Meteorological and management influences on seasonal variation in phosphorus fractions extracted from soils in western Ireland David Styles ⁎ , Catherine Coxon School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland Received 3 May 2007; received in revised form 31 July 2007; accepted 19 August 2007 Available online 21 September 2007 Abstract Soils from ten grassland sites and one forest site in western Ireland were sampled monthly for 12–17 months. Moist, composite samples were analysed for labile phosphorus (P) fractions extractable with the Olsen’s reagent and water (P w ). Molybdate analysis before and after extractant digestion was used to define inorganic and organic P fractions: Olsen P and Olsen molydate unreactive P (MUP); P w dissolved reactive P (DRP) and dissolved unreactive P (DUP). All-soil P fractions exhibited significant covariance with a seasonal sine-curve parameter in ANCOVA, and this seasonal component of variation was statistically similar among soils for all parameters except Olsen P. Olsen P fractions followed a distinct seasonal trend of maximum winter and minimum summer concentrations. This was more pronounced for the Olsen MUP fraction, which displayed strong positive covariation with soil total N content, moisture content, and preceding effective precipitation ( p = b 0.0001, 0.0027 and b 0.0001 respectively), and negative covariation with preceding temperature ( p = 0.0011) — consistent with expected patterns of biological cycling. Seasonal variation in both P w fractions followed an inverse trend to Olsen P variation, resulting in differing summer and winter Olsen P– P w relationships. P w DRP exhibited negative covariation with effective precipitation in the week, but not month, prior to sampling and with pH ( p = 0.005 and 0.003 respectively), indicating a possible wash-out effect and physico-chemical control. P w DUP exhibited strong positive covariation with temperature during the preceding month ( p = 0.0004), indicating biochemical control, and was significantly and positively associated with grazing occurrence ( p b 0.0001). Spikes in soil Olsen P and P w fractions were attributed to an intensive grazing event during spring, and manure application during winter, but there was no peak in soil P following summer fertiliser application. It was concluded that less labile and organic P fractions buffer soluble and inorganic P fractions throughout the year. © 2007 Elsevier B.V. All rights reserved. Keywords: Soluble phosphorus; Olsen phosphorus; Seasonal variation; Meteorology; Grazing 1. Introduction 1.1. Extractable soil phosphorus fractions Published literature has been somewhat limited regarding quantification of seasonal variation in soil phosphorus (P) characteristics, largely due to high field-scale soil-sample vari- ability and the confounding effect of standard soil drying preparation prior to laboratory analyses (Pote et al., 1999). Di- verging or conflicting seasonal patterns have been observed among different studies for soil test P (STP), especially Olsen P (e.g. Perrott et al., 1990; Tate et al., 1991a; Magid and Nielsen, 1992; McDowell and Trudgill, 2000), and soil soluble P (e.g. Kuo and Jellum, 1987; Sharpley et al., 1995; Pote et al., 1999; Chen et al., 2003). Differences may be attributable to varying climates, soil types and land uses, but also the oft-used pre-analytical procedure of soil drying. Soil drying solubilises organic and microbial P that is otherwise insoluble under field-moist con- ditions (Turner and Haygarth, 2003; Turner et al., 2003; Styles and Coxon, 2006; Styles et al., 2006). Soil test P, predominantly Olsen P in Europe, is often used as an indicator of soluble P loss risk to freshwater in environmental analyses, based on ratios to soluble P (Daly et al., 2002; Styles et al., 2006), or ‘change-points’ (thresholds above which soluble P increases more rapidly: Heckrath et al., 1995; Meissner et al., 1998; Hesketh and Brookes, 2000; McDowell and Sharpley, 2001). Quantification of seasonal and management-related variation in STP and P Geoderma 142 (2007) 152 – 164 www.elsevier.com/locate/geoderma ⁎ Corresponding author. Tel.: +353 1 896 3068; fax: +353 1 896 1147. E-mail address: dstyles@tcd.ie (D. Styles). 0016-7061/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2007.08.008