Summary 1.Increasing plant species richness often increases biomass production in nutrient-poor seminatural grasslands. If such positive diversity-productivity effects also apply to nutrient-rich agricultural grasslands, mixtures could improve resource-use efficiency in the vast area used for forage production. We therefore quantified the diversity-productivity effects in nutrient-rich agricultural grasslands using four-species grass-legume mixtures. 2. The sown overall density and species proportions of Lolium perenne , Dactylis glomerata , Trifolium pratense and Trifolium repens were varied in a 3-year field experiment to investigate the effects of species richness (1, 2, 4 species) and species proportion (0, 3, 10, 25, 40, 50, 70, 90, 100% sowing proportion) on productivity under a nitrogen fertilization of 50, 150 or 450 kg N ha − 1 year − 1 . 3. The four-species mixtures reached up to twice the yield of the average of the four species' monocultures (overyielding up to 106%), predominantly due to combining grass and legume species. Mixtures were up to 57% more productive than the most productive monoculture (transgressive overyielding). Both these diversity-productivity effects appeared across a broad range of species proportions and persisted at the two lower levels of N fertilization for 3 years. Mixtures fertilized with 50 kg N ha− 1 year − 1 produced yields comparable to grass monocultures fertilized with 450 kg N ha − 1 year − 1 , if the legume proportion was about 50 to 70%. Diversityproductivity effects were reduced at the highest level of N fertilization, where they virtually disappeared in the third year. Increased N fertilization also accelerated the observed general trend towards D. glomerata dominated and legume-poor swards. 5. Synthesis and applications . Diversity-productivity effects led to consistent transgressive overyielding in intensively managed grasslands, suggesting a highly increased resource-use efficiency in mixtures. Performance better than monocultures can be achieved with grass-legume mixtures that have a low number of species, across a wide range of species proportions and in nutrient-rich conditions. Processes such as niche complementarity and positive interspecific interactions leading to diversity effects proved to be highly relevant and widely applicable for intensive forage production. Such diversity-productivity effects could allow reduced inputs of N fertilizer without loss of productivity in different grassland production systems.

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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...

Phosphorus availability may shape plantmicroorganism-soil interactions in forest ecosystems. Our aim was to quantify the interactions between soil P availability and P nutrition strategies of European beech (Fagus sylvatica) forests. We assumed that plants and microorganisms of P-rich forests carry over mineral-bound P into the biogeochemical P cycle (acquiring strategy). In contrast, P-poor ecosystems establish tight P cycles to sustain their P demand (recycling strategy). We tested if this conceptual model on supply-controlled P nutrition strategies was consistent with data from five European beech forest ecosystems with different parent materials (geosequence), covering a wide range of total soil P stocks (160-900 g P m -2 ; \1 m depth). We analyzed numerous soil chemical and biological properties. Especially P-rich beech ecosystems accumulated P in topsoil horizons in moderately labile forms. Forest floor turnover rates decreased with decreasing total P stocks (from 1/5 to 1/40 per year) while ratios between organic carbon and organic phosphorus (C:P org ) increased from 110 to 984 (A horizons). High proportions of fine-root biomass in forest floors seemed to favor tight P recycling. Phosphorus in fine-root biomass increased relative to microbial P with decreasing P stocks. Concomitantly, phosphodiesterase activity decreased, which might explain increasing proportions of diester-P remaining in the soil organic matter. With decreasing P supply indicator values for P acquisition decreased and those for recycling increased, implying adjustment of plantmicroorganism-soil feedbacks to soil P availability. Intense recycling improves the P use efficiency of beech forests.