Rhizosphere effects on Cu availability and fractionation in sewage sludge‐amended calcareous soils

Rhizosphere processes have a major impact on copper (Cu) availability and its fractions in soils. A greenhouse experiment with wheat was performed to investigate availability (using seven chemical procedures) and fractionation of Cu in the rhizosphere of ten agricultural soils (Typic Calcixerepts) amended with sewage sludge (1% w/w) using rhizoboxes. The results show that available Cu concentrations in rhizosphere soils were significantly (P < 1%) lower than in bulk soils. In comparison with the bulk soils, in the rhizosphere soils the concentration of Cu associated with organic matter and residual Cu increased by 24 and 4%, respectively, whereas exchangeable Cu, Cu associated with iron‐manganese oxides, and Cu associated with carbonate decreased by 20, 14, and 12%, respectively. Dissolved organic carbon (DOC) and Cu associated with iron‐manganese oxides and Cu associated with organic matter in the rhizosphere and bulk soils were significantly correlated (P < 5%). The results show that the differences between rhizosphere and bulk soils in chemical conditions such as DOC concentrations can change the proportion of soil Cu fractions and, therefore, Cu availability for wheat in calcareous soils amended with sewage sludge. The results show that the wheat root‐induced modifications of chemical and biological soil conditions do not only lead to Cu depletion in mobile soil Cu fractions, but also to modification in soil Cu fractions which are commonly considered as more stable.     

水稻子实对不同形态重金属的累积差异及其影响因素分析

在分析成都平原核心区土壤重金属(Cd、Cr、Pb、Cu、Zn)全量、各形态含量及相应点位种植的水稻子实重金属含量的基础上,通过统计分析、空间插值及线性回归方程的模拟,研究了土壤Cd、Cr、Pb、Cu、Zn全量的空间分布状况、各形态重金属含量统计特征,以及水稻子实对重金属各形态的累积差异及其影响因素。结果表明,成都平原水稻土重金属污染较轻,除Cd外,均低于国家土壤环境质量二级标准。土壤中重金属的可交换态含量均较低,Cd主要以铁锰氧化态存在,Cr、Cu、Zn、Pb主要以残渣态存在。水稻子实对5种重金属的累积效应顺序为:Cd>Zn>Cu>Pb>Cr。与水稻重金属累积关系密切的重金属活性形态(可交换态、碳酸盐结合态、铁锰氧化物结合态和有机物结合态)主要有:Cd的碳酸盐结合态、Cr的可交换态、Pb的有机物结合态和Cu的碳酸盐结合态含量;Zn各活性形态对水稻子实含量的影响不明显。土壤理化性质对不同活性形态重金属元素的影响效应各不相同。活性态Cd主要受有机质、pH和容重的影响;活性态Cr与pH、有机质、CEC和容重密切相关;活性态Pb与有机质、容重、中细粉粒、砂粒等均有密切的关系;Cu的活性主要受粘粒、有机质含量的影响;Zn的有效性主要受pH、有机质、砂粒、容重的影响。总的看来,对土壤Cd、Cr、Pb、Cu、Zn各活性形态含量影响效应较强的是有机质、pH、容重,而与土壤吸附性能密切相关的颗粒组成、CEC的影响不甚明显。     

苏南经济快速发展区土壤Ni的形态分布影响因素——以昆山市为例

以江苏省昆山市为典型区,通过现场采样及室内分析测定,定量研究了几种因素对农田土壤Ni形态分布的影响。结果表明：（1）土壤有效态Ni含量为1.31 mg.kg-1,土壤全Ni含量为40.95 mg.kg-1,土壤Ni的活化率为3.38%。（2）土壤重金属Ni各形态含量相对大小为残渣态（36.20 mg.kg-1）〉有机质结合态（2.80 mg.kg-1）〉铁锰氧化物结合态（1.31 mg.kg-1）〉可交换态（0.54 mg.kg-1）、碳酸盐结合态（0.10 mg.kg-1）,残渣态含量明显高于其他形态,达88.16%。（3）pH值是影响可交换态Ni含量的最主要因素,达极显著负相关水平;全Ni含量是影响碳酸盐结合态Ni含量、铁锰氧化物结合态Ni含量和残渣态Ni含量的最主要因素,达极显著正相关水平;有机质含量是影响有机质结合态Ni含量的最主要因素,呈显著正相关水平。（4）〈0.01 mm粘粒含量是影响可交换态Ni含量的重要因素,有机质含量是碳酸盐结合态Ni含量的重要影响因素,pH值和有机质含量都是影响铁锰氧化物结合态Ni含量的重要因素,〈0.01 mm粘粒含量、pH值都是影响有机质结合态Ni含量的重要因素,pH值是影响残渣态Ni含量的重要因素。     

不同有机肥中Cu、 Zn在农田土壤中的有效性与形态归趋

【目的】畜禽粪便有机肥的施用是造成我国农田土壤重金属污染的重要原因之一。本文选用两种典型规模化养殖场畜禽粪便有机肥,研究其在石灰性土壤和酸性土壤上施用1年后Cu、 Zn的有效性和形态归趋,为客观评价畜禽粪便有机肥中重金属进入土壤后的环境行为和生态风险提供理论依据。【方法】采用温室土壤培养试验,在石灰性土壤和酸性土壤上分别设对照(CK)、 施2%鸡粪(CM2%)、 施5%鸡粪(CM5%)、 施与CM2%、 CM5%含等量Cu、 Zn的重金属无机盐溶液(CS2%、 CS5%)、 施2%猪粪(PM2%)、 施5%猪粪(PM5%)、 施与PM2%、 PM5%含等量Cu、 Zn的重金属无机盐溶液(PS2%、 PS5%)9个处理,每个处理设3次重复,在温室条件下培养1年。测定土壤pH值, EDTA提取有效态Cu、 Zn以及采用改进Tessier连续提取法提取的各形态Cu、 Zn的含量,分析鸡粪、 猪粪及等量无机盐溶液中Cu、 Zn进入土壤后的有效性和形态归趋。【结果】施用鸡粪和猪粪1年后,石灰性土壤的pH值降低,酸性土壤的pH值升高,施用5%猪粪时石灰性土壤pH值降低了0.23个单位,酸性土壤pH值升高了0.87个单位。施入鸡粪、 猪粪1年后,石灰性土壤中有效态Zn和酸性土壤中有效态Cu、 Zn含量显著增加,施用5%猪粪时酸性土壤中有效态Cu含量增加了1.95倍,施用5%鸡粪时2种土壤中有效态Cu的含量均显著低于等量无机盐。施用鸡粪和等量无机盐后,2种土壤中交换态和有机结合态Cu的含量显著增加,交换态、 碳酸盐结合态、 有机结合态和铁锰氧化物结合态Zn的含量显著增加; 施用猪粪和等量无机盐后,2种土壤中交换态、 铁锰氧化物结合态和有机结合态Cu的含量显著增加,碳酸盐结合态和铁锰氧化物结合态Zn的含量显著增加。【结论】施用鸡粪、 猪粪提高了石灰性土壤中Zn和酸性土壤中Cu、 Zn的有效性,高用量条件下鸡粪中Cu的有效性低于等量无机盐。1年后, 通过畜禽粪便有机肥带入2种土壤中的Cu 主要以交换态和有机结合态的形式存在,Zn则主要以碳酸盐结合态、 铁锰氧化物结合态和有机结合态的形式存在。2种土壤上有机肥带入的Cu、 Zn转化为铁锰氧化物结合态的比例低于等量无机盐,2种有机肥带入2种土壤中的Cu转化为交换态和有机结合态的比例高于等量无机盐。鸡粪带入的Zn转化为交换态的比例在酸性土壤中低于等量无机盐,但在石灰性土壤中则高于等量无机盐。     

Poorly crystalline iron and aluminium oxides contribute to the carbon saturation and sorption of dissolved organic carbon in the soil

Soil carbon (C) saturation implies an upper limit to a soil's capacity to store C depending on the contents of silt + clay and poorly crystalline Fe and Al oxides. We hypothesized that the poorly crystalline Fe and Al oxides in silt + clay fraction increased the C saturation and thus reduced the capacity of the soil to sorb additional C input. To test the hypothesis, we studied the sorption of dissolved organic carbon (DOC) on silt + clay fractions (<53 µm) of highly weathered oxic soils, collected from three different land uses (i.e., improved pasture, cropping and forest). Soils with high carbon saturation desorbed 38% more C than soils with low C saturation upon addition of DOC, whereas adsorption of DOC was only observed at higher concentration (>15 g kg^?1). While high Al oxide concentration significantly increased both the saturation and desorption of DOC, the high Fe oxide concentration significantly increased the desorption of DOC, supporting the proposition that both oxides have influence on the DOC sorption in soil. Our findings provide a new insight into the chemical control of stabilization and destabilization of DOC in soil.     

Assessment of zinc availability to rice in mollisols of North India

Abstract

The objectives of this study were to (1) characterize zinc (Zn) fractions and their relation to Zn extracted with mixed‐bed ion exchange resin capsules and (2) assess the relationships between the latter and Zn uptake by rice in 12 Mollisols from North India. The Resin Adsorption Quantity (RAQ) of Zn was measured after 1 and 14 days of anaerobic incubation. Six organic and inorganic Zn fractions were determined on anaerobic soil. Zinc uptake by rice was studied in a greenhouse experiment. Soil Zn fractions under reduced conditions followed the order residual Zn (80%)>carbonates and amorphous oxides bound Zn (12%)>weakly organically bound Zn (3%)>crystalline oxides bound Zn (2%)>strongly organically bound Zn (2%)>water soluble + exchangeable Zn (1%). RAQ‐Zn was best correlated with Zn bound to carbonates and amorphous oxides. Due to negative interactions between bicarbonate and Zn uptake, correlations between relative dry matter yield or total Zn uptake and the different Zn fractions, DTPA‐Zn measured on dry soil, or RAQ Zn were not significant. Adjusting soil test values according to soil pH improved the prediction of relative dry matter yield, but further studies are required to determine whether the resin capsule can be used as a soil test for Zn in calcareous soils.     

外源铜和镍在土壤中的化学形态及其老化研究

采用连续提取法测定了外源铜和镍进入田间土壤后的化学形态分布,比较研究了这2种重金属在3种不同类型土壤(红壤,水稻土和潮土)中随老化时间的形态转化和分布.结果表明,外源铜以残留态(40%～60%)和EDTA可提取态(40%)为主;随老化时间,EDTA可提取态、易还原锰结合态及铁铝氧化态向残留态转化;外源镍在酸性红壤中以可交换态(40%)和残留态(30%～50%)为主,在中性水稻土中以EDTA可提取态(30%)和残留态(30%～50%)为主,在碱性潮土中以铁铝氧化态(20%)和残留态(40%)为主.随老化时间,水溶态、可交换态、EDTA可提取态等向残留态转化.土壤pH较低时水溶态和可交换态含量较高,但是同时随老化时间的降低量也明显;pH较高时有利于易还原锰结合态和有机质结合态的转化.     

河北主要土壤中Cd和Pb的形态分布及其影响因素

采用网室盆栽试验和大田取样 ,运用连续提取方法 ,研究了河北平原潮土和潮褐土两种土壤中Cd、Pb的化学形态特征及与其影响因素的关系。结果表明 :随着Cd、Pb污染程度的增加 ,其交换态有增加趋势。当高浓度重金属污染土壤时 ,Cd(潮土 >1mgkg- 1、潮褐土 >5mgkg- 1)主要以交换态存在 ,并表现为 :交换态 >碳酸盐结合态 >铁锰氧化物结合态 >有机结合态 >残留态 ;Pb主要以碳酸盐结合态和铁锰氧化物结合态存在。在低浓度重金属污染的土壤中 ,Cd (潮土 <1mgkg- 1、潮褐土 <5mgkg- 1)的残留态、有机结合态成倍增加 ,甚至超过交换态 ,表现为 :残留态 >碳酸盐结合态 >有机结合态 >交换态 >铁锰氧化物结合态 ;Pb主要以铁锰氧化物结合态和残留态存在。Cd、Pb在土壤中的分布与土壤的pH值 ,有机质含量密切相关。     

Changes in aluminum pools of andisols due to soil acidification

Abstract

Aluminum concentrations in soil solutions are not only controlled by inorganic clay minerals but also by organically bound aluminum. The objective of this study was to determine which pools contribute to Al dissolution. Soil samples were taken at various distances from tree trunks and at various depths at the Rolling Land Laboratory (RLL), Hachioji, Tokyo. Selective dissolution techniques were used to analyze the changes in pools of solid-phase aluminum. Soil pH values around Hinoki cypresses were in the aluminum buffer range. Exchangeable aluminum contents in soils under Hinoki cypresses were 104 mmol_c kg-^?1 on the average. This value was similar to that of the cation exchange capacity (CEC) of Andisols at RLL at a soil pH of 4. The relationship between the soil pH and exchangeable, organically bound, and amorphous aluminum pools showed that dissolved aluminum ions in the soil solution were primarily derived from the amorphous Al pool. Dissolved aluminum ions were substituted with base cations of soils, resulting in the increase of the content of exchangeable Al and/or the formation of complexes with organic matter which increased the proportion of organically bound Al pools. Increase in the proportion of organically bound Al pools indicated the importance of complexation with soil organic matter for controlling the aluminum concentration in the soil solution.     

外生菌根诱导油松根际土壤重金属形态变化

To understand the role of ectomycorrhizas in improving the tolerance of its host to excessive heavy metals in soil, this study was conducted to exam the patterns of four fractions （the exchangeable, the carbonate-bound, the Fe-Mn oxide- bound and the organically bound） of both Cu and Cd in the rhizosphere of Chinese pine （Pinus tabulaeformis） seedlings grown in excessive Cu and Cd environment. The results showed that the speciation of Cu and Cd in the rhizosphere was significantly influenced by inoculation of ectomycorrhizal fungus Boletus edulis. Compared to the rhizosphere, the content of exchangeable Cu slightly decreased in the mycorrhizosphere of the seedlings grown in 166 and 400 mg kg^-1 Cu contaminated soil, whereas the exchangeable Cd in the mycorrhizosphere decreased remarkably to only 33% and to 60% that of the rhizosphere at 0.75 and 1.50 mg kg^-1 Cd levels, respectively. These indicate the potential capacity of mycorrhizas to alleviate the damage of heavy metals to the host plants by reducing the bioavailability of heavy metals in soil. Distribution of the 4 tested fractions of Cu and Cd at different contamination levels showed that there was a strong tendency of changing from loosely associated fractions to strongly associated fractions in the mycorrhizosphere. The most stable Cd fraction, organically bound Cd, was significantly larger in the mycorrhizosphere than in the rhizosphere at different Cd contamination levels. This phenomenon was also observed for Cu but the difference was not statistically significant.     

Organic Amendment Effects on the Transformation and Fractionation of Aluminum in Acidic Sandy Soil

Addition of organic amendments can alleviate the level of aluminum (Al) phytotoxicity in acid soils by affecting the nature and quantity of Al species. This study evaluated the transformation of Al in an acidic sandy Alaquod soil amended with composts (10 and 50 g kg^?1 soil of yard waste, yard + municipal waste, GreenEdge^®, and synthetic humic acid) based on soil Al fractionation by single and sequential extractions. Though the organic compost amendments increased total Al in soil, they alleviated Al potential toxicity in acidic soil by increasing soil pH and converting exchangeable Al to organically bound and other noncrystalline fractions, stressing the benefits of amending composts to improve acid soil fertility. The single‐extraction method appears to be more reliable for exchangeable Al than sequential extraction because of the use of nonbuffered pH extract solution.     

Aluminum Forms in Solid Phase of Soils Developed over Schists as a Function of Land Use

ABSTRACT

Land use may modify certain soil properties while soil physicochemical characteristics can influence metal partitioning in soils. Therefore, the total content and various forms of aluminum (Al) in solid phase of schist-developed topsoils (0–20 cm) in NW Spain under different land uses (i.e., forest, pasture, and cultivation) were evaluated to identify the Al-bearing phases. Aluminum fractionation was performed, using a six-step sequential extraction procedure with ammonium acetate, hydroxylamine hydrochloride, ammonium oxalate in darkness, hydrogen peroxide, ammonium oxalate under ultraviolet radiation, and acid digestion. Mean concentrations of total Al were similar in the soils under three land uses. Mean percentage of the various Al forms in all soils were in the following order: residual fraction > amorphous compounds > crystalline compounds > water-soluble/exchangeable/specifically adsorbed > bound to oxidizable organic matter > manganese oxides. The forest soils contained considerably higher contribution of amorphous compounds (16.3%) to total Al concentration compared with the soils under other two uses (mean about 9%). Maximum mean concentration of exchangeable Al was also observed in forest soils (mean 8.8% of total Al vs. about 4% in pasture soils and cultivated soils); this is attributed to lower pH and higher organic matter content of the forest soils. Thus, this study revealed the impact of land use on the Al-bearing phases and, hence, in its bioavailability to plants.     

Chemical Speciation of Heavy Metals in the Fractionated Rhizosphere Soils of Sunflower Cultivated in a Humic Andosol

Chemical speciation and bioaccumulation factor of iron (Fe), manganese (Mn), and zinc (Zn) were investigated in the fractionated rhizosphere soils and tissues of sunflower plants grown in a humic Andosol. The experiment was conducted for a period of 35 days in the greenhouse, and at harvest the soil system was differentiated into bulk, rhizosphere, and rhizoplane soils based on the collection of root-attaching soil aggregates. The chemical speciations of heavy metals in the soil samples were determined after extraction sequentially into fractions classified as exchangeable, carbonate bound, metal–organic complex bound, easily reducible metal oxide bound, hydrogen peroxide (H₂O₂)–extractable organically bound, amorphous mineral colloid bound, and crystalline Fe oxide bound. Iron and Zn were predominantly crystalline Fe oxide bound in the initial bulk soils whereas Mn was mainly organically bound. Heavy metals in the exchangeable form accumulated in the rhizosphere and rhizoplane soils, comprising <4% of the total content, suggesting their relatively low availability in humic Andosol. Concentrations of organically bound Fe and Mn in soils decreased with the proximity to roots, suggesting that organic fraction is the main source for plant uptake. Concentrations of Mn and Zn in the metal–organic complex also decreased, indicating a greater ability of sunflower to access Mn from more soil pools. Sunflower showed bioaccumulation factors for Zn, Fe, and Mn as large as 0.39, 0.05, and 0.04 respectively, defining the plant as a metal excluder species. This result suggests that access to multiple metal pools in soil is not necessarily a major factor that governs metal accumulation in the plant.     

南方红壤区典型母质发育土壤硒素特征及影响因素

为探究南方红壤区不同母质土壤硒素特征及理化性质对硒有效性的影响,以广西典型红壤区不同母质土壤为研究对象,通过测定硒形态特征及理化性质,并利用AMOS路径分析模型及土壤培养试验,量化了不同因子对不同母质土壤有效硒的影响。结果表明：南方红壤区不同母质土壤总硒含量均高于0.4 mg/kg,其中以花岗岩母质土壤总硒含量最高,以砂页岩母质土壤有效硒含量最高,且变幅最大。土壤硒形态主要以残渣态和有机结合态为主,不同形态硒含量由高到低均表现为：残渣态硒>有机结合态硒>铁锰氧化物结合态硒>交换态硒>可溶态硒。AMOS路径分析模型结果表明,影响不同母质土壤硒形态分配贡献的主要因素是全磷、有机质和pH,第四纪红色黏土母质土壤中的全氮及砂页岩母质土壤中的速效氮的影响也值得关注。利用贡献较大的理化因子开展土壤培养试验,外源理化因子添加后均在一定的时间及剂量条件下出现土壤有效硒含量最大值,且不同母质土壤获得的最佳因子条件不同。因此,不同土壤母质因土壤理化性质的不同,其硒形态特征存在较大差异,不同理化因子作用于土壤后,土壤硒会因土壤环境的变化而重新分配。     

海南岛砖红壤中铅、镉的化学形态与转化

采用土培实验和连续提取.原子吸收分光光度法,研究了重金属Pb、Cd在海南岛花岗岩砖红壤中的形态组成、外源Pb、Cd污染及化学修复剂磷、钙、硫对土壤重金属形态的影响.结果表明:在供试原土壤中,重金属Pb的化学形态以结合态和残余态为主,土壤有效态Pb含量较低,其中残余态Pb>有机质结合态Pb>铁锰氧化物结合态Pb>碳酸盐结合态Pb>交换态Pb>水溶态Pb,说明土壤Pb的环境风险较低;重金属Cd的化学形态以铁锰氧化物结合态和碳酸盐结合态为主,土壤中交换态Cd含量较高,其中铁锰氧化物结合态Cd>碳酸盐结合态Cd>交换态Cd>有机结合态Cd>残余态Cd>水溶态Cd,说明土壤Cd的环境风险较高.当外源Pb、Cd污染土壤时,有铁锰结合态Pb>残余态Pb>有机态Pb>碳酸盐结合态Pb>交换态Pb>水溶态Pb,交换态Cd>铁锰氧化物结合态Cd>碳酸盐结合态Cd>残余态Cd>有机态Cd>水溶态Cd的趋势.向污染土壤施加化学改良剂过磷酸钙、硫化钠和石灰,能显著降低水溶态Pb、Cd和交换态Pb、Cd的含量,并使有机结合态Pb、碳酸盐结合态Pb和铁锰氧化物结合态Pb含量下降,但残余态Pb、碳酸盐结合态Cd、铁锰氧化物结合态Cd和有机态Cd有增加的趋势,残余态Cd的含量基本稳定.     

施用水稻秸秆对酸性红壤铝形态的动态影响

采用室内培养试验研究了施用秸秆对酸性红壤pH值和高活性的交换性铝、吸附态羟基铝和有机络合态铝的动态变化。结果表明，土壤pH值和各形态铝在前30天内变化较大，而后达到稳定。土壤pH值在前10天内快速提高，之后缓慢下降至第30天渐趋平衡，土壤交换性铝则是先降低，10天后逐渐增加至稳定状态。添加秸秆后，土壤吸附态羟基铝发生复杂的水解和聚合反应，变化不大。有机络合态铝在可溶性有机质和有机酸等小分子化合物的联合作用下，先增加而后降低。由于秸秆作用的时间较短，不能使有机质积累，故有机络合态铝不稳定易于变化，不能起到很好的络合铝的作用。     

氮锌配施对冬小麦根土界面锌有效性及形态分级的影响

【目的】 氮能够促进冬小麦根系对锌的吸收及在籽粒中的积累。研究氮锌配施对冬小麦根土界面锌有效性及形态分级的影响,有助于探究氮锌配施促进冬小麦吸收锌的可能机制,为合理施用氮肥来提高冬小麦籽粒锌含量提供一定的理论依据。 【方法】 以冬小麦为试材进行了根箱培养试验。分别设置三个氮水平 (0、0.2和0.4 g/kg) 和两个锌水平 (0和10 mg/kg),分析了冬小麦地上部锌含量、根际土和非根际土有效锌含量、pH以及六种锌形态含量。 【结果】 氮锌配施 (N_0.2Zn₁₀和N_0.4Zn₁₀) 处理显著提高了冬小麦地上部干物质重和锌含量。在不施锌 (Zn₀) 条件下,N_0.4处理显著提高根际土壤的有效锌含量;在Zn₁₀条件下,N_0.4和N_0.2处理均显著降低根际土有效锌含量,却提高了非根际土有效锌含量。无论施锌与否,N_0.4和N_0.2处理均显著降低根际土壤的pH,但对非根际土壤的pH影响不大。在Zn₀条件下,N_0.4和N_0.2处理显著降低了根际土壤交换态锌、碳酸盐结合态锌及非根际土氧化物结合态锌含量,提高了非根际土交换态锌、根际与非根际土壤残渣态锌含量。在Zn₁₀条件下,N_0.4和N_0.2处理显著提高了根际和非根际土交换态锌、非根际土松结有机态和紧结有机态锌及根际土残渣态锌含量,降低了根际土松结有机态、碳酸盐结合态锌及根际与非根际土壤残渣态锌含量。 【结论】 氮锌配施提高冬小麦锌含量,促进冬小麦锌的吸收,可能是由于氮锌配施与冬小麦根系共同作用降低了根际土壤pH,促进土壤中锌从松结有机态和碳酸盐结合态向交换态转化,从而提高了土壤锌的有效性。      

Forms of Al in soil and soil solution in a long‐term fertilizer application experiment

Under the conditions of a long‐term fertilizer experiment, this study aimed to determine the contents of total and exchangeable aluminium in soil as well as the Al concentration in the soil solution. Additionally, Al speciation was evaluated with the use of the MINTEQA2 software. The results obtained indicated that under the conditions of long‐term application of different mineral fertilizers or farmyard manure, the soil reaction changed to a great extent (pH 3.58–6.78). At the same time, the content of total Al in soil fluctuated from 18.85 to 22.13 g/kg and that of exchangeable Al ranged from 1.42 to 102.66 mg/kg. The concentration of Al in the soil solution was highly differentiated (5.19–124.07 μmol/L) as well as that of free aluminium ions (Al³⁺) (0–16.9 μmol/L). In acidic soils, aluminium complexes with organic matter are the predominant forms of Al in the soil solution. In soils with neutral soil reaction, there were no free aluminium ions. Soil liming and addition of organic amendment were the treatments that restricted the presence of toxic aluminium forms in soil.     

Effect of pH on chemical forms and plant availability of cadmium,zinc, and lead in polluted soils

The effect of pH on chemical forms and plant availability of heavy metals in three polluted soils was investigated. The soils were adjusted to pH values of 7.0, 6.0, and 4.5, then sequentially extracted so that Cd, Zn, and Pb could be partitioned into five operationally defined chemical fractions: exchangeable, carbonate, Fe-Mn oxide, organic, and residual. Kidney beans were grown in the soils to investigate plant availability of the metals in relation to changes of their levels in chemical forms resulting from alteration of soil pH. Alteration of pH resulted in changes of chemical forms of the metals in the soils, and at lower pH the changes were more significant. When soil pH values were decreased from 7.0 to 4.55, levels of Cd, Zn, and Pb in exchangeable form increased, decreased in carbonates and decreased slightly in Fe-Mn oxide forms. Their levels in organic and residual forms were unchanged. Although concentration of metals in plants increased with reduction in soil pH values, dry matter yields were also restricted, so that the amount of metal uptake were almost similar. The uptake rate of the metals in the exchangeable + carbonate forms was the same for the three elements in all the cases.