Extraction of inorganic forms of translocated Al, Fe and Si from a podzol Bs horizon

A range of extractants commonly used in soil studies, both to clean up the crystalline clay minerals by dissolving poorly ordered phases, and to assist in the classification of soils, has been used to assess their efficiency in extracting inorganic forms of translocated Al, Fe and Si from a podzol Bs horizon. A 4 h extraction with ammonium oxalate pH 3 is the most efficient single procedure for these phases. It is concluded that satisfactory characterization of a podzol B horizon requires the use of two extractants: acid oxalate to assess the total amounts of Al, Fe and Si in non-crystalline weathering products, and pyrophosphate to assess the amounts in organic forms. Dithionite-citrate-bicarbonate gives a measure of total free iron oxides, but extracts only an ill-defined fraction of the allophane-imogolite complex.     

Iron reduction and changes in cation exchange capacity in intermittently waterlogged soil

The long-term effects of intermittent flooding on soil properties were studied in field experiments on a Vertisol cropped with rice in Senegal. The dominant clay minerals were smectite and kaolinite. When the soil was reduced after flooding, its cation exchange capacity (CEC) increased to twice that of its oxidized, unflooded state. Mössbauer spectroscopy showed an increase in smectite structural Fe^II upon reduction, which explained a part of the increase in CEC. The rest of the increase was attributed to the removal of iron oxyhydroxide coatings by reductive dissolution. The reduction and dissolution of oxides under the field conditions were substantiated by analysis of the surfaces of vermiculites buried in the Ap horizons of the cropped and the non-cropped soils. The redox-induced CEC changes were found to be reversible after 22 cycles of rice cropping. Nevertheless, the structural Fe and free Fe contents of the rice field Ap horizon were less than those of soil in uncropped neighbouring land, suggesting that inundation induced weathering and eluviation of the minerals. The observed changes in CEC and related redox reactions may substantially modify proton, anion and cation balances in intermittently flooded soils.     

Soil and sediment properties affecting the accumulation of mercury in a flood control reservoir

Mercury accumulations in some fish species from Grenada Lake in north Mississippi exceed the Food and Drug Administration standards for human consumption. This large flood control reservoir serves as a sink for the Skuna and Yalobusha River watersheds whose highly erodible soils contribute to excessively high sediment yields and impaired water quality. This study was conducted to characterize the distribution of total Hg in watershed soils and determine the relationship between the easily transportable clay, organic C (OC), and Fe oxide fractions and the movement of Hg from upland sources to reservoir sinks. Cores were collected from soils, of different land-use, representative of the three soil orders (Alfisols, Entisols, and Vertisols) found in the watersheds. Sediment cores were collected from the Yalobusha River and Grenada Lake. In the laboratory, soil cores were sampled by horizon while sediment cores were sampled in 10 cm increments. These samples were characterized for total Hg, particle size distribution, OC, Fe oxide contents, and pH. Mercury concentrations ranged from 10 to 112 µg kg^− 1 in the soil profiles, with average regression coefficient (r²) values of 0.104, 0.362, and 0.06 for Hg versus clay, OC, and Fe oxides, respectively. River sediment cores had Hg concentrations ranging from 0 to 38 µg kg^− 1, and significant (1% level) r² values of 0.611, 0.447, and 0.632 versus clay, OC, and Fe oxides, respectively. Mercury concentrations in the lake sediment ranged from 0 to 125 µg kg^− 1. The r² values for Hg versus clay, OC, and Fe oxides in the lake sediment were 0.813, 0.499, and 0.805, respectively, all significant at the 1% level. These results indicate that total Hg is poorly correlated with the clay, OC, and Fe oxide fractions at depth in the soil profiles because maximum Hg concentrations occur in the surface horizons due to atmospheric in-fall. The statistically significant r² values for Hg versus these components in the sediment cores are the result of particulate clay, Fe oxides, and finely divided OC sorption of Hg from solution during the runoff and sediment transport process. The higher correlations for the lake sediment reflect an enrichment of the Hg-laden clay fraction relative to stream sediment through flocculation and sedimentation processes in the slack-water environment of the reservoir.     

Speciation of phosphorus in temperate zone forest soils as assessed by combined wet‐chemical fractionation and XANES spectroscopy

Phosphorus availability in terrestrial ecosystems is strongly dependent on soil P speciation. Here we present information on the P speciation of 10 forest soils in Germany developed from different parent materials as assessed by combined wet‐chemical P fractionation and synchrotron‐based X‐ray absorption near‐edge structure (XANES) spectroscopy. Soil P speciation showed clear differences among different parent materials and changed systematically with soil depth. In soils formed from silicate bedrock or loess, Fe‐bound P species (FePO₄, organic and inorganic phosphate adsorbed to Fe oxyhydroxides) and Al‐bound P species (AlPO₄, organic and inorganic phosphate adsorbed to Al oxyhydroxides, Al‐saturated clay minerals and Al‐saturated soil organic matter) were most dominant. In contrast, the P speciation of soils formed from calcareous bedrock was dominated (40–70% of total P) by Ca‐bound organic P, which most likely primarily is inositol hexakisphosphate (IHP) precipitated as Ca₃‐IHP. The second largest portion of total P in all calcareous soils was organic P not bound to Ca, Al, or Fe. The relevance of this P form decreased with soil depth. Additionally, apatite (relevance increasing with depth) and Al‐bound P were present. The most relevant soil properties governing the P speciation of the investigated soils were soil stocks of Fe oxyhydroxides, organic matter, and carbonate. Different types of P speciation in soils on silicate and calcareous parent material suggest different ecosystem P nutrition strategies and biogeochemical P cycling patterns in the respective ecosystems. Our study demonstrates that combined wet‐chemical soil P fractionation and synchrotron‐based XANES spectroscopy provides substantial novel information on the P speciation of forest soils.     

PARTITIONING OF HEAVY METALS IN PODZOL SOILS CONTAMINATED BY MINE DRAINAGE WATERS,DALARNA, SWEDEN

The discharge of acidic mine drainage waters onto a hillslope in Dalarna, central Sweden, has lead to the contamination of the podzol soils with Cu, Fe, Ni, Pb, Zn and sulfate. Samples from contaminated and reference soils have been collected for chemical and mineralogical analyses. Jarosite is identified by x-ray diffraction analysis as a precipitate in the upper horizons (A, E, B) of the contaminated soils, where the soil acidity (pH_KCl～2.6) promotes jarosite stability. The sequential chemical extraction of soil samples indicates that, in the reference A horizon, Cu, Pb, Ni and Zn are bound primarily to cation exchange sites and organic matter. In the A horizon of the contaminated soils closest to the rock dump, metal partitioning is dominated by the Fe oxide fractions, despite the high organic matter content; Pb is almost completely bound to crystalline Fe oxides, possibly adsorbed to Fe oxides or occuring in a jarosite solid solution. In the reference B and C horizons, Cu, Ni and Zn are primarily adsorbed/coprecipitated in the Fe oxide fractions, while Pb remains with a large fraction bound to organic matter. In the Fe-rich B horizon of the contaminated soils, the partitioning of the metals in cation exchange sites and to organic matter has greatly increased relative to the reference soils, resulting from the mobilization of organo-metal complexes down the profile.     

平原区水田改林地后土壤黏土矿物及氧化铁的变化

近年来水田改为林地在我国南方地区非常普遍,为了解这种转变对土壤矿物(黏土矿物和氧化铁)演变的影响,在浙江省平原地区构建了4个水田改林地系列土壤,采用野外调查和室内分析相结合的方法,研究水田改林地后土壤剖面中氧化铁形态和黏土矿物类型的演变规律。结果表明,改林地后,土壤剖面中全铁含量变化不明显,耕作层和犁底层土壤游离氧化铁有轻微增加趋势。改林地15～20 a后,4个系列土壤耕作层活性铁和活化度降幅分别在18.0%～38.4%和24.7%～48.9%;耕作层土壤铁氧化物的晶胶比增幅在0.73倍～1.62倍;耕作层土壤亚铁含量明显下降,降幅最高达95.8%,变异系数达到143.9%;耕作层络合铁降幅在21.3%～36.2%,并与有机质呈极显著正相关(P0.01)。改林地后,犁底层土壤中绿泥石相对含量及其与高岭石的比值都呈降低趋势,其他黏土矿物相对含量变化规律不明显,土壤有机质显著下降,土壤明显酸化。土壤铁氧化物形态和数量的变化对土壤结构以及土壤重金属的迁移转化等产生重要影响。     

Adsorption and Desorption of Copper in Pasture Soils

Abstract

Copper (Cu) is bound strongly to organic matter, oxides of iron (Fe) and manganese (Mn), and clay minerals in soils. To investigate the relative contribution of different soil components in the sorption of Cu, sorption was measured after the removal of various other soil components; organic matter and aluminum (Al) and Fe oxides are important in Cu adsorption. Both adsorption and desorption of Cu at various pH values were also measured by using diverse pasture soils. The differences in the sorption of Cu between the soils are attributed to the differences in the chemical characteristics of the soils. Copper sorption, as measured by the Freundlich equation sorption constants [potassium (K) and nitrogen (N)], was strongly correlated with soil properties, such as silt content, organic carbon, and soil pH. The relative importance of organic matter and oxides on Cu adsorption decreased and increased, respectively, with increasing solution Cu concentrations. In all soils, Cu sorption increased with increasing pH, but the solution Cu concentration decreased with increasing soil pH. The cumulative amounts of native and added soil Cu desorbed from two contrasting soils (Manawatu and Ngamoka) during desorption periods showed that the differences in the desorbability of Cu were a result of differences in the physico‐chemical properties of the soil matrix. This finding suggests that soil organic matter complexes of Cu added through fertilizer, resulted in decreased desorption. The proportions of added Cu desorbed during 10 desorption periods were low, ranging from 2.5% in the 24‐h to 6% in the 2‐h desorption periods. The desorption of Cu decreased with increasing soil pH. The irreversible retention of Cu might be the result of complex formation with Cu at high pH.     

Lability of soil organic carbon in tropical soils with different clay minerals

Soil organic carbon (SOC) storage and turnover is influenced by interactions between organic matter and the mineral soil fraction. However, the influence of clay content and type on SOC turnover rates remains unclear, particularly in tropical soils under natural vegetation. We examined the lability of SOC in tropical soils with contrasting clay mineralogy (kaolinite, smectite, allophane and Al-rich chlorite). Soil was sampled from A horizons at six sites in humid tropical areas of Ghana, Malaysian Borneo and the Solomon Islands and separated into fractions above and below 250 μm by wet sieving. Basal soil respiration rates were determined from bulk soils and soil fractions. Substrate induced respiration rates were determined from soil fractions. SOC lability was significantly influenced by clay mineralogy, but not by clay content when compared across contrasting clay minerals. The lability of SOC was lowest in the allophanic and chloritic soil, higher in the kaolinitic soils and highest in the smectitic soil. Our results contrast with conventional concepts of the greater capacity of smectite than of kaolinite to stabilize SOC. Contents of dithionite-citrate-bicarbonate extractable Fe and Al were inversely related to SOC lability when compared across soil types. A stronger inverse correlation between content of ammonium-oxalate extractable Fe and SOC lability was found when considering the kaolinitic soils only and we conclude that the content of active Fe (hydr-) oxides controls SOC stabilization in the kaolinitic soils. Our results suggest that the validity of predictive models of SOC turnover in tropical soils would be improved by the inclusion of soil types and contents of Fe and Al (hydr-) oxides.     

Poorly crystalline mineral phases protect organic matter in acid subsoil horizons

Soil minerals are known to influence the biological stability of soil organic matter (SOM). Our study aimed to relate properties of the mineral matrix to its ability to protect organic C against decomposition in acid soils. We used the amount of hydroxyl ions released after exposure to NaF solution to establish a reactivity gradient spanning 12 subsoil horizons collected from 10 different locations. The subsoil horizons represent six soil orders and diverse geological parent materials. Phyllosilicates were characterized by X-ray diffraction and pedogenic oxides by selective dissolution procedures. The organic carbon (C) remaining after chemical removal of an oxidizable fraction of SOM with NaOCl solution was taken to represent a stable organic carbon pool. Stable organic carbon was confirmed as older than bulk organic carbon by a smaller radiocarbon (¹⁴C) content after oxidation in all 12 soils. The amount of stable organic C did not depend on clay content or the content of dithionite–citrate-extractable Fe. The combination of oxalate-extractable Fe and Al explained the greatest amount of variation in stable organic C (R² = 0.78). Our results suggest that in acid soils, organic matter is preferentially protected by interaction with poorly crystalline minerals represented by the oxalate-soluble Fe and Al fraction. This evidence suggests that ligand exchange between mineral surface hydroxyl groups and negatively charged organic functional groups is a quantitatively important mechanism in the stabilization of SOM in acid soils. The results imply a finite stabilization capacity of soil minerals for organic matter, limited by the area density of reactive surface sites.     

Methylmercury Production and Accumulation in Sediments and Soils of an Amazonian Floodplain – Effect of Seasonal Inundation

This study investigated the spatialand seasonal variations of MeHg concentrations andburdens of different sediments and soils of theTapajós river floodplain, one of the majorclear-water tributaries of the Amazon. The smallfloodplain of the Tapajós is typical of Amazonianfloodplain ecosystems. The studied lakes are borderedby inundated forest (igapó), while floatingmacrophyte mats (Paspalum sp.) develop at themargin of lakes during the flooded season. During theflood, we observed very low MeHg concentrations in theopen water lake sediments (<0.5 ng g^-1 d.w or<0.5 μg m^-2 cm^-1 d.w.) as compared to thesemi-aquatic sediments of the macrophyte zone (0.2–1.4 ng g^-1 d.w or 1–3 μg m^-2cm^-1 d.w.) and the igapó semi-terrestrial soils (0.2–3 ng g^-1 d.w or2–12 μg m^-2 cm^-1 d.w.). The litter horizon fromthe igapó soils showed the highest value of MeHg(4–8 ng g^-1 d.w.) representing 0.2–2 μg m^-2cm^-1 d.w. at the sediment/water interface during theaquatic phase. The inundation had no effect on theconcentrations and burdens of MeHg in the sediment ofthe central part of the lake. The inundation had aclear effect on the methylation of Hg at the surfaceof semi-aquatic shoreline sediments (macrophyte zone)and semi-terrestrial forest soils, where MeHgconcentrations and burdens appeared to be 3 timesgreater following inundation. In all cores, total Hgconcentrations follow those of Fe and Aloxy-hydroxides, whereas the MeHg concentrations arelinked to organic matter quality and quantity. It issuggested that organic matter and inundation controlMeHg production and accumulation at the benthicinterface. These results confirm previousobservations, in the same study area, of net²⁰³Hg methylation potentials. The fresh andlabile organic matter in the litter of the igapóforest appears as the most important factor leading tosignificant enrichment of MeHg in these particularterrestrial/aquatic sedimentary environments.     

Temporary storage of soil organic matter and acid neutralizing capacity during the process of pedogenetic acidification of forest soils in Kinki District,Japan

Abstract

Pedogenetic acidification processes in forest soils derived from sedimentary rocks under mesic and thermic soil temperature regimes (MSTR and TSTR; corresponding to mean annual soil temperatures of 8–15°C and 15–22°C, respectively) in the Kinki District were investigated based on titratable alkalinity and acidity characteristics and soil solution composition. According to statistical analyses of the soil properties, the titratable alkalinity required to acidify soils to pH 3.0 was considered to be derived from reactions occurring at the surface of amorphous Al oxides, while titratable acidity at a pH ranging from 5.5 to 8.3 results from dissociation of acidic functional groups of soil humus and/or deprotonation of oxide surfaces. These reactions were generally more prevalent in MSTR soils. Based on the soil solution composition and titratable alkalinity and acidity in the soil profiles, two processes were postulated for pedogenetic acidification, that is, eluvi-illuviation of inorganic Al followed by subsequent adsorption of dissolved organic carbon (DOC) onto the precipitates of Al hydroxides and comigration of Al and DOC in the form of organo-mineral complexes. Both processes were conspicuous in MSTR soils and significantly contributed to soil organic matter storage in the subsoil layers. Pedogenetic acidification in forest soils with MSTR was characterized by an accumulation of acidity in the form of amorphous compounds and/or organo-mineral complexes in the B horizon. It seems, to some extent, similar to podzol formation, at least in terms of Al translocation. Amorphous Al hydroxides protect against further acidification through protonation and/or partial monomerization and can, thus, be regarded as a temporary storage of acid neutralizing capacity of the soil, which would be otherwise leached out directly from the soil profile. In contrast, the acid-buffering reactions of TSTR soils seemed to occur, if at all, mostly at or near the soil surface and the contribution of the B-horizon soils was limited.     

Podzolization and clay migration in spodosols of eastern France

Field observations and laboratory analyses of podzols developed on sandy Triassic parent material in the Vosges have demonstrated clay accumulation in the upper parts of spodic horizons, especially in the B_h horizons. To see whether clay accumulation corresponds to real clay illuviation, two types of podzol profiles have been studied. The first is an iron podzol, called “podzol forestier”, because it has been entirely developed under climax forest environment. The second, an iron-humus podzol, called “podzol de dégradation”, was first developed under forest and more recently (the last 1000 to 2000 years) under heath vegetation. Methods used in the study of the two profiles were the “isoquartz” alteration balance, clay mineral identification by X-ray analyses and micromorphholigical examination.For the podzol forestier, the data strongly support a hypothesis of illuviation of very fine micaceous clays (vermiculite), especially the ferriargillans in the B_h horizon. Consequently, it is believed that illuviation of the greatest part of clays promoted differentiation of podzol horizons. Podzolization is strongly redistributive.In the podzol de dégradation, the identified clay minerals are the same as in the podzol forestier but the pedological features in the B_h horizon correspond to those of classic spodic horizons (mixed concentration and grain coatings). Moreover, podzolization is geochemically strongly subtractive in this second type of podzol.Inasmuch as the podzols de dégradation are developed from the podzols forestiers, the authors define an evolutionary time-sequence: the first phase of forest soil genesis is regulated by the double process of clay illuviation and redistributive podzolization. The following degradation phase connected to the particular nature of heath vegetation and to abundant chelating organic compounds induces the disruption of the ferriargillans previously formed in the B_h horizon, the formation of an agglomeroplasmic fabric microstructure and above all, the elimination of hydrolyzed products of minerals from the profile. During this last phase, a strongly subtractive podzolization prevails over clay illuviation.     

Effects of organic matter addition on phosphorus availability to flooded and nonflooded rice in a P‐deficient tropical soil: a greenhouse study

Addition of organic matter (OM) to flooded soils stimulates reductive dissolution of Fe(III) minerals, thereby mobilizing associated phosphate (P). Hence, OM management has the potential to overcome P deficiency. This study assessed if OM applications increases soil or mineral fertilizer P availability to rice under anaerobic (flooded) condition and if that effect is different relative to that in aerobic (nonflooded) soils. Rice was grown in P‐deficient soil treated with combinations of addition of mineral P (0, 26 mg P/kg), OM (0, ~9 g OM/kg as rice straw + cattle manure) and water treatments (flooded vs nonflooded) in a factorial pot experiment. The OM was either freshly added just before flooding or incubated moist in soil for 6 months prior to flooding; blanket N and K was added in all treatments. Fresh addition of OM promoted reductive dissolution of Fe(III) minerals in flooded soils, whereas no such effect was found when OM had been incubated for 6 months before flooding. Yield and shoot P uptake largely increased with mineral P addition in all soils, whereas OM addition increased yield and P uptake only in flooded soils following fresh OM addition. The combination of mineral P and OM gave the largest yield and P uptake. Addition of OM just prior to soil flooding increased P uptake but was insufficient to overcome P deficiency in the absence of mineral P. Larger applications of OM are unlikely to be more successful in flooded soils due to side effects, such as Fe toxicity.     

DISTRIBUTION OF PYROPHOSPHATE-EXTRACTABLE IRON AND ORGANIC CARBON IN SOILS OF VARIOUS GROUPS

Potassium pyrophosphate (0.1m ) removes very little Fe from crystalline Fe oxides at pH 10, but peptizes finely divided hydrous amorphous oxides and organic matter in soils. Fe and C contents of extracts from each horizon of twenty-six British soil profiles show distinctive patterns, independent of the residual dithionite-soluble Fe. Thus extracts of humus Fe podzols have maximum Fe and C in the B horizon, peaty gley podzol has maximum Fe in the B horizon but maximum C in the surface. These groups are differentiated from non-podzols which have maximum pyrophosphate extractable Fe and C in the surface horizon, decreasing with depth. Intermediate patterns help to quantify differences in soils of classes having properties of more than one soil group.     

Zur pedochemie des berylliums — untersuchungen einer bodengesellschaft im gebiet des bärhaldegranits (südschwarzwald)

The profile distribution of Be is given for a toposequence of podzols, brown earths, stagnogley, forest bog and “Ockererde” (slope gley with infiltrated Al, Mn and Fe). Analysed were the parent rock, fine earth, four fractions of mineral soil (fine skeleton, sand, silt, clay) and the needles of spruce trees.Beryllium is involved in selective weathering processes of elements. Accordingly, the Be-content increases from the fine skeleton to sand, rock, silt and clay, progressively. Through pedogenetic enrichment, Be accumulates in the mica-rich silt fraction.Uptake of Be by spruce is very low even from these acid soils. The accumulation in the needles becomes greater with increasing age.The pedochemical behaviour of Be is very similar to that of Al.Be is incorporated very little in the organic matter cycle, and is therefore not enriched in the soil humus.The brown earth has lost Be, especially in the A_h-horizon.In the A_e-horizon of the podzol, loss of Be is even greater; the loss is partly offset by Be-enrichment in the B-horizon.In the profiles of depressions, the Be-content is high and is ascribed to lateral flow from the slopes. Be-enrichment is especially high in horizons with a pH of about 5.The Be-balance of the watershed indicates an export of about 10% of the original amount.     

Molybdenum Determination in Mehlich‐1 and Mehlich‐3 Soil Test Extracts and Molybdenum Adsorption in Brazilian Soils

Abstract

The extractant Mehlich‐1 is routinely used in Brazil for determination of soil nutrients, whereas Mehlich‐3 has been suggested as a promising extractor for soil fertility evaluation. Both were used for extraction of molybdenum (Mo) in Brazilian soils with Mo dosage by the KI+H₂O₂ method. The Langmuir and Freundlich isotherms were used to study soil Mo adsorption. Mehlich‐1 extracted more Mo than Mehlich‐3 in soils with high contents of organic matter, clay, and iron (Fe) oxides. Mehlich‐3 and Mehlich‐1 extractions correlated positively and significantly with amorphous Fe oxides, crystalline Fe oxides, and organic matter. Molybdenum recovering rates correlated to crystalline Fe oxides and clay contents but not to organic matter, pH, and Mo adsorption capacity. Amorphous and crystalline Fe oxides, clay, and organic matter were responsible for most of the Mo adsorption. The Langmuir isotherm described better the Mo adsorption to soil amorphous Fe oxides and organic matter than the Freundlich isotherm.     

Reversibility of soil solution acidity and of sulfate retention in acid forest soils

To quantify the effects of reduced sulfate input on the chemistry of soil solution and soil S storage in acid forest soils, an experiment with undisturbed soil columns from two different sites was implemented. The acid cambisol of the Solling is subjected to a high sulfate input and especially the B-horizon has a high sulfate content. On the contrary, the podzol of the Fuhrberg site is subjected to low input and has low sulfate content. Undisturbed soil columns were taken from both sites and were irrigated at 6 °C with a precipitation rate of 3 mm d^?1 over 10 mo. In treatment No. 1, an artificial throughfall with pH 5.2 and reduced sulfate load (45 μmol L^?1) was applied. In treatment No. 2, an artificial througfall representing a high sulfate deposition (427 μmol L^?1, pH 3.2) was used. In case of the Solling soil, the pH of soil solution was unaffected by treatments during the entire experiment. Alkalinity of the soil solution was slightly increased in treatment No. 1 at a depth of 20 cm. While treatment No. 1 resulted in a reduction of the sulfate concentrations of the soil solution in the top soil, sulfate concentrations were unaffected at a depth of 40 cm. The B-horizon of the Solling soil prevented deacidification of the soil solution by desorption of previously stored sulfate. In case of the Fuhrberg soil, treatment No. 1 resulted in reduced sulfate concentrations of the soil solution even in deeper soil layers with concentrations approaching input levels. The pH of the solution was slightly elevated and the alkalinity of the solution increased. Organic S compounds in the soil seemed to have no influence on sulfate release in either soils.     

我国几种地带性土壤无机胶体的表面电荷特性

研究了用返滴定法测定我国七个不同地带土壤无机胶体的表面电荷特性。结果表明 :( 1 )返滴定法测定的供试土壤无机胶体可变电荷量随纬度升高而逐渐上升。这主要与供试土壤粘土矿物、铁铝氧化物组成及含量有关。一般来说 ,土壤层状硅酸盐粘土含量越多 ,永久负电荷量和其可变负电荷量越大 ;铁铝氧化物含量越高 ,可变负电荷量越低。( 2 )七个地带性土壤无机胶体的Qv pH曲线上在pH5左右有一峰值 ,这可能与供试土壤胶体中铁铝氧化物的ZPC及供试土样层状硅酸盐的边面Si OH基吸附H 及Al OH基离解H 有关     

Nitrogen mineralization in sub‐tropical paddy soils in relation to soil mineralogy,management, pH,carbon, nitrogen and iron contents

The nitrogen (N) requirement for paddy rice cultivated in Bangladesh amounts to approximately 80 kg N ha^?1. Lack of knowledge on N mineralization from soil organic matter leads farmers to meet this N requirement exclusively by costly mineral fertilizers, which have typically an efficiency of less than 40%. We assessed to what extent routinely analysed soil properties (N and carbon (C), texture, pH, extractable iron (Fe), aluminium (Al) and manganese (Mn), soil mineralogy and length of the annual inundation period) are able to predict net aerobic and anaerobic N mineralization in paddy soils. Both soil N and C correlated positively with the aerobic but not with the anaerobic N mineralization rate. Instead, relative anaerobic N mineralization showed a significant negative correlation with soil N content. We observed no significant influence of clay mineralogy on soil N mineralization. Aerobic but not anaerobic N mineralization increased with length of the annual inundation period while the proportion of the soil N that was mineralized during 120 days decreased. The large clay content of fields that are inundated for 9–10 months annually explains the co‐occurrence of large soil N contents and relatively small N mineralization rates in these fields. However, variation in texture did not explain variation in N mineralization of soils with inundation periods of 3–8 months. Instead, the anaerobic N mineralization correlated positively with Na pyrophosphate‐extractable Fe and negatively with pH (both at P < 0.01). Thus, pH and Fe content, rather than soil N content, clay mineralogy or texture, explained the substantial variation in anaerobic N mineralization of paddy soils in Bangladesh inundated for 3–8 months. It is not known if these relationships between net evolution of ammonium in soil and pH and Fe content are causal or indirect. Elucidation of these mechanisms would greatly further our comprehension of the biochemistry of the young ‘floodplain soils' with relatively low content of pedogenic oxides throughout southeast Asia.     

Implications of iron solubilization on soil phosphorus release in seasonally flooded forests of the lower Orinoco River, Venezuela

The microbial reduction of Fe oxides is thought to contribute with the release of P in sedimentary environments. However, secondary reactions of the bioproduced Fe(II) with P in solution, can lead to a decrease in the soluble P concentration. In this study, we examined how the reduction of Fe(III) affects the soluble P concentration, when the soils of a seasonally flooded forest gradient are subjected to anaerobic conditions. Soil samples were collected during the dry season from two zones subjected to different flooding intensity: MAX and MIN zones that were inundated 8 and 2 months per year, respectively. When anaerobic conditions were applied to soils from both zones, a clear stimulatory effect on the Fe(III) reduction was observed. However, bioproduced Fe(II) underwent secondary chemical reactions, masking the extend of Fe(III) reduction of these soils. Iron was reduced mainly during the first 15 days of the anaerobic incubation and it was stimulated by a pulse of labile carbon. Iron dissolution did not lead to an increase of the soluble P content. However, in both zones P was high and positively correlated with Fe(II), implying that soil P mobilization was linked to Fe dissolution. In the MIN zone, soluble P concentration decreased, probably as a consequence of the secondary reactions of solubilized P with other non-redox sensitive soils elements. Fe solubilization also had an effect on the activity of acid phosphatase and consequently in the solubilization of P from the organic pool. In conclusion, the P cycle in these soils is strongly coupled to C and Fe cycles.