Sorghum and soybean response to natural and modified phosphate rock on acid soils in Venezuela

Abstract

An agronomic evaluation of phosphate rock (PR) on acid soils of Guarico and Anzoategui States was conducted to measure the effect on soil available phosphorus (P), grain yields of sorghum (Sorghum bicolor (L.) Moench) and soybean [Glycine max (L.) Merr.], and P absorption by these crops. Field experiments were established using a complete randomized block design with three replications. The treatments used were: triple superphosphate (TSP), North Carolina phosphate rock (NCRPR), compacted NCRPR with TSP at 60–40% and 70–30% (NCRPR 60/40 NCRPR 70/30), compacted Venezuelan Monte Fresco phosphate rock 60–40% and 70–30% (MFPR 60/40, MFPR 70/30), finely ground Monte Fresco phosphate rock (MFPR), finely ground Navay phosphate rock (NPR), and a check plot with no P source added. Five rates were applied, 0, 50,100,150, and 200 P₂O₅/ha, broadcasted and incorporated prior to planting. A basic fertilization of N, K, S, Mg, and Zn was uniformly applied to ensure an adequate supply of these nutrients. Soybean seeds were inoculated with Bradyrhizobium japonicum. Soil samples were taken before planting and at harvest, and plant samples taken at critical growth stage (flowering) for tissue analysis, Crop grain yields were measured at 12% moisture content. The relative agronomic effectiveness (RAE) was calculted by the formula: RAE (%( = Yield of the rock ? Yield of check plot/Yield of TSP ? Yield of check plot × 100 Maximum yields in all cases were obtained with TSP, however, in most cases there were no statistically significant differences between TSP and NCRPR as received or compacted (60/40 and 70/30). The compacted Venezuelan phosphate rocks also gave significantly higher yield and RAE responses showing the potential of these P sources for annual crops in the country. The finely ground Venezuelan phosphate rocks increased yields and RAE as compared to the check plot, but they gave the lowest response of all P sources. Soil available P after harvest had a tendency to increase where any source of P was applied or when the rates increased. The increment in soil available P was more evident when TSP and NCRPR as received and compacted were used. The lowest increments were reached with finely ground Navay phosphate rock. Leaf P concentration values were between the sufficiency range, except for those treatment where Monte Fresco and Navay finely ground phosphate rocks were applied at the rate of 50 kg P₂O₅/ha and for the check plot.     

Comparative Effects of Two Forage Species on Rhizosphere Acidification and Solubilization of Phosphate Rocks of Different Reactivity

ABSTRACT

Dissolution of phosphate rocks (PR) in soils requires an adequate supply of acid (H⁺) and the removal of the dissolved products [calcium (Ca^{2 +}) and dihydrogen phosphate (H₂PO₄ ^?)]. Plant roots may excrete H⁺ or OH^? in quantities that are stoichiometrically equal to excess cation or anion uptake in order to maintain internal electroneutrality. Extrusion of H⁺ or OH^? may affect rhizosphere pH and PR dissolution. Differences in rhizosphere acidity and solubilization of three PRs were compared with triple superphosphate between a grass (Brachiaria decumbens) and a legume (Stylosanthes guianensis) forage species at two pH levels (4.9 and 5.8) in a phosphorus (P)-deficient Ultisol with low Ca content. The experiment was performed in a growth chamber with pots designed to isolate rhizosphere and non-rhizosphere soil. Assessment of P solubility with chemical extractants led to ranking the PRs investigated as either low (Monte Fresco) or high solubility (Riecito and North Carolina). Solubilization of the PRs was influenced by both forage species and mineral composition of the PR. The low solubility PR had a higher content of calcite than the high solubility PRs, which led to increased soil pH values (> 7.0) and exchangeable Ca, and relatively little change in bicarbonate-extractable soil P. Rhizosphere soil pH decreased under Stylosanthes but increased under Brachiaria. The greater ability of Stylosanthes to acidify rhizosphere soil and solubilize PR relative to Brachiaria is attributed to differences between species in net ion uptake. Stylosanthes had an excess cation uptake, defined by a large Ca uptake and its dependence on N₂ fixation, which induced a significant H⁺ extrusion from roots to maintain cell electroneutrality. Brachiaria had an excess of anion uptake, with nitrate (NO₃ ^?) comprising 92% of total anion uptake. Nitrate and sulfate (SO₄ ^{2 ?}) reduction in Brachiaria root cells may have generated a significant amount of cytoplasmic hydroxide (OH^?), which could have increased cytoplasmic pH and induced synthesis of organic acids and OH^? extrusion from roots.     

Solubilization of inorganic phosphates and plant growth promotion by <Emphasis Type="Italic">Aspergillus niger</Emphasis>

Two of 187 fungal isolates (Aspergillus niger 1B and 6A) displaying superior phosphate (P) solubilization and hydrolytic enzyme secretion were studied using P forms of calcium (Ca-P), iron (Fe-P), and aluminum (Al-P). Phosphate solubilization in a sucrose-basal salt (SB) broth was increased and pH decreased by both isolates. In Ca-P medium, solubilization for 6A was approximately 322 μg P mL⁻¹ and pH decreased by 4.2 units to 2.3 in 72 h. However, when pH value of the SB broth was lowered to 2.5 using HCl, 65.3  ±  0.4 μg mL⁻¹ of P was released from Ca-P, whereas trace amounts of P were released from Fe-P and Al-P. Both isolates displayed enhanced Al-P solubilization using NH₄Cl rather than KNO₃ as the N source; final pH values were not significantly different. With Ca-P, gluconic acid was predominantly produced by 1B and 6A, whereas oxalic acid predominated with Fe-P and Al-P. Addition of gluconic acid (final concentration of 8.5 μmol mL⁻¹) to Ca-P-supplemented SB lowered pH (2.9) and solubilized phosphate (146.0 ± 1.0 μg mL⁻¹). Similarly, addition of oxalic acid (final concentration 6.6 μmol mL⁻¹) to Ca-P- and Fe-P-amended media solubilized P (60.2 ± 0.9 and 21.6 ± 2.1 μg mL⁻¹, respectively), although these quantities were significantly lower than those detected in unamended SB. The presence of unidentified P solubilized compound(s) in the dialyzed (MW>500) supernatant warrants further study. In pot experiments, significant increases in plant (Brassica chinensis Linn.) dry weight and N and P contents were observed with the addition of isolate 6A, when a small amount of organic fertilizer together with either rock phosphate (South African apatite) or Ca-P served as the main P sources.     

Effect of various microorganisms on phosphorus uptake from insoluble Ca‐phosphates by cucumber plants

Rhizospheric microorganisms can increase P availability to plants. The objective of this work was to elucidate the effects of two plant growth promoting rhizobacteria and biocontrol agents (Bacillus subtilis QST713 and B. Amyloliquefaciens FZB24), a biocontrol agent (the fungus Trichoderma asperellum T34), and Aspergillus niger CBS513.88 on P uptake from insoluble Ca‐phosphates by plants. An experiment involving microbial cultivation in liquid media and three involving cultivation of cucumber plants in a siliceous growing medium fertilized with 40 mg P kg^?1 as phosphate rock (PR), a calcareous medium supplied with the same fertilizer, and one fertilized with KH₂PO₄ or PR at 200 mg P kg^?1 were conducted. In spite of the observed PR solubilization in liquid culture, not all the microorganisms improved P uptake by plants from this P source. The effect of each microorganism also differed depending on the plant‐growing medium, revealing that its P‐solubilizing activity was affected by pH and P concentration in the medium. Overall, best results were obtained with Bacillus subtilis QST713 which increased P uptake from the siliceous growing medium and the calcareous medium fertilized with 200 mg P kg^?1. Improved P nutrition of plants was the result not only of increased P solubilization, but also of enhanced ability of plants to absorb P. The other microorganisms studied provided less promising results despite the P mobilizing strategies they exhibited in the liquid culture (acidification and organic anion exudation). Therefore, these cannot be the only mechanisms contributing to P uptake by plants. Our results support the ability of B. subtilis QST713 to enhance the use of PR as a P source for calcareous soils or to improve uptake of residual P in the form of sparingly soluble Ca‐phosphates.     

Absorption of Ammonia Released from Poultry Manure to Soil and Bark and the Use of Absorbed Ammonia in Solubilizing Phosphate Rock

Composting systems were designed to utilize ammonia (NH₃) released during composting of poultry manure to solubilize phosphate rock (PR). The NH₃ released from decomposing manure was allowed to pass through columns containing soil or bark materials mixed with North Carolina phosphate rock (NCPR) at a rate of 1 mg P g^?1. After eight weeks of incubation, the columns were dismantled and the forms of P and N in PR/soil or PR/bark mixtures were measured. The dissolution of PR was determined from the increases in the amount of soluble and adsorbed P (resin plus NaOH extractable P) or from the decreases in the residual apatite P (HC1 extractable P).

The amounts of NH₄^+-N in the soil and bark columns increased due to absorption of the NH₃ released from poultry manure. No nitrification of absorbed NH₃ occurred, however, unless the soil or bark were reinoculated with a fresh soil solution and incubated for further six weeks.

In the absence of NH₃ absorption, soil and bark materials dissolved approximately 33 percent and 82 percent of NCPR, respectively. The higher dissolution of NCPR in bark was attributed to its higher exchangeable acidity and Ca sink size. There was no increase in NCPR dissolution during the initial NH₃ absorption phase (36 percent and 85 percent dissolution in soil and bark respectively), which may be due to the absence of nitrification. However, during subsequent reincubation when nitrification occurred, the final dissolution of NCPR in the NH₃ treated soil and bark was slightly higher (41 percent and 100 percent, respectively). Protons (H⁺) are released during the oxidation of NH₄⁺ to NO_3? (nitrification) which promote the dissolution of PR. However, most of the H⁺ released during nitrification was involved with soil and bark pH buffering reactions. Only five to 10 percent was involved in PR solubilization in PR/soil mixtures whereas about 50 percent was involved in PR/bark systems.

Bark covers for poultry manure and poultry manure compost heaps have the potential to reduce NH₃ loss and conserve N and may be useful for other purposes such as PR solubilization.     

Assessment of phosphate solubility in forest and highly fertilized Andisols and Andic soils

Abstract

Phosphate solubility in Andisols and Andic soils (forest and fertilized) has been studied in soil solutions at two soil:solution ratios (1:2.5 and 1:25). Forest soils approached the variscite solubility (pIAP=30.4±0.3) while fertilized soils were closer to the amosphous analog of variscite (pIAP= 29.8±0.2). In some samples, phosphorus (P) activities were consistent with simultaneous equilibrium between aluminum (Al) and iron (Fe) phosphates. The dilution ratio, DR=10(P)_1:25/(P)_l:25, was 10.0±0.4 for all samples. This suggests a dissolution of phosphate minerals attaining apparent saturation in 24 h. However, the high content of dissolved organic carbon and Al in aqueous extracts also pointed to the existence of soluble ternary complexes of P with Al(Fe)‐humus complexes. These results could be consistent with simultaneous control between the dissolution of variscite‐like minerals and humus‐Al(Fe)‐P associations. Nevertheless, DR seems a valuable criterion to assess soil P status, especially in fertilized soils with high active Al and Fe contents.     

Influence of low-molecular-weight organic acids on the solubilization of phosphates

A range of low-molecular-weight organic acids were identified in rhizosphere soil, leaf litter, and poultry manure compost. Laboratory and greenhouse experiments were carried out to examine the effects of seven low-molecular-weight organic acids on phosphate adsorption by soils, and the solubilization and plant uptake of P from soil pre-incubated with monocalcium phosphate and North Carolina phosphate rock. Acetic, formic, lactic (monocarboxylic), malic, tartaric, oxalic (dicarboxylic), and citric (tricarboxylic) acids were used in the study. The addition of organic acids decreased the adsorption of P by soils in the order tricarboxylic acid>dicarboxylic acid>monocarboxylic acid. The decreases in P adsorption with organic acid addition increased with an increase in the stability constant of the organic acid for Al (logK _Al). Organic acids extracted greater amounts of P from soils meubated with both monocalcium phosphate and phosphate rock than water did. Although more phosphate was extracted by the organic acids from monocalcium phosphate — than from phosphate rock — treated soils in absolute terms, when the results were expressed as a percentage of dissolved phosphate there was little difference between the two fertilizers. The amount of P extracted by the organic acids from both fertilizers increased with an increase in logK _Al values. The addition of oxalic and citric acids increased the dry matter yield of ryegrass and the uptake of P in soils treated with both fertilizers. The agronomic effectiveness of both fertilizers increased in the presence of organic acids and the increase was greater with the phosphate rock than with the monocalcium phosphate. The results indicated that organic acids increase the availability of P in soils mainly through both decreased adsorption of P and increased solubilization of P compounds.     

Residual effects of organic acid-treated phosphate rocks on some soil properties and phosphate availability

The effects of the application of organic acid-treated phosphate rocks on the growth and nutrient uptake of Italian rye grass (Lolium multiflorum Lam. cv. Tachiwase) and some properties of the soil were evaluated in a greenhouse pot experiment. Phosphate rocks (PRs) collected from six countries; China, Florida (USA), Jordan, Sri Lanka, Togo, and Tanzania, were treated with 1 M oxalic or tartaric acid at the ratio of 2.5 mL g^-1 PR. The organic acid-treated PRs, containing 12–31% water soluble P, were applied to a granitic regosol (pH 5.8) at 200 mg P pot^-1 (4 kg soil). Untreated PRs and single superphosphate (SSP) were included in the treatments. Italian ryegrass was grown for 175 dafter planting (DAP) with ample supply of other nutrients and water. Shoots were harvested at 56, 119, and 175 DAP and the soils were analyzed for pH and Olsen-P after the experiment. Application of organic acid-treated PRs consistently increased the dry matter yield and P uptake of the plants compared with the application of untreated PRs at each harvest, but they were less effective than SSP. A larger amount of P (calculated per unit water-soluble P applied) was recovered from the organic acid-treated PRs than from SSP. The amount of residual extractable P in the soils with the organic acid-treated PRs was about the same as or significantly larger than that in the soil treated with SSP. Soil pH was also significantly higher than in the control and SSP soils. The results suggest that organic acids could be used to improve the P availability of PRs to plants with favorable residual effects in terms of available P and soil pH, without exerting any adverse effects on plant growth or nutrient acquisition.     

VA菌根真菌对石灰性土壤不同形态磷酸盐有效性的影响

用32P示踪法研究了VA菌根真菌对石灰性土壤不同形态磷酸盐有效性的影响。结果表明，VA菌很真菌显著增加了玉米吸收肥料和土壤的磷量。菌根植物和非菌根植物都可不同程度地吸收利用土壤中的Ca2-P、Ca8-P、Fe-P和Al-P，VA菌根真菌增加了玉米对它们的吸收。试验结果还表明，施Ca10-P时接种VA菌很真菌对玉米生长的促进作用比施用其它磷酸盐明显，但Ca10-P不能直接被玉米植株所利用。说明VA菌根真菌能提高土壤中的有效性磷（Ca2-P和Al-P）和缓效性磷（Ca8-P和Fe-P）的有效性。     

Solubility and application effects of African low-grade phosphate rock calcinated with potassium carbonate

This article indicates solubility improvement of African low-grade phosphate rock (PR) through calcination with potassium carbonate. We showed calcination with potassium carbonate made its citric acid solubility up to 100%, and reinforcement of its application effect. In sub-Saharan Africa, although many phosphate deposits have been found, farmers are facing high prices of P fertilizers because of the low solubility of the African PRs. We previously reported PR calcination with Na carbonate improves the solubility, but their application produced limited crop growth, especially in upland conditions. It was speculated soil Na accumulation caused plant growth inhibition. Therefore, we tried to elucidate the effect of calcination with potassium carbonate on PR solubility and its application effects for lowland rice and maize through pot experiments. We used Kodjari PR produced in Burkina Faso for the calcination. Powdered PRs were mixed with K₂CO₃ in five doses to achieve the target K₂O compositions of 200, 250, 300, 350, and 400 g kg^-1. The pelletized PR-K₂CO₃ mixtures were calcined at 900, 1000, 1050, and 1100 °C for 10 min. The pot experiments were conducted for 56 days, monitoring the growth of rice and maize under several application rates. Calcination with K carbonate generated K-rhenanite (KCaPO₄) and kaliophilite (KAlSiO₄), and the solubility reached about 100% in 20 g L^-1 citric acid and about 40% in water. This shows that K carbonates behave like Na carbonate in the solubilization of Kodjari PRs. Although the calcinated Burkina PR (CB) application significantly reduced plant growth when it was applied at an excessive rate, reduced application rates yielded comparable plant growth to that of triple super phosphate (TSP). K carbonate calcination deterred Na accumulation in the soil, and it was effective for plant growth. However, CB easily causes plant damage in excessive application, and it may contain phytotoxicant substances.     

Solubilization of zinc phosphate by a strain of Pseudomonas fluorescens isolated from a forest soil

 A strain of Pseudomonas fluorescens, able to solubilize zinc phosphate, was isolated from a forest soil. Colonies of the microorganism produced clear haloes on solid medium incorporating zinc phosphate, but only when glucose was provided as the carbon source. Solubilization of zinc phosphate occurred by both an increase in the H⁺ concentration of the medium, probably a consequence of ammonia assimilation, and the production of gluconic acid. High concentrations of gluconic acid were produced when P. fluorescens 3a was cultured in the presence of zinc phosphate. Although under some conditions gluconic acid is purportedly able to solubilize metals by the formation of chelates, no evidence of zinc chelation was obtained in our experiments. Furthermore, the increased Zn²⁺ concentration caused by the solubilization process resulted in the manifestation of toxic effects on the culture. A sample of the culture, sonicated to disrupt cells, still possessed the ability to produce gluconic acid from glucose, in the presence and absence of zinc phosphate. The lack of gluconic acid overproduction in cultures of P. fluorescens 3a which were not amended with zinc phosphate suggests that at least some of the glucose oxidation required for the zinc solubilization occurred as a result of the toxic stress caused by the high Zn²⁺ concentration. Received: 16 December 1997     

C、N源及C/N比对微生物溶磷的影响

以不同的氮源 (NH4+、NO3- 、尿素 )、不同的碳源 (葡萄糖、蔗糖、糖蜜和淀粉 )及碳氮比 (34∶1、20∶1、5∶1)为培养基研究不同C、N源和C/N比对微生物溶磷的影响。结果发现 ,曲霉 2TCiF2和 4TCiF6在以NO3-为氮源的培养基中表现出强的解磷活力 ,而节杆菌 1TCRi7和 1TCRi14的溶磷活性则在NO3-存在时降低 ,青霉 1TCRiF5、2TCRiF4、肠杆菌 1TCRi15和欧文氏菌 4TCRi2 2则只有在供给NH4+时 ,才具有溶解磷矿粉的能力。加入少量可溶性磷对大多数微生物的溶磷能力没有显著的影响。曲霉 2TCiF2在蔗糖为碳源时溶磷活力最高 ,节杆菌 1TCRi7只有在葡萄糖为碳源时才具有溶磷能力。培养基的C/N比越高 ,曲霉和欧文氏杆菌的溶磷活力越高 ,而青霉和肠杆菌则在C/N比最低时 ,其溶磷活力最强。这些微生物之所以具有溶解磷矿粉的能力 ,主要是由于分泌有机酸 ,但非有机酸物质的络合和螯合作用 ,可能在肠杆菌和欧文氏菌溶磷中起重要作用。氮源、碳源和碳氮比极大地影响微生物的代谢 ,尤其对分泌有机酸等物质的种类可能产生很大的影响。     

An evaluation of three Thai phosphate rocks for agronomic use based upon their chemical and mineralogical properties

Ratchaburi phosphate rock (Rat-PR), Kanchanaburi-PR (Kan-PR), and Roi-Et-PR (Roi-PR) from Thailand, and North Carolina-PR (NCR-PR), were evaluated in the laboratory for agronomic use. NCR-PR consisted mostly of apatite (unit-cell a-value 9.336 Å, c-value 6.889 Å). Rat-PR contained apatite (a-value 9.428 Å and c-value 6.882 Å) and calcite as the main minerals. Kan-PR consisted mostly of apatite (a-value 9.406 Å and c-value 6.888 Å), crandallite, and calcite. Roi-RP consisted mostly of quartz and variscite. Dissolution kinetics of PRs in 2% formic acid (2% FA), 2% citric acid (2% CA), neutral ammonium citrate (NAC), alkaline ammonium citrate (AAC), and deionized (DI)-water were determined. The dissolution rate of phosphate from PRs in 2%FA, 2%CA, NAC, and DI-water was NCR-PR > Rat-PR > Kan-PR > Roi-PR. As Roi-PR is very poorly soluble, it is clearly unsuitable for direct application to soil.     

VARIATION IN PHOSPHORUS EFFICIENCY AMONG BRASSICA CULTIVARS II: CHANGES IN ROOT MORPHOLOGY AND CARBOXYLATE EXUDATION

Increased phosphorus (P) efficiency is needed to sustain agriculture productivity on soils with low available P. Significant differences were found among Brassica cultivars for growth, P utilization, and remobilization under P deficiency (see our companion paper, Aziz et al., 2011a). To identify the possible mechanisms of P acquisition from low soluble P compounds, four cultivars (‘Rainbow’, ‘CON-1’, ‘Dunkeld’, and ‘Peela Raya’) were selected to ascertain the relationship of their differential P acquisition and growth with their root length in soil and with organic acid release pattern in solution culture experiments. For this purpose their growth and P acquisition from phosphate rock (PR) was compared with calcium di-hydrogen phosphate (Ca-P) when adding uniform dose of 100 mg P kg^?1 soil separately from the two sources. Biomass accumulation, root length, root fineness, plant P uptake and ash alkalinity was significantly (P < 0.01) different in plants of all the four cultivars when supplied with PR or Ca-P in soil. Minimum biomass produced by ‘Peela Raya’ grown with either P source was followed by ‘CON-1’, ‘Dunkeld’, and ‘Rainbow’ in ascending order. Shoot dry matter production had a significant positive correlation with root dry matter production (r = 0.85, P < 0.01), root length (r = 0.59, P < 0.05) and root P uptake (r = 0.95, P < 0.01). Cultivars varied significantly for organic acid secretion in solution culture experiment. Higher quantities of secreted citric acid, malic acid and tartaric acid in solution culture experiment were measured for ‘Rainbow’ and ‘Dunkeld’ cultivars. Efficient performance of these two cultivars for growth and P uptake was associated with their longer roots and more secretion of organic acids especially citric acid.     

Biological P cycling is influenced by the form of P fertilizer in an Oxisol

Phosphate rock (PR) is an alternative fertilizer to increase the P content of P-deficient weathered soils. We evaluated the effects of fertilizer form on indicators of biological cycling of P using an on-farm trial on a Rhodic Kandiudox in western Kenya. Treatment plots were sampled after 13 cropping seasons of P applications as Minjingu phosphate rock (PR) or as triple super phosphate (?TSP) (50 kg P ha^?1 season^?1), as well as a P-unfertilized control (0 kg P ha^?1 season^?1). Soils (0–15 and 15–30 cm) were analyzed for microbial biomass P (P_mic), activities of acid phosphomonoesterase, alkaline phosphomonoesterase, and phosphodiesterase, and sequentially extractable P fractions. P additions as Minjingu PR yielded 299% greater P_mic than TSP at 0–15-cm depth despite similar labile P concentrations in the two P fertilization treatments and stimulated activities of acid phosphomonoesterase (+39%). When added in the soluble form of TSP, a greater percentage of total soil P was present in mineral-bound forms (+33% Fe- and Al-associated P). Higher soil pH under Minjingu PR (pH 5.35) versus TSP (pH 5.02) and the P-unfertilized treatment (pH 4.69) at 0–15-cm depth reflected a liming effect of Minjingu PR. The form of P fertilizer can influence biological P cycling in weathered soils, potentially improving P availability under Minjingu PR relative to TSP via enhanced microbial biomass P and enzymatic drivers of P cycling.     

Effect of P‐solubilizing Azotobacter chroococcum on N,P, K uptake in P‐responsive wheat genotypes grown under greenhouse conditions

Natural and mutant strains of A. chroococcum were isolated from Indian soils. Their ability to dissolve phosphate and their phytohormone production were tested under in vitro conditions. In addition the effect of bacterial inoculation of Azotobacter on N, P, K uptake by three P responsive wheat genotypes (Triticum aestivum L.) under greenhouse conditions at five nutrient levels (Control, 90 kg N ha^—1, 90 kg N + 26 kg P ha^—1, 120 kg N ha^—1 and 120 kg N + 26 kg P ha^—1) was studied. In vitro phosphate solubilization and growth hormone production by mutant strains was more than by the soil isolates. Inoculation of wheat varieties with the soil isolates and mutant strains of A. chroococcum showed greater NPK uptakes as compared with parent soil isolates. Mutant strains M15 and M37 were proved to be the most effective for all three wheat varieties with regard to NPK uptake as well as root biomass production under greenhouse conditions.     

K3解磷菌的解磷机理及其对缓冲容量的响应

以对磷酸三钙具有高效溶解作用且对玉米苗生长有促生效果的假单胞菌K3为模式菌株,采用NBRIP液体培养基研究了解磷菌K3的解磷机制及缓冲容量对其解磷量的影响。结果表明,解磷菌K3液体摇瓶培养7 d后,培养液中水溶性磷从6.54 μg/mL增加至655.23 μg/mL,pH从7.00降至3.99。高效液相色谱测定发现,K3菌液中的主要代谢产物为苹果酸、乳酸和草酸,浓度分别为47.39 mmol/L、25.67 mmol/L和1.89 mmol/L。人工模拟K3菌株产生的有机酸及调节培养基不同pH值对磷酸三钙溶解度影响的试验表明,有机酸的螯合作用是解磷细菌K3菌株解磷的主要机理,而调节培养基pH对解磷的作用有限。液体摇瓶和土培试验结果显示,土壤缓冲容量对K3解磷菌的解磷效应有显著的抑制作用。     

Aluminium phosphates as products of transformations of fertilizer phosphorus in an acid soil

The interactions of double superphosphate in an acid soddy-podzolic soil initially give rise to R-amorphous aluminium phosphates. These, however, have small admixtures of cryptocrystalline phases, indicating the beginning of the formation of aluminium phosphate minerals. Model experiments have demonstrated that minerals of the variscite group, mainly metavariscite, can be synthesized in the zone of reaction of the fertilizer P₂O₅ in an acid soil. Metavariscite, variscite and aluminium phosphates of various degrees of crystallization were studied with respect to the solubility and availability of their phosphorus to plants. The results of the experiment show metavariscite to have better solubility and higher availability to plants than variscite. Crystal aluminium phosphates seem to satisfy about 10–25% of the phosphorus needs of crops. Newly precipitated aluminophosphates have much higher availability.     

Phosphate solubilization by a wild type strain and UV-induced mutants of Aspergillus tubingensis

Phenotypic mutants of Aspergillus tubingensis were obtained by UV irradiation and phosphate solubilization ability of these mutants were studied and compared with the wild type strain. Low phosphate solubilizing mutant was also selected in this study. Among the different mutants, AtM-5 and AtM-2 showed highest P solubilization when tri-calcium phosphate and rock phosphate were used as P source compared to the wild type strain and other mutants. These mutants also showed maximum acid phosphatase and phytase activity. These results suggest that P solubilization by these isolates is due to lowering of pH of the culture filtrate and also the activity of acid phosphatase and phytase.     

Isolation and characterization of phosphate-solubilizing bacteria from betel nut (Areca catechu) and their effects on plant growth and phosphorus mobilization in tropical soils

Phosphate-solubilizing bacteria (PSB) were isolated and characterized from the rhizosphere and bulk soils of Areca catechu plants. A long history of phosphate fertilizer use has elicited a direct effect on the incidence of soil PSB. Their abundance and ability to solubilize insoluble phosphate were significantly greater (P?<?0.0001) in soils with low available phosphorus (P) content than in other soil types. Three efficient PSB strains, namely, ASL12, ASG34, and ADH302, were identified as Acinetobacter pittii, Escherichia coli, and Enterobacter cloacae by characterizing 16S rRNA sequences and biochemical characteristics; they produced gluconic acid at concentrations of 7862.4, 4306.5, and 2663.8 mg L^?1, respectively. The highest amount of solubilized P was determined in Pikovskaya (PVK) medium for the bacterial strain ASL12. The secretion of gluconic acid was related to the available P of rhizosphere soils and P solubilization. Under shaded conditions, the application of these three strains significantly improved plant height, shoot and root dry weight, and nutrient uptake of A. catechu seedlings. A further increase in P solubilization was observed by co-inoculating the three strains and also applying tricalcium phosphate (TCP) or aluminum phosphate (AP). A significant (P?<?0.05) correlation was also observed between P-solubilization activity and A. catechu plant growth in pot experiments. Thus, the three strains can be potentially applied as inoculants in tropical and aluminum-rich soils.