Water-soluble Al inhibits methane oxidation at atmospheric concentration levels in Japanese forest soil

The effect of aluminium on methane oxidation was examined from incubation experiments involving the addition of several concentrations of Al solution (0.1, 0.2, 0.5, 1, 3 and 5 mM) to two soil samples that possessed different CH₄ oxidation potential. Atmospheric CH₄ oxidation activity was inhibited by the addition of as little as 0.1 mM Al solution (approximately 0.5 μg of Al per gram dry weight soil) to a forest soil that contained low water-soluble Al and possessed a high CH₄ oxidation potential. Our results indicate that Al inhibition of CH₄ oxidation activity is concentration-dependant after a certain time and the inhibition increases gradually over time until at least 96 h have elapsed. We also found that relatively small amounts of Al additions, such as 10-20 μg per gram dry weight of soil, halved the CH₄ oxidation rate compared to the control, regardless of the original CH₄ oxidation potential of the soil. Since the Al concentrations used in our experiment are often observed in forest soils, we can assume that Al acts as an important inhibitor of CH₄ oxidation in forest soils under natural conditions. The sharp falls and a continuous decrease in CH₄ oxidation rate in other forest samples with the addition of deionized water implies that the water-soluble Al contained in soils contributes to the inhibition of CH₄ oxidation rate. This result suggests that precipitation causes a relatively prolonged inhibition of CH₄ oxidation in soils containing a high concentration of water-soluble Al.     

Methane oxidation in a temperate coniferous forest soil: effects of inorganic N

Methane oxidation rates were measured in soils obtained from a coniferous forest in northern England. The effects of depth and added K⁺ (K₂SO₄), NH₄⁺ ((NH₄)₂SO₄) and NO₃⁻ (KNO₃) on potential CH₄ oxidation were investigated in a series of laboratory incubations. The humus (H) layer soil showed much greater CH₄ oxidation rates than the other soil layers, with maximal rates of 53 and 226 ng CH₄ gdw⁻¹ h⁻¹ when incubated with initial 10 and 1000 μl CH₄ l⁻¹, respectively. Additions of the solutes K⁺, NH₄⁺ and NO₃⁻ showed differing degrees of inhibition on CH₄ oxidation, which varied with the initial CH₄ concentration, the ion added, and the ion concentration. In general, inhibition by the ions was slightly greater for incubations with an initial concentration of 1000 μl CH₄ l⁻¹ than for 10 μl CH₄ l⁻¹ under otherwise identical conditions. For K⁺ and NH₄⁺ treatments, inhibitory rates were usually less than 15%, but at high K⁺ and NH₄⁺ concentrations inhibition could reach 50%, the inhibitory effects of NH₄⁺ were consistently slightly greater than those of K⁺ at the same concentration. In marked contrast to NH₄⁺, NO₃⁻ showed a very strong inhibitory effect. Added NO₃⁻ and NO₂⁻ produced via added NO₃⁻ reduction in anaerobic ‘microsites’ are probably toxic to CH₄-oxidizing bacteria. These results, together with those from other reports, suggest that NO₃⁻ may have a greater importance in the inhibition of CH₄ oxidation in forest soils than that attributed to NH₄⁺ and needs to be investigated in a wide range of soil types from various forests.     

Influence of N and non-N salts on atmospheric methane oxidation by upland boreal forest and tundra soils

 The short-term (24 h) and medium-term (30 day) influence of N salts (NH₄Cl, NaNO₃ and NaNO₂) and a non-N salt (NaCl) on first-order rate constants, k (h^–1) and thresholds (C_Th) for atmospheric CH₄ oxidation by homogenized composites of upland boreal forest and tundra soils was assessed at salt additions ranging to 20 μmol g^–1 dry weight (dw) soil. Additions of NH₄Cl, NaNO₃ and NaCl to 0.5 μmol g^–1 dw soil did not significantly decrease k relative to watered controls in the short term. Higher concentrations significantly reduced k, with the degree of inhibition increasing with increasing dose. Similar doses of NH₄Cl and NaCl gave comparable decreases in k relative to controls and both soils showed low native concentrations of NH₄ ⁺-N (≤1 μmol g^–1dw soil), suggesting that the reduction in k was due primarily to a salt influence rather than competitive inhibition of CH₄ oxidation by exogenous NH₄ ⁺-N or NH₄ ⁺-N released through cation exchange. The decrease in k was consistently less for NaNO₃ than for NH₄Cl and NaCl at similar doses, pointing to a strong inhibitory effect of the Cl^– counter-anion. Thresholds for CH₄ oxidation were less sensitive to salt addition than k for these three salts, as significant increases in C_Th relative to controls were only observed at concentrations ≥1.0 μmol g^–1 dw soil. Both soils were more sensitive to NaNO₂ than to other salts in the short term, showing a significant decrease in k at an addition of 0.25 μmol NaNO₂ g^–1 dw soil that was clearly attributable to NO₂ ^–. Soils showed no recovery from NaCl, NH₄ ⁺-N or NaNO₃ addition with respect to atmospheric CH₄ oxidation after 30 days. However, soils amended with NaNO₂ to 1.0 μmol NaNO₂ g^–1 dw showed values of k that were not significantly different from controls. Recovery of CH₄-oxidizing ability was due to complete oxidation of NO₂ ^–-N to NO₃ ^–-N. Analysis of soil concentrations of N salts necessary to inhibit atmospheric CH₄ oxidation and regional rates of N deposition suggest that N deposition will not decrease the future sink strength of upland high-latitude soils in the atmospheric CH₄ budget. Received: 30 April 1999     

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- and long-term effects of nitrogen fertilization on methane oxidation in three Swedish forest soils

 Under normal conditions, CH₄, one of the most important greenhouse gases, is subject to biological oxidation in forest soils. However, this process can be negatively affected by N amendment. The reported experiment was conducted in order to study the short- and long-term effects of N amendment on CH₄ oxidation in pine (Pinus sylvestris L.) forest soils. Soil samples were taken from three experimental sites, two of which had been amended with N once, over 20 years earlier, while the third had been amended 3 weeks earlier. The soil samples were incubated fresh at 15  °C at ambient CH₄ concentrations (ca. 1.8 ppmv CH₄). The variation in CH₄-turnover rates was high within the treatments: CH₄ was produced [up to 22.6 pmol CH₄ g dry wt. soil^–1 h^–1] in samples from the recently amended site, whereas it was consumed at high rates (up to 431 pmol CH₄ g dry wt. soil^–1 h^–1) in samples from the plot that had received the highest N amendment 27 years before sampling. Although no significant differences were found between N treatments, in the oldest plots there was a correlation between consumption of atmospheric CH₄ and the total C content at a depth of 7.5–15 cm in the mineral soil (r ²=0.74). This indicates that in the long-term, increased C retention in forest soils following N amendment could lead to increased CH₄ oxidation. Received: 3 September 1997     

Dimethyl sulphoxide (DMSO) and dimethyl sulphide (DMS) as inhibitors of methane oxidation in forest soil

Dimethyl sulphoxide (DMSO) at 14 mM inhibits CH₄ oxidation in forest soil, but the inhibition mechanism is unknown. When soil slurries are incubated in gas tight flasks, there is a lag of about 45 h before DMSO inhibits CH₄ oxidation. We tried to determine if some metabolic compound derived from DMSO, as a result of microbial activity, is responsible for the inhibition. Dimethyl sulphide (DMS) accumulated in the sealed flasks up to 5-83 μl l⁻¹ in the headspace during a 2-week period. DMS at 1 μl l⁻¹ in the headspace (0.64 μM in soil-water slurry) had a negligible effect on CH₄ oxidation but 50 μl l⁻¹ DMS (32 μM) inhibited CH₄ oxidation completely. However, the inhibition by DMSO was already evident after 45 h, when DMS concentrations were generally non-inhibiting (0.1-0.7 μl l⁻¹). DMSO was also shown to inhibit CH₄ oxidation when the DMS produced was continuously removed. Results suggest that the production of DMS from DMSO makes a minor contribution to the inhibition of CH₄ oxidation by DMSO with incubation times relevant in CH₄ oxidation studies.     

Methane oxidation in boreal forest soils: kinetics and sensitivity to pH and ammonium

In soil incubation experiments we examined if there are differences in the kinetic parameters of atmospheric methane (CH₄) oxidation in soils of upland forests and forested peatlands. All soils showed net uptake of atmospheric CH₄. One of the upland forests included also managed (clear-cut with or without previous liming or N-fertilization) study plots. The CH₄ oxidation in the forested peat soil had a higher K_m (510 μl l⁻¹) and V_max (6.2 nmol CH₄ cm⁻³ h⁻¹) than the upland forest soils (K_m from 5 to 18 μl l⁻¹ and V_max from 0.15 to 1.7 nmol CH₄ cm⁻³ h⁻¹). The forest managements did not affect the K_m-values. At atmospheric CH₄ concentration, the upland forest soils had a higher CH₄ oxidation activity than the forested peat soil; at high CH₄ concentrations the reverse was true. Most of the soils oxidised CH₄ in the studied pH range from 3 to 7.5. The pH optimum for CH₄ oxidation varied from 4 to 7.5. Some of the soils had a pH optimum for CH₄ oxidation that was above their natural pH. The CH₄ oxidation in the upland forest soils and in the peat soil did not differ in their sensitivities to (NH₄)₂SO₄ or K₂SO₄ (used as a non-ammonium salt control). Inhibition of CH₄ oxidation by (NH₄)₂SO₄ resulted mainly from a general salt effect (osmotic stress) though NH₄⁺ did have some additional inhibitory properties. Both salts were better inhibitors of CH₄ oxidation than respiration. The differences in the CH₄ oxidation kinetics in the forested peat soil and in the upland forest soils reveal that there are differences in the physiologies of the CH₄ oxidisers in these soils.     

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.     

Ethylene production and decomposition in soils

Six soils differing in texture and use were investigated for their ability to produce and decompose ethylene. In addition, changes in methane and CO₂ concentrations were monitored. The effects of organic amendments and different water tensions were studied, and a method using low concentrations of acetylene as an inhibitor of ethylene degradation was tested. Possible reduction of acetylene to ethylene was identified by the use of CO or NH₄ ⁺-N, of which the latter turned out to be the more reliable method. This reduction only occurred in a grassland soil. Under aerobic soil conditions, gross ethylene production rates of up to 4.7pmol g^–1 h^–1 could be measured. Highest ethylene production and lowest ethylene decomposition was detected in a spruce forest soil. Fine textured soils produced more ethylene than coarse textured soils. Amended soils produced more ethylene at –100kPa and –5kPa than at 0kPa water tension. Ethylene decomposition was most effective in soils from deciduous woodlands and reached rates of up to 137pmol g^–1 h^–1. Parallels between ethylene and methane decomposition were observed. The addition of 5mgg^–1 glucose and 1mgg^–1 methionine not only promoted ethylene production but also inhibited ethylene decomposition. Received: 4 April 1997     

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.     

Consequences of nitrogen fertilization on soil methane consumption in a productive temperate deciduous forest

To investigate the consequences of long-term N additions on soil CH₄ dynamics, we measured in situ CH₄ uptake rates, soil profiles and kinetics parameters during the growing season in a temperate deciduous forest in northwestern Pennsylvania (Allegheny College Bousson Environmental Forest). Measurements were made in control and adjacent plots amended with 100 kg N ha^–1 year^–1 for 8 years. We found that the in situ consumption rates were 0.19±0.02 (mean±SE) for the control and 0.12±0.01 mg CH₄–C m^–2 h^–1 for the N treatment, indicating that consumption had been reduced by 35% after 8 years of N amendments. Despite the large difference in rates of consumption, there were no differences in the CH₄ concentration profiles between the control and N-amended plots. Laboratory incubations of CH₄ consumption throughout the soil column (organic horizon and mineral soil depths) showed that rates were greatest in the organic horizon of both control and N-amended soils, although consumption was reduced by 42% in the N-amended plot. However, the rate in the organic horizon was only about 50% the rate measured in organic horizons at other temperate forests. The apparent K_m [K_m(app)] value in the organic horizon of the control plot was fourfold less than the K_m(app) value in the organic horizon of another temperate forest, but similar to the K_m(app) values in adjacent plots amended with N for a decade. Unlike results for other temperate forests, K_m(app) values at Bousson generally did not decrease with soil depth. These results indicate that N cycling strongly controls the CH₄-consuming community, and suggest that alterations of the N cycle due to N deposition or addition may alter rates and the location of CH₄ consumption by soils, even in soils with high N content and cycling rates.     

Suppression of methane oxidation in aerobic soil by nitrogen fertilizers,nitrification inhibitors,and urease inhibitors

Concentrations of CH₄, a potent greenhouse gas, have been increasing in the atmosphere at the rate of 1% per year. The objective of these laboratory studies was to measure the effect of different forms of inorganic N and various N-transformation inhibitors on CH₄ oxidation in soil. NH ₄ ⁺ oxidation was also measured in the presence of the inhibitors to determine whether they had differential activity with respect to CH₄ and NH ₄ ⁺ oxidation. The addition of NH₄Cl at 25 g N g^-1 soil strongly inhibited (78–89%) CH₄ oxidation in the surface layer (0–15 cm) of a fine sandy loam and a sandy clay loam (native shortgrass prairie soils). The nitrification inhibitor nitrapyrin (5 g g^-1 soil) inhibited CH₄ oxidation as effectively as did NH₄Cl in the fine sandy loam (82–89%), but less effectively in the sandy clay loam (52–66%). Acetylene (5 mol mol^-1 in soil headspace) had a strong (76–100%) inhibitory effect on CH₄ consumption in both soils. The phosphoroamide (urease inhibitor) N-(n-butyl) thiophosphoric triamide (NBPT) showed strong inhibition of CH₄ consumption at 25 g g^-1 soil in the fine sandy loam (83%) in the sandy clay loam (60%), but NH ₄ ⁺ oxidation inhibition was weak in both soils (13–17%). The discovery that the urease inhibitor NBPT inhibits CH₄ oxidation was unexpected, and the mechanism involved is unknown.     

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.     

施用铵态氮对森林土壤硝态氮和铵态氮的影响

对取自武夷山的红壤、黄壤、黄壤性草甸土分别在对照(CK,N 0 mg/kg)、低氮(LN,N 50 mg/kg)、高氮(HN,N 100 mg/kg)3种氮(N)水平处理下开展培养实验,研究施加NH4+-N对森林土壤N转化的短期影响.结果表明,添加NH4+-N可显著(p＜0.05)降低土壤NO3--N含量4.5％～25.7％,但LN与HN处理差异不显著,NO3--N降低可能与NO3--N反硝化和异氧还原有关;然而,黄壤性草甸土NO3--N没有降低.与培养前比较,在第56天红壤NO3--N含量显著增加5倍左右;桐木关黄壤增加40％左右,而黄冈山25 km黄壤仅在CK处理下增加16％,但是黄壤性草甸土显著降低;结果显示LN与HN处理土壤NO3--N含量变化幅度小于CK.与CK相比,LN和HN处理红壤NH4+-N分别显著(p＜0.05)升高24.1％ ～ 96.5％和68.7％～114.1％,且随培养进行没有累积,可能与微生物固N有关;桐木关NH4+-N分别升高17.6％ ～ 39.6％和37.6％～95.8％ (p＜0.05),LN处理黄冈山25 km黄壤NH4+-N只有第7天升高17.8％ (p＜0.05),HN处理第7、14、28、42天显著升高17.5％～48.6％(p＜0.05).LN处理黄壤性草甸土的NH4+-N在前3周显著降低11.6％～28.5％ (p＜0.01); HN处理在第7天和14天分别降低10.8％(p＜0.01)和7.5％,但是在第28～56天显著增加17.6％～20.4％(p=0.002).随着培养进行,CK处理红壤NH4+-N逐渐降低,桐木关黄壤、黄冈山25 km黄壤和黄壤性草甸土升高;LN和HN处理黄壤和黄壤性草甸土NH4+-N逐渐升高.可见,不同海拔土壤类型对NH4+-N添加响应存在差异.     

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     

Microbial ethylene production and inhibition of methanotrophic activity in a deciduous forest soil

Production of C₂H₄, but not of CH₄, was observed in anoxically incubated soil samples (cambisol on loamy sand) from a deciduous forest. Ethylene production was prevented by autoclaving, indicating its microbial origin. Ethylene production gradually decreased from 4 to 12 cm soil depth and was not affected by moisture or addition of methionine, a possible precursor of C₂H₄. Oxidation of atmospheric CH₄ in soil samples was inhibited by C₂H₄. Ethylene concentrations of 3, 6 and 10 μl l⁻¹ decreased CH₄ uptake by 21, 63 and 98%, respectively. Methionine and methanethiol, a possible product of methionine degradation, also inhibited CH₄ oxidation. Under oxic conditions, C₂H₄ was consumed in the soil samples. Ethylene oxidation kinetics exhibited two apparent K_m values of 40 μl l⁻¹ and 12,600 μl l⁻¹ suggesting the presence of two different types of C₂H₄-oxidizing microorganisms. Methanotrophic bacteria were most probably not responsible for C₂H₄ oxidation, since the maximum of C₂H₄ oxidation activity was localized in soil layers (2-8 cm depth) above those (8-10 cm depth) of CH₄ oxidation activity. Our observations suggest that C₂H₄ production in the upper soil layers inhibits CH₄ oxidation, thus being one reason for the localization of methanotrophic activity in deeper soil layers.     

Effect of tree species on methane and ammonium oxidation capacity in forest soils

High and low affinity methane oxidation potentials were measured for soils under five fully replicated land-use treatments over an entire calendar year. Simultaneous measurements of soil nitrification potential in replicate soil samples were also made. Both high and low affinity CH₄ oxidation were significantly reduced in the nitrate-rich soils under alder, compared to the other four vegetation treatments (oak, Norway spruce, Scots pine and grass). However, the effect of land-use was less for high affinity methanotrophy than for low affinity CH₄ oxidation. Nitrification rates were highest in alder soils, with the greatest potential for oxidation occurring in the top 5 cm of the soil. No significant relationship between potential nitrification rate and low affinity CH₄ oxidation was seen. However, a significant negative relationship between nitrification and high affinity CH₄ oxidation was identified. We found vegetation type to be a key determinant of soil-mediated CH₄ and oxidation, but found no evidence for significant CH₄ oxidation by nitrifying bacteria.     

Strong inhibitory effect of nitrate on atmospheric methane oxidation in forest soils

We investigated inhibitory effects of ammonium and nitrate on soil methane oxidation in two forests. Mineral soils in a forest with low nitrogen-deposition had much higher methane oxidation rates than those observed in nitrogen-saturated forest soils with higher nitrogen-deposition. Nitrate addition to soils from a low nitrogen-deposition site exhibited a stronger inhibitory effect on methane oxidation than either ammonium or potassium. The decrease in soil pH accompanied by the addition of nitrate was not responsible for this repression. Our results indicate that even a level of nitrate observed in N-saturated forests (dozens of μg N g soil⁻¹) exerts a strong inhibitory effect on the oxidation of atmospheric methane, although the mechanism remains unresolved.     

Soluble organic nitrogen in temperate forest soils

Very few studies have been related to soluble organic nitrogen (SON) in forest soils. However, this nitrogen pool could be a sensitive indicator to evaluate the soil nitrogen status. The current study was conducted in temperate forests of Thuringia, Germany, where soils had SON (extracted in 0.5 M K₂SO₄) varying from 0.3 to 2.2% of total N, which was about one-third of the soil microbial biomass N by CFE. SON in study soils were positively correlated to microbial biomass N and soil total N. Multiple regression analysis also showed that mineral N negatively affected SON pool. The dynamics of the SON was significantly affected by mineralization and immobilization. During the 2 months of aerobic incubation, the SON were significantly correlated with net N mineralization and microbial biomass N. SON extracted by two different salt solution (i.e. 1 M KCl and 0.5 M K₂SO₄₎ were highly correlated. In mineral soil, SON concentrations extracted by 1 M KCl and 0.5 M K₂SO₄ solutions were similar. In contrast, in organic soil layer the amount of KCl-extractable SON was about 1.2-1.4 times higher than the K₂SO₄-extractable SON. Further studies such as the differences of organic N form and pool size between SON and dissolved organic N (DON) are recommended.     

The influence of land use and pesticides on methane oxidation in some Belgian soils

 In a first experiment, the effect of land use on the uptake rate of atmospheric CH₄ was studied in laboratory incubations of intact soil cores. A soil under deciduous forest showed the highest CH₄ oxidation. Its overall CH₄ uptake during the measuring period (202 days) was 1.03 kg CH₄ ha^–1. Natural grassland showed the second highest CH₄ oxidizing capacity (0.71 kg CH₄ ha^–1). The overall amount of CH₄ uptake by fertilized pasture was 0.33 kg CH₄ ha^–1. CH₄ oxidation in arable soils with different fertilizer treatments varied between 0.34 and 0.37 kg CH₄ ha^–1. Undisturbed soils had a higher CH₄ uptake capacity than agricultural soils. The moisture content of the soil was found to be an important parameter explaining temporal variations of CH₄ oxidation. Different methods of fertilization which had been commenced 10 years previously were not yet reflected in the total CH₄ uptake rate of the arable soil. In a second experiment, a number of frequently used pesticides were screened for their possible effect on CH₄ oxidation. In a sandy arable soil lenacil, mikado and oxadixyl caused significantly reduced CH₄ oxidation compared to the control. Under the same conditions, but in a clayey arable soil, mikado, atrazine and dimethenamid caused a reduction of the CH₄ uptake. In a landfill cover soil, with a 100-fold higher CH₄ oxidation rate, no inhibition of CH₄ oxidation was observed, not even when the application rate of pesticides was tenfold higher than usual. Received: 1 December 1998