Effect of organic matter on the distribution, extractability and uptake of cadmium in soils

Distribution and plant uptake of soil Cd as influenced by organic matter and soil type were investigated in a greenhouse experiment. Three soils (a sand, sandy loam and clay loam) were used. The rates of organic matter in its moist state added were 0,20,40, 80, 160 and 320 g kg^-1 of the air-dried soil on mass basis. Ryegrass (Lolium multörum L.) was used as a test crop. Soil Cd was analysed by a sequential extraction technique and by extraction with 1 M NH₄NO₃ and 0.005 M DTPA. The exchangeable fraction of Cd as determined by 1 M MgCl₂ in the sequential extraction procedure increased, whereas the Fe-Mn oxidebound fraction decreased, with increasing levels of organic matter addition in all three soils. The dry matter yields of ryegrass were not affected by the addition of organic matter, but the Cd concentrations in both cuts of ryegrass decreased with increasing amounts of organic matter added. The plant Cd was highly but negatively correlated to soil CEC. At any level of organic matter addition, the decrease in Cd concentration of ryegrass was in the order: sand > sandy loam > clay loam.     

The influence of pH,soil type and time on adsorbtion and uptake by plants of Cd added to the soil

Adsorption of Cd by two soils and its uptake by perennial ryegrass (Lolium perenne) and winter rape (Brassica napus) as a function of pH (pH 4 to 7) and the amount of Cd added to the soil (0 to 5 mg kg^?1 soil) were studied in a 2-yr pot experiment. In the soils, the more soluble fractions of Cd increased as the pH was lowered. Increasing the pH from 5 to 7 by adding CaO invariably reduced the Cd-content of ryegrass plants, but this decrease was less consistent where the pH had only been increased to 6. In some cases, acidifying the soil with S to reach a pH of 4 also led to a decrease in plant Cd-content. The Cd-content of rapeseed plants was markedly higher at pH 4 than at pH 5. Plant damage at low pH was observed in this crop. Water-leachable and CaCl₂-extractable soil Cd levels as well as plant uptake were higher in the sand soil than in the clay soil, whereas 1M NH₄AcO (buffered at pH 4.8 and 7) extracted roughly equal amounts from both soils. Adding more Cd to the soil did not change the relation between Cd levels in soil and those in plants; instead the amounts of Cd in both increased in direct proportion to the amounts added. Fixation of added Cd apparently did not occur continuously at any pH or Cd-level during the 2-yr period, but seasonal variations in solubility and uptake were observed.     

Effects of nitrogen-containing fertilizers on solubility and plant uptake of cadmium

In a pot experiment, three N-fertilizers, differing in NO^? ₃ and NH⁺ ₄; content, were compared in terms of their effects on the extractability of soil Cd in 1M ammonium acetate at pH 7 and on the uptake of native and added Cd by winter rape (Brassica napus L. var. oleifera Metzger). In another similar experiment the Cd-availability in soils receiving NPK-fertilizer applied in a concentrated, granulated form was compared to that in soils receiving a uniform application of dissolved NPK-fertilizer. Both loamy sand and clay soils were used. With the N-fertilizers levels of extractable soil Cd and plant Cd-contents were lowest in the nitrate of lime treatment, highest in the ammonium sulphate treatment and intermediate in the nitro-chalk treatment. The addition of 1 mg kg^?1 DW Cd to the soil increased Cd-levels but did not change the response pattern. There was a strong association between the effects of the various fertilizers on Cd availability and their effects on soil pH: the stronger the acidifying effect the higher the Cd availability. When applied at a low dose, granular NPK was more effective than dissolved NPK at enhancing plant uptake of Cd from both soils, but this was not the case when applied at a higher dose to the sand. The two forms of NPK differed little in their effect on extractable soil Cd. Plant uptake of Cd was greater from soils receiving granular NPK with a high Cd-content than from those receiving granular NPK low in Cd. In both experiments extractable Cd was taken up to a greater extent from the sand than from the clay. An increase in fertilizer dose generally resulted in an increase in levels of soil-Cd and in Cd concentrations in the plants.     

A field study on factors influencing Cd levels in soils and in grain of oats and winter wheat

The influence of selected factors on Cd levels in soils and in grain of oats and winter wheat was investigated. Soil and grain were sampled at sites randomly distributed over Sweden. Organic soils generally had higher Cd contents and lower pH levels than mineral soils, and plants growing in organic soils tended to have higher Cd contents than plants growing in mineral soils. In mineral soils the amount of soil Cd extractable in 2M HNO₃ was positively correlated with the pH and the contents of organic matter and clay. The studied variable best correlated with the Cd content of oat grain grown on mineral soils was the pH (negative correlation). Soil contents of organic matter, clay, HNO₃-extractable Cd and Zn were also found to be significantly related to the Cd content in a stepwise regression analysis. In winter wheat grain, Cd content was best correlated with the HNO₃-extractable Cd (positive correlation). Additional significant factors were pH, grain yield and contents of organic matter, clay and HNO₃-extractable Zn. In winter wheat the presence of Zn reduced Cd uptake, and vice versa; no such mutually antagonistic relationship was apparent in oats. In oats, but not in wheat, it was possible to predict most of the differences in grain Cd content, caused by the factors described above, based on the variation in CaCl₂-extractable soil Cd. Analysis of samples from field trials indicated that there were differences in Cd content between varieties of both crops. Variation in factors described generally explained most of the differences in soil- and grain Cd levels between regions.     

Native soil organic matter as a decisive factor to determine the arbuscular mycorrhizal fungal community structure in contaminated soils

The present study of arbuscular mycorrhizal (AM) fungi is focused on the identification of AM ecotypes associated with different plants species (Poa annua, Medicago polymorpha, and Malva sylvestris) growing in three contaminated soils with different organic matter, phosphorus, and trace element (TE; Cu, Cd, Mn, and Zn) contents. Soils were amended with biosolid and alperujo compost. Shifts in AM fungal community structure, diversity, richness, root colonization, and plant TE uptake were evaluated. Soil properties and plant species had a significant effect on AM fungal community composition as well as on root colonization. However, AM fungal diversity and richness were only affected by soil properties and especially by soil organic matter that was a major driver of AM fungal community. As soil quality increased, Glomeraceae decreased in favor of Claroideoglomeraceae in the community, AM fungal diversity and richness increased, and root colonization decreased. No effect due to amendment (exogenous organic matter) addition was found either in AM fungal parameters measured or TE plant uptake. Our results revealed that the role of TE contamination was secondary for the fungal community behavior, being the native organic matter content the most significant factor.     

Retention of copper, cadmium and zinc in soil and its textural fractions influenced by long-term field management

The present study investigated the impact of long‐term soil management on the metal retention capacity of soil. We examined the sorption behaviour of Cu, Cd and Zn in soils and in the various particle‐size fractions of these soils, which had been amended with farmyard manure, mineral fertilizers or were fallow for 38 years in a long‐term field experiment. The soils investigated contained different amounts and origins of organic matter and differed in soil pH, but the mineral phase showed less response to the different soil managements. Batch adsorption and desorption experiments as well as a sequential fractionation schema, which defines seven geochemical fractions, were used to investigate the retention properties of soil. Sequential extraction was conducted with original as well as with metal‐spiked soils. Results showed that amounts of Cu, Cd and Zn retained differed by a factor of more than 3 among the treatments in the long‐term field experiment, when a massive concentration of metal was added to soil. An increased sorption on smaller particle size fractions occurred (clay ≫ silt > fine sand ≥ coarse sand) due to the larger surface area as well as the greater carbon content in the smaller fractions. Soil sorption behaviour in another long‐term field experiment was estimated based on the present particle‐sorption data. Differences in the sorption behaviour were related to differences in soil mineralogy and amounts of Fe‐ and Mn‐oxides. Fractionation of the original and the metal‐spiked soil underlined the contribution of organic matter to sorption capacity (sequence: Cu ≫ Cd > Zn). Different organic matter contents and a different soil pH considerably changed the amounts of metals in the defined geochemical fractions. Freshly added Cu, Cd and Zn ions were found mainly in more mobile fractions. In contrast, metals in non‐spiked soils appeared in less‐mobile fractions reflecting their long‐term sorption behaviour.     

Cadmium soil sorption at low concentrations: VIII. correlation with soil parameters

Cadmium distribution coefficients, K _d were determined at low Cd concentrations (solute: 0.2 to 3.0 μg Cd dm^?3, soil: 0.044 to 1.1 mg Cd kg^?1) for 63 Danish agricultural soils. The K _d values ranged from 15 to 2450 L kg^?1. About 40% of the soils had K _d values below 200 L kg^?1. The observed K _d values correlated very well with soil pH (r ² = 0.72). Introducing soil organic matter content as a second parameter improved the correlation some (r ² = 0.79). No further improvements were obtained by introducing traditional soil parameters as clay, silt, fine sand, coarse sand and CEC or ‘reactive’ parameters as oxyhydroxides of Mn, Fe and Al. The identified regression equation for predicting K _d values indicates that K _d approximately doubles for each 0.5 unit increase in pH or 2% increase (weight basis) in organic matter content.     

In-Situ Immobilization of Cadmium and Lead by Different Amendments in Two Contaminated Soils

Different soil amendments, including 1% zeolite, 1% bentonite,5% Penghu soil (PHS), 5% Penghu soil + 1% manganese oxide (PHS + MO), 1% MO, and 1.5% silicate slag (SS), were used to immobilize Cd and Pb in two contaminated soils evaluated by single and sequential extractions and by uptake of Chinese cabbage (Brassica Chinensis L.). Results indicated that the PHS and MO significantly (p < 0.05) reduced the 0.1 M HCl extractable Cd and Pb in the two contaminated soils. Allamendment treatments did not change the organic and residual amounts of Cd and Pb in soils A and B, but the PHS andPHS + MOtreatments significantly reduced the exchangeable amounts of Cdand Pb in the two soils as measured by sequential extraction. Thecombination of PHS and MO amendments was associated with ahigh pH value and negative soil surface charge showed the best immobilizing efficiency of Cd and Pb in this study. All soil amendments investigated did not increase the dry matter weight of the plant, and most of them decreased the uptake of Cd and Pb, especially for the PHS and MO. The PHS and MO treatments reduced the extractability of Cd and Pb in two soils and theiruptake by the plant, but only the Pb content in Chinese cabbagefrom the amended soils was less than the background levels of heavy metals in leaf vegetables of Taiwan.     

Straw incorporation and soil organic matter in macro-aggregates and particle size separates

Two field experiments in which straw has been removed or incorporated for 17 yr (loamy sand) and 10 yr (sandy clay loam) were sampled to examine the effect of straw on the C and N contents in whole soil samples, macro-aggregate fractions and primary particle-size separates. The particle size composition of the aggregate fractions was determined. Aggregates were isolated by dry sieving. Straw incorporation increased the number of 1–20 mm aggregates in the loamy sand but no effect was noted in the sandy clay loam. Straw had no effect on the particle size composition of the various aggregate fractions. After correction for loose sand that accumulated in the aggregate fractions during dry sieving, macro-aggregates appeared to be enriched in clay and silt compared with whole soil samples. Because of the possible detachment of sand particles from the exterior surface of aggregates during sieving operations, it was inferred that the particle size composition of macro-aggregates is similar to that of the bulk soil. The organic matter contents of the aggregate fractions were closely correlated with their clay + silt contents. Differences in the organic matter content of clay isolated from whole soil samples and aggregate fractions were generally small. This was also true for the silt-size separates. In both soils, straw incorporation increased the organic matter content of nearly all clay and silt separates; for silt this was generally twice that observed for clay. The amounts of soil C, derived from straw, left in the loamy sand and sandy clay loam at the time of sampling were 4.4 and 4.5 t ha^?1, corresponding to 12 and 21% of the straw C added. The C/N ratios of the straw-derived soil organic matter were 11 and 12 for the loamy sand and sandy clay loam, respectively.     

水稻子实对不同形态重金属的累积差异及其影响因素分析

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

Solubility control of Cu, Zn, Cd and Pb in contaminated soils

We developed a semiempirical equation from metal complextion theory which relates the metal activity of soil solutions to the soil's pH, organic matter content (OM) and total metal content (MT). The equation has the general form: where pM is the negative logarithm (to base 10) of the metal activity, and a, b and c are constants. The equation successfully predicted free Cu²⁺ activity in soils with a wide range of properties, including soils previously treated with sewage sludge. The significant correlation of pCu to these measured soil properties in long-contaminated soils suggests that copper activity is controlled by adsorption on organic matter under steady state conditions. An attempt was made from separate published data to correlate total soluble Cu, Zn, Cd and Pb in soils to soil pH, organic matter content and total metal content. For Cu, the total Cu content of the soil was most highly correlated with total soluble Cu. Similarly, total soluble Zn and Cd were correlated with total metal content, but were more strongly related to soil pH than was soluble Cu. Smaller metal solubility in response to higher soil pH was most marked for Zn and Cd, metals that tend not to complex strongly with soluble organics. The organic matter content was often, but not always, a statistically significant variable in predicting metal solubility from soil properties. The solubility of Pb was less satisfactorily predicted from measured soil properties than solubility of the other metals. It seems that for Cu at least, solid organic matter limits free metal activity, whilst dissolved organic matter promotes metal solubility, in soils well-aged with respect to the metal pollutant. Although total metal content alone is not generally a good predictor of metal solubility or activity, it assumes great importance when comparing metal solubility in soils having similar pH and organic matter content.     

Solubility,sorption and desorption of native and added cadmium in relation to properties of soils in New Zealand

Solution cadmium (Cd) concentrations and sorption and desorption of native and added Cd were studied in a range of New Zealand soils. The concentration of Cd in solution and the concentrations and patterns of native soil Cd desorbed and added Cd sorbed and desorbed varied greatly between the 29 soils studied. Correlation analysis revealed that pH was the most dominant soil variable affecting solution Cd concentration and sorption and desorption of native and added Cd in these soils. However, organic matter, cation exchange capacity (CEC) and total soil Cd were also found to be important. Multiple regression analysis showed that the log concentration of Cd in solution was strongly related to soil pH, organic matter and total Cd, which in combination explained 76% of the variation between soils. When data from the present study were combined into a single multiple regression with soil data from a previously published study, the equation generated could explain 81% of the variation in log Cd solution concentration. This reinforces the importance of pH, organic matter and total Cd in controlling solution Cd concentrations. Simple linear regression analysis could at best explain 53% of the total variation in Cd sorption or desorption for the soils studied. Multiple regression analysis showed that native Cd desorption was related to pH, organic matter and total Cd, which in combination explained 85% of the variation between soils. For sorption of Cd (from 2 μg Cd g^–1 soil added), pH and organic matter in combination explained 75% of the variation between soils. However, for added Cd desorption (%), pH and CEC explained 77%. It is clear that the combined effects of a range of soil properties control the concentration of Cd in solution, and of sorption and desorption of Cd in soils. The fraction of potentially desorbable added Cd in soils could also be predicted from a soil’s K_d value. This could have value for assessing both the mobility of Cd in soil and its likely availability to plants.     

土壤主要理化性质对湘粤污染农田镉稳定效果的影响

利用盆栽试验研究了稳定剂(石灰、海泡石联合施用)对湖南、广东两省区不同性质土壤上生长的小青菜(Brassica chinensis L.)生物量、重金属吸收以及土壤pH和重金属提取态含量的影响,探讨了影响镉(Cd)稳定修复效果的土壤性质参数。结果表明:施加稳定剂对增加酸性土壤上小青菜生物量效果显著,土壤pH、有机质(OM)、全量Cd和黏粒是影响小青菜生物量变化的主要因素;土壤pH、阳离子交换量(CEC)、OM、黏粒是影响小青菜Cd含量变化的主要因素;土壤pH、CEC、全量Cd和黏粒是影响土壤提取态Cd含量变化的主要因素。     

Nitrogen in particle size fractions of soils incubated for five years with 15N-ammonium and 14C-hemicellulose

Four soils with a range of clay and silt contents were incubated for 5 a with ¹⁵N-labelled (NH₄)SO₄ and ¹⁴C-labelled hemicellulose and then fractionated according to particle size by ultrasonic dispersion and sedimentation. The distribution of labelled and native N between clay, silt and sand fractions was determined and elated to previous results on the C distributions. Between 29% and 48% of the added N was found in organic form. The ¹⁵N atom percentage excess decreased in the order: clay > whole soil > silt > sand. For both clay and silt, the enrichment factor for labelled and native N decreased with increasing fraction weight. Clay enrichment was higher for labelled than for native N, the converse being true for silt. The distribution of whole soil labelled organic N was: clay 77–91%, silt 4–11%, and sand <0.5%. Corresponding values for native N were 69–74%, 16–22%, and 1–2%, respectively. All soils had higher proportions of labelled than of native N in the clay, the converse was true for the silt. The C/N ratio of the native silt organic matter was higher and that of clay organic matter lower than whole soil C/N ratios. Differences between the C/N ratio distributions of native and labelled organic matter were small. The relative distribution of labelled N and C was very similar confirming that the turnover of C and N in soil organic matter is closely interrelated.     

Support Vector Machine and Nonlinear Regression Methods for Estimating Saturated Hydraulic Conductivity

Pedotransfer functions (PTFs) are widely used for hydrological calculations based on the known basic properties of soils and sediments. The choice of predictors and the mathematical calculus are of particular importance for the accuracy of calculations. The aim of this study is to compare PTFs with the use of the nonlinear regression (NLR) and support vector machine (SVM) methods, as well as to choose predictor properties for estimating saturated hydraulic conductivity (K_s). K_s was determined in direct laboratory experiments on monoliths of agrosoddy-podzolic soil (Umbric Albeluvisol Abruptic, WRB, 2006) and calculated using PTFs based on the NLR and SVM methods. Six classes of predictor properties were tested for the calculated prognosis: K_s-1 (predictors: the sand, silt, and clay contents); K_s-2 (sand, silt, clay, and soil density); K_s-3 (sand, silt, clay, soil organic matter); K_s-4 (sand, silt, clay, soil density, organic matter); K_s-5 (clay, soil density, organic matter); and K_s-6 (sand, clay, soil density, organic matter). The efficiency of PTFs was determined by comparison with experimental values using the root mean square error (RMSE) and determination coefficient (R²). The results showed that the RMSE for SVM is smaller than the RMSE for NLR in predicting K_s for all classes of PTFs. The SVM method has advantages over the NLR method in terms of simplicity and range of application for predicting K_s using PTFs.     

基于小白菜Cd吸收推算土壤Cd安全阈值

Cadmium(Cd), a common toxic heavy metal in soil, has relatively high bioavailability, which seriously threatens agricultural products. In this study, 8 different soils with contrasting soil properties were collected from different regions in China to investigate the Cd transfer coefficient from soil to Chinese cabbage(Brassica chinensis L.) and the threshold levels of Cd in soils for production of Chinese cabbage according to the food safety standard for Cd. Exogenous Cd(0–4 mg kg~(-1)) was added to the soils and equilibrated for 2 weeks before Chinese cabbage was grown under greenhouse conditions. The influence of soil properties on the relationship between soil and cabbage Cd concentrations was investigated. The results showed that Cd concentration in the edible part of Chinese cabbage increased linearly with soil Cd concentration in 5 soils, but showed a curvilinear pattern with a plateau at the highest dose of exogenous Cd in the other 3 soils. The Cd transfer coefficient from soil to plant varied significantly among the different soils and decreased with increasing soil p H from 4.7 to 7.5. However, further increase in soil pH to 8.0 resulted in a significant decrease in the Cd transfer coefficient. According to the measured Cd transfer coefficient and by reference to the National Food Safety Standards of China, the safety threshold of Cd concentration in soil was predicted to be between 0.12 and 1.7 mg kg~(-1) for the tested soils. The predicted threshold values were higher than the current soil quality standard for Cd in 5 soils, but lower than the standard in the other 3 soils. Regression analysis showed a significant positive relationship between the predicted soil Cd safety threshold value and soil p H in combination with soil organic matter or clay content.     

Phospholipid fatty acid profiles and xylanase activity in particle size fractions of forest soil and casts of Lumbricus terrestris L. (Oligochaeta,Lumbricidae)

Earthworms are key agents in organic matter decomposition, as they remove surface plant litter material and mix it with mineral soil. Plant litter material is comminuted in the gizzard of anecic earthworms and this is enhanced if sand particles are available. We hypothesize that this comminution of soil and litter will result in changes in the distribution of soil organic matter and soil microorganisms in the different particle-size fractions. We investigated soil organic matter content, xylanase- and microbial activity and community structure in bulk soil and particle size fractions of Lumbricus terrestris L. casts and in soil with and without the addition of beech litter.Earthworm gut passage did not affect the particle-size distribution but the content of soil organic matter was decreased in the fine sand fraction in treatments without litter (−6.80%) and increased in treatments with litter (+33.23%). The soil organic matter content of the clay fraction tended to be higher in earthworm casts. Xylanase activity was at a maximum in the fine sand fraction, lower in the coarse sand fraction and at a similar minimum in the silt- and clay-sized fraction. In the coarse sand fraction of the cast and litter treatments xylanase activity was increased by 39.1% and 124.8%, respectively. In the silt-sized fraction of casts the addition of litter increased xylanase activity (+58.6%) whereas, in casts without litter it was decreased (−36.25%). In the particle size fractions of casts, the content of bacterial PLFAs was decreased in the fine sand fraction and tended to be decreased in the clay fraction compared to the respective fractions in soil. In the silt fraction the fungal-to-bacterial PLFA ratio was higher in casts than in soil.We conclude that earthworms stabilize soil organic matter in cast aggregates predominantly by increasing the soil organic matter content in the clay fraction where it becomes protected against microbial attack. Organic matter in the coarse and fine sand fractions is decomposed primarily by fungi; xylanase is very active in these sand fractions and incorporation of litter into these fractions by the earthworms increased fungal biomass. Comminution of litter during passage through the earthworm gut increased the biomass and activity of fungi also in the silt fraction. The use of PLFA profiles in combination with other quantitative microbial methods improves the understanding of stabilizing and mobilizing processes in earthworm casts.     

Effects of addition of maize litter and earthworms on C mineralization and aggregate formation in single and mixed soils differing in soil organic carbon and clay content

C mineralization and aggregate stability directly depend upon organic matter and clay content, and both processes are influenced by the activity of microorganisms and soil fauna. However, quantitative data are scarce. To achieve a gradient in C and clay content, a topsoil was mixed with a subsoil. Single soils and the soil mixture were amended with 1.0 mg maize litter C g soil⁻¹ with and without endogeic earthworms (Aporrectodea caliginosa). The differently treated soils were incubated for 49 days at 15 °C and 40% water holding capacity. Cumulative C mineralization, microbial biomass, ergosterol content and aggregate fractions were investigated and litter derived C in bulk soil and aggregates were determined using isotope analyses. Results from the soil mixture were compared with the calculated mean values of the two single soils. Mixing of soil horizons differing in carbon and clay content stimulated C mineralization of added maize residues as well as of soil organic matter. Mixing also increased contents of macro-aggregate C and decreased contents of micro-aggregate C. Although A. caliginosa had a stimulating effect on C mineralization in all soils, decomposition of added litter by A. caliginosa was higher in the subsoil, whereas A. caliginosa decreased litter decomposition in the soil mixture and the topsoil. Litter derived C in macro-aggregates was higher with A. caliginosa than with litter only. In the C poor subsoil amended with litter, A. caliginosa stimulated the microbial community as indicated by the increase in microbial biomass. Furthermore, the decrease of ergosterol in the earthworm treated soils showed the influence of A. caliginosa on the microbial community, by reducing saprotrophic fungi. Overall, our data suggest both a decrease of saprotrophic fungi by selective grazing, burrowing and casting activity as well as a stimulation of the microbial community by A. caliginosa.     

Potassium release from sand,silt and clay fractions in calcareous soils of southern Iran

The release of non-exchangeable potassium from 24 calcareous soils of divergent mineralogy, from southern Iran, was examined. Sand, silt and clay particles were fractionated after dispersion with an ultrasonic probe. Samples were extracted with 0.01 M CaCl₂ for 30 successive 2-h periods. The clay fraction released the largest amount of K in each soil. Cumulative K released ranged from 175 to 723, 35 to 128, and 71 to 146 mg kg^?1 contributing 20–90, 4–39 and 2–54% for clay, silt and sand fractions, respectively. The lower proportion of K released from sand and silt fractions can be explained by the presence of a high content of CaCO₃ and quartz in these fractions. The release kinetics for the non-exchangeable K data showed that parabolic diffusion and power function were the best fitting kinetic models. This indicated that slow diffusion of K from the mica interlayer positions is the main rate-controlling process. Cumulative K released and constant b values of parabolic diffusion model correlated significantly with the mica content of the clay fraction.     

3种有机酸对伴矿景天修复效率及土壤微生物数量的影响

伴矿景天(Sedumplumbizincicola)是一种Cd和Zn的超积累植物,常用于Cd污染土壤的植物修复。有机酸能够提高土壤重金属的有效性,促进植物对重金属的积累,对重金属污染土壤的植物修复效率具有强化作用,并对土壤微生物数量有重要影响。以河潮土和红黄泥为供试土壤,探讨了乙二胺四乙酸(EDTA)、柠檬酸、草酸对伴矿景天修复效率和土壤微生物数量的影响。结果表明,有机酸能显著提高土壤有效态Cd含量,柠檬酸处理的效果最好,河潮土和红黄泥中有效态Cd含量较单种伴矿景天分别增加72.73%,12.99%(P<0.05);伴矿景天地上部Cd含量在河潮土和红黄泥中以EDTA处理最高,在河潮土和红黄泥中分别比单种伴矿景天增加99.24%和33.32%;与单种伴矿景天相比,添加有机酸处理河潮土和红黄泥中伴矿景天修复效率显著提高。添加有机酸比单种伴矿景天显著增加土壤中微生物数量,其中柠檬酸处理河潮土中细菌和真菌数量分别增加34.38%和68.42%(P<0.05),草酸处理红黄泥中放线菌数量增加150.00%。研究结果可为重金属污染土壤的植物强化修复提供理论支撑。