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...

A sequential alkali extraction procedure followed by ultrafiltration and quantitative "P nuclear magnetic resonance (NMR) spectroscopy was used to examine organic phosphorus in a Brown Chernozem, and an adjacent Gleysol developed under native prairie vegetation, and a Grey Luvisol formed under aspen forest in Saskatchewan, Canada. Differences in the nature of organic P in the native soils were related to moisture status and vegetation. In the grassland soils, a greater proportion of orthophosphate diester P was found in the bottom-slope Gleysol. This difference was partly attributed to less favourable conditions for mineralization in the bottom slope soil compared with the mid-slope Brown Chernozem. Teichoic acid P occurred only in the native Grey Luvisol (NMR 6 p.p.m. 0.36-0.95) under boreal forest and not under native grassland. At all three sites, soils under long-term cultivation were also examined and while orthophosphate monoester P (83.4-94.6% total Po), orthophosphate diester P (3.9-8.7% total Po) and teichoic acid P (12.7% total Po in forested Grey Luvisol) were detected in native soils, only orthophosphate monoester P was found in the corresponding soils that had been cultivated for 70-80 years. These findings suggest that orthophosphate diester P and teichoic acid P are more readily mineralized in the soil environment than orthophosphate monoester P forms. I N T R O D U C T I O NInitial investigations into the use of "P nuclear magnetic resonance (NMR) spectroscopy in the analysis of phosphorus (P) in soils showed that several different inorganic and organic P species (i.e. inorganic orthophosphate P, orthophosphate monoester P, orthophosphate diester P, polyphosphate P, pyrophosphate P and phosphonate P) could be determined quantitatively

Intact lysimeters (50 cm diameter, 70 cm deep) of silt loam soil under permanent grassland were used to investigate preferential transport of phosphorus (P) by leaching immediately after application of dairy effluent. Four treatments that received mineral P fertilizer alone (superphosphate at 45 kg P ha À1 year À1 ) or in combination with effluent (at $ 40-80 kg P ha À1 year À1 ) over 2 years were monitored. Losses of total P from the combined P fertilizer and effluent treatments were 1.6-2.3 kg ha À1 (60% of overall loss) during eight drainage events following effluent application. The rest of the P lost (40% of overall loss) occurred during 43 drainage events following a significant rainfall or irrigation compared with 0.30 kg ha À1 from mineral P fertilizer alone. Reactive forms of P (mainly dissolved reactive P: 38-76%) were the dominant fractions in effluent compared with unreactive P forms (mainly particulate unreactive P: 15-56%). In contrast, in leachate following effluent application, particulate unreactive P was the major fraction (71-79%) compared with dissolved reactive P (1-7%). The results were corroborated by 31 P nuclear magnetic resonance analysis, which showed that inorganic orthophosphate was the predominant P fraction present in the effluent (86%), while orthophosphate monoesters and diesters together comprised up to 88% of P in leachate. This shows that unreactive P forms were selectively transported through soil because of their greater mobility as monoesters (labile monoester P and inositol hexakisphosphate) and diesters. The short-term strategies for reducing loss of P after application of dairy effluent application should involve increasing the residence time of applied effluent in the soil profile. This can be achieved by applying effluent frequently in small amounts.

Ecosystem process rates typically increase after plant invasion, but the extent to which this is driven by (i) changes in productivity, (ii) exotic species’ traits, or (iii) novel (non-coevolved) biotic interactions has never been quantified. We created communities varying in exotic plant dominance, plant traits, soil biota, and invertebrate herbivores and measured indicators of carbon cycling. Interactions with soil biota and herbivores were the strongest drivers of exotic plant effects, particularly on measures of soil carbon turnover. Moreover, plant traits related to growth and nutrient acquisition explained differences in the ways that exotic plants interacted with novel biota compared with natives. We conclude that novel biological interactions with exotic species are a more important driver of ecosystem transformation than was previously recognized.