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Manure Phosphorus and Surface Water Protection I: Basic Concepts of Soil and Water P

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Phosphorus (P) in Water Bodies

Phosphorus transport to surface water is primarily due to runoff and erosion, and may consist of soluble, labile and stable forms of soil P. Small amounts of P may enter surface waters due to downward movement through the surface profile, followed by lateral subsurface flow, to emerge as seepage or in springs whose water flows to the surface water body.

The P fractions entering water bodies are classed differently than soil P fractions. The three types of P in water are:  dissolved P, bio-available P, and sediment P.

Dissolved P is considered to be P that passes through a 0.45-micron filter paper and is immediately available to aquatic vegetation.

Discussion Question:
Which soil P fraction is the near equivalent of dissolved P in water?

Bio-available, or algal-available P, is a combination of dissolved P, plus the estimated portion of sediment P to be released within a short time, e.g., 45 hours, after the particles enter surface water. Bio-available P is not immediately available, but is expected to be available to aquatic vegetation within a few days or weeks. Bio-available P in water is commonly measured as P adsorped during shaking with a strip of filter paper impregnated with iron oxide.

Discussion Question:
Which soil P fraction is the near equivalent of bio-available P in water?

Discussion Question:
Why is iron oxide used in this water test?

Sediment P, or particulate P, is strongly bound to organic or mineral sediment (e.g,. clay particles). It is not readily available, but much of it may become available within a few years.

Phosphorus in surface water strives for equilibrium between dissolved P, bio-available P, sediment P, and P in living tissue (biomass) (Fig. 9).  Conversion of sediment-bound P to P available to vegetation is affected by several factors (Fig. 10).

Figure 9.  Water P forms and reactions (Graphic courtesy of Dusti Torrey)

Figure 10.  Phosphorus dynamics in water bodies that cause sediment P to become available to aquatic organisms. Arrows of heavier weight indicate a shift in the reaction to increase the concentration of a P fraction. (Image by Charles Wortmann)

More conversion of sediment P to bio-available P is expected when dissolved P concentrations are low to maintain equilibrium between the P forms. Some algae produce phosphatase which reacts with organic sediment P to release it to inorganic dissolved forms. Vertical cycling of water, e.g., due to seasonal temperature differences with depth of the water, will favor conversion of sediment P to dissolved P. Bottom-feeding fish, such as carp, and motor-boat traffic, cause much resuspension of sediment P, driving to reaction to release bio-available and dissolved P. Therefore, in the short term, dissolved and bio-available P fractions are of greatest concern. In the long term, however, we need to be concerned about the relatively large amount of total P entering surface waters where P is still a limiting nutrient to algal growth.


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