Titratable Acidity and Alkalinity of Red Soil Surfaces

The surfaces of red soils have an apparent amphotenc character, carrying titratable acidity and titratable alkalinity simultaneously. The titratable acidity arises from deprotonation of hydroxyl groups of hydrous oxide-type surfaces and dissociation of weak-acid functional groups of soil organic matter, while the titratable alkalinity is derived from release of hydroxyl groups of hydrous oxide-type surfaces. The titratable acidity and titratable alkalinity mainly depended on the composition and content of iron and aluminum oxides in the soils. The results showed that the titratable acidity and titratable alkalinity were in significantly positive correlation not only with the content of amorphous aluminum oxide(Alo) and iron oxide(Feo) extracted with acid ammonium oxalate solution, free iron ox-ide(Fed) extracted with sodium dithionite-citrate-bicarbonate (DCB) and clays, but also with the zero point of charge (ZPC) of the samples. Organic matter made an important contribution to the titratable acidity.     

铁氧化物与土壤表面电荷性质的关系

The relationship between iron oxides and surface charge characteristics in variable charge soils (latosol and red earth) was studied in following three ways.(1)Remove free iron oxides (Fed) and amorphous iron oxides (Feo) from the soils with sodium dithionite and acid ammonium oxalate solution respectively.(2) Add 2% glucose (on the basis of air-dry soil weight) to soils and incubate under submerged condition to activate iron oxides,and then the mixtures are dehydrated and air-dried to age iron oxides.(3) Precipitate various crystalline forms of iron oxides onto kaolinite.The results showed that free iron oxides (Fed) were the chief carrier of variable positive charges.Of which crystalline iron oxides (Fed-Feo) presented mainly as discrete particles in the soils and could only play a role of the carrier of positive charges,and did little influence on negative charges.Whereas the amorphous iron oxides (Feo),which presented mainly fas a coating with a large specific surface area,not only had positive charges,but also blocked the negative charge sites in soils.Submerged incubation activated iron oxides in the soils,and increased the amount of amorphous iron oxides and the degree of activation of iron oxide,which resulted in the increase of positive and negative charges of soils.Dehydration and air-dry aged iron oxides in soils and decreased the amount of amorphous iron oxides and the degree of activation of iron oxide,and also led to the decrease of positive and negative charges.Both the submerged incubation and the dehydration and air-dry had no significant influence on net charges.Precipitation of iron oxides onto kaolinite markedly increased positive charges and decreased negative charges.Amorphous iron oxide having a larger surface area contributed more positive charge sites and blocked more negative charge sites in kaolinite than crystalline goethite.     

Effect of Electrolytes on Surface Charge Characteristics of Red Soils

The zero point of charge (ZPC) and the remaining charge σp at ZPC are two important parameters characterizing surface charge of red soils.Fourteen red soil samples of different soil type and parent material were treated with dithionite-citrate-dicarbonate (DCB) and Na2CO3 respectively.ZPC and σp of the samples in three indifferent electrolytes (NaCl,Na2SO4,and NaH2PO4) were determined.Kaolinite was used as reference.The results showed that ZPC of red soils was affected by the composition of parent materials and clay minerals and in significantly positive correlation with the content of total iron oxide (Fet),free iron oxide (Fed),amorphous iron oxide (Feo),aluminum oxide (Alo) and clay,but it was negatively correlated with the content of total silica (Sit).The σp of red soils was also markedly influenced by mineral components.Organic components were also contributing factor to the value of σp.The surface charges of red soils were evidently affected by the constitution of the electrolytes.Specific adsorption of anions in the electrolytes tended to make the ZPC of red soils shift to a higher pH value and to increase positive surface charges of the soils,thus leading to change of the σp value and decrease of the remaining net negative charges,even to the soils becoming net positive charge carriers.The effect of phosphate anion was greater than that of sulfate ion.     

Aggregate Stability and Its Relationship with Some ChemicalProperties of Red Soils in Subtropical China

The stability of aggregates in the surface soil is crucial to soil erosion and runoff generation. Thus, to understand the stability and the breakdown mechanisms of soil aggregates as well as the relationship between aggregate stability and selected soil chemical properties, such as different forms of Fe and A1 oxides, organic matter, CEC and clay content, the aggregates of slightly and severely eroded red soils derived from Quaternary red clay in subtropical China were analyzed using the routine wet sieving and the Le Bissonnais methods. The results indicated that the aggregates of the severely eroded soils were more stable than those of the slightly eroded soils. Different aggregate breakdown mechanisms resulted in different particle size distribution. The slaking from entrapped air in aggregates severely destroyed the soil aggregates,especially in the slightly eroded soils. Meanwhile, mechanical breakdown and microcracking had little effect on the aggregates compared to slaking. The fragments resulting from slaking were mainly microaggregates that increased in size with increasing clay content. The main fragment size of the slightly eroded soils was 1.0-0.2 mm, while for the severely eroded soils it was 5.0-2.0 mm and 1.0-0.5 mm. Overall, more than 20% of the fragments were smaller than 0.2 mm.In addition, aggregate stability was positively and often significantly correlated with Fed, Ald, Feo and clay content, but significantly and negatively correlated to SOC.     

Charge Properties and Clay Mineral Composition of Tianbao Mountains Soils

The clay mineral association,oxides of clay fraction and surface charge properties of 7 soils,which are developed from granite,located at different altitudesof the Tianbao Mountains were studied.Results indicate that with the increase in altitude,1) the weathering process and desilicification of soil clay minerals became weaker,whereas the leaching depotassication and the formation process of hydroxy-aluminum interlayer got stronger;2)the contents of amorphous and complex aluminum and iron,and the activity of aluminum and iron oxides for soil clay fraction increased;and 3) the amount of variable negarive charge,anion exchange capacity and the values of PZC and PZNC also increased.The activity of aluminum and iron oxides,the accumulation of aluminum,and surface charge characteristics and their relation to clay oxides of the vertical zone soils were observed and recorded.     

Contribution of Iron and Aluminum Oxides to Electrokinetic Characteristics of Variable Charge Soils in Relation to Surface Charge

The contribution of iron and aluminum oxides to electrokinetic characteristics of variable charge soils was studied through determination of electrophoretic mobilities of the red soils treated with either removal of iron oxides or coating of aluminum oxides,and of those deferrated under natural conditions.After removal of the iron oxides,zeta potentials of the latosol and the red earth decreased obviously with a shift of IEP to a lower pH,from 6.4 to 5.3 and 4.1 to 2.4 for the former and the latter,respectively,and the electrokinetic change for the latosol was greater than for the red earth.Zeta potentials of the kaolinite sample increased markedly after coated with iron oxides.The striking effect of iron oxides on electrokinetix properties of the soils was also demonstrated by the electrokinetic differences between the samples from the red and white zones of a plinthitic horizon formed naturally,and between the samples from the gley and bottom horizons of a paddy soil derived from a red earth.The coatings of aluminum oxides on the latosol and the yellow earth made their zeta potentials rise pronouncedly and their IEFs move toward higher pHs,from 6.2 to 6.8 and 4.3 to 5.3 for the former and the latter,respectively.The samples with different particle sizes also exhibited some electrokinetic variation.The experiment showed that the effects of iron and aluminum oxides were closely related to the pH and type of the soils.     

长期地下水位管理及施用秸秆对中国亚热带水稻土团聚体形成的影响

Soil organic carbon(SOC) and iron(Fe)-oxides are important contributors of aggregate stability in highly weathered soils, and they are influenced by groundwater management and straw application. A 30-year plot experiment with early rice(Oryza sativa L.)-late rice-winter fallow rotations was conducted using a upland clay soil in cement pools under shallow groundwater table at a depth of 20 cm(SGT) and deep groundwater table at a depth of 80 cm(DGT) to simulate the groundwater tables of two types of important paddy soils, gleyed paddy soils and hydromorphic paddy soils, respectively, in subtropical China. Soil redox potential(Eh) was measured in situ, and 0–20 cm soil samples were collected for the analyses of soil Fe-oxides, SOC, and aggregates under SGT or DGT with different straw application treatments, in order to evaluate the interaction of groundwater management and straw application on paddy soil aggregation and the relative importance of SOC or Fe-oxides on soil aggregation. The results showed that soil Eh was restricted by irrigation, and its variation was more significant under DGT than under SGT. The decreased soil Eh or reduced drying and wetting cycles under SGT resulted in more SOC accumulation with the straw application, had no effect on soil free Fe-oxides(Fed), significantly increased the amorphous Fe-oxide(Feo) and complex Fe-oxide contents, but decreased the crystalline Fe-oxide content(Fed–Feo). The soils under DGT had more macroaggregates than those under SGT, but the difference decreased with the straw application. It could be concluded that soil Fe-oxides were the principal contributing factor to the aggregation of paddy soils in subtropical China and SOC was also an important contributing factor.     

亚热带土壤不同矿物组分中铬的吸附

Safe application of chromium (Cr)-containing organic industrial wastes to soil requires considering the ability of the soil to adsorb Cr.In this study,the maximum Cr adsorption capacity was assessed for the bulk samples and their clay and iron-free clay fractions of four subtropical soils differing in mineralogy.To this end,the samples were supplied with Cr(Ⅲ) nitrate solutions at pH 4.5 or 5.5.The results of Cr(Ⅲ) adsorption fitted to a Freundlich equation and the adsorption capacity was positively correlated with soil organic matter and iron oxide contents.The clay fractions adsorbed more Cr per unit mass than the bulk soils and the iron-free clay fractions.The Cr(Ⅲ) adsorption capacity increased with increasing soil pH due to more charges on adsorbing surfaces.Our results suggest that the soils rich in organic matter and iron oxides and having a pH above 4.5 are suitable for application of Cr(Ⅲ)-loaded industrial wastes.     

铵、钾同时存在时, 土壤对铵的优先吸附

The water stability of aggregates in various size classes separated from 18 samples of red soils under different managements, and the mechanisms responsible for the formation of water-stable soil aggregates were studied. The results showed that the water stability of soil aggregates declined with increasing size, especially for the low organic matter soils. Organic matter plays a key role in the formation of water-stable soil aggregates. The larger the soil aggregate size, the greater the impact of organic matter on the water stability of soil aggregates. Removal of organic matter markedly disintegrated the large water-stable aggregates (> 2.0 mm) and increased the small ones (< 0.25-0.5mm) to some extent, whereas removal of free iron(aluminium) oxides considerably destroyed aggregates of all sizes, especially the < 0.25-0.5 mm classes. The contents of organic matter in water-stable aggregates increased with aggregate sizes. It is concluded from this study that small water-stable aggregates (< 0.25-0.5 mm) were chiefly cemented by Fe and Al oxides whilst the large ones (> 2.0 mm) were mainly glued up by organic matter. Both free oxides and organic matter contribute to the formation and water stability of aggregates in red soils.     

Distribution of iron and manganese oxides in Haploxeralfs and Rhodoxeralfs and their relation to the degree of soil development and soil colour

The status and the distribution of iron and manganese oxides were studied in six soil profiles from Thrace region (Greece), classified as Rhodoxeralfs and Haploxeralfs. The acid ammonium oxalate and the dithionite-citrate-bicarbonate methods were used to extract the free iron and manganese oxides from the soils. In both great groups the Fed and Mnd values are higher than Feo and Mno values, respectively, indicating that a considerable fraction is present in crystalline form. Both Feo/Fed and Fed-Feo values suggest that, in the study area, Haploxeralfs are less developed soils than Rhodoxeralfs. The manganese oxides present in crystalline forms (Mnd-Mno) were more in Rhodoxeralfs than in Haploxeralfs, suggesting that the crystallinity of manganese oxides increases, with the degree of soil development. Significant correlations were found of Redness Rating to Fed-Feo and to Feo/Fed ratio, positive and negative respectively, leading to the conclusion that soil red colouration increases with the degree of soil development.     

Characterization of iron oxides in Fe-rich concretions from an imperfectly-drained Greek soil: a study by selective-dissolution techniques and X-ray diffraction

Fe-rich concretions commonly occur in Greek soils with alternating drying and waterlogging periods. This study was conducted to characterize the iron oxides in Fe-rich concretions from the upper solum of an Alfisol with seasonal perched water table by the combination of selective dissolution and X-ray diffraction (XRD) techniques. The results showed that more than 75% of the total iron (Fet) was associated with the crystalline and the amorphous Fe oxides, indicating a strong accumulation of free iron oxides (Fed) in concretions. Amorphous iron compounds (Feo) was present at high concentrations and fluctuated with profile depth. The active Fe ratio (Feo/Fed) values that ranged from 0.35 to 0.41 reflected an association of poorly crystalline goethite with some ferrihydrite. The XRD data showed that the Fe-rich concretions consisted of quartz, feldspars, illite and gypsum. The mineralogy of iron oxides in concretions was determined by comparison of XRD patterns for dithionite-citrate-bicarbonate (DCB) treated (deferrated) and untreated (non-deferrated) samples. Poorly crystalline goethite as demonstrated by broad lines in the diffraction patterns and ferrihydrite were the iron oxides detected in the concretions. This mineral assemblage appears to be related to the pedoenvironmental conditions in which the concretions were formed and indicates that the mechanisms governing the formation of crystalline Fe oxides from ferrihydrite are retarded by the presence of crystallization inhibitors.     

海南岛北部玄武岩上土壤发生研究Ⅱ.铁氧化物特征

研究了海南岛北部玄武岩上发育的不同风化成土年代土壤系列的铁氧化物特征。研究表明,随风化成土时间增加,土壤发生过程中形成的游离铁（Fed）以及铁游离度（Fed/Fet）增大,铁活化度（Feo/Fed）降低。穆斯堡尔谱分析显示风化成土时间从9×104a、64×104a至 181× 104a,土壤黏粒中形成的赤铁矿含量占铁氧化物的比例从16％、25％增至 48％。风化成土时间越短,土壤中针铁矿结晶程度越差。X-射线衍射结果表明土壤细土和黏粒中赤铁矿含量与风化成土时间呈正比增加。同时,土壤有磁铁矿含量减少、磁赤铁矿增加的趋势。     

珠江三角洲平原不同种植年限土壤铁氧化物特征研究

珠江三角洲平原具有上千年的围垦历史,其土壤发生演变过程深受人为作用影响,开展此区域不同种植年限土壤中铁氧化物形态特征和分布规律的研究,能够揭示人为耕种下土壤发生演变过程。以珠江三角洲平原不同种植年限的土壤剖面为对象,研究了滨海沉积物、河流冲积物和三角洲沉积物发育的土壤及黏粒中全铁、游离铁含量变化及其影响因素。结果表明:随着种植年限的增加,河流冲积物、三角洲沉积物发育土壤中游离铁(Fe_d)向土体下部淀积深度逐渐增加,黏粒中游离铁(Fe_(d(clay)))含量在水耕氧化还原层中呈减小趋势,而滨海沉积物发育的土壤Fe_d含量及淀积深度均有所减小。随着种植年限的增加,滨海沉积物发育的土壤全铁(Fe_t)和游离铁(Fe_d)在黏粒中的富集程度呈增大趋势,而河流冲积物、三角洲沉积物发育的土壤Fe_t和Fe_d富集程度逐渐减小。土壤Fe_d与Fe_t、黏粒游离铁(Fe_(d(clay)))与黏粒全铁(Fe_(t(clay)))均呈极显著正相关;全铁富集率(Fe_(t(clay))/Fe_t)、游离铁富集率(Fe_(d(clay))/Fe_d)均与Fe_(t(clay))、Fe_(d(clay))、黏粒铁游离度(Fe_(d(clay))/Fe_(t(clay)))呈极显著正相关,与Fe_t、Fe_d、土壤铁游离度(Fe_d/Fe_t)、黏粒含量呈极显著负相关,且Fe_(t(clay))/Fe_t与Fe_(d(clay))/Fe_d呈极显著正相关,表明土壤铁氧化物在黏粒中的富集以Fe_d为主,且铁氧化物的富集程度受土壤黏粒含量的影响。     

水耕人为土时间序列铁氧化物与磁化率演变特征

以我国亚热带地区不同母质起源的水耕人为土时间序列为研究对象,分析不同形态铁氧化物和磁化率随成土时间的动态演变特征及其影响因素。结果表明,石灰性母质起源的水耕人为土0~120 cm土体中全铁(Fet)、游离铁(Fed)和游离度(Fed/Fet)随时间序列演变均逐渐增加,0~50 a内Fet、Fed和Fed/Fet增加速率分别为3.2 t hm~(-2) a~(-1)、1.2 t hm~(-2) a~(-1)和0.04%a~(-1),50~1 000 a内Fet、Fed和Fed/Fet增加速率分别为0.1 t hm~(-2) a~(-1)、0.15 t hm~(-2) a~(-1)和0.01%a~(-1);而酸性母质起源的水耕人为土0~120 cm土体中Fet、Fed和Fed/Fet随时间序列演变均逐渐下降,0~60 a内Fet、Fed和Fed/Fet下降速率分别为0.2 t hm~(-2) a~(-1)、0.5 t hm~(-2) a~(-1)和0.03%a~(-1),60~300 a内Fet、Fed和Fed/Fet下降速率分别为0.9 t hm~(-2) a~(-1)、1.2 t hm~(-2) a~(-1)和0.06%a~(-1)。土壤pH、Eh、以及外源铁输入与土体内铁淋失的相对强度是控制不同母质水耕人为土中铁氧化物转化速率与途径的主要因素。石灰性母质起源的水耕人为土中不同磁学指标随时间演变分为三个阶段:0~50 a内表现为质量磁化率(MS)、饱和等温剩磁(SIRM)和软剩磁(IRMs)的急剧降低;50~300 a内表现为MS、SIRM和IRMs的持续、缓慢降低以及硬剩磁(IRMh)的相对稳定发展;300~1 000 a内表现为MS、SIRM和IRMs的持续、缓慢降低以及IRMh的快速下降。酸性母质起源的水耕人为土0~20 cm和20~120 cm土壤中磁学指标演变呈现截然不同的两个阶段:0~60 a 0~20 cm内MS,SIRM和IRMs的急剧降低,IRMh具有明显增加;而20~120 cm内MS、SIRM和IRMs缓慢下降,IRMh明显降低。60~300 a 0~20 cm内不同磁学指标变化幅度均很小,而20~120 cm内IRMh相对比较稳定,MS、SIRM和IRMs在种稻150 a后快速下降。淹水还原条件下亚铁磁性矿物的破坏是不同母质水耕人为土演变过程中磁性衰减的主要机制。     

Mineralogy and Arsenic Mobility in Arsenic-rich Brazilian Soils and Sediments (11 pp)

Background   Soils and sediments in certain mining regions of Brazil contain an unusually large amount of arsenic (As), which raises concerns that mining could promote increased As mobility, and thereby increase the risks of contaminating water supplies. Objectives   The purpose of this study was to identify the most important factors governing As mobility in sediments and soils near three gold-mining sites in the State of Minas Gerais, Brazil. Methods   Surface and sub-surface soil samples were collected at those sites and characterized by chemical and mineralogical analyses. Oxalate (Feo) and citrate-bicarbonate-dithionite (Fed) iron contents were determined by atomic absorption spectroscopy (AAS). Arsenic mobilization was measured after incubating the samples in a 2.5 mM CaCl2 solution under anaerobic conditions for 1, 28, 56, 84, or 112 days. The solution concentrations of As, Fe, and Mn were then measured by inductively coupled plasma-mass spectrometry (ICP-MS) and AAS, respectively. Results and Discussion   Results indicated that As mobilization is largely independent of both the total As and the Feo/Fed ratio of the solid phase. Soluble As is roughly controlled by the Fe (hydr)oxide content of the soil, but a closer examination of the data revealed the importance of other highly weathered clay minerals and organic matter. Large amounts of organic matter and a low iron oxide content should favor As leaching from soils and sediments. Under reducing conditions, As is mobilized by the reductive dissolution of Fe and/or Mn oxides. However, released As may be readsorbed depending on the sorptive properties of the soil. Gibbsite is particularly effective in adsorbing or readsorbing As, as is the remaining unreduced fraction of the iron (hydr)oxides. Conclusion and Outlook   In general, low soluble As is related to the presence of gibbsite, a large amount of iron oxides, and a lack of organic matter in the solid phase. This has environmental significance because gibbsite is thermodynamically more stable than Fe oxides under anaerobic conditions, such as those found in waterlogged soils and lake sediments.     

THE COVARIANCE OF PHOSPHATE SORPTION WITH OTHER SOIL PROPERTIES IN SOME BRITISH AND TROPICAL SOILS

Phosphate sorption capacity estimated by Piper's (1942) ‘anion exchange capacity’ and Bache and Williams's (1971) phosphate sorption index were correlated with soil pH, clay, organic matter, ‘free iron oxides’ and ‘extractable aluminium’ (McLean et al., 1958) for topsoil and subsoil samples from twenty tropical and twenty British acidic soil profiles. These two groups of soils did not differ significantly in phosphate sorption. Extractable aluminium and free iron oxide were well correlated with phosphate sorption, free iron oxide being superior to aluminium in freely drained British soils but not in poorly drained ones. Organic matter content correlated well with phosphate sorption for the poorly drained British soils, and for the tropical soils when sorption capacitywas measured using a high phosphate concentration.     

What Constitutes Plant-Available Molybdenum in Sandy Acidic Soils?

Molybdenum (Mo) is critical for the function of enzymes related to nitrogen cycling. Concentrations of Mo are very low in sandy, acidic soils, and biologically available Mo is only a small fraction of the total pool. While several methods have been proposed to measure plant-available Mo, there has not been a recent comprehensive analytical study that compares soil extraction methods as predictors of plant Mo uptake. A suite of five assays [total acid microwave digestion, ethylenediamenetetraaacetic acid (EDTA) extraction, Environmental Protection Agency (EPA) protocol 3050B, ammonium oxalate extraction, and pressurized hot water] was employed, followed by the determination of soil Mo concentrations via inductively coupled mass spectroscopy. The concentrations of soil Mo determined from these assays and their relationships as predictors of plant Mo concentration were compared. The assays yielded different concentrations of Mo: total digest > EPA > ammonium oxalate ≥ EDTA > pressurized hot water. Legume foliar Mo concentrations were most strongly correlated with ammonium oxalate–extractable Mo from soils, but an oak species showed no relationship with any soil Mo fraction and foliar Mo. Bulk fine roots in the 10- to 30-cm soil horizon were significantly correlated with the ammonium oxalate Mo fraction. There were significant correlations between ammonium oxalate Mo and the oxides of iron (Fe), manganese (Mn), and aluminum (Al). Results suggest that the ammonium oxalate extraction for soil Mo is the best predictor of plant-available Mo for species with high Mo requirements such as legumes and that plant-available Mo tracks strongly with other metal oxides in sandy, acidic soils.