Effect of fertilizer application on methane emission/production in the paddy soils of Taiwan

Methane production in three types of rice paddy soil was investigated under greenhouse conditions. The amount of methane produced during the first crop season (March to July) was 2–6 times higher than that in the second crop season (August to December). Application of organic fertilizer hastened the drop in redox potential and increased methane production and emission. Methane production also increased with the depth of soil with high values in soil samples from 18 to 30cm depth. Methane production in the first crop season was 18.0, 54.3 and 49.4mgcm^–3 for 6tha^–1 straw application for Linkou, Tzawchyau and Jiaushi soils, respectively. The value was 33.4mgcm^–3 for the second crop season in Jiaushi soil. Methane emission was high during the flowering and maturity stages in the first crop season and the values were high during the tillering and flowering stages in the second crop season. Methane emission was high in Tzawchyau and Jiaushi soils in the first crop season. Methane emission rate reached a maximum from 12 noon to 3p.m. due to high temperature and a minimum at 3 to 6a.m. in both planted and unplanted soils. Received: 17 September 1996     

Methane concentrations and methanotrophic community structure influence the response of soil methane oxidation to nitrogen content in a temperate forest

Methane oxidation in forest soils removes atmospheric CH₄. Many studies have determined methane uptake rates and their controlling variables, yet the microorganisms involved have rarely been assessed simultaneously over the longer term. We measured methane uptake rates and the community structure of methanotrophic bacteria in temperate forest soil (sandy clay loam) on a monthly basis for two years in South Korea. Methane uptake rates at the field site did not show any seasonal patterns, and net uptake occurred throughout both years. In situ uptake rates and uptake potentials determined in the laboratory were 2.92 ± 4.07 mg CH₄ m⁻² day⁻¹ and 51.6 ± 45.8 ng CH₄ g⁻¹ soil day⁻¹, respectively. Contrary to results from other studies, in situ oxidation rates were positively correlated with soil nitrate concentrations. Short-term experimental nitrate addition (0.20-1.95 μg N g⁻¹ soil) significantly stimulated oxidation rates under low methane concentrations (1.7-2.0 ppmv CH₄), but significantly inhibited oxidation under high methane concentrations (300 ppmv CH₄). We analyzed the community structures of methanotrophic bacteria using a DNA-based fingerprinting method (T-RFLP). Type II methanotrophs dominated under low methane concentrations while Type I methanotrophs dominated under high methane concentrations. Nitrogen addition selectively inhibited Type I methanotrophic bacteria. Overall, the results of this study indicate that the effects of inorganic N on methane uptake depend on methane concentrations and that such a response is related to the dissimilar activation or inhibition of different types of methanotrophic bacteria.     

Soil contamination with emissions of non-ferrous metallurgical plants

The upper soil horizons are strongly contaminated in the area influenced by the Mid-Urals copper smelter. In the technogenic desert and impact zones, the contents of a number of elements (Cu, Zn, As, Pb, P, and S) by many times exceed their clarke values and the maximum permissible concentrations (or provisional permissible concentrations). The degree of technogeneity (Tg) for these elements is very high in these zones. In the far buffer zone, Tg is about zero for many elements and increases up to Tg = 27–42% for four heavy elements (Cu, Zn, Pb, and As) and up to 81–98% for P and S. The buffer capacity of the humus horizon depends on the soil’s location within the technogeochemical anomaly and also on the particular pollutant. In the impact zone, it is equal to 70–77% for lead and arsenic, although other technogenic elements (Zn, Cr, S, and P) are poorly retained and readily migrate into the deeper horizons (the buffer capacity is equal to 14–25%). Nearly all the heavy metals enter the soil in the form of sulfides. The soils in the area affected by the Noril’sk mining and smelting metallurgical enterprise are subdivided into two groups according to the degree of their contamination, i.e., the soils within Noril’sk proper and the soils in its suburbs to a distance of 4–15 km. The strongest soil contamination is recorded in the city: the clarke values are exceeded by 287, 78, 16, 4.1, and 3.5 times for Cu, Ni, Cr, Fe, and S, respectively. The major pollutants enter the soil from the ferruginous slag. The soil’s contamination degree is lower in the suburbs, where heavy metal sulfides reach the soils with the aerial emission from the enterprise.     

Activation of the biochemical processes in an oil-contaminated soil using a light-correcting film and humic acids

It was shown that the use of a light-correcting film as a covering material for an oil-contaminated soil in combination with humic acids increased the number of the main physiological groups of the soil microorganisms responsible for the development of the soil’s fertility (heterotrophic bacteria, actinomycetes, and micromycetes) by 60–100 times. The activity of the soil enzymes (catalase, dehydrogenase, polyphenoloxidase, peroxidase, and urease) increased by 3–6 times. The biochemical oxidation of oil hydrocarbons in the soil became significantly more intense.     

Production,oxidation, emission and consumption of methane by soils: A review

Methane emission by soils results from antagonistic but correlated microbial activities. Methane is produced in the anaerobic zones of submerged soils by methanogens and is oxidised into CO₂ by methanotrophs in the aerobic zones of wetland soils and in upland soils. Methanogens and methanotrophs are ubiquitous in soils where they remain viable under unfavourable conditions. Methane transfer from the soil to the atmosphere occurs mostly through the aerenchyma of aquatic plants, but also by diffusion and as bubbles escaping from wetland soils. Methane sources are mainly wetlands. However 60 to more than 90 % of CH₄ produced in the anaerobic zones of wetlands is reoxidised in their aerobic zones (rhizosphere and oxidised soil-water interface). Methane consumption occurs in most soils and exhibits a broad range of values. Highest consumption rates or potentials are observed in soils where methanogenesis is or has been effective and where CH₄ concentration is or has been much higher than in the atmosphere (ricefields, swamps, landfills, etc.). Aerobic soils consume atmospheric CH₄ but their activities are very low and the micro-organisms involved are largely unknown. Methane emissions by cultivated or natural wetlands are expressed in mg CH₄·m^–2·h^–1 with a median lower than 10 mg CH₄·m^–2·h^–1. Methanotrophy in wetlands is most often expressed with the same unit. Methane oxidation by aerobic upland soils is rarely higher than 0.1 mg CH₄·m^–2·h^–1. Forest soils are the most active, followed by grasslands and cultivated soils. Factors that favour CH₄ emission from cultivated wetlands are mostly submersion and organic matter addition. Intermittent drainage and utilisation of the sulphate forms of N-fertilisers reduce CH₄ emission. Methane oxidation potential of upland soils is reduced by cultivation, especially by ammonium N-fertiliser application.     

Nickel adsorption in two Oxisols and an Alfisol as affected by pH,nature of the electrolyte,and ionic strength of soil solution

Background, aim, and scope  The retention of potentially toxic metals in highly weathered soils can follow different pathways that variably affect their mobility and availability in the soil–water–plant system. This study aimed to evaluate the effects of pH, nature of electrolyte, and ionic strength of the solution on nickel (Ni) adsorption by two acric Oxisols and a less weathered Alfisol. Materials and methods  The effect of pH on Ni adsorption was evaluated in surface and subsurface samples from a clayey textured Anionic ‘Rhodic’ Acrudox (RA), a sandy-clayey textured Anionic ‘Xantic’ Acrudox (XA), and a heavy clayey textured Rhodic Kandiudalf (RK). All soil samples were equilibrated with the same concentration of Ni solution (5.0 mg L⁻¹) and two electrolyte solutions (CaCl₂ or NaCl) with different ionic strengths (IS) (1.0, 0.1 and 0.01 mol L⁻¹). The pH of each sample set varied from 3 to 10 in order to obtain sorption envelopes. Results and discussion  Ni adsorption increased as the pH increased, reaching its maximum of nearly pH 6. The adsorption was highest in Alfisol, followed by RA and XA. Competition between Ni²⁺ and Ca²⁺ was higher than that between Ni²⁺ and Na⁺ in all soil samples, as shown by the higher percentage of Ni adsorption at pH 5. At pH values below the intersection point of the three ionic strength curves (zero point of salt effect), Ni adsorption was generally higher in the more concentrated solution (highest IS), probably due to the neutralization of positive charges of soil colloids by Cl⁻ ions and consequent adsorption of Ni²⁺. Above this point, Ni adsorption was higher in the more diluted solution (lowest ionic strength), due to the higher negative potential at the colloid surfaces and the lower ionic competition for exchange sites in soil colloids. Conclusions  The effect of ionic strength was lower in the Oxisols than in the Alfisol. The main mechanism that controlled Ni adsorption in the soils was the ionic exchange, since the adsorption of ionic species varied according to the variation of pH values. The ionic competition revealed the importance of electrolyte composition and ionic strength on Ni adsorption in soils from the humid tropics. Recommendations and perspectives  The presence of NaCl or CaCl₂ in different ionic strengths affects the availability of heavy metals in contaminated soils. Therefore, the study of heavy metal dynamics in highly weathered soils must consider this behavior, especially in soils with large amounts of acric components.     

Effect of differently post-treated dewatered sewage sludge on β-glucosidase activity,microbial biomass carbon,basal respiration and carbohydrates contents of soils from limestone quarries

This work has evaluated the effects of thermally dried (TDS) or composted (CDS) dewatered sewage sludge on β-glucosidase activity, total (TCH) and extractable (ECH) carbohydrate content, microbial biomass carbon and basal respiration of soils from limestone quarries under laboratory conditions. Two doses (low and high) of the dewatered sludge (DS) or of the respective TDS or CDS were applied to a clayey and a sandy soil, both coming from working quarries. The soil mixtures and the controls (soils with no added sludge) were incubated for 9 months at 25°C and 30% of field capacity. The addition of sludge increased all the studied soil parameters, and the increase depended on the amount of sludge. Except in the case of TCH and ECH, the enhancing effect decreased with time, but at the end of incubation, parameters of the treated soils were higher than those of the control. The rank order of the initial stimulating effect was soil–TDS ≥ soil–DS ≥ soil–CDS, and probably, this order depended on the proportion of stable organic matter, which was the lowest in the TDS. Values of metabolic quotient (qCO₂) were higher at the lower dose, and they did not change during incubation in the CDS-treated soils. Both TCH and ECH were the parameters with the greatest significant sludge and dose effects. Basal respiration, microbial biomass carbon and β-glucosidase activity were the best measured parameters in distinguishing the long-term effects of the three sludge types over the soils.     

Methane-oxidizing activities and methanotrophic populations associated with wetland rice plants

Acetylene up to 500 μl l^–1 did not affect methane formation in anoxic soil up to 12 h, but further incubation for 1 week showed strong inhibition of methanogenesis. To ascertain the extent of the oxidation of methane produced from rice-planted pots, the effect of acetylene on methane emission was studied. Two rice varieties (Toyohatamochi and Yamahikari) were grown in a greenhouse in submerged soil in pots. At about maximum tillering, heading, and grain-forming stages, methane fluxes were measured. Flux measurement was performed for 3 h from 6 pm, then acetylene at 100 μl l^–1 was added to some of the pots. At 6 a.m. the following day, methane fluxes were again measured for 3 h. Only at maximum tillering stage of the variety Toyohatamochi was a significant increase (1.4 times) in methane flux caused by acetylene observed, whereas in the other treatments no significant increase in methane fluxes by acetylene could be defected. To ascertain the activity of methane monooxygenase (MMO), propylene oxide (PPO) formation from propylene was measured with excised roots and a basal portion of stems of the rice plants grown on the submerged soil. A level of 0.1–0.2 μmol PPO h^–1 plant^–1 was recorded. The roots showed the highest PPO formation per gram dry matter, followed by basal stems. Methane oxidation was roughly proportional to PPO formation. Soluble MMO-positive methanotroph populations were measured by plate counts. The number of colony-forming units per gram dry matter was 10⁶–10⁵ in roots, and 10⁴–10³ in basal stems. These results indicate the possibility of methane oxidation in association with wetland rice plants. Received: 26 October 1995     

Carbon dioxide,methane, and nitrous oxide fluxes in soil catena across the right bank of the Oka River (Moscow oblast)

The flux rates of carbon dioxide, methane, and nitrous oxide in the soils on autonomous, transitional, transitional-accumulative, and accumulative positions of a catena on the Oka River’s right bank (Moscow oblast) were assessed using the chamber method. The lowest rate of C-CO₂ emission (18.8–29.8 mg/m² per hour) was found for the gray forest soil in the autonomous position, and the highest rate (52.4–66.1 mg/m² per hour) was found for the alluvial meadow soil of the accumulative landscape. In the summer, the uptake of methane from the atmosphere exceeded its release from the soil at all the points of the catena (9–38 μg/m² per hour). The highest rate of the C-CH₄ uptake was observed for the soil in the transitional position. In the fall, the soils in the autonomous, transitional, and transitional-accumulative positions served as a sink of C-CH₄, and the soil of the accumulative position was a source of methane emission. The rate of the N-N₂O emission from the catena soils increased when going from the autonomous position to the accumulative one (0.41–11.2 μg/m² per hour). The spatial variation of the C-CO₂, C-CH₄, and N-N₂O fluxes within the catena was 33, 172, and 138%, respectively. The upper (0- to 10-cm) soil layer made the major contribution to the emission of carbon dioxide. This soil layer was characterized by its C-CH₄ uptake, and the emission of methane was typical for the deeper (0- to 20-cm) layer. The layers deeper than 10 and 20 cm emitted more N-N₂O than the surface layer.     

Analysis of solutions of the equation for the convective diffusion of ions in soil

New analytical solutions are proposed for the problem of the convective-dispersion transfer of salts in a soil layer of finite and semifinite thickness at the boundary conditions of the first and third kinds on the soil’s surface under the alternating (pulsating) impact of washing water. Numerical studies (computer experiments) of the salt transfer equation were performed for the equilibrium sorption and linear sorption isotherms in soils with deep and shallow groundwater tables and for nonequilibrium irreversible sorption (characterized by biological transformation in the soil solution following the first-order kinetics) to examine the effect of the upper and lower boundary conditions on the transfer of salts in a soil. The analysis revealed relationships between the parameters of the convective diffusion equation (for which simpler equations were proposed for calculating the irrigation rates from the average salt content in the soil layer before and after washing), which included the filtration rate, the physicochemical features of the soil and salts, and the degree and depth of the preset desalination. The use of the initial and permissible salt concentrations averaged for the studied layer was substantiated for the first time. After wide experimental validation, the obtained equations can be used in soil-reclamation practice.     

Short-term kinetic response of enhanced methane oxidation in landfill cover soils to environmental factors

 This paper aims at a better understanding of methane oxidation under conditions that are representative of landfill cover soils. The kinetics of methane oxidation were studied in landfill cover soils that had been exposed to high methane mixing ratios. This was done in batch experiments, under various environmental conditions. V _max increased exponentially with temperature in the range 5–35  °C, with a Q ₁₀ value of 2.8. K _m increased approximately linearly in this range from 1.2 μM to 7 μM. Consequently, the influence of temperature on methane consumption was more pronounced at high concentrations than at low concentrations. The inhibition by ammonium of methane consumption was much stronger after 6–7 months of exposure to high methane mixing ratios than after 5–7 weeks of exposure, indicating that there was a shift of dominating methanotrophic species in soils after long exposure times. Additions of nitrifying sludge or compost to soils initially inhibited methane oxidation, followed by a stimulation after a few days. Received: 19 May 2000     

Short-term effects of nitrogen on methane oxidation in soils

 The short-term effects of N addition on CH₄ oxidation were studied in two soils. Both sites are unfertilized, one has been under long-term arable rotation, the other is a grassland that has been cut for hay for the past 125 years. The sites showed clear differences in their capacity to oxidise CH₄, the arable soil oxidised CH₄ at a rate of 0.013 μg CH₄ kg^–1 h^–1 and the grassland soil approximately an order of magnitude quicker. In both sites the addition of (NH₄)₂SO₄ caused an immediate reduction in the rate of atmospheric CH₄ oxidation approximately in inverse proportion to the amount of NH₄ ⁺ added. The addition of KNO₃ caused no change in the rate of CH₄ oxidation in the arable soil, but in the grassland soil after 9 days the rate of CH₄ oxidation had decreased from 0.22 μg CH₄ kg^–1 h^–1 to 0.13 μg CH₄ kg^–1 h^–1 in soil treated with the equivalent of 192 kg N ha^–1. A ¹⁵N isotopic dilution technique was used to investigate the role of nitrifiers in regulating CH₄ oxidation. The arable soil showed a low rate of gross N mineralisation (0.67 mg N kg^–1 day^–1), but a relatively high proportion of the mineralised N was nitrified. The grassland soil had a high rate of gross N mineralisation (18.28 mg N kg^–1 day^–1), but negligible nitrification activity. It is hypothesised that since there was virtually no nitrification in the grassland soil then CH₄ oxidation at this site must be methanotroph mediated. Received: 31 October 1997     

Kinetics and binding capacity of six soils for structurally defined hydrolyzable and condensed tannins and related phenols

Purpose  

We investigated tannin–soil interactions by assessing the kinetics of sorption and sorption capacities, and their relationship to the chemical properties of six polyphenolic compounds and the textures of six soils. We developed a new extraction procedure for recovering tannins from soil samples by successive extraction with solvents of decreasing polarity.     

Methane uptake in Swedish forest soil in relation to liming and extra N-deposition

Methane uptake to soil was examined in individual chambers at three small forest catchments with different treatments, Control, Limed and Nitrex sites, where N-deposition was experimentally increased. The catchments consisted of both well-drained forest and wet sphagnum areas, and showed uptake of CH₄ from the ambient air. The lowest CH₄ uptakes were observed in the wet areas, where the different treatments did not influence the uptake rate. In the well-drained areas the CH₄ uptakes were 1.6, 1.4 and 0.6 kg ha^–1 year^–1 for the Limed, Control and Nitrex sites, respectively. The uptake of methane at the well-drained Nitrex site was statistically smaller than at the other well-drained catchments. Both acidification and increase in nitrogen in the soil, caused by the air-borne deposition, are the probable cause for the reduction in the methane uptake potential. Uptake of methane was correlated to soil water content or temperature for individual chambers at the well-drained sites. The uptake rate of methane in soil cores was largest in the 0- to 10-cm upper soil layer. The concentration of CH₄ in the soil was lower than the atmospheric concentration up to 30 cm depth, where methane production occurred. Besides acting as a sink for atmospheric methane, the oxidizing process in soil prevents the release of produced methane from deeper soil layers reaching the atmosphere. Received: 27 September 1996     

Labile organic C and N mineralization of soil aggregate size classes in semiarid grasslands as affected by grazing management

Soil labile organic carbon (C) oxidation drives the flux of carbon dioxide (CO₂) between soils and the atmosphere. However, the impact of grazing management and the contribution soil aggregate size classes (ASCs) to labile organic C from grassland soils is unclear. We evaluated the effects of grazing intensity and soil ASC on the soil labile organic C, including CO₂ production, microbial biomass C, and dissolved organic C and nitrogen (N) mineralization in topsoils (0–10 cm) in Inner Mongolia, Northern China. Soil samples were separated into ASCs of 0–630 μm [fine ASC (fASC)], 630–2000 μm [medium ASC (mASC)] and >2000 μm [coarse ASC (cASC)]. The results showed that heavy grazing (HG) and continuous grazing (CG) increased soil labile organic C significantly compared to an ungrazed site since 1999 (UG99) and an ungrazed site since 1979 (UG79). For winter grazing site (WG), no significant differences were found. CO₂ production was highest in cASC, while lowest in fASC. Microbial biomass C and dissolved organic C showed the highest values in mASC and were significantly lower in fASC. Grazing increased N mineralization in bulk soils, while it exhibited complex effects in the three ASCs. The results suggest that the rate of C mineralization was related to the rate of N accumulation. To reduce CO₂ emission and nutrient loss, and to improve soil quality and productivity, a grazing system with moderate intensity is suggested.     

Pyrene effects on methanotroph community and methane oxidation rate,tested by dose–response experiment and resistance and resilience experiment

Purpose  

Methanotrophs are an important group of bacteria that can metabolize methane. Polycyclic aromatic hydrocarbons (PAHs) are widespread contaminants and present in all ecosystems. We hypothesize that PAHs may affect methanotrophs and methane oxidation. In this study, we assessed dose–response curves for the inhibition of methane oxidation and methanotrophs diversity by pyrene, and resistance and resilience of soil methane oxidation rate and methanotrophs composition in response to pyrene contamination.     

Speciation and biochemical transformations of sulfur and copper in rice rhizosphere and bulk soil—XANES evidence of sulfur and copper associations

Purpose  

Contamination of heavy metals in soil and its subsequent accumulation along the food chain is a potential risk to human health. Cu speciation in soil–plant system, particularly on the availability to plant roots, has obtained great attention. X-ray absorption near-edge structure spectroscopy (XANES) provides information about the bonding of Cu soil components at the molecular scale. In paddy soils, changes of redox conditions led to microbially mediated sulfur transformation, thus affecting heavy metal behavior. The objective of this work was to investigate how sulfur transformation in a paddy soil affected Cu biogeochemical processes.     

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.     

Short-term carbon mineralization in saline–sodic soils

Previous studies have shown that carbon (C) mineralization in saline or sodic soils is affected by various factors including organic C content, salt concentration and water content in saline soils and soil structure in sodic soils, but there is little information about which soil properties control carbon dioxide (CO₂) emission from saline-sodic soils. In this study, eight field-collected saline–sodic soils, varying in electrical conductivity (EC_e, a measure of salinity, ranging from 3 to 262 dS m⁻¹) and sodium adsorption ratio (SAR_e, a measure of sodicity, ranging from 11 to 62), were left unamended or amended with mature wheat or vetch residues (2% w/w). Carbon dioxide release was measured over 42 days at constant temperature and soil water content. Cumulative respiration expressed per gram SOC increased in the following order: unamended soil<soil amended with wheat residues (C/N ratio 122)<soil with vetch residue (C/N ratio 18). Cumulative respiration was significantly (p < 0.05) negatively correlated with EC_e but not with SAR_e. Our results show that the response to EC_e and SAR_e of the microbial community activated by addition of organic C does not differ from that of the less active microbial community in unamended soils and that salinity is the main influential factor for C mineralization in saline–sodic soils.     

Methane uptake in salt-affected soils shows low sensitivity to salt addition

Methane oxidation in aerated soils is a significant sink for atmospheric methane (CH₄). Salt-affected soils are extensively present and constitute about 7% of total land surface. However, our knowledge about CH₄ turnover between the atmosphere and the saline soils is very limited. In order to evaluate the potential of CH₄ consumption in saline soils, CH₄ fluxes were measured in intact cores of the slightly (ECe = 3.2 mS cm⁻¹), moderately (ECe = 7.1 mS cm⁻¹) and extremely (ECe = 50.7 mS cm⁻¹ and 112.6 mS cm⁻¹) saline soils from the Yellow River Delta, China. CH₄ uptake of cores from the slightly saline soil ranged from 14 to 24 μg CH₄-C m⁻² h⁻¹, comparable to those in the non-saline forest soils with similar texture. CH₄ uptake of cores from the moderately saline soil was only about 6% of that in the slightly saline soil. CH₄ uptake was too low to be measurable in the extremely saline soil. Compared with the non-saline soil, CH₄ uptake in the saline soils was much less sensitive to salt, suggesting the higher salt-tolerance of CH₄ oxidizers in the saline soil. The result also indicated an underestimate in CH₄ uptake for the naturally-occurring saline soils by adding salt to non-saline soils. These results should be useful to study the global CH₄ budget and to explore the physiological and ecological characteristics of methanotrophic bacteria in the salt-affected soils.