Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma Soil apatite loss rate across different parent materials Ayaz Mehmood a , Mohammad Saleem Akhtar b,⁎ , Muhammad Imran b , Shah Rukh b a Department of Agricultural Sciences, University of Haripur, Pakistan b Department of Soil Science, PMAS-Arid Agriculture University Rawalpindi, Pakistan ARTICLE INFO Editor: M. Vepraskas Keywords: Pedogenic iron oxides Phosphorus fractions Iron adsorbed and occluded-P Fe-P 50 Apatite exponential decay Soil weathering ABSTRACT Apatite is a lithogenic soil mineral and the primary source of phosphorus (P) that limits crop production worldwide. Apatite differs with sediment geology, and soil processes redistribute the P forms. Whether soil apatite loss with soil formation differs in sediment needs further research. The objectives of this study were to (i) determine the relationship between soil genesis and the distribution of P fractions in soils formed in different parent materials, and (ii) determine the extent to which apatite loss rate with weathering differs due to parent material. Triplicate profiles for three soils representing sequences of development in loess, alluvium, shale re- siduum, and sandstone residuum were analyzed for various soil P forms and related parameters at genetic horizon levels. The labile P fractions, secondary phosphates of iron and aluminum, and apatite-P varied with parent materials and soil weathering. Apatite-P decreased exponentially with the sum of iron adsorbed and occluded P in soil, and fit the equation M (Y) =M o [1-exp.(-λY)] where M o is initial apatite-P in the sediment, M (Y) is the current apatite-P content in soil, Y is the cumulative iron sorbed P, and -λ, an empirical decrease-rate constant. The apatite loss model fits well for the all soils, except the shale soils that contained lithogenic iron oxides. At the Alfisols development stage, apatite-P loss was 48% in loess, 72% in sandstone and 93% in alluvium out of 1455, 675 and 945 kg ha -1 per 0.9 m that arrived with the sediments, respectively. Fe-P 50 , the level of iron sorbed P content in soil, was 55.7 for loess, 46.8 for sandstone, and 20.4 kg ha -1 per 0.9 m for alluvium, suggest the highest stability of apatite occurred in loess. The model provides a measure to compare kinetics of soil apatite dissolution under soil genesis independent of time in term of weathering. 1. Introduction Phosphorus (P) is an essential plant-nutrient that is deficient in its bioavailable form in soils worldwide. Apatite, Ca 10 (PO 4 ) 6 (F, OH), is the primary source of P in terrestrial ecosystems and a ubiquitous mineral that weathers under biogeochemical processes in sediments and soil, dissolving to release P as a secondary precipitate with varying levels of lability (Yang, Post, Thornton, & Jain, 2013). The present day dis- tribution patterns of P forms within soil profiles are thought to be as- sociated primarily with pedogenic processes. At early stages of devel- opment, the soils differing in parent materials exhibit distribution of P forms closely related to the lithology (Xiao, Anderson, & Bettany, 1991). Formation of secondary phosphates in soil and sediments at the expense of apatite has been modeled (Walker & Syers, 1976). Since rainfall controls apatite dissolution rate mainly by its effect on soil biota and chemical kinetics (Yang & Ding, 2001), the relief of landscape be- comes an important factor for P transformations in subhumid and semiarid areas (Akhtar et al., 2014). Whether soil apatite loss rate differs with lithology under varying landscape relief needs further investigation. Apatite decrease with soil development has been studied on chronological scales i.e. the same parent material deposited at different times in the past (Shah, 1966; Tan, 1971; Eger, Almond, & Condron, 2011 and others). The Walker and Syers model was adopted extensively to fit P changes during pedogenesis on chronological scales in humid ecosystems (Walker & Syers, 1976; Crews et al., 1995; Wardle, Walker, & Bardgett, 2004). Apatite weathering studied under extremely slow soil development in a desert chronosequence lead to the conclu- sion that the Walker and Syers model may not fit under aridic soil moisture regime (Lajtha & Schlesinger, 1988). Later, Selmants and Hart (2010) argued that the chronosequence investigated by Lajtha and Schlesinger (1988) had minimal soil development due to the short time span. Selmants and Hart (2010) concluded that though the reduced water input slowed apatite breakdown, the depletion in apatite and formation of secondary phosphates with age will fit the Walker and Syers model. The soil properties acquired as a function of relief and time are equally valid (Jenny, 1941). Apatite depletion in several toposequences http://dx.doi.org/10.1016/j.geoderma.2017.09.036 Received 5 July 2017; Received in revised form 13 September 2017; Accepted 23 September 2017 ⁎ Corresponding author. E-mail addresses: ayaz.gill@uoh.edu.pk (A. Mehmood), msakhtar@uaar.edu.pk (M.S. Akhtar). Geoderma 310 (2018) 218–229 Available online 30 September 2017 0016-7061/ © 2017 Elsevier B.V. All rights reserved. MARK