Phosphorus in soil solution

Vaz, M.D. Ron, Edwards, A.C., Shand, C.A. and Cresser, M. (1992) Determination of dissolved organic phosphorus in soil solutions by an improved automated photo-oxidation procedure. Talanta, 39, 1479-1487. 

Pant, H.K., Edwards, A.C. and Vaughan, D. (1994) Extraction, molecular fractionation and enzyme degradation of organically associated phosphorus in soil solutions. Biology and Fertility of Soils 1 7, 1 95-200. 

Pant et al. (1994) reported that organic phosphorus in soil solution is associated with a... 

The availability of organic and condensed phosphorus in soil solution and other extractable soil phosphorus fractions has also been investigated using commercially available phosphatase enzymes (Fox and Comerford, 1992 Pant et al., 1994 Shand and Smith, 1997 Otani and Ae, 1999 Hayes et al., 2000a Hens and Merckx, 2001 Turner et al., 2002a Toor et al.,... 

Turner, B.L., Baxter, R. and Whitton, B.A. (2003b) Nitrogen and phosphorus in soil solutions and drainage streams in Upper Teesdale, northern England implications of organic compounds for biological nutrient limitation. Science of the Total Environment 314/31 SC, 153-1 70. 

Forms of Organic Phosphorus in Soil Solution and Runoff... 

Rowland, A.P. and Haygarth, P.M., Determination of total dissolved phosphorus in soil solutions. Journal of Environmental Quality 26, 410, 1997. 

The growth of ectomycorrhizal trees is frequently improved by their increased phosphorus (P) accumulation (3), and this, in turn, is related to the intensity of the mycorrhizal infection. Ectomycorrhizal fungi solubilize insoluble forms of A1 and Ca phosphates as well as inositol hexaphosphates, though a wide interstrain variability has been recorded (112). These complex P forms are digested by the secretion of extracellular acid and alkaline phosphomono- and phosphodi-ester-ases. Pi in soil solutions is easily taken up by ectomycorrhizal hyphae and then translocated to the host roots. Its absorption and efflux are probably regulated... 

In the method for extractable phosphorus [62, 64-66] the phosphorus is extracted from the soil at 20 1°C with sodium bicarbonate solution at pH8.5. After filtration and release of carbon dioxide the extracts are introduced into a flow-injection system for the determination of phosphate. Phosphate is determined by reaction with vanadomolybdate and the yellow colour evaluated at 410nm. Between 20 and lOOOmg kg-1 phosphorus in soil has been determined using this method. 

Transport of contaminants by surface runoff is illustrated in the experimental results of Turner et al. (2004), which deal with the colloid-mediated transfer of phosphorus (P) from a calcareous agricultural land to watercourses. Colloidal molybdate-reactive phosphorus (MRP) was identified by ultrafiltration associated with particles between l am and Inm in diameter. Colloidal P compounds can constitute a substantial component of the filterable MRP in soil solution and include primary and secondary P minerals, P occluded or adsorbed on or within mineral or organic particles, and biocolloids (Kretzschmar et al. 1999). 

Phosphorus occurs in various soil fractions as soil minerals combined with Ca, Fe, Al, which are of low solubility bound to particle surfaces of, e.g. sesquioxides, calcite, to Al on humus surfaces in soil solution in the organic matter, primarily as esters. 

Knight, W. G., Allen, M. F., Jurinak, J. J. Dudley, L. M. (1989). Elevated carbon-dioxide and solution phosphorus in soil with vesicular-arbuscular mycorrhizal western wheatgrass. Soil Science Society of America Journal, 53, 1075-82. 

Plants can also balance their uptake of different nutrients through their production of enzymes and other compounds that help to make specific nutrients more available. Nitrate reductase is required to assimilate NO3 into plant biomass, and its production is triggered by the presence of NO3 in soil solution. Phosphorus limitation induces production of root phosphatase enzymes that cleave organically bound PO4, or side-rophores, which solubihze mineral phosphorus by chelating with other minerals that bind to PO4, such as iron. 

This picture of a substantial phosphorus constraint is drastically altered when the presence of the inorganic labile (i.e., sorbed) phosphorus pool is taken into account (Scenario C). Desorption of phosphate occurs in response to increased rates of removal from the soil solution. Consequently, the reduction in soil solution phosphorus concentration over 1730 levels is only 9% for 1981-1990. This contrasts with the 25% reduction in Scenario B. Consequently, the enhancements of Gp and Np are more similar to the no-P-constraint case, though a full expression of the COi-in-duced growth response is still not possible. Accordingly, the rate of net carbon accumulation by the ecosystem is 6.5 mol m - year", substantially more than Scenario B, but about 20% less than what is modeled to be the case if no phosphorus limitations to plant production occurred. 

This chapter reviews the abiotic processes that can lead to the stabilization of organic phosphorus in soils and the aquatic environment. In particular, we examine the role of adsorption to soil minerals, complex-ation reactions, precipitation with polyvalent cations and the incorporation of organic phosphorus into humic substances in stabilizing organic phosphorus. We then discuss the effects of soil solution chemistry on these reactions, as well as the effects of these reactions on the environment. 

It is generally considered that plants obtain phosphorus exclusively as phosphate anions from soil solution. However, in most soils, phosphorus acquisition by plants is limited by low concentrations of phosphate in soil solution (typically less than 5 p-M), low diffusion rates, or a limited capacity to replenish phosphate levels in soil solution (Bieleski, 1973). Soil solution also contains significant amounts of organic (and condensed) phosphorus compounds at concentrations that may be appreciably higher than that of phosphate (Ron Vaz et al., 1993 Shand et al., 1994). Despite this, the contribution of non-phosphate forms of phosphorus to plant nutrition is not well understood and currently there is no evidence to suggest that plants are able to take up such forms of phosphorus directly. 

The major pools and associated transformations of phosphorus in the soil-plant system are presented in Fig. 13.1. These show that the distribution, dynamics and availability of phosphorus in soil are controlled by a combination of biological, chemical and physical processes. Soil solution is the primary source of phosphorus for plants and microorganisms, and most phosphorus is taken up as phosphate (HPO , H2P0 ). The equilibrium concentration of phosphate present in soil solution is very low (<5 fxM) and phosphate removed by plant and microbial uptake must be continually replenished from the inorganic, organic and microbial phosphorus pools. This is especially important in agroecosystems where demand for phosphorus is high and annual off-farm transfer of phosphorus in produce commonly exceeds 20 kg P/ha (Stevenson and Cole, 1999). 

If added solution phosphorus concentration is lower than the concentration of phosphorus in soil pore water (e.g., rainwater), then the soil releases or desorbs phosphorus until new equilibrium is maintained. For any given soil, at some critical concentration, net adsorption equals zero, which means that adsorption equals desorption and the system is at equilibrium as indicated by EPCg (equilibrium phosphorus concentration). At this point, soil exhibits maximum capacity for buffering phosphorus in soil pore water. In this region, the system reattains equilibrium conditions, even if soils are loaded with or depleted of phosphorus in soil pore water. If the water entering a wetland has a phosphorus concentration below EPCq, then that soil releases phosphorus or serves as a source of phosphorus to the water column or soil pore water. If the water entering a wetland has phosphorus concentration higher than EPCq, then that soil adsorbs or retains phosphorus or serves as a sink for added phosphorus. 

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