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1.
Recent investigations have shown that phosphate (P) mobilization by root exudates is an important feature of genotypes to acquire P even in soils of low‐P availability. We, therefore, investigated P mobilization processes in the rhizosphere of red clover (Trifolium pratense L.) and black mustard (Brassica nigra L.) on a humic podzol. As measured by the Kuchenbuch‐Jungk method (Kuchenbuch and Jungk, 1982), both species accumulated similar quantities of citrate (12 μmol/g soil) in the rhizosphere in about 1 mm distance from the soil‐root interface. Despite of similar concentrations of P‐mobilizing citrate in the rhizosphere of both species, red clover took up nearly the two‐fold of P compared to black mustard. Differences in rhizosphere pH were determined between both species. Black mustard did not acidify the rhizosphere, whereas red clover decreased the pH in the rhizosphere from 5.8 to about 4.0 (in 0.01M CaCl2). The simultaneous acidification and excretion of citrate compared to citrate excretion alone had consequences for P mobilization processes in the rhizosphere. Phosphate mobilization from the soil solid phase was higher at higher pH. Thus, the citrate‐induced P desorption was not the limiting step in P acquisition by red clover and black mustard. Calculations of P distribution in the soil solution between free ortho‐P and humic‐associated P showed that at higher pH most of the P was associated with dissolved humic substances, whereas at pH < 5, most of the P was present as free ortho‐P. These P species can readily be taken up by the roots whereas humic‐associated P must probably be desorbed from the humic surface before uptake. Phosphate species calculations, therefore, explained the higher P uptake of red clover compared to black mustard. Aluminum species distribution calculations in the soil solution further show that even at pH < 5.0 in the soil solution, citrate strongly complex Al and thereby reduce the activity of monomeric Al species. The excretion of citrate can, therefore, counteract the root induced acidification of the rhizosphere with respect to Al toxicity.  相似文献   

2.
Billions of dollars are spent annually in Brazil on imported phosphorus (P) fertilizers for agricultural crops produced on Oxisols. Phosphate fixation by Oxisols is a main limitation to crop productivity because these highly weathered soils have a high phosphate adsorption capacity. The objective was to determine whether reaction of an Acric Oxisol with citrate increases P availability. Columns of P-amended subsoil material were leached with 1 mM citrate solution, and effluent was monitored. Solid-phase speciation was measured for different stages of leaching using X-ray absorption near edge structure (XANES) spectroscopy at the P K-edge. When soil columns were leached with up to 56 column pore volumes of 1 mM citrate solutions, no P was detected in column effluent samples, whereas aluminum (Al) and iron (Fe) increased up to 30 and 1.4 µmol L?1, respectively. The XANES analysis suggested that with increased leaching, a minor proportion of phosphate adsorbed on Fe-oxides increased as phosphate adsorbed on Al-oxides decreased. The results indicated that citrate tends to induce minor species redistribution of phosphate between Al- and Fe-oxide bound forms, but P mobilization was trivial compared with complexometric mobilization of Fe and Al.  相似文献   

3.
R.J. Haynes  R.S. Swift 《Geoderma》1985,35(2):145-157
The effects of air-drying field-moist soils on the adsorption and desorption of added phosphate and on the levels of extractable native soil phosphate were examined using the A and B horizons of a group of four acid soils.Air-drying increased the capacity of all the soil samples to adsorb phosphate. At an equilibrium solution concentration of 0.5 μg P ml?1, the increase in the quantity of phosphate adsorbed following drying ranged from 23% to 70% of that adsorbed by the moist samples. Considerable hysteresis in phosphate adsorption—desorption isotherms was observed for both moist and dried soil samples indicating that the additional phosphate adsorbed by the dried samples was held with the same strength as that held by the moist samples.Air-drying the soil samples caused a small decrease in soil pH of approximately 0.1 pH unit and a general increase in levels of EDTA-extractable Fe, Al and organic matter. Quantities of native soil phosphate extractable with EDTA, resin and NaHCO3 were also increased. Concentrations of oxalate- and pyrophosphate-extractable Fe and Al and exchangeable Al were, however, unaffected by drying.It was also shown that when the phosphate content of NaHCO3 extracts is measured using the conventional molybdenum blue method, orthophosphate plus a differing amount of acid-hydrolysable organic P present in the extract is measured.  相似文献   

4.
Rhizosphere processes strongly influence the availability of phosphorus (P) to plants. Organic ligands that are exuded from the root surface mobilize phosphorus by dissolution of P minerals or by desorption of adsorbed phosphate. We developed a mechanistic model to study the mobilization of phosphate sorbed on goethite by the exudation of citrate and consequent uptake of phosphate by the root. The use of a model allows the effects of the organic anion and pH on P desorption to be separated. The model is also used to predict concentration profiles developing around the root for phosphate, citrate (with or without accounting for degradation) and pH, providing insight into the processes that occur in the rhizosphere. Results of model calculations show that with larger rates of citrate exudation, greater P availability is predicted. Exudation at a rate of 0.5 μmol citrate m–1 root day–1, which is in the range found for P-deficient plants, increased P availability almost 2-fold at fairly large phosphate loading of goethite (1.9 μmol m–2) and almost 30-fold at small phosphate loading (1.3 μmol m–2). Competitive adsorption causes a much greater relative increase in the phosphate concentration in solution at small than at large phosphate loading, which explains this result. Simultaneous acidification of the rhizosphere results in a smaller P mobilization than at a fixed pH of 5, as a result of the pH dependence of phosphate adsorption in the presence of citrate. Sorption of citrate increases its persistence against microbial decay, and hence has a positive effect on the mobilization of adsorbed phosphate.  相似文献   

5.
Abstract

The immobilization of P is a significant fertility limitation of Andisols in Central America. It is believed that soil Al and Fe fractions have an important influence on P availability. This study was conducted to obtain information on the various forms of Al and Fe in ten pedons derived from volcanic ash in Panama and Costa Rica. Correlations between these Al and Fe fractions and P immobilization were measured by different methods.

The Al and Fe fractions, extracted by acid ammonium oxalate, 4M KOH, sodium pyrophosphate and dithionite‐citrate, were determined, and the correlations between these fractions, P immobilization, and other soil properties made.

It was observed that oxalate extractable Al correlated significantly with P immobilization, which had values of over 85% by the New Zealand method and 96% by the isotherm method. The Al concentration corresponding to this immobilization was over 2%. The difference between oxalate extractable Al and pyrophosphate extractable Al (inorganic Al fraction) correlated with P immobilization also. The pyrophosphate extractable Al fraction had the lowest concentrations and did not correlate with P immobilization. The NaF‐pH and dithionite‐citrate extractable Al correlated significantly with P immobilization.

Oxalate extractable Fe correlated with the P retained by the isotherm method, but dithionite‐citrate extractable Fe was negatively correlation with P immobilized. No correlations were found between the pyrophosphate extractable Fe and other soil properties.  相似文献   

6.
Abstract

Surface horizons from Podzolic and Gleysolic soils were collected in various parts of the province of Quebec, Canada, and equilibrated with various amounts of KH2PO4 in 0.01 M CaCl2 for 48 hours. P sorption data conformed to the linear form of the Langmuir and Freundlich equations. P solubility isotherms showed evidence of hydroxyapatite formation in most samples studied, whereas equilibration solutions of only few samples were saturated with respect to either dicalcium phoshate dihydrate or octocalcium phosphate. These reaction products were associated to soil pH and levels of added phosphate. The average values of the Langmuir sorption maximum for these studied Gleysolic and Podzolic samples were 763 and 1096 μg/g respectively. These values were higher than those obtained by the segmented and modified Freundlich models.

Relationships between the soil characteristics and P sorption parameters were evaluated by regression analysis. Among all variables, oxalate‐extractable Fe plus Al content of the Podzolic samples and the ratio of oxalate—extractable Al to clay of the Gleysolic samples gave the best significant correlation coefficients. Furthermore, soil pH and various ratios such as pyrophosphate‐extractable Fe and Al, oxalate‐extractable Fe and organic matter to clay were found to be significantly correlated only with the P sorption parameters of the Gleysolic samples.  相似文献   

7.
Abstract

Phosphate (P) sorption characteristics of six natural Ghanaian Oxisols, selected because of their hydrological and topographical suitability for agriculture, were evaluated. Availability of P appears to be adequate for half of the soils as suggested by the Bray P1 test and determination of the standard P requirement (SPR), i.e., the amount of P sorbed at a concentration of 0.2 ppm P (6.46 μM). The SPR was found to be very closely related to Pmax (Langmuir P sorption capacity), which in turn, was significantly correlated with oxalate‐extractable aluminum (Al) (Alo) and iron (Fe) (Feo) and related (not significantly) to the difference between dithionite‐citrate‐bicarbonate‐extractable Fe (Fed) and oxalate‐extractable Fe. Accordingly, Pmax is fairly well predicted by the model of Borggaard: Pca]e=0.211#lbÀlo+0.115#lbFeo+ 0.05#lb(Fed‐Feo)+0.3, except for one soil strongly enriched in Fe oxides, mainly goethite. This goethite was found by X‐ray diffraction analysis to consist of crystals larger than normally found for pedogenic Fe oxides. The difference between Pmax and Pcalc for this soil could, therefore, be attributed to the occurrence of these large Fe oxide crystals, because P sorption will decrease with increasing crystal size (decreasing specific surface area).  相似文献   

8.
The effect of humic substances on P-availability in soil is still debated. Therefore, the effect of model humic substances synthesized from hydroquinone on P, Fe, and Al solubility in a strong P fixing Oxisol and on P acquisition by red clover and ryegrass was investigated. After 4 months of incubation, P concentration of soil solution had increased by a factor of > 10 at the highest humic level (50 g humics kg?1 soil), accompanied by a similar increase in Fe and Al concentrations. Soil samples with 0, 10, 30, 50 g humics kg?1 soil were planted with red clover and ryegrass. Red clover showed a small increase of shoot yield and a moderate increase of P uptake after humics addition. High humics levels increased slightly Fe concentration in the shoots but strongly that of Mn leading to Mn toxicity. Ryegrass showed a strong increase in shoot yield after humics addition of about 150 % at the highest humics level compared to the control without humics. At each humic level, P application (100 mg kg?1 soil) had no effect on P uptake of red clover and a small effect on P uptake by ryegrass. The relatively small effect of humics and P application on shoot yield of clover compared to grass can be explained by chemical P mobilization of red clover via exudation of citrate (about 12 μmol citrate g?1 soil). This agrees with the finding that P solubility increased in the soil under red clover but not under ryegrass from the first to the second harvest, indicating that red clover mobilized P.  相似文献   

9.
铝和镉胁迫对两个大麦品种矿质营养和根系分泌物的影响   总被引：7，自引：0，他引：7  
A hydroponic experiment was carried out to study the effect of aluminum （Al） and cadmium （Cd） on Al and mineral nutrient contents in plants and Al-induced organic acid exudation in two barley varieties with different Al tolerance. Al- sensitive cv. Shang 70-119 had significantly higher Al content and accumulation in plants than Al-tolerant cv. Gebeina, especially in roots, when subjected to low pH （4.0） and Al treatments （100 μmol L^-1 Al and 100 μmol L^-1 Al ＋1.0 μmol L^-1 Cd）. Cd addition increased Al content in plants exposed to Al stress. Both low pH and Al treatments caused marked reduction in Ca and Mg contents in all plant parts, P and K contents in the shoots and leaves, Fe, Zn and Mo contents in the leaves, Zn and B contents in the shoots, and Mn contents both in the roots and leaves. Moreover, changes in nutrient concentrations were greater in the plants exposed to both Al and Cd than in those exposed only to Al treatment. A dramatic enhancement of malate, citrate, and succinate was found in the plants exposed to 100 μmol L^-1 Al relative to the control, and the Al-tolerant cultivar had a considerable higher exudation of these organic acids than the Al-sensitive one, indicating that Al-induced enhancement of these organic acids is very likely to be associated with Al tolerance.  相似文献   

10.
Root-induced changes in the rhizosphere may affect mineral nutrition of plants in various ways. Examples for this are changes in rhizosphere pH in response to the source of nitrogen (NH4-N versus NO3-N), and iron and phosphorus deficiency. These pH changes can readily be demonstrated by infiltration of the soil with agar containing a pH indicator. The rhizosphere pH may be as much as 2 units higher or lower than the pH of the bulk soil. Also along the roots distinct differences in rhizosphere pH exist. In response to iron deficiency most plant species in their apical root zones increase the rate of H+ net excretion (acidification), the reducing capacity, the rate of FeIII reduction and iron uptake. Also manganese reduction and uptake is increased several-fold, leading to high manganese concentrations in iron deficient plants. Low-molecular-weight root exudates may enhance mobilization of mineral nutrients in the rhizosphere. In response to iron deficiency, roots of grass species release non-proteinogenic amino acids (?phytosiderophores”?) which dissolve inorganic iron compounds by chelation of FeIII and also mediate the plasma membrane transport of this chelated iron into the roots. A particular mechanism of mobilization of phosphorus in the rhizosphere exists in white lupin (Lupinus albus L.). In this species, phosphorus deficiency induces the formation of so-called proteoid roots. In these root zones sparingly soluble iron and aluminium phosphates are mobilized by the exudation of chelating substances (probably citrate), net excretion of H+ and increase in the reducing capacity. In mixed culture with white lupin, phosphorus uptake per unit root length of wheat (Triticum aestivum L.) plants from a soil low in available P is increased, indicating that wheat can take up phosphorus mobilized in the proteoid root zones of lupin. At the rhizoplane and in the root (root homogenates) of several plant species grown in different soils, of the total number of bacteria less than 1 % are N2-fixing (diazotrophe) bacteria, mainly Enterobacter and Klebsiella. The proportion of the diazotroph bacteria is higher in the rhizosphere soil. This discrimination of diazotroph bacteria in the rhizosphere is increased with foliar application of combined nitrogen. Inoculation with the diazotroph bacteria Azospirillum increases root length and enhances formation of lateral roots and root hairs similarly as does application of auxin (IAA). Thus rhizosphere bacteria such as Azospirillum may affect mineral nutrition and plant growth indirectly rather than by supply of nitrogen.  相似文献   

11.
The solubility of Al and Fe in soil is of relevance for their toxicity and availability, respectively, to plant roots. Humic substances as the main part of stable soil organic matter and citrate which is often excreted by P deficient plants are strong complexants of Al and Fe(III). Therefore, equations were developed to calculate the Al and Fe(III) species distribution in the soil solution in the presence of humic substances and citrate as organic ligands. Calculations in the pH range 4.0–7.0 showed that at higher pH humic-Al complexes were the most important species whereas AlOH-citrate? dominated between pH 4.0 and 5.4. Free monomeric Al and AlSO4+ were of minor relevance. Iron(III) species calculations showed that humic-Fe complexes were the main species in the pH range 4.0–7.0. But if mugineic acid, a Fe complexing phytosiderophore released into the rhizosphere by graminaceous plant species, was present in the soil solution (10?6 M), Fe-mugineic acid complexes accounted for most of the Fe in solution. Fe-citrate? was relevant at lower pH but contributed little to Fe(III) species at pH > 6.0. The results demonstrate the strong importance of the considered organic ligands for Fe and Al in the soil solution.  相似文献   

12.
Phosphorus is one of the most limiting macronutrients for plant productivity in agriculture worldwide. The main reasons are the limited rock phosphate reserves and the high affinity of phosphate (P) to the soil solid phase, restricting the P availability to the plant roots. Plants can adapt to soils low in available P by changing morphological or/and physiological root features. Morphological changes include the formation of longer root hairs and a higher root : shoot ratio both parameters increasing the root surface which provides the shoot with P. This may be successful if the P availability in soil, i.e., the P concentration of the soil solution is not extremely low (> 1–2 µM P). If the P concentration of the soil solution is lower, the diffusive flux to the root surface will be very low and may not satisfy the P demand of the shoots. Under these conditions plants have developed strategies to increase the rhizosphere soil solution concentration by secreting mobilizing agents. The most effective way of P mobilization is the release of di‐ and tricarboxylic acid anions, especially oxalate and citrate. Citrate can accumulate in the rhizosphere up to concentrations up to 80 µmol g?1 soil. Cluster root formation is an efficient way of carboxylate accumulation in the cluster root rhizosphere improving P mobilization. Cluster roots strongly improve the acquisition of the mobilized P. Considering a single root, around 80–90% of the mobilized P diffuses away from the root. From the rhizosphere of cluster roots, most of the mobilized P is taken up by the cluster roots. Both, the strong accumulation of carboxylates in and the effective P uptake from the cluster‐root rhizosphere are the basis of the unique ability of P acquisition by cluster root‐forming plants. Plants that do not form cluster roots, e.g., red clover, can also accumulate carboxylates in the rhizosphere. Red clover accumulates high quantities of citrate in the rhizosphere soil. Model calculations show that the release of citrate by red clover roots and its accumulation in the rhizosphere strongly improve P acquisition by this plant species in various soils. Similar results are obtained with alfalfa. In sugar beet, oxalate release can strongly contribute to P acquisition. In summary, P acquisition can be strongly improved by the release of carboxylates and should be taken as a challenge for basic and applied research.  相似文献   

13.
Several plant species excrete di- and tricarboxylic acids as a reaction to P and Fe deficiency. Therefore, the effect of citric acid on P, Al, and Fe soil solution concentrations of three soils was investigated in an incubation experiment. Citric acid additions of 10–50 μmole/g soil increased P, Al, and Fe solubility. The effect was strongly pronounced in the podzol with humic-P complexes as the main P fraction. The maximum increase was about 80 μmole P/L in the luvisol, 500 μmole P/L oxisol, and 3000 μmole P/L in the podzol. The Fe and Al solubility increased parallel to that of P with a ratio of Fe+Alsolubized/Psolubized of 10 or more. The solubization effect by citric acid was strong even after 90 days. Possible solubization reactions are discussed.  相似文献   

14.
Abstract

Different forms of soil aluminum (Al) are involved in the retention of anions and cations, phytotoxicity of Al in acid soils, CEC reduction and soil physical properties such as aggregate stability and water infiltration. Therefore it is desirable to quantify the different forms of Al in soil especially acidic soils. A rationale was developed from a literature survey to identify the following fractions of Al: (a) exchangeable quantified by 1M KC1 extraction; (b) organic bound quantified by 0.1M CuCl2 + 0.5M KCl extraction; (c) sorhed Al extractable with 1M NE4OAc at pH 4.0; (d) amorphous Al oxide and hydroxide and amorphous aluminosilicates (if present) extractable with 0.2M ammonium oxalate at pH 3.0; and (e) interlayered Al extractable with 0. 33M sodium citrate at pH 7.3. Pools (a), (b), and (c) are extracted sequentially. Amorphous Al oxide and hydroxide (pool d) is calculated from ammonium oxalate extractable Al minus (a + b + c). Interlayered Al is calculated from sodium citrate extractable Al minus ammonium oxalate extractable Al. The latter two extractions are done on separate subsamples of soils. From preliminary studies and data for 13 soil samples it is suggested that this fractionation of soil Al is more meaningful than that obtained by the KCl ‐> K4P2O7 ‐> ammonium oxalate > citrate‐bicarbonate‐dithionite extraction sequence.  相似文献   

15.
The long-term fertilization results in accumulation of phosphorus especially in the top layer of the soils. Inundation of agricultural lands leads to a switch to anaerobic soil condition, causing reduction of iron and leaching of phosphate simultaneously. From the ecological and environmental perspective, high nutrients flux especially phosphorus will increase the possibility of eutrophication in aquatic system. The fern Azolla had a good potential to adsorb phosphorus, it also has distinctive nitrogen-fixing capacity. We conducted a 10-week aquarium experiment to investigate the phosphorus release capacity from two agricultural soils in the Netherlands with different Fe and P concentrations but comparable Fe/P ratios. Besides, the research questions rose to whether Azolla could use the mobilized phosphate released from the soils for growth. We also tried to find an effective indicator to estimate the actually phosphate mobilization from sediment to water layer. Results showed that the soils with high Fe and P concentrations had higher phosphate release rate compared with the soil with low Fe and P concentrations. Pore water Fe: PO43? ratios were valid to identify P release to surface water, when the Fe: PO43? ratios less than 8 mol mol?1 substantial phosphorus mobilization occurred. The conclusions showed that the actual mobilization of phosphate is more important than the phosphorus retained in the sediments for the internal PO43? fluxes. From 10-week experimental results, we found that Azolla can reuse the phosphate retained in soils thus removed the mobilized phosphate in a moderately low surface water nutrient loading.  相似文献   

16.
Equal or comparable contents of Fe and Al extractable by Tamm and Mehra-Jackson solutions have been revealed in all the horizons of a loamy mucky-peaty-podzolic gley soil on binary deposits. The content of Si extractable by the Mehra-Jackson solution has exceeded that of oxalate-soluble Si by an order of magnitude. The distributions of Al in the Tamm solutions from the entire soil and its fractions of 1–5 and >5 μm are of accumulative type with a maximum in the mucky H horizon and a gradual decrease of the content with depth in relation with the analogous distribution of Al-organic complexes. The maximum content of oxalate-soluble Al in the clay fraction has been found in the eluvial ELg horizon, which can be due to the partial dissolution of Al hydroxide interlayers in soil chlorites. The distribution of Fe in the entire soil has two maximums, in the H horizon due to the accumulation of Fe-organic complexes and in the concretion-rich ELnn,g horizon due to the accumulation of Fe hydroxides. Depletion of oxalate-soluble Fe in the eluvial ELg horizon has been observed in all the fractions, which can be related to its mobilization and removal under strongly acidic conditions and the development of reductive processes, as well as the enrichment of the concretion-rich horizon with these compounds because of an increase in pH and the development of conditions favorable for water stagnation and Fe segregation.  相似文献   

17.
Model calculations were made in order to quantify the effect of carboxylate excretion on phosphate (P) uptake by a single root. The uptake of chemically mobilized P increased exponentially with increasing concentration of adsorbed citrate or oxalate in soil because of the exponential relationship between adsorbed carboxylate and the solubilizing effect of carboxylate on P. The effect of local citrate excretion compared with uniform citrate excretion along the whole root was also calculated. Local exudation increased the uptake of chemically mobilized P because the higher concentration of citrate increases the solubilization of P. Additionally the effect of citrate excretion by root clusters e.g. proteoid roots was evaluated. Uptake of chemically mobilized P by root clusters was much higher than that of single roots, especially if the ratio of P buffering to citrate buffering was high. This is often the case in P fixing soils where by definition P buffering is high and citrate buffering is low because of the short time of reaction between root excreted citrate and rhizosphere soil. The reason for the superiority of cluster roots lies in the fact that most of the mobilized P is transported away from a single root to be absorbed by neighbouring roots in the clusters. This phenomenon demonstrates the strong ecological significance of cluster roots in relation to nutrient mobilization. The calculations on the effect of oxalate excretion by sugar beet roots on the uptake of mobilized P show that under P fixing conditions the influx of mobilized P will exceed that of P transported by diffusion to the root surface by a factor of 1.5—6.0.  相似文献   

18.
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19.
We extracted molybdenum (Mo) from eight acid forest soils (19 A, E, and B horizons) in NE-Bavaria and from one site in the Ore Mountains, using (1) anion exchange-resin, (2) 0.2 M ammonium oxalate, and (3) ascorbic acid/ammonium oxalate. The Mo concentrations in the anion exchange-resin fraction varied between 5 and 28 μg kg-1. Oxalate-extractable Mo ranged from 44 to 407 μg kg-1 and after reduction of iron (Fe) with ascorbic acid, 135 to 1071 μg Mo kg-1 were extracted. The lowest concentrations of Mo were measured in acid and sesquioxide impoverished E horizons. The total concentrations of Mo in spruce needles correlated with ion exchange resin extractable Mo, indicating that this fraction represents Mo readily available to plants. The Mo and Fe dissolution kinetics during oxalate extraction were studied on 8 of the soil samples to obtain further information on Mo mobilization. Oxalate extractable iron (Feo) was mobilized within a few hours. A first order equation was applicable to the Fe dissolution kinetics with the rate constants ranging between 0.9 and 9.0 h-1. The mobilization of Mo occurred in two distinct stages. An initially rapid dissolution was followed by a further increase in extractable Mo but with slower kinetics. A combined first order-diffusion equation was found to be appropriate for modelling the results. The first order rate constants for Mo mobilization ranged from 0.6 to 11.4 h-1. However, correlations between the rates of reaction of Mo and Fe could not be established, indicating that Mo is either not distributed equally along Fe minerals or that there is another pool, possibly the organic substance of the soil, from which Mo is extractable by oxalate.  相似文献   

20.
Abstract

Highly calcareous soils are abundant in Iran. The calcium carbonate equivalent (CCE) of these soils reach up to 650 g kg?1. Although phosphorus (P) fertilizer is being widely used in these soils, little information, if any, is available about P status in such soils. The objectives of this study were to 1) determine inorganic P forms in 18 surface soils of southern Iran, 2) study P readsorption during different stages of fractionation schemes, 3) assess the ability of NaOH to extract aluminum (Al)‐P, and 4) evaluate the relationships between P availability indices and inorganic P forms. Eighteen soil samples with a wide range of physicochemical properties were selected for this study. Inorganic P forms was determined by sequential extraction with NaHCO3, NH4OAc, NH4F, NaOH, citrate dithionite (CD), and H2SO4, which are referred to as Ca2‐P, Ca8‐P, Al‐P, Fe‐P, occluded P (O‐P), and Ca10‐P. Phosphorus readsorption in different stages was determined by 1 M MgCl2. Furthermore, a fractionation scheme without an NH4F step was used to evaluate the ability of NaOH to extract Al‐P. NaHCO3 (Olsen‐P) and MgCl2‐extractable P (Exch‐P) were regarded as P-availability indices. The abundance of different P forms was in the order Ca2‐P<Fe‐P<Al‐P<O‐P<Ca8‐P<Ca10‐P. Ca2‐P was highly correlated with Olsen‐P and Exch‐P. Ca2‐P, Olsen‐P, and Exch‐P showed a relationship with CCE, citrate–bicarbonate–dithionite extractable Fe (Fed), and Al (Ald). Phosphorus readsorption appeared to be important only in the Ca8‐P step, and the content of readsorbed P was related to Ca8‐P, CCE, and clay content of the soils. In the present study, Al‐P and Fe‐P accounted for 10 and 5% of the sum of the inorganic P fractions, respectively, and Fe‐P showed a strong relationship with Feo, whereas Al‐P showed a significant relationship with oxalate‐extractable Al (Alo) and Ald. It was found that one extraction with NaOH is not a good indicator for Fe‐ and Al‐P, and the ability of NaOH to extract Al‐P was reduced with increase in Al‐P content.  相似文献   

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