come existing obstacles to better manure management (e.g., incentives and disincentives).
Composting is another potential management tool to improve manure distribution. Although composting tends to increase the phosphorus concentration of manures, the volume is reduced and thus transportation costs are reduced. Additional markets are also available for composted materials. Finally, there is interest in using some manures as sources of bioenergy. For example, dried poultry litter can be burned directly or converted by pyrolytic methods into oils suitable for use to generate electric power. Liquid wastes can be digested anaerobically to produce methane that can be used for heat and energy. As the value of clean water and cost of sustainable manure management is realized, it is expected that alternative entrepreneurial uses for manure will be developed, become more cost-effective, and, thus, create expanding markets. Research is needed to speed the development of these types of technologies and approaches.
Once water and sediment begin to move in the landscape, taking with them the phosphorus originally applied as fertilizer and/or manure, the quantities that reach the stream can be reduced by any feature that slows flow and/or encourages infiltration or sediment trapping. Such transport management measures include terracing, contour tillage, cover crops, buffer strips, riparian zones, and impoundments. These transport measures are generally more efficient at reducing particulate phosphorus rather than dissolved phosphorus. However, such approaches only work where subsurface pathways of phosphorus loss are unimportant. Furthermore, by encouraging infiltration of surface runoff, which may be enriched with phosphorus, the problem is simply translated from surface to subsurface delivery. While uptake by plant roots and adsorption onto soil particles may delay the delivery of phosphorus to surface waters, such mechanisms may be ineffective in soils with a high hydraulic conductivity (e.g., sands) or where macropore or drainflow is important.
For nitrogen, losses can also be reduced by improved water management, including adoption of controlled drainage or sub-irrigation methods, switching from furrow irrigation to surge irrigation or sprinkler irrigation with fertigation, and the use of irrigation scheduling techniques (Skaggs and Gilliam 1981). Nitrate losses can also be reduced by control of water table depth by managing tile drain spacing and depth and by control structures on the tile drain outlets, to limit tile flow when the potential for nitrate may be greatest (Gilliam et al. 1979; Kladivko et al. 1991; Zucker