Catalysis of Dissolved and Adsorbed Iron in Soil Suspension for Chromium（Ⅵ） Reduction by Sulfide

The kinetics of Cr（Ⅵ） reduction by sulfide in soil suspensions with various pHs, soil compositions, and Fe（Ⅱ） concentrations was examined using batch anaeroblc experimental systems at constant temperature. The results showed that the reaction rate of Cr（Ⅵ） reduction was in the order of red soil 〈 yellow-brown soil 〈 chernozem and was proportional to the concentration of HCl-extractable iron in the soils. Dissolved and adsorbed iron in soil suspensions played an important role in accelerating Cr（Ⅵ） reduction. The reaction involved in the Cr（Ⅵ） reduction by Fe（Ⅱ） to produce Fe（ⅡI）, which was reduced to Fe（Ⅱ） again by sulfide, could represent the catalytic pathway until about 70% of the initially present Cr（Ⅵ） was reduced. The catalysis occurred because the one-step reduction of Cr（Ⅵ） by sulfide was slower than the two-step process consisting of rapid Cr（Ⅵ） reduction by Fe（Ⅱ） followed by Fe（Ⅲ） reduction by sulfide. In essence, Fe（Ⅱ）/Fe（Ⅲ） species shuttle electrons from sulfide to Cr（Ⅵ）, facilitating the reaction. The effect of iron, however, could be completely blocked by adding a strong Fe（Ⅱ）-complexing ligand, 1,10-phenanthroline, to the soil suspensions. In all the experiments, initial sulfide concentration was much higher than initial Cr（Ⅵ） concentration. The plots of In e[Cr（Ⅵ）] versus reaction time were linear up to approximately 70% of Cr（Ⅵ） reduction, suggesting a first-order reaction kinetics with respect to Cr（Ⅵ）. Elemental sulfur, the product of sulfide oxidation, was found to accelerate Cr（Ⅵ） reduction at a later stage of the reaction, resulting in deviation from linearity for the In c[Cr（Ⅵ）] versus time plots.     

土壤悬浮液中溶解和吸附性铁离子对硫化物还原Cr(VI)的催化作用

The kinetics of Cr(Ⅵ) reduction by sulfide in soil suspensions with various pHs, soil compositions, and Fe(Ⅱ) concentrations was examined using batch anaerobic experimental systems at constant temperature. The results showed that the reaction rate of Cr(Ⅵ) reduction was in the order of red soil < yellow-brown soil < chernozem and was proportional to the concentration of HCl-extractable iron in the soils. Dissolved and adsorbed iron in soil suspensions played an important role in accelerating Cr(Ⅵ) reduction. The reaction involved in the Cr(Ⅵ) reduction by Fe(Ⅱ) to produce Fe(Ⅲ), which was reduced to Fe(Ⅱ) again by sulfide, could represent the catalytic pathway until about 70% of the initially present Cr(Ⅵ) was reduced. The catalysis occurred because the one-step reduction of Cr(Ⅵ) by sulfide was slower than the two-step process consisting of rapid Cr(Ⅵ) reduction by Fe(Ⅱ) followed by Fe(Ⅲ) reduction by sulfide. In essence, Fe(Ⅱ)/Fe(Ⅲ) species shuttle electrons from sulfide to Cr(Ⅵ), facilitating the reaction. The effect of iron, however, could be completely blocked by adding a strong Fe(Ⅱ)-complexing ligand, 1,10-phenanthroline, to the soil suspensions. In all the experiments, initial sulfide concentration was much higher than initial Cr(Ⅵ) concentration. The plots of lnc[Cr(Ⅵ)] versus reaction time were linear up to approximately 70% of Cr(Ⅵ) reduction, suggesting a first-order reaction kinetics with respect to Cr(Ⅵ). Elemental sulfur, the product of sulfide oxidation, was found to accelerate Cr(Ⅵ) reduction at a later stage of the reaction, resulting in deⅥation from linearity for the lnc[Cr(Ⅵ)] versus time plots.     

VA菌根对土壤酸度的耐性

The ammonification,nitrifcation and denitrification in maize rhizosphere of alluvial sols were compared with those in the bulk soil after exposure to different kinds of heavy metals.The addition of cadmium at low levels (2 mg kg^-1 and 5 mg kg^-1）could stimulate the ammonification and nitrification in the soils,while inhibition influences were found at high levels of Cd addition (10 mg kg^-1 and 20mg kg^-1），The relationship between microbial activity and cadmium concentration varied with the kind of microorganisms.The nitrifying bacteria were more sensitive to cadmium pollution than the ammonifying bacteria.When Cd( Ⅱ），Cu（Ⅱ）and Cr（Ⅵ）wer compared at the same addition concentration of 20 mg kg^-1 soil,Cd(Ⅱ） was the most effective inhibitor of ammonification and denitrification. among the three investigated heavy metals,and Cr（Ⅵ）had the most strong inhibitory influence on the nitrifying bacteria.The microbial activities in rhizosphere were higher than those in the bulk soil for most of the treatments.Cr(Ⅵ)proved to be the most effective in enhacing the microbial activities in rhizosphere,and this could be caused by the poitive reduction of Cr(Ⅵ）to Cr(Ⅲ）in rhizosphere,and the relatively sufficient existence of organic matter which intensified the adsorption of the metal,It seemed that the rhizosphere had some mitigation effect on heavy metal toxicity.     

Elemental Sulfur Oxidation by Thiobacillus spp. and Aerobic Heterotrophic Sulfur-Oxidizing Bacteria

The prediction of the oxidation rate of elemental sulfur (S 0 ) is a critical step in sulfur (S) fertilizer strategy to supply plant-available sulfur. An incubation experiment was conducted to determine the rate and amount of S 0 oxidation in relation to the contribution of Thiobacillus spp. and aerobic heterotrophic S-oxidizing bacteria. After 84 days, 16.3% and 22.4% of the total S 0 applied to the soil were oxidized at 20 and 30-C, respectively. The oxidation of S 0 proved to be a two-step process with a rapid oxidation during the first 28 days and a slow oxidation from then on. The highest oxidation rate of 12.8 μg S cm-2 d-1 was measured during the first two weeks at 30-C. At 20-C the highest oxidation rate of 10.2 μg S cm-2 d-1 was obtained from two to four weeks after start of the experiment. On an average the soil pH declined by 3.6 and 4.0 units after two weeks of experiment. At the same time the electric conductivity increased nine times. With the oxidation of S 0 the population of Thiobacillus spp. and aerobic heterotrophic S-oxidizing bacteria increased. The corresponding values for Thiobacillus spp. and aerobic heterotrophic S-oxidizing bacteria increased from 2.9 × 10 5 and 1.4 × 10 5 g-1 soil at the start of the experiment to 4 × 10 8 and 5.6 × 10 8 g-1 soil 14 days after S 0 application, respectively. No Thiobacillus spp. was present eight weeks after S 0 application. The results suggested that oxidation of residual S 0 completely relied on aerobic heterotrophic S-oxidizing bacteria.     

磷酸铁盐涂层: 控制黄铁矿氧化的新手段

A novel coating technique was develped for controlling pyrite oxidation .The technique involved leaching pyrite particles with a solution containing low concentrations of phosphate and hydrogen peroxide.During the leaching rpocess,the iron released from pyrite by hydrogen proxide was precipitated by phosphate as a ferric phosphate coating .This coating was shown to be able to effectively prevent pyirte from oxidation and it could be established at the expense of only surface portions of pyrite.The emergence of this technique could provide a unique potential route for abating acid mine draingage and reclaiming sulfide-containing degraded mining land.     

以Mg2+/Co2+为模板, 钴掺杂对层状锰氧化物向钙锰矿转化的影响

Cobalt(Co)exists in significant quantities in naturally occurring manganese(Mn)oxides and alters the growth of Mn oxide crystals.Four-layered Mn oxides,Na-buserite(Na-bus)and three Co-doped Na-buserite samples prepared from oxidation of Mn(OH)2 with 5%,10%,and 20% Co/(Mn+Co)molar ratios(5Co-Na-bus,10Co-Na-bus,and 20Co-Na-bus),were used to prepare todorokite,a common Mn oxide on the Earth’s surface,using Mg2+/Co2+ ions as a template.The results showed that todorokites could be obtained by reflux treatment of Mg2+-exchanged non-doped Na-buserite and three Co-doped Na-buserites at atmospheric pressure.However,the formation of todorokites was prohibited by reflux treatment of Co2+-exchanged Na-bus,5Co-Na-bus,and 10Co-Na-bus samples.Instead,todorokite was obtained by the reflux treatment of Co2+-exchanged 20Co-Na-bus samples under atmospheric pressure.X-ray photoelectron spectroscopy analysis showed that doped Co existed as Co3+in the MnO6 layers of doped Na-buserites.The amount of substituted Co3+ in the MnO6 layers may play a key role in the conversion of buserite to todorokite using Co2+ ions as a template.     

Chemical oxidation of arsenic in the environment and its application in remediation: A mini review

Arsenic(As) contamination in soil and water poses a serious threat to the ecosystem health and human beings, and is of widespread concern. The main As species found in soil and water are arsenite As(Ⅲ) and arsenate As(Ⅴ). Because As(Ⅲ) is more toxic and often more mobile than As(Ⅴ), many remediation strategies aim to oxidize As(Ⅲ) to As(Ⅴ). In the environment, the reduction of As(Ⅴ) under anaerobic conditions is mainly mediated by microorganisms, but the oxidation of As(Ⅲ) under aerobic conditio...     

无定形锰氧化物在氮素损失中的作用

Studies have been made,by ^15N-tracer technique on nitrogen loss resulting from adding amorphous manganese oxide to NH4^ -N medium under anaerobic conditions.The fact that the total nitrogen recovery was decreased and that ^15NO2,^15N2O,^15N^14NO,^15NO,^15N2 and ^15N^14N were emitted has proved that,like amorphous iron oxide,amorphous manganese oxide can also act as an electron acceptor in the oxidation of NH4^ -N under anaerobic conditions and give rise to nitrogen loss.This once again illustrates another mechanism by which the loss of ammonium nitrogen in paddy soils is brought about by amorphous iron and manganese oxides.The quantity of nitrogen loss by amorphous manganese oxide increased with an increase in the amount of amorphous manganese oxide added and lessened with time of its aging.The nitrogen loss resulting from amorphous manganese oxide was less than that from amorphous iron oxide.And the nitrogen loss resulting from amorphous manganese oxide was less than that from amorphous iron oxide.And the nitrogen loss by cooperation of amorphous manganese oxide and microorganisms (soil suspension) was larger than that by amorphous manganese oxide alone.In the system,nitrogen loss was associated with the specific surface ares and oxidation-reduction of amorphous manganese oxide.However,their quantitative relationship and the exact reaction processes of nitrogen loss induced by amorphous manganese oxide remain to be further studied.     

除草剂二氯喹啉酸对水稻田土壤酶活性短期影响

Birnessite occurs in a wide variety of natural environments, and plays an important role in soil chemistry. A modified Stāhli procedure was used to synthesize sodium birnessite in an alkali medium by O2 oxidation. The effects of preparative parameters on the synthesis of birnessite, such as pretreatment on solutions with N2, reaction temperature, O2 flow rate, fluxion velocity of the reaction suspension, and dehydration conditions were investigated. The fluxion velocity of the reactive suspension and O2 flow rate significantly influenced the synthesis of birnessite. Vigorous stirring raised the fluxion velocity of the reaction suspension and easily allowed synthesis of pure crystalline birnessite. However pretreatment of the reacting solutions with N2 and the reaction temperature had little effect on the synthesis. Diffusion of O2 was the controlling step during the course of oxidation. The optimum synthetic conditions for pure birnessite were: a NaOH to Mn molar ratio of 13.7, an O2 flow rate of 2 L rain^-1, and oxidation for 5 hours with vigorous stirring at normal temperatures. The chemical composition of the synthesized pure birnessite was Na0.25MnO2.07-0.66H2O.     

以地表温度-植被指数特征空间为基础基于MODIS数据进行土壤水分监测研究

Soil moisture has been considered as one of the main indicators that are widely used in the fields of hydrology, climate, ecology and others. The land surface temperature-vegetation index （LST-VI） space has comprehensive information of the sensor from the visible to thermal infrared band and can well reflect the regional soil moisture conditions. In this study, 9 pairs of moderate-resolution imaging spectroradiometer （MODIS） products （MOD09A1 and MODllA2）, covering 5 provinces in Southwest China, were chosen to construct the LST-VI space, and then the spatial distribution of soil moisture in 5 provinces of Southwest China was monitored by the temperature vegetation dryness index （TVDI）. Three LST-VI spaces were constructed by normalized difference vegetation index （NDVI）, enhanced vegetation index （EVI）, and modified soil-adjusted vegetation index （MSAVI）, respectively. The correlations between the soil moisture data from 98 sites and the 3 TVDIs calculated by LST-NDVI, LST-EVI and LST-MSAVI, respectively, were analyzed. The results showed that TVDI was a useful parameter for soil surface moisture conditions. The TVDI calculated from the LST-EVI space （TVDIE） revealed a better correlation with soil moisture than those calculated from the LST-NDVI and LST-MSAVI spaces. From the different stages of the TVDIE space, it is concluded that TVDIE can effectively show the temporal and spatial differences of soil moisture, and is an effective approach to monitor soil moisture condition.     

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.     

内源性土壤苏云金杆菌(BRC-ZYR2)对含氧铬还原的影响研究

Chromium（Cr） may cause losses in the yield of field plant, which is one of the favorite habitats of Bacillus thuringiensis（Bt）. The purposes of our study were to assess the Cr（VI）-resistance and Cr（VI）-reducing abilities of an indigenous soil isolate of Bt and to determine the factors governing Cr（VI） reduction. Towards this end a novel dichromate-reducing Bt BRC-ZYR2, characterized with insecticidal crystal proteins（ICPs）, was isolated from a uranium deposit. Minimum inhibitory concentrations（MICs） of Cr（VI） were determined by broth dilution method and the concentrations of Cr（VI） and total Cr in the supernatant were quantified colorimetrically using 1,5-diphenylcarbazide（DPC） reagent and a mixture of sulfuric-nitric acids, respectively. The isolate contained five ICP genes（cry1Ba, cry1 Bb, cry1Be/cry1 Bf, cry9 Ca and cry9Da） and exhibited a high level of Cr（VI） resistance with MICs of 150 mg L-1at pH 7.0 and 30？C, and 500 mg L-1under optimal conditions（pH 9.0 and 40？C）. The total Cr concentration was similar to initial concentration of Cr（VI） under the optimal condition, suggesting that the essential removal of the Cr（VI） was dependent on Bt reduction. Under optimal conditions, the initial Cr（VI） concentrations from 25 to 75 mg L-1significantly decreased in 24 h after incubation. Addition of Mn2＋, Co2＋, Mo2＋and Cu2＋activated Bt-mediated Cr（VI） reduction, while Zn2＋, Ni2＋and glucose were found to inhibit the reduction. Our results indicated that this isolate could be a promising biopesticide with the potential for both insect biocontrol and Cr bioremediation in the field.     

运用岩石热解产生的S3和S4信号进行土壤有机质特征研究

Variations in the abundance of soil organic matter（SOM） constituents with different stability have a major impact on important environmental processes, e.g., carbon dioxide（CO2） fluxes between the soil and the atmosphere. Recently, besides the bulk Rock-Eval（RE） data, the mathematical deconvolution of the signals derived from hydrocarbon-like compounds released by thermal cracking of SOM during RE pyrolysis has been increasingly used to estimate the relative contribution of the major SOM classes differing in origin and preservation. This study applied the mathematical deconvolution of the S3 and S4 signals of carbon monoxide（CO） and CO2, produced both by the pyrolysis of the oxygen-containing moieties and by the oxidation of the residual highly resistant organic matter, to characterize the stability of these components. Our results suggested that the stability of the oxygen-containing moieties was controlled by the precursor material and was strongly affected by the land use and the presence of humic substances in the surface horizon of some main soil types in Hungary. In consistence with the bulk RE data, results of the mathematical deconvolution also proved to be diagnostic markers for discriminating the aquatic or terrigenous plants as the main sources of SOM. The mathematical deconvolution of S4 signals derived from the highly resistant SOM fraction allowed us to quantify the contribution of constituents with different stability. Furthermore, the results of this study displayed that the stability of this highly abundant SOM fraction in the surface soil samples depended on source biomass and intensity of leaching.     

连续三年添加制革厂堆肥后土壤微生物生物量研究

Brazilian industry produces huge amounts of tannery sludge as residues, which is often disposed by landfilling or land application. However, consecutive amendment of such composted industrial wastes may cause shifts in soil microbial biomass （SMB） and enzyme activity. This study aimed to evaluate SMB and enzyme activity after 3-year consecutive composted tannery sludge （CTS） amendment in tropical sandy soils. Different amounts of CTS （0.0, 2.5, 5.0, 10.0, and 20.0 t ha-1） were applied to a sandy soil. The C and N contents of SMB, basal and substrate-induced respiration, respiratory quotient （qCO2）, and enzyme activities were determined in the soil samples collected after CTS amendment for 60 d at the third year. After 3 years, significant changes were found in soil microbial properties in response to different CTS amounts applied. The organic matter and Cr contents significantly increased with increasing CTS amounts. SMB and soil respiration peaked following amendment with 10.0 and 20.0 t ha-1 of CTS, respectively, while qCO2 was not significantly affected by CTS amendment. However, soil enzyme activity decreased significantly with increasing CTS amounts. Consecutive CTS amendment for 3 years showed inconsistent and contrasting effects on SMB and enzyme activities. The decrease in soil enzyme activities was proportional to a substantial increase in soil Cr concentration, with the latter exceeding the permitted concentrations by more than twofold. Thus, our results suggest that a maximum CTS quantity of 5.0 t ha-1 can be applied annually to tropical sandy soil, without causing potential risks to SMB and enzyme activity.     

万寿菊：铬植物修复的潜在价值

Environmental pollution with chromium(Cr) is harmful to humans, animals and plants, while in plants it causes diminished growth,anatomical alterations and death. In the present study, the potential value of marigold(Tagetes erecta) in the phytoremediation of Cr has been investigated. The randomized experimental design involved the exposure of plants to nutrient solutions containing 0.00,0.04, 0.08, 0.12, 0.16 or 0.24 mmol L~(-1)Cr(Ⅲ). Chromium toxicity was observed at Cr(Ⅲ) concentrations ≥ 0.12 mmol L~(-1) as demonstrated by diminished growth of the aerial parts and reduced density of the root system. Increasing Cr(Ⅲ) concentrations in the nutrient solution resulted in a higher bioaccumulation of total Cr in the tissues, although translocation from roots to aerial parts was not efficient(maximum value of 25% at 0.12 mmol L~(-1)Cr(Ⅲ)). The Cr bioaccumulation was up to 11-fold greater in roots than in the aerial parts. Tagetes erecta exhibited leaf plasticity when exposed to Cr, indicating the existence of a tolerance mechanism to Cr in this species. Chromium caused a reduction in xilem vases, resulting in a plastic effect in T. erecta leaves that increased the metal tolerance in culture solution. Tagetes spp. are potential Cr hyperaccumulators; at Cr(Ⅲ) concentrations up to 0.12 mmol L~(-1), the plants accumulated levels above that proposed for hyperaccumulators and still maintained a considerable growth and even flourished. However, this study was conducted in nutrient solution, and studies on species confirmation as Cr hyperaccumulator should be conducted in soils for further clarification.     

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.     

铜对土壤中功夫菊酯和氯氰菊酯迁移和扩散的影响

Repeated applications of bordeaux mixture （a blend of copper sulfate and calcium hydroxide） and pyrethroid insecticides （Pys） have led to elevated copper （Cu） and Pys concentrations in vineyard surface soils. To understand the potential influence of Cu on the fate of Pys in the soil environment, we selected two Pys, cypermethrin （CPM） and lambda-cyhalothrin （λ-CHT）, and two typical Chinese vineyard soils, Haplic Acrisol and Luvic Phaeozem, as experimental samples. The dissipation experiment was conducted at room temperature in the dark, and the transport of both Pys through the soils was investigated using soil thin-layer chromatography. The results showed that the transport of Pys in both soils increased as the Cu²⁺ concentration increased from 0 to 100 mg L^-1 , and Pys were more transportable in Haplic Acrisol （HA） than in Luvic Phaeozem （LP） under the same experimental conditions. For CPM, only 100 mg L^-1 of Cu²⁺ significantly （P<0.05） increased Pys transport through both soils relative to water. Lambda-CHT was significantly （P<0.05） transported through HA by all the Cu²⁺ concentrations compared to water, and all but the 1 mg L^-1 of Cu²⁺ significantly （P<0.05） increased the transport of λ-CHT through LP relative to water. However, the dissipation rates of CPM and λ-CHT decreased with the addition of Cu to soils. Our findings suggest that the risk of groundwater contamination by Pys increases in the soils with elevated Cu concentrations.     

Changes of Bacterial Community Structure in Copper Mine Tailings After Colonization of Reed （Phragmites communis）

Soil samples were collected from both bare and vegetated mine railings to study the changes in bacterial communities and soil chemical properties of copper mine tailings due to reed （Phragmites communis） colonization. The structures of bacterial communities were investigated using culture-independent 16S rRNA gene sequencing method. The bacterial diversity in the bare mine tailing was lower than that of the vegetated mine tailing. The former was dominated by sulfur metabolizing bacteria, whereas the latter was by nitrogen fixing bacteria. The bare mine tailing was acidic （pH = 3.78）, whereas the vegetated mine tailing was near neutral （pH ： 7.28）. The contents of organic matter, total nitrogen, and ammonium acetate-extractable potassium in vegetated mine tailings were significantly higher than those in the bare mine railings （P 〈 0.01）, whereas available phosphorus and electrical conductivity were significantly lower than those in the bare mine tailings （P 〈 0.01）. The results demonstrated that 16S rRNA gene sequencing could be successfully used to study the bacterial diversity in mine tailings. The colonization of the mine tailings by reed significantly changed the bacterial community and the chemical properties of tailings. The complex interactions between bacteria and plants deserve further investigation.     

气候和土壤质地对土壤有机碳影响的区域差异研究

The agricultural soil carbon pool plays an important role in mitigating greenhouse gas emission ana unaerstanamg the son orgamc carbon-climate-soil texture relationship is of great significance for estimating cropland soil carbon pool responses to climate change. Using data from 900 soil profiles, obtained from the Second National Soil Survey of China, we investigated the soil organic carbon （SOC） depth distribution in relation to climate and soil texture under various climate regimes of the cold northeast region （NER） and the warmer Huang-Huai-Hai region （HHHR） of China. The results demonstrated that the SOC content was higher in NER than in HHHR. For both regions, the SOC content at all soil depths had significant negative relationships with mean annual temperature （MAT）, but was related to mean annual precipitation （MAP） just at the surface 0-20 cm. The climate effect on SOC content was more pronounced in NER than in HHHR. Regional differences in the effect of soil texture on SOC content were not found. However, the dominant texture factors were different. The effect of sand content on SOC was more pronounced than that of clay content in NER. Conversely, the effect of clay on SOC was more pronounced than sand in HHHR. Climate and soil texture jointly explained the greatest SOC variability of 49.0% （0-20 cm） and 33.5% （20-30 cm） in NER and HHHR, respectively. Moreover, regional differences occurred in the importance of climate vs. soil texture in explaining SOC variability. In NER, the SOC content of the shallow layers （0-30 cm） was mainly determined by climate factor, specifically MAT, but the SOC content of the deeper soil layers （30-100 cm） was more affected by texture factor, specifically sand content. In HHHR, all the SOC variability in all soil layers was predominantly best explained by clay content. Therefore, when temperature was colder, the climate effect became stronger and this trend was restricted by soil depth. The regional differences and soil depth influence underscored the importance of explicitly considering them in modeling long-term soil responses to climate change and predicting potential soil carbon sequestration.     

影响土壤中铬, 锰的氧化还原反应的环境因素

Disposal of chromium(Cr) hexavalent form,Cr(VI),in soils as additions in organic fertilizers,liming materials or plant nutrient sources can be dangerous since Cr(VI) can be highly toxic to plants,animals,and humans.In order to explore soil conditions that lead to Cr(VI) generation,this study were performed using a Paleudult(Dystic Nitosol) from a region that has a high concentration of tannery operations in the Rio Grande do Sul State,southern Brazil.Three laboratory incubation experiments were carried out to examine the influences of soil moisture content and concentration of cobalt and organic matter additions on soil Cr(VI) formation and release and manganese(Mn) oxide reduction with a salt of chromium chloride(CrCl 3) and tannery sludge as inorganic and organic sources of Cr(III),respectively.The amount of Cr(III) oxidation depended on the concentration of easily reducible Mn oxides and the oxidation was more intense at the soil water contents in which Mn(III/IV) oxides were more stable.Soluble organic compounds in soil decreased Cr(VI) formation due to Cr(III) complexation.This mechanism also resulted in the decrease in the oxidation of Cr(III) due to the tannery sludge additions.Chromium(III) oxidation to Cr(VI) at the solid/solution interface involved the following mechanisms:the formation of a precursor complex on manganese(Mn) oxide surfaces,followed by electron transfer from Cr(III) to Mn(III or IV),the formation of a successor complex with Mn(II) and Cr(VI),and the breakdown of the successor complex and release of Mn(II) and Cr(VI) into the soil solution.