Phosphorous desorption kinetics under saturated and field capacity condition in a calcareous soil.
Desorption behavior is of important processes controlling nutrient mobility and bioavailability in the environment . Phosphorous (P) release rate and capacity from soils would insight invaluable information on the P fate and transport as a plant nutrient and potential risk of surface and ground waters contamination [1, 8]. Thus, in order to improve nutrient management, risk assessments, and predictions about the mobility of contaminants, it is critical that time-dependent P desorption behavior be understood, as well as the mechanisms of desorption reactions under various soil conditions . Although soil P dynamics mostly controlled by the soil pH and carbonates in calcareous soils , soil moisture regime is known to affect P transformation through reduction/oxidation processes [13,16]. Anaerobic soil conditions increase soluble P and Fe in soils via dissolution of Ca-P due to increased C[O.sub.2] concentration as well as reduction in Fe-P minerals . Ma et al. (2010) reported that the Olsen-P of soil decreased after each field saturation process and increased after each draining process . Thus, in time dependent investigations it is critical to consider the initial sample condition in each time series as well as initial soil moisture, but due to the implications during the moist soil analysis the samples may be air dried before extraction. The authors believed that this pretreatment can affect P behavior so the aim of this study were to investigate 1) P desorption behaviors in two different soil moisture regimes (field capacity and water saturated); and 2) the air drying pretreatment effect of soil samples before extraction on kinetics models of P desorption in a calcareous soil.
MATERIALS AND METHODS
Soil sample preparation and analysis:
A bulk soil sample from calcareous soils in an arid region of Khuzestan Province, Southwestern Iran was collected and transported to laboratory. The climate is varied from semiarid to arid with a mean annual precipitation and class A pan evaporation of 240 and 3000 mm, respectively . The soil was air dried, ground, passed through a 2-mm sieve and some analysis were made by standard methods including: particle size distribution (hydrometer method) ; Soil Organic Matter (SOM) content (Walkley-Black procedure; ); Soil pH (glass electrode in saturation extracts); and electrical conductivity by a conductivity meter. Calcium carbonate equivalent (CCE) was determined by neutralization with hydrochloric acid , NaHCO3-P was extracted using Olsen et al., (1954) procedure and determined by Murphy and Riley (1962).
In desorption experiment phosphorous was added (100 mg P [kg.sup.-1] soil, as K[H.sub.2]P[O.sub.4]) to polyethylene pots containing 100 g soil and incubated for two months at 25[degrees], Gn two subset including two different moisture condition (water saturated and field capacity). After 2 months soil phosphorous was extracted using 0.01 M Ca[Cl.sub.2] at 10 different shaking time (15, 30, 60, 120, 240, 480, 960, 1920, 3840 and 7680 min) and determined using spectrophotometer . Before extracting the soil P each soil samples for each time was divided to two subset one extracted in initial soil moisture and other was dried in oven at 50[degrees]C then after extracted for bicarbonate extractable P.
The suitability of eight kinetics models (Table 1) for describing the data trend was tested using least-square regression analysis. High values of the coefficient of determination ([R.sup.2]) and low standard error of estimate were used as indices for comparing the validation and suitability of models. The standard error of the estimate was obtained as follows:
SE = [square root of [summation][(q - q').sup.2]/(N - 2)]
where q and q' are the measured and calculated amounts of Cd in soil at time t, respectively and N is the number of measurements.
RESULTS AND DISCUSSION
The studied soil was highly calcareous with 47 % calcium carbonate equivalent. A sandy loam soil with the clay, silt, and sand percentage of 10, 22 and 68, respectively. The pH was 7.1 and electrical conductivity was measured 1.16 dS [m.sup.-1]. Bicarbonate extractable P was low (5.4 mg [kg.sup.-1]) in the studied soil.
Several parameters including pH, clay type and content, carbonates, organic matter, redox condition, aging time, and complexing agents influenced the release of sorbed P from soil particles . The release of P, by means of 0.01 M Ca[Cl.sub.2] extraction is presented Fig. 2. It is clear that the amount of released P was increased with increasing time and equilibrium is reached in almost 16 h, followed by a slight decrease in desorption. The experimental data showed that air drying of the soil samples before extraction resulted in increased desorbed P in field saturation and field capacity water regimes in almost all extraction times (Table 2).
The results revealed that the amounts of desorbed P were significantly greater in the samples which were air dried before extraction comparing to those extracted in initial soil moisture in two water regime of field capacity and field saturation (Table 2). Generally, the amounts of released P were higher in soils incubated under field saturation compared to those for field capacity. In the first extraction time (15 min) the amount of released P in field saturated soil was 0.683 mg [kg.sup.-1] which increased by air drying the sample before extraction to 1.224 mg [kg.sup.-1]. As well as in field capacity condition released P was 0.646 mg [kg.sup.-1] when samples were in field capacity at time of extraction which increased to 1.650 mg [kg.sup.-1] when air dried and extracted for P. It is revealed that there is a direct correlation between P solubility in soil and soil redox conditions. The availability and solubility of P can be affected in cyclically waterlogged and drained environments [3, 14].
Table 3 shows the correlation coefficients obtained from the application of several kinetics models to describe P release pattern for four different moisture conditions. The zero-, first-, second-, third-, and parabolic equations poorly described the extraction kinetics based on [R.sup.2]. However, the two constant rate (Exponential), and simple Elovich equations were the suitable models in describing the extraction kinetics of P from treated soils. Moreover Exponential and simple Elovich equations fitted better in soils treated under field saturation (Table 3). Anaerobic soil conditions increase oxalate-soluble P and Fe in soils . Ma et al. (2010) reported that the Olsen-P of soil decreased after each inundating process and increased after each draining process .
The values of the rate parameters for the six equation used for describing P release from three soils under different treatments are shown in Table4. From the two constant rate equations, the rate equation (b) for P release was used to compare the relative rate of P releasing from solid phase to the bulk solution under two moisture regimes. The b values averaged 0.082 for field saturated soils, which decreased to 0.042 in soils incubated under field capacity.
Our results implied that under the anaerobic condition of field saturation, phosphorus release increased in terms of amounts and rate compared to field capacity conditions. Previous findings related the elevated soluble P in saturation condition to releasing P from Ca-P and Fe-P minerals. We also implied that pretreatment of moist soil samples (air drying) before extraction resulted in change in amounts and rates of P release and would complicate the comparison of inter laboratory data. The authors believed that a universally accepted procedures in terms of reagents, extractions conditions such as temperature, reagent concentration, solid to solution ratio, treatment time, pH control, and shaking time are required for desorption studies that data to be comparable inter laboratory.
Received 4 September 2013
Received in revised form 24 October 2013
Accepted 5 October 2013
Available online 14 November 2013
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Ali Khanmirzaei, Hossein Akbari, Shekoofeh Rezaei
Soil Science Department, Karaj branch, Islamic Azad University, Karaj, Iran
Corresponding Author: Ali Khanmirzaei, Soil Science Department, Karaj branch, Islamic Azad University, Karaj, Iran
Table 1: kinetics models used for describing the data trend in desorption study Model equation First-order reaction ln[q.sub.t] = ln[q.sub.0]-[k.sub.1]t Second-order reaction 1/[q.sub.1] = 1/[q.sub.0] - [k.sub.2]t Third-order reaction 1/[q.sub.t.sup.2] = 1/ [q.sub.0.sup.2]-[k.sub.3]t Parabolic [q.sub.t] = [q.sub.0] + [k.sub.p][t.sup.0.5] Exponential [q.sub.t] = a[t.sup.b] Elovich [q.sub.t] = 1/ [[beta].sub.s]ln([[alpha].sub.s] [[beta].sub.s]) + 1/[[beta].sub.s]lnt Model parameters First-order reaction [k.sub.1] first-order rate constant ([h.sup.-1]) Second-order reaction [k.sub.2] Second-order rate constant [[(mg.[kg.sup.-1]).sup.-1]] Third-order reaction [k.sub.3] Third-order rate constant [[(mg.[kg.sup.-1]).sup.-2][h.sup.-2]] Parabolic [k.sub.p] diffusion rate constant ([h.sup.-1]) Exponential a sorption magnitude constant Elovich [[alpha].sub.s] initial sorption constant (mg.[kg.sup.-1][h.sup.- 1]) and ps sorption rate constant[[(mg.[kg.sup.-1]).sup.1] [q.sub.0] and [q.sub.t] are the amount of sorption (mg [kg.sup.-1]) at time zero and t respectively. Table 2: The Ca[Cl.sub.2]-extractable P in two moisture regimes pretreated before extraction (moist and air dried) at different extraction times. Moisture Time (min) regime Pretreatment 15 30 120 480 p Field Moist 0.683 0.591 0.906 0.887 Saturation Air dried 1.224 1.612 1.675 1.671 Field Moist 0.646 0.518 0.729 0.752 Capacity Air dried 1.650 1.522 1.725 1.594 Moisture Time (min) regime Pretreatment 960 1920 3840 7680 Field Moist 0.969 1.023 0.962 1.105 Saturation Air dried 1.982 2.140 2.216 2.099 Field Moist 0.944 0.679 0.825 0.816 Capacity Air dried 1.851 1.977 1.860 1.526 Table 3: Coefficients of determination ([R.sup.2]) for the fitted functions of pretreated soils under two moisture regimes. Moisture regime Equations pretreatment Zero First Second Third Field moist 0.48 * 0.41 0.36 0.30 Saturation Air dried 0.44 0.41 0.36 0.32 Field Capacity moist 0.17 0.18 0.19 0.19 Air dried 0.25 0.26 0.26 0.27 Moisture regime Equations pretreatment Parabolic Exponential Elovich Field moist 0.66 0.81 0.84 Saturation Air dried 0.68 0.85 0.87 Field Capacity moist 0.29 0.49 0.47 Air dried 0.25 0.57 0.57 * Coefficients of determination ([R.sup.2]) Table 4: Major parameters of the kinetics equations of P desorption from soils incubated under field saturation and field capacity pretreatment First a b Field Moist 0.79 5.1 x [10.sup.-5] Saturation Air dried 1.64 4.8 x [10.sup.-5] Field Moist 9 2.9 x [10.sup.-5] Capacity Air dried 9 1.7 x [10.sup.-5] pretreatment Second a b Field Moist 0.78 -6.0 x [10.sup.-5] Saturation Air dried 1.62 2.7 x [10.sup.-5] Field Moist 0.68 4.3 x [10.sup.-5] Capacity Air dried 8 -9.8 x 10-6 pretreatment Third a b Field Moist 0.77 -0.00014 Saturation Air dried 1.59 3.2 x [10.sup.-5] Field Moist 0.67 -0.00013 Capacity Air dried 1.68 -1.2 x [10.sup.-5] pretreatment Parabolic Exponential a b a b Field Moist 0.73 0.005 0.52 0.084 Saturation Air dried 1.51 0.009 1.09 0.081 Field Moist 0.66 0.002 0.51 0.056 Capacity Air dried 1.69 0.00003 1.46 0.029 pretreatment Elovich [alpha] [beta] Field Moist .51 14.3 Saturation Air dried 3.54 7.16 Field Moist 3.97 25.7 Capacity Air dried 9.46 19.5
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|Author:||Khanmirzaei, Ali; Akbari, Hossein; Rezaei, Shekoofeh|
|Publication:||Advances in Environmental Biology|
|Date:||Oct 1, 2013|
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