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Soil responds dynamically to the addition of phosphates by establishing exchanges among the di †erent reservoirs of P existing in the soil. T he aim of this work was to Ðnd out the e †ects of surface applied fertilization with triple superphosphate at two concentrations (50 and 100 kg P ha ] 1 ) in soils of the Sierra de Gata mountains (Spanish Central System) on forms of soil P, the availability of P for plants, and the impact on aboveground production of the tree cover and grasses. Soil P was deter ± mined according to Hedley, and P availability was quantiÐ ed using ion exchange membranes. T he e †ects of P fertilization on P forms were observed mainly in the Ðrst 10 cm of the study soils ; there were no signiÐcant di †erences at greater depths because of the high P sorption capacity of these soils. W ith a moderate fertilization of 50 kg P ha ] 1 only two of the four studied soils showed a signiÐcant increase in labile P contents and of P adsorbed by sesquioxides. An intense fertilization (100 kg P ha ] 1 ) had signiÐcant e †ects on the distribution of soil P, independently of the initial available P of soils. However, fertilization with triple superphosphate did not signiÐcantly a †ect the aerial production of the tree cover (except in the Ðrst year), whereas it did a †ect herbaceous production at two sites, possibly because they have a larger root mass near the soil surface.

-The amount, quality, and turnover of soil P is influenced by climate and changes in soil management. The objectives of this study were to evaluate the influence of edaphic properties, mean annual precipitation, and P-fertilization on soil organic P. 31 P-NMR spectroscopy was applied to investigate P forms of forest soils of the Central Western Spain. The concentrations of NaOHextractable inorganic-P were significantly higher in fertilized than in natural soils. Monoester-P was the dominant organic-P species in both natural and fertilized soils, representing between 19 and 54% of NaOH-extractable P. The highest concentrations of monoester-P were observed in the soil with higher content of Fe oxides. The high charge density of monoester-P allows rapid adsorption on soil minerals and extensive interaction with sesquioxides that protect inositols from degradation. Diester-P represented between 3 and 17% of alkali-extractable P, reflecting a relatively low microbial activity in the soils on schists with a high content of Al and Fe oxides.organic P / P-fertilization / forest soil / diester-monoester-P / 31 P-NMR Résumé -Caractérisation du phosphore et de ses fractions par la technique du NMR dans des sols forestiers fertilisés et nonfertilisés. La teneur, la qualité et le turnover du P du sol sont influencés par le climat et les changements dans la gestion du sol. L'objectif de cette étude a été d'évaluer l'influence des propriétés édaphiques, de la pluviométrie moyenne annuelle et de la fertilisation phosphatée sur le P organique du sol. La spectroscopie NMR a été appliquée pour rechercher les formes du P dans des sols forestiers du Centre Ouest de l'Espagne. La concentration en P inorganique extrait avec NaOH a été significativement plus haute dans le sol fertilisé que dans le sol naturel. Le monoester phosphorique (représentant entre 19 et 54 % du P extractable avec NaOH) est la forme dominante de P organique, aussi bien dans le sol naturel que dans le sol fertilisé. La plus haute concentration de monoester phosphorique a été observée dans le sol avec la plus haute teneur en oxydes ferriques. La haute densité de charge du monoester phosphorique permet sa rapide adsorption sur les composés mineraux du sol et son interation marquée avec les sesquioxydes, protégeant ainsi de la dégradation les inositols. Les diesters phosphoriques représentent entre 3 et 17 % du P extractable avec NaOH, montrant par là une relativement basse activité microbienne dans les sols sur schistes avec une haute teneur en oxydes aluminiques. P organique / fertilisation phosphorique / sols forestiers / esters phosphoriques / NMR du 31 P Ann. For. Sci. 58 (2001) [89][90][91][92][93][94][95][96][97][98] 89

No abstract

Classical chemical extraction methods to assess nutrient availability are often inadequate to detect in situ temporal and spatial soil-nutrient changes produced by fertilization. The objective of this work was to develop and evaluate an in situ soil test suitable for estimating the soil nutrient status in both climax and fertilized forests. Simultaneous extraction of soil phosphate, nitrate, and sulfate by means of anion-exchange membranes (AEM) placed in situ is described. These anions were analyzed simultaneously with a Dionex chromatograph. Membranes were placed at soil depths of 2.5, 10, 20, and 30 cm, in natural and P-fertilized forests with minimal disturbance of the soil. Seasonal patterns of ion sorption presumably reflect the effect of a combination of several factors: soil temperature and moisture, microbial and root activities, and ion mobility. Ion sorption by the AEM was strongly affected by soil water regime. All nutrient ions sorbed by the AEM diminished in summer. Phosphate concentration in the soil varied significantly with parent material, depth, season, and, as expected, P fertilization. Nitrate and sulfate contents at the different soil subhorizons were not affected by P fertilization. The different anion levels found at different soil subhorizons are a consequence of both the decreasing of the soil organic matter content and hence enzymatic activities, and an increase in soil fixation phenomena at depth.