubo G. 2001. Comparative evaluation on nitrogen saturation of forest catchments in Japan and North America. Proceedings of Acid Rain 2000, Japan. Water, Air and SOil Pollution
in press. Stoddard JL. 1994. Long-term changes in watershed retention of nitrogen: its causes and consequences. In Environmental Chemistry of Lakes and Reservoirs. Advances in Chemistry Series 237 . Baker LA (ed.), American Chemical Society
: Washington, DC
, USA; 223284. Triska FJ, Duff JH, Avanzino RJ. 1993. The role of water exchange between a stream channel and hyporheic zone in nitrogen cycling at the terrestrialaquatic interface. Hydrobiologia 251: 167 184. Vitousek PM, Aber J, Howarth RW, Likens GE, Matson PA, Schindler DW, Schlesinger WH, Tilman GD. 1997. Human alteration of the global nitrogen cycle
: causes and consequences. Ecological Applications 7: 737 750.
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