Soil apatite loss rate across different parent materials

Geoderma 310 (2018) 218–229

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Soil apatite loss rate across different parent materials a

Ayaz Mehmood , Mohammad Saleem Akhtar a b

b,⁎

b

, Muhammad Imran , Shah Rukh

MARK b

Department of Agricultural Sciences, University of Haripur, Pakistan Department of Soil Science, PMAS-Arid Agriculture University Rawalpindi, Pakistan

A R T I C L E I N F O

A B S T R A C T

Editor: M. Vepraskas

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 residuum, 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) = Mo[1-exp.(− λY)] where Mo 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-P50, 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.

Keywords: Pedogenic iron oxides Phosphorus fractions Iron adsorbed and occluded-P Fe-P50 Apatite exponential decay Soil weathering

1. Introduction Phosphorus (P) is an essential plant-nutrient that is deficient in its bioavailable form in soils worldwide. Apatite, Ca10(PO4)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 distribution patterns of P forms within soil profiles are thought to be associated primarily with pedogenic processes. At early stages of development, 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 becomes 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 conclusion 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

Corresponding author. E-mail addresses: [email protected] (A. Mehmood), [email protected] (M.S. Akhtar).

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 Available online 30 September 2017 0016-7061/ © 2017 Elsevier B.V. All rights reserved.

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has been reported, and found that in humid climates where excessive precipitation masks the effect of landscape relief, apatite depletion occurs at the same rate regardless on steep slopes or on level plains (Adams & Walker, 1975). Relief may however be a prominent factor in subhumid and semiarid climates where the distribution of rainfall intensity shifts the udic-ustic boundary (Cochran, 2010). The wide plains and depression areas in level plains have greater soil moisture than the shoulder and backslopes under the same rainfall regimes. In loess plain, distribution of soil apatite and the change of secondary phosphates were different on the wide and flat land surfaces than on the slopes (Akhtar et al., 2014). In the loess plain, soil development occurred to almost similarly depth but deep Bt was found on the wide flat land and deep Bk/BCk on slopes, suggesting nominal difference in residence time and a dominant effect of relief. Apatite and secondary phosphate content also varied with relief along a toposequence in Brazilian loess in a semiarid climate (Araujo, Schaefer, & Sampaio, 2004). Phosphorus transformation models for time-dependent soil development have been presented (Walker & Syers, 1976; Crews et al., 1995; Eger et al., 2011; Selmants & Hart, 2010; Yang et al., 2013; Turner & Laliberté, 2015). The Walker and Syers (1976) model conceptualizes apatite to be the dominant P form in young soils. With advanced weathering, secondary phosphates of iron and aluminum become dominant, and organic P increases early in forming soils, and then stabilizes later. In the model, an exponential decay of apatite is implicit, suggesting that rate of apatite mass change is proportional to the mass per unit volume in the pedon (Porder, Vitousek, Chadwick, Chamberlain, & Hilley, 2007; Boyle, Chiverrell, Norton, & Plater, 2013). Eger et al. (2011) similarly observed that apatite depletion best fits to a negative logarithmic function. These models assume soil development as a function of time, and the other soil forming factors have largely been ignored. Iron release from the weathering of primary minerals, precipitation as hydroxides and oxyhydroxides, and the hydrolytic breakdown of apatite are concurrent processes under soil formation (Churchman & Lowe, 2010). These soil processes occur irrespective of the nature of soil forming factor. The linear reservoir concept may be adopted for modeling average mass of apatite remaining in a soil from the fixed quantity laid with the sediments. Apatite decreases exponentially a function of soil development that can be represented by a measureable soil property. Phosphorus adsorbed and occluded in the pedogenically produced iron oxides (a measureable pedogenic property) as an independent variable relates to the current apatite-P content through Eq. 1 (Akhtar et al., 2014).

1979), and replace Y1/2 with the term Fe-P50. Fe-P50 is defined as the unit iron oxides sorbed P formed during pedogensis (MM− 1or ML− 3) when soil apatite depletes by 50% of what was in the parent material. Fe-P50 (or Y1/2) can be calculated from the following equation (Thomas & Finney, 1995):

M(Y) = Mo e−λY

Soils at three distinct stages of development were taken from four parent materials occurring in the Pothwar Plateau (Pakistan). The alluvium, loess and sandstone derived soils developed under semiarid to sub-humid climate where rainfall vary from 500 to 750 mm per annum. The shale derived soil developed under humid climate having rainfall more than 1000 mm per annum. The soil profiles sections with genetic horizons are given in Fig. 1 with USDA soil classification (great group). In the loess of Pleistocene period, Typic Haplustalfs (Chakwal soil series) occurs on level plains, Udic Haplustepts (Rawalpindi series) on gently sloping areas, and Typic Ustorthents (Rajar soil series) on steep gullied land. Typic Haplustalfs are deeply decalcified, Udic Haplustepts are decalcified to 60 cm depth, and Typic Ustorthents are least developed with no cambic B horizon. Alluvium occurs as narrow strips along the water courses in the area that are tributaries of the Indus River, and the soil development in the alluvial sediments relates to the age of deposition. Typic Haplustalfs (Gujranwala soil series) occurs in Pleistocene period alluvium, Fluventic Haplustepts (Argan soil series) in the Holocene period and Typic Ustifluents (Shahdra soil series) in Recent and active floodplain. Typic Haplustalfs are deeply decalcified, Fluventic Haplustepts are calcareous and Typic Ustifluents are the least developed with no structural cambic B horizon. In the sandstone of Siwalik group of formations of Pliocene period (de Terra & de Chardin,

Fe − P50 (Y1 2) = −

ln 2 λ

(2)

Phosphorus fractionation schemes have been adopted to quantify apatite and the secondary precipitates in soils when measurement using X-ray diffraction is not as effective. Specific for the calcareous soils, Jiang and Gu (1989) fractionation scheme partitions soil P into the inorganic P equivalents, including dicalcium phosphate and octacalcium phosphate, phosphate adsorbed on iron oxides, phosphates occluded in the crystalline iron oxide, aluminum phosphate adsorbed on aluminum hydroxides surfaces, and P occurring as apatite in soil. This scheme with modifications has been used successfully (Samadi & Gilkes, 1998; Shen et al., 2004; Memon, Akhtar, Memon, & Stuben, 2011 and several others). The soil P fractionation helps predict apatite-P transformations and P bioavailability. The fractionation scheme of Hedley, Stewart, and Chauhan (1982) has been fundamental for understanding P dynamics in non-calcareous soils (Cross & Schlesinger, 1995; Yang & Post, 2011; da Silva et al., 2017; Margenot et al., 2017 and several other). Dominant soil parent materials in Pothwar plateau (Pakistan) is the loess deposited on sandstone and shale whose outcrops provide site for residual soils; and the alluvium occurs as narrow terraces along the courses of Indus River tributaries (Mian & Syal, 1985). Since mineralogical composition, including apatite, varies with soil parent materials, the soils sequences (toposequences in case of loess, shale and sandstone residuum and the chronosequence in case of alluvium) provide an opportunity to further the work of Akhtar et al. (2014) where a toposequence in loess was investigated. We hypothesize that apatite stability changes across the diverse parent materials, and its comparison can be made under various weathering factors without reference to the age of sediments. The objectives of this study were to determine the quantitative distribution of soil apatite and secondary phosphates, the extent of apatite loss at various stages of development, and to compare the apatite stability during soil development on each parent material. 2. Materials and methods 2.1. Site and soil description

(1)

where Mo is initial apatite-P content in the parent material (MM− 1) at the time of deposition, M(Y), the current apatite-P content (MM− 1), Y, the cumulative P adsorbed and occluded on iron oxides (MM− 1), and -λ, the decay rate constant. The assumption of the model is that iron hydroxides and oxyhydroxides are formed under iron release from the weathering of primary minerals. The cumulative iron oxides can be estimated by selecting soils at progressively greater stages of development in a toposequence or a chronosequence of constant lithology (Smeck, Torrent, & Barron, 1994). The assumption on iron hydroxides and oxyhydroxides to have been formed by pedogenesis also precludes the gleyed soils developed under submerged conditions. Oxidation of Fe2 + released from primary minerals and formation of new sparingly soluble Fe(III) oxides occur only under aerobic conditions. Addition of phosphorous fertilizer changes dicalcium and octacalcium phosphates that are not included in the Eq. 1. Further, the exponential decay functions against time help calculate half-life independent of initial concentration (Lottermoser, 2010 for radioactive elements; Gibson & Burns, 1977 for pesticides in soil). Since the term half-life implies the change specifically on a time scale, we adopt the concept of “Enzyme Unit” used in biochemistry (NC-IUB, 219

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Fig. 1. Profiles photographs of selected parent material soils indicating genetic horizons and USDA soil classification (great group).

dichromate (K2Cr2O7) (Nelson & Sommers, 1996). Extractable organic carbon (OCext) was determined by consumption of K2Cr2O7 (Nelson & Sommers, 1982) after extraction with potassium sulfate (K2SO4). Soil iron oxides were dissolved by sodium dithionite in sodium citrate and sodium bicarbonate buffer solution, and iron and aluminum in the extract were measured by atomic absorption spectroscopy (Mehra & Jackson, 1960). Separately, poorly crystalline soil iron oxides were determined by extraction in acidified ammonium oxalate solution (Jackson, Lim, & Zelazny, 1986).

1936), Udic Haplustalfs (Kahuta soil series) is formed on gently sloping and level areas, Typic Haplustalfs (Balkasar soil series) in low lying parts of the gently sloping and level areas and Lithic Ustipsamments (Qazian soil series) occurs in the foothills. Udic Haplustalfs are deeply decalcified with an argillic B horizon; Typic Haplustalfs are decalcified up to 70 cm depth with an argillic B horizon and Lithic Ustipsamments have no cambic B horizon. In Murree formation of Miocene period that comprises of shale-sandstone interbedding (Bossart & Ottiger, 1989), Typic Hapludolls (Murree soil series) occurs on the mountain tops, Typic Haplustepts (Tirnul soil series) at the bottom of troughs within the mountain ridges and Typic Udorthents (Ghoragali soil series) on the mountain slopes. Typic Hapludolls were decalcified and had high organic matter content at the surface under high rainfall and coniferous forest, the Tirnul soils were moderately deep with a cambic B horizon, and Typic Udorthents had no B horizon.

2.3. Sequential extraction of soil phosphorus The Jiang and Gu (1989) fractionation scheme was used for P fractionation. Briefly, 0.25 N NaHCO3 (pH 7.5) was used to extract di‑calcium phosphate, CaH2PO4, referred as Ca2-P, the 0.5 N NH4acetate (pH 4.2) treatment extracts octa‑calcium phosphate, Ca4H2(PO4)3·2.5H2O, referred as Ca8-P, the 0.5 N ammonium fluoride (pH 8.2) treatment desorbs phosphate from aluminum hydroxides surfaces (Al-P), the sodium hydroxides‑sodium carbonate solution mixture desorbed phosphate from surfaces of the iron oxides (Fe-P), and the sodium dithionite and sodium citrate, buffered with sodium bicarbonate, dissolve iron oxides and release the P occluded in crystalline iron oxides. The residue from the last step was treated with 0.5 M sulfuric acid, which dissolves apatite. Separately, total P was determined by digestion in perchloric acid (Olsen & Sommers, 1982). The difference in the sum of inorganic P and total P gives an estimate of the organic P fraction.

2.2. Soil Sampling and Characterization Three replicate profiles for each soil were dug at separate locations. Soil samples were taken at genetic horizon level after profile description (FAO, 1990). Soil bulk density (ρb) was measured by taking 5 cm high with 5 cm inner diameter core from the center of each horizon. The soil contained in a fixed volume of core was oven dried at 105 °C until constant weight (Blake & Hartge, 1986). Soil samples were air dried, crushed to pass through a 2-mm sieve, and analyzed for texture, pH, CaCO3, sodium dithionite extractable iron and aluminum, and total and dissolved organic carbon. The soil particle size distribution was determined by dispersion in sodium hexametaphosphate, and pH was of saturated soil paste (Mclean, 1982). Soil CaCO3 was determined by acetic acid consumption (Loeppert, Hallmark, & Koshy, 1984). Total organic carbon (TOC) was determined by wet digestion in potassium

2.4. Statistical analysis The variance in the P fractions at horizons level was analyzed using 220

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the multivariate analysis of variance (MANOVA) in GLM procedure of SAS version 9.3 (SAS Institute Inc., 2003). Class variables were ‘parent material’ and ‘soil’ nested within the parent materials while the measurements at various horizons were multiple dependent variables. The parent material means over the soils were compared using least significant difference (LSD).

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3.1. Soil characteristics The soils represented early stages of development with weak to moderate horizon differentiation. Most soils were dark brown to yellowish brown with neutral to slightly basic pH, except for the reddishbrown shale derived soils. The Entisols were calcareous without a distinct zone of lime accumulation while the Haplustalfs/Hapludolls were decalcified to a variable depth, and had a distinct zone of lime accumulation. The loess derived soils were silty clay loam to silt loam in texture, and the alluvial soils were sandy loam to silty clay loam and exhibited the depositional breaks. The sandstone derived soils were loamy sand to sandy clay loam, except for a few clay loam horizons caused by shale interbedding. Similarly, the shale derived soils were mostly silty clay where sandstone interbedding resulted in coarse texture in a few horizons. Lithological continuity as determined by a change in the sand/silt ratio (Smeck & Wilding, 1980) is an essential assumption to determine in situ soil minerals weathering. Lithological breaks (discontinuities) were frequent in the alluvial soils. The loess and sandstone derived profiles were continuous in lithology, except that one profile each from Chakwal and Rawalpindi soil series had lithological discontinuity at the very last horizon level associated with loess deposition on Siwalik sandstone or on interbedded shale-sandstone. The shale-derived soils had high extractable iron, and the crystal phase's hematite and ferrihydrite were observable under a transmission electron microscope. The extractable iron content increased with soil development in alluvium, loess and sandstone derived soils, and was highest at Bt horizon ascribed to translocation under pedogenesis (data not presented). Similarly, oxalate extractable iron was high in shale derived soils and increased with development in all the parent materials with largest accumulation in the Bt horizons. With improbable occurrence of crystalline aluminum hydroxides at the pH range, high extractable aluminum found mostly in the surface soils of the Alfisols and Mollisols was ascribed to aluminum associated with exposed edges of organic matter (Larssen et al., 1999).

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Total phosphorus (mg kg ) Fig. 2. Total phosphorus distribution with depth in the parent materials which decreased toward the surface (except A/Ap) in loess, shale, and sandstone. The dotted lines show means for the respective parent material.

(n ≈ 9) for the parent materials, varied significantly as the hypothesis of no depth × parent-material interaction was rejected through the test statistics Wilks' Lambda from the MANOVA (p ≥ 0.002) (Fig. 2). The pattern of total P distribution with depth for the soils representing the genetic stage was consistent across the parent materials (Fig. 3). Total P decreased toward the surface (except Ap/A horizon) in all relatively higher development level soils whereas the soils of early stage development remained unchanged with soil depth in all parent materials except alluvium. 3.3. Distribution of soil phosphorus fractions 3.3.1. Organic phosphorus Organic P, determined by subtracting the sum of inorganic P fractions from total P, ranged from 25 to 240 mg kg− 1, and the mean differed significantly with the parent materials (p ≥ 0.0001) given in Table 1. The alluvial soils had greater mean organic P content than the shale and loess soils. The sandstone derived soils contained the lowest organic P content. Organic P increased toward the surface in the shale and alluvium (Fig. 4). These soils were under mixed coniferous forest

3.2. Total phosphorus Total soil P values ranged from 170 to 1010 mg kg− 1 soil. The loess and alluvial soils had significantly greater mean total P (n ≈ 75) than the sandstone derived soils (p ≥ 0.0001) whereas the shale and alluvial soils had non-significant difference in total P content (Table 1). The vertical distribution pattern of total P, averaged for different depths Table 1 Mean soil phosphorus fractions variation in soil parent materials. Parent material

Total Pa

Org-Pb

Apatite-Pc

Fe-P

Occl-P

Al-P

Ca8-P

Ca2-P

11.55a 9.25b 5.50c 6.00c

13.10ab 12.00b 13.90a 6.80c

15.00a 13.10ab 6.80c 9.95bc

4.20a 3.65ab 3.10ab 2.15b

−1

Alluvium Loess Shale Sandstone

585ab 630a 500b 375c

155a 135b 135b 110c

350b 420a 305c 225d

mg kg 21.65bc 32.40a 24.10b 20.45c

N.B.: the means with the same letter are not significantly different. a Total phosphorus determined by HClO4 digestion (Olsen & Sommers, 1982). b Organic phosphorus by subtraction of total inorganic phosphorus from total phosphorus. c The inorganic phosphorus fractions determined by Jiang and Gu (1989) fractionation scheme: Apatite-P, Ca2-P, dicalcium phosphate equivalents; Ca8-P, octacalcium phosphate equivalents; Al-P, aluminum phosphates; Fe-P, iron oxides surface adsorbed P; Occl-P, iron oxides occluded phosphorus.

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Fig. 3. Total phosphorus distribution with depth in soils of different stages of development in each parent material: (a) alluvium, (b) loess, (c) sandstone, and (d) shale, (mean n 3, error bar one standard error).

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Total phosphorus (mg kg-1) parent materials had deep apatite-P depletion, and the soils at early stages of development had almost uniform apatite-P content throughout soils profile (Fig. 6). Comparing the stages of soil development, the depletion in the sandstone and alluvial soils was deeper than the loess and shale derived soils.

and commercial vegetable garden along the water-courses. At deeper depths variation was limited in all the parent material (data not presented). 3.3.2. Apatite-P Apatite-P ranged from 20 to 700 mg kg− 1 soil, and the variation in distribution pattern was ascribed to the parent material and conditions for soil formation in each parent material. The loess soils contained significantly greater mean (n ≈ 75) apatite-P content (420 mg kg− 1) followed by those derived soils from alluvium, shale and sandstone with means 350, 305 and 225 mg kg− 1, respectively (p ≥ 0.0001). The vertical distribution pattern of mean apatite-P varied with the parent material (Fig. 5). The hypothesis of no depth ∗ parent-material interaction was rejected through the test criteria Wilks' Lambda (p ≥ 0.0002). In the surface three horizons, overall the loess and alluvial soils had non-significant differences but both had greater apatite-P than the shale and sandstone derived soils. Average depletion in apatite-P was deeper in the loess where depletion implies lesser apatite-P than the parent materials. Relatively weathered soils in respective

3.3.3. Phosphorus sorbed on metal oxides Phosphorus sorbed on metal oxides includes three fractions: (i) P adsorbed on iron oxides surfaces that ranged from 3 to 80 mg kg− 1, (ii) P occluded in the crystalline iron oxides that ranged from 1 to 32 mg kg− 1, and (iii) P adsorbed on aluminum hydroxides surfaces that ranged from 2 to 35 mg kg− 1. The metal oxides sorbed P fractions varied significantly with the source of parent material (p ≥ 0.0001). Mean iron oxides adsorbed P in the parent materials when averaged over stages of soil development (n ≈ 75) was ordered as loess > shale = alluvium > sandstone. Parent material mean iron oxides occluded P was ordered as alluvium > loess > shale = sandstone, and aluminum hydroxides adsorbed P was ordered as shale = alluvium > loess > sandstone (Table 1). 222

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Organic phosphorus (mg kg ) Fig. 4. Organic phosphorus distribution with depth in the parent materials showing and an increase toward the surface in all parent materials. The dotted line shows mean for the respective parent material.

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600

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Apatite-P (mg kg ) Fig. 5. Apatite-P distribution with depth in the parent materials indicating a deeper depletion in loess followed by the sandstone and shale derived soils, and the alluvial soils had uniform distribution. An error bar represents one standard error, a dotted line the mean for respective parent material, and an arrow the point where it deviates the respective mean.

The vertical distribution of the metal oxides sorbed P fractions in the profiles varied with the parent material. The hypotheses of no depth ∗ parent-material interactions were rejected p ≥ 0.04 for iron oxides surface adsorbed and p ≥ 0.0001 for occluded P. The change in vertical distribution pattern of aluminum hydroxides surface adsorbed P due to the parent material was non-significant (p ≥ 0.42). Iron oxides surface adsorbed P increased toward the soil surface with a lesser change in the sandstone and alluvial soils (Fig. 7a). Iron oxides occluded P increased toward surface in all parent materials with a large difference in the alluvial soils and the least in the loess soils (Fig. 7b). The aluminum hydroxides adsorbed P fraction had the lowest abundance in the sandstone throughout the depth examined, and increased toward the surface (Fig. 7c). The vertical distribution of the metal oxides sorbed P fractions in the soils at different stages of development also changed differently in the parent materials. The hypothesis of no depth ∗ soil (pm) interaction could not be rejected for the iron and aluminum hydroxides adsorbed P fractions, and rejected for the iron oxides occluded P fraction (MANOVA test criteria Wilks' λ p ≥ 0.014). Although accumulation of the metal oxides sorbed P fractions was limited to surface horizon in all parent materials, the content increased toward the surface in the soils at later stages of development. The iron and aluminum hydroxides surface adsorbed P fractions in the relatively weathered soils were greater than the soils at early stage development, irrespective of parent material (data not presented). The iron oxides occluded P increased toward the surface in the soils of relatively later developmental stage in the alluvial soils but not in loess derived soils (Fig. 8).

76 mg kg− 1 soil. The surface samples of two Argan profiles and one of each the Chakwal and Tirnul soil series profiles had exceptionally high calcium phosphates. The mean calcium phosphates (n ≈ 75) differed significantly in different parent material derived soils (p ≥ 0.0001) given in Table 1. The alluvial soils had higher concentration of calcium phosphates than loess soils that were at par with the alluvial and sandstone derived soils. Calcium phosphate accumulation was limited to soil surface in each of the four parent materials averaged over the soils of respective parent material (data not presented), and the alluvial soils had greater mean (n = 9) than the loess, shale and sandstone derived at respective depths. Further, calcium phosphate accumulation at surface existed in all soils irrespective of genetic stage and parent materials. 4. Discussion Conforming several previous studies, the loess soils contained greater total P and apatite-P followed by the alluvium, shale and sandstone derived soils (Fenton, 1999; Rao, Chen, Luo, & Liu, 2006; Porder & Ramachandran, 2013; Yang et al., 2013). Since the sand particle fractions contains lower apatite-P than the corresponding silt fractions isolated from the same soil (Akhtar et al., 2014), the greater apatite in the loess may relate to the dominance of silt particles. Loess particles are dominantly 10 to 30 μm size, picked-up by the cyclonic winds from the plains of ancient rivers and deposited during the last Glaciations (Brinkman & Rafiq, 1971; Mian & Syal, 1985). The distribution of P in the parent materials conforms to those presented on a global scale (Yang et al., 2013). Previously, apatite-P of 680 mg kg− 1 was reported for undeveloped loess (Akhtar et al., 2014), 580 mg kg− 1 for highly developed loess soils (Hartmann, Durr, Moosdorf, Meybeck, & Kempe, 2012), 190 mg kg− 1 for coarse textured sand dunes

3.3.4. Calcium phosphates Calcium phosphates include: (i) P extracted with NH4-acetate referred as octacalcium phosphate (Ca8-P), Ca8H2(PO4)6.5H2O equivalent and (ii) P extracted by 0.25 M NaHCO3 (pH 7.5) referred as dicalcium phosphate (Ca2-P). The dicalcium phosphate fraction ranged from 0.40 to 15 mg kg− 1 and the octacalcium phosphate fraction from 1 to 223

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Fig. 6. Apatite-P distribution with depth in soils of different stages of development in each parent material: (a) alluvium, (b) loess, (c) sandstone, and (d) shale. The value at each depth is mean of three values, an error bar one standard error, and the dotted line mean of respective parent material.

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Apatite-phosphorus (mg kg-1) (Syers & Walker, 1969), 230 mg kg− 1 for coarse texture greywacke derived alluvium (Shah, 1966), and 396 mg kg− 1 for sandstone. Apatite remained the largest contributor to total P in all the soils. Further, the inclusion of organic P with apatite (Syers, Williams, Campbell, & Walker, 1967) improved correlation coefficient r2 0.87 to 0.96 for explaining variability in total soil P. Apatite-pH and apatite-CaCO3 had positive correlations (r2 0.78 and 0.58, respectively). Soil CaCO3 buffers pH change, and as a result protects apatite from hydrolytic breakdown (Guidry & Mackenzie, 2003). The conclusion that deeper depletion of apatite-P (and total P) have occurred in the loess compared to other three parent materials, may be a misleading suggestion when the parent material mean was over three development stages. Separate comparison among Alfisols (and Mollisols from shale) of different parent materials (and similar comparison among Inceptisols and among Entisols), gave a better estimation of depletion depth in the parent materials. In fact a deeper depletion was observed in the sandstone and alluvium compared to the loess and shale soils. The coarse textured sandstone soils allow more percolation of rainwater causing deeper weathering and apatite depletion (Jenny, 1941; Syers & Walker, 1969; Araujo et al., 2004). Apatite depletion is attributed to faster fluid fluxes in sandstone and alluvium that enhance hydrolytic

breakdown (Hausrath, Navarre-Sitchler, Sak, Williams, & Brantley, 2011). Since percolation through shale is minimal, the acidic leachate from forest litter would cause apatite breakdown but the effect should be limited to the surface. Iron oxides' surface adsorbed P was greater in the loess- and shalederived soils while iron oxides occluded P content was greater in the alluvial and sandstone-derived soils. It was ascribed to pedogenesis and the parent material in case of shale. The loess derived soils contain poorly crystalline ferrihydrite and the shale had hematite, ferrihydrite, and lepidocrosite while the alluvial soils and loess soils at early developmental stages exhibit goethite crystallization (Mehmood et al., 2015). Occlusion P is due to twining structures of goethite crystals (Memon et al., 2011). Surface adsorbed P correlates with the amount of oxalate extractable iron (r2 = 0.70 comparable with Galvez, Barron, & Torrent, 1999). Shale derived soils had significantly greater dithionite extractable iron than alluvium but lower occluded P content. The presence of ferrihydrite and hematite in shales and star shaped goethite in the alluvial soils may explain the difference in iron P fraction (Memon et al., 2011; Schwertmann & Taylor, 1989; McBride, 1994; Smeck et al., 1994). 224

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0

(c)

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120

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20

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50 0

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-1

Metal oxides sorbed-phosphorus (mg kg ) Fig. 7. Metal oxides sorbed phosphorus distribution with depth in the soil parent materials: (a) iron oxides surface adsorbed phosphorus, (b) iron oxides occluded phosphorus and (c) aluminum oxides surfaces adsorbed phosphorus. Iron oxides surface adsorbed P accumulation was deeper in the loess and shale derived soils than the alluvial and sandstone derived soils, occluded P accumulation was deeper in the alluvium and loess soils than sandstone and shale derived soils (dotted lines show mean for the respective parent material).

Taking the apatite content that arrived with the sediments (Mo) as a fixed quantity and the existing apatite M(y) reached after soil weathering, the apatite loss from the soil can be calculated by rearranging the Eq. 1 as.

Dicalcium and octacalcium phosphates in calcareous soils are metastable P forms and highly abundant in soils under intensive cultivation and fertilizer input. Dicalcium phosphates, phosphate adsorbed on surface of organic matter, and iron oxides are the main pools of bioavailable phosphates. The octacalcium phosphate fraction may transform to dicalcium phosphate under P depletion (Jiang & Gu, 1989; Liu, He, Colombo, & Violante, 1999; Sharpley, 2000; Samadi & Gilkes, 1998). Octacalcium phosphates tend to form, particularly if soluble phosphate has been introduced by high fertilizer application (Sposito, 1984). Memon et al. (2011) reported high octacalcium phosphate in alluvial soils than the loess and shale soils. Our results here support those previously showing a relationship between soil development stages and P forms in soils (Walker & Syers, 1976). Soil P occurs primarily as apatite during the early stages of soil development, decreases with weathering while the organic P and occluded P fractions increase under soil abiotic and biotic processes. The released P when taken up by the soil biota becomes part of the organic P reservoir and, partly, the released P is sorbed onto the surface of secondary minerals to become non-occluded P (Yang & Post, 2011; Crews et al., 1995; Cross & Schlesinger, 1995).

M(Y) ⎞ 1 ln ⎛1 − =− Y Mo ⎠ λ ⎝ ⎜

⎟

(3)

where Y is the cumulative iron oxides sorbed P. Here it was a mean of n ≈ 18 from three replicated profiles, each with six horizons, representing a soil at a particular development stage (or soil order); and with other two soils, it would represent a sequence of soil with progressively increasing development (Smeck et al., 1994). Fig. 10 is a plot of Y against ln(1-M(y)/Mo) for the four parent materials. The apatite loss model fits well in all the parent materials except the shale. The shales contain significant amount of lithogenic iron oxides contrary to the assumption of iron oxides being mainly pedogenic. The Apatite-P content arrived with the sediment was determined through optimizing r2 by substituting values for Mo. Hence calculated Mo was 670 mg kg− 1 for loess, 510 mg kg− 1 for the alluvium and 315 mg kg− 1for the sandstone. The measured values for the almost all development stages at deeper depth were close to calculated values (Mo) from the model that verify the model results. The measured values for the alluvial Haplustalfs at depth of 2 m and sandstone at depth of 2.2 m were 477 and 335 mg kg− 1, respectively while the calculated values of Mo were 510 and 315 mg kg− 1 for the alluvium and sandstone, respectively. Similarly, Ustorthents of loess soils at the depth of 160 cm had 690 mg kg− 1 while calculated Mo for loess soils was 670 mg kg− 1. These Mo values translate into 1455 kg for loess, 945 kg for alluvium, and 675 kg for sandstone per hectare. The values are close to the apatite-P content reported for loess (Rao et al., 2006; Hartmann et al., 2012; Yang et al., 2013; Akhtar et al., 2014), and for alluvium and sandstone (Shah, 1966; Walker & Syers, 1976; Boyle et al., 2013). The loss of apatite-P, the difference in Mo and the mean apatite-P in respective parent material based on the average of three profiles for each development stage, increased with soil development from Entisols, to Alfisols conforming with previous studies (Selmants & Hart, 2010; Akhtar et al., 2014). The calculated apatite-P loss in the loess derived soils was 48% in Typic Haplustalfs, 45% in Udic Haplustepts, and only 13% in Typic Ustorthents. Assuming the loess sediments to be of same age, the differences in apatite was apparently driven by the reliefcontrolled soil development. The Haplustalfs and Udic Haplustepts are developed in wide level plains and in depressions area of level plains where rainwater infiltration was high compared to the Ustorthents.

4.1. Comparative depletion and Fe-P50 The concentration of apatite-P (and iron sorbed P) was expressed as mass (kg). A uniform depth of 15 cm for each horizons and measured bulk density for the respective was used in the mass calculation. The apatite-P and iron sorbed P contents are in kg per hectare to a cumulative depth of 90 cm. The current apatite content plotted against the iron oxides' sorbed P revealed an exponential weathering or a “decay function” (Fig. 9). The abscissa is P sorbed on pedogenic iron oxides rather than time used earlier by Walker and Syers (1976), Lajtha and Schlesinger (1988), Porder et al. (2007), Selmants and Hart (2010), Eger et al. (2011), Boyle et al. (2013) and Yang et al. (2013). The function fits better for the soils derived from the loess, sandstone and alluvium. All the Entisols samples of respective parent material plotted close since the change in iron sorbed P was minimum due to lack of vertical development. Whereas soil development caused a vertical variability in profiles of Alfisols and Mollisols leading the spread in the plots along abscissa. A similar relationship for iron oxides sorbed P with apatite has likewise been reported (Delgado, Ruiz, Campillo, Kassem, & Andreu, 2000). The plot of apatite-P asymptotic to abscissa (iron oxides surfaces adsorbed and occluded P) indicates an equilibrium between input of P and soil apatite (Walker & Syers, 1976). 225

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Fig. 8. The distribution of occluded phosphorus with depth in soils at three stages of development in each parent material: (a) alluvium, (b) loess, (c) sandstone, and (d) shale (mean n 3, an error bar one standard error).

0 (b)

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Occluded phosphorus (mg kg-1) dryer soil moisture regimes (Selmants & Hart, 2010), and similarly organic P also initially increased but decreased with extreme weathering (Yang et al., 2013). Though the P fractions occluded in iron and aluminum equally increased with development, the use of iron oxides surface adsorbed and occluded is preferred for two reasons: (i) aluminum hydroxides do not exist during initial stages of development if soil pH is controlled by free lime, and P adsorbed on aluminum was confounded with P on aluminum from exposed edges of organic matter (Samadi & Gilkes, 1998) and (ii) the combined iron oxides surface adsorbed and occluded P has a continuous increasing trend over a wide range of soil development. It implies that the model can be tested for the development stages beyond Alfisols. The concept is legitimate since all the fundamental principles of conservation of matter and energy are applied, and would be applicable within its assumptions when a description of detailed physical, biological, and chemical processes in the soil are complex. The question is addressed here whether the apatite loss rate differs with the sediments geology. Soil apatite-P loss per unit production of iron adsorbed and occluded P represents its depletion rate that is independent

Apatite-P loss in the sandstone derived soils, again related the soil relief, and was 44% in Udic Haplustalfs, 19% in Typic Haplustalfs, and only 1% in Lithic Ustipsamments. The mean apatite loss in the alluvial soils was 93% in Typic Haplustalfs, 6% in Fluventic Haplustepts, and only 4% in Typic Ustifluents. The alluvial soils represent a chronosequence. The Haplustalfs are formed in the Pleistocene age sediments, Haplustepts in Holocene sediments and Ustifluents in Recent and active floodplain. Therefore, it is effective-rainfall driven depletion of apatite from loess and sandstone and age of the sediments in case of alluvium. Our model conforms to the conceptual framework of Walker and Syers that the primary source of all soil P is apatite initially, and it decreases exponentially with soil development. The plot of apatite-P becoming asymptotic to abscissa with iron oxides sorbed P fully conforms Walker and Syers (1976) paradigm that the occluded P associated with iron (and aluminum) increases steadily with soil age as previously reported by Crews et al. (1995), Samadi and Gilkes (1998), Cross and Schlesinger (1995), Smeck et al. (1994) and several others. The secondary precipitates with calcium initially increased and eventually decreased though the decrease may be fast in udic and slow ustic and

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1500

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900 600 300 0 0

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0

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Fig. 9. Relationship of apatite- and iron oxides sorbed‑phosphorus suggesting an exponential decrease in apatite-P with an increase in iron oxides sorbed phosphorus in selected parent materials. The values are in kg ha− 1 where ha, hectare is 100 by 100 m and cumulative depth 6 horizons was 90 cm.

in Haplustalfs for both the parent materials i.e. 93% in Haplustalfs of alluvium versus 48% in Haplustalfs of loess. Both the Haplustalfs are of same age (the loess and Old River terraces are of Pleistocene epoch), which strengthens the hypothesis of more stability of apatite in the loess than alluvium. Rapid leaching of the weathering products out of profile under high infiltration rate in the lighter texture soils (Jenny, 1941) may be related with the low stability of apatite in sandstone and alluvium. High fluid fluxes in alluvium and sandstone are responsible for less stability of apatite (Hausrath et al., 2011).

of initial apatite content in the parent materials and chronology of sediments. The exponential decrease with iron oxides sorbed P in soil implies a continuous rate change that is dependent on the mass present. Therefore, Fe-P50, the iron oxides sorbed P level when soil apatite reduces by 50% of original content (Eq. 2) is valid derivation, and it varied with the sediments geology. Fe-P50 is 55.7 kg in the loess, 46.8 kg in the sandstone and 20.4 kg in the alluvium when apatite will reach the level of Mo/2. It is apparent that apatite is more stable in the loess than in sandstone and alluvium, which conforms to the differences of apatite loss 0.0 Typic Haplustalfs

-1.5

Fig. 10. Plot of ln(1-M(y)/Mo) versus cumulative iron oxides sorbed‑phosphorus for the parent materials: depicting a fit to the linear reservoir model except for the shale derived soils containing iron oxides derived from red shale.

Typic Haplustalfs Udic Haplustepts

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Alluvium

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ln(1-My/Mo) = 0.03 (iron oxides sorbed P) - 3.70 R² = 0.99

ln(1-My/Mo) = 0.16 (iron oxides sorbed P) - 12.83 2

R =1

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Typic Haplustalfs

-3.0 ln(1-My/Mo) = 0.11 (iron oxides sorbed P) - 8.53 R² = 0.99

-4.5

ln(1-My/Mo) = 0.009 (iron oxides sorbed P) - 1.34 2

R = 0.02

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granulatus (Gervais, 1847), on aggregation, enzymatic activity, and phosphorus fractions in the soil. Geoderma 289, 135–141. de Terra, H., de Chardin, P.T., 1936. Observations on the upper Siwalik formation and later pleistocene deposits in India. Proc. Am. Philos. Soc. 76, 791–822. Delgado, A., Ruiz, J.R., Campillo, M.C.D., Kassem, S., Andreu, L., 2000. Calcium and iron related phosphorus in calcareous and calcareous marsh soils: sequential chemical fractionation and 31P nuclear magnetic resonance study. Commun. Soil Sci. Plant Anal. 31, 2483–2499. Eger, A., Almond, P.C., Condron, L.M., 2011. Pedogenesis, soil mass balance, phosphorus dynamics and vegetation communities across a holocene soil chronosequence in a super-humid climate, South Westland, New Zealand. Geoderma 163, 185–196. FAO, 1990. Guidelines for soil description. Third addition (revised). In: Soil Resources, Management and Conservation Services, Land and Water Development Division. FAO, Rome. Fenton, T.E., 1999. Phosphorus in Iowa soils. Agronomy Department, Iowa State University. http://extension.agron.iastate.edu/NPknowledge/pubs/phosias.pdf. Galvez, N., Barron, V., Torrent, J., 1999. Preparation and properties of hematite with structural phosphorus. Clay Clay Miner. 47, 375–385. Gibson, W.P., Burns, R.G., 1977. The breakdown of malathion in soil and soil components. Microb. Ecol. 3, 219–230. Guidry, M.W., Mackenzie, F.T., 2003. Experimental study of igneous and sedimentary apatite dissolution: control of pH, distance from equilibrium, and temperature on dissolution rates. Geochim. Cosmochim. Acta 16, 2949–2963. Hartmann, J., Durr, H.H., Moosdorf, N., Meybeck, M., Kempe, S., 2012. The geochemical composition of the terrestrial surface (without soils) and comparison with the upper continental crust. Int. J. Earth Sci. 101, 365–376. Hausrath, E.M., Navarre-Sitchler, A.K., Sak, P.B., Williams, J.Z., Brantley, S.L., 2011. Soil profiles as indicators of mineral weathering rates and organic interactions for a Pennsylvania diabase. Chem. Geol. 290, 89–100. Hedley, M.J., Stewart, J.W.B., Chauhan, B.S., 1982. Changes in inorganic and organic soil phosphorus fractions by cultivation practice and by laboratory incubations. Soil Sci. Soc. Am. J. 46, 970–976. Jackson, M.L., Lim, C.H., Zelazny, L.W., 1986. Oxides, hydroxides and aluminosilicates. In: Klute, A. (Ed.), Methods of Soil Analysis. Part 1. ASA No.9. Madison, Wisconsin, pp. 101. Jenny, H., 1941. Factors of Soil Formation. McGraw-Hill, New York. Jiang, B., Gu, Y., 1989. A suggested fractionation scheme of inorganic phosphorus in calcareous soils. Fert. Res. 20, 159–165. Lajtha, K., Schlesinger, W.H., 1988. The biogeochemistry of phosphorus cycling and phosphorus availability along a desert soil chronosequence. Ecology 69, 24–39. Larssen, T., Vogt, R.D., Seip, H.M., Furuberg, G., Liao, B., Xiao, J., Xiong, J., 1999. 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5. Conclusion The changes in various soil P forms with soil development have been studied on chronological scales that makes it difficult to assess the rate of change for the parent material that have same chronology yet differ in development due to other soil factors (relief, organism, and climate). Three profiles for each of Entisols, Inceptisols, and Alfisols/ Mollisols (in case of shale) represented by a soil in loess, shale and sandstone (as toposequences) and alluvium (as chronosequences) were investigated for P forms at the genetic horizon levels. The loess and alluvium contained greater total P than shale and sandstone throughout the depth. Organic P increased toward the surface in all the parent materials but more pronounced accumulation was in case of shale and loess. The Haplustalfs/Hapludolls (soils at the greater stages of development) in the respective parent material had deep apatite-P depletion and the soils at early stages of development had almost uniform apatiteP content throughout soils profile. The metal oxides sorbed P accumulation was limited to surface in all parent materials, and greater increase was occurred in Haplustalfs and Hapludolls. The heavily fertilized profiles had high accumulation of dicalcium phosphate and octacalcium phosphate. Soil apatite-P plot against iron oxides P (surface adsorbed and occluded) fit an exponential decay function. The model enabled us to calculate the relative apatite-P loss in all the parent materials, except the shale due violation of the assumption. Fe-P50, the iron oxides sorbed P when apatite reduces by 50% of the mass arrived with the sediment was 55.7 kg for loess compared to 46.8 kg for sandstone and 20.4 kg for alluvium. The study suggests a greater stability of apatite in loess compared to alluvium and sandstone. Acknowledgment The authors wish to thank the Higher Education Commission, Pakistan for financial support through HEC project no. 1038 and Indigenous Ph.D. Fellowship Program. The authors also deeply acknowledge the contribution of anonymous reviewers and Professor Matthew A. Jenks, Division of Plant and Soil Sciences, West Virginia University, Morgantown, WV 26506-6108, USA for their very valid suggestions which greatly improved the manuscript. References Adams, J.A., Walker, T.W., 1975. Some properties of a chrono-toposequence of soils from granite in New Zealand, 2. Forms and amounts of phosphorus. Geoderma 13, 41–51. Akhtar, M.S., Imran, M., Mehmood, A., Memon, M., Rukh, S., Kiani, G.S., 2014. Apatite loss in Pothwar Loess Plain (Pakistan) fits a simple linear reservoir model. Pedosphere 24, 763–775. Araujo, M.S.B., Schaefer, C.E.R., Sampaio, E.V.S., 2004. Soil phosphorus fractions from toposequences of semi-arid Latosols and Luvisols in northeastern Brazil. Geoderma 119, 309–321. Blake, G.R., Hartge, K.H., 1986. Bulk density. Methods of soil analysis, part e1. J. Soil Sci. Soc. Am. 363–376 Madison, USA. Bossart, P., Ottiger, R., 1989. Rocks of the Murree Formation in Northern Pakistan: Indicators of a descending foreland basin of late Paleocene to middle Eocene age. Eclogae Geol. Helv. 82, 133–165. Boyle, J.F., Chiverrell, R.C., Norton, S.A., Plater, A.J., 2013. A leaky model of long-term soil phosphorus dynamics. Glob. Biogeochem. Cycles 27, 516–525. Brinkman, R., Rafiq, M., 1971. Landforms and Soil Parent Materials in West Pakistan. Soil Survey Project of Pakistan, Lahore-Dacca. Churchman, G.J., Lowe, D.L., 2010. Alteration, formation, and occurrence of minerals in soils. In: Huang, P.M., Li, Y., Sumner, M.E. (Eds.), Hand Book of Soil Science, 2nd Ed. Properties and Processes Vol 1. CRC Press (Taylor and Francis), Boca Raton, pp. 20.1–20.72. Cochran, C.C., 2010. Soil moisture-temperature correlation and classification model. In: 19th World Cong. Soil Sci., Soil Solutions for a Changing World, 1–6 August 2010, Brisbane, Australia, (Published on DVD). Crews, T.E., Kitayama, K., Fownes, J.H., Riley, R.H., Herbert, D.A., Mueller-Dombois, D., Vitousek, P.M., 1995. Changes in soil phosphorus fractions and ecosystem dynamics across a long chronosequence in Hawaii. Ecology 76, 1407–1424. Cross, A.F., Schlesinger, W.H., 1995. A literature review and evaluation of the Hedley fractionation: applications to the biogeochemical cycle of soil phosphorus in natural ecosystems. Geoderma 64, 197–214. da Silva, V.M., Antoniolli, Z.I., Jacques, R.J.S., Ott, R., Rodrigues, P.E.D., Andrade, F.V., Passos, R.R., de Mendonça, E., 2017. Influence of the tropical millipede, Glyphiulus

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