Effects of parent material and land use on soil phosphorus forms in Southern Belgium

1. a a a ab ab ab ab ab ab ab ab b a b b bc cd d d d d d d d Effects of parent material and land use on soil phosphorus forms in Southern Belgium Renneson1 M., Dufey2 J., Bock1 L. and Colinet1 G. 1 University of Liege (Belgium) - Gembloux Agro-Bio Tech – Soil Science Unit. Passage des Déportés, 2. B-5030 Gembloux - Malorie.Renneson@ulg.ac.be 2 University of Louvain-la-Neuve (Belgium) – Soil Sciences Unit. INTRODUCTION AND OBJECTIVES 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. MATERIALS AND METHODS 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 100-120 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 : pHwater and pHKCl, total organic carbon (TOC), cation exchange capacity (CEC) and particle size distribution. Available phosphorus (Pav) (Lakanen and Erviö, 1971) and total phosphorus (Ptot) (NF X 31-147, 1996) were determined. Inorganic phosphorus (Pinorg) was extracted by H2SO4 (1:40, w:v) while organic phosphorus (Porg) was determined by difference of Ptot by Pinorg. Fig. 1: Location of the parent materials in Walloon Region. 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. There is a large range of variation for studied soil properties On average, deep soil P amounts to half of surface P. The ratio Porg/Pinorg is relatively variable (from 7.9 to 74%), with a mean of 30% close to values from Conesa & Fardeau (1994). Influence of land use and parental material in surface horizon Fig. 2: Particle size distribution. Influence of parent material on deeper horizon 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, Pav represented 9% of Ptot. 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 andFig. 6. a a b Statistical analyses have shown differences between parent materials for Ptot, Pinorg and Porg contents, Pinorg percentage but also for edaphic properties. But, despite the diversity of geological contexts , the Ptot differences are relatively limited (Fig. 3), excepted for sandy loamy soils which presented low Ptot and Porg contents. a ab b a a b Fig. 5: Difference of total, inorganic and organic P according to land use without consideration of parent materials. 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. a a a ab ab b b b Fig. 4: P availability according to parent materials. Fig. 3: Difference of total P content according to parent materials. b Fig. 6: Differences of Ptot, Pinorg and Porg means between land use modalities and crops. ANOVA. 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. Ptot = 453 + 25 Fetot - 20.7 clay + 124 TOC (R² = 64.6%) Pinorg = -174 + 0.657 Ptot +40.6 pHwater -5.68 CEC (R² = 92.6%) Porg = 250 + 0.379 Ptot +2.28 clay - 48.9 pHwater (R² = 80.8%) CONCLUSIONS 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. Fardeau JC, Conesa AP (1994). Le phosphore In ‘Pédologie Tome 2. Constituants et propriétés du sol’. (Eds B Bonneau, M Souchier) pp. 649-658. (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, 223-232. This research has been financed by the « Fonds pour la formation à la Recherche dans l’Industrie et dans l’Agriculture » (FRIA) 19th World Congress of Soil Science – Brisbane – 1-6 August 2010 Soil solutions for a changing world