Phosphorus losses from agricultural soil to water bodies are mainly related to the excessive accumulation of available P in soil as a result of long-term inputs of fertilizer P. Since P is a nonrenewable resource, there is a need to develop agricultural systems based on maximum P use efficiency with minimal adverse environmental impacts. This requires detailed understanding of the processes that govern the availability of P in soil, and this paper reviews recent advances in this field. The first part of the review is dedicated to the understanding of processes governing inorganic P release from the solid phase to the soil solution and its measurement using two dynamic approaches: isotope exchange kinetics and desorption of inorganic P with an infinite sink. The second part deals with biologically driven processes. Improved understanding of the abiotic and biotic processes involved in P cycling and availability will be useful in the development of effective strategies to reduce P losses from agricultural soils, which will include matching crop needs with soil P release and the development of appropriate remediation techniques to reduce P availability in high P status soils. SOILS contain between 100 and 3000 mg P kg" 1 soil, most of which is present as orthophosphate compounds. The proportion of total soil P present in organic forms ranges from 30 to 65% (Harrison, 1987). The soil solution in agricultural soils, which is the main source of P for plant roots, contains between 0.01 and 3.0 mg PL"1 . The quantity of P present in the soil solution represents only a small fraction of plant needs, and the remainder must be obtained from the solid phase by a combination of abiotic and biotic processes. The pro- cesses involved in soil P transformation are precipitation-dissolution and adsorption-desorption which control the abiotic transfer of P between the solid phase and soil solution, and biological immobilization-mineralization processes that control the transformations of P between inorganic and organic forms (Fig. 1).Phosphorus losses from soils occur by leaching at very low rates in undisturbed ecosystems (Walker and Syers, 1976; St. Arnaud et al., 1988; Frossard et al., 1989; Letkeman et al., 1996). The implementation of intensive agricultural production has markedly increased P losses from soils through increased runoff, erosion and leaching, which in turn can have adverse effects on water quality. These losses are further increased by the excessive accumulation of bioavailable P in the upper soil horizons, due either to application of inorganic and/or organic P fertilizers in excess of plant needs and/or to inappropriate fertilizer applications (Braun et al., 1994; Beaton et al., 1995; Sharpley et al., 1995; Sharpley and Rekolainen, 1997; Sibbesen and Runge-Metzger, 1995; Sibbesen and Sharpley, 1997; Daniel et al., 1998; van der Molen et al., 1998).It is the hypothesis of this paper that an efficient way of reducing P losses to the environment while maintaining an optimum plant production is to co...
Methods are needed for the rapid characterization of the phytoavailable fraction of trace elements in soils to assess the risk of contamination of the food chain and evaluate the impact of waste management practices. This investigation was undertaken to study the phytotavailability of Ni in soils using the isotopic exchange method. Isotopic exchange of 63Ni2+ ions was studied in two soils, a silt loam and a clayey muck, and the isotopic composition of Ni in the soil solution was determined. Plant tests were conducted on the same soils amended with 63Ni2+, and the isotopic composition of Ni in the plant tissues was also measured. The isotopic composition of Ni in soil extracted by DTPA at the end of the culture was also determined. Results showed that the isotopic composition of Ni in the soil solution was identical to the isotopic composition of Ni taken up by plants during the same time. The Ni in the plant came exclusively from the pool of the isotopically exchangeable Ni of the soil. Also, under these experimental conditions, DTPA extracted mainly isotopically exchangeable Ni, reinforcing the validity of this chemical to assess the phytoavailability of Ni. Besides, the phytoavailable soil Ni could be characterized with the intensity, quantity, and capacity factors deduced from isotopic exchange kinetics. The kinetics for which parameters were obtained from short‐term experiments were successfully extrapolated to longer times of exchange corresponding to plant growth which demonstrated that isotopic exchange kinetics is an appropriate tool to assess the truly phytoavailable Ni in soils. This technique requires more data from many different soils before it can be used for routine measurements.
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