Demonstration of two different H2-oxidizing activities in soil using an H2 consumption and a tritium exchange assay

H₂-oxidizing activities were assayed in slurries of four soils by measuring the consumption of H₂ and the exchange of ³H₂ with H₂O at increasing mixing ratios of H₂ or ³H₂. Both H₂ consumption and ³H₂ exchange were abolished by autoclaving or the addition of formaldehyde. The rates of H₂ consumption and ³H₂ exchange were proportional to the quantity of soil used. Both activities increased with increasing concentrations of H₂ or ³H₂ and displayed biphasic kinetics, demonstrating the existence of two different H₂-oxidizing activities, one with a relatively low K _m and V _max, and a second with a relatively high K _m und V _max. The first type of activity was characteristic of abiontic soil hydrogenases, and the second of aerobic H₂-oxidizing bacteria. In contrast to H₂ consumption, which required the presence of either O₂ or ferricyanide, ³H₂ exchange operated equally well without an external electron acceptor. The ³H₂ exchange assay may thus be particularly useful for enrichment of soil hydrogenases which have not yet been isolated and for which no natural electron acceptor is known.     

Effect of long-term field exposure of soil to acetylene on nitrification,denitrification, and acetylene consumption

Coated CaC₂ is a newly developed product which can supply nitrification-inhibiting quantities of C₂H₂ (1–10 Pa) to the soil, throughout a cropping season. This method of applying C₂H₂ to the soil maintains C₂H₂ in the soil continuously for several months. It is not know whether these low C₂H₂ concentrations alter soil microbial processes. A field study was initiated to determine the effect of supplying C₂H₂ to a clay soil, using coated CaC₂, on soil respiration, denitrification, nitrification, and C₂H₂ consumption. The C₂H₂ consumption rate increased with length of soil exposure to C₂H₂ (r ²=0.59). The rates of CO₂ production (r ²=0.88) and denitrification (r ²=0.86) were both highly correlated with the C₂H₂ consumption rates. The nitrifier potential decreased to a minimum of 21% of the control after 3 months of C₂H₂ treatment. After this time, nitrifier activity increased to 41% of the control after 11 months of treatment. This increase was due to increased C₂H₂ consumption in the soil. After 3 months of continuous application of C₂H₂ to the soil, the C₂H₂ concentrations were generally below that necessary to inhibit nitrification. No adaptation to the C₂H₂ by nitrifiers was found. Repeating these measurements 1 year later showed that soils previously exposed to C₂H₂ retained their enhanced C₂H₂ oxidation capacity and the capacity to use C₂H₂ to increase denitrification. Nitrification potentials remained about 50% lower in soils exposed to C₂H₂ a year earlier compared to soils not previously exposed to C₂H₂.     

Responses of ethylene and methane consumption to temperature and pH in temperate volcanic forest soils

There is limited knowledge about the consumption and interaction of methane (CH₄) and ethylene (C₂H₄) in forest soils under disturbances of temperature and acidification. Temperate volcanic forest topsoils (0‐5 cm) sampled under different tree species (e.g. Pinus sylvestris, Cryptomeria japonica and Quercus serrata) were used to study the capacities for CH₄ and C₂H₄ consumption and their sensitivity to temperature and pH. We also studied the responses of soil nitrogen (N) transformations to temperature and relationships to consumption of both CH₄ and C₂H₄. The C₂H₄ consumption rates increased with temperature up to 35^oC, whereas the optimum temperature for CH₄ consumption rates was approximately 25^oC. Both Q₁₀ values and activation energies for CH₄ consumption rates over the range 5 to 25^oC were larger than corresponding values for C₂H₄ consumption rates. The rates of nitrous oxide (N₂O) and nitric oxide (NO) evolution and net N mineralization in the soils increased exponentially with temperature up to 35^oC, with relatively large Q₁₀ values and activation energies for NO evolution. In these forest topsoils, rates of CH₄ and C₂H₄ consumption at pH < 4.0 were negligible, and the pH optimum for both consumptions varied from 5.5 to 6.2. Most of the tested forest soils had an optimum pH for CH₄ and C₂H₄ consumption that was above natural pH values, which indicated that soil acidification would inhibit CH₄ and C₂H₄ consumption in situ. There was a high rate of net C₂H₄ evolution from forest soils acidified experimentally to pH < 4.0, particularly from Cryptomeria japonica forest soil, and 67% of the variation in C₂H₄ evolution rates could be accounted for by the increase in soil water‐soluble organic carbon concentrations. Previous studies have shown that addition of C₂H₄ in headspace gases can inhibit atmospheric CH₄ consumption in such forest soils. Hence, the evolution of C₂H₄ from temperate volcanic forest soils at decreasing pH can exacerbate inhibition of the soil atmospheric CH₄ consumption in situ.     

Hydrogen consumption and carbon monoxide production in soils with different properties

 Soils are the dominant sink in the global budget of atmospheric H₂, and can be an important local source of atmospheric CO. In order to understand which soil characteristics affect the rates of H₂ consumption and CO production, we measured these activities in 16 different soils at 30% and 60% of their maximum water holding capacity (whc). The soils were obtained from forests, meadows and agricultural fields in Germany and exhibited different characteristics with respect to texture, pH, total C, substrate-induced respiration (SIR), respiration, total and inorganic N, N mineralization, nitrification, N₂O production and NO turnover. The H₂ consumption rate constants were generally lower at 60% than at 30% whc, whereas the CO production rates were not influenced by the whc. Spearman correlation analysis showed that H₂ consumption correlated significantly (r>0.5, P<0.05) at both water contents only with SIR and potential nitrification. The correlation with these variables that are largely dominated by soil microorganisms is consistent with our understanding that atmospheric H₂ is oxidized by soil hydrogenases. Multiple regression analysis and factor analysis gave similar results. Production of CO, on the other hand, was significantly correlated to soil total C, respiration, total N and NH₄ ⁺. The correlation with these variables that are largely dominated by a soil's chemical composition is consistent with our understanding that CO is produced by chemical oxidation of soil organic C. CO production was also influenced by soil usage, with rates increasing in the order: arable<meadow<forest. H₂ consumption was not influenced by soil usage. Received: 28 October 1999     

Production and consumption of ethylene in temperate volcanic forest surface soils

To date our knowledge is limited with regard to the cycling of ethylene (C₂H₄) in temperate forest soils containing volcanic ash, and the effect of forest‐to‐orchard conversion on its cycling. We studied ethylene accumulation in such forest soils by oxic and anoxic incubations, along with the stimulatory effect of glucose addition on soil C₂H₄ accumulation. We also studied the effect of antibiotics and autoclaving on C₂H₄ production and consumption by volcanic forest soils, and the cycling of C₂H₄ and CH₄ in surface soils after conversion of a Japanese cedar forest to an orchard. Ethylene production and consumption by forest surface soils results from a microbial process, and soil streptomycin‐sensitive bacteria make a minor contribution. Soil C₂H₄ accumulation was much larger during anoxic than during oxic incubation, which indicates that anoxic conditions can induce C₂H₄ accumulation in forest soils. Glucose addition as a carbon source can sharply increase C₂H₄ accumulation rates in the anoxic and oxic forest soils during the first week of incubation. However, there was no difference in total C₂H₄ accumulation in the amended and non‐treated soils after 35 days of anoxic incubation. Ethylene production of the 0–5 cm and 5–10 cm soils beneath forest and orchard showed the greatest rate after 2 weeks of anoxic incubation when soil CH₄ production started to increase sharply, and later it was strongly suppressed. The forest‐to‐orchard conversion showed little influence on the CH₄ production of surface soils during short‐term anoxic incubation, but significantly reduced soil C₂H₄ production. The conversion also significantly decreased the consumption of soil CH₄ and C₂H₄, the former more than the latter. Soil properties such as total C, water‐soluble organic C and pH contribute to the consumption and production of C₂H₄ in the 0–5 cm and 5–10 cm soils, and there are the parallels between CH₄ and C₂H₄ consumption in soils, which suggests the presence of similar microorganisms. Long‐term anoxic conditions of in situ surface upland soils are normally not prevalent, so it can be reasonably concluded that there is a larger C₂H₄ accumulation rather than CH₄ accumulation in surface soils beneath forest and orchard after heavy rainfall, especially beneath forest.     

Carbon monoxide as an inhibitor of N2ase activity(C2H2) in control measurements of endogenous formation of ethylene by forest soils

Two problems with the recently suggested method to measure endogenous formation of C₂H₄ in an atmosphere enriched with C₂H₂ and CO in studies of N₂ase activity (C₂H₂) in forest soils were analysed, namely the effect of consumption of CO during incubation and the effect of water-saturated conditions.After an initial addition of 100 ml C₂H₂ and 20 ml CO 1^?1 to soil incubation vessels, CO was gradually consumed and followed by a recovery of N₂ase activity when the concentration of CO was lower than about 10 ml 1^?1. The shortest period within which this concentration was achieved was 1 day when incubating fresh soil cores at 15°C, and it was concluded that longer incubations should be avoided.The inhibition of N₂ase activity by CO was strongly suppressed when all soil pores were filled with water. Dissolved inorganic N (0.1% of dry mass soil) was much more efficient in inhibiting N₂ase activity under such conditions.     

Decomposition of atmospheric hydrogen by soil microorganisms and soil enzymes

The decomposition of atmospheric hydrogen in different types of soil was measured. The decomposition of H₂ was apparently a first-order reaction. H₂ decomposition activity was proportional to the amount of soil with maximum activities at soil water contents of approx. 6–11% (w/w). The activity was lower under anaerobic conditions, but was constant between 1–20% O₂. It was destroyed by autoclaving and was partially inactivated by fumigation with NH₃, CHC1₃ or acetone, by u.v. irradiation and by treatment with NaCN or NaN₃, indicating that biological processes in the soil were responsible for the observed H₂ decomposition. Treatment of soil with toluene or CHCl₃ caused only a partial inactivation. Incubation of soil in the presence of streptomycin or actidione reduced H₂ decomposition by less than 50%, whereas CO consumption was abolished. The H₂ decomposition rates showed H₂ saturation curves with apparent Michaelis-Menten kinetics. Cooperative effects were not observed. V_max was reached at approx. 200 μl1^?1. The K_m values for H₂ were in the range of 30μl 1^?1, but increased to higher values, when the soil had been pretreated with high H₂ mixing ratios. Apparently, the observed H₂ decomposition by soil is not only due to the activity of viable microorganisms, but soil enzymes as well.     

Nitrous Oxide Production and Consumption Potential in an Agricultural and a Forest Soil

Both a laboratory incubation experiment using soils from an agricultural field and a forest and field measurements at the same locations were conducted to determine nitrous oxide (N₂O) production and consumption (reduction) potentials using the acetylene (C₂H₂) inhibition technique. Results from the laboratory experiment show that the agricultural soil had a stronger N₂O reduction potential than the forest soil, as indicated by the N₂O/N₂ ratio in denitrification products. Without C₂H₂ inhibition, N₂O could reach a maximum concentration of 51 and 296 ppmv in headspace of the agricultural and forest soil slurries, respectively. Addition of glucose decreased the maximum N₂O concentration to 22 ppmv in headspace of the agricultural soil slurries, but increased to 520 ppmv in the forest soil slurries. Addition of exogenous N₂O did not change such N₂O accumulation maxima during the incubations. The field measurements show that average N₂O emission rates were 0.56 and 0.59 kg N ha^?1 in the agricultural field and forest, respectively. When C₂H₂ was provided in the field measurements, N₂O emission rates from the agricultural field and forest increased by 38 and 51%, respectively. Nitrous oxide consumption under elevated N₂O condition (about 300 ppmv) was found in all five agricultural field measurements, but only in three of the six forest measurements under the same conditions. Field measurements agreed with the laboratory experiment that N₂O reduction activity, which plays a critical role in abating N₂O emissions from soils, largely depended on soil characteristics associated with land use.     

Effects of chemical and heat treatments on ethylene production in soil

Factors influencing C₂H₄ production in a silt loam were investigated in an effort to determine the source of this gas in soil. Air-dried samples of soil in glass vials were moistened to about ?10 kPa, sealed with rubber septa, and incubated at 30 or 35°C with an original atmosphere of air or O₂-free N₂. C₂H₄ concentrations in the vials were determined by gas chromatography.Addition of the antibacterial agents chloramphenicol or novobiocin to the soil inhibited C₂H₄ production, whereas the antifungal agent cycloheximide had no effect. Sodium azide and sodium cyanide also reduced C₂H₄ production. Treatment of the soil with moist heat (i.e. passing a steam-air mixture through it) at 80°C for 30 min failed to reduce the ability of the soil to produce C₂H₄ during subsequent incubation at 30°C, but autoclaving it twice at 121°C prevented C₂H₄ production. As with nonheated soil, C₂H₄ production from soil treated at 80°C was prevented by novobiocin but not by cycloheximide. Only about 10% of the bacteria isolated from nontreated soil were spore-formers. In contrast, 95–98% and possibly more of the bacteria isolated from heat-treated soil were spore-formers, including those in soil which was heat-treated and then incubated moist at 30°C for an additional 3 days before dilution plating. Addition of methionine had no effect on the production of C₂H₄ in anaerobic soil, whereas ethionine, chlorogenic acid, and ethylenediaminetetraacetic acid (EDTA) all enhanced C₂H₄ production. Ethionine, but not chlorogenic acid or EDTA, also resulted in considerable C₂H₄ accumulation in autoclaved soil; the C₂H₄ detected in ethionine-amended soil was apparently nonmicrobial in origin. Soil samples incubated at constant temperatures of 30, 50, or 70°C all produced C₂H₄.The results collectively indicate that C₂H₄ in soil is most likely produced by facultative or strictly anaerobic bacteria which are probably spore-formers and may also be thermophilic. Several isolates of spore-forming bacteria were inoculated into autoclaved soil, but none produced appreciable amounts of C₂H₄ under the test conditions.     

The influence of moisture level,light, aeration and glucose upon acetylene reduction by a black earth soil

The influence of moisture level, light, aeration, and glucose upon C₂H₂ reduction by a clay soil was studied. C₂H₂ reduction was greater in 15 g samples of the air-dried soil moistened with 9 ml water than in samples moistened with 5 ml water. Illumination of the soil led to significantly more C₂H₂ reduction than when the soil samples were incubated in the dark. The addition of glucose to the soil increased C₂H₂ reduction. C₂H₂ reduction by soil samples incubated under an aerobic gas phase (either Ar + O₂, 80; 20; or air) was not significantly different from that of samples incubated under an anaerobic gas phase (Ar). Several questions regarding the use of the C₂H₂ reduction technique in assessing nitrogen fixation by natural ecosystems are raised.     

Nitrogen monoxide production and consumption in an organic soil

 Factors controlling NO production, consumption, and emission rates were examined in an organic soil. Emission rates were measured in the enclosed headspaces of intact soil cores under three fertilisation treatments (unfertilised or 100 kg N ha^–1 as NH₄Cl or as NaNO₃), with and without the nitrification inhibitor C₂H₂ (20–70 μl l^–1). Nitrification was always the main source of NO emitted across the soil surface, even when the soil was nearly saturated. Fertilisation of soil with NH₄Cl increased NO emission both by stimulating NO production from nitrification, and by decreasing the NO consumption rate constant. Addition of NaNO₃ also stimulated the production of NO and N₂O during nitrification in aerobic soil slurry experiments. This effect was eliminated by adding C₂H₂ and was therefore not related to denitrification. In loose soil samples, the increase in NO-N production after NH₄Cl addition represented as much as 26% of the added N. However, in intact cores, 95% of the NO produced through nitrification was oxidised within the soil column rather than emitted to the atmosphere. We concluded that nitrification is the primary NO source from this organic soil, that surface NO emissions are much lower than gross NO production rates, and that gaseous N oxide (NO and N₂O) losses during nitrification can be affected by both soil NH₄ ⁺ and NO₃ ^–. Received: 15 December 1998     

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.     

Ethylene oxidation,atmospheric methane consumption,and ammonium oxidation in temperate volcanic forest soils

Temperate volcanic forest surface soils under different forest stands (e.g., Pinus sylvestris L., Cryptomeria japonica, and Quercus serrata) were sampled to study the kinetics of ethylene (C₂H₄) oxidation and the C₂H₄ concentrations that effectively inhibit oxidation of atmospheric methane (CH₄) and nitrification. The kinetics of C₂H₄ oxidation in temperate volcanic forest soils was biphasic, indicating that at least two different microbial populations, one with low and another with high apparent K _m values, were responsible for ethylene oxidation. Methane consumption activity and ammonium oxidation of soil were inhibited by adding ethylene. Added C₂H₄ at concentrations of 3, 10, and 20 μl C₂H₄ per liter in the headspace gas respectively reduced by 20%, 50%, and 100% atmospheric CH₄ consumption by soil, and these values were much smaller than those inhibiting ammonium oxidation in these forest soils; thus, the CH₄ consumption activity was more sensitive to the addition of C₂H₄ than ammonium oxidation. Previous studies have shown that accumulation of C₂H₄ in such volcanic forest soils within 3 days of aerobic and anaerobic incubations can reach a range from 0.2 to 0.3 and from 1.0 to 3.0 μl C₂H₄ per liter in the headspace gas, respectively. It is suggested that C₂H₄ production beneath forest floors, particularly after heavy rain, can to some extent affect the capacity of forest surface soils to consume atmospheric CH₄, but probably, it has no impact on ammonium oxidation.     

Contributions from different microbial processes to N2O emission from soil under different moisture regimes

Nitrous oxide emissions from a sandy-loam textured soil wetted to matric potentials of either-1.0 or-0.1 kPa were determined in laboratory experiments in which the soil was incubated in air (control), air plus 10 Pa C₂H₂ (to inhibit nitrification), 100 kPa O₂ (to suppress denitrification), 10 kPa C₂H₂ (to inhibit N₂O reduction to N₂ in denitrification) or following autoclaving. The total N₂O production, consumption and net N₂O emission from the soils together with the contributions to N₂O emission from different processes of N₂O production were estimated. The rate of N₂O production was significantly greater in the wetter soil (282 pmol N₂O g^-1 soil h^-1) than in the drier soil (192 pmol N₂O g^-1 soil h^-1), but because N₂O consumption by denitrifiers was also greater in the wetter soil, the net N₂O emissions from the wetter and the drier soils did not differ significantly. Non-biological sources made no significant contribution to N₂O emission under either moisture regime and biological processes other than denitrification and nitrification made only a small contribution (1% of the total N₂O production) in the wetter soil. Denitrifying nitrifiers were the predominant source of N₂O emitted from the drier soil and other (non-nitrifying) denitrifiers were the predominant source of N₂O emitted from the wetter soil.     

Hydrogen oxidation in soil following rhizobial H2 production due to N2 fixation by a Vicia faba-Rhizobium leguminosarum symbiosis

Summary The rate of H₂ release from broad beans (Vicia faba) infected with Rhizobium leguminosarum Hup^- was much faster than from beans infected with the Hup⁺ strain. Acetylene reduction and H₂ release were abolished by cutting the plants down, by incubation in darkness, or after the addition of ammonium, indicating that the H₂ was released by N₂-fixing bacterial symbionts. In laboratory cultures using non-sterile soil, the bean plants released H₂ until an equilibrium between H₂ production and H₂ oxidation was reached. The H₂ equilibrium concentration was higher in Hup^--infected bean cultures (about 3 ppm H₂ in the gas phase) than in Hup⁺-infected cultures (0.3 ppm H₂) because of the higher H₂ production. The H₂ release from Hup^--infected bean cultures in sterile soil did not reach equilibrium. An equilibrium occurred, if Knallgas bacteria were added. However, the equilibrium value was higher (13 ppm H₂) than in non-sterile soil, which seemed to be more efficient at H₂ oxidation. The Knallgas bacteria exhibited a relatively high K _m for H₂ (> 1300 ppmv H₂); this activity was observed in unplanted non-sterile soil, and in nonsterile soil planted with Hup⁺-infected beans or planted with Hup^--infected beans which had been cut down before being assayed. All these soils also showed a second, low-K _m (<50 ppm) level of H₂ oxidation activity, which was presumably due to abiontic soil enzymes. In contrast, only one level of activity, which had an intermediate K _m (about 200 ppm H₂), was observed when the soil was planted with Hup^--infected beans. The origin of this activity, which was only observed in the presence of intact, H₂-producing beans, is still unknown.     

How much oxygen is needed for acetylene to be consumed in soil?

Purpose  

Acetylene (C₂H₂) is employed for the quantification of important biological processes such as nitrogen fixation, nitrous oxide reduction, ammonium and methane oxidation, and methanogenesis. Although acetylene is not a natural product, the ability of bacteria to grow on C₂H₂ is a phenomenon common to soils and sediments. Our experiment was designed to study the modification of CO₂ production, O₂ uptake and microbial biomass (C_mic) in soil in response to the consumption of added acetylene.     

Factors regulating acetylene reduction assay for measuring heterotrophic nitrogen fixation in water-logged soils

In the C₂H₂-C₂H₄ assay for measurement of heterotrophic N₂ fixation in water-logged soils, the diffusion of C₂H₂ into the soil and the recovery of C₂H₄ from it are critical factors regulating the assay result. To establish an C₂H₂-C₂H₄ assay technique suitable for waterlogged soils, the C₂H₂-reducing activities (ARA), assayed by varying the method of assay gas filling, the pC₂H₂ of the assay gas, the duration of assay incubation and of soil vibration before the gas sampling, were compared.

A maximum ARA was measured when the following set of procedures were applied to the soil sample in assay flasks: 1) a 4-fold repetition of I-min evacuation under 0.01 atmospheric pressure and the subsequent I-min filling under 1 atmospheric pressure with assay gas at pC₂H₂ of 0.1 atm, 2) an assay incubation for 3 hr, and 3) a sampling of an aliquot of the headspace gas after strongly vibrating the flask for 1 min.

The ARA measured by this technique was several times larger than those measured by the techniques hitherto applied, and corresponded to an almost 80% of the V _max of the sample. This technique was, therefore, proposed for the assay of heterotrophic N₂ fixation in waterlogged soils.

A striking depression of ARA in the soil sample prepared with agitation indicated that a microbial ecosystem established in the soil should be kept as undisturbed as possible throughout the C₂H₂-C₂H₄ assay.     

Oxygen and redox potential gradients in the rhizosphere of alfalfa grown on a loamy soil

Oxygen (O₂) supply and the related redox potential (E_H) are important parameters for interactions between roots and microorganisms in the rhizosphere. Rhizosphere extension in terms of the spatial distribution of O₂ concentration and E_H is poorly documented under aerobic soil conditions. We investigated how far O₂ consumption of roots and microorganisms in the rhizosphere is replenished by O₂ diffusion as a function of water/air‐filled porosity. Oxygen concentration and E_H in the rhizosphere were monitored at a mm‐scale by means of electroreductive Clark‐type sensors and miniaturized E_H electrodes under various matric potential ranges. Respiratory activity of roots and microorganisms was calculated from O₂ profiles and diffusion coefficients. pH profiles were determined in thin soil layers sliced near the root surface. Gradients of O₂ concentration and the extent of anoxic zones depended on the respiratory activity near the root surface. Matric potential, reflecting air‐filled porosity, was found to be the most important factor affecting O₂ transport in the rhizosphere. Under water‐saturated conditions and near field capacity up to –200 hPa, O₂ transport was limited, causing a decline in oxygen partial pressures (pO₂) to values between 0 and 3 kPa at the root surface. Aerobic respiration increased by a factor of 100 when comparing the saturated with the driest status. At an air‐filled porosity of 9% to 12%, diffusion of O₂ increased considerably. This was confirmed by E_H around 300 mV under aerated conditions, while E_H decreased to 100 mV on the root surface under near water‐saturated conditions. Gradients of pO₂ and pH from the root surface indicated an extent of the rhizosphere effect of 10–20 mm. In contrast, E_H gradients were observed from 0 to 2 mm from the root surface. We conclude that the rhizosphere extent differs for various parameters (pH, Eh, pO₂) and is strongly dependent on soil moisture.     

Soil Reserves of Potassium: Release and Availability to Lolium perenne in Relation to Clay Minerals in Six Cropland Soils from Eastern China

An 8‐month greenhouse experiment with ryegrass (Lolium perenne L.) examined the relationship between the dynamics of potassium (K) reserves in soil and changes in clay minerals, using X‐ray diffraction. The capacity of soil to release K was consistent with its ability to supply it and was the highest in the soil samples collected from Laiyang (H₁T₂a), Harbin (H₁T₁) and Beibei (H₂T₄), followed by the sample from Jiangyan (H₁T₂b), and the lowest in the samples from Gao'an (H₂T₃a) and Wangcheng (H₂T₃b). Removal of soil K decreases the intensity of reflections for illite and increases that for interstratified clay minerals. The centre of gravity values of the clay fractions was significantly negatively correlated to the depletion of soil K reserves, as ascertained through chemical extraction or through plant action. The quantitative regression equations between the centre of gravity values and the dynamics of soil reserves of K can be used for predicting the release and plant availability of K even without growing a crop. Copyright © 2017 John Wiley & Sons, Ltd.     

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.