The forms, amounts and distribution of phosphorus (P) were determined in 28 calcareous soils from South-Western Australia to evaluate soil P status and their contribution to soil P tests. The soils under study exhibited wide ranges in content of total P (TP), organic P (org.-P), inorganic P (Pi) fractions and of P extracted by soil tests for plant available P. The values of TP ranged from 33 to 392 mg/kg with a mean of 176 mg P/kg soil. Org.-P constituted a considerable portion of TP (mean 35%).i The mean ratio of organic C/ org. P was 287. In general, the distribution of P; fraction was CaZ-P, 15; Cab-P, 27; AI-P, 26; FeP, 14; occluded-P (O-P), 20, Calo-P, 16 mg P/kg soil, accounting respectively for 13%, 23%, 22%, 12%, 17%, and 13% of total Pi. The pattern of distribution of the Pi fractions, however, differed between virgin and cultivated soils. For virgin soils, the average relative abundance of Pi in the secondary Ca-P [(Ca2-P)+(Ca8-P)], nonoccluded Fe- and Al-P [(AIP)+(Fe-P)J and acid extractable P (primary Ca phosphate (Ca10-P)) was 2.5:2:1 and in cultivated soils was 2:2.5:1, respectively. Statistical analysis indicates that the Ca2-P, Al-P, Fe-P and Calo-P fractions made significant contributions to available P as estimated by the Olsen and Colwell methods. Stepwise regression showed that most of variation in OIsen-P (90%) and Colwell-P (82%) was accounted for by Ca2-P. Scanning electron microscopy (SEM) showed that P was uniformly distributed in the soil matrix to the limit of the spatial resolution and sensitivity of this technique. There were no local concentrations of P or spatial associations of other elements with P.

In this research, two separate experiments were carried out as factorial on the basis of completely randomized design with three replications. In the first experiment, the effects of cultivation, salinity, and phosphorus (P) fertilizer on available-P and inorganic P fractions were investigated with three factors of cultivation type at six levels (No cultivation, wheat monoculture, lathyrus monoculture, vetch monoculture, wheat-Lathyrus intercropping, wheat-vetch intercropping), P fertilizer at two levels (zero and 80 mg P/kg) and soil salinity (ECe) at two levels (0.75 and 7.5 dS/m) under greenhouse conditions. The second experiment was conducted to investigate the effects of ageing, salinity, and P fertilizer on Olsen-P and inorganic P fractions with three factors of time at two levels (zero and 90 days), P at two levels (zero and 80 mg P/kg), and ECe at two levels (0.75 and 7.5 dS/m) under laboratory conditions. Results showed that the cultivation of wheat, lathyrus, and vetch in both monoculture and intercropping conditions decreased Olsen-P, NH4F-P, NaOH-P, H2SO4-P, and sum of P fractions. Under P using conditions, available-P in the rhizosphere soil was more than the one in bulk soil and in intercroping it was more than the one in monoculture. In 90 days after P using, NaCl salinity decreased NH4Cl-P, NH4F-P, and NaOH-P while increased Olsen-P and H2SO4-P compared to non-saline conditions. Aging after P using decreased NH4Cl-P and Olsen-P and increased NH4F-P, NaOH-P, and H2SO4-P. Under P using conditions, Olsen extractant may underestimate soil P availablity for wheat, lathyrus, and vetch plants at both monoculture and intercropping conditions. Generally, intercropping of wheat-Lathyrus and wheat-vetch and using 80 mg P fertilizer per kg of soil can be recommedded under similar conditions.

Effects of different soil moisture regimes and P application on available and inorganic P forms were studied in four paddy soils, north of Iran. A factorial experiment in a completely randomized design with two replications was performed in a laboratory study. Factors of soil at four levels (two acid and two alkaline-calcareous soils), soil moisture contents at three different levels (air-dry, 40% Saturation Percentage (SP), and submerged) as well as P fertilizer at two levels (with and without P fertilizer application). Phosphorus fertilizer was added at a rate of 40 mg P per kg of soil as monocalcium phosphate. The treatments were incubated for one month at 25oC and then analyzed for P fractions employing the method of Jiang and Gu (1989) for calcareous soils and of Kuo (1996) for non-calcareous soils. Available P, was determined through Olsen method. The results indicated that the level of available P, in all soils, increased on the average more than two times following submergence. In acidic soils, the level of available P increased more than that in alkaline-calcareous soils following submergence. After one month of incubation at 40% SP and with no P application, in three out of four soils, the available-P increased, while in one soil it did not change significantly. The average recovery of added P under submerged conditions was lower than that under unsubmerged conditions. The average recovery of added P in alkaline-calcareous soils was higher than that in acidic soils. Effects of soil moisture regimes and P fertilizer on all inorganic P fractions were significant in all soils studied. Higher levels of added P were converted to Fe-P under unsubmerged conditions. Higher amounts of added P were transformed to Al-P in alkaline-calcareous soils relative to acidic soils under both nonsubmerged and submerged conditions. Higher levels of added P were converted to Fe-P in acidic soils relative to alkaline-calcareous ones under at both nonsubmerged vs. submerged conditions. In air dried alkaline-calcareous soils, the following order was observed for inorganic phosphorus fractions: Fe-P<Ca2-P<Ca8-P<Al-P<Ca10-P; and in acidic soils, under the same conditions, the order observed was as follows: NH4Cl-P<Al-P<Fe-P<Ca10-P.

In a greenhouse experiment, changes in inorganic phosphorus fractions as well as their availability in the rhizosphere of rice (Oryza sativa c.v. Khazar) crop, following an application of 40 mg P (as monocalcium phosphate per kg of soil) were investigated in paddy soils north of Iran. The study was performed as a 14×2 factorial in a completely randomized design of two factors namely: soil at 14 levels (10 calcareous vs. 4 non-calcareous) and cultivation at two levels (cultivated vs. uncultivated). Each level of soil and cultivation were performed in examined two replicates. Following a period of three months, specially designed soil tubes, buried in soil at the beginning of the experiment, were simultaneously drawn out of the cultivated and of the uncultivated pots and then immediately analyzed for P fractionation. The method of Jiang and Gu (1989) was employed for determinations in calcareous soils, of Kuo (1996) for non-calcareous soils) while the available-P was determined through Olsen method. The results indicated that in all the 14 soils, the available soil P (Olsen-P) in the rhizosphere of rice was significantly lower than that in the non-rhizosphere (uncultivated soil). But, following the plant P uptake and its addition to the extractable-P (Olsen-P) in the rhizosphere soil, the total available-P in the rhizosphere soil was higher than that in the non-rhizosphere one. In calcareous soils, dicalcium-phosphates, octacalcium-phosphates, and Al-phosphates (Al-P) in the rhizosphere of the rice crop were significantly lower than those in the non-rhizosphere soil; whereas iron oxides occluded-phosphates and Fe-phosphates (Fe-P) in the rhizosphere of rice did not significantly differ from those in the non-rhizosphere (uncultivated) soil. Only in two calcareous soils, the apatite-P in the rhizosphere of rice was significantly lower than that in the non-rhizosphere soil. In non-calcareous and acidic soils, readily soluble-P, Fe-P, and Al-P in the rhizosphere of the rice crop were significantly lower than those in the non-rhizosphere soil. Apatite-P and Fe oxide occluded-P did not significantly differ from those in the non-rhizosphere soil. The results of the research finally revealed that the Olsen extractant may underestimate the availability of P under laboratory (air dry) conditions.

To evaluate the relationships between soil inorganic phosphorus P (Pi) fractions, the soil P test and plant parameters such as plant P uptake, dry matter yield, tissue P concentration and relative yield, glasshouse experiments and chemical analyses were conducted on 13 calcareous soils from six agricultural and seven adjacent bushland (virgin soil) sites. Four rates of P (0, 15, 30, 60 mg/kg soil) were applied as reagent-grade KH2PO4 to the soils in a randomised complete block design with three replications. Perennial ryegrass (Lolium perenne cv. Roper) was grown and forage was harvested five times over a period of 210 days. Successive harvesting resulted in the depletion of plant available P as measured by NaHCO3-extractable P, which coincided with the decrease in the plant dry matter yield and P uptake. After five harvests, the order of reduction in Pi fractions induced by cropping without added P was Ca10-P>Al-P>Ca2-P>Ca8-P>occluded-P>Fe-P for the virgin soils and Ca2-P>Al-P>Ca10-P>Ca8-P>Fe-P>occluded-P for the agricultural soils. The order of abundance of Pi fractions for P treated-soils was non-occluded Al and Fe phosphate (Al-P+Fe-P)>secondary Ca-bound P (Ca2-P+Ca8-P)>acid-extractable P (Ca10- P)>occluded-P for both virgin and agricultural soils. Although a marked proportion of added P was transformed into less soluble Al and Fe phosphates, successive harvesting had depleted considerable percentages of these fractions. Highly significant (p<0.001) relationships were found for P uptake vs. Olsen-P, P uptake vs. Pi fractions (Ca2-P, Ca10-P, Al-P, Ca8-P, Fe-P) and Olsen-P vs. Pi fractions. NaHCO3-extractable P seems to be adequate for evaluating plant available P in calcareous soils. However, the closer relationship for the Fe-P fraction vs plant P uptake than for Olsen-P versus plant P uptake indicates that NaHCO3 may not provide the best estimate of plant available P for calcareous soils. Using stepwise regression analysis, it was found that the Ca2-P fraction was most predictive of P uptake (60%), total dry matter (68%), relative yield (74%) and Olsen-P (69%), followed by the Fe-P fraction.

In order to study the effect of different sources of sulfur on inorganic phosphorus fractions in a calcareous soil, an incubation experiment was conducted in a completely randomized design with five treatments in three replications. Treatments were including: control (without sulfur compounds), Al2(SO4)3, FeSO4, S + Ti (sulfur + thiobacillius) and H2SO4 at 2% (w/w). Amendments were incubated at 25± 1 ° C for a period of 12 days. Then, available-P (Olsen-P) and different forms of inorganic phosphorus were determined by sequential extraction method. The application of sulfur compounds caused significant changes (p≤ 0. 05) in the amount of P-Olsen and inorganic forms of phosphorus (Ca2-P, Ca8-P, Al-P, Fe-P, Ca10-P) compared to control. The amount of available phosphorus in H2SO4, Al2(SO4)3 and S + Ti compared to control increased 240, 70 and 50%, respectively. Also, these amendments caused to insoluble forms of phosphorus, Ca8-P and Ca10-P, significantly reduced and in more soluble forms of Ca2-P and available-P could be used. In the amendments, FeSO4 caused 30% decrease in available-P and 4% increase in apatite compared to control. The results of correlation studies showed that available phosphorus (Olsen extracted phosphorus) had a significant correlation with Ca2-P, Ca8-P, Ca10-P and residual-P; the results of this study showed the effect of sulfur compounds on the distribution of phosphorus in different forms, so it is expected that Sulfur sources affect the usability and phosphorus chemistry...

The objectives of the present study were to evaluate the effect of heating on various inorganic phosphorus (P) fractions and sorption characteristics of highly calcareous soils of Kohgyloyeh and Boier-Ahmad province. Treatments were consisted of 5 soils and 4 heating treatment (untreated, 200, 350 and 550oC) for two hour. P fractionation included successive extraction with NaHCO3 (NaHCO3-P); pedogenic Ca-P (NH4OAc-P+MgCl2-P); NH4F (NH4F-P); NaOHNa2CO3 (HC-P), citrate-dithionite-bicarbonate (CBD-P), and H2SO4 (H2SO4-P) carried on 1 g sample in duplicate. P sorption was studied by equilibrating 2 g of samples with 20 ml solution containing 5 to 400 mg P kg-1 soil with 0.01 M KCl as background solution in duplicate. Results indicated that generally high temperatures (350 and 550o C) significantly increased NaHCO3-P, pedogenic Ca-P and NH4F-P. H2SO4-P significantly decreased in 350oC but significantly increased by 550oC heating. These changes indicate the transformation of inorganic P fractions to each other in studied temperatures in addition to transformation of organic P to inorganic P. Quantity of P sorption in the unity of P concentration (KF) decreased by 550oC in soils with lower CCE and higher Fe oxides, while it increased in soils with higher CCE and lower Fe oxides content. This effect is probably due to the transformation of crystalline Fe oxides to forms with lower specific surface area, which regualte P retention in the low concentration of P.

Effects of rhizosphere of rice plant on the native inorganic phosphorus fractions in the paddy soils of north of Iran were investigated in a greenhouse experiment. The study was performed as a 14x2 factorial experiment in a randomized complete block design with 2 factors of soil (10 calcareous and 4 noncalcareous), and cultivation (cultivated and uncultivated), each level with 2 replicates. Bags of soil samples, buried in experimental pots (cultivated and uncultivated), were taken out simultaneously after a 3 month period, and their inorganic P fractions measured by the sequential extraction methods of Jiang and Gu (1989) in calcareous soils, and Kuo (1996) in noncalcareous soils. The available P in all soils was measured by Olsen method. In a different experiment, a special pot of 1 m2x 30 cm depth was designed to measure the variation in available P with distance from the plant stem. In this pot, the plant spacing was 25 cm, and bags of soil samples were buried in the soil at different distances from the plant stem. The results of these experiments are as follows: 1- In all 14 soils, the Olsen-P in the rhizosphere of rice was significantly lower than that in the uncultivated soils. 2- In calcareous soils, the dicalcium phosphates, the octacalcium phosphates, the AI-phosphates, the P in apatites, and the occluded-P in the rhizosphere of rice were significantly lower than those in the uncultivated soils; whereas the Fe-phosphates was not significantly different in the two conditions. 3- In noncalcareous soils, the readily soluble-P, the Fe-phosphates, and the AI-phosphates in the rhizosphere of rice were significantly lower than those in the uncultivated soils; whereas the occluded-P and the apatites were not significantly different in the two conditions 4- The Olsen-P of the soil between the two hills of rice plant in the special pot experiment, did not vary with the distance from the plant stem, whereas, the Olsen-P in the cultivated soil was significantly lower than that in the uncultivated soil. 5- In all soils, the pH of the water on the surface of the soils in the cultivated pots was significantly lower than that in the uncultivated pots at the end of the growth period.

In order to investigate the effects of corn-wheat rotationon the available-P and inorganic phosphorus fractions of soil, with and without P application, agreenhouse study was carried out as a 2 x3 x2 factorial experiment in a randomized complete blocks design. Treatments included two types of soil, 3 levels of P, and two planting treatments (corn-wheat rotation vs fallow land), with 3 replications. In a period of 5 months, variations of Olsen-P and soil inorganic phosphorus fractions were studied. The results indicated that effects of soil P application and planting treatment on Olsen-P and soil inorganic phosphorus fractions were significant.Within creasing P application, soil inorganic phosphorus fractions increased, but, with balanced P application, P fixation in soil was decreased. In corn-wheat rotation, suitable P application rates at the time of planting corn will also meet the requirement of the next crop of wheat.

Soil carbon changes is one of the most important indicators impacts of climate change impacts on soil genesis. Study of soil carbon, including organic and inorganic carbon (carbonates) and its impact on other soil characteristics in different climates, is essential for the proper management of soil carbon on a global scale. It is too important the balance between different parts of carbon sources in the environment. In current study, organic and inorganic carbon complex with primary particles were studied in 9 profiles a climosequence including three climates arid, semi-arid and semi-humid with typic aridic, dry xeric and typic xeric moisture regimes and mesic and thermic temperature regimes. Results showed that the amount of organic carbon in all three components decreases with increasing depth and also clay component has more organic carbon content in all depths compared with other components of soil. Contrast to organic carbon, inorganic carbon content is lower in surface horizon compared with the subsurface in all three components of particle size and increases with increasing depth. The avearage of organic carbon components of particle size in three studied regions showed that the following trend clay (1. 1%) > silt (0. 68%) > sand (0. 23%), while inorganic carbon in the soil with trend silt (14. 41%) > sand (12. 11%) > clay (8. 14%).