Current knowledge of processes such as denitrification and sediment P adsorption that are critical to stream N and P buffering capacity indicate that these processes are strongly influenced by water residence times and the accumulation of fine textured organic rich sediments in streams. Channelisation of small headwater streams to increase drainage capacity tends to reduce water contact time with the stream bed and enhance the removal of fine sediments. In contrast, management manipulations that reduce water velocity such as reductions of channel slope or the construction of small ponds would tend to increase water retention times and the accumulation of organic matter and thus enhance the stream nutrient buffering capacity. The afforestation of stream riparian zones to
enhance the buffering capacity of the stream side environment (Petersen et al., 1992; Muscutt et al.,
1993) can also improve the efficiency of stream N and P removal by increasing the supply of terrestrial carbon to stream sediments.
Macrophytes play a significant role in nitrate uptake in some streams and can also stimulate denitrification by encouraging significant deposition of organic matter and increasing the supply of nitrate in sediments by nitrification activity in the root zone (Christensen and Sorensen, 1988). The establishment and maintenance of macrophyte communities in small streams unshaded by adjacent riparian vegetation can therefore also increase the stream nutrient removal capacity during low flows. The costs involved in reduced drainage capability on agricultural land can mitigate against stream management practices that increase water retention times in order to maintain or enhance the nutrient buffering capacity of small headwater streams. It is therefore important to consider the benefits of stream N and P removal processes in making catchment management decisions. It is also essential to integrate the management of stream buffers with other potential buffer zones such as riparian zones and small on-stream ponds as part of a more comprehensive approach to the management of nutrient fluxes within the landscape.
CONCLUSIONS
There is now considerable evidence that streams play a significant role in regulating the flux of N and P during base flows particularly in summer, although the annual capacity to remove N and P is often low. Nevertheless, many aspects of streams as N and P buffers are inadequately understood. The influence of the hyporheic zone on stream water chemistry in relatively eutrophic agricultural streams is still unknown, as is the capacity of this subsurface environment to act as a buffer for nitrate rich groundwater which discharges directly through the stream bed. Further research is needed to evaluate P transport and retention processes in small agricultural streams which do not receive large point sewage inputs. Most studies of nutrient removal in headwater streams are from temperate landscapes and little information is available in tropical environments. A more detailed understanding of the potential role of in-stream and hyporheic environments as buffer zones in a wider range of streams and landscape settings can contribute to the successful management of N and P in catchments.
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