There is increasing concern about water quality in rivers. Researchers are looking for ways to prevent pollution, such as nitrogen in agricultural runoff, from entering streams. Researchers suggest that one way to reduce the amount of nitrogen entering the streams is to maintain a vegetative riparian zone along the banks. They believe that these riparian zones act as "nutrient filters," regulating the nutrients that enter the stream from both forested and agricultural landscapes.
Riparian zones occur along rivers, and act as a transition zone between the terrestrial and the aquatic ecosystems. The vegetation in these riparian zones influences the river by shading and by providing organic matter (Gregory et al., 1991.) The riparian zones may also trap nutrients, such as nitrogen, which otherwise would have gone directly into the stream. One way they may do this is by trapping sediment, including adsorbed phosphorus, when it is eroded downslope (Hill, lecture.) Another way they may do this is by intercepting the nutrient-rich groundwater as it flows from upslope agricultural fields towards the river (Gregory et al., 1991.) When the groundwater flows through the root zone of the vegetation, the vegetation might take the nutrients up, since it requires them for growth. This might remove many nutrients that the groundwater would have otherwise carried into the stream (Gregory et al., 1991.) For example, in Maryland, riparian forests removed more than 75 percent of the dissolved nitrate from cropland runoff, before it could reach the river (Gregory et al., 1991.) In Georgia's coastal plains, the riparian forest removed more than 65 percent of the nitrogen and more than 30 percent of the phosphorus from the soil solution, nutrients that had come from nearby agricultural land (Gregory et al., 1991.) Although uptake by vegetation only temporarily removes nitrogen, the removed nitrogen might go to the river eventually, as the vegetation decomposes. However, there is potential for managers of riparian zones to solve this problem by harvesting some of the vegetation (Hill, lecture), thus removing the excess nitrogen from that ecosystem.
Although uptake by vegetation is important, the main process by which the riparian zone removes nitrogen is by microbial denitrification. Since the product of denitrification is nitrogen gas, this effectively removes the nitrogen from the ecosystem (Haycock, 1993.) Many factors contribute to make riparian zones suitable sites for denitrification. One factor is that flooding of the zone provides sediment containing organic carbon, an important energy source for denitrification. The flooding may also saturate the soil, producing anaerobic conditions: denitrification occurs in the absence of oxygen. The vegetation is also a source of organic carbon for the denitrification process, and when roots decay, they can provide organic carbon at depths within the soil (Haycock et al., 1993.) So, riparian zones can be important sites for nitrogen removal, thus acting as nutrient filters to the stream. In fact, since most of the nitrate is usually removed within the first five to ten meters of the riparian zone (Hill, lecture), it may be capable of removing much more (Hill, 1996.)
Although riparian zones can be important for nutrient removal, the presence of a vegetative riparian zone does not guarantee that it will be an effective nutrient buffer (Haycock et al., 1993.) For example, groundwater might flow at a depth where it will not interact with the vegetation, then discharge up through the stream bed (Hill, 1996.) If the water does not flow through an area where there is a source of organic carbon for energy, denitrification of the water will not occur. The groundwater may also flow through an area that is not saturated. If there is not sufficiently low oxygen, denitrification will not occur. If the groundwater flows through the riparian zone at a high velocity, it may not have a long enough residence time for plant uptake or denitrification to occur. Also, if there is a large volume of water flowing through the riparian zone at once, all of it cannot interact with the riparian zone. As well, low temperatures could reduce the efficiency of the microbial processes (Haycock et al., 1993.) For example, in a riparian zone at the Nottawasaga River near Alliston, there is sandy soil. The groundwater can flow at depth, and is flowing through soil with low organic matter, and there is oxygen present. This riparian zone, then, does not have much denitrification (Hill, lecture.) Even riparian zones that are suitable for denitrification might be variable in time. For example, if there was high snowmelt producing overland flow over cold soils, not much denitrification would occur (Haycock et al., 1993.) So, not all riparian zones are effective nutrient buffers to streams.
In conclusion, riparian zones can be important and effective at filtering nutrients, preventing excess nutrients such as nitrogen from entering the stream through groundwater. However, not all riparian zones are suitable sites for denitrification, and therefore are not important nutrient filters. Suitable riparian zones for denitrification will have: permeable soils with a shallow impermeable layer, forcing the groundwater to be in contact with the vegetation root zone; low to moderate groundwater flow, so that there is maximum residence time (Hill, 1996); seasonally variable flow (Hill, lecture) so that anaerobic conditions occur within the root zone; a supply of organic carbon, either from the vegetation or from deposited river sediments; and a lack of oxygen, for example in waterlogged areas. If managers are interested in reducing nitrogen inputs to a river, they need to determine if the riparian zone is a suitable site for denitrification. If so, they can encourage the preservation of that riparian zone. Under the right conditions, riparian zones can be very important nutrient filters, and thus important tools in reducing nutrient inputs to streams.
Gregory, Stanley V., Frederick J. Swanson, W. Arthur McKee, and Kenneth W. Cummins. 1991. An Ecosystem Perspective of Riparian Zones: Focus on links between land and water. Bioscience, 41(8):540-549.
Haycock, Nick E., Gilles Pinay and Charles Walker. 1993. Nitrogen Retention in River Corridors: European Perspective. Ambio, 22:340-346.
Hill, Alan R. 1996. Nitrate Removal in Stream Riparian Zones. Journal of Environmental Quality, 25:743-754.
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