Conclusion Ammonia (NH3), nitrate (NO3) and orthophosphate (PO4) are the principal nonpoint source forms of nitrogen and phosphorus. They may be released to surface waters by dissolved transport in subsurface water or by dissolved and particulate transport in overland flow. Particulate transport includes the adsorption of these ions to inorganic particles as well as the adsorption to, or combination with, organic materials. Nitrate has a low displacing capacity and is consequently very mobile, being subject to dissolved losses. Phosphate has a strong displacing capacity causing it to be readily adsorbed on sediment. Ammonia tends to accumulate in the forest floor litter or upper soil and may be adsorbed as well as dissolved before mineralization.
Studies of sediment-nutrient relationships on agricultural catchments revealed that overland flow is a common occurrence, contributing substantial amounts of particulate nitrogen and phosphorus when soil conservation practices are poor. The release of nitrogen and phosphorus depends on the sequence of cropland management practices proceeding storm events.
The distinction between dissolved and particulate nutrient transport is rarely reported in the literature for forested catchments. Studies in the Hubbard-Brook Experimental Forest and H.J. Andrews Experimental Forest present contrasting results regarding the percentage of phosphorus transported by sediment following timber harvest. These differences can be attributed to regional differences in the forest soils, specifically soil depth, content of clays and organic matter (i.e., CEC) and the rate of nitrification. Studies of nutrient release from forested catchments which do not account for the particulate component may provide misleading results. Because streambank erosion is dominant in undisturbed forests, the phosphorus-scavenging effect of this sediment should be investigated because of the typical low concentrations of phosphorus in the streambank sediment and the subsequent adsorption of dissolved phosphate. The results summarized for agricultural catchments above may also apply to forested catchments.
Thus, a review of sediment-nutrient studies on agricultural and forested catchments reveals no constant proportion of nitrogen or phosphorus transported by either sediment or solution, although trends are apparent. This observation undermines the usefulness of any nonpoint source models for nitrogen and phosphorus discharge, especially from forested catchments, unless the models can account for synoptic climatic conditions (e.g. frozen soils, storm rainfall energy, snowmelt), the geographic variability in catchment controls on the sources, adsorption, and movement of these ions and the effect of contrasting cropping patterns and conservation practices. Nitrogen and phosphorus export also changes over time as a part of ecosystem succession (Reiners, 1981), another factor not incorporated into nonpoint source modelling. Before sediment can be used as an indicator of nitrogen and phosphorus loads from forested catchments, the transport of these nutrients in subsurface and overland flow must be quantified, with distinction between the dissolved and particulate component. The proportion of nutrients transported by sediment can then be related to observed levels of the various ions to develop a factor quantifying the importance of this transport process depending on catchment factors, synoptic climatology, and the sequence of silvicultural practices.
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