Dissolution of synthetic crystalline and noncrystalline iron oxides by organic acids

Thirteen organic acids and fulvic acid were used in dissolution experiments with laboratory-synthesized noncrystalline Fe oxide, hematite and goethite to determine the effect of the organic acid properties on the kinetics and pH-dependence of oxide dissolution. After 100 h of reaction the noncrystalline oxide had completely dissolved at pH 3.5 in the presence of oxalic, malonic, malic, citric and tartaric acids as well as EDTA, DTPA and NTA. Acetic, lactic, salicylic, phthalic and fulvic acids were much less reactive. Dissolution was generally lower at pH 5.5 than at pH 3.5. Hematite and goethite were solubilized to a much lesser extent than the noncrystalline oxides, presumably due to lower surface areas and differences in type and arrangement of surface hydroxyl and oxygen groups. Oxide solubilities predicted using formation constants of Fe-organic acid complexes approximately predicted dissolution measured at 100 h for the different acids used with the noncrystalline material, whereas slow reaction times limited dissolution of hematite and goethite to levels far below those predicted. The differences observed in pH-dependence and kinetics were explained on the basis of adsorption onto variable-charge surfaces and as steric limitations for the large chelating acids.     

Factors Influencing the Dissolution of Phosphate Rock in a Range of High P‐Fixing Soils from Cameroon

Abstract

A laboratory incubation experiment was conducted to evaluate the soil factors that influence the dissolution of two phosphate rocks (PRs) of different reactivity (Gafsa, GPR, reactive PR; and Togo‐Hahotoe, HPR, low reactivity PR) in seven agricultural soils from Cameroon having variable phosphorus (P)‐sorption capacities, organic carbon (C) contents, and exchangeable acidities. Ground PR was mixed with the soils at a rate of 500 mg P kg^?1 soil and incubated at 30°C for 85 days. Dissolution of the PRs was determined at various intervals using the ΔNaOH‐P method (the difference of the amount of P extracted by 0.5 M NaOH between the PR‐treated soils and the control). Between 4 and 27% of HPR and 33 and 50% of GPR were dissolved in the soils. Calcium (Ca) saturation of cation exchange sites and proton supply strongly affected PR dissolution in these soils. Acid soils with pH‐(H₂O)<5 (NKL, ODJ, NSM, MTF) dissolved more phosphate rock than those with pH‐(H₂O)>5 (DSC, FGT, BAF). However, the lack of a sufficient Ca sink in the former constrained the dissolution of both PRs. The dissolution of GPR in the slightly acidic soils was limited by increase in Ca saturation and that of HPR was constrained by limited supply in protons. Generally, the dissolution of GPR was higher than that of HPR for each soil. The kinetics of dissolution of PR in the soils was best described by the power function equation P=At^B. More efficient use of PR in these soils can be achieved by raising the soil cation exchange capacity, thereby increasing the Ca sink size. This could be done by amending such soils with organic materials.     

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.     

红壤中低分子量有机酸的吸附动力学

低分子量有机酸在土壤中广泛存在尤其是在根际土壤中,其主要来源于动植物残体的分解、微生物的代谢、植物根系的分泌和土壤中有机物的转化等。由于羧基的强络合作用,低分子量有机酸一旦进入土壤将很快被土壤吸附,影响土壤的表面电荷性质和动电性质,进而影响其在土壤中的作用。关于低     

大豆和小麦对矿物钾的活化作用研究

大豆、春小麦均能有效地活化利用白云母、钾长石中的结构性钾,结构钾释放量为19.53mg kg-1~45.46mg kg-1。春小麦和大豆根系分泌的有机酸主要为草酸、酒石酸和柠檬酸,其次为乳酸、乙酸和苹果酸。草酸溶液酸性较强,络合能力也强,其活化钾的能力最强。苹果酸、酒石酸、柠檬酸、乙酸和乳酸亦可有效地促进白云母和钾长石的分解,但活化土壤钾的能力比较低。低分子量有机酸作用下矿物钾的释放是酸性水解和络合溶解双重作用的结果。草酸、酒石酸、柠檬酸是影响大豆、春小麦吸收利用矿物钾的主要低分子量有机酸。     

Low molecular weight organic acids released from roots of durum wheat and flax into sterile nutrient solutions

Knowledge of the composition and quantity of organic substances released from roots of different plant species is necessary for understanding the chemical and biological processes in the rhizosphere. The present study was undertaken to quantify low molecular weight organic acids (LMWOAs) released from roots of five cultivars/lines of durum wheat (Triticum turgidum var. durum L.): Kyle, Sceptre, DT618, DT627, and DT637 and four cultivars/ lines of flax (Linum usitatissiumum L.): Somme, Flanders, AC Emerson, and YSED 2. Plants were grown in sterile nutrient solution cultures and amounts of organic acids exuded by roots were analyzed by gas chromatography. The LMWOAs varied significantly among both durum wheat and flax cultivars and oxalic, malonic, fumaric, succinic, acetic, malic, citric and tartaric acids were detected in root exudates of both species. Generally, oxalic and acetic acids were predominant in durum wheat exudates and oxalic, acetic and malic acids were predominant in flax root exudates. High oxalic acid concentrations occurred in root exudates of durum wheat cultivars DT627 and DT637, and flax cultivar YSED 2. Compared with the other durum wheat cultivars, Kyle released the lowest total amount of LMWOAs, whereas among the flax cultivars, YSED 2 had the highest total amount of acids secreted from roots. The data showed that the release of LMWOAs from roots was cultivar dependent. The results provide valuable background information for studying the role of root exudates in soil‐plant relationships.     

红壤主要土壤组分对低分子量有机酸吸附的研究

通过一次平衡法研究了双氧水去除有机质、添加1%腐殖酸和DCB法(连二亚硫酸钠-柠檬酸钠-碳酸氢钠)去除游离氧化铁、铝对红黏土发育的红壤吸附草酸、柠檬酸、酒石酸和苹果酸的影响。结果表明,去除有机质后,由于受溶液pH、表面吸附点位变化、土壤结构变化、表面基团活性变化、草酸根生成等多种因素的影响,红壤对低分子量有机酸的吸附量虽略有增加,但增加并不明显。添加腐殖酸培养一个月后,由于受土壤表面电荷变化、吸附点位覆盖、氧化铁活性改变、基团质子化等因素的影响,红壤对低分子量有机酸的吸附量虽有轻微的减少,但减少也不明显。去除占土壤总量3.79%的游离氧化铁、铝后,土壤表面正电荷将会显著减少,而导致红壤对低分子量有机酸的吸附量显著减少,其减少程度因有机酸种类而异,与原土对有机酸的最大吸附量(Sm)大小顺序相反。     

低分子有机酸去除土壤中重金属条件的研究

采用化学萃取实验,以湖南郴州柿竹园和湖南衡阳水口山矿区的重金属污染农田土壤为研究对象,采用柠檬酸、草酸、酒石酸作为低分子有机酸萃取剂,在一定的条件下对污染土壤中重金属进行萃取实验,确定各个单因素的适宜条件。结果表明,对于湖南郴州和衡阳两个污染地区土壤运用化学萃取技术萃取重金属来进行土壤修复是实际可行的;柠檬酸、酒石酸、草酸对各种土壤中的重金属都表现出了良好的萃取能力,是高效的土壤重金属萃取剂;单因素的适宜萃取条件为100mmol·L^-1有机酸溶液,固液比1∶5,恒定温度35℃,pH为3,反应时间24h,且萃取率随着电解质浓度的增大而提高;土壤中重金属存在形态与萃取效果有一定的相关性,稳定态（残渣态、硫化物和有机结合态、铁-锰氧化物结合态）金属含量高,表现为较低的萃取率;反之,萃取率高;柠檬酸、草酸、酒石酸3种萃取剂均能有效地降低衡阳污染土壤中的重金属浓度,3种萃取剂的萃取效率依次为酒石酸〉草酸〉柠檬酸。     

低分子量有机酸对不同合成磷源的释磷效应

采用化学浸提方法,研究了5种低分子量有机酸(草酸、柠檬酸、酒石酸、苹果酸、乙酸)对不同合成磷源的释磷效应。结果表明:供试有机酸(低浓度乙酸除外)均能促进不同合成磷源(DCP、OCP、FA、Fe-P、A l-P)中磷素的释放;其释磷效果与有机酸的种类和浓度有关,强弱顺序依次为柠檬酸、草酸、酒石酸、苹果酸、乙酸,有机酸浓度越高其释磷效果越好;有机酸与氟磷灰石(FA)反应后,溶液pH值升高。低浓度条件下pH值变化较大,而高浓度条件下pH值变化较小。     

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.     

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.     

Dissolution of rock phosphate in silage leachate

Abstract

Following a favourable crop response to rock phosphate (RP) in a long term fertility trial in DeKalb, IL, an experiment was designed to dissolve RP in silage leachate on the premise that silage has a pH of 4.0 to 4.2 and that once drained of its leachate, it can be used for animal feed. Two types of RP, Brown Sugar RP and Ruhm's RP, having 30% P₂O₅ were dissolved in pure organic acids similar to those present in silage leachates. The amount of dissolved phosphorus (P) from RP in organic acids increased with organic acid:RP ratio. The highest amount of P dissolution was found with butyric acid. Thus, butyric acid with Brown Sugar RP, 2519, 1500, and 710 μg P/g RP were dissolved at 800, 400, and 200 acid:RP ratio, respectively; with Ruhm's RP the values were 3900, 1988, and 835, μg P/g RP, respectively. After dissolution of the two types of RP in the acids, there was an increase in solution pH of 1 to 2 units. Of the three types of silage leachates used, grass silage leachate had the highest acidity and calcium (Ca) concentration followed by high moisture and low moisture corn silage leachates, respectively. Soluble P concentration was five times higher in high moisture corn and grass leachates than in low moisture corn silage leachate. There was a decrease in acidity, Ca, and P content with the number of successive leachings. In low moisture corn silage leachate, 13.9 and 3.05 mg P/g Brown Sugar RP and 14.4 and 3.05 mg P/g Ruhm's RP were solubilized in the 1st and 2nd leaching, respectively. The study shows that corn low moisture silage leachate can dissolve RP efficiently and that it is a good, promising, and environmentally safe method for RP dissolution.     

绿肥腐解液中有机酸组成对铝磷和铁磷活化能力的影响

  【目的】  研究绿肥还田腐解过程中产生的有机酸阴离子对土壤中难溶性磷素的活化效果,以深化理解绿肥作物提高土壤养分供应能力的机理。  【方法】  供试6种绿肥作物为紫云英 (Astragalus sinicus L.)、肥田萝卜 (Raphanus sativus var. Longipinnatus)、二月兰 (Orychophragmus violaceus)、双低甘蓝型油菜 (Brassica napus L.)、双高甘蓝型油菜 (Brassica napus L.) 和芥菜型油菜 (Brassica juncea)。将绿肥植株切碎后,置于离心管中,加入土壤浸提液于室外背光处密闭腐解45天,采用离心法提取植株腐解液。测定腐解液中10种有机酸组分 (丙二酸、苹果酸、柠檬酸、草酸、乳酸、酒石酸、乙酸、琥珀酸、马来酸和富马酸阴离子) 含量。采用室内模拟方法,分别用6种绿肥腐解液溶解难溶性铝磷 (Al-P,AlPO₄) 和铁磷 (Fe-P,FePO₄·2H₂O),测定溶液中的无机磷含量。  【结果】  二月兰腐解液中10种有机酸组分总含量最高,其次为肥田萝卜、双低甘蓝型油菜和芥菜型油菜,双高甘蓝型油菜和紫云英腐解后腐解液中较低。所有绿肥作物腐解液中10种有机酸组分含量大小依次为酒石酸、丙二酸、柠檬酸、琥珀酸、乳酸、苹果酸、乙酸、草酸、马来酸和富马酸阴离子。其中酒石酸和丙二酸阴离子占上述10种主要有机酸总量的15%以上,马来酸和富马酸阴离子占比最低,不到总量的1%。不同绿肥腐解液对Al-P和Fe-P的活化能力有显著差异,肥田萝卜对两种难溶性磷素的活化能力最高,理论上1000 kg肥田萝卜绿肥可活化0.6～1.2 kg的Al-P和Fe-P,其次为紫云英和芥菜型油菜,对Al-P和Fe-P的活化量均约为0.6～1.0 kg,而二月兰的活化量最低,约为0.2～0.6 kg。对不同有机酸含量与难溶性磷活化量进行相关性分析发现,Al-P活化量与柠檬酸、苹果酸和草酸阴离子的含量呈现正相关关系,Fe-P的活化量与酒石酸的含量呈现正相关关系。  【结论】  与Al-P和Fe-P的活化密切相关的是有机酸中含有较高的柠檬酸、草酸、苹果酸和酒石酸。供试6种绿肥作物腐解液中的10种有机酸的总含量及各有机酸组分的占比差异很大。二月兰虽然有机酸总量高,但是柠檬酸、草酸和酒石酸比例均较低;而肥田萝卜腐解液中的酒石酸含量及其占比高于其他绿肥,且柠檬酸和琥珀酸占比较高,对Fe-P的活化能力强;芥菜型油菜和紫云英腐解液中草酸、柠檬酸和琥珀酸占比高于其他绿肥,对Al-P的活化能力强。供试6种绿肥中以肥田萝卜腐解液的活化能力最强,芥菜型油菜和紫云英腐解液的活化能力次之。     

低分子量有机酸对高岭石中铝释放的影响

选择了几种土壤中可能存在的低分子量脂肪羧酸 ,研究了它们对高岭石中铝释放的影响。结果表明 ,有机酸可以通过络合作用促进高岭石中铝的释放。几种有机酸对体系中可溶性铝影响的大小顺序为 :草酸 >柠檬酸 >丙二酸 >苹果酸 >乳酸。草酸、柠檬酸和乳酸对可溶性铝释放的促进作用随体系pH的升高而减小 ,其中草酸体系中可溶性铝随pH的变化幅度最大。在苹果酸体系中 ,可溶性铝随pH的升高而稍有增加。体系中的可溶性铝随有机酸浓度的增加而增加 ,而交换性铝随柠檬酸浓度的增加而减少 ,先随苹果酸浓度增加而增加 ,然后又逐渐减少。与对照相比 ,柠檬酸和草酸使交换性铝的量减小 ,苹果酸和乳酸在低pH下使交换性铝明显增加 ,而苹果酸在较高pH下使交换性铝减少。有机酸影响释放出的铝在固液相间的分配比 ,苹果酸在低pH下使体系中释放出的大部分铝以交换性形态存在 ,而在较高pH下 ,大部分铝以可溶形态存在。在草酸体系中 ,释放出铝的大部分都以可溶形态存在。不同有机酸的不同表现与体系中铝的溶解平衡、铝的吸附 -解吸平衡、有机酸的吸附 -解吸平衡、有机酸的离解平衡和铝与有机酸的络合 -离解平衡有关。     

外源有机酸对美人蕉耐性和Cr吸收、迁移的影响

采用营养液培养试验,研究外源有机酸(柠檬酸、草酸和酒石酸)对Cr污染水体中美人蕉(Canna indicaL.)的耐性、对Cr的吸收和迁移能力的影响。结果表明,各有机酸均不同程度地增强了美人蕉的耐性,促进Cr吸收和向地上部迁移。不同有机酸和同种有机酸不同浓度(4,8,16 mmol/L)的影响效果不尽相同。就耐性特征而言,16 mmol/L的草酸能够明显提高美人蕉生物量和促进根系生长,其影响效果要大于柠檬酸和酒石酸;就Cr吸收和迁移特性而言,影响大小为柠檬酸>酒石酸>草酸,尤以16 mmol/L的柠檬酸效果最为显著,而酒石酸和草酸分别在8 mmol/L,16 mmol/L时最利于Cr的吸收和迁移。因此,在本试验条件下16 mmol/L的柠檬酸较适合Cr污染水体的植物修复。     

有机酸和磷对两种污染土壤铅的释放作用研究

用土壤培养和化学浸提法研究了不同低分子量有机酸和磷对污染土壤中铅释放的影响。结果表明,随有机酸浓度增加,铅污染红壤、棕壤中可溶出铅量均增加。当供试有机酸浓度≥1mmol·L-1时,相同浓度有机酸溶出铅量为柠檬酸〉乙酸〉草酸;当有机酸浓度≤0.5mmol·L-1时,溶出铅量为草酸〉柠檬酸〉乙酸。将2g·kg-1磷及50mmol·kg-1有机酸与铅污染红壤以不同方式混合后,柠檬酸处理的溶出铅比率（在污染土壤中加入P、有机酸后溶出铅含量与单加有机酸溶出铅含量之比）为66%（先加有机酸再加磷）、58%（有机酸与磷同时加入）、70%（先加磷再加有机酸）,草酸处理（方式同上）的溶出铅比率为90%、89%、94%,乙酸处理（方式同上）的为10%、8%、10%。铅污染棕壤上,以上处理的溶出铅比率分别为106.46%、104.43%、105.19%（柠檬酸）;43%、48%、58%（草酸）;38%、42%、55%（乙酸）。供试条件下,红壤最低溶出铅比率低于棕壤。     

红壤中低分子量有机酸的吸附特征

应用平衡法研究了红壤对草酸、柠檬酸、酒石酸和苹果酸的吸附行为。结果表明:红壤对有机酸的吸附量随着有机酸平衡溶液浓度的增加而增加,有机酸种类不同,吸附量也不同。有机酸在红壤中的吸附行为均可以很好地用Langmuir、Freundlich和Temkim方程描述,其中以Langmuir方程的拟合效果最好。红壤对有机酸均具有较高的吸附率,在0.5～20mmolL-1初始浓度范围内,草酸、柠檬酸、酒石酸和苹果酸的吸附率最大分别可达89.2%、77.5%、67.4%和47.1%。红壤对柠檬酸、酒石酸和苹果酸的吸附率均随着有机酸初始浓度的增加先急剧下降(<5mmolL-1),之后缓慢下降至趋于平稳;草酸的吸附率则随着初始浓度的增加而增加,在3mmolL-1左右时达到最大,随后降低至趋于平稳。     

有机酸对红壤磷素潜在环境风险的影响

从两个方面初步探讨了有机酸对红壤磷素潜在环境风险的影响,一是有机酸对红壤固磷能力的影响,二是有机酸对红壤固定磷向液相释放的影响。研究结果表明不同类型有机酸在这两个方面的影响是不同的,以柠檬酸为代表的二羧酸可以显著减少红壤对磷的固定量,大大提高了土壤向水体释放磷的能力,增大了土磷的潜在环境风险;而以草酸为代表的三羧酸则可以不同程度上增加土壤的固磷能力,且可使已经存在于液相中的磷再次被吸附,进而减缓或消除土磷了的潜在环境风险。     

The possible role of organic acids as allelochemicals in Tamarindus indica L. leaves

Tamarindus indica L. is well known for its acidic nature and allelopathic potential, but to date, little is known about its organic acids playing their role as allelochemicals. Hence, in the present study, identification, quantification, and contribution of organic acids present in its leaf extract were conducted using the principle of bioassay-guided procedure. High pressure liquid chromatography identified four organic acids, viz. citric, malic, oxalic, and tartaric acids, in its leaf aqueous extract with the predominance of oxalic acid (7.5 g kg^?1 leaves fresh weight) followed by tartaric acid (7.3 g kg^?1). The allelopathic activity of identified acids and aqueous extract was evaluated on lettuce seedlings growth based on the specific activity (EC₅₀). The crude extract reduced radicle growth more adversely than hypocotyl at the concentration of 2.5 g L^?1 (EC₅₀). It hindered the normal physiological growth process through weak and curly seedlings, and necrosis of their tips. Among the identified acids, oxalic acid had the highest specific activity (40 mg L^?1) and citric acid had the lowest (>1000 mg L^?1). As a consequence of its high contents, the total activity, a function of specific activity and concentration, of oxalic acid (188) was found higher followed by tartaric acid (146). The contribution of both acids influencing the specific activity of the crude extract was then turned out to be 74%. To the best of our knowledge, this is the first report identifying the oxalic and tartaric acids as growth inhibitors in tamarind leaves and quantifying their contribution in its allelopathic expression. Based on the total activity, the results suggested that oxalic and tartaric acids are the major allelochemicals in tamarind leaves. The allelopathic potential of these acids might promote the development of natural herbicides as an alternative to the synthetic ones in a most sustainable manner.     

低分子量有机酸对红壤磷酸单酯酶活性的影响

采用室内模拟试验研究了低分子量有机酸(柠檬酸、草酸、苹果酸、酒石酸)对红壤磷酸单酯酶活性的影响。结果表明,低浓度有机酸(<1μmol g-1)对磷酸单酯酶活性有促进作用,且促进作用大小依次为柠檬酸≈草酸>苹果酸>酒石酸;而高浓度有机酸(>5μmol g-1)则为抑制作用,且抑制作用大小依次为柠檬酸>草酸>苹果酸>酒石酸。当体系pH趋向酶促反应最佳pH时,磷酸单酯酶活性增强;反之,当体系pH远离酶促反应最佳pH时,磷酸单酯酶活性减弱。有机酸根一方面通过羧基的辅助作用提高磷酸单酯酶活性;另一方面通过释放土壤A l3+、Fe3+等金属离子,对土壤磷酸单酯酶活性有一定的抑制作用。