Specific surface area and surface charges of some argentinian soils

The specific surface areas of nine argentinian soils obtained by adsorption followed the order N₂ < ethylene glycol < H₂O, attributed to the presence of smectites (verified by XRD analysis) and some organic coating. The H₂O₂ treatment of these soils modified the former order of surface area determined by different absorbents. This order was modified because the mineral surface was evidenced and an increase of cation adsorption was produced by organic matter removal. This fact was supported by the decrease of PZC values of soils after H₂O₂ treatment. The specific surface area of initial soils obtained by water and ethylene glycol adsorptions showed a good correlation with carbon content and CEC of untreated soils and with the PZC of protonated soils.     

Adsorption of the insecticidal protein of Bacillus thuringiensis subsp. kurstaki by minerals: effects of inorganic salts

The persistence of the insecticidal protein of Bacillus thuringiensis (Bt) is enhanced by the reactivity with soil particles and may constitute a hazard to the soil ecosystem; however, studies on the fate of the Bt toxin in soil, especially in the presence of inorganic salts, are limited. The effects of different concentrations of KNO₃, KH₂PO₄ and NH₄H₂PO₄ on the adsorption of Bacillus thuringiensis toxin by kaolinite, montmorillonite, goethite and silicon dioxide were investigated. Results showed that small salt concentrations tended to enhance toxin adsorption, whereas large concentrations (> 10 mmol litre^?1) inhibited sorption. Similar results were observed regardless of the order in which toxin and inorganic salt were added. The degree to which individual salts affected adsorption decreased in the sequence for minerals, goethite > kaolinite ≥ montmorillonite > silicon dioxide, and for ions, H₂PO₄^? > NO₃^?, NH₄⁺ > K⁺. Our results indicate that inorganic salts can markedly influence the adsorption of Bt toxin by soil minerals. This investigation will help in evaluating the behaviour and fate of Bt toxins in the soil environment.     

Adsorption and Desorption of Cry1Ab Proteins on Differently Textured Paddy Soils

In recent years, selected cry genes from Bacillus thuringiensis (Bt) encoding the production of Cry proteins (Bt toxins) have been engineered into crop plants (Bt-crops). Through the cultivation of Bt crops and the application of Bt pesticides, Cry proteins could be introduced into arable soils. The interaction between the proteins and soils was analyzed in this study to investigate the affinity of Cry proteins in paddy soil ecosystems. Four Paddy soils were selected to represent different soil textures. Cry proteins were spiked in soils, and the amount of protein adsorbed was measured over 24 h. Desorption of Cry1Ab proteins from paddy soils was performed by washing with sterile Milli-Q water (H₂O_MQ), and subsequently extracted with an extraction buffer. The paddy soils had a strong affinity for Cry1Ab proteins. Most of the Cry1Ab proteins added (> 98%) were rapidly adsorbed on the paddy soils tested. More Cry1Ab proteins were adsorbed on non-sterile soils than on sterile soils. Less than 2% of the adsorbed Cry1Ab proteins were desorbed using H₂O_MQ, while a considerable proportion of the adsorbed proteins could be desorbed with the buffer, ranging from 20% to 40%. The amount of proteins desorbed increased with the increases in the initial amount of Cry1Ab proteins added to the paddy soils. The concentration of Cry1Ab proteins desorbed from the paddy soils was higher for sterile soils than non-sterile ones. Our results indicate that Bt toxins released via the cultivation of Bt crops, the application of Bt pesticides can be adsorbed on paddy soils, and soil texture could impose an impact on the adsorption capability.     

Short communication Estimation of the specific surface area of soil particles by adsorption of polyvinylalcohol in aqueous suspension

Adsorption of polyvinylalcohol (PVA) in aqueous suspension has been used to measure the specific surface area (SSA) of a silicon dioxide, a goethite, a clay, and a sample of a topsoil, the latter before and after treatment with hydrogen peroxide. Surface areas were calculated from each of the plateaus of the isotherms derived from the Langmuir equation fitted to the data, using a value of 0.04268 nm² for the molecular area of a monomer of PVA. We compared these SSA values with those measured by N₂ adsorption. The SSA values of the silicon dioxide, the goethite and the clay are in excellent agreement with the corresponding N₂‐BET areas. The removal of organic matter by H₂O₂ from the topsoil sample led to a marked increase in the SSA measured by the BET‐method. For this sample, the SSA measured by PVA was considerably larger than the one that was obtained by the BET‐method and showed only a little change after removal of SOM.     

Adsorption of the Bacillus thuringiensis toxin (Cry1Ab) on Na‐montmorillonite and on the clay fractions of different soils

The adsorption of the toxin from Bacillus thuringiensis (Bt‐toxin), which is synthesized in genetically modified maize, on sterilized Na‐montmorillonite and on H₂O₂‐treated and untreated clay fractions of three soils from different sites were studied. All adsorption isotherms can be described by a linear isotherm. Although all clay fractions from the different soils show nearly the same mineralogical composition, we found different affinities ranging from k = 47.7 to k = 366.7 of the adsorbates for the Bt‐toxin. The H₂O₂‐treated clay fractions show no correlation between the adsorption affinity and the amount of soil organic matter. On the other hand, there is a correlation between the content of organic carbon and the adsorption affinity of the untreated clay fractions. This can be explained by the fact that due to the coatings of soil organic matter on aggregates, the Bt‐toxin polymers are not able to adsorb within the clay aggregates.     

Ionic-strength and pH effects on the sorption of cadmium and the surface charge of soils

Two Oxisols (Mena and Malanda), a Xeralf and a Xerert from Australia and an Andept (Patua) and a Fragiaqualf (Tokomaru) from New Zealand were used to examine the effect of pH and ionic strength on the surface charge of soil and sorption of cadmium. Adsorption of Cd was measured using water, 0.01 mol dmp^?3 Ca(NO₃)₂, and various concentrations of NaNO₃ (0.01–1.5 mol dm^?3) as background solutions at a range of pH values (3–8). In all soils, the net surface charge decreased with an increase in pH. The pH at which the net surface charge was zero (point of net zero charge, PZC) differed between the soils. The PZC was higher for soils dominated by variable-charge components (Oxisols and Andept) than soils dominated by permanent charge (Xeralf, Xerert and Fragiaqualf). For all soils, the adsorption of Cd increased with an increase in pH and most of the variation in adsorption with pH was explained by the variation in negative surface charge. The effect of ionic strength on Cd adsorption varied between the soils and with the pH. In Oxisols, which are dominated by variable-charge components, there was a characteristic pH below which increasing ionic strength of NaNO₃ increased Cd adsorption and above which the reverse occurred. In all the soils in the normal pH range (i.e. pH>PZC), the adsorption of Cd always decreased with an increase in ionic strength irrespective of pH. If increasing ionic strength decreases cation adsorption, then the potential in the plane of adsorption is negative. Also, if increasing ionic strength increases adsorption below the PZC, then the potential in the plane of adsorption must be positive. These observations suggest that, depending upon the pH and PZC, Cd is adsorbed when potential in the plane of adsorption is either positive or negative providing evidence for both specific and non-specific adsorption of Cd. Adsorption of Cd was approximately doubled when Na rather than Ca was used as the index cation.     

Carbon content in soil particle size and consequence on cation exchange capacity of Alfisols

Abstract

Many of the cultivated soils of sub‐Saharan Africa typically have a surface horizon low in clay and with a low cation exchange capacity (CEC). In these soils, CEC is largely due to the soil organic matter (SOM). Measurements made on long‐term trials show that changes in CEC and SOM are positively correlated to one another, but not of same magnitude, suggesting that not all of the SOM plays an equal role as regards the soil CEC. To study the influence of the different SOM size fractions on the CEC, soils with or without application of manure or compost coming from trials in Chad and Côte d'Ivoire were separated without destruction of the SOM into five organo‐mineral fractions: “coarse sand”;, “fine sand”;, “coarse silt”;, “fine silt”;, and “clay”; made up of particles of sizes between 2,000 and 200, 200 and 50, 50 and 20, 20 and 2, and 2 and 0 μm, respectively. Fractionation was carried out by mechanical dispersion of the soil, wet sieving of the fractions larger than 20 μm, and decanting of the “clay”; and “fine silt”; fractions. The CEC of these fractions increases inversely with their size. The “clay”; fraction which contains half of the SOM contributes about 80% of the CEC of the soils. The CEC of the fractions is largely a function of their carbon (C) content, but the organic CEC per unit C of the “clay”; fraction appears to be four times greater than that of the other fractions (1,000 as against 270 cmol_c kg^‐1). Applications of manure or compost increase the CEC of the soils by increasing the soil C only when this C increase concerns the fine fractions of the SOM.     

胡敏酸对铵钾在粘土矿物上交互作用的影响

Interaction of ammonium (NH+4) and potassium (K+) is typical in field soils. However, the effects of organic matter on interaction of NH+4 and K+have not been thoroughly investigated. In this study, we examined the changes in major physicochemical properties of three clay minerals (kaolinite, illite, and montmorillonite) after humic acid (HA) coating and evaluated the influences of these changes on the interaction of NH+4 and K+on clay minerals using batch experiments. After HA coating, the cation exchange capacity (CEC) and specific surface area (SSA) of montmorillonite decreased significantly, while little decrease in CEC and SSA occurred in illite and only a slight increase in CEC was found in kaolinite. Humic acid coating significantly increased cation adsorption and preference for NH+4, and this effect was more obvious on clay minerals with a lower CEC. Results of Fourier transform infrared spectrometry analysis showed that HA coating promoted the formation of H-bonds between the adsorbed NH+4 and the organo-mineral complexes. HA coating increased cation fixation capacity on montmorillonite and kaolinite, but the opposite occurred on illite. In addition, HA coating increased the competitiveness of NH+4 on fixation sites. These results showed that HA coating affected both the nature of clay mineral surfaces and the reactions of NH+4 and K+with clay minerals, which might influence the availability of nutrient cations to plants in field soils amended with organic matter.     

苏南水稻土对铜离子专性吸附的初步研究

六十年代以来,土壤对多价离子特别是重金属离子的专性吸附,已日益成为土壤化学领域中一个受人注意的问题。土壤胶体中铁、锰、铝、硅等氧化物及其水合物和土壤有机质是专性吸附的主要载体^{[12,16,17,19,20,21]},它们对许多重金属元素的专性吸附所引起的富集过程,起着控制这些金属元素在土壤溶液和海水中浓度的作用^[9,15,18];一些微盒营养元素在土壤中的移动及其对植物的有效性,也深受土壤中氧化物胶体的专性吸附的影响^[11,14,21]。因此,铁、锰等水合氧化物对重金属离子的专性吸附的研究,不仅是土壤化学领域,而且也是土壤环境保护和地球化学等领域中的重要研究课题。     

Comparison of chemical fractionation methods for isolating stable soil organic carbon pools

Stable soil organic matter (SOM) is important for long‐term sequestration of soil organic carbon (SOC), but the usefulness of different fractionation methods to isolate stable SOM is open to question. We assessed the suitability of five chemical fractionation methods (stepwise hydrolysis, treatment with H₂O₂, Na₂S₂O₈, NaOCl, and demineralization of the NaOCl‐resistant fraction (NaOCl + HF)) to isolate stable SOM from soil samples of a loamy sand and a silty loam under different land use regimes (grassland, forest and arable crops). The apparent C turnover time and mean age of SOC before and after fractionation was determined by ¹³C and ¹⁴C analysis. Particulate organic matter was removed by density fractionation before soils were exposed to chemical fractionation. All chemical treatments induced large SOC losses of 62–95% of the mineral‐associated SOC fraction. The amounts of H₂O₂‐ and Na₂S₂O₈‐resistant SOC were independent from land use, while those of NaOCl‐ (NaOCl + HF)‐ and hydrolysis‐resistant SOC were not. All chemical treatments caused a preferential removal of young, maize‐derived SOC, with Na₂S₂O₈ and H₂O₂ being most efficient. The mean ¹⁴C age of SOC was 1000–10000 years greater after chemical fractionation than that of the initial, mineral‐associated SOC and mean ¹⁴C ages increased in the order: NaOCl < NaOCl + HF ≤ stepwise hydrolysis ≪ H₂O₂≈ Na₂S₂O₈. None of the methods appeared generally suitable for the determination of the inert organic matter pool of the Rothamsted Carbon Model. Nonetheless, our results indicate that all methods are able to isolate an older, more stable SOC fraction, but treatments with H₂O₂ and Na₂S₂O₈ were the most efficient ones in isolating stable SOM.     

Carbon storage in loess derived surface soils from Central Germany: Influence of mineral phase variables

The influence of the soil mineral phase on organic matter storage was studied in loess derived surface soils of Central Germany. The seven soils were developed to different genetic stages. The carbon content of the bulk soils ranged from 8.7 to 19.7 g kg^—1. Clay mineralogy was confirmed to be constant, with illite contents > 80 %. Both, specific surface area (SSA, BET‐N₂‐method) and cation exchange capacity (CEC) of bulk soils after carbon removal were better predictors of carbon content than clay content or dithionite‐extractable iron. SSA explained 55 % and CEC 54 % of the variation in carbon content. The carbon loadings of the soils were between 0.57 and 1.06 mg C m^—2, and therefore in the ”︁monolayer equivalent” (ME) level. The increase in SSA after carbon removal (ΔSSA) was significantly and positively related to carbon content (r² = 0.77). Together with CEC of carbon‐free samples, ΔSSA explained 90 % of the variation in carbon content. Clay (< 2 μm) and fine silt fractions (2—6.3 μm) contained 68—82 % of the bulk soil organic carbon. A significantly positive relationship between carbon content in the clay fraction and in the bulk soil was observed (r² = 0.95). The carbon pools of the clay and fine silt fractions were characterized by differences in C/N ratio, δ¹³C ratio, and enrichment factors for carbon and nitrogen. Organic matter in clay fractions seems to be more altered by microbes than organic matter in fine silt fractions. The results imply that organic matter accumulates in the fractions of smallest size and highest surface area, apparently intimately associated with the mineral phase. The amount of cations adhering to the mineral surface and the size of a certain and specific part of the surface area (ΔSSA) are the mineral phase properties which affect the content of the organic carbon in loess derived arable surface soils in Central Germany most. There is no monolayer of organic matter on the soil surfaces even if carbon loadings are in the ME level.     

Predicting carbon content in illitic clay fractions from surface area, cation exchange capacity and dithionite-extractable iron

We used the specific surface area (SSA), the cation exchange capacity (CEC) and the content of dithionite‐extractable iron (Fe_d) to predict the content of organic carbon in illitic clay fractions of topsoils from loess. We determined SSA (BET‐N₂ method) and CEC of clay fractions after removing organic C or reducing oxides or both. The CEC and the SSA of the carbon‐ and oxide‐free clay fraction explained 56% and 54% of the variation in C content, respectively. The Fe_d content of the clay fractions was strongly and negatively related to the C content, and with the SSA of the carbon‐free clay fraction it predicted C content almost completely (R² = 0.96). The results indicate that the amount of cations adhering to the silicate clay minerals and the size of the silicate mineral surface area are important properties of the mineral phase for the storage potential of C. The reason for the negative relation between iron oxides and C content remains unclear.     

Transgenic Bt plants decompose less in soil than non-Bt plants

Bt plants are plants that have been genetically modified to express the insecticidal proteins (e.g. Cry1Ab, Cry1Ac, Cry3A) from subspecies of the bacterium, Bacillus thuringiensis (Bt), to kill lepidopteran pests that feed on corn, rice, tobacco, canola, and cotton and coleopteran pests that feed on potato. The biomass of these transgenic Bt plants (Bt+) was decomposed less in soil than the biomass of their near-isogenic non-Bt plant counterparts (Bt−). Soil was amended with 0.5, 1, or 2% (wt wt⁻¹) ground, dried (50 °C) leaves or stems of Bt corn plants; with 0.5% (wt wt⁻¹) ground, dried biomass of Bt rice, tobacco, canola, cotton, and potato plants; with biomass of the near-isogenic plants without the respective cry genes; or not amended. The gross metabolic activity of the soil was determined by CO₂ evolution. The amounts of C evolved as CO₂ were significantly lower from soil microcosms amended with biomass of Bt plants than of non-Bt plants. This difference occurred with stems and leaves from two hybrids of Bt corn, one of which had a higher C:N ratio than its near-isogenic non-Bt counterpart and the other which had essentially the same C:N ratio, even when glucose, nitrogen (NH₄NO₃), or glucose plus nitrogen were added with the biomass. The C:N ratios of the other Bt plants (including two other hybrids of Bt corn) and their near-isogenic non-Bt counterparts were also not related to their relative biodegradation. Bt corn had a significantly higher lignin content than near-isogenic non-Bt corn. However, the lignin content of the other Bt plants, which was significantly lower than that of both Bt and non-Bt corn, was generally not statistically significantly different, although 10-66% higher, from that of their respective non-Bt near-isolines. The numbers of culturable bacteria and fungi and the activity of representative enzymes involved in the degradation of plant biomass were not significantly different between soil amended with biomass of Bt or non-Bt corn. The degradation of the biomass of all Bt plants in the absence of soil but inoculated with a microbial suspension from the same soil was also significantly less than that of their respective inoculated non-Bt plants. The addition of streptomycin, cycloheximide, or both to the soil suspension did not alter the relative degradation of Bt+ and Bt− biomass, suggesting that differences in the soil microbiota were not responsible for the differential decomposition of Bt+ and Bt− biomass. All samples of soil amended with biomass of Bt plants were immunologically positive for the respective Cry proteins and toxic to the larvae of the tobacco hornworm (Manduca sexta), which was used as a representative lepidopteran in insect bioassays (no insecticidal assay was done for the Cry3A protein from potato). The ecological and environmental relevance of these findings is not clear.     

Changes in forest soil organic matter pools after a decade of elevated CO2 and O3

The impact of rising atmospheric carbon dioxide (CO₂) may be mitigated, in part, by enhanced rates of net primary production and greater C storage in plant biomass and soil organic matter (SOM). However, C sequestration in forest soils may be offset by other environmental changes such as increasing tropospheric ozone (O₃) or vary based on species-specific growth responses to elevated CO₂. To understand how projected increases in atmospheric CO₂ and O₃ alter SOM formation, we used physical fractionation to characterize soil C and N at the Rhinelander Free Air CO₂-O₃ Enrichment (FACE) experiment. Tracer amounts of ¹⁵NH₄⁺ were applied to the forest floor of Populus tremuloides, P. tremuloides-Betula papyrifera and P. tremuloides-Acer saccharum communities exposed to factorial CO₂ and O₃ treatments. The ¹⁵N tracer and strongly depleted ¹³C-CO₂ were traced into SOM fractions over four years. Over time, C and N increased in coarse particulate organic matter (cPOM) and decreased in mineral-associated organic matter (MAOM) under elevated CO₂ relative to ambient CO₂. As main effects, neither CO₂ nor O₃ significantly altered ¹⁵N recovery in SOM. Elevated CO₂ significantly increased new C in all SOM fractions, and significantly decreased old C in fine POM (fPOM) and MAOM over the duration of our study. Overall, our observations indicate that elevated CO₂ has altered SOM cycling at this site to favor C and N accumulation in less stable pools, with more rapid turnover. Elevated O₃ had the opposite effect, significantly reducing cPOM N by 15% and significantly increasing the C:N ratio by 7%. Our results demonstrate that CO₂ can enhance SOM turnover, potentially limiting long-term C sequestration in terrestrial ecosystems; plant community composition is an important determinant of the magnitude of this response.     

Ammonia volatilization from urea-treated pasture and tillage soils: effects of soil properties

Mean NH₃ losses after nine days incubation at 18°C and 60% FC were 3.1±2.9% and 7.6±6.0% of applied urea-N from the pasture and tillage counterparts of 10 soil series. These losses were highly correlated with buffered CEC and maximal pH values (pH_m) generated three days after urea application. NH₃ volatilization was apparently controlled by buffered CEC and initial pH (R²= 72–87%) and was related to variations in soil organic matter and texture (R²= 77–81%). Losses in the acid pasture soils were attributed largely to initial pH differences, and in the tillage soils to buffered CEC only. Evolution was greater from the tillage than from the pasture equivalent in eight series. This was attributed to differences in CEC, including buffered CEC and pH-dependent charge, caused by differences in OM content primarily but also in texture between the two soil groups. Differences in NH₃ evolution from urea in pasture and tillage soils, in general, are not related to pH differences.     

Differential response of mineral-associated organic matter in tropical soils formed in volcanic ashes and marine Tertiary sediment to treatment with HCl,NaOCl, and Na4P2O7

Quantitative knowledge of the amount and stability of soil organic matter (SOM) is necessary to understand and predict the role of soils in the global carbon cycle. At present little is known about the influence of soil type on the storage and stability of SOM, especially in the tropics. We compared the amount of mineral-associated SOM resistant to different chemical treatments in soils of different parent material and mineralogical composition (volcanic ashes – dominated by short-range-order aluminosilicates and marine Tertiary sediments – dominated by smectite) in the humid tropics of Northwest Ecuador. Using ¹³C isotope analyses we traced the origin of soil organic carbon (SOC) in mineral-associated soil fractions resistant to treatment with HCl, NaOCl, and Na₄P₂O₇ under pasture (C₄) and secondary forest (C₃). Prior to chemical treatments, particulate organic matter was removed by density fractionation (cut-off: 1.6 g cm^?3). Our results show that: (1) independent of soil mineralogical composition, about 45% of mineral-associated SOC was resistant to acid hydrolysis, suggesting a comparable SOM composition for the investigated soils; (2) oxidation by NaOCl isolated a SOM fraction with enhanced stability of mineral-bound SOM in soils developed from volcanic ashes; while Na₄P₂O₇ extracted more SOC, indicating the importance of Al-humus complexes in these soils; and (3) recently incorporated SOM was not stabilized after land use change in soils developed from volcanic ashes but was partly stabilized in soils rich in smectites. Together these results show that the employed methods were not able to isolate a SOM fraction which is protected against microbial decay under field conditions and that the outcome of these methods is sensitive to soil type which makes interpretation challenging and generalisations to other soils types or climates impossible.     

Boron adsorption and desorption in some acid soils of West Bengal,India

Laboratory and greenhouse experiments were conducted to determine the influence of soil properties on adsorption and desorption of boron (B) as well as to estimate the degree of reversibility of adsorption reactions. The utility of Freundlich and Langmuir equations for characterizing the plant availability of applied B in soils was established using soybean [Glycine max (L.) Merr.] as a test crop. The adsorption-desorption study revealed that Fe₂O₃ and clay were primarily responsible for retaining added B in all the 25 different soils under investigation. Organic carbon, pH and cation exchange capacity (CEC) positively influenced the adsorption of B while free Fe₂O₃, organic carbon and clay retarded release of B from these soils. The degree of irreversibility (hysteresis) of B adsorption/desorption increased with increase in organic carbon and CEC of these soils. Freundlich isotherm proved more effective in describing B adsorption in soils as compared to Langmuir equation. The split Langmuir isotherm demonstrated that any of the adsorption maxima, calculated from lower, upper or entire isotherm, could be of practical use. Contrary, bonding energy coefficient, calculated either at lower or higher equilibrium concentration failed to show any practical benefit. Regression models as a function of B application rate and adsorption equation parameters to predict B uptake from applied B, demonstrated the utility of Langmuir and Freundlich equation parameters.     

Characterization of Pterocarpus macrocarpus (pradoo wood) biochar and its effect on the retention properties of sandy soils in Northeast Thailand

Pradoo wood biochar has been tested in order to explore sustainable solutions to the development of agriculture on poor sandy soils in marginal areas in Northeast Thailand. Some basic physicochemical properties of biochar, for example pore size distribution, cation exchange capacity (CEC), specific surface area (SSA), and water and nutrient adsorption, were determined and compared to soil properties in order to determine appropriate biochar application to soil. Pradoo wood biochar showed important adsorption properties with high SSA, CEC and nutrient adsorption. The water retention properties were also improved on the dry end of the water retention curve. Phosphorous and ammonium adsorption–desorption isotherms were established and their respective affinity for the biochar surface was quantified, by the means of a retention index and thermodynamical parameters. We found that despite excellent retention properties, biochar needs to be added in large amounts (between 10 and 70 kg m⁻²) to soil to be able to modify noticeably the resulting soil properties.     

Influence of organic matter on selenite sorption by Andosols

Abstract

The influence of soil organic matter on selenite sorption was investigated in the selenite adsorption capacity and the surface particle charge change by ligand exchange reaction using the hydrogen peroxide (H₂O₂) treatment and the ignition treatment of two Andosols. The removal of organic carbon (C) in soils accelerated selenite sorption, implying that organic matter of soils had negative influence on the selenite adsorption on the soils. Positive charge decrease on soil particles, concomitant proton consumption, and release of silicon (Si), sulfate (SO₄ ^2‐), and organic C were observed in selenite sorption by the soils. The development of surface particle negative charge with selenite sorption was smaller in the H₂O₂‐treated soil than in the original soils and was scarcely observed in the ignition‐treated soil. It can be assumed that the increase of negative charge by selenite sorption was attributed to new negative sites borne by released insoluble organic matter and negative charge development directly by selenite sorption was small.     

Microbial activity and bacterial composition of H2-treated soils with net CO2 fixation

The effects of H₂ gas treatment of an agricultural soil cultivated previously with a mixture of clover (Trifolium pratense) and alfalfa (Medicago sativa) on CO₂ dynamics and microbial activity and composition were analyzed. The H₂ emission rate of 250 nmol H₂ g⁻¹ soil h⁻¹ was similar to the upper limit of estimated H₂ amounts emitted from N₂ fixing nodules into the surrounding soil ([Dong, Z., Layzell, D.B., 2001. H₂ oxidation, O₂ uptake and CO₂ fixation in hydrogen treated soil. Plant and Soil 229, 1-12.]). After 1 week of H₂ supply to soil samples simultaneously with H₂ uptake net CO₂ production declined continuously and this finally led to a net CO₂ fixation rate in the H₂-treated soil of 8 nmol CO₂ g⁻¹ soil h⁻¹. The time course of H₂ uptake and CO₂ fixation in the soils corresponded with an increase in microbial activity and biomass of the H₂-treated soil determined by microcalorimetric measurements, fluorescence in situ hybridization analysis (FISH) and DNA staining (DAPI). Shifts in the bacterial community structure caused by the supply of H₂ were recorded. While the H₂ treatment stimulated β-and γ-subclasses of Proteobacteria, it had no significant effect on α-Proteobacteria. In addition, FISH-detectable bacteria of the Cytophaga-Flavobacterium-Bacteroides phylum increased in numbers.