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Relationship Between Phosphatase Activity and Phosphorus Fractions in Agricultural Soils



Ridvan Kizilkaya , Fethi Bayrakli and Abdulkadir Surucu
 
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ABSTRACT

Soil phosphatases play a major role in the mineralization processes of organic phosphorus. The activity of soil phosphatases can be influenced by numerous factors and soil physico-chemical properties play a key role among them. Our research adds to the growing knowledge on soil acid (ACPA), neutral (NEPA) and alkaline phosphatase with some soil properties and total P (Ptotal), organic P (Porg), bioavailable P and inorganic phosphorus (Pinorg) fractions such as soluble and loosely bound phosphorus (Psoluble), aluminum phosphates (Al-P), iron phosphates (Fe-P), reductant soluble phosphorus (Preductant) calcium phosphates (Ca-P) and strongly adsorbed Fe and Al phosphates (Fixed-P) fractions in agricultural soils of Çarşamba Plain, Turkey. ALPA, 24.6-138.6 μg phenol g-1 3 h-1, was significantly higher than NEPA, 11.5-84.1 μg phenol g-1 3 h-1 and ACPA, 8.6 - 64.1 μg phenol g-1 3 h-1, in the studied soils. Ptotal contents in the soils ranged from 400.8 - 745.0 μg P g-1 with an average 570 μg P g-1. Phosphorus was mostly concentrated in the Pinorg (57%) although it was also present in Porg (12%) and Presidual (31%). On the average, percent of Pinorg associated with different fractions in these soils was in the decreasing order of: Ca-P > Fixed-P > Preductant > Al-P > Fe-P > Psoluble. On the basis of results obtained in this study, phosphatase activies showed significant correlations with the Ptotal, Porg, bioavailable P and some Pinorg fractions (Ca-P and Psoluble). These results suggested that Ca-P is major Pinorg compound on the effects of soil phosphatase activity in agricultural soils of Çarşamba Plain, Turkey.

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Ridvan Kizilkaya , Fethi Bayrakli and Abdulkadir Surucu , 2007. Relationship Between Phosphatase Activity and Phosphorus Fractions in Agricultural Soils. International Journal of Soil Science, 2: 107-118.

DOI: 10.3923/ijss.2007.107.118

URL: https://scialert.net/abstract/?doi=ijss.2007.107.118

INTRODUCTION

The element of phosphorus is essential for plant growth and metabolism. It is generally accepted that plant roots take up phosphorus as soluble inorganic phosphate (Bieleski and Ferguson, 1983). Since a large proportion of the P in soil is organically bound (a very important amount of P in soils especially of arid climates is bound inorganically), the mineralization of this organic fraction is major agricultural importance (Speir and Ross, 1978). Several enzymes are involved in the decomposition of organic phosphorus compounds. Those enzymes that catalyze the hydrolysis of both P and anhydrides of H3PO4 esters are commonly called phosphatases (Alexander, 1977). Phosphatases (orthophosphoric monoester phosphohydrolase, E.C. 3.1.3) are important in soils because phosphatase catalyze the hydrolysis of organic esters and anhydrides of H3PO4 to orthophosphate; thus they form an important link between plant-unavailable and soluble P fractions in soil (Amador et al., 1997). Phosphatase activities in soil can be associated with active cells (animal, plant, microbial), entire dead cells and cell debris as well as being complexed with clay minerals and humic colloids (Pascual et al., 2002). In addition, the sorption of phosphatases on clay, oxides or humic substances can change enzyme conformation and reduce activity (Dick and Tabatabai, 1987; Nannipieri et al., 1988). Phosphatases are distinguishable not only by the chemical nature of the substrates hydrolyzed but also by pH ranges for their optimal activity. Among them are: acid phosphatase, optimal pH 4-6; neutral phosphatase, optimal pH 7; and alkaline phosphatase, optimum pH 8-10 (Speir and Ross, 1978).

Phosphatase activity is affected by soil physico-chemical (clay content, soil moisture, soil depth, temperature, organic matter, pH and nutrients) and biological (microbial population and their activities) properties (Speir and Ross, 1978) and these properties play a key role among them. As far as physico-chemical soil properties are concerned, numerous studies have focused on the carbon content and its positive impact phosphatase activity (e.g., Herbien and Neal, 1990; Pagliai and De-Nobili, 1993; Marinari et al., 2000), relationships between organic matter and the other elements in the organic bounds (e.g., N and P) and pH. Relationships between phosphatase activities and total P, organic P available P have been described by Gavrilova et al. (1973), Speir and Ross (1978), Haynes and Swift (1988) and Nahas et al. (1994). On the contrary, little information on the relationships between phosphatase activities and inorganic P fractions in soils is available. In moderately well drained soils with a relatively low level of inorganic P, Amador et al. (1997) has observed a positive correlation between inorganic P and phosphatase activity. The high concentration of inorganic P in soils has been shown to reduce phosphatase activity. For example, orthophosphate inhibited phosphatase activity in soils (Juma and Tabatabai, 1978), as well as the synthesis and catalytic action of phosphatases of microorganisms in soil (Woolhouse, 1969). Chen (2003) suggested that the positive correlations between phosphatase activities (acide and neutral) and inorganic P fractions (iron and aluminium phosphates) in acidic Chinese forest soils.

There is currently great interest in the use of extracellular enzymes as biological indicators of soil quality, because they are relatively simple to determine, have microbial ecological significance, are sensitive to environmental stress and respond rapidly to changes in land management (Dick, 1997; Yakovchenko et al., 1996). Phosphatase activity may be a particularly useful enzyme for soil quality monitoring because of its central role in soil organic matter cycling, which is generally regarded as an important component of soil quality. Research has shown that phosphatase is the most abundant and easily detected of the enzymes involved in organic P compounds decomposition in soil and is rarely substrate limited, thus making it ideal to examine the importance of soil P status. Indeed, it provides an early indication of changes in organic P and organic matter status and turnover (Gavrilova et al., 1973; Speir and Ross, 1978).

The adoption of soil phosphatase activities in soil quality monitoring requires information on activities from a wide range of soil types and land uses under steady-state conditions, in addition to mechanistic understanding of how soil properties control these activities (Trasar-Cepeda et al., 2000). The aim of this study were, (I) to determine phosphatases (acide, neutral and alkaline) in wide range of agricultural soils with contrasting total P, organic P and inorganic P fractions such as aluminium phosphates, iron phosphates, calcium phosphates, strongly adsorbed Fe and Al phosphates, phosphorus involved in reductant and slightly bounded phosphorus, but under similar land use and (ii) to investigate the relations between activity of the phosphatases and total P, organic P, inorganic phosphors fractions in those soils.

MATERIALS AND METHODS

Study Sites
The study sites, Çarşamba plain (Latitude, 41°11’N; longitude, 36°45’W), are located in the Black Sea Region, Northern Turkey (Fig. 1). Soil parent materials are alluvial in sampling sites; deposited by Yeşilırmak River and also on recent stream outwash generally. As choosing sites, we considered consistency in soil texture (sandy loam to clay loam) and slope (0-3%).


Image for - Relationship Between Phosphatase Activity and Phosphorus Fractions in Agricultural Soils
Fig. 1: Location of the sampling sites in Çarşamba Plain, Turkey

Historically, all sampling sites were managed as agricultural land for several hundred years. This management regime included the use of intensively cultivated vegetable fields and cereals such as wheat and corn have been allocated in these sites with more intensive cultivation practices and were irrigated. All sites face with chemical fertilizer (e.g., diammonium phosphate, triple super phosphate, compose fertilizer and calcium ammonium nitrate) and herbicides applications for 30-35 years. The climate is semi humid with monthly mean temperatures ranging from 9.6°C in January to 19.3°C in July. The annual mean temperature is 14.3°C and annual mean precipitation 1045.3 mm (Anonymous, 2000).

Soil Sampling
In May 2000, 16 sampling sites were collected randomly from the agricultural soils of Çarşamba Plain. Samples of 0-20 cm depth below the litter (or where soil surface where no litter existed) were taken form five points within each plot using a 20x5 cm soil corer. The five replicate samples were homogenized by hand mixing. Major live plant materials (roots and shoots) and pebbles in each sample were separated by hand and discarded. About 1 kg mixed samples were returned to the laboratory in polyethylene bags at 4°C. The samples were brought to the laboratory on the same day and kept in the refrigerator at 4°C for no longer than 72 h prior to phosphatase analysis. These soil samples were used for analyzing physico-chemical properties and total, organic and inorganic phosphorus fractions and phosphatase activities.

Soil Physico-chemical Analyses
Prior to physico-chemical analysis, all soil samples were air dried at approximately 25°C, lightly ground and sieved through a 2 mm stainless steel sieve. All analyses were performed on the size fraction smaller than 2 mm and stored at room temperature. Selected soil physico-chemical properties were determined by means of appropriate methods: soil particle size distribution by the hydrometer method (Bouyoucos, 1951), pH in 1: 2.5 (w/v) in soil: water suspension by pH-meter (Peech, 1965) and Cation Exchange Capacity (CEC) by Bower method (Rowell, 1996), CaCO3 content by Scheibler calcimeter (Soil Survey Staff, 1993). Bioavailable phosphorus (Olsen-P) was extracted by shaking 2.5 g of soil for 30 min with 50 mL of 0.5 M NaHCO3 (Olsen et al., 1954). An aliquot of the extract was analyzed for P with molybdenum blue method (ascorbic acid method) proposed by Murphey and Riley (1962). Whole soil samples were sieved through a 150 μm mesh to determine total organic carbon by the wet oxidation method (Walkley-Black) with K2Cr2O7 (Rowell, 1996).

Phosphorus
The soil samples were air-dried, then passed through 2 mm sieves for determination of total, organic and inorganic P fractions as soil physico-chemical properties.

Total and Organic Phosphorus
Total phosphorus (Ptotal) was determined perchloric acid (HClO4) digestion method according to Olsen and Sommers (1982). Organic phosphorus (Porg) content was measured by the ignition method according to Saunders and Williams (1955).

Inorganic Phosphorus Fractions
The procedure of Chang and Jackson (1957) and Peterson and Corey (1966) selected for this study, is designed to separate inorganic phosphorus (Pinorg) fractions in to six operationally defined fractions: soluble and loosely bound phosphorus, aluminum phosphates, iron phosphates, reductant soluble phosphorus, calcium phosphates and strongly adsorbed Fe and Al phosphates fractions. A summary of the procedure is as follows:

One gram of each soil is weighed into 100 mL centrifuge tube and the following fractions obtained.

Soluble and Loosely Bound Phosphorus (Psoluble)
Soil samples extracted with 50 mL of 1 M NH4CI for 30 min. Centrifuging and decanting the supernatant into a 50 mL volumetric flask and bringing to volume with deionized water (extract A).

Aluminum Phosphates (Al-P)
The residue from Psoluble is extracted with 50 mL of 0.5 M NH4F (pH 8.2) for 1 h. Centrifuging and decanting the supernatant into a 100 mL volumetric flask (extract B).

Iron Phosphates (Fe-P)
After washing the soil sample twice with 25 mL portion of saturated NaCI and centrifugation combining the washing with extract B and bringing to volume. The residue from Al-P is extracted with 50 mL 0.1 M NaOH for 17 h. Centrifuging and decanting the supernatant solution into a 100 mL volumetric flask (extract C). Washing the soil twice with 25 mL portions of saturated NaCI and centrifuging. Combining the washing with extract C and bringing to volume.

Reductant Soluble Phosphorus (Preductant)
The residue from Fe-P is heating in water bath at 85°C with 40 mL of 0.3 M Na3 C6H5O7 and 5 mL of 1 M NaHCO3 and adding 1.0 g of Na2 S2O4 and stirring rapidly to extract Preductant. Continuing to heat for 15 min and then centrifuging. Decanting the supernatant solution into a 100 mL volumetric flask (extract D). Washing the soil twice with 25 mL portion of saturated NaCI and centrifuging. Combining the washings with extract D and diluting D to volume.

Calcium Phosphates (Ca-P)
The residue from Preductant is extracted with 50 mL of 0.25 M H2SO4 for 1 h. Centrifuging the suspension for 10 min and decanting the supernatant into a 100 mL volumetric flask (extract E). Washing the soil twice with 25 mL portions of saturated NaCl and centrifuging. Combining the washings with the extract E and diluting to volume.

Strongly Adsorbed Fe and Al Phosphates (Fixed-P)
The residue from Ca-P is extracted with 50 mL of 0.1 M NaOH for 1 h. Centrifuging and decanting the supernatant into a 50 mL volumetric flask and bringing to volume with deionized water (extract F).

Phosphorus concentrations in all the extracts were carried out using the molybdenum blue method (ascorbic acid method) proposed by Murphey and Riley (1962). The amount of inorganic P (Pinorg) was calculated based on the sum of Psoluble, Al-P, Fe-P, Preductant, Ca-P and Fixed-P. Residual phosphorus (Presidual) was calculated as follows; Presidual = Ptotal-(Porg +Pinorg). Results were expressed as μg P g-1.

Phosphatases
The phosphatase activities were determined for the each of the field-moist soil. All results are expressed on a moisture-free basis. Moisture was determined after drying at 105°C for 48 h.

Acid, neutral and alkaline phosphatase activities (ACPA, NEPA and ALPA, respectively) were assayed by Hoffmann’s method (Hoffmann, 1968; Alef and Nannipieri, 1995). Two milliliter toluene, 20 mL buffer (acetate buffer, pH 5.0 to acid phosphatase; citrate buffer, pH 7.0 to neutral phosphatase; borate buffer, pH 9.6 to alkaline phosphatase) and 10 mL of 0.675% disodiumphenyl phosphate substrate solution were added in to the 10 g soil and the samples were incubated for 3 h at 37°C. The volume was made up to 100 mL with distilled water at 37°C. Following filtration through Whatman No. 42 filter papers, 1 mL of filtrate was diluted to 10 mL with distilled water and 5 mL of borate buffer and 1 mL of 2.6 dibromchinon chlorimid solution were added. The volume was made up to 50 mL with distilled water. Finally the formation phenol was determined spectrophotometrically at 578 nm. Acid, neutral and alkaline phosphatase activity was expressed as μg hydrolyzed phenol g-1 dry soil for 3 h at 37°C.

Statistical Analysis
All data were analyzed using SPSS 11.0 statistical software (SPSS Inc.). The significance of differences between the different soils or sampling sites was tested by one-way ANOVA. Pearson correlation coefficients and P-values were calculated for all possible variable pairs. The asterisks *and ** indicate significance at p<0.05 and 0.01, respectively.

RESULTS

Soil Physico-chemical Properties
The sixteen sampling sites representing mineral soils were collected from agricultural land. For all sites, the five replicate sampling points showed different values in soil physico-chemical properties. The mean value of soil pH was 7.35 (slightly alkaline), which has slightly above the well-established values (6.7-7.3) in cultivated soils of this area of Turkey Table 1. All soils were clay loam with >27% clay content, except for few sites that were sandy clay loam. Soils were moderately low in total organic carbon. The content of CaCO3 ranged from low to moderate (8-15%) in these soils. CEC was ranged from 32.5 to 42.2 cmol(+) kg-1.


Table 1: Maxima, minima, means and standard deviations (S.D.) of the soil physico-chemical properties studied
Image for - Relationship Between Phosphatase Activity and Phosphorus Fractions in Agricultural Soils

Image for - Relationship Between Phosphatase Activity and Phosphorus Fractions in Agricultural Soils
Fig. 2: Percentage of phosphorus fractions in soils studied, (a) Porg, Presidual and Pinorg by Ptotal (b) Ca-P, Fe-P, Al-P, Fixed-P, Preductant and Psoluble by Pinorg

The Contents of Soil Phosphorus Fractions
Like soil physico-chemical properties, P distribution showed different values in Ptotal, Porg and inorganic P fractions ralative to sampling sites. Bioavailable phosphorus contents ranged from low (<8 μg P g-1) to marginal (8-15 μg P g-1). Ptotal contents ranged from 400.78 to 745.03 μg P g-1; Porg contents ranged from 32.96 to 100.08 μg P g-1; Pinorg contents ranged from 131.63 to 504.84 μg P g-1; Presidual contents ranged from 46.09 to 227.43 μg P g-1. The amounts of phosphorus in Porg, Pinorg and Presidual fractions were 12.0, 57.3 and 30.7% of Ptotal, respectively (Fig. 2a). Al-P contents ranged from 9.56 to 26.77 μg P g-1; Fe-P contents ranged from 9.69 to 21.77 μg P g-1; Ca-P contents ranged from 67.42 to 235.09 μg P g-1; Fixed-P contents ranged from 31.45 to 131.83 μg P g-1; Psoluble contents ranged from 2.40 to 12.00 μg P g-1; Preductant contents ranged from 11.08 to 77.38 μg P g-1. The amounts of phosphorus in Al-P, Fe-P, Ca-P, Fixed-P, Psoluble and Preductant fractions were 5.1, 4.7, 49.7, 25.3, 2.3 and 13.1% of the total Pinorg On the average, percent of Pinorg associated with different fractions in the sixteen sampling sites was in the following order: Ca-P>Fixed-P>Preductant>Al-P>Fe-P>Psoluble (Fig. 2b). Analysis of variance (ANOVA) indicated significant differences in Ptotal, Porg and inorganic P fractions among different soil samples (p<0.01).

Phosphatase Activities
ALPA, 64.8 μg phenol g-1 3 h-1 (range 24.6 to138.6, S.D. 16.8), was significantly higher than NEPA, 47.5 μg phenol g-1 3 h-1 (range 11.5 to 84.1, S.D. 22.3) and ACPA, 37.4 μg phenol g-1 3 h-1 (range 8.6 to 64.1, SD 16.8), in the studied soils (Fig. 3). ANOVA indicated that there were significant differences in all phosphatase (ACPA, NEPA and ALPA) in the different soils (p<0.01).

The results indicated that there was a statistically significant pearson correlations between physico-chemical properties and soil phosphatase activities. ACPA, NEPA and ALPA were strongly positively correlated with bioavailable P and soil organic matter (Table 2). Pearson correlation analysis indicates highly significant positive relationships among phosphatase activities (ACPA, ALPA and NEPA), Ptotal, Porg, Ca-P and Psoluble but not significantly correlated with the other inorganic P fractions (Al-P, Fe-P, Fixed-P, Preductant) (Table 3).


Table 2: Correlation coefficients between phosphatase activity and soil physico-chemical properties
Image for - Relationship Between Phosphatase Activity and Phosphorus Fractions in Agricultural Soils
*,** indicate significent at p<0.05 and 0.01 level

Image for - Relationship Between Phosphatase Activity and Phosphorus Fractions in Agricultural Soils
Fig. 3: Distribution of phosphatase activity (ACPA, ALPA and NEPA) in soils

Table 3: Correlation coefficients among Ptotal, Porg and inorganic phosphorus fractions with phosphatase activity in soils studied
Image for - Relationship Between Phosphatase Activity and Phosphorus Fractions in Agricultural Soils
**: Significant at 0.01 level

DISCUSSION

The Contents of Soil Phosphorus
Mean Ptotal in soil studied was 570.01 μg P g-1 (Fig. 2a). The phosphorus content of common soils varies from 100 to 2000 μg P g-1 (Kabata-Pendias and Pendias, 1992). Soils can be classified according to Tripathi et al. (1970) as low (<600 μg P g-1) to moderate (600-1000 μg P g-1). Bayraklı (1975) determined that Ptotal contents were 641 μg P g-1 in Bayburt Plain soils, were 521 μg P g-1 in Erzincan plain soils and were 676 μg P g-1 in Rize region soils, Turkey.

Mean Porg in soil studied was 68.3 μg P g-1 (Fig. 2a). Most soils contain between 50 to 500 μg P g-1. The average content of Porg in cultivated soils ranges from 5-50 % of Ptotal (Adepetu and Corey, 1976; Harrison, 1987). In this research the contents of Porg was 12% of the Ptotal.

Mean Pinorg in soil studied was 326.5 μg P g-1 (Fig. 2a). Pinorg was mostly concentrated in the Ca-P fraction, although it was also present in the other fractions. A small percentage of Pinorg was associated with Psoluble, Al-P and Fe-P fractions (Fig. 2b). Similarly, in the study of alluvial soils by Sing et al. (1968), also found that the most Pinorg was associated with calcium phosphates (Ca-P) and only very low amounts of P was in soluble and slightly bounded phosphorus (Psoluble) forms. Uriyo and Kasseba, (1973), Bayraklı (1975) and Udo and Ogunwale, (1977) also found a majority of the Pinorg in soils to be associated with the Ca-P. Nearly all Pinorg exists in the form of orthophosphates, derivatives of phosphoric acid (Black, 1968). The compounds are believed to be chiefly phosphates of calcium, aluminum and iron with minor proportions of others. Some of the Pinorg is present in the lattices of silicate minerals and as inclusions in minerals, e.g., in quartz crystals (Black, 1968). The initial form of Pinorg is predominantly some form of the mineral apatite, perhaps most commonly calcium fluorapatite (Larsen, 1967). Approximately 90% of the soil phosphorus occurs in soluble or fixed forms (primary phosphate minerals, humus P, phosphates of Ca, Fe, Al, phosphates fixed by colloidal oxides and silicate minerals). On the contrary, only a small fraction of phosphorus occurs in labil forms (Fox and Kamprath, 1970).

Phosphatase Activities and Their Relationship to Physico-chemical Properties and Different P Fractions in Soil
The pH values of soil samples varied from 6.00 to 8.40 with a mean of 7.35. ALPA was higher than the ACPA and NEPA in the investigated soil sites (Fig. 3), because the soil reaction was slightly alkaline in nature. ACPA, NEPA and ALPA are common in nature; pH optima are generally within the ranges pH 4-6, 7 and 8-10, respectively (Speir and Ross, 1978). Soil organic matter and bioavailable P contents gave the significant positive correlations with ACPA, NEPA and ALPA. On the contrary, phosphatase activities were not significantly correlated with the pH, CaCO3 and clay content. In agricultural soils phosphatase activity is affected by physico-chemical soil properties. As far as chemical characteristics are concerned, numerous studies have focused on carbon content and its positive impact on phosphatase activity (Jordan and Kremer 1994; Pascual et al., 2002), relationships between organic matter content and bioavailable phosphorus content. Similarly, Aon and Colaneri (2001) described positive correlations of phosphatase activity with organic matter and a negative relationship with soil pH. In this research no correlation was found between phosphatase activity and clay content. Various studies have shown either positive or negative correlations between the above soil properties and phosphatase activity. Generally, soil phosphatase activity is related to content of organic matter and bioavailable P in soil.

Phosphatase activities (ACPA, NEPA and APA) gave significant positive correlations with Ptotal and Porg but phosphatase activity not significantly correlated with Presidual (Table 3). The relationship of phosphatase activity to soil Ptotal, Porg has been the subject of several studies. Marinari et al. (2000) described positive correlations of phosphatase activity with total organic matter and organic phosphorus. On the contrary, Nahas et al. (1994) suggested that the activity of phosphatase correlated with organic matter and total phosphorus content, but not with organic phosphorus content. In most studies, however, significant positive correlations have been found between phosphatase activity and Porg. (Gavrilova et al., 1973).

ACPA, NEPA and ALPA were strongly positively correlated with Ca-P and Psoluble (P<0.01) but were also not significantly correlated with Al-P, Fe-P, Preductant and Fixed-P (Table 3). Correlation coefficients between phosphatase activity and Pinorg fractions had not been measured in earlier studies, probably because of the difficulty of measuring soil phosphate. However, correlations between phosphatase activity and the so-called plant available fraction of inorganic phosphate, variously called available-P, mobile-P and soluble-P, had been measured contradictory results. Hofmann and Kasseba (1962) found a highly significant correlation between phosphatase activity and soluble-P in German soils but an equally significant positive correlation between these factors in Egyptian soils. Haynes and Swift (1988) described negative correlations of phosphatase activity with available phosphorus. Nahas et al. (1994) suggested that the activity of phosphatase not correlated with available phosphorus. Inverse relationships have found by other workers (Speir and Ross, 1978) found positively correlations between inorganic PO42- anions and phosphatase activity.

In literatures it was shown that phosphatase activity is directly related to organic phosphorus and bioavailable P content in soil. The strong correlation among Ca-P, Ptotal with activity of phosphatases (ACPA, NEPA and APA) may be sourced by relationship among CA-P, Ptotal with Porg and bioavailable-P. The relationships among the Ptotal, Porg, Ca-P, Psoluble with bioavailable P in soil studied are given Table 4.

The Porg content of a soil, could be affected by parent material, climatic zone and human activities such as cultivation, land use and fertilizer, is related to some soil physico-chemical and biological properties. The contents of organic matter and Porg in soil studied were correlated positively (r = 0.589**). In addition, organic matter and Porg were positively correlated with the bioavailable P. A significant relationship between the Porg and the available form of soils has been found by a number of researchers (Adepetu and Corey, 1976; Harrison, 1987). A large reservoir of Porg exists which is unavailable to alive. Here, the microbial oxidation of organic substrates is an important supplementary source of bioavailable P.


Table 4: Pearson correlation coefficients and regression equations among Ptotal, Porg, Ca-P, Psoluble with bioavailable P in soil studied
Image for - Relationship Between Phosphatase Activity and Phosphorus Fractions in Agricultural Soils
**: Significent at 0.01 level

As might be expected, there is no direct relationship Ca-P and Ptotal with the bioavailable P. However, positive correlation was also found Ca-P and Ptotal was significantly correlated with bioavailable P (Table 4). As soils develop, the total P undergoes changes that are related to the weathering environment. We expect the amount and forms of Ptotal to change with soil-forming processes. It was demonstrated that the driving force for conversion of primary to secondary and occluded forms is weathering and available of the soil. In addition, bioavailable P concentrations might be largely controlled by solubility of P minerals that are dominated by Ca-P in neutral to high pH soils studied. Psoluble (ammonium chloride extractable phosphorus content) can be easily desorbed from soil and thus may be also considered bioavailable (Maida 1978). For this reason, the correlations between bioavailable P and Psoluble are expected results in soils.

In summary, in this study has shown that ACPA, NEPA and ALPA were correlated to Ptotal, Porg, Ca-P, Psoluble, bioavailable P contents in soils studied but not significantly correlated with Presidual, Al-P, Fe-P and Fixed-P. These results obtained that Ca-P is major Pinorg compound on the effects of soil phosphatase activity in agricultural soils of Çarşamba Plain, Turkey. In addition, further studies are needed to explore the relevancy in using phosphatase activity to reflect different type soils and climatic zone.

ACKNOWLEDGMENT

This work was supported by the OMU Center of Scientific Research (Z-286).

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