Accumulation of organo‐metallic complexes in laterites and the formation of Aluandic Andosols in the Nilgiri Hills (southern India): similarities and differences with Umbric Podzols

We have investigated the speciation and distribution of iron (Fe) and aluminium (Al) between minerals and organic species in A and B horizons of two Aluandic Andosols with X‐ray diffraction, thermal analyses, visible diffuse reflectance and Fourier transform infrared (FTIR) spectroscopies, together with selective and total chemical extractions. The two Aluandic Andosols of the Nilgiri Highlands (south India) have formed at the expense of intensively weathered lateritic formations of the Eocene. Data revealed that Al and Fe were predominantly stored in end‐weathering products of laterites, mainly as gibbsite and Fe (hydr)oxides in B horizons of the Aluandic Andosols. These secondary minerals are gradually replaced by organo‐metallic complexes in the topsoil A horizons exhibiting andic properties. We then indicate that formation of the organo‐metallic complexes results from weathering of the dominant crystalline Al‐ and Fe‐(hydr)oxides mediated by the organic ligands and complexation of the polyvalent metals following the accumulation of organic matter. Such weathering and complexation mechanisms are therefore similar to those recently ascribed to deferralitization and the formation of freely drained Umbric Podzols (Humus‐Podzols) in the upper Amazon Basin. In the present case, large supplies of both Al‐ and Fe‐bearing minerals provide large metal:carbon ratios that prevent the mobility of the organo‐metallic complexes and induce the formation of Aluandic Andosols rather than Podzols.     

Podzol: Soil of the Year 2007. A review on its genesis,occurrence, and functions

The Podzol has been elected “Soil of the year 2007” in Germany. This article reviews the present knowledge on the development, functions, and threats of Podzols. The main theories on mobilization and transport of organic matter, Fe, Al, and Si are (1) metal‐organic migration, (2) metal reduction, and (3) inorganic sol migration. Immobilization theories include precipitation or polymerization due to increasing pH/abundance of base cations with depth, mechanical filtering in soil pores, oxidation of metal‐organic complexes, biodegradation of the organic part, decreasing C‐to‐metal ratios during translocation, adsorption to soil particles, and flocculation at the point of zero charge. Podzolization is discussed also on the catena scale, where vertical and lateral translocation processes (across pedon boundaries) need to be considered to understand Podzol patterns in landscapes. Chronosequence studies show that incipient podzolization usually becomes visible between 100 and 500 y and mature Podzols develop in 1,000–6,000 y. The occurrence of Podzols worldwide is concentrated mainly on the boreal zone and mountain regions within the humid temperate zone. Smaller Podzol areas are found in some perhumid tropical and subtropical regions. In Germany, Podzols occur in the Alps, in the glaciofluvial valleys and heathlands of N Germany, and in the mountain ranges. They fulfil several ecological functions, especially for groundwater recharge. Main threats for these mostly sandy soils are wind erosion and surface mining of sand. Two pedons which were chosen to represent the “Soil of the year 2007” are presented. Finally, some conclusions about podzolization processes are drawn, which may explain the diverse observations reported in the literature.     

Bodenentwicklung am Rande oligotropher Moore im Raum Berlin

Soil development in the surrounding of oligotrophic mires in the Berlin region Polygenetic soils, surrounding oligotrophic kettle hole mires in the valley and aeolian sand areas of the Berlin region, were investigated. The typical soil catena is formed by the sequence of Ombric Histosol (Niedermoor), Ombric Histosol/Albi‐gleyic Podzol (Moor‐Podsol‐Gley), Albi‐gleyic Podzol (Nasspodsol‐Gley), Gleyic Podzol (Podsol‐Gley), and Dystri‐gleyic Arenosol (Gley‐Podsol‐Braunerde) (German soil classifications in parenthesis). Field and laboratory work showed, that the investigated soils were strongly related to each other and that their development depends on the trophy of the mire and groundwater fluctuations during the Holocene. Compared with the Bh‐horizon of terrestrial soils the Gh‐horizon is nearly free of Fe and Mn, but very rich in pedogenic Al‐oxides and rich in organic matter. The genesis of the soils is explained as follows: 1. The development of different Gleyic Podzols was due to rise of groundwater. Consequently the Bh and Bs horizons of Podzols surrounding the mire were converted to Gh and Gr horizons. 2. Humic substances and Al in the Gh and Gr horizons were not re‐mobilized due to the rise of groundwater, whereas Fe and Mn were reduced and removed by groundwater. 3. At the periphery of the mire Fe was enriched in the Go horizon of the Gley‐Podzols but not Mn. 4. The fact that the mire is completely surrounded by Podzol‐Gleys, indicates, that movement of the groundwater from the central parts of mires towards the periphery is an essential pedogenetic factor.     

Podzolization as a deferralitization process: a study of an Acrisol–Podzol sequence derived from Palaeozoic sandstones in the northern upper Amazon Basin

Morphological, geochemical and mineralogical studies were carried out in a representative soil catena of the low‐elevation plateaux of the upper Amazon Basin to interpret the steps and mechanisms involved in the podzolization of low‐activity clay soils. The soils are derived from Palaeozoic sandstones. They consist of Hydromorphic Podzols under tree savannah in the depressions of the plateaux and predominantly of Acrisols covered by evergreen forest elsewhere. Incipient podzolization in the uppermost Acrisols is related to the formation of organic‐rich A and Bhs horizons slightly depleted in fine‐size particles by both mechanical particle transfer and weathering. Weathering of secondary minerals by organic acids and formation of organo‐metallic complexes act simultaneously over short distances. Their vertical transfer is limited. Selective dissolution of aluminous goethite, then gibbsite and finally kaolinite favour the preferential cheluviation of first Fe and secondly Al. The relatively small amount of organo‐metallic complexes produced is related to the quartzitic parent materials, and the predominance of Al over Fe in the spodic horizons is due to the importance of gibbsite in these low‐activity clay soils. Morphologically well‐expressed podzols occur in strongly iron‐depleted topsoils of the depression. Mechanical transfer and weathering of gibbsite and kaolinite by organic acids is enhanced and leads to residual accumulation of sands. Organo‐metallic complexes are translocated in strongly permeable sandy horizons and impregnate at depth the macro‐voids of embedded soil and saprolite materials to form the spodic Bs and 2BCs horizons. Mechanical transfer of black particulate organic compounds devoid of metals has occurred later within the sandy horizons of the podzols. Their vertical transfer has formed well‐differentiated A and Bh horizons. Their lateral removal by groundwater favours the development of an albic E horizon. In an open and waterlogged environment, the general trend is therefore towards the removal of all the metals that have initially accumulated as a response to the ferralitization process and have temporarily been sequestrated in organic complexes in previous stages of soil podzolization.     

Silicon dynamics in the rhizosphere: Connections with iron mobilization

Silicon (Si) is a beneficial element for plants as it increases their resistance to several biotic and abiotic stresses. In the rhizosphere, root exudates, especially when released by nutritionally stressed plants, promote the mineral weathering and, consequently, influence Si biogeochemistry. This study aims at evaluating the mineralogical alterations in the rhizosphere of Fe‐deficient or Fe‐sufficient barley plants grown either in a natural or in an artificial calcareous soil, focusing on the dynamics of both Fe and Si. After 6 d of soil–plant contact, X‐ray diffraction (XRD) analysis of rhizosphere soil samples of Fe‐deficient plants revealed, for both natural and artificial soil, a decrease of amorphous phases and an increase of smectite compared to the unplanted soil. Root exudates released by Fe‐deficient plants were most likely the main responsible for the weathering of the amorphous phases by a ligand controlled dissolution mechanism. When the soil–plant contact was prolonged up to 12 d, plants overcame Fe nutritional stress and their effect on soil mineralogy completely changed, as proved by the considerable increase of amorphous and decrease of smectite. Smectite decrease might evidence the effort of plant to mobilize Si and micronutrients other than Fe from the soil through the exudation of organic ligands. When the artificial soil was treated with Fe‐sufficient barley plants, the mineral weathering trend appeared reversed compared to the experiments with Fe‐deficient plants. Plant nutritional status regulates the root exudation pattern and, consequently, drives mineral weathering processes in the rhizosphere. Barley has shown to be able to mobilize Si from smectite, yet depending on its Fe supply and proving the strict connection between Si and Fe dynamics in the rhizosphere.     

Mobilization of dissolved organic matter, aluminium and iron in podzol eluvial horizons as affected by formation of metal-organic complexes and interactions with solid soil material

Interactions with dissolved organic matter (DOM) are generally believed to play a crucial role in the translocation of Al and Fe in acid sandy soils. Binding of Al and Fe to DOM affects their mobility in soils by altering sorption equilibria of charged sites on solid soil material, inducing precipitation of organo‐metallic complexes and preventing the formation of inorganic Al and Fe phases. The relative importance of the different processes, especially with respect to the translocation of Al, Fe and organic matter in podzols, remains unresolved. We determined the effect of the presence of solid soil material from the eluvial (AhE and AE, respectively) horizons of a Fimic Anthrosol and a Haplic Podzol on the metal‐to‐organic carbon (M/C) ratio in solution and the formation of dissolved organic Al and Fe complexes. Furthermore, we assessed the resulting influence on the mobilization of Al, Fe and DOM. Even under considerable metal loading, the M/C ratios and ‘free’ metal fractions in solution remained low and relatively constant, due to an apparent buffering by the solid phase and the formation of organo‐metal complexes in solution. The M/C ratios remained so low that significant precipitation of organo‐metal complexes due to saturation with metals was not found. The apparent buffering by the solid phase can be explained by a strong release of organic matter from solid soil material and adsorption of non‐complexed Al and Fe on solid organic matter upon metal addition. Adsorption of organo‐metal complexes most likely played only a minor role. The observations confirm the expected mobilization of Al, Fe and DOM in eluvial horizons and seem to indicate that even under fluctuating input of Al, Fe and DOM the soil solution will have a constant composition with respect to M/C ratios and percentage of Al and Fe present in dissolved organo‐metal complexes.     

Mechanisms controlling the mobility of dissolved organic matter, aluminium and iron in podzol B horizons

The processes governing the (im)mobilization of Al, Fe and dissolved organic matter (DOM) in podzols are still subject to debate. In this study we investigated the mechanisms of (im)mobilization of Al, Fe and organic matter in the upper and lower B horizons of two podzols from the Netherlands that are in different stages of development. We equilibrated batches of soil material from each horizon with DOM solutions obtained from the Oh horizon of the corresponding soil profiles. We determined the amount of (im)mobilized Al, Fe and DOM after addition of Al and Fe at pH 4.0 and 4.5 and initial dissolved organic carbon (DOC) concentrations of 10 mg C litre^?1 or 30 mg C litre^?1, respectively. At the combination of pH and DOC concentrations most realistic for the field situation, organic matter was retained in all horizons, the most being retained in the lower B horizon of the well‐developed soil and the least in the upper B horizon of the younger profile. Organic matter solubility seemed to be controlled mainly by precipitation as organo‐metal complexes and/or by adsorption on freshly precipitated solid Al‐ and Fe‐phases. In the lower B horizons, at pH 4.5, solubility of Al and Fe appeared to be controlled mainly by the equilibrium with secondary solid Al‐ and Fe‐phases. In the upper B horizons, the solubility of Al was controlled by adsorption processes, while Fe still precipitated as inorganic complexes as well as organic complexes in spite of the prevailing more acidic pH. Combined with a previous study of eluvial horizons from the same profiles, the results confirm the important role of organic matter in the transport of Al and Fe to create illuvial B horizons initially and subsequently deepening and differentiating them into Bh and Bs horizons.     

Mobilization of Zn upon waterlogging riparian Spodosols is related to reductive dissolution of Fe minerals

Contaminated riparian soils can release metals to surface water. Periodic waterlogging affects metal mobility but the processes and soil factors governing net trends are not well understood. Experiments were combined with geochemical modelling to identify processes explaining the dynamics of zinc (Zn) in contaminated soils following waterlogging. Samples were collected from 12 Spodosols near streams in a metal‐contaminated area and four similar but uncontaminated soils were sampled in a reference area. Air‐dried samples were submerged and incubated under N₂. The soil redox potential decreased from 470 mV initially to approximately 30 mV over 2 months. The pore‐water Zn concentrations surprisingly increased over the same period by, on average, a factor of 18 (range 0.6–80; immobilization in one soil only) despite an increase in pH of 1.8 units, on average. Dissolved organic matter (DOM) in the soil solution increased during waterlogging but the observed increase in Zn solubility could not be explained by increased complexation with DOM, because the estimated Zn²⁺ activity also increased by a factor of 18 on average (range 0.2–82). Speciation modelling suggests that Zn mobilization during waterlogging results from Fe²⁺ displacing sorbed Zn²⁺ from particulate organic matter and from dissolution of Zn‐bearing Fe/Mn oxyhydroxides. This hypothesis is supported by the significant positive correlation (r = 0.87, n = 13) between the factor change in pore‐water Zn concentration and the ratio of dithionite‐extractable Fe to organic carbon content. These results show that Fe dynamics are important for predicting the fate of trace metals in anoxic soils.     

A carbon balance model for organic layers of acid forest soils

Organic layers of acid forest soils are highly dynamic carbon reservoirs. During forest succession the stored amount of organic carbon (OC) changes drastically. Because of feedback between OC storage in organic layers and in mineral soils and other compartments of the environment (plant, atmosphere, and groundwater), there is a strong need for applicable carbon balance models, particularly for organic layers. In this paper a simplified model for the carbon balance of organic layers (CABOLA model) of acid forest soils is presented. The model considers two horizons, the L and O horizon. Decomposition and transport processes are described by first order differential equations. C input into the organic layer is due to litter fall onto the L horizon. The governing equations are solved by integration. To demonstrate the model's capability of simulating the OC dynamics of organic layers, data on OC storage in organic layers of acid sandy forest soils with deep groundwater tables (Podzols) under pine stands were used. Together with literature data and some assumptions, these data were used for a first, rough estimation of the model parameters. Model calculations confirm that the CABOLA model is in principle able to simulate the dynamics of OC storage in organic layers during forest succession. Nevertheless, intensive research efforts will be necessary to independently parameterize the model for broad applications.     

Physiochemical Processes Affecting the Geochemistry of Carbonate Aquifer of Southeastern Al-Ain Area, United Arab Emirates (UAE)

Detailed hydrogeochemical analyses were carried out in order to delineate the physiochemical processes and paleohydrogeological regime of groundwater flow of southeastern Al-Ain area and its surrounding desert fringes. Groundwater composition and salinization could be a result of integrated cycles of successive physiochemical processes of mixing of seawater, salt dissolution, and flushing by recharge water from the infiltration of rainfall in the interdune areas and gravel plains and also from Jabal Hafit lying to the southeast, and sewage effluent water from the residence areas to the northwest. In addition to that, ion exchange upon flushing, salt sieving from deep aquifers, and evaporation and accumulation along the wetland zones are factors affecting the groundwater of the area. Human and anthropogenic activities might have negative impact on the quality of groundwater in the region. Both Zn and Fe are the most detectable heavy metals in the analyzed samples, and they reach as maximum as 2,277 ??g/l and 2,902.6 ??g/l for Zn and Fe, respectively, along the north western localities. The source of these metals is most probably due to contamination threats by the sewage effluent or industrial wastes in these localities. Reducing environment releases the toxic and heavy metals in soluble compounds in case of high organic matters. A strong relationship appears between Fe and Zn ion concentrations in groundwater and the type of well construction materials (casings and screens). Also, a positive correlation is observed between the concentrations of Zn in water wells and in plants surrounding the wells. The increase of chromium concentrations is observed in the north of the study area, and this increase might be ascribed to the weathering of olivine and pyroxence of Oman Mountain in the northeast and might be possibly due to using the manure or animal wastes for agriculture activities.     

Transfer of dissolved Al, Fe and Si in two Amazonian forest environments in Brazil

The balance of Si, Fe and Al in the soil solution determines more or less the course of soil formation in the tropics. We have tried to improve understanding of the processes by studying fluxes of dissolved Si, Fe and Al from the atmosphere, through the canopy and the soil, and to the groundwater, in two distinct Amazonian ecosystems, one a typical rainforest, developed on a Ferralsol, the other a so-called ‘Campinarana’ forest, developed on a Podzol. The Si, Fe and Al in the rain and the throughfall and the stemflow were measured throughout a year, and the leaching of Si, Fe and Al through the upper soil and at the groundwater level were estimated. The annual balance showed that stemflow inputs were negigible compared with the contributions from throughfall and rain. The inputs of Fe, Al and Si to the topsoil from the rain and from dust and biological release in the canopy were small but not negligible. These sources contributed more aluminium in the Campinarana than in the rainforest. The rainfall constituted the main input of dissolved Fe and Al to the topsoil in both ecosystems. The element balances in the soil horizons confirmed that the present functioning of the Ferralsol results in aluminization and desilicification. We also found that the elements are transported in micropore flow and on translocated particles, as well as in freely percolating water.     

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.     

Sodium hypochlorite oxidation reduces soil organic matter concentrations without affecting inorganic soil constituents

Oxidative treatment can isolate a stable organic matter pool in soils for process studies of organic matter stabilization. Wet oxidation methods using hydrogen peroxide are widely used for that purpose, but are said to modify poorly crystalline soil constituents. We investigated the effect of a modified NaOCl oxidation (pH 8) on the mineral composition of 12 subsoils (4.9–38.2 g organic C kg^?1) containing varying amounts of poorly crystalline mineral phases, i.e. 1.1–20.5 g oxalate‐extractable Fe kg^?1, and of different phyllosilicate mineralogy. Post‐oxidative changes in mineral composition were estimated by (i) the determination of elements released into the NaOCl solution, (ii) the difference in dithionite‐ and oxalate‐extractable Si, Al and Fe, and (iii) the specific surface areas (SSAs) of the soils. The NaOCl procedure reduced the organic C concentrations by 12–72%. The amounts of elements released into the NaOCl extracts were small (≤ 0.14 g kg^?1 for Si, ≤ 0.13 g kg^?1 for Al, and ≤ 0.03 g kg^?1 for Fe). The SSA data and the amounts of dithionite‐ and oxalate‐extractable elements suggest that the NaOCl oxidation at pH 8 does not attack pedogenic oxides and hydroxides and only slightly dissolves Al from the poorly crystalline minerals. Therefore, we recommend NaOCl oxidation at pH 8 for the purpose of isolating a stable organic matter pool in soils for process studies of organic matter stabilization.     

Potential Groundwater Contamination with Toxic Metals in and Around an Abandoned Zn Mine, Korea

This study evaluated potential groundwater contamination with toxic metals in and around an abandoned zinc mine in Korea. Water levels in the mine waste dump indicated occurrence of a losing stream during the period of peak stream flow as a result of snowmelt runoff, which posed the threat of groundwater pollution. The pH values for the groundwaters were near neutral to slightly basic, with a slight increase of the values along the stream flow direction. Higher values of electrical conductivity were observed in the mine area. High dissolved oxygen concentrations clearly indicated an oxygenated groundwater environment. High concentration of sulfate and most dominant Ca-SO₄ type groundwaters represent effects of mine drainage and sulfide minerals. In the mine area, groundwater contamination by Zn, Al, Fe, and Mn was observed. Most of the toxic metals decreased with distance from the mine, some have decreased gradually or others more suddenly although some metals were also found in high concentrations- in downgradient area. Levels of toxic metals were relatively low in groundwaters downgradient of the mine, which may be due to the high pH and highly oxygenated conditions, and mixing with metal-free waters.     

Mobilization of aluminium, iron and silicon by Picea abies and ectomycorrhizas in a forest soil

Weathering of soil minerals is a key determinant of ground and surface water quality and is also important in pedogenic and rhizosphere processes. The relative importance of biotic and abiotic studies in mineral weathering, however, is poorly understood. We investigated the impact of Picea abies seedlings, an ectomycorrhizal fungus and humic acid on the solubilization of aluminium (Al), iron (Fe) and silicon (Si) in an E horizon forest soil over 10 months. Elemental budgets were constructed based upon losses in drainage water, accumulation in plants and changes in the pools of exchangeable ions. Plants and mycorrhizas or both had a significant effect on the total amounts of Al, Fe and Si mobilized from the soil. Significantly larger amounts of Al and Fe were recovered in plants than those lost in drainage water, whereas the opposite trend was true for Si. The continual addition of dissolved organic matter to the soil in the form of humic acid had an effect only on mobilization of Fe, which increased due to larger plant uptake and an increase in the exchangeable pool. The mobilization of Fe and Si were positively correlated with hyphal length, soil respiration and concentrations of oxalate in the soil solution, and mobilization of Al was strongly correlated with plant weight. Scanning electron microscopy revealed that most fungal hyphae were associated with mineral surfaces with little occupation of cracks and micropores within mineral grains. Evidently ectomycorrhizas have important impacts on mineral dissolution and the chemistry of forest soils.     

Genetic relationships between ferralsols,podzols and white kaolin in Amazonia

White kaolin has frequently been observed to be associated with ferralsol‐podzol soil systems in Amazonia. In order to evaluate whether such systems favour kaolin genesis and to identify the associated genetic processes, we studied soil organization, mineralogy and groundwater properties of a ferralsol‐podzol soil system with white kaolin located in the High Rio Negro Basin, Brazil. We found that the kaolin was situated near the ferralsol‐podzol transition and that its thickness was related to the depth of landscape incision by regressive erosion. The kaolin was characterized by silicon, iron and titanium (Ti) leaching and aluminium (Al) absolute accumulation. The groundwater that percolates from the podzol to the kaolin can enhance kaolinite precipitation, by supplying Al originating from kaolinite dissolution in the overlying Bh, and kaolin bleaching, by low pH and Eh of the percolating waters favouring iron reduction. The system dynamics imply that the quartz dissolution rate in the kaolin is of at least the same order of magnitude as the kaolinite dissolution rate in the overlying Bh. Within the whole system, Ti appeared to be very mobile.     

祁连山区成土过程驱动的土壤地形序列土壤有机碳固存特征

土壤有机碳（SOC）含量和动态是有机碳输入输出平衡和土壤固持能力共同作用的结果，前者主要受生物气候条件控制，而后者则主要受黏粒及无机矿物等土壤理化属性的影响。本研究通过沿海拔梯度选择表土地形序列作为“自然试验场”，探究祁连山区SOC含量及组分变化的控制因素。研究发现：该地形序列土壤母质主要来源于黄土沉积，土壤黏土矿物以绿泥石和水云母为主，指示该区域的整体弱风化特征。在巨大的水热梯度影响下，地形序列内土壤成土强度差异明显，低海拔地区土壤含有碳酸盐，随着海拔上升，碳酸盐物质逐渐淋失，SOC和铁铝氧化物含量增加。进一步分析发现，铁铝氧化物是SOC含量及组分方差的主要解释变量。偏相关分析显示，当控制铁铝氧化物后，气候对SOC含量及组分的影响不显著。这表明气候对SOC的影响可能主要通过影响土壤属性，造成铁铝氧化物等属性的差异，间接影响SOC的长期固存，且该机制主要作用于矿物结合态有机碳（MOC）组分。本研究对理解SOC固存及其对气候变化的动态响应有重要启示。     

Dissolved and Particulate Metals in the Mero River (NW Spain): Factors affecting Concentrations and Load during Runoff Events

This article examines the metal [aluminum (Al), iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn)] runoff dynamics in the Mero River (northwestern Spain). At the catchment outlet, metal concentrations, suspended solids, dissolved organic carbon, pH, and discharge were determined during three runoff events. The river drains an agroforestry catchment of 65 km² with relatively high livestock density. Dissolved and particulate metal concentrations increased during the events with respect to pre-event concentrations, but the increase of the dissolved fraction is relatively small compared to that of the particulate fraction. The dissolved metal concentration peaks appeared after the hydrograph peak, suggesting transport associated with subsurface flow. For particulate metals, peaks usually occurred during the falling limb of hydrograph, implying distant-river source of metals. Particulate forms represented more than 90% of total Al, Fe, and Mn load, whereas for Cu and Zn its contributions were 52–76% and 31–56%, respectively. The high positive correlations of all particulate metals, except Zn, with suspended solid concentrations indicate that these constituents play a major role in transport of metals. Soil erosion is the main process responsible for causing elevated Al, Fe, and Mn concentrations in the river during rainfall–runoff events while Zn and Cu come mainly from the addition of slurries and manures to farmland.     

A reassessment of podzol formation processes

Translocated (oxalate-soluble) Al and Fe are present predominantly in inorganic forms in the B₂ horizons of the five pcdzol profiles examined: A1 as imogolite and proto-imogolite allophanes, and Fe as a separate oxide phase. Below the top few cm of the B₂ horizon, over 75 per cent of the extractable (acid-plus alkali-soluble) organic matter is present as Al-fulvates, largely sorbed on allophanic material. The Bh horizons of the Iron Humus Podzol and Iron Podzol intergrades are distinguished by very high levels of organically bound Fe (soluble in EDTA solution), five to ten times more than in immediately adjacent A₂ or B₂ horizons, and also by larger humic acid contents than in comparable B₂, levels in typical Iron Podzols. Inorganic forms of translocated Al and Fe are probably absent from two of the three Bh horizons examined, and also from the Bhg horizon overlying the thin iron pan in the Peaty Podzol. The organic matter in this Bhg horizon is saturated with Al rather than Fe. Chemical and physical processes which could lead to evolution of a profile along the genetic sequence, Iron Podzol, Iron Humus Podzol, Peaty Podzol, are postulated. During the formation of an Iron Podzol, positively charged inorganic sols carry aluminium, silicon and iron from the A₂ and deposit them in the B₂ horizon; subsequently, with the development of an H layer, colloidal humus migrates through the A₂ and precipitates on the positive colloids at the top of the B₂ horizon to form a Bh horizon, in which remobilized ferric species are trapped by the organic matter. In higher rainfall areas, occasional waterlogging above the oxide-impregnated B₂ leads to a thin iron pan, separating permanently oxidizing conditions below from seasonally waterlogged and reducing conditions above.