Aging effects on phosphorus transformation rate and fractionation in some calcareous soils M. Jalali ⁎, F. Ranjbar Department of Soil Science, College of Agriculture, Bu-Ali Sina University, Hamadan, Iran abstract article info Article history: Received 3 March 2009 Received in revised form 19 November 2009 Accepted 27 November 2009 Available online 4 January 2010 Keywords: Phosphorus Calcareous soil Kinetics Fractionation P transformation rate Rate of phosphorus (P) transformation in soils can significantly influence P fertility of soils. The transfor- mation rate and fractionations of P in 20 calcareous soils of varying properties were investigated. Phosphorus was added to the samples at rates of 200 mg kg -1 soil. The samples were incubated for 3, 24, 168, 336, 504, 720, 1440, and 2160 h at 25 °C and Olsen-P was determined after each incubation period. The P release in the studied soils was initially rapid followed by a slower release that lasted up to 2160 h. The transformation rate of Olsen-P for soils was estimated by best fitted kinetic equation (parabolic) for above incubation periods. There were differences in the rates at which redistribution took place between soils and P. Phosphorus in control and amended soils were fractionated before and after 2160 h incubation by sequential extraction procedure, in which the P fractions were experimentally defined as exchangeable (KCl–P), Fe- and Al-bound (NaOH–P), Ca-bound (HCl–P), and residual P (Res-P) fractions. The results showed a sharp decrease in Olsen-P within 3 h after P addition. Relative NaOH–P in amended soils ranged from 6.6 to 13.5%. Relative HCl–P, Res-P and KCl–P ranged from 49.9 to 71.2, 13.5 to 26.7, and 7.4 to 13.3%, respectively. There were changes in the proportional distribution of P in all the soils during 2160 h of incubation with amended P. In general the proportions of P associated with the most weakly bound fraction (KCl–P) tended to decrease, with corresponding increases in the NaOH–P and HCl–P fractions during the incubation. The principal component analysis showed that the first four components explained 77.1% of the overall variation. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Phosphorus (P) is one of the major nutrients limiting crop production. One method to raise soil available P to the critical crop level is by addition of P fertilizer. However, continued long-term application of fertilizers can lead to P accumulation in surface horizons greater than that required for optimum plant growth, thus increasing the potential for P loss to surface waters and eutrophication (McDowell et al., 2001; Sui et al., 1999). Soil P reserves make an important contribution to plant P supply. In intensively cultivated and fertilized soils, P availability has increased, as measured by soil test methods (Sui et al., 1999; Jalali, 2007). Some agricultural soils can now be classified as overfertilized due to a steady increase in available P resulting from application of fertilizer P in the past (Sharpley and Smith, 1989). Many soils in Iran have received large amounts of P fertilizer and consequently contained high level of available P (Jalali, 2007). The release of organically and inorganically bound nutrients like P, which can then be utilized by plants, is particularly important in agriculture. For optimal nutrition of crop, the replenishment of a P- depleted soil solution is affected predominately by the release of P from clay minerals and organic matter. Hughes et al. (2000) indicated that the release rate of adsorbed P directly affects the P supply to plants. In addition, the release of P may have a significant impact on the surface and ground water quality. Thus, knowledge of the rate of release of P from soils is important for long-term planning of crop production while minimizing the impact on the surface and groundwater quality. Phosphorus in soil is considered to be distributed among several geochemical forms that include soil solution and exchangeable phase, organic matter phase, Ca-bound phase, and Fe-Al- bound phase and residual phase (Hedley et al., 1982a,b). The degree of P association with different geochemical forms strongly depends upon physico-chemical properties of different soil types (Tiessen et al., 1984), climate, and management practices (Motavalli and Miles, 2002). Various P fractions have remarkable differences in mobility, bioavailability and chemical behaviors in soil and can be transformed under certain conditions (Sharpley et al., 2000). Sequential extraction technique is used to define them in soils both as qualitatively as well as quantitatively. Such information is potential useful for predicting bioavailability, P leachability and transformations of P between chemical forms in agricultural and polluted soils (Hedley et al., 1982a,b; Sui et al., 1999). In contrast to analysis of total P, redistribution among various fractions is important for P mobility. The Ca-bound, Fe–Al oxide bound, and organic matter bound fractions could be considered relatively active depending on the actual physical and chemical properties of soil. Geoderma 155 (2010) 101–106 ⁎ Corresponding author. Tel.: +98 811 4224090; fax: +98 811 4224012. E-mail address: Jalali@basu.ac.ir (M. Jalali). 0016-7061/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2009.11.030 Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma