Study of the relevance of phosphorus saturation in Walloon Region (Southern Belgium)

1. Study of the relevance of phosphorus saturation in Walloon Region (Southern Belgium) Renneson M., Vandenberghe C., Marcoen JM., Bock L. and Colinet G. University of Liege (Belgium) - Gembloux Agro-Bio Tech – Soil Science Unit. Passage des Déportés, 2. B-5030 Gembloux - Malorie.Renneson@ulg.ac.be INTRODUCTION AND OBJECTIVES MATERIALS AND METHODS Agriculture is partly responsible for the eutrophication of surface waters due to overfertilization and phosphorus losses. Assessment of the adequacy of agricultural practices to the sensibility of the physical environment should rely on indicators which can be easily determined. The degree of phosphorus saturation (DPS) in soils is often proposed and is already used in some countries for environmental purposes. The adequacy of this indicator to the Walloon situation had to be assessed before large-scale survey. This study aims to verify the dependency of DPS to soil type and agricultural soil fertility management. Six reference farms were selected to represent the range of environmental conditions in terms of geology and soil fertility management from the Belgian Loamy and Sandy-loamy regions and the Condroz (three important crop production regions) (Fig. 1). These farms are included in a network of reference farms which are supervised for nitrate management. Soil samples were analyzed for oxalate extractable iron, aluminium and phosphorus content (Feox, Alox and Pox). Available phosphorus (Pav) was determined by lakanen-Erviö method (CH3COO-NH4 + EDTA, pH 4.65). Total organic carbon, CEC, total N and clay content were predicted by near-infrared spectroscopy Fig. 1: Location of the studied farms in Wallonia a DETERMINATION INFLUENCE OF SOIL PROPERTIES ON SORPTION CAPACITY The Degree of Phosphorus Saturation (DPS) is calculated by the following equation: DPS = 100 * Pox / a (Feox + Alox) (%) A prior assessment of a scale factor was necessary to calibrate the a value to the regional natural environment, even if a 0.5 value is largely used. This parameter has been determined by the standardized one-point short-term isotherm method (Bache & Williams, 1971). Our results varied from 0.43 to 0.93, with a mean value of 0.66 and a standard deviation of 0.13. These values are close to values from other studies. Clayey soils showed a higher a (0.88 ± 0.06) than the other soils. Dominant soil order is Luvisol, but some Fluvisols, Regosols and Leptosols were also observed. Compared to regional values for cultivated soils, the mean Pav (12.8 mg P/100g) appeared high. The phosphorus sorption capacity (PSC =a[Feox + Alox]) ranged from 64 to 144 mmol.kg-1 and depended on soil texture. It was higher in clayey or silty soils than in sandy soils. These latter soils are therefore more vulnerable to soil losses. Drainage had also an influence on DPS, PSC and Pox, which were smaller in poor drainage situations. High PSC values explained the low DPS values observed in some parcels, even if P content was high. Indeed, in these soils, P fixation capacity is important and P is well linked. Various parameters were correlated with DPS, like pHwater (-0.478***), CEC (-0.514***) or clay content (-0.644***). The use of multiple regression with these edaphic factors didn’t really improve the evaluation of DPS or other associated parameters. So, simple regressions can be used. AGRICULTURAL DEGREE OF PHOSPHORUS SATURATION • DPS values ranged from 12.9 to 63.8%, with a mean of 32% (Fig. 2). So, 77% of our samples are above the Dutch environmental threshold of 25% proposed by Breeuwsma et al. (1995) and 20% should be classified as saturated according to Flemish legislation (40%, Chardon & Schoumans, 2007). However, until now, no study has been made in Walloon Region to rely DPS with measurements of P transfers from soil to surface waters. • DPS were compared to following parameters to verify the dependency of DPS to agricultural soil fertility management: • A 5 years P balance for each parcel: no clear relationship could be found with DPS. • Pav values: a significant relationship (0.595***) between DPS and Pav has shown that DPS was relevant to illustrate soil • fertility and growth potential for crops. This relationship can be improved when the correlation analysis take the • texture into account. • Fertility classes etablished according to Pav and pH thresholds used in Walloon Region: a good relationship (-0.689***) • between DPS and fertility classes showed that the P saturation gives similar agronomic advice than it is made • routinely (Fig. 3). a a a a a a a ab b ab b b Fig. 2: Means and standard deviations of oxalate phosphorus (orange) and degree of phosphorus saturation (green) per farm. The ratio Pav/Poxevaluates the fraction of sorbed P which is more susceptible to desorb and migrate to plants or in the soil. The higher the PSC, the smallest the proportion of available P to sorbed P for the plant growth but also for the environment. This suggests that the binding strength depends upon the number of sites and that availability of P is improved with soil saturation. The figure below also allows one to distinguish intensive pig farming from other cultural systems (Fig. 4). Fig. 3: Boxplot of the degree of phosphorus saturation according to fertility classes. Fig. 4: Relationship between Pav/Pox andphosphorus sorption capacity according to cultural system. CONCLUSIONS Used in many countries, the Degree of Phosphorus Saturation had to be adapted to be useful in Southern Belgium. An a value of 0.66 appears more relevant to Walloon situation than 0.5 used in many studies. DPS is an environmental indicator which is hardly interpretable because it depends both on soil properties (mainly texture, drainage and pH) and P fertilization management. Although DPS is well correlated with Pav, it is not very agronomically transparent. Indeed, to explain DPS behaviour, it has to be divided into Pox and PSC. So, the use of DPS in Walloon Region is possible but requires other studies to understand better the DPS fate and establishe agronomical and environmental thresholds, eventually according to regions. Bache BW, Williams EG (1971). A phosphate sorption index for soils. Journal of Soil Science22, 289-301. Breeuwsma A, Reijerink JGA, Schoumans OF (1995). Impact of manure on accumulation and leaching of phosphate in areas of intensive livestock farming. p. 239-251. In ‘Animal waste and the land water interface’. (Ed. Steele K) pp. 239-251. (Lewis Publishing: New York). Chardon WJ, Schoumans OF (2007). Soil texture effects on the transport of phosphorus from agricultural land in river deltas of Northern Belgium, The Netherlands and North-West Germany. Soil Use and Management23 (Supplement 1), 16-24. 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