Nutrient Cycling at the Land - Water Interface : the Importance of the Riparian Zone !

-Riparian ecosystems are sites of important biogeochemical processes that affect the composition and structure of the streamside biota as well as aquatic systems. Microbial activity, coupled with the slow diffusion of oxygen in waterlogged riparian soils, causes anaerobic conditions and reduction in reduction­ oxidation (redox) potential. Redox potentials provide a useful measurement as to the intensity of anaerobic conditions and the degree of chemical transformation in riparian zones. Under anaerobic conditions, biogeo­ chemical cycles differ greatly from the aerobic conditions of surrounding uplands. Different plant species are adapted to survive in varying levels of reduced waterlogged conditions such that different plant communities occupy sites of different redox potential. For example, Kentucky bluegrass Poa pratensis-timothy Phleum pratense communities occurred in areas ofhigh redox potential (well drained soils) (570 m V), beaked sedge Carex rostrata in moderate redox potential (-48 mV), and tall mannagrass Glyceria grandis (-164 mV) in waterlogged, anaerobic soils. Recent research has shown that low redox potentials result in denitrification and that phospho­ rus immobilization processes occur within intact riparian ecosystems, thereby greatly influencing water quality. The effects of grazing or other anthropogenic disturbances can alter these biogeochemical cycles resulting in drastic alterations in riparian vegetation composition and productivity, aquatic ecosystems, and water quality. Given these important linkages to resource productivity, the effect oiland use activities on biogeochemical cycles of riparian/stream ecosystems should be considered. Riparian ecosystems perform a variety of functions essential to a healthy instream community and high water quality (Swanson et al. 1982). Brinson et al. (1981) esti­ mated that over 70% of riparian communities have been altered and less than 2% of the land area in the USA con­ sists of intact natural riparian communities. Major losses of riparian bottom land forest have occurred as a result of lumbering, drainage for agriculture (Korte and Fredrick­ son 1977; McGill 1979) or channelization (Funk and Robin­ son 1974; Barclay 1978; Triska et al.1982). Historical activ­ ities such as debris removal, beaver Castor canadensis trapping, and grazing reduced the biological integrity of riparian zones before extensive research began (Harman et al. 1986; Naiman et al. 1986). These changes have greatly altered riparian ecosystem structure and function thereby greatly changing biotic interrelationships between ripar­ ian and aquatic ecosystems. Livestock are believed to be attracted to riparian areas by the availability of water, shade, thermal cover, and quality and quantity of forage (Kauffman and Krueger 1984). Although riparian meadows occupy only 1-2% ofthe interior northwestern USA, they account for 81% of the forage removed by livestock (Roath and Krueger 1982). In reviewing Iivestock-riparian relationships, Kauffman (1988), separated the impacts of excessive grazing in the riparian zone into four components: (1) compaction ofsoils which increases runoff and decreases water availability to plants, (2) herbage removal which lowers plant vigor and changes competitive interactions among species, (3) physi­ cal damage to vegetation by rubbing, trampling, and browsing, and (4) changes in fluvial processes which may lower water tables and/or cause a decline in invasion sites for woody species. Dahm et al. (1987) have concluded that human influen­ <:.~~ \~~~~~"'t \."'t~}>}>\.~'i., <:"~~~~~\;"'L~\.;"<:>~, ~~<!. 'i."'t~'L""~'b\ ~~'<.\~ acted to uncouple riparian and stream ecosystem inter­ actions.This has important implications as riparian zones are physical, biological, and chemicallinks between terres ~rial and aquatic environments. Important, yet little stud­ Ied ecological processes of riparian zones are the biogeo­ l:rechnical Paper No. 8724, Oregon Agricultural Experiment Sta­ '»tW, £"a-m:rlWs-. 61 chemical cycles that influence both riparian and aquatic systems. Riparian areas are unique in that they contain large areas of anaerobic (saturated) soil. In saturated soils, measurement of reduction-oxidation (redox) potential pro­ vides a useful indicator ofthe intensity of anaerobic condi­ tions and therefore the degree of chemical transformation and the biogeochemical pathway operating in the riparian zone. Nutrient inputs resulting from the biogeochemical pro­ cesses of riparian zones not only affect the associated aquatic community, but also the downstream industrial, recreational, and agricultural water users as well (Coats et al. 1976; Rhodes et al. 1985). Pristine or intact riparian zones have been found to function as important sites for denitrification (Coats et al. 1976; Lowrance et a11984; Jac­ obs and Gilliam 1985). Additionally, riparian zones are important phosphorus sinks where phosphorus adsorbed to clay particles is deposited during high flow or trapped in runofffrom surrounding uplands allowing time for plant uptake or microbial use (Yarbro 1979; Cooper and Gilliam 1987; Cooper et al. 1987). Given the critical role biogeo­ chemical cycles play in riparian ecosystem structure and function, management strategies will need to be designed to insure these ecosystems are functioning in a dynamic equilibrium.