effect of irrigation water quality on the leaching and desorption of phosphorous from soil

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Effect of Irrigation Water Quality on theLeaching and Desorption of Phosphorousfrom SoilM. Jalali a & Z. Kolahchi aa Department of Soil Science, College of Agriculture , Bu-Ali SinaUniversity , Hamadan, IranPublished online: 12 Aug 2009.

To cite this article: M. Jalali & Z. Kolahchi (2009) Effect of Irrigation Water Quality on the Leachingand Desorption of Phosphorous from Soil, Soil and Sediment Contamination: An International Journal,18:5, 576-589

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Soil and Sediment Contamination, 18:576–589, 2009Copyright © Taylor & Francis Group, LLCISSN: 1532-0383 print / 1549-7887 onlineDOI: 10.1080/15320380903113451

Effect of Irrigation Water Quality on the Leachingand Desorption of Phosphorous from Soil

M. JALALI AND Z. KOLAHCHI

Department of Soil Science, College of Agriculture, Bu-Ali Sina University,Hamadan, Iran

Knowledge of the rate of decrease of nutrients from soils resulting from poor waterquality application is essential for long-term planning of crop production while mini-mizing the impact on groundwater quality. In this study, we examined the effect of Ca2+

concentration of irrigation water on phosphorus (P) leaching and kinetic release incolumns of sandy soil. Phosphorous sorption in the presence of CaCl2 solutions withCa2+ concentrations of 3, 5, 10, and 15 mM CaCl2 was determined to understand thetransport and leaching of P in the sandy soil. The geochemical Visual MINTEQ was usedto calculate saturation indices. A considerable number of leachate samples containedP at concentrations that could cause eutrophication. Total P leached from soil due toapplication of different CaCl2 solutions ranged from 1.7 to 1.8 kg ha−1 after 20 porevolumes had passed through the soil. Comparison of the leaching experiments resultswith the kinetic desorption data indicated that leaching removed on average 50 timesless P than cumulative P desorbed by successive extractions with different CaCl2 solu-tions. Leaching in presence of different CaCl2 solutions was controlled by rate-limiteddissolution of calcium hydroxyappatite and ß-tricalcium phosphate.

Keywords calcareous soils, irrigation water quality, nutrient leaching, phosphorous

Introduction

Losses of nutrients from agricultural land have been identified as one of the main causalfactors in degrading water quality (Boesch et al., 2001; Maguire and Sims, 2002). Amongsoil nutrients, phosphorous (P) is usually considered to be highly immobile in soils andits movement into groundwater has generally been considered to be insignificant becauseit is fixed by soil colloids or organic matter (Rowell, 1994). But, in some circumstancessubsurface pathways can play an important role in P leaching losses from agriculture,especially where soil P concentrations are already high, soil P sorption capacities are low(sandy soils and high organic matter), irrigation water is enriched with P, and drainage ratesin soils are high (Heckrath et al., 1995; Sims et al., 1998; Maguire and Sims, 2002).

In arid and semi-arid regions, groundwater is commonly the only source of irrigation,and its quality is usually poor because of limited rainfall and high rates of evaporation.Thus, a high proportion of low- to medium-quality groundwater is used for irrigation.Such water may contain large quantities of soluble salts, predominately calcium (Ca2+) andsodium (Na+) ions. Some agricultural soils can now be classified as overfertilized due to a

Address correspondence to M. Jalali, Department of Soil Science, College of Agriculture, Bu-AliSina University, Hamadan, Iran. E-mail: [email protected]

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Effect of Irrigation Water Quality on Phosphorous Leaching 577

steady increase in available P resulting from application of fertilizer P in the past (Sharpleyand Smith, 1989). Phosphorous fertilizer is used annually and plant uptake and microbialimmobilization can not remove the entire P from the solution. Long-term fertilizationof course- to medium-textured soils could increase downward P mobility (Mozaffari andSims, 1994; Zhang et al., 1995). Scalenghe et al. (2002) indicated that long-term repeatedapplications of fertilizers and livestock wastes have resulted in a general increase in theP status of soil. As a consequence, many agricultural soils are now considered to be apotential diffuse source of P to surface waters (Kumar et al., 1994). Very low concentrationsof P can cause environmentally significant nutrient enrichment of surface water. In somecircumstances concentrations as low as 20 µg P l−1 are sufficient to cause algal growth andeutrophication (Powlson, 1998).

Many soils in Iran have received large amounts of P-fertilizer and consequently con-tained high level of available P (Jalali, 2006b). The quality of groundwaters in the Hamadan,western Iran, varies from useable to hazardous (Jalali, 2002). In addition, in the studiedarea Ca2+ is the dominant ion in wells water, representing on average 43.6% of all cations(Jalali, 2002). Its concentration in irrigation water varies from 0.01 to 11.3 mM.

While potassium (K+) leaching as a result of irrigation water quality has been thesubject of several studies, which related leaching of K+ from soil with various indicesof irrigation waters (Jalali and Rowell, 2003; Kolahchi and Jalali, 2006), their effects onP losses remain relatively unknown. An understanding of P-Ca exchange is necessary topredict how much P will precipitate and subsequently how much P will leach when waterswith different Ca2+ concentrations are applied. Therefore, this study was conducted toevaluate the impact of using such water on leaching and kinetic release of P from a sandysoil, using column leaching and batch method, respectively.

Column studies have frequently been used to provide information about element releaseand transport in soil, chemistry of soil and leachates, and to carry out kinetics and massbalance studies (Voegelin et al., 2003; Qureshi et al., 2004). Therefore, they were used inthe present study to examine the downward movement of P due to application of differentCa2+ concentrations in a sandy soil.

Materials and Methods

Soil

The test soil was taken from the 0 to 30-cm layer of a typical sandy loam (157 g kg−1

clay, 620 g kg−1 sand and 223 g kg−1 silt) of the Azandarian Series that is continuouslycultivated (for grapes [Vitis vinifera L.]) in Hamadan, western Iran. It was classified asa Typic Calcixerolic Xerochrept according to USDA classification (Kolahchi and Jalali,2006), and had a pH (H2O) of 7.1, organic matter content of 37.1 g kg−1, exchange-able K+ of 0.52 cmolc kg−1, CEC of 12.6 cmolc kg−1, and carbonate calcium equiva-lent of 47 g kg−1. The Olsen P before and after addition of P was 22 and 36 mg kg−1,respectively.

The soil was air-dried and passed through a 2-mm mesh sieve before being storedin polyethylene bags. Olsen P was determined using a soil to solution ratio 1:20 and30 min shaking (Olsen and Sommers, 1982). Phosphorus in extractant was determinedusing the ammonium molybdate-ascorbic acid method described by Murphy and Riley(1962).

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578 M. Jalali and Z. Kolahchi

Incubation Study

For the incubation study, the soil was treated with 100 mg P kg−1 as K2HPO4 in solution.The applied P (280 kg P ha−1, assuming 2800 t soil ha−1) is higher than the average Pcurrently being used (40 kg P ha−1) in Iran (Jalali, 2007). High rate of application was usedbecause it can occur where a large amount of total P is placed on a small area of soil, e.g.,band application. The soil was placed in plastic bags and incubated for 21 days at 25 ◦C.The appropriate amount of water was added to bring the soil to the estimated field capacity.The sample was kept moist by adding distilled water as needed. After the given time, soilwas taken and air dried before being used for leaching, kinetic, and sorption experiments.Sub-samples were analyzed for Olsen P.

Soil Column Leaching Experiment

The leaching columns consisted of Pyrex tubes of 30 cm length and an internal diameterof 4.8 cm. The soil was seated at a height of 10 cm by uniform tapping with a woodenrod to achieve a uniform bulk density of 1.3 g cm−3. The soil was retained by a WhatmanNo. 42 filter paper supported by a piece of nylon mesh base. After packing, the soil wascovered with a filter paper to protect the surface from disturbance. Phosphorous was leachedwith CaCl2 solutions of various concentrations (3, 5, 10, and 15 mM CaCl2). ExtractableCaCl2-P has previously been shown to be a useful indicator for P leaching (McDowell etal., 2001). Robbins et al. (1999) indicated that 10 mM CaCl2 has comparable ionic strengthto natural soil solutions, thus the leachability of CaCl2 should be similar to that for thenatural soil solution. Dissolution of calcite and gypsum may provide Ca2+ concentrationlower and higher than 10 mM, respectively. Thus, a range of Ca2+ concentration was usedto simulate soil pore water in a wide range of soils.

Leaching experiments were conducted under saturation condition. The study was con-ducted in two replicates at room temperature (22–24◦C). The solution level was maintainedat approximately 5 cm above the soil surface within the soil column to maintain the effluentflow at an average of 2.5 ± 0.1 ml min−1. The water level varied slightly according tooutflow rate. Pore volume (PV) of soil columns is the volume of water-filled pores in thecolumn and was calculated from value for the bulk density and particle density (2.65 g cm−3)of the soil in the column (Rowell, 1994). Therefore, the pore volume of the columns wastaken to be 92 ml. The columns were leached with 20 pore volumes. Effluents from eachleaching stage were collected in 23–46-ml lots and were analyzed for P colorimetricallyby the molybdate blue method (Murphy and Riley, 1962). The quantity of leached P wascalculated for each treatment from the P concentration and the volume of leachate fraction.

Saturation Indices

The saturation indices (SI) for P in leachates were calculated using geochemical specia-tion model Visual MINTEQ version 2.30 (Allison et al., 1991). Visual MINTEQ 2.30 is ageochemical equilibrium computer program that has an extensive thermodynamic databasethat allows for the calculation of speciation, solubility, and equilibrium of solid and dis-solved phases of minerals in an aqueous solution (Gustafsson, 2005). The saturation indicesdescribe quantitatively the deviation of water from equilibrium with respect to dissolvedminerals. Saturation indices were calculated by using the following equation:

SI = log(IAP/Keq) (1)

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where IAP signifies the ion activity product and Keq is the equilibrium constant of the solidphase. If the water is exactly saturated with the dissolved mineral, SI equals to zero. Positivevalues of SI indicate saturation, and a negative one indicates undersaturated (Appelo andPostma, 1996). Although it is likely that there are several P minerals present in the soil inaddition to Ca-P minerals, the high concentration of Ca2+ in the applied CaCl2 solutions,and the low solubility of Ca-P minerals, suggest that Ca-P minerals are most likely present(Hansen and Strawn, 2003). In addition to the measurement of P concentration in theleachates, pH, EC, and Ca2+ was measured. For Visual MINTEQ input, the ionic strengthwas calculated using following equation (Griffin and Jurinak, 1973):

I = 0.013EC (2)

Kinetics of P Release

Kinetics of P release was studied by successive extraction (Abekoe and Sahrawat, 2001;Shariatmadari et al., 2006; Jalali, 2006a) with different CaCl2 solutions. A total of 6successive 15-min extracts was conducted at 25 ± 2◦C. The experiment was conducted intwo replicates. The release of P with time was fitted by using the following equations:

Parabolic diffusion model q = a + bt1/2 (3)

Power function equation ln q = ln a + b ln t (4)

Elovich equation q = a + b ln t (5)

First order ln(q0 − qt ) = a − bt (6)

where q is the amount of released P, qt is the cumulative P released at time t, t, the time ofrelease, q0 is the maximum P released, a and b are constants. An important term of theseequations is the constant b, which is indicative of the release rate of P. These mathematicalmodels were tested by least square regression analysis to determine which equation bestdescribed P release from soil. Coefficients of determination (R2) were obtained by leastsquare regression of measured versus predicted values. Standard errors of the estimate werecalculated by:

SE =[∑

(q − q∗)2

(n − 2)

]1/2

(7)

where q and q∗ represent the measured and predicted P released, respectively, and n is thenumber of data points evaluated. Zero order and second order models were also tested butdid not fit the data and therefore are not discussed.

Isotherm Experiments

To determine P sorption, 2.5 g of soil was placed into a 50-ml centrifuge tube and mixedwith 25 cm3 of varying equilibrating solutions with the following range of K2HPO4 con-centrations: 0, 10, 20, 30, 40, and 50 mg l−1 P. The equilibrating solutions contained 3, 5,10, and 15 mM CaCl2. Suspensions were shaken for 1 h. This procedure was performed in

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duplicate. The amount of P sorbed by soil was calculated with (Rowell, 1994):

S = (C0 − Ce) V

W(8)

where S is the amount of P sorbed (mg kg−1), C0 is the concentration of soluble P added in theinitial solution (mg l−1), Ce is the concentration of P in the solution at equilibrium (mg l−1),V is the solution volume (l), and W is the weight of air-dried soil (kg). The correspondingsorption isotherms for P in each CaCl2 solution has been quantitavely described by fittingthe experimental data to the Freundlich sorption isotherm using log-transformed form:

LogSi = log KF + n log Ce (9)

where KF (l kg−1) is the Freundlich distribution coefficient and n is a empirical constantthat typically has a value of less than 1. The goodness of fit for all equations was estimatedby the coefficient of determination. Phosphorous buffer capacity (PBC) was determinedfor each CaCl2 solution from the slope of the line obtained by plotting P sorbed (mg kg−1)against log Ce according to Bertrand et al. (2003) and Abekoe and Sahrawat (2001). Thepoint where the isotherm crosses the abscissa is the concentration of P in solution causingno adsorption or desorption of P (equilibrium P concentration).

Results and Discussion

Phosphorous Leaching

The results of the leaching are presented as breakthrough curves (graphs showing therelationships between concentrations of cations/anions and cumulative water passing outthe columns). The peak P concentration was greater in the leachate from the lower Ca2+

solution (3 mM) than from the higher Ca2+ solution (15 mM) (Figure 1). The relatively

0

0.02

0.04

0.06

0.08

0.1

0.12

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0.18

0.2

0 3 6 9 12 15 18 21

Pore volumes

Co

nce

ntr

atio

n o

f P

(m

g l-1

) 3 mM CaCl2

5 mM CaCl2

10 mM CaCl2

15 mM CaCl2

Figure 1. Breakthrough curves for P from the soil leached with different CaCl2 solutions.

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Table 1The amount of P leached from columns of studied soil using different CaCl2 solutions

5 PV 10 PV 15 PV 20 PVExtractionmedia (mM mg kg mg kg mg kg mg kgCaCl2) column−1 ha−1 column−1 ha−1 column−1 ha−1 column−1 ha−1

3 0.084 0.463 0.167 0.922 0.233 1.288 0.331 1.834a†

5 0.081 0.446 0.161 0.889 0.224 1.239 0.315 1.741b

10 0.078 0.432 0.155 0.856 0.217 1.200 0.308 1.703c

15 0.077 0.428 0.154 0.851 0.215 1.191 0.306 1.696c

†Values followed by same letters within a column indicate no significant difference at p ≤ 0.05.

high initial concentration of P in the leachate decreased to more stable values after 4–5pore volumes and can be attributed to a slow dissolution of the P salts. The P concentrationremained below 0.20 mg l−1 for all Ca2+ solutions and the amount of P leached wassignificantly different (P < 0.05) in these solutions (Table 1). The P concentrations in theleachate in different CaCl2 solutions are greater than the recommended guideline (0.10 mgP l−1) in the Netherlands as an environmental upper limit for P in shallow groundwaters(Breeuwsma et al., 1995). In surface water, 0.01 to 0.05 mg l−1 total solution P mayresult in considerable phytoplankton production (Grobbelaar and House, 1995). Accordingto USEPA surface water quality guidelines, the maximum contaminant limit of P forsurface water quality standard is 0.05 mg l−1 (USEPA, 1986). Although groundwater Pconcentrations are generally low, the results of leaching of this sandy soil showed the highrisk for P transfer into groundwater in concentrations exceeding the groundwater qualitystandard.

Phosphorus release was rapid during the first few hours, but then slows with time.Similar results for P release were observed by Dev et al. (1990) and Abekoe and Sahrawat(2001). Abekoe and Sahrawat (2001) observed that there was a sharp initial increase in Prelease in the first 2 h followed by a continued slow release up to 16 h.

Distilled water can be used to represent the relatively low-salinity water source fromrainfall or snowmelt in arid and semi-arid regions. Although distilled water and rain don’tnecessarily have similar Ca2+ concentrations, when they enter the soil CaCO3 dissolutionis the main source of Ca2+ for displacing other cations from exchange sites (Kolahchi andJalali, 2007). They used distilled water for leaching of K+ from the same soil used here.They indicated that the Ca2+ concentration reached to a constant value of about 0.8 mMafter 11 pore volumes. The soil contains CaCO3 (47 g kg−1) that can solubilize to supplyCa2+ to decrease P solubility. In the field, where CO2 concentration in the soil air is higherthan that in the atmosphere, the dissolution of calcite can be enhanced, leading to leachingof P more likely similar to 3 mM CaCl2 solution.

Several mechanisms have been proposed to explain the slow release of P in soils, suchas a bonding change with time (Kafkafi et al., 1967) and solid-state diffusion (Barrow,1974). The initial fast release is considered to represent true P desorption on mineralsurface. Dissolution of various P mineral phases, desorption of P from mineral surfaces,and release of P from organic matter are the common P-release mechanisms observed insoils (Hansen and Strawn, 2003). Thus, it can be suggested that the initial fast release may

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reflect the surface desorption and subsequent decrease in the P release with time may reflectthe diffusion-dissolution of P from the porous solids.

The mobility of P in soil is low compared with other nutrients due to high capacityof soil material to adsorb P. Sandy soils usually have a very small sorption capacity for Pbecause of low clay content. Phosphorus leaching can occur in sandy soils (Guertal et al.,1991) and other types of soils with high P levels due to continuous P application (Heckrathet al., 1995). Transport of P from agricultural land in sandy soil areas has been reported tocause contamination of ground and surface waters (McComb and Davis, 1993).

Phosphorous concentrations in drainage waters can be related to the levels of P by soiltest (McDowell et al., 2001). Extractable Olsen P has previously been shown to be a usefulindicator for P leaching (Heckrath et al., 1995). In general, a plot of CaCl2-P against soiltest P (Olsen or Mehlich-3 P) exhibits a point, termed a change point, above which CaCl2-Pincreases much more rapidly per unit of increase in soil test than if it is below that point(McDowell et al., 2001). They investigated the existence of a change point in soil P releasefor a wide range of variously managed soils. They found the change points from 20 to112 mg Olsen P kg−1. The Olsen P concentration 36 mg kg−1 is in the range of the changepoints found by McDowell et al. (2001), indicating potential loss of P to drainage waters.

The amount of P leached when the soil was leached with different CaCl2 solutionsvaried from 1.7 to 1.8 kg ha−1, when 20 pore volumes had passed through the column(Table 1). The total P leached in the leaching experiments was less than the Olsen P(1.13–1.22%) in the soil. The soil showed a significant decrease in extractable P after21 days incubation (86% of total P addition). Del Campillo et al. (1999) suggested a valueof 0.44 kg P ha−1 year−1 (1 kg P2O5 ha−1 year−1), equivalent to the P exported by 300 mmof drainage water containing 0.14 mg P l−1, as an environmentally acceptable value of Pleaching. The amount of P leached with the different CaCl2 solutions in the present study(Table 1) is, however, greater than the suggested value. The relatively low leaching of Pin 15 mM CaCl2 is likely to be due to the high concentration of soluble Ca2+ used in thesolutions. In calcareous soils, precipitation of insoluble Ca-P is supposed to be a majorfactor in the time loss of P availability (Freeman and Rowell, 1981).

The leaching data for the soil leached with different CaCl2 solutions were plottedas saturation indices of several common Ca-P minerals. It has been shown that in soilswith high amounts of exchangeable Ca2+, insoluble Ca-P precipitates control P availability(Tunesi et al., 1999). These minerals play a significant role in the overall leaching of P and,therefore, it is important to investigate leaching behavior with respect to these minerals. Atthe beginning of the leaching experiments, for the 3, 5, and 10 mM CaCl2, the leachates areundersaturated with respect to ß-tricalcium phosphate (ß-TCP) (Figure 2), while at the endof leaching experiments, the leachates in 5 and 10 mM CaCl2 solutions are saturated withrespect to ß-TCP. In 15 mM CaCl2 solution, the leachates are saturated with respect of ß-TCP through the leaching experiment (SI varied from 0.15 to 0.80) (Figure 2). The leachatesin all CaCl2 leaching solutions were oversaturated with respect to calcium hydroxyapatite(HY) through the entire leaching experiments (SI varied from 4.85 to 8.57 for differentCaCl2 solutions) (Figure 2). Similar to our results, Robbins et al. (1999) and Hansen andStrawn (2003) observed that manure-amended calcareous soils had aqueous Ca2+ and Pconcentrations between the solubility of the minerals octa calcium phosphate and ß-TCP.Thus, it suggested that calcium HY and ß-TCP minerals are controlling the equilibriumCa2+ and P solution concentrations through the leaching experiments.

Although leachate P from topsoil columns exceeded water quality guidelines, this doesnot mean that similar P concentrations will be found in water percolating into shallowgroundwaters. Other factors, such as P sorption by subsoils, can decrease P concentrations

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Figure 2. Calculated MINTEQA2 saturation indices (SI) for ß-TCP and hydroxyapatite that controlP concentration in the initial (i) and final (f) leachates for different CaCl2 leaching solutions.

during subsurface transport. Also, direct extrapolation from effluent concentrations ob-served from laboratory columns to field conditions is questionable as variation may occurwith respect to soil depth, number of preferential pathways, adsorption/desorption anddissolution/precipitation. In the field, dissolution of CaCO3 is probably one source ofthe released Ca2+. Weatherable silicate plagioclase feldspars are also a major source ofCa2+ and Mg2+. Agricultural activity is associated with elevated levels of Ca2+ and Mg2+

(Wayland et al., 2003). They reported increasing dissolution of soil minerals, such as cal-cite, dolomite, and potassium feldspar, during soil cultivation. Mineralization of organicN to nitrate, and weathering of minerals, will maintain the concentration of Ca2+ in soilsolution. Therefore, in the field conditions it is likely that, increased Ca2+ concentrationsmay reduce P leaching.

In addition, the column leaching experiments were conducted under saturated flow,which is different from unsaturated flow normally occuring in field situation. The leachingrate was faster than that in the field; only about 2.2–50.5 h was required for 5 pore volumesto pass through the column. This was not sufficiently slow to ensure equilibration (the soilis sandy loam, and the same flow rate can be expected to occur in the field).

Kinetics of P Desorption

Phosphorous desorption by successive extraction with different CaCl2 solutions was shownin Figure 3. The P desorption during six successively extractions with different CaCl2solutions varied from 18 mg kg−1 in 5 mM CaCl2 to 29 mg kg−1 in 10 mM CaCl2 solution.Comparison of the Olsen P result with the kinetic desorption data indicates that the Olsen P

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0

5

10

15

20

25

30

35

0 20 40 60 80 100

Time (min)

Cu

mu

lati

ve P

rel

ease

d (

mg

kg

-1)

3 mM CaCl2

5 mM CaCl2

10 mM CaCl2

15 mM CaCl2

Figure 3. Cumulative amounts of P released by time in different CaCl2 soultions. Error bars representone standard deviation either side of the mean.

extraction removed on average 1.56 times more P than cumulative P desorbed by successiveextractions with different CaCl2 solutions. Shariatmadari et al. (2006) studied releasekinetics and availability in calcareous soils of selected arid and semi-arid toposequences inIran. They found that the P released during 72 h extractions with 10 mM CaCl2 varied from9 to 42 mg kg−1. Despite low desorption, at the end of the six successive extractions withdifferent CaCl2 solutions, the average solution P maintained by soil (0.18 mg P l−1) wasmore than the critical level of 0.1 mg P l−1. Rate of P release can be related to their mobilityand availability. Kinetic of P desorbed can provide useful information for assessing the riskof soil P as the potential sources of eutrophication in aquatic systems.

Different kinetic models were used to describe desorption of P from the soil(Table 2). A linear fit to the parabolic diffusion model indicates that desorption of P isa diffusion controlled process (Jardine and Sparks, 1984). The P desorption rates rangedfrom 2.8 to 4.7 mg kg−1 h−1/2 (Table 2). The fit of the data to power equation yielded astraight line. The fit between the Elovich model and experimental data is reflected by thecoefficients of determination shown in Table 3. The constant b represents the slope and canbe used to compare the general desorbability of different treatments which ranged from 2.4to 2.7 mg kg−1 h−1. The difference between b values indicates that ease of desorption of Pfrom soil in presence of different CaCl2 solutions is different. Similar results were obtainedby Chien and Clayton (1980) and Hansen and Strawn (2003), who observed that P releasedata was successfully described by the Elovich equation. Also, the cumulative P was fittedto the first order model. Although all these models described the P release kinetics, compar-isons of r and SE values (Table 2) indicated that the power function and parabolic diffusionbest represented the cumulative release of P. Similar results were obtained by Shariatmadariet al. (2006), who observed that P release data was successfully described by the Elovichequation, power function and parabolic diffusion. Since both parabolic diffusion and powerfunction models describe the rate process, then the latter may also represent slow diffusionof P from soil (Havlin et al., 1985).

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Tabl

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990.

050.

112.

70.

968.

712.

8−9

.40.

980.

81−0

.03

3.3

0.97

0.12

101.

36−2

.60.

990.

110.

062.

70.

9716

.00

4.73

−16.

80.

990.

71−0

.03

3.8

0.97

0.12

150.

93−1

.08

0.98

0.10

0.10

2.4

0.98

12.2

03.

04−7

.70.

990.

65−0

.03

3.3

0.96

0.14

585

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Table 3Phosphorous isotherm parameters and proportion of Olsen P in solution

Extraction PBC Equilibroummedia [(mg kg−1)/ KF P concentration(mM CaCl2) (log10 mg l−1)] n (l kg−1) (mg l−1)

3 166 0.710 10.5 0.965 188 0.719 14.7 0.85

10 206 0.728 19.0 0.7315 224 0.699 15.1 1.05

Sorption Isotherms

Phosphorous sorption was determined to understand the transport and leaching of P in thesandy soil. Figure 4 shows P exchange isotherm for soil with different CaCl2 concentrations.It is clear from the isotherms that with increasing Ca2+ concentration from 3 to 15 mM,more P was held on soil. The lower sorption in low CaCl2 concentrations maintains highersolution concentrations of P. The adsorption rate was high for small concentrations of addedP. At a rate of 10 mg l−1, the amounts of P adsorbed were between 62 to 76% of added P. Atthe high P rate (50 mg l−1), however, the amounts of P adsorbed were 73 to 80% of addedP. This shows that a great proportion of the added P is adsorbed at high P concentrations.The release of P from soil (not P-treated) ranged from 1.9 to 2.8 mg kg−1. Equilibrium Pconcentration of the soils (i.e. no net desorption or sorption) ranges from 0.73 to 1.05 mgl−1 with an average value of 0.90 mg l−1 (Table 3).

The Freundlich equation adequately described sorption isotherms. The coefficients ofdetermination ranged from 0.97 to 0.99. There have been reported successful representation

-50

0

50

100

150

200

250

300

350

400

450

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Equilibrium concentration of P (mg l-1)

Ad

so

rped

P (

mg

kg

-1)

3 mM CaCl2

5 mM CaCl2

10 mM CaCl2

15 mM CaCl2

Figure 4. Phosphorous isotherms for the soil with a range of CaCl2 concentration. Error bars repre-sent one standard deviation either side of the mean.

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of P sorption by soils with the Freundlich equation (Pal et al., 1999; Bertrand et al., 2003).The Freundlich constants KF and n, which represent the intercept and slope of the log-transformed sorption isotherm, may be taken as measures of the extent of adsorption andindicator of the homogeneity of the sorption sites, respectively. The exponent term n in theFreundlich relationship describing solid phase P and solution P at equilibrium in most soilsare less than 1 and the Freundlich coefficient KF ranged from 10.5 to 19.0 (Table 3).

The PBC calculated for each CaCl2 solution from the slope of the line obtained byplotting P sorbed against log Ce ranges from 166 to 224 l kg−1 (Table 3). Phosphorousbuffer capacities of soil in different CaCl2 solutions were classified as high according to theclassification of Moody and Bolland (1999). Phosphorous buffer capacity is the capacityof the soil to resist to a change of the soil solution P concentration, following an output ofP from (or an input of P into) the soil (Rowell, 1994). Phosphorous buffering capacitiescan be related to both plant nutrition and environmental pollution. In general, fine-texturedsoils have higher PBC. Therefore, a high PBC is indicative of the continuing availabilityof adequate P over a long period of cropping, whereas a low PBC indicates the need forfrequent fertilization. In addition, the greater PBC of the soil, the higher the P rate requiredto increase P concentration in soil solution. In comparison with 15 mM CaCl2, the 3 mMCaCl2 solution appeared to have a lower capacity to retain P (166 l kg−1) and the tendencyfor P to be lost by leaching is greater. Thus, the application of P fertilizers to most sandysoils with low clay content and small PBC, in which P does not interact strongly with the soilmatrix results in localized increase of P concentration in the soil solution and subsequentlyP will be leached by rainfall or irrigation water.

Conclusions

Application of solutions having different Ca2+ concentrations to a sandy loam soil indicatedthat leaching of P is initially rapid, followed by a slow release. Phosphorus leaching fromsoil is attributable to several different processes with soil solution concentration of P beingcontrolled primarily by less soluble P minerals such as HY and ß-TCP. The kinetic modelparameters and leaching P losses can be used in transport models to predict P loss fromsoil, which, in turn, can be used to identify areas where P applications in fertilizers andmanures should be controlled to protect water quality.

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effect of irrigation water quality on the leaching and desorption of phosphorous from soil

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