. 2005. Soil and fertilizer phosphorus: Effects on plant P supply and mycorrhizal development. Can. J. Plant Sci. 85: 3-14. Plants require adequate P from the very early stages of growth for optimum crop production. Phosphorus supply to the crop is affected by soil P, P fertilizer management and by soil and environmental conditions influencing P phytoavailability and root growth. Phosphorus uptake in many crops is improved by associations with arbuscular mycorrhizal fungi. Cropping system and long-term input of P through fertilizers and manures can influence the amount and phytoavailability of P in the system and the development of mycorrhizal associations. Optimum yield potential requires an adequate P supply to the crop from the soil or from P additions. Where early-season P supply is low, P fertilization may improve P nutrition and crop yield potential. Alternately, under low-P conditions, encouragement of arbuscular mycorrhizal associations may enhance P uptake by crops early in the growing season, improving crop yield potential and replacing starter fertilizer P applications. Soil P supply that exceeds P requirements of the crop may preclude mycorrhizal development. To encourage arbuscular mycorrhizal association, threshold levels of soil solution P that restrict mycorrhizal development must not be exceeded. Sustainable P management practices must be applied both in conventional and in alternative biologically based agricultural systems.Key words: Microbiology, fertility, colonization Grant, C., Bittman, S., Montreal, M., Plenchette, C. et Morel, C. 2005. Le phosphore dans le sol et les engrais : incidence sur l'absorption du P par les plantes et sur le développement des mycorhizes. Can. J. Plant Sci. 85: 3-14. Les plantes ont besoin d'une quantité suffisante de P dès le début de leur croissance si l'on veut que leur culture donne un rendement optimal. Les apports de P dépendent de la concentration de cet élément dans le sol, de l'application d'engrais phosphatés et des conditions environnementales qui affectent la quantité de P disponible pour la plante et la croissance des racines. Chez maintes cultures, l'association avec des mycorhizes à arbuscules améliore l'absorption du phosphore. Les pratiques agricoles et l'apport prolongé de P résultant de l'application d'engrais et de fumier peuvent modifier la quantité de P dont les plantes disposent dans le milieu et le développement d'une symbiose avec les mycorhizes. Pour atteindre son meilleur rendement, la culture a besoin de tirer une quantité suffisante de P du sol ou des amendements. Quand la concentration de P ne suffit pas au début de la période végétative, la fertilisation facilite parfois l'assimilation de cet élément et accroît le rendement potentiel de la culture. Dans les mêmes conditions, favoriser l'association avec les mycorhizes à arbuscules peut aider la plante à mieux absorber le P au début de la saison de croissance, ce qui en accroîtra le rendement éventuel et remplacera l'épandage initial d'engrais phosphatés. Lorsqu'il contient plus de P...

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Phosphorus (P) is often a limiting factor of forest growth but our knowledge of the processes governing P availability in forest soils is rather limited. In the present work, we combined a isotopic dilution method with extraction methods to evaluate the P status in Pinus pinaster plantation forests on highly P-deficient soils. Total, organic, and inorganic P, dissolved and diffusive P, i.e. ionic P species that can be transferred from the solid phase to the soil solution due a gradient of concentration, were determined to a soil depth of 120 cm in a gradient of 18 forest sites (seven humid sites, five mesic sites, and six dry sites). Our objective was to assess the potential contribution of organic and inorganic P to plant available P. Based on results and our original assumptions, we observed that the contribution of organic P fractions (mineralization of soil organic P) to P availability related to the contribution of inorganic P fractions (diffusive P for durations up to 1 year) was predominant in litter, less important in top soil horizons, and negligible at depths below 30 cm. This was partly due to a decreasing proportion of organic P and an increasing proportion of diffusive P with soil depth. Owing to a very low amount of diffusive P in the top soils in dry sites, the relative contribution of organic P was actually higher in these sites than in humid and mesic sites, despite a lower overall organic P fraction. The combination of extraction and isotopic dilution methods in our study shed new light on P status in this forest range. In particular, these methods enable assessment of both the size of the pools and their dynamic fractions.

Adequate phosphorus (P) nutrition during early stages is critical for maize growth. Our objective was to evaluate the relative contribution of seed P reserves and exogenous P to maize nutrition during early growth stages. Seedlings were grown with labeled nutrient solution ( 32 P). Seedlings were harvested periodically over the course of the three-week study. Initially, 87% and 77% of the total C and N in seeds were located in the endosperm, whereas 86% of seed P was located in the scutellum as phytate. Up to the 7th day after sowing, 96% of phytate was hydrolyzed. Hydrolyzed forms of P were temporarily stored in the seed before being translocated to growing organs, suggesting that the hydrolysis of phytate was not a limiting step for P supply to seedlings. Significant P uptake by roots was observed from the 5th day after sowing on. Both sources of P supplied roots and leaves, with a slightly higher proportion of P from seed reserves going to leaves rather than to roots. Of total seed P, 60% and 92% was exported towards newly growing seedlings till 7th and 17th days after sowing and ceased to be a significant source of P for growth thereafter. We conclude that although both P supply processes overlap in time, seed P was the main P source during early growth stages.