Effects of parent material and land use on soil phosphorus forms in Southern Belgium Renneson 1 M., Dufey 2 J., Bock 1 L. and Colinet 1 G. 1 University of Liege (Belgium) - Gembloux Agro-Bio Tech – Soil Science Unit. Passage des Déportés, 2. B-5030 Gembloux - 2 University of Louvain-la-Neuve (Belgium) – Soil Sciences Unit. MATERIALS AND METHODS Soil phosphorus faces both environmental and agronomic issues because it is responsible for eutrophication of surface waters and, at the same time, it is an essential element for plants growth. It is therefore important to deepen our knowledge about the quantities and forms of P in Walloon soils and the part of P content due to the background characteristics. INTRODUCTION AND OBJECTIVES Influence of parent material on deeper horizon Influence of land use and parental material in surface horizon CONCLUSIONS There is a large range of variation for studied soil properties ParametersRange of values pH water Clay content % TOC % CEC cmol+ kg -1 P av mgP kg -1 P total mgP kg -1 Al total g kg -1 Fe total g kg -1 Twelve types of parent materials have been selected in Walloon Region. For each one, 10 fields have been chosen (Fig. 1). Soil types were observed by augering and soil samples were taken in surface horizon (0-15 cm) and in the deep cm horizon. They were located in 76 crop fields, 15 temporary grasslands and 29 pastures, chosen according to regional land use distribution. Different parameters were determined to characterize edaphic conditions : pH water and pH KCl, total organic carbon (TOC), cation exchange capacity (CEC) and particle size distribution. Available phosphorus (P av ) (Lakanen and Erviö, 1971) and total phosphorus (P tot ) (NF X , 1996) were determined. Inorganic phosphorus (P inorg ) was extracted by H 2 SO 4 (1:40, w:v) while organic phosphorus (P org ) was determined by difference of P tot by P inorg. On average, deep soil P amounts to half of surface P. The ratio P org /P inorg is relatively variable (from 7.9 to 74%), with a mean of 30% close to values from Conesa & Fardeau (1994). Statistical analyses have shown differences between parent materials for P tot, P inorg and P org contents, P inorg percentage but also for edaphic properties. But, despite the diversity of geological contexts, the P tot differences are relatively limited (Fig. 3), excepted for sandy loamy soils which presented low P tot and P org contents. The land use effect adds to parent material influence on surface P con- tent. P availability, however, depends mainly upon parent material (Fig. 4). In sandy-loamy and loamy soils, the higher P availability is attributed to weaker sorption capacities. On ave- rage, P av represented 9% of P tot. The statistical analysis of land use effect on P content should integer the influence of parent materials. Other- wise, conclusions might differ as can be seen in Fig. 5 and Fig. 6. The fate of P in the soil is under the dependence of soil characteristics and agronomic practices. Parent materials have an influence on both surface and deep horizons, explained by Fe and Al contents mainly. In surface, land use also influences P content and availability. So, the management of phosphorus resources in cultivated soils has to take into account the sub-regional specificities of soil parent materials and land use. Indeed, taking into account parent material modifies conclusion concerning difference of P content according to land use. Finally, comparison between surface and deep P content should allow to distinguish between P management and geochemical influences. Fig. 1: Location of the parent materials in Walloon Region. Fig. 2: Particle size distribution. Fig. 3: Difference of total P content according to parent materials. Fig. 4: P availability according to parent materials. Fig. 5: Difference of total, inorganic and organic P according to land use without consideration of parent materials. Fig. 6: Differences of P tot, P inorg and P org means between land use modalities and crops. ANOVA. The difference between surface and deep P content can be attributed to effects of biogeochemical cycle and fertilization inputs. To determine the geochemical effect, regression models for P content in deep horizon can be applied to surface. The difference or the ratio between observed and predicted P contents permits to estimate the importance of fertilization and other inputs. ab a b a a The influence of parent materials can be seen in deeper horizon but also in surface horizon. Surface P content is also influenced by land use. Fardeau JC, Conesa AP (1994). Le phosphore In ‘Pédologie Tome 2. Constituants et propriétés du sol’. (Eds B Bonneau, M Souchier) pp (Masson publishing: Paris). Lakanen E, Erviö R (1971). A comparison of eight extractants for the determination of plant available micronutrients in soils. Acta Agralia Fennica 123, Significant relationships have been found between total P and total Fe and Al contents, which are indicators of the quantity of P-adsorption sites. But, on contrary of other studies, no relationship has been observed with clay content or Ca. Multiple linear regressions can be usefull to predict soil P content from more easily measurable or more available soil properties. b b bc a cd d d d d d d d ab b b b a b a a P tot = Fe tot clay TOC (R² = 64.6%) P inorg = P tot pH water CEC (R² = 92.6%) P org = P tot clay pH water (R² = 80.8%) b a a a ab b b a a