Responses of methane oxidation to temperature and water content in cover soil of a boreal landfill

Methane oxidation in a cover soil of a landfill located in a boreal climate was studied at temperatures ranging from 1-19 °C and with water content of 7-34% of dry weight (dw), corresponding to 17-81% of water-holding capacity (WHC) in order to better understand the factors regulating CH₄ oxidation at low temperatures. CH₄ consumption was detected at all the temperatures studied (1-19 °C) and an increase in CH₄ consumption rate in consecutive incubations was obtained even at 1 °C, indicating activation or increase in enzymes and/or microorganisms responsible for CH₄ oxidation. CH₄ consumption was reduced with low water content (17%WHC) at all temperatures. The response of CH₄ consumption to temperature was high with Q₁₀ values from 6.5 to 8.4 and dependent on water content: at 33%WHC or more an increase in water content was accompanied by a decrease in Q₁₀ values. The responses of CH₄ consumption to water content varied at different temperatures so that at 1-6 °C, CH₄ consumption increased along with water content (33-67%WHC) while at 12-19 °C the response was curvilinear, peaking at 50%WHC. CH₄ consumption was less tolerant (higher Q₁₀ values; 6.5-8.4) of low temperatures compared to basal respiration (Q₁₀ values for CO₂ production and O₂ consumption 3.2-4.0). Overall, the present results demonstrate the presence of CH₄-oxidizing microorganisms, which are able to consume CH₄ and to be activated or grow at low temperatures, suggesting that CH₄ oxidation can reduce atmospheric CH₄ emissions from methanogenic environments even in cold climates.     

Micro-scale CO2 and CH4 dynamics in a peat soil during a water fluctuation and sulfate pulse

We investigated the response of CO₂ and CH₄ production to a water table fluctuation and a SO₄²⁻ pulse in a bog mesocosm. Net gas production rates in the mesocosm were calculated from concentration data by diffusive mass-balances. Incubation experiments were used to quantify the effect of SO₄²⁻ addition and the distribution of potential CO₂ and CH₄ production rates. Flooding of unsaturated peat resulted in rapid depletion of O₂ and complex patterns of net CH₄, CO₂, and H₂S production. Methane production began locally and without a time lag at rates of 3-4 nmol cm⁻³ d⁻¹ deeper in the peat. Similar rates were determined after a time lag of 10-60 days in the surface layers, whereas rates at lower depths declined. Net CO₂ production was largest immediately after the water table position was altered (100-300 nmol cm⁻³ d⁻¹) and declined to −50-50 nmol cm⁻³ d⁻¹ after a few weeks. SO₄²⁻ addition (500 mM) significantly increased potential CH₄ production rates in the surface layer from an average of 132-201 nmol cm⁻³ d⁻¹ and reduced it below from an average of 418-256 nmol cm⁻³ d⁻¹. Our results suggest that deeper in the peat (40-70 cm) under in situ conditions, methanogenic populations are less impaired by unsaturated conditions than in the surface layers, and that at these depths after flooding the substrate availability for CH₄ and DIC production is significantly enhanced. They also suggest that methanogenic and SO₄²⁻-reducing activity were non-competitive in the surface layer, which might explain contradictory findings from field studies.     

Saccharolytic activity and its role as a limiting step in methane formation during the anaerobic degradation of rice straw in rice paddy soil

Rice straw polysaccharides are one of the major C sources for CH₄ formation in anoxic rice paddy soils. We investigated the initial step of straw degradation by measuring the substrate-saturated activities of the polysaccharolytic enzymes #-glucosidase, exo-#-1,4-glucanase and xylosidase using substrates labelled with methylumbelliferone (MUF). The actual activity of the enzymes was measured by the release of reducing sugars after the inhibition of microbial carbohydrate uptake by toluene. The substrate-saturated enzyme activities increased during the first 11 days of incubation, while the actual activities decreased, presumably due to the decreasing access of straw polysaccharides to hydrolytic enzymes. The temporal progress of polysaccharide hydrolysis, transient accumulation of fermentation products and CH₄ production indicated five distinct phases. In phase I (<8 h), the fermentation of sugar monomers released by hydrolysis of polysaccharides was limiting. In phase II (2 and acetate accumulated since the activity of methanogens was low, though increasing exponentially. In phase III (days 3-10), H₂ was also consumed by respiratory processes (e.g. SO₄^2- reduction) so that H₂-dependent methanogenesis sometimes became substrate limited. However, acetate still accumulated, probably due to the limiting activity of acetoclastic methanogens. In phase IV (days 10-18), methanogenic activity was no longer limited and acetate was depleted to low concentrations. In phase V (>day 18), the methanogenic degradation of straw reached a quasi-steady state, when polysaccharide hydrolysis became the rate-limiting step for CH₄ formation.     

Influence of warming and enchytraeid activities on soil CO2 and CH4 fluxes

To determine the sum of ‘direct’ and ‘indirect’ effects of climatic change on enchytraeid activity and C fluxes from an organic soil we assessed the influence of temperature (4, 10 and 15 °C incubations) on enchytraeid populations and soil CO₂ and CH₄ fluxes over 116 days. Moisture was maintained at 60% of soil dry weight during the experimental period and measurements of enchytraeid biomass and numbers, and CO₂ and CH₄ fluxes were made after 3, 16, 33, 44, 65, 86 and 116 days. Enchytraeid population numbers and biomass increased in all temperature treatments with the greatest increase produced at 15 °C (to over threefold initial values by day 86). Results also showed that enchytraeid activity increased CO₂ fluxes by 10.7±4.5, 3.4±4.0 and 26.8±2.6% in 4, 10 and 15 °C treatments, respectively, with the greatest CO₂ production observed at 15 °C for the entire 116 day incubation period (P<0.05). The soil respiratory quotient analyses at lower temperatures (i.e. 4-10 °C) gave a Q₁₀ of 1.7 and 1.9 with and without enchytraeids, respectively. At temperatures above 10 °C (i.e. 10-15 °C) Q₁₀ significantly increased (P<0.01) and was 25% greater in the presence of enchytraeids (Q₁₀=3.4) than without (Q₁₀=2.6). In contrast to CO₂ production, no significant relationships were observed between net CH₄ fluxes and temperature and only time showed a significant effect on CH₄ production (P<0.01).Total soil CO₂ production was positively linked with enchytraeid biomass and mean soil CO₂-C production was 77.01±6.05 CO₂-C μg mg enchytraeid tissue⁻¹ day⁻¹ irrespective of temperature treatment. This positive relationship was used to build a two step regression model to estimate the effects of temperature on enchytraeid biomass and soil CO₂ respiration in the field. Predictions of potential CO₂ production were made using enchytraeid biomass data obtained in the field from two upland grassland sites (Sourhope and Great Dun Fell at the Moor House Nature Reserve, both in the UK). The findings of this work suggest that a 5 °C increase in atmospheric temperature above mean ambient temperature could have the potential to produce a significant increase in enchytraeid biomass resulting in a near twofold increase in soil CO₂ release from both soil types. The interaction between temperature and soil biology will clearly be an important determinant of soil respiration responses to global warming.     

Loss of low molecular weight dissolved organic carbon (DOC) and nitrogen (DON) in H2O and 0.5 M K2SO4 soil extracts

Soil extracts are routinely used to quantify dissolved organic nutrient concentrations in soil. Here we studied the loss and transformation of low molecular weight (LMW) components of DOC (¹⁴C-glucose, 1 and 100 μM) and DON (¹⁴C-amino acid mixture, 1 and 100 μM) during extraction of soil (0-6 h) with either distilled water or 0.5 M K₂SO₄. The extractions were performed at 20 °C, at 4 °C, or in the presence of an inhibitor of microbial activity (HgCl₂ and Na-azide). We showed that both glucose and amino acids became progressively lost from solution with increasing shaking time. The greatest loss was observed in H₂O extracts at 1 μM for both substances (>90% loss after 15 min). Lower temperature (4 °C) and presence of K₂SO₄ both resulted in reduced loss rates. The presence of microbial inhibitors effectively eliminated the loss of glucose and amino acids. We conclude that microbial transformation of LMW-DOC and DON during H₂O or K₂SO₄ extraction of soil may affect the estimation of their concentrations in soil. This finding has significant implications for methods that rely on chemical extractions to estimate LMW-C components of DOC and DON.     

Elevated CO2 concentration and nitrogen fertilisation effects on N2O and CH4 fluxes and biomass production of Phleum pratense on farmed peat soil

The effects of elevated CO₂ supply on N₂O and CH₄ fluxes and biomass production of Phleum pratense were studied in a greenhouse experiment. Three sets of 12 farmed peat soil mesocosms (10 cm dia, 47 cm long) sown with P. pratense and equally distributed in four thermo-controlled greenhouses were fertilised with a commercial fertiliser in order to add 2, 6 or 10 g N m⁻². In two of the greenhouses, CO₂ concentration was kept at atmospheric concentration (360 μmol mol⁻¹) and in the other two at doubled concentration (720 μmol mol⁻¹). Soil temperature was kept at 15 °C and air temperature at 20 °C. Natural lighting was supported by artificial light and deionized water was used to regulate soil moisture. Forage was harvested and the plants fertilised three times during the basic experiment, followed by an extra fertilisations and harvests. At the end of the experiment CH₄ production and CH₄ oxidation potentials were determined; roots were collected and the biomass was determined. From the three first harvests the amount of total N in the aboveground biomass was determined. N₂O and CH₄ exchange was monitored using a closed chamber technique and a gas chromatograph. The highest N₂O fluxes (on average, 255 μg N₂O m⁻² h⁻¹ during period IV) occurred just after fertilisation at high water contents, and especially at the beginning of the growing season (on average, 490 μg N₂O m⁻² h⁻¹ during period I) when the competition of vegetation for N was low. CH₄ fluxes were negligible throughout the experiment, and for all treatments the production and oxidation potentials of CH₄ were inconsequential. Especially at the highest rates of fertilisation, the elevated supply of CO₂ increased above- and below-ground biomass production, but both at the highest and lowest rates of fertilisation, decreased the total amount of N in the aboveground dry biomass. N₂O fluxes tended to be higher under doubled CO₂ concentrations, indicating that increasing atmospheric CO₂ concentration may affect N and C dynamics in farmed peat soil.     

Effect of temperature on reduction of iron and production of carbon dioxide and methane in anoxic wetland rice soils

 Wetland rice soils from Italy (Pavia) and the Philippines (Bugallon, Luisiana, Maligaya) were incubated under anoxic conditions at 31 different temperatures ranging from 4.7  °C to 49.5  °C. Production of CO₂ was most intensive at the beginning of the incubation (0–4 days) and was predominantly coupled to the reduction of free Fe(III). The optimum temperature for these processes was between 32  °C and 41  °C. After 9–16 days, CO₂ production rates had decreased and the available Fe(III) had been completely reduced at the optimum temperatures. However, Fe(III) was still available at temperatures below and above the optimum. Maximum CH₄ production rates were observed after 4–16 days (except in soil from Maligaya) with temperature optima between 32  °C and 41  °C, similar to those for CO₂ production and Fe reduction. Since ongoing Fe reduction is known to suppress CH₄ production, the temperature range of optimum CH₄ production was restricted to those temperatures at which Fe(III) had already been depleted. Nevertheless, the temperature characteristics of both CO₂ and CH₄ production often exhibited two temperature optima at some time during the incubation, suggesting a complex pattern of adaptation of the methanogenic microbial community to temperature. When available Fe(III) was completely depleted by anoxic pre-incubation at 30  °C, CH₄ was produced at a constant rate (steady state conditions) which increased with increasing temperature. Steady state CH₄ production reached a first maximum at about 40  °C, but increased further up to at least 50  °C, suggesting the presence of thermophilic microorganisms whose activity was apparently masked when Fe had not been completely reduced. The apparent activation energy of CH₄ production at steady state ranged between 48 kJ mol^–1 and 65 kJ mol^–1. Received: 26 August 1999     

Methane uptake in soils from Pinus radiata plantations, a reverting shrubland and adjacent pastures: Effects of land-use change, and soil texture, water and mineral nitrogen

Afforestation and reforestation of pastures are key land-use changes in New Zealand that help sequester carbon (C) to offset its carbon dioxide (CO₂) emissions under the Kyoto Protocol. However, relatively little attention has been given so far to associated changes in trace gas fluxes. Here, we measure methane (CH₄) fluxes and CO₂ production, as well as microbial C, nitrogen (N) and mineral-N, in intact, gradually dried (ca. 2 months at 20 °C) cores of a volcanic soil and a heavier textured, non-volcanic soil collected within plantations of Pinus radiata D. Don (pine) and adjacent permanent pastures. CH₄ fluxes and CO₂ production were also measured in cores of another volcanic soil under reverting shrubland (mainly Kunzea var. ericoides (A. Rich) J. Thompson) and an adjacent pasture. CH₄ uptake in the pine and shrubland cores of the volcanic soils at field capacity averaged about 35 and 14 μg CH₄-C m⁻² h⁻¹, respectively, and was significantly higher than in the pasture cores (about 21 and 6 μg CH₄-C m⁻² h⁻¹, respectively). In the non-volcanic soil, however, CH₄-C uptake was similar in most cores of the pine and pasture soils, averaging about 7-9 μg m⁻² h⁻¹, except in very wet samples. In contrast, rates of CO₂ production and microbial C and N concentrations were significantly lower under pine than under pasture. In the air-dry cores, microbial C and N had declined in the volcanic soil, but not in the non-volcanic soil; ammonium-N, and especially nitrate-N, had increased significantly in all samples. CH₄ uptake was, with few exceptions, not significantly influenced by initial concentrations of ammonium-N or nitrate-N, nor by their changes on air-drying. A combination of phospholipid fatty acid (PLFA) and stable isotope probing (SIP) analyses of only the pine and pasture soils showed that different methanotrophic communities were probably active in soils under the different vegetations. The C18 PLFAs (type II methanotrophs) predominated under pine and C16 PLFAs (type I methanotrophs) predominated under pasture. Overall, vegetation, soil texture, and water-filled pore space influenced CH₄-C uptake more than did soil mineral-N concentrations.     

Microbial decomposition of skeletal muscle tissue (Ovis aries) in a sandy loam soil at different temperatures

A laboratory experiment was conducted to determine the effect of temperature (2, 12, 22 °C) on the rate of aerobic decomposition of skeletal muscle tissue (Ovis aries) in a sandy loam soil incubated for a period of 42 days. Measurements of decomposition processes included skeletal muscle tissue mass loss, carbon dioxide (CO₂) evolution, microbial biomass, soil pH, skeletal muscle tissue carbon (C) and nitrogen (N) content and the calculation of metabolic quotient (qCO₂). Incubation temperature and skeletal muscle tissue quality had a significant effect on all of the measured process rates with 2 °C usually much lower than 12 and 22 °C. Cumulative CO₂ evolution at 2, 12 and 22 °C equaled 252, 619 and 905 mg CO₂, respectively. A significant correlation (P<0.001) was detected between cumulative CO₂ evolution and tissue mass loss at all temperatures. Q₁₀s for mass loss and CO₂ evolution, which ranged from 1.19 to 3.95, were higher for the lower temperature range (Q₁₀(2-12 °C)>Q₁₀(12-22 °C)) in the Ovis samples and lower for the low temperature range (Q₁₀(2-12 °C)<Q₁₀(12-22 °C)) in the control samples. Metabolic quotient and the positive relationship between skeletal muscle tissue mass loss and cumulative CO₂ evolution suggest that tissue decomposition was most efficient at 2 °C. These phenomena may be due to lower microbial catabolic requirements at lower temperature.     

Methane oxidation in temperate soils: effects of inorganic N

Additions of inorganic nitrogen (N) to an oak soil with significant potential for methane (CH₄) oxidation resulted in differential reduction in CH₄ oxidation capacity depending on N species added. Nitrate, rather than nitrite or ammonium, proved to be the strongest inhibitor of CH₄ oxidation in oak soil. Both high (CH₄ at 10 μl l⁻¹) and low (CH₄ at 5 ml l⁻¹) affinity CH₄ oxidation in oak soil was completely inhibited at a nitrate concentration similar to that present in an alder soil from the same experimental site. The alder soil showed no capacity for low affinity CH₄ oxidation. A ‘low nitrate’ forest soil (oak) showed high affinity, low capacity CH₄ oxidation upto around 1 ml l⁻¹ CH₄, above which both high and low affinity CH₄ oxidation became apparent following a lag phase, indicating either an induced high affinity uptake mechanism or the existence of distinct low affinity and high affinity methanotroph populations. High affinity CH₄ oxidation became saturated at CH₄ concentrations >500 μl l⁻¹, while low affinity CH₄ oxidation became saturated at ∼30 ml l⁻¹ CH₄. In a ‘high nitrate’ forest soil (alder), CH₄ oxidation appeared to be due to high affinity CH₄ oxidation only and became undetectable at CH₄ concentrations >5 ml l⁻¹.     

Effect of summer throughfall exclusion, summer drought, and winter snow cover on methane fluxes in a temperate forest soil

Soil moisture strongly controls the uptake of atmospheric methane by limiting the diffusion of methane into the soil, resulting in a negative correlation between soil moisture and methane uptake rates under most non-drought conditions. However, little is known about the effect of water stress on methane uptake in temperate forests during severe droughts. We simulated extreme summer droughts by exclusion of 168 mm (2001) and 344 mm (2002) throughfall using three translucent roofs in a mixed deciduous forest at the Harvard Forest, Massachusetts, USA. The treatment significantly increased CH₄ uptake during the first weeks of throughfall exclusion in 2001 and during most of the 2002 treatment period. Low summertime CH₄ uptake rates were found only briefly in both control and exclusion plots during a natural late summer drought, when water contents below 0.15 g cm⁻³ may have caused water stress of methanotrophs in the A horizon. Because these soils are well drained, the exclusion treatment had little effect on A horizon water content between wetting events, and the effect of water stress was smaller and more brief than was the overall treatment effect on methane diffusion. Methane consumption rates were highest in the A horizon and showed a parabolic relationship between gravimetric water content and CH₄ consumption, with maximum rate at 0.23 g H₂O g⁻¹ soil. On average, about 74% of atmospheric CH₄ was consumed in the top 4-5 cm of the mineral soil. By contrast, little or no CH₄ consumption occurred in the O horizon. Snow cover significantly reduced the uptake rate from December to March. Removal of snow enhanced CH₄ uptake by about 700-1000%, resulting in uptake rates similar to those measured during the growing season. Soil temperatures had little effect on CH₄ uptake as long as the mineral soil was not frozen, indicating strong substrate limitation of methanotrophs throughout the year. Our results suggest that the extension of snow periods may affect the annual rate of CH₄ oxidation and that summer droughts may increase the soil CH₄ sink of temperate forest soils.     

Uncoupling of microbial CO2 production and release in frozen soil and its implications for field studies of arctic C cycling

Knowledge of seasonal trends and controls of soil CO₂ emissions to the atmosphere is important for simulating atmospheric CO₂ concentrations and for understanding and predicting the global carbon cycle. This is particularly the case for high arctic soils subject to extreme fluctuating environmental conditions. Based on field measurements of soil CO₂ efflux, temperature, water content, pore gas composition in soil and frozen cores as well as detailed temperature experiments performed in the laboratory, we evaluated seasonal controls of CO₂ effluxes from a well-drained tundra heath site in NE-Greenland. During the growing season, near-surface temperatures correlated well with observed CO₂ effluxes (r²>0.9). However, during intensive thawing of near-surface layers we observed up to 1.5-fold higher effluxes than expected due to temperature alone. These high rates were consistent with high CO₂ concentrations in frozen soil (>10% CO₂) and suggested a spring burst event during soil thawing and a corresponding trapping of produced CO₂ during winter. Laboratory experiments revealed that microbial soil respiration continued down to a least −18 °C and that up to 80% of the produced CO₂ was trapped in soil at temperatures between 0 and −9 °C. The trapping of CO₂ in frozen soil was positively correlated with soil moisture (r²=0.85) and led to an abrupt change of the temperature sensitivity (Q₁₀) observed for soil CO₂ release at 0 °C with Q₁₀ values below 0 °C being up to 100-fold higher than above 0 °C. The results of sub-zero CO₂ production allowed us to predict the microbial soil respiration throughout the year and to evaluate to what extent burst events during thawing can be explained by the release of CO₂ being produced and trapped during winter. Taking only the upper 20 cm of the soil into account, winter soil respiration accounted for about 40% of the annual soil respiration. At least 14% of the winter CO₂ production was trapped during the winter 2000-2001 and observed to be released upon thawing. Thus, the site-specific winter soil respiration is an important part of the annual C cycle and CO₂ trapping should be accounted for in future field and modelling studies of soil respiration dynamics in arctic ecosystems. In conclusion, we have discovered a soil moisture dependent uncoupling of CO₂ production and release in frozen soils with important implications for future field studies of Arctic C cycling.     

Temperature and stubble management influence microbial CO2-C evolution and gross N transformation rates

Few studies have examined the kinetics of gross nitrogen (N) mineralization, immobilization, and nitrification rates in soil at temperatures above 15 °C. In this study, ¹⁵N isotopic pool dilution was used to evaluate the influence of retaining standing crop residues after harvest versus burning crop residues on short-term gross N transformation rates at constant temperatures of 5, 10, 15, 20, 30, and 40 °C. Gross N mineralization rates calculated per unit soil organic carbon were between 1 and 7 times lower in stubble burnt treatments than in stubble retained treatments. In addition, significant declines in soil microbial biomass (P=0.05) and CO₂-C evolution (P<0.001) were associated with stubble burning. Immobilization rates were of similar magnitude to gross N mineralization rates in stubble retained and burnt treatments incubated between 5 and 20 °C, but demonstrated significant divergence from gross N mineralization rates at temperatures between 20 and 40 °C. Separation in the mineralization immobilization turnover (MIT) in soil at high temperatures was not due to a lack of available C substrate, as glucose-C was added to one treatment to test this assumption. Nitrification increased linearly with temperature (P<0.001) and dominated over immobilization for available ammonium in soil incubated at 5 °C, and above 20 °C indicating that nitrification is often the principal process controlling consumption in a semi-arid soil. These findings illustrate that the MIT at soil temperatures above 20 °C is not tightly coupled, and consequently that the potential for loss of N (as nitrate) is considerably greater due to increased nitrification.     

Short-term temperature dependence of heterotrophic soil respiration after one-month of pre-incubation at different temperatures

Quantification of microbial activities involved in soil organic carbon (SOC) decomposition is critical for the prediction of the long-term impact of climate change on soil respiration (SR) and SOC stock. Although the temperature sensitivity of SR is especially critical in semi-arid regions, such as North West Tunisia, where the SOC stock is low, little research has been carried out in these environments. More needs to be known about factors, such as SOC availability that influence temperature sensitivity. In this study, soil samples were incubated with and without glucose addition for 28 days after a 28-day pre-incubation period. Pre-incubation and incubation was carried out at 20 °C, 30 °C, 40 °C and 50 °C. Respiration measurements were taken with temperature, glucose addition and incubation time as independent variables. The highest pre-incubation temperature reduced the temperature sensitivity of SR during the subsequent incubation period, both with and without glucose addition. Soil samples pre-incubated at 50 °C had the lowest SR at all subsequent incubation temperatures and the lowest temperature sensitivity of SR, even after glucose addition. However, after glucose addition, the effect of a high pre-incubation temperature on soil respiration lasted only two days. Measuring the water-soluble carbon (WSC) in soil samples suggested that the high pre-incubation temperature may have killed part of the microbial biomass, modified microbial communities or solubilized SOC. For quantifying the possible effect of global warming, in particular heat waves, on soil respiration in the soil studied, the results indicate a moderate response of soil respiration to temperature at high temperatures, as shown by Q₁₀ close to 1.7, even in the range 40-50 °C.     

The inhibitory effect of difluoromethane on CH4 oxidation in reconstructed peat columns and side-effects on CO2 and N2O emissions at two water levels

Methane emissions from soils are the net result of two processes: methane (CH₄) production and CH₄ oxidation. In order to understand how both processes respond to environmental changes, it is necessary to distinguish between CH₄ production and oxidation. In bacterial cultures and small soil samples, difluoromethane (CH₂F₂) was found to inhibit CH₄ oxidation reversibly, without affecting CH₄ production. Hence, CH₂F₂ allows the study of CH₄ production directly and of CH₄ oxidation indirectly. To our knowledge, however, the inhibitory effect of CH₂F₂ within soil columns has not yet been evaluated. We therefore tested which CH₂F₂ concentration is needed for complete inhibition of CH₄ oxidation in reconstructed 28 cm high peat soil columns under different water levels (WL). We found that soil columns require considerably higher headspace CH₂F₂ concentrations for complete inhibition of CH₄ oxidation than small soil samples. Inhibition remained complete until ca. 24 h after CH₂F₂ exposure. Then, the inhibitory effect diminished. The time needed for the inhibitory effect to disappear depended on WL; at a low WL of −15 cm, the inhibitory effect declined slowly and oxidation rates recovered by 90% only after 12 days. At WL = −5 cm, CH₄ oxidation recovered much faster (90% recovery after ca. 3 days). Last, CH₂F₂ addition significantly decreased the N₂O emissions, whereas CO₂ emissions remained unaltered.     

Responses of soil organic matter and greenhouse gas fluxes to soil management and land use changes in a humid temperate region of southern Europe

We studied the effects of soil management and changes of land use on soils of three adjacent plots of cropland, pasture and oak (Quercus robur) forest. The pasture and the forest were established in part of the cropland, respectively, 20 and 40 yr before the study began. Soil organic matter (SOM) dynamics, water-filled pore space (WFPS), soil temperature, inorganic N and microbial C, as well as fluxes of CO₂, CH₄ and N₂O were measured in the plots over 25 months. The transformation of the cropland to mowed pasture slightly increased the soil organic and microbial C contents, whereas afforestation significantly increased these variables. The cropland and pasture soils showed low CH₄ uptake rates (<1 kg C ha⁻¹ yr⁻¹) and, coinciding with WFPS values >70%, episodes of CH₄ emission, which could be favoured by soil compaction. In the forest site, possibly because of the changes in soil structure and microbial activity, the soil always acted as a sink for CH₄ (4.7 kg C ha⁻¹ yr⁻¹). The N₂O releases at the cropland and pasture sites (2.7 and 4.8 kg N₂O-N ha⁻¹ yr⁻¹) were, respectively, 3 and 6 times higher than at the forest site (0.8 kg N₂O-N ha⁻¹ yr⁻¹). The highest N₂O emissions in the cultivated soils were related to fertilisation and slurry application, and always occurred when the WFPS >60%. These results show that the changes in soil properties as a consequence of the transformation of cropfield to intensive grassland do not imply substantial changes in SOM or in the dynamics of CH₄ and N₂O. On the contrary, afforestation resulted in increases in SOM content and CH₄ uptake, as well as decreases in N₂O emissions.     

Microbial activity in soils frozen to below −39 °C

Recent research on life in extreme environments has shown that some microorganisms metabolize at extremely low temperatures in Arctic and Antarctic ice and permafrost. Here, we present kinetic data on CO₂ and ¹⁴CO₂ release from intact and ¹⁴C-glucose amended tundra soils (Barrow, Alaska) incubated for up to a year at 0 to −39°C. The rate of CO₂ production declined exponentially with temperature but it remained positive and measurable, e.g. 2-7 ng CO₂-C cm⁻³ soil d⁻¹, at −39 °C. The variation of CO₂ release rate (v) was adequately explained by the double exponential dependence on temperature (T) and unfrozen water content (W) (r²>0.98): v=A exp(λT+kW) and where A, λ and k are constants. The rate of ¹⁴CO₂ release from added glucose declined more steeply with cooling as compared with the release of total CO₂, indicating that (a) there could be some abiotic component in the measured flux of CO₂ or (b) endogenous respiration is more cold-resistant than substrate-induced respiration. The respiration activity was completely eliminated by soil sterilization (1 h, 121 °C), stimulated by the addition of oxidizable substrate (glucose, yeast extract), and reduced by the addition of acetate, which inhibits microbial processes in acidic soils (pH 3-5). The tundra soil from Barrow displayed higher below-zero activity than boreal soils from West Siberia and Sweden. The permafrost soils (20-30 cm) were more active than the samples from seasonally frozen topsoil (0-10 cm, Barrow). Finding measurable respiration to −39 °C is significant for determining, understanding, and predicting current and future CO₂ emission to the atmosphere and for understanding the low temperature limits of microbial activity on the Earth and on other planets.     

Acid hydrolysis to partition plant material into decomposable and resistant fractions for use in the Rothamsted carbon model

Using various plant materials, we identified two conceptual pools of plant litter, decomposable plant material (DPM) and resistant plant material (RPM), in the Rothamsted Carbon Model (RothC) by comparing the default proportions of DPM and RPM in the RothC and proportions in plant material fractions as determined by two-step acid hydrolysis with H₂SO₄. We collected 37 plant samples from 15 species at six sites on arable land, grassland, or forest in Japan. Carbon in the plant materials was divided into three pools by acid hydrolysis: (a) Labile Pool I (LP I), obtained by hydrolysis with 5 N H₂SO₄ at 105 °C for 30 min; (b) Labile Pool II (LP II), obtained by hydrolysis with 26 N H₂SO₄ at room temperature overnight, and then with 2 N H₂SO₄ at 105 °C for 3 h; and (c) Recalcitrant Pool (RP), the unhydrolyzed residue. The average proportion of LP I in crops and grasses was 59%, which was the same as the proportion of DPM defined in the RothC as the default value for crops and grasses. The remaining 41% (23% LP II+18% RP) was consequently the same as the RPM proportion defined in the RothC. Similarly, the average proportion of LP I in all tree leaves (19%) was very close to the proportion of DPM in the RothC (20%) for trees. These results indicate that DPM in the RothC can be identified as LP I from the acid hydrolysis analysis and RPM as LP II+RP. We conclude that, at least theoretically, the use of an independent DPM:RPM ratio, as determined by acid hydrolysis analysis for each plant material, should enable more reliable modeling of SOM dynamics than the use of default DPM:RPM values provided by the model, even though the practical advantages of this method require further evaluation.     

Kinetics of bacterial death by heated dolomite powder slurry

The kinetics of the bactericidal action of dolomite powders heated at 600-1000 °C against Escherichia coli and Staphylococcus aureus were investigated. Dolomite powder heated to at least 700 °C exhibited bactericidal action, and the process of bacterial death in the heated dolomite powder slurries followed first-order reaction kinetics. The value of the death rate constant (k) increased with dolomite powder concentration, and the dilution coefficient (n), which indicates the dependence of k on the reagent concentration, was measured. The n values of the powder heated at 700 °C and at temperatures >900 °C were almost identical to those of MgO and CaO, respectively. This suggests that the first emergence of bactericidal action at 700 °C corresponds to generation of MgO while that at temperatures >900 °C is due to generation of CaO. The slurry temperature significantly affects the bactericidal action. The slope of the Arrhenius plot of k for E. coli and S. aureus grown at 37 °C exhibited a discontinuous point at approximately 22 °C, where a change in the value of activation energy for bacterial death occurred. This temperature corresponds to that of the phase transition of cell membrane lipids.