The Freundlich adsorption isotherm constants and prediction of phosphorus bioavailability as affected by different phosphorus sources in two Kansas soils

Chemosphere 99 (2014) 72–80

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The Freundlich adsorption isotherm constants and prediction of phosphorus bioavailability as affected by different phosphorus sources in two Kansas soils Mustafa N. Shafqat a,b,⇑, Gary M. Pierzynski b a b

Department of Biosciences, COMSATS Institute of Information Technology, Park Road, Chak Shahzad, Islamabad, Pakistan Department of Agronomy, Kansas State University, Manhattan, KS 66506, USA

h i g h l i g h t s  Freundlich constants were used for predicting corn growth parameters.  All P sources reduced levels of Freundlich K, increased 1/n and EPC0 in both soils.  Hog manure had smaller values of Freundlich K in the P deficient Eram-Lebo soil.  Corn biomass, tissue P and P uptake were increased by P sources in both soils.  Freundlich constants had relationships with agronomic parameters in low P soil.

a r t i c l e

i n f o

Article history: Received 6 May 2013 Received in revised form 29 September 2013 Accepted 4 October 2013 Available online 13 November 2013 Keywords: Freundlich adsorption coefficients Animal manure phosphorus Soil phosphorus bioavailability Corn phosphorus uptake

a b s t r a c t Phosphorus (P) adsorption onto soil constituents influences P bioavailability from both agronomic and environmental perspectives. In this study, the P availability from different P sources along with utility of Freundlich adsorption coefficients on the predictability of various crop growth parameters were assessed. Two soils were amended with 150 mg P kg 1 each from six different P sources comprised of manures from two types of ruminants animals, three types of monogastric animals, and inorganic P fertilizer. Corn (Zea mays) was grown and harvested seven times under greenhouse conditions to remove P from the P amended treatments. The application of all P sources reduced the value of Freundlich K and increased the value of Freundlich 1/n and equilibrium P concentration (EPC0) in both soils compared to the un-amended control before cropping. The swine (Sus scrofa) manure (HM) resulted in significant smaller values of Freundlich K and larger values of 1/n in the P deficient Eram-Lebo soil compared to other P sources while, the opposite was true for the turkey (Meleagris gallopava) litter (TL) in the Ulysses soil. The corn biomass, tissue P concentration and P uptake were significantly influenced by all P sources during the first harvest and the total P uptake during seven harvests in both soils compared to the control treatment. Both Freundlich coefficients had strong relationships with the aforementioned corn parameters in the P deficient Eram-Lebo soil while, strength of the association was weak or missing in the Ulysses soil which had optimum levels of antecedent P. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Phosphorus (P) adsorption to the soil surfaces is an important reaction that controls P bioavailability. Silicate clays; aluminum, iron, and manganese oxides; and calcium carbonate (CaCO3) are involved in surface P adsorption (Singh and Gilkes, 1991; Nair et al., 1999; Samadi and Gilkes, 1999). Other soil factors such as pH, soil organic matter content (SOM), moisture, temperature and contact ⇑ Corresponding author at: Department of Biosciences, COMSATS Institute of Information Technology, Park Road, Chak Shahzad, Islamabad, Pakistan. Tel./fax: +92 51 8318471. E-mail address: [email protected] (M.N. Shafqat). 0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.10.009

time between P and soil constituents also play a significant role in controlling surface adsorption reactions (Sharpley and Ahuja, 1982; Barrow, 1984; Hue, 1991). The most commonly used mathematical models which fit adsorption data are the Langmuir and Freundlich equations. The parameters of these equations do not have any mechanistic significance but still have practical utility in comparing P retention in different soils (Sposito, 2008). Another utility of the P adsorption isotherm is that it provides an estimate of the equilibrium P concentration (EPC0). This is the P concentration in soil solution with no net adsorption or desorption (Sharpley et al., 1994). Numerous studies have indicated that animal waste and biosolids application increases the EPC0, and decreases the

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affinity of soil constituents for P adsorption (binding intensities) resulting in soils having less ability to sorb additional added P and maintain higher soil solution P concentrations (Sui and Thompson, 2000; Jiao et al., 2007). Numerous factors govern the crop P bioavailability in soils amended with animal manures and biosolids. The production of organic acids in soils during microbial decomposition of the carbon-rich soil amendments also interacts with adsorbing surfaces and may also act as chelating agents and thus increase the solubility of P in soil (Inskeep and Silvertooth, 1988; Bermudez et al., 1993). Pierzynski et al. (1990) found that poorly crystallized secondary P precipitates controlled P solubility in historically waste amended soils. Soil properties that influence P bioavailability include the amount and type of clay, pH, redox potential, initial soil test phosphorus (STP), SOM, microbial number and types, the presence of Al and Fe oxide surfaces under acidic conditions, and the amount of CaCO3 in alkaline soils (Sanyal and Datta, 1991; Stevenson and Cole, 1999; Zheng et al., 2003; Sposito, 2008). The total P content of the animal manures depends upon the age of the animal, diet, digestive system, and waste processing methods (Barnett, 1994; Poulsen, 2000; Dou et al., 2002). Young animals might require more supplementary P in the form of feed additives to meet the demands for their rapid body growth and therefore would also secrete more P in their manures (Knowlton et al., 2004). Unlike ruminants, monogastric animals such as swine and turkey lack phosphatase and phytase enzymes necessary for the digestion of organic P (Po) in the forages and thus are dependent on supplementary inorganic P (Pi) additions in the form of salts in their diets and consequently would also excrete substantial amounts of P in their manures (Maguire et al., 2003). Waste handling and processing methods such as the addition of bedding materials, liming materials, composting, and chemical treatments such as the addition of oxides and hydroxides of aluminum and iron are common practice and might decrease waste volume and convert P to less soluble forms that would ultimately lead to reduced degradation of surface waters after land application (Hinedi et al., 1989; Maguire et al., 2001; Kalbasi and Karthikeyan, 2004; Hunger et al., 2004; Seiter et al., 2008). Most animal P sources are currently disposed of on the assumption that P contained therein would behave similarly once added to soil. Therefore, a greenhouse study could offer an opportunity to study the influence of variations in the nature of P and other characteristics in animal manures on soil P adsorption and consequently its effects on corn P uptake. In this study we tested the following hypotheses: (A) The magnitude of P adsorption would be similar for all P sources in both soils both before and after crop P removal; (B) There would be no difference between P sources from ruminant and monogastric animals on corn biomass, leaf tissue P concentration, and P uptake; (C) The Freundlich adsorption coefficients would have similar predictability of various corn growth parameters in both soils.

2. Materials and methods 2.1. The collection of soils and P sources The six P sources consisted of two types of ruminants waste from cattle (Bos taurus) manures (CM1 and CM2), three types of monogastric wastes comprised of HM, TL, and biosolids (SS) collected from various parts of the Kansas. Triple super phosphate (TSP) was also included as inorganic P source for comparative purposes. Both cattle manures were collected from field stockpiles, fresh HM and TL was collected from nearby farms. The TL had substantial quantities of the bedding material mixed with it. Lastly, the solid SS was collected from a wastewater treatment facility which used an aerobic digestion process. Selected chemical characteristics of organic P (Po) sources are presented in Table 1. The two soils belonged to Eram-Lebo clay loam (fine, mixed, active, thermic Aquic Argiudolls) and Ulysses silt loam (fine-silty, mixed, superactive, mesic Aridic Haplustolls) with initial Bray1P levels, soil pH, and SOM were 4.5 and 23 mg P kg 1, 6.2 and 7.7 g kg 1, 28 and 16 g kg 1 , respectively. The soil texture was silty clay and loam for the Eram-Lebo and Ulysses soil samples, respectively. The Po sources and soil samples were air dried, ground, and sieved through <2 mm sieve and stored at room temperature before use in the laboratory. 2.2. Greenhouse study Briefly, two kg of each soil was amended with 150 mg P kg 1 from each of the six P sources and placed into the plastic pot lined with plastic bags and incubated at 200 g kg 1 moisture content overnight. The control treatment, with no P added, was also prepared in like manner for both soils. The experimental design was randomized complete block with three replications. Soil samples (50 g) were collected from each pot prior to the first planting of corn and were designated as T0. Subsequently, ten seeds for corn hybrid (Pioneer-4662 developed by DuPont Pioneer, Johnston, Iowa, USA) were sown and thinned to six plants per pot after 10 days of germination. Nitrogen and potassium (K) were applied at 150 mg N kg 1 and 60 mg K kg 1 soil. The N was applied from urea in three equal splits, at sowing, and 10 and 20 days after emergence while all K was applied in the form of potassium sulfate at the time of sowing. All pots were brought to 20% gravimetric moisture content twice weekly and an equal volume of water was added to each pot in between weighings. Sixteen h of supplementary light was maintained throughout the study. Corn was harvested 35 days after sowing during each crop cycle. A total of seven harvests were collected during the entire study with N and K added each time as described above. Soil samples collected both before the first crop was planted and after the seventh harvest (T7) were subsequently air dried and ground to pass less than 2 mm sieve and stored at room temperature before analysis. Only the above

Table 1 Selected characteristics of organic phosphorus sources used in the study. P sources

Total Pa

Labile Pib mg kg

CM1 CM2 TL HM SS a

8800 3800 21000 17000 27000

Labile Poc

1

Humic Poc mg kg

4178 2021 5325 9009 7597

(47)d (53) (25) (53) (28)

109 210 683 179 915

(1.2)d (5.5) (3.3) (1.1) (3.4)

Total Ca

1

Total Na %

486 (5.5)d 232 (6.1) 321 (1.5) 736 (4.3) 2951(10.9)

23.8 7.7 27.9 40.0 31.1

1.98 0.77 1.71 4.51 5.02

From Shafqat and Pierzynski (2011). Labile Pi pools consisted of sum of 0.1 M CaCl2 and 0.5 M NaHCO3-P extracted sequentially and data was taken from Shafqat and Pierzynski (2011). Labile Po pool was determined in 0.5 M NaHCO3 extraction while humic-Po was determined in 0.1 M NaOH pools in sequential order. d Values inside parentheses indicate % of respective fraction of the total P. Abbreviations Pi = inorganic P, Po = organic P, C = carbon, N = nitrogen, CM1 = cattle manure 1, CM2 = cattle manure 2, TL = turkey litter, HM = swine manure, SS = biosolid. b

c

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ground portion of the corn was harvested while the roots were remained in the pots to simulate field soil conditions. Mineralization of Po contained in roots might contribute bioavailable P to each crop in the cycle with exception to the first but that would be constant for all treatments in the study and still enable us to monitor bioavailable P for different P source treatments. With exception to the corn planted after the first harvest, the soils were thoroughly mixed before planting for the remaining crops. Plant samples were briefly rinsed in distilled water, blotted dry, and put into the oven at 60 °C and dried to a constant mass. The complete description of experiment is available elsewhere (Shafqat and Pierzynski, 2011).

and time was included as the subplot factor to allow for the determination of the effect of time for a given P source and adsorption parameter. The data regarding total corn P uptake for the sum of all seven harvests and after the first harvest were analyzed using a factorial treatment structure comprised of two levels of soil and 7 P source treatments (2  7) in a randomized complete block design. In the case of significant interaction effects between soil and P sources, simple effects were monitored and means were separated using LSD procedures at a = 0.05, otherwise the main effect of soils and P sources are discussed.

3. Results and discussion 2.3. Soil and plant analysis The ground samples (0.25 g) of corn biomass were digested with concentrated H2SO4 and H2O2 and subsequently run on inductively coupled plasma emission spectrometer (ICP) for the determination of P concentration and subsequent calculation of corn P uptake. Corn P uptake in each harvest was also pooled for all seven harvests to determine total corn P uptake in both soils and expressed in units of mg P kg 1 soil. This value of corn P uptake would be one half of original total corn P uptake per pot which had 2 kg soil. Justification of expressing total uptake in this unit was to equate it to the units of P dose application which was 150 mg P kg 1 soil for each of the P source. 2.4. Phosphorus adsorption isotherms A 1.5-g air dried sample was weighed into a 50-mL centrifuge tube with a screw cap and equilibrated with 30-mL of one of nine P solutions having 0, 0.05, 0.1, 0.2, 0.4, 0.8, 1.2, 1.8, or 2.6 mM P. Phosphorus solutions were prepared by dissolving monobasic potassium phosphate in 0.01 M CaCl2. To inhibit the microbial activity in each tube, three drops of chloroform were added. The tubes were shaken for 24 h for equilibration purposes and subsequently were centrifuged at 5400g for 15 min and the supernatant was collected for P determination. The amount of P sorbed was calculated from the difference between the initial solution P concentration and the concentration of P in the equilibrium solution. The P concentration in the extracts was determined by using the method of Murphy and Riley (1962) with absorption measured using a spectrophotometer at a wavelength at 880 nm. The sorption data were fit to the Freundlich model (A = KC1/n), because it produced the least residual sum of squares compared to the simple and two surface Langmuir equations. In the Freundlich model, A represents the amount of P sorbed by the solid phase (mg kg–1); C is the solution P concentration (mg L 1) after equilibration, while K and 1/n are the Freundlich constants. The Freundlich K refers to the ratio of P adsorbed to P in the soil solution, while 1/n describes the non-linearity of the adsorption curve (Schwarzenbach et al., 1993). The EPC0 was computed from P adsorption isotherm when it dissected the x-axis containing aforementioned equilibrium P solutions. So this was P concentration where neither adsorption nor desorption was taking place. 2.5. Statistical analysis Analysis of variance was done using Proc GLM in SAS (SAS, 2009) at each harvest for total corn biomass, corn tissue P concentration, and P uptake. The means were separated at a = 0.05 by using the least significant difference (LSD) procedure. Similarly, the data from the P adsorption study was statistically analyzed for each soil separately and means were separated at a = 0.05 using LSD. A split plot design was used to study the time effect in adsorption studies. Phosphorus source was used as the main plot factor

3.1. Phosphorus source effects on P adsorption before (T0) and after (T7) crop P removal The P adsorption isotherm for selected P sources in the Ulysses soil is presented in Fig. 1a. The application of all P sources at the level of 150 mg P kg–1 significantly decreased the Freundlich K values when compared to the control treatment in the Ulysses soil (Table 2). The lowest K values were found for CM1, HM, and TSP, while TL produced the highest K value, which was significantly different from all treatments except that of CM2. After crop P removal (T7), the Freundlich K value significantly increased in all treatments for the Ulysses soil compared to T0 (Table 2). Triple super phosphate produced the highest Freundlich K value at T7 and was significantly different from all other treatments in the study. All P sources in the Ulysses soil significantly increased the 1/n value compared to the control treatment at T0 (Table 2). Cattle manure 1 produced the highest value which was significantly higher than the control, CM2, TL and SS treatments. Crop P removal decreased the value of 1/n in all treatments although significant differences between treatments were not found (Table 2). The Freundlich equation is widely used in P adsorption studies. Van Bladel and Moreale (1977) define Freundlich K and 1/n constants as adsorption capacity and intensity respectively. Holford (1982) suggested that the reciprocal of the Freundlich constant (1/n) showed poor correlation with P uptake in plants. He was in favor of using Freundlich K as both capacity and affinity parameter. However, Schwarzenbach et al. (1993) defined Freundlich K as ratio of the amount of substance adsorbed to that in solution, provided the Freundlich exponent is equal to 1. If the Freundlich exponent (1/ n) were less than 1, it would indicate that as sorbate concentrations increased, sorption of additional molecules on the solid surface would become more difficult. The application of all P sources significantly increased EPC0 compared to the control treatment for the Ulysses soil (Table 2). The highest EPC0 was produced by TSP, which was significantly different from all other treatments. The smallest increase in EPC0 was observed with TL, which was also significantly different from all other P treatments as well as from the control. The threshold level of solution P for eutrophication is 0.03 mg L 1 (Pierzynski et al., 2004). Therefore, the application of all P sources produced EPC0 values above the threshold level in the Ulysses soil where this P could be transported as soluble P to nearby streams and might contribute eutrophication. The effects of P source on the Freundlich K and 1/n parameters were similar for the Eram-Lebo soil (Table 2) as observed for the Ulysses soil, except that much higher Freundlich K and smaller 1/ n values were observed for this P deficient soil. This suggests that more P was sorbed and less was present in the soil solution. Further, much lower EPC0 values were also found in this soil (Table 2). The values of EPC0 were significantly higher for all treatments receiving P as compared to the control at T0 but without having any environmental concerns in this P deficient soil. The TSP

75

500

(a)

-1

P adsorbed (mg kg )

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400 300 Control HM TSP Control HM TSP

200 100 0 -100 0

10

20

30

40

50

60

70

-1

Equilibrium P concentration (mg L ) 100 Coefficients: b[0]= 294.2 p=0.0026 b[1]= -1.29 p=0.0075 r ² = 0.79

P uptake (mg P kg-1 )

80

(b)

60

40

20

Coefficients: b[0] = 124.3 p = 0.0013 b[1] = -0.59 p= 0.0029 r ² = 0.85

0

150

160

Harvest 1 During seven harvest linear regression

170

180

190

200

210

Freundlich K Fig. 1. (a) Phosphorus adsorption isotherm for selected P sources in the Ulysses soil. (b) The relationship between Freundlich constant (K) and P uptake in the Eram-Lebo soil. Abbreviations, HM = swine manure, TSP = triple super phosphate.

Table 2 Influence of P sources on Freundlich constants and on Equilibrium phosphorus concentration (EPC0) both prior to crop P removal (T0) and after seventh harvest (T7) in the Ulysses and Eram-Lebo soil. Treatment

T0

T7

T0 vs. T7

T0

T7

Freundlich K

T0 vs. T7

T0

T7 EPC0 (lg L

Freundlich 1/n

T0 vs. T7 1

)

Ulysses soil Control CM1 CM2 TL HM SS TSP

a

45 a 25 d 32 bc 33 b 26 d 30 c 26 d

51 50 52 53 49 55 60

c c bc bc c b a

* * * * * * *

0.56 0.73 0.65 0.63 0.70 0.65 0.72

0.52 NSb 0.56 0.54 0.53 0.55 0.53 0.53

c a b b a b a

* * * * * * *

27 e 420 c 350 c 210 d 550 b 390 c 2070 a

EPC0 (lg L

Eram-Lebo soil Control CM1 CM2 TL HM SS TSP

204 173 184 174 156 180 166

a cd b bcd e bc ed

251 226 243 224 219 224 214

a b a b bc b c

NS

0.35 0.40 0.39 0.37 0.42 0.38 0.40

e bc bcd d a cd b

0.33 0.35 0.36 0.34 0.36 0.35 0.37

d b b c b b a

* * * * * * *

0.0 d 12.3 bc 2.46 cd 1.16 d 13.9 b 1.53 cd 25.2 a

*

0.0 c 17 a 14 ab 6.0 bc 19 a 18 a 1.0 c

0.0 NS 0.34 0.15 0.0 0.21 0.0 1.10

* * * * * * 1

) NS *

NS NS *

NS *

a

Means with same letter within column in a given soil are not significantly different at P < 0.05. NS = non significant. Significant difference was seen for a given P adsorption parameter in a given P source treatment when both times were compared. Abbreviations CM1 = cattle manure 1, CM2 = cattle manure 2, TL = turkey litter, HM = swine manure, SS = biosolid, TSP = triple superphosphate. b *

produced the highest EPC0 but the concentration was much lower than the Ulysses soil. At T7, no significant treatment effects were found. The EPC0 decreased significantly with cropping for the CM1, HM, and TSP treatments. The affinity of soil to sorb P was decreased by all P sources prior to crop P removal and one could expect more P in soil solution

compared to the control treatment in both soils. Goyne et al. (2008) explained the role of competition between dissolved organic matter and P for sorption sites and this was responsible for higher soil solution P concentrations. In their study, soil P sorption was positively correlated with clay content and negatively correlated with Bray 1 extractable P. These findings are similar in this study

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where the Eram-Lebo soil, with low initial P levels and high clay content, had more adsorption and lower solution P concentrations compared to the Ulysses soil. Large Freundlich K values after the seventh harvest in both soils suggested that crop P removal significantly decreased the solution P concentrations and, thus, the soil solid phases had a greater affinity for P. Increased K and decreased 1/n after cropping reflects the availability of high energy binding sites after crop P uptake in both soils. Sui and Thompson (2000) also suggested that the application of biosolids decreased the P binding intensities at both high and low binding sites when P adsorption data was fit to two surface Langmuir equation. In both soils, HM, CM1 and TSP produced some of the highest values of 1/n and lowest K values compared to the other P sources suggesting significantly more soil solution P concentrations. This could partly be attributed to the presence of large proportions of their total P in labile inorganic P pool which were 53% and 47% for the HM and CM1 respectively plus having large amounts of total C (40% and 24%, respectively) (Table 1). The influence of total C content was more conspicuous in case of CM2 which had large proportion of labile P (53%) but significantly more adsorption than the aforementioned treatments owing to lowest amount of total C (7.7%). The influence of carbon was more pronounced in P deficient Eram-Lebo soil where larger competition from the soluble organics with the labile P resulted in less P adsorption and consequently more soil solution P, even higher than the TSP treatment. Despite the competing effects of soluble organic compounds which might be furnished by these P sources, it seemed that labile inorganic P concentration was much more useful in explaining the adsorption behavior in both soils with CM2 as the only exception. Among the soil characteristics, clay content, initial levels of STP and SOM

might had bigger role in explaining in variation seen in much larger values of Freundlich K and smaller values of 1/n in the Eram-Lebo soil compared to the Ulysses soil. The clay content, STP and SOM levels were 400 and 190 g kg 1, 4.5 and 23 mg P kg 1, and 28 and 16 g kg 1 for the Eram-Lebo and Ulysses soil respectively. High clay content (400 g kg 1) in Eram-Lebo soil might offer much larger surface area for P adsorption reactions along with low concentrations of antecedent P (4.5 mg kg 1) would suggest that most P adsorption sites might be empty. Similarly, high SOM content of Eram-Lebo soil plus soluble organic compounds added through animal P sources might also alter the ratio of Pi/Po which strongly affected the P adsorption behavior compared with the response in the Ulysses soil. Moreover, high soil pH of Ulysses soil (7.7) likely contribute more OH concentrations which might also be competing with orthophosphate ions for surface adsorption reactions which might be less important in slightly acidic EramLebo soil (soil pH 6.2). The HM and TSP produced significantly higher EPC0 than other P sources in both soils. However, the magnitude was many times less in the high clay content, P deficient Eram-Lebo soil, than in the Ulysses soil owing to greater adsorption of P in the former soil. These results are in agreement to those reported by Sui and Thompson (2000). 3.2. Phosphorus source effects on corn biomass, tissue P concentration, and P uptake during seven harvests in the Eram-Lebo soil Application of all P sources resulted in significant increases in corn biomass compared to the control treatment (Fig. 2a). However, large increases in biomass were seen for the first four

(a)

(b)

(c)

(d)

Fig. 2. (a) Influence of P sources on corn biomass (g pot 1) in Eram-Lebo soil. (b) Influence of P sources on corn biomass (g pot 1) in the Ulysses soil. (c) Influence of P sources on corn tissue P concentration (g P kg 1) in the Eram-Lebo soil. (d) Influence of P sources on corn tissue P concentration (g P kg 1) in the Ulysses soil. Hypotheses of non significant difference between various P source treatments in a given harvest were rejected at p < 0.05. The difference between any two treatments mean larger than least significant difference (LSD) value in a given harvest would be considered as significantly different from each other. Abbreviations, ns = non-significant, CM1 = cattle manure 1, CM2 = cattle manure 2, TL = turkey litter, HM = swine manure, SS = biosolid, TSP = triple super phosphate. The number 1 through 7 on x-axis represents corn harvest with each 35 days after sowing.

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harvests only. The HM produced the highest corn biomass of 20 g pot 1 which was significantly different from all other treatments after harvest 1. The lowest yield of biomass was 10.6 g pot 1 in SS treatment after the same harvest which was statistically similar to the TSP treatment (11.4 g pot 1) but significantly different from all other treatments. Both cattle manures resulted in identical biomass (15.1 g pot 1) during the first harvest and were significantly different from all other treatments but statistically similar to the TL (13.5 g pot 1). Contrary to the first harvest, SS resulted in the highest corn biomass of 17.7 g pot 1 during the second harvest and was significantly different from all treatments while TL, TSP, HM, and CM2 produced the lowest amounts of biomass ranging from 9.9 (TL) to 12.7 g pot 1 (CM2) with no significant differences between them. All P sources resulted in near identical corn biomass after the third harvests, though a significant difference was seen from harvest 4 through 6 in this experiment. However, large decreases in biomass were monitored (up to 50%) for all treatments when we moved from harvest 4 to 5 suggesting that P availability might have become a limiting factor for the growth of these plants. Large effects of P sources on corn biomass were only seen during the first two harvests that also correlated well with data on corn tissue P concentrations (Fig. 2c). All P sources resulted in significant increases in tissue P concentration after harvest 1 and 2 with SS being the only exception after harvest 2 where it was non-significantly different with the control treatment. Among the P sources, the highest tissue P concentration was 4.6 g kg 1 in the TSP treatment and significantly different from all other P sources after harvest 1. However, among the animal P sources, HM had the highest P concentrations of 3.5 g kg 1 which was significantly higher than in SS and CM1 which had 2.8 and 2.7 g kg 1, respectively. The relative differences were different after the second harvest where HM had the highest concentrations at 3.3 g kg 1 and that was significantly different from CM2 and SS which had 2.5 and 1.7 g kg 1, respectively. Previously, Barber (1979) also observed that most crop plants, when grown in soils with plenty of bioavailable P, would likely show luxury consumption. A sudden drop in P concentration was noticed (up to 50%) after harvest 3 relative to harvest 2 for the most treatments with exception from SS and the control. We did not detect any differences in treatments between harvest 3 and 5 in the study. The basic trends in corn P uptake were similar to corn biomass (Figs. 2a and 3a). The maximum corn P uptake was seen during the first harvest and it steadily decreased throughout the successive harvests with a large abrupt decrease after the 4th harvest. Moreover, larger effects of P sources on crop P uptake were only seen

(a)

during the first two harvests, in agreement with the results on corn biomass and tissue P concentrations. However, HM resulted in the highest corn P uptake of 35.2 mg P kg 1 and SS with the lowest amount of 14.8 mg kg 1 after the first harvest and were significantly different from each other and from all other treatments. The P sources from ruminants had similar corn P uptake while significant differences were found between the monogastric P sources with HM as the highest, followed by TL and SS. The HM was the only treatment that had P uptake which was significantly better than TSP after the first harvest, while CM2 was similar to TSP and rest of the P sources having significantly less P uptake than the TSP treatment. Most P sources performed near identically after harvest 2. The only exception was TSP which had some of the lowest P uptake (12.5 mg P kg 1). Most P sources had similar P uptake with significantly more P uptake than the control treatment during the remaining harvests. 3.3. Phosphorus source effects on corn biomass, tissue P concentration, and on P uptake during seven harvests in the Ulysses soil Application of all P sources at 150 mg P kg 1 resulted in significant increase in corn biomass compared to the control treatment in each harvest (Fig. 2b). The highest corn biomass was 22 g pot 1 in the HM treatment after harvest 1 which was significantly different from all other treatments, while the lowest biomass was 14 g pot 1 in the TSP treatment, followed by CM1 and TL which produced 16 g pot 1 and were statistically similar to each other but significantly different from all other treatments. A sharp decrease in corn biomass was seen during the second harvest for all the treatments, similar to the Eram-Lebo soil. This decrease in biomass after harvest 2 could possibly be related to the fact that the soil was not mixed prior to sowing of the second crop. Soil settling might have caused an increase in bulk density resulting in clogging of soil pores and thus poor soil aeration. The effects were more pronounced in Ulysses soil as it had less SOM content (16 g kg 1) when compared to the Eram-Lebo soil (28 g kg 1). Further, this effect was more pronounced for TSP and in the control treatments for both soils than in pots which received Po treatments. The addition of organic C with P might have maintained more favorable soil physical properties for plant growth. However, from the second harvest onwards, this problem was alleviated by mixing/homogenization of soils in all pots prior to sowing of next crop. Both cattle manures (CM1 and CM2) along with HM and SS treatment produced identical corn biomass from harvest 3 to 7. However, TL and TSP treatments had significantly lower biomass than the other P sources and were statistical similar to each other.

(b)

Fig. 3. (a) Influence of P sources on corn P uptake/removed (mg P kg 1) in the Eram-Lebo soil. (b) Influence of P sources on corn P uptake/removed (mg P kg 1) in the Ulysses soil. Hypotheses of non significant difference between various treatments in a given harvest were rejected at p < 0.05. The difference between any two treatments mean larger than least significant difference (LSD) value in a given harvest would be considered as significantly different from each other. Abbreviations, ns = non-significant, CM1 = cattle manure 1, CM2 = cattle manure 2, TL = turkey litter, HM = swine manure, SS = biosolid, TSP = triple super phosphate. The number 1 through 7 on x-axis represents corn harvest with each 35 days after sowing.

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All P sources resulted in significantly higher tissue P concentrations than the control treatment (Fig. 2d). The only exceptions were harvest 5 and 6, CM1 treatment in the harvest 1 and 2, TL and TSP in harvest 4, and TL in harvest 7 where these treatments were non-significantly different from each other and with the control treatment. The highest tissue concentration was 4.8 g kg 1 produced by HM after the 1st harvest which was significantly different from all other treatments in the same harvest. Like the Eram-Lebo soil, P source effects were more pronounced during the first two harvests which became less conspicuous after the third harvest. The concentrations continuously decreased as the harvest number increased but Ulysses soil maintained a relatively higher tissue P concentration in comparison to the Eram-Lebo soil until after harvest 3. Total P uptake was highest in the first four harvests, and thereafter it continued to decrease until the end of the study. Relative to the Eram-Lebo soil, higher P uptake was seen during all harvests and with all P sources (Fig. 3b). Like the Eram-Lebo soil, HM resulted in the highest amount of P uptake (53 mg kg 1) and was significantly different from all treatments during the first harvest. The lowest uptake was 23 mg P kg 1 with the TL which was also significantly different from all other treatments but was statistically similar to CM1 and TSP which also had P uptake of 25 mg P kg 1. In subsequent harvests, the differences among the P sources became less and both cattle manures along with HM and SS had identical uptake values. The TL and TSP had the lowest uptake throughout the study. Both TL and TSP have relatively high labile Pi and this P might have reacted with abundant Ca and Mg in this soil producing secondary P minerals of lower solubility (Lindsay, 2001). 3.4. Phosphorus source effects on total P uptake during entire study and the first harvest The corn had taken up almost similar amounts of P during seven harvests in both un-amended soils (20 mg P kg 1); despite having different initial STP values (Table 3). Previously, Xiao et al. (2012) found that Pi, Po, available P (AP) and TP was significantly positively correlated with SOM and clay content in the soils. Despite having low initial STP (4.5 mg kg 1) in Eram-Lebo soil, it furnished similar amounts of P for crop uptake as seen in Ulysses soil owing to higher clay and SOM contents which might have maintained larger reserve of P. We also arrived at similar findings by using bio concentration factor (BCF) approach (ratio of P concentration (mg kg 1) in tissue/available P concentration (mg kg 1) in the soil) as proposed by Xiao et al. (2011). Application of all P sources resulted in significantly higher total corn P uptake when compared to the controls in both soils. However, significant differences did exist when different P sources were compared with each other in a given soil or when a given P source was compared between both soils. Among the animal P sources, HM resulted in 115.2 and 89.2 mg P kg 1 total P uptake in the Ulysses and EramLebo soils respectively, and were significantly different form all treatments in a given soil as well as with each other. The TL was the only P source which resulted in near identical values of total P uptake in both soil (64 mg P kg 1). However, SS in Eram-Lebo soil also produced values of total P uptake similar to TL (64.7 mg P kg 1). Moreover, TL and TSP treatments were also statistically similar in the Ulysses soil as well. The total P uptake was nearly similar for both cattle manures in the respective soils but CM2 had significantly higher P uptake in the Ulysses soil (90.1 mg P kg 1) compared to the Eram-Lebo soil (76.7 mg kg 1). In fact, all P sources resulted in significantly more P uptake in the Ulysses than in the Eram-Lebo soil with the exception of the TL and TSP treatments, where the former was nearly identical, and the latter was significantly less in the Ulysses soil

Table 3 Interactive effecta of soil and P sources on total bioavailable P (total P uptake) measured as actual crop uptake (mg P kg 1)b during sevenc and first harvest in the study. Treatments

Ulysses soil

Eram-Lebo soil mg P kg

1

Seven harvests CM1 CM2 TL HM SS TSP Control

d

CM1 CM2 TL HM SS TSP Control

25.0 dc 32.3 b 22.7 cde 52.3 a 31.1 b 24.7 cd 7.9 g

84.8 bc 90.1 b 64.1 de 115.2 a 91.1 b 62.1 e 19.9 f

76.2 76.7 64.3 89.2 64.7 74.6 20.7

c c de b de dc f

First harvest 20.6 de 23.3 cde 19.9 e 35.2 b 14.8 f 25.9 c 4.1 g

Abbreviations CM1 = cattle manure 1, CM2 = cattle manure 2, TL = turkey litter, HM = swine manure, SS = biosolid, TSP = triple superphosphate. a Soil  P source interaction effect on total P bioavailability (seven harvests) and during first harvest is significant at P < 0.0001. b Total P uptake was calculated per kg of soil and making it consistent with the unit of amended dose of P (150 mg P kg 1). c During all seven harvest (from harvest 1 through 7 added together). d Mean with same letter/letters in columns and across rows in a given harvest are not significant at P = 0.05.

(62.1 mg P kg 1) than in the Eram-Lebo soil (74.6 mg P kg 1). Therefore, the total P uptake among the monogastric P sources was highest for HM in both soils followed by SS in the Ulysses soil and TL and SS in the Eram-Lebo soils which resulted in near identical values of total P uptake (64 mg P kg 1). The partial support for higher total P uptake in cattle manures and significantly lower amount in TL was provided from the study of Cooperband and Good (2002) where they detected higher levels of water soluble P in cattle manure treatments than in TL and attributed it to the presence of biogenic phosphate minerals in the litter. Moreover, Turner and Leytem (2004) fractionated total P in HM, CM and in TL into P fractions of different bioavailability by using deionized water, 0.5 M NaHCO3 (pH 8.5), 0.1 M NaOH, and 1 M HCl in sequential order. The order of the proportions of total P in easily bioavailable fractions (water and NaHCO3) was HM > CM > TL which represented 78%, 55% and 25%, respectively, of total P. The order of total P removal in the form of corn uptake in this study was HM > CM > TL in both soils. In fact, the total P uptake when expressed as percentage of total P added in the HM treatment was 59% and 76% in the Eram-Lebo and Ulysses soils, respectively. These proportions for the both cattle manures was nearly 50% in Eram-Lebo soil while for the Ulysses soil it was around 57%. However, the proportion of the total P added that was removed by the crop for TL was 42%. It does seem that extracting P from animal waste amended soils with water and subsequently with NaHCO3 could furnish useful information on the magnitude of total P that might be taken up if crops were to grow on these soils. The magnitude of P uptake during the first harvest is important from the environmental stand point. Crop uptake of soluble P will reduce the probability that P might end up in runoff generated from the rainfall or irrigation events. Basic trend in P uptake during the first harvest was similar to the total P uptake during entire study (Table 3). A significantly greater P uptake was seen in all P source treatments in the Ulysses soil than was observed in the Eram-Lebo soil. However, there were a few exceptions which include the TL, CM1 and TSP treatments which resulted in similar

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M.N. Shafqat, G.M. Pierzynski / Chemosphere 99 (2014) 72–80 Table 4 Slope and regression coefficient (R2) for the relationships between crops P related parameters and Freundlich equation constants for both soils. Response variable

Eram-Lebo soil Freundlich K aa

bb c

Corn biomass-harvest 1 Total corn biomass (harvest 1–7)e P concentration-harvest 1 P concentration mean (harvest 1–7) P uptake Harvest 1 Total P uptake (harvest 1–7)

Ulysses soil Freundlich K R2

d

60 246 10.6 4.7 124 294

0.27 0.97 0.04 0.016 0.59 1.29

a

0.69 0.64 0.58 0.77 0.85 0.79



33 158 5.1nsf 3.4 69 182

0.57 2.8 0.06ns 0.04 1.3ns 3.4

Freundlich 1/n a Corn biomass-harvest 1 Total corn biomass (harvest 1–7) P concentration-harvest 1 P concentration mean (harvest 1–7) P uptake harvest 1 Total P uptake (harvest 1–7)



58 175ns 6.7ns 2.4ns 129 273



184 645 25ns 10.7 389 877

0.59 0.73 0.29 0.58 0.46 0.62

Freundlich 1/n R2

b

R2

b 

0.74 0.65 0.46 0.74 0.86 0.84

a

cx2

bx 

471 1815 26ns 29ns 920ns 2324ns



1457 5565 86ns 92ns 2811ns 7101ns

R2 

1083 4074 61ns 67ns 2068ns 5214ns

0.85 0.87 0.32 0.75 0.56 0.75

EPC0 P concentration-harvest 1 P concentration mean (harvest 1–7) a b c d e f

2.5 1.6

0.074 0.023

0.67 0.55

Intercept of the linear equation. Slope of the linear equation. Slope and intercept followed by () were significant at P < 0.01. Slope and intercept followed by () were significant at P < 0.05. Harvest 1–7 means during all harvest. ns = non significant.

uptake values in both soils. The highest values of P uptake were again seen in HM treatments, 52.3 and 35.2 mg P kg 1 in the Ulysses and Eram-Lebo soil, respectively, and were significantly different from all other treatments in the respective soils. The P uptake from CM2 (32.3 mg P kg 1) and SS (31.1 mg P kg 1) in the Ulysses soil was significantly higher than the respective treatments in the Eram-Lebo soil. In fact, SS resulted in the lowest amount of P uptake (14.8 mg P kg 1) in the Eram-Lebo soil when compared to all P sources in both soils. 3.5. Predictability of corn growth parameters from Freundlich constants Prior to crop P removal in both soils, we monitored the utility of Freundlich constants (K and 1/n parameters) in predicting corn biomass, P concentration in plant tissue, and total P uptake (Table 4 and Fig. 1b). The regression coefficients for the relationship between Freundlich K and P uptake were 0.85 and 0.79 after harvest 1 and 7, respectively. A similar trend was seen for the corn biomass both after harvest 1 and total biomass after 7 harvests. However, the opposite was true in case of P concentration in corn leaves with a smaller R2 (0.58) after harvest 1 compared to mean tissue P concentration during entire study (0.77). The values of regression coefficient for the aforementioned relationships were relatively smaller in the Ulysses soil. Since Freundlich K refers to the ratio of adsorbed P to that present in the soil solution, larger values were seen in Eram-Lebo soil (Table 2) than in the Ulysses soil suggesting more P was present in the adsorbed state than present in the soil solution. This is consistent with the fact that the Eram-Lebo soil was extremely P deficient with a high clay content compared to the Ulysses soil, which would favor P adsorption. The presence of strong relationships between various crop parameters and Freundlich K in the Eram-Lebo soil suggests the usefulness of this coefficient in both short-term (single harvest) and long term (multiple harvests) predictability of P bioavailability, corn biomass, and tissue P concentrations. Such relationships were weaker for the

Ulysses soil. The Ulysses soil already had its P in the optimum range and most of the surface area was already occupied by antecedent P, thus further addition of P from either manures or from fertilizers would only increase solution P which is readily available for luxury consumption. Freundlich 1/n also showed strong regression coefficients for most crop related variables in the Eram-Lebo soil, although the relationships were reciprocal in nature compared to the Freundlich K. The only significant relationship in Ulysses soil was between Freundlich 1/n and corn biomass at both harvest 1 and for total biomass where R2 values were 0.87 and 0.85 respectively when fitted to a quadratic function. Various factors might influence the Freundlich K and 1/n parameters as we have seen significant variation in both parameters in both soils (Table 2). These variations might be attributed to the presence of variable amounts of organic C as well as labile organic C, and ratio of Pi to Po between the P sources. Moreover, enhanced microbial growth plus metabolically generated various soluble organic compounds might compete with orthophosphates for the adsorption site on the layer silicate clays. These results suggest limited predictability of crop growth and P related parameters from Freundlich constants. The relationships may be more favorable in P deficient soils such as in Eram-Lebo. The EPC0 values also had a relationship between P concentrations in the leaf at harvest 1 and with total mean concentrations during seven harvests in P deficient soils as well (Table 4). 4. Conclusions On the basis of evidence provided by our data, we reject our hypotheses of no difference between the P sources from the monogastric, ruminants and inorganic P fertilizer on corn biomass, tissue P concentration, and on P uptake during various harvests in the study. In fact large variations not only existed between the P sources but significant variations were also seen within the monogastric and ruminants P sources during various harvests, especially during the first few harvests. We also rejected

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the hypothesis that the magnitude of P adsorption would be similar for all P sources in both soils both before and after crop P removal and also rejected the third hypothesis and found that both Freundlich adsorption coefficients had shown better predictability of various corn growth parameters in the P deficient EramLebo soil despite having significant variation in their absolute values as dictated by physico-chemical characteristics of the P sources. However, such relationships were either weak or totally absent in the Ulysses soil which had higher antecedent levels of STP. Therefore, one might use these Freundlich constants in predicting the P bioavailability from the various P sources applied to P deficient soils and this information might prove useful in environmental as well as from agronomic perspectives. However, further studies with the inclusion of many soils with diverse physico-chemical characteristics will be needed to understand wider applicability of these coefficients in predicting P bioavailability using diverse range of crop plants. Acknowledgements This work was partly supported by Ministry of Education, Government of Pakistan, and Department of Agronomy, Kansas State University, USA. Special thanks to my daughter Husna for motivation and my son Ahsan for editing this paper. References Barber, S.A., 1979. Soil phosphorus after 25 years of cropping with five rates of phosphorus application. Commun. Soil Sci. Plant Anal. 10, 1459–1468. Barnett, G.M., 1994. Phosphorus forms in animal manure. Bioresour. Technol. 49, 139–147. Barrow, N.J., 1984. Modeling the effects of pH on phosphate sorption by soils. J. Soil Sci. 35, 283–297. Bermudez, D., Juarez, M., Sanchez-Andreu, J., Jorda, J., 1993. Role of EDDHA and humic acids on the solubility of soil phosphorus. Commun. Soil Sci. Plant Anal. 24, 673–683. Cooperband, L.R., Good, L.W., 2002. Biogenic phosphate minerals in manure: implications for phosphorus loss to surface waters. Environ. Sci. Technol. 36, 5075–5082. Dou, Z., Knowlton, K., Kohn, R., Wu, Z., Satter, L., Zhang, G., Toth, J., Ferguson, J., 2002. Phosphorus characteristics of dairy feces affected by diets. J. Environ. Qual. 31, 2058–2065. Goyne, K.W., Jun, H.J., Anderson, S.H., Motavalli, P.P., 2008. Phosphorus and nitrogen sorption to soil in the presence of poultry litter derived dissolved organic matter. J. Environ. Qual. 37, 154–163. Hinedi, Z., Chang, A., Lee, R., 1989. Characterization of phosphorus in sludge extracts using phosphorus-31 nuclear magnetic resonance spectroscopy. J. Environ. Qual. 18, 323–329. Holford, I.C.R., 1982. The comparative significance and utility of the Freundlich and Langmuir parameters for characterizing sorption and plant availability of phosphate in soils. Aust. J. Soil Res. 20, 233–242. Hue, N.V., 1991. Effects of organic acids/anions on P sorption and phytoavailability in soils with different mineralogies. Soil Sci. 152, 463–471. Hunger, S., Cho, H., Sims, J., Sparks, D., 2004. Direct speciation of phosphorus in alum-amended poultry litter: solid-state 31P NMR investigation. Environ. Sci. Technol. 38, 674–681. Inskeep, W.P., Silvertooth, J.C., 1988. Inhibition of hydroxyapatite precipitation in the presence of fulvic, humic, and tannic acids. Soil Sci. Soc. Am. J. 52, 941–946. Jiao, Y., Whalen, J.K., Hendershot, W.H., 2007. Phosphate sorption and release in a sandy loam soil as influenced by fertilizer sources. Soil Sci. Soc. Am. J. 71, 118– 124.

Kalbasi, M., Karthikeyan, K., 2004. Phosphorus dynamics in soils receiving chemically treated dairy manure. J. Environ. Qual. 33, 2296–2305. Knowlton, K.F., Radcliffe, J.S., Novak, C.L., Emmerson, D.A., 2004. Animal management to reduce phosphorus losses to the environment. J. Anim. Sci. 82, E173–E195. Lindsay, W.L., 2001. Chemical Equilibria in Soils. The Blackburn Press New Jersey, USA. Maguire, R., Sims, J., Dentel, S., Coale, F., Mah, J., 2001. Relationships between biosolids treatment process and soil phosphorus availability. J. Environ. Qual. 30, 1023–1033. Maguire, R.O., Sims, J.T., McGrath, J.M., Angel, C.R., 2003. Effect of phytase and vitamin D metabolite (25Oh-D3) in turkey diets on phosphorus solubility in manure-amended soils. Soil Sci. 168, 421–433. Murphy, J., Riley, J., 1962. Modified single solution method for the determination of phosphorus in natural waters. Anal. Chem. Acta 27, 31–36. Nair, V.D., Villapando, R.R., Graetz, D.A., 1999. Phosphorus retention capacity of the spodic horizon under varying environmental conditions. J. Environ. Qual. 28, 1308–1313. Pierzynski, G.M., Logan, T.J., Traina, S.J., Bigham, J.M., 1990. Phosphorus chemistry and mineralogy in excessively fertilized soils: solubility equilibria. Soil Sci. Soc. Am. J. 54, 1589–1595. Pierzynski, G.M., Sims, J.T., Vance, G.F., 2004. Soils and Environmental Quality, third ed. CRC Press Boca Raton, USA. Poulsen, H.D., 2000. Phosphorus utilization and excretion in pig production. J. Environ. Qual. 29, 24–27. Samadi, A., Gilkes, R.J., 1999. Phosphorus transformations and their relationships with calcareous soil properties of South Western Australia. Soil Sci. Soc. Am. J. 63, 809–815. Sanyal, K.S., Datta, S.K., 1991. Chemistry of phosphorus transformations in soil. Adv. Soil Sci. 16, 1–120. SAS Institute Inc., 2009. SAS Software: Changes and Enhancements through Release 9.12; SAS Institute: Cary, NC, USA. Schwarzenbach, R.P., Gschwend, P.M., Imboden, D.M., 1993. Environmental Organic Chemistry. John Wiley & Sons, New York, USA. Seiter, J., Kristin, E., Staats, B., Vogel, M., Sparks, D., 2008. XANES spectroscopic analysis of phosphorus speciation in alum-amended poultry litter. J. Environ. Qual. 37, 477–485. Shafqat, M.N., Pierzynski, G.M., 2011. Bioavailable phosphorus in animal waste amended soils: using actual crop uptake and p mass balance approach. Environ. Sci. Technol. 45, 8217–8224. Sharpley, A.N., Ahuja, L.R., 1982. Effects of temperature and soil water content during incubation on the desorption of phosphorus from soil. Soil Sci. 133, 350– 355. Sharpley, A.N., Sims, J.T., Pierzynski, G.M., 1994. Innovative soil phosphorus availability indices: Assessing inorganic phosphorus. In soil testing: Prospects for improving nutrient recommendations. SSSA Special Publication 40. Soil Science Society of America, 677 S. Segoe Rd., Madison, WI 53711, USA. Singh, B., Gilkes, R.J., 1991. Phosphorus sorption in relation to soil properties for the major soil types of South-western Australia. Aust. J. Soil Res. 29, 603–618. Sposito, G., 2008. The Chemistry of the Soils, second ed. Oxford University Press, New York, USA. Stevenson, F.J., Cole, M.A., 1999. Cycles of Soil, second ed. John Wiley & Sons, New York, USA. Sui, Y., Thompson, M.L., 2000. Phosphorus sorption, desorption, and buffering capacity in a biosolids-amended Mollisol. Soil Sci. Soc. Am. J. 64, 164–169. Turner, B.L., Leytem, A.B., 2004. Phosphorus compounds in sequential extracts of animal manures: chemical speciation and a novel fractionation procedure. Environ. Sci. Technol. 38, 6101–6108. Van Bladel, R., Moreale, A., 1977. Adsorption of herbicides-derived p-chloroaniline residue in soils: a predictive equation. J. Soil Sci. 28, 93–102. Xiao, R., Bai, J., Zhang, H., Gao, H., Liu, X., Wilkes, A., 2011. Changes of P, Ca, Al and Fe contents in fringe marshes along a pedogenic chronosequence in the Pearl River estuary, South China. Cont. Shelf Res. 31, 739–747. Xiao, R., Bai, J., Gao, H., Huang, L., Deng, W., 2012. Spatial distribution of phosphorus in marsh soils of a typical land/inland water ecotone along a hydrological gradient. Catena 98, 96–103. Zheng, Z., Simard, R.R., Parent, L.E., 2003. Anion exchange and Mehlich III phosphorus in Humaquepts varying in clay content. Soil Sci. Soc. Am. J. 67, 1287–1295.