Physical Properties of Vetch Seed

The physical properties of vetch seed were evaluated as a function of moisture content. The average length, width and thickness were 5·19, 4·33 and 3·63 mm, respectively, at 10·57% dry basis (d.b.) moisture content. In the moisture range from 10·57 to 20·63% d.b., studies on rewetted vetch seed showed that the thousand seed mass increased from 55·47 to 59·03 g, the projected area from 23·52 to 29·05 mm², the sphericity from 0·837 to 0·859, the true density from 1286·2 to 1369·9 kg m⁻³, the porosity from 33·08% to 39·68% and the terminal velocity from 9·94 to 10·33 m s⁻¹. The static coefficient of friction of vetch seed increased the linearly against surfaces of four structural materials, namely, rubber (0·350–0·387), aluminium (0·319–0·367), stainless steel (0·202–0·328) and galvanised iron (0·312–0·361) as the moisture content increased from 10·57 to 20·63% d.b. The bulk density and the shelling resistance decreased from 860·8 to 826·2 kg m⁻³ and from 148·73 to 62·68 N, respectively, with an increase in moisture content from 10·57 to 20·63% d.b.     

PH—Postharvest Technology: Airflow Resistance of Green Gram

Pressure drops were measured in clean green gram beds at moisture content in a range of 8·36–16·65% d.b. for superficial air velocities which ranged between 0·0104 and 1·0875 m³ s⁻¹ m⁻² at bed depths of 0·2 to 0·6 m with bulk density ranging from 760 to 855 kg m⁻³. The airflow resistance of green gram increased with the increase in airflow rate, bulk density, bed depth and decreased moisture content. Results indicated that 1% increase in moisture content decreased the pressure drop by 2·43% whereas, 1% increase in bulk density increased the pressure drop by 6·6%. Modified Shedd's equation, Hukill and Ives equation and modified Ergun equation all with two parameters were examined. Airflow resistance was accurately described by modified Shedd's equation and an empirical equation, composed of airflow rate, moisture content and bulk density.     

Physical Properties of Raw and Parboiled Paddy

The physical properties namely, size and shape, bulk density, true density, and angle of repose at moisture contents ranging from 7·19 to 28·28% d.b. for raw paddy (IR-36) and from 12·24 to 43·53% d.b. for parboiled paddy were determined using standard techniques. In the case of raw paddy, the thousand grain weight increased from 23·04 to 27·16 g with an increase in moisture content from 10·45 to 32·13% d.b. Bulk density and angle of repose increased from 522 to 566 kg/m³ and 42·35 to 49·30°, respectively, with an increase in moisture content from 7·19 to 27·86% d.b. True density and porosity decreased from 1405 to 1348 kg/m³ and from 62·84 to 58·01% respectively, with an increase in moisture content from 7·19 to 27·86% (d.b.). In the case of parboiled paddy, bulk density and angle of repose increased from 507 to 564·8 kg/m³ and 39·90 to 43·89°, respectively, with an increase in moisture content from 12·24 to 43·53% d.b. True density and porosity decreased from 1411 to 1342 kg/m³ and from 64·08 to 57·91% respectively, with an increase in moisture content from 12·24 to 43·53%. The physical properties were linearly dependent upon moisture content.     

PH—Postharvest Technology: Thermodynamics of Moisture Sorption in Sesame Seed

Experimental data on the sorption isotherms of sesame seed were used to determine the thermodynamic functions (heat of vaporisation, spreading pressure, net integral enthalpy and entropy). The heat of vaporisation decreased with increase in moisture content and approached the latent heat of pure water at moisture content between 18 and 21% dry basis. The spreading pressure increased with increase in water activity and was not significantly affected by temperature. Net integral enthalpy decreased with increase in moisture content, and became asymptotic as the moisture content of 12% was approached. Net integral entropy decreased with increase in moisture content to a minimum value of 0·138 J kg⁻¹K⁻¹ at moisture content of about 3·7%. It then increased with moisture content to a maximum of about 0·63 J kg⁻¹K⁻¹ at about 12% moisture content and thereafter, remained nearly constant.     

Sorption Isotherms of Tomato Slices and Onion Shreds

Experiments were conducted to determine the equilibrium moisture content of tomato (cv Roma) slices and white onion shreds at temperatures of 30, 40 and 50°C and relative humidities of 15–85% obtained from an air conditioning unit. The samples were placed on trays and the equilibrium moisture contents were determined once the samples attained the constant mass. The equilibrium moisture content–equilibrium relative humidity data were fitted to the modified Henderson, Chung-Pfost, modified Halsey, modified Oswin, and modified GAB models. The modified Henderson and modified Halsey models fitted well for the tomato slices and onion shreds, respectively, based on the coefficient of regression, mean relative percent error and standard error of moisture. The values of these parameters were, 0·985, 2·58 and 0·52, respectively, for the modified Henderson model for the sliced tomatoes and 0·991, 2·01 and 0·42, respectively, for the modified Halsey model for onion shreds.     

An Observational Method for the Assessment of Biogas Production from an Anaerobic Waste Stabilisation Pond treating Farm Dairy Wastewater

A biogas production assessment method based on the visual monitoring of biogas evolution events in an anaerobic waste stabilisation pond was developed and applied to an anaerobic pond treating farm dairy wastewater in New Zealand. Major biogas-induced perturbations at the pond surface were classified as either type 1 or 2 events and other observed biogas activities as small bubble events. Mean counts of types 1 and 2 events varied from 7·3 to 30·0 per hour and 4·3–34·0 per hour, respectively, over the pond surface and the frequency of events decreased as both organic loading and temperature increased. Preliminary estimates of areal gas production rates, obtained using the observational method, ranged from 0·002 to 0·015 m³ m⁻² day⁻¹ for major eruptions and 0·0004–0·024 m³ m⁻² day⁻¹ for small bubble events, giving a total range of 0·002–0·039 m³ m⁻² day⁻¹. Pond temperatures at 2·75 m depth showed relatively minor fluctuations on a diurnal basis and ranged between 13 and 15°C from days 1–60, reaching a maximum of 24°C at day 190. Refinements proposed for future method development include an increased number and range of event categories, the automatic recording of events and the use of an improved cover. Further work is required to assess the general applicability of the method to anaerobic ponds.     

Mathematical Modelling of Vacuum Pressure on a Precision Seeder

The purpose of this research was to determine the optimum vacuum pressure of a precision vacuum seeder and to develop mathematical models by using some physical properties of seeds such as one thousand kernel mass, projected area, sphericity and kernel density. Maize, cotton, soya bean, watermelon, melon, cucumber, sugarbeet and onion seeds were used in laboratory tests. One thousand kernel mass, projected area, sphericity and kernel density of seeds varied from 4·3 to 372·5 g, 5–77 mm², 38·4–85·8% and 440–1310 kg m⁻³, respectively. The optimum vacuum pressure was determined as 4·0 kPa for maize I and II; 3·0 kPa for cotton, soya bean and watermelon I; 2·5 kPa for watermelon II, melon and cucumber; 2·0 kPa for sugarbeet; and 1·5 kPa for onion seeds.The vacuum pressure was predicted by mathematical models. According to the results, the final model could satisfactorily describe the vacuum pressure of the precision vacuum seeder with a chi-square of 2·51×10⁻³, root mean square error of 2·74×10⁻² and modelling efficiency of 0·99.     

Evaluation of Air Oven Moisture Content Determination Methods for Rough Rice

The effects of air oven drying temperature and drying duration on moisture content determination of medium grain rough rice were studied for moisture content levels from 10·2 to 32·5% (w.b.). Six different methods were compared in this study. A moisture content determination model for an air oven was developed to describe the effect of drying temperature, drying time, and sample grinding on moisture content determination. Drying whole kernels for 72 h at 105°C had the smallest standard deviation. Regression equations obtained from the measured data were used to validate the model developed in this study. This model could be employed to evaluate the moisture determination methods for other varieties of rough rice and other crops obtained from previous literature.     

Effect of time of mulching on soil temperature and moisture regime and emergence, growth and yield of white yam in western Nigeria

Time of mulching can influence the growth environment and performance of white yam (Dioscorea rotundata Poir). An on-farm trial was conducted during the 1988–1989 and 1989–1990 seasons (October–August) in Nigeria to determine the effect of time of mulching (October–February) on the hydrothermal regime and emergence, growth and tuber yield of white yam. Application of 12.5 mg ha⁻¹ of dry Eupatorium odoratum L. (Syn. Chromolaena odorata L.) mulch on top of the mounds significantly improved soil moisture content of the 15 cm surface layer by 50–120 g kg⁻¹ and decreased the maximum soil temperature by 2–7°C at 15 cm depth in the early growing season (March–April). The emergence and development of yam seedlings were significantly lower in unmulched plots than in mulch-treated plots. Mulching significantly increased tuber yield by about 10–15 mg ha⁻¹ season⁻¹. Plots mulched in October–December were more moist by 20–60 g kg⁻¹ and cooler by 1–3°C, and had 27–44% greater emergence than those mulched in January or February. The number of leaves per plant, vine diameter and leaf area index were also significantly greater in plots mulched in October–December than plots mulched in January or February. Consequently, shoot dry weight was about 28–36% greater in yam mulched in October–December than in yam mulched in February. However, the time of mulching had no effect on soil moisture of the surface layer at the beginning of the rainy season (April), on yam emergence in May and on tuber yield and yield components. Although time of mulching did not significantly affect tuber yield, the increase (10–15%) in the tuber yield of yam mulched in December–February compared to the yam mulched in October or November was considerable. It was concluded that yam planted in October, just before the rain stops, can be mulched in January or February without detrimental effect on emergence, growth and tuber yield.     

Burrow building in seabird colonies: a soil-forming process in island ecosystems

Soil modification via biopedturbation by burrow-building seabirds was examined in a Mediterranean, island ecosystem. Physical and chemical soil properties were compared between a colony of Wedge-tailed Shearwaters (Puffinus pacificus) and adjacent heath across a 14-month period. When compared to heath soil, the biopedturbated soil was 28% drier (6.04±5.40 vol%), had increased bulk density (by 29% to 1.30±0.11 g cm⁻³, 51% porosity), wetting capacity (by 83% to 0.55±0.83 molarity of ethanol droplet), hydraulic conductivity (by 266% to 398.91±252.04 mm h⁻¹), and a greater range in soil surface temperature (31.7±6.2 °C diurnally to 18.3±3.2 °C nocturnally). Soil penetration resistance was reduced by 26% at a depth of 0–100 mm (326.5±122.4 kPa) and by 55% at 500–600 mm (1116.8±465.0 kPa). Colony soil also had increased levels of nitrate (by 470%), phosphorous (118%), ammonium (102%), sulphur (69%), and potassium (34%), decreased levels of iron (by 50%) and organic carbon (61%), was more alkaline, and had a 78% greater conductivity. Shearwaters deposited guano at a rate of 234.4 kg ha⁻¹ yr⁻¹ (dry mass). Chemical analysis of guano equated this to 50.9, 5.7, 5.5, and 3.6 kg ha⁻¹ yr⁻¹ of nitrogen, potassium, sulphur, and phosphorous, respectively. Experimentally constructed burrows demonstrated that digging alone can alter physical and chemical soil factors, but that changes in the nutrient profile of colony soils are predominantly guano-driven. We argue that the physical impact of seabirds on soil should not be overlooked as a soil-forming and ecosystem-shaping factor in island ecosystems, and that biopedturbation can exert major bottom-up influences on insular plant and animal communities.     

Conservation of soil moisture by different stone covers on alpine talus slopes (Lassen, California)

This study reports on the influence of stone covers with different clast sizes on the soil moisture of alpine talus slopes in Lassen (California). Fifteen four-plot sets were sampled in the dry season (July 1990) in sandy areas and in talus covered with pebbles, cobbles, or blocks between 2740 and 2775 m. Three depths (0–5, 5–10, 10–15 cm) were sampled. Field moisture content increased gradually with depth in all soil profiles, and also in plots covered by increasingly larger rocks. Surface soils in sand areas were very dry, but under rocks had water contents 6 to 14 times greater. Differences among plots decreased with depth, but subsoil samples in sand were still drier than those beneath any stone cover at similar depths. Blocks were most effective in conserving moisture; water content below them was higher than even in deep (10–15 cm) sand soils. Soil temperatures were recorded in sand and under blocks for an 11-day period. Minima were not significantly different, but average maxima were 5.6°C lower under blocks than in sand, which reached highs 4.4°C lower than the air. Differences in soil moisture among talus types are ascribed to lower evaporation losses under stones, due to both disruption of capillarity by the coarse particles, which prevented water flow to the talus surface, and to their efficient reduction of maximum temperatures. An irrigation experiment was conducted at 2110 m on a steep talus on the Chaos Crags from July 18 to Aug. 2, 1993. Four 100×75 cm plots with the same surface types than at Lassen received 22.5 mm water; moisture content was then periodically sampled. Watering produced similar water distributions among soil depths and talus types to those in Lassen. Evaporation occurred quickly in bare soils due to high air and soil temperatures. The sand surface was already dry 2 days after watering, but stone-covered plots remained moist until day 15, when soils under blocks still retained 77–97% of the water content (percent by weight) at the start of the test.     

Real-time Measures of Canopy Size as a Basis for Spatially Varying Nitrogen Applications to Winter Wheat sown at Different Seed Rates

Experiments at two sites growing winter wheat show that in order to manage a wheat canopy more effectively, the use of specific remote sensing techniques both to monitor crop canopy expansion, and to determine variable nitrogen applications at key timings is required. Variations in seed rate were used to achieve a range of initial crop structures, and treatments were compared to standard farm practice. In the first year, the effect of varying seed rate (250, 350 and 450 seeds m⁻²) on crop structure, yield components and grain yield, was compared to the effects of underlying spatial variation. Plant populations increased up to the highest rate, but shoot and ear populations peaked at 350 seeds m⁻². Compensation through an increased number of grains per ear and thousand grain weight resulted in the highest yield and gross margin at the lowest seed rate. In later experiments, the range of seed rates was extended to include 150 seeds m⁻², each sown in 24 m wide strips split into 12 m wide halves. One half received a standard nitrogen dose of 200 kg [N] ha⁻¹, the other a variable treatment based on near ‘real-time’ maps of crop growth. Both were split into three applications, targeted at mid-late tillering (early March), growth stages GS30-31 (mid April) and GS33 (mid May). At each timing, calibrated aerial digital photography was used to assess crop growth in terms of shoot population at tillering, and canopy green area index at GS30-31 and GS33. These were compared to current agronomic guidelines. Application rates were then varied below or above the planned amount where growth was above- or below-target, respectively. In the first field, total nitrogen doses in the variable treatments ranged from 188 to 243 kg [N] ha⁻¹, which gave higher yields than the standards at all seed rates in the range 0·36–0·78 t ha⁻¹ and gross margins of £17 to £60 ha⁻¹. In the second field, variable treatments ranged from 135 to 197 kg [N] ha⁻¹ that resulted in lower yields of −0·32 to +0·30 t ha⁻¹. However, in three out of the four seed rates, variable treatments produced higher gross margins than the standard, which ranged from £2 to £20 ha⁻¹. In both fields, the greatest benefits were obtained where the total amount of applied nitrogen was similar to the standard, but was applied variably rather than uniformly along the strips. Simple nitrogen balance calculations have shown that variable application of nitrogen can have an overall effect on reducing the nitrogen surplus by one-third.     

Estimate of N2-fixation in waste-derived compost after treatment with glucose

It has been shown previously that treatment of grass swards cultured in waste-derived compost and treated with 8–24 g (5–15 g·L⁻¹) glucose over 8 weeks resulted in a substantial increase of shoot dry matter and total nitrogen content, while a growth inhibiting effect was observed in unamended field soil. The improved sward performance in compost was associated with a significant expansion of the aerobic, diazotrophic bacterial population in the growth medium. The purpose of the present experiment was to measure the contribution of N₂-fixation to the improved growth. Glucose treatment resulted in the fixation of at least 128 mg total nitrogen per pot (equivalent to 14.5 mg N fixed per g C respired; 104 kg·ha⁻¹), with 32 % made available to shoots for growth over the following 6 months. There was no evidence of accelerated soil organic matter decomposition that could account for the increased N availability.

Résumé

Nous avons montré précédemment que les pelouses prairiales arrosées avec des eaux issues de compost traitées avec 8–24 g de glucose (5–15 g·L⁻¹) pendant 8 semaines voient une augmentation substantielle de la matière sèche et de la teneur en azote des pousses alors qu'un effet inhibiteur de croissance est observé sur les sols non traités. L'amélioration des performances prairiales sont associées à une augmentation significative des population de bactéries aérobies fixatrices d'azote libre. Le but de l'expérimentation présentée ici était d'évaluer la contribution de la fixation d'azote dans l'amélioration de la croissance. Le traitement par le glucose conduit à la fixation d'au moins 128 mg d'azote par pot (équivalent à 14,5 mg d'azote fixé par g de C respiré; 104 kg·ha⁻¹), dont 32 % utilisable pour la croissance des pousses durant les 6 mois suivant. Aucune accélération de la décomposition de la matière organique n'est détectée, qui pourrait contribuer à cette augmentation d'azote disponible.     

校正温度和盐度对单电容传感器的影响

Several newly developed capacitance sensors have simplified real-time determination of soil water content.Previous work has shown that salinity and temperature can affect these sensors,but relatively little has been done to correct these effects.The objectives of this study were to evaluate the effect of media temperature and salinity on the apparent water content measured with a single capacitance sensor (SCS),and to mitigate this effect using a temperature dependent scaled voltage technique under laboratory conditions.A column study was conducted containing two media:pure deionized water and quartz sand under varying water contents (0.05 to 0.30 cm³ cm^-3) and salinity (0 to 80 mmol L^-1).Media temperature was varied between 5 and 45℃ using an incubator.The SCS probes and thermocouples were placed in the middle of the columns and were logged at an interval of 1 minute.There was strong negative correlation between sensor reading and temperature of deionized water with a rate of -0.779 mV ℃^-1.Rates of SCS apparent output were 0.454 and 0.535 mV ℃^-1 for air in heating and cooling cycles,respectively.A similar positive correlation with temperature was observed in sand at different water contents.The SCS probe was less sensitive to temperature as salinity and water content increased.Using a temperature-corrected voltage calibration model,the effect of temperature was reduced by 98%.An analytical model for salinity correction was able to minimize the error as low as ± 2% over the salinity level tested.     

SE—Structures and Environment: Biofiltration of Odour and Ammonia from a Pig Unit—Biofiltration of Odour and Ammonia from a Pig Unit—a pilot-scale Study

A pilot-scale biofiltration unit was constructed at a pig finishing building on the University College Dublin research farm. The biofiltration system was investigated over three trial periods. Exhaust air from a single pen was extracted by a variable speed centrifugal fan and passed through a humidifier and biofilter. A 0·5 m depth of woodchips of over 20 mm screen size was used as the biofilter medium. The moisture content of the medium was maintained at 64±4% (wet weight basis) for trial one and 69±4% (wet weight basis) for trials two and three using a load cell method. The volumetric loading rate varied from 769 to 1898 m³ [air] m⁻³ [medium] h⁻¹ during the three trial periods. Odour and ammonia removal efficiencies ranged from 77 to 95% and 54 to 93%, respectively. The pH of the biofilter leachate remained between 6 and 8 throughout the experimental periods. The pressure drop across the biofilter ranged from 14 to 64 Pa. It is concluded that a wood chip media particle size >20 mm is suitable for use in biofiltration systems on intensive pig production facilities. This will minimize the pressure drop on the system fans to reduce overall operation costs. It is recommended that a filter bed moisture content (wet weight basis) of greater than 63% be used to maintain overall efficiency. An efficient air moisturizing system (humidification and bed sprinkling) along with a properly designed air distribution system must be incorporated in the overall design when operating at such high volumetric loading rates.     

Shrinkage Evaluation of Five Different Varieties of Coffee Berries during the Drying Process

The objective of this work was to evaluate the effect of moisture content variation on reduction of superficial area, volume and equivalent sphere diameter of coffee berries. Four varieties of Coffea arabica (cv Catuaí Vermelho, Catuaí Amarelo, Mundo Novo and Catimor) and one variety of Coffea canephora (cv Conilon) were used. From the results obtained, it was concluded that moisture content in the coffee berries affects its physical properties causing significant decrease of the superficial area, volume and diameter of the equivalent sphere during a drying process. The varieties of coffee had different shrinkage behaviour. The Conilon coffee had the highest level of berry shrinkage, the volume decreased 35% during the drying process from 1·38 to 0·12 dry basis (d.b.) berry moisture content. The shrinkage behaviour during the drying process was well explained by a polynomial model with coefficient of determination greater than 90%.     

Mechanical Behaviour of Hazelnut under Compression Loading

Aci Findik, Cakildak, Tombul, and Güney Karasi varieties of hazelnut were loaded between two parallel plates to determine the specific deformation, rupture force, and rupture energy required to initiate shell and kernel rupture. The tests were carried out at two deformation rates of 0·52, and 0·91 mm s⁻¹, four moisture contents of 6, 11, 15, and 18% w.b., and three axes (X,Y,Z). The X-axis is the longitudinal axis through the hilum, while the Y-axis is the transverse axis containing the major dimension at right angles to the longitudinal axis, and the Z-axis is the transverse axis containing the minimum dimension. Physical characteristics of nut and kernel such as mass, dimensions, geometric mean diameter and sphericity were determined. Specific deformation and rupture energy of the shell generally increased in magnitude with an increase in moisture content while rupture force decreased for compression along the X- and Y-axis. The highest nut shell specific deformation, rupture force and rupture energy among the four varieties were obtained for Aci Findik nuts loaded along the Z-axis at a deformation rate of 0·52 mm s⁻¹. The Tombul and Güney Karasi varieties had the highest kernel rupture force and rupture energy, respectively.     

Effect of Changing Temperature on the Deterioration of Soya Beans

Deterioration rates as indicated by carbon dioxide evolution for soya bean (Glycine max L. Merr.) stored under changing temperature conditions were quantified and compared with those predicted using equations. Experiments included soya bean moisture contents of 18, 22, and 26% (wet basis), constant storage temperatures of 15, 20, 25, and 30°C, and cyclical storage temperatures that changed between 15 and 25°C and between 20 and 30°C on a 24 h basis. Also, the growth of micro-organisms was identified after 10 days from the treatments by using the pour plate method.The results indicated an increase in deterioration by increasing storage temperature and moisture content of soya bean. Equations of carbon dioxide weight versus time for each moisture content and storage temperature were fitted. The longest allowable storage time to reach 0·5% dry matter loss (1132 h) occurred at lower moisture content and lower constant storage temperature, while the shortest allowable storage time (170 h) occurred at higher moisture content and higher constant storage temperature. The allowable storage times for soya bean stored under cyclical temperatures were close to the allowable storage time for soya bean stored at a constant temperature equal to the average cyclical temperature. Microbial infection levels increased with increasing storage temperature and moisture content. The increasing rate of micro-organism growth decreased by increasing the storage temperature over 25°C. However, this increasing rate of micro-organism growth for soya bean exposed to a cyclical storage temperature was usually lower than that for soya bean held at constant storage temperatures of about 20°C (the average of 15 and 25°C) and 25°C (the average of 20 and 30°C).     

Cesium and soil carbon in a small agricultural watershed

Scientific, political, and social interests have developed recently in the concept of using agricultural soils to sequester carbon. Studies supporting this concept indicate that soil erosion and subsequent redeposition of eroded soils in the same field may establish an ecosystem disequilibrium that promotes the buildup of carbon on agricultural landscapes. The problem is to determine the patterns of soil erosion and redeposition on the landscape and to relate these to soil carbon patterns. Radioactive ¹³⁷cesium (¹³⁷Cs) can be used to estimate soil erosion patterns and, more importantly, redeposition patterns at the field level. The purpose of this study was to determine the relationship between ¹³⁷Cs, soil erosion, and soil carbon patterns on a small agricultural watershed. Profiles of soils from an upland area and soils in an adjacent riparian system were collected in 5 cm increments and the concentrations of ¹³⁷Cs and carbon were determined. ¹³⁷Cs and carbon were uniformly mixed in the upper 15–20 cm of upland soils. ¹³⁷Cs (Bq g⁻¹) and carbon (%) in the upland soils were significantly correlated (r²=0.66). Carbon content of the 0–20 cm layer was higher (1.4±0.3%) in areas of soil deposition than carbon content (1.1±0.3%) in areas of soil erosion as determined by the ¹³⁷Cs technique. These data suggest that measurements of ¹³⁷Cs in the soils can be useful for understanding carbon distribution patterns in surface soil. Carbon content of the upland soils ranged from 0.5 to 1.9% with an average of 1.2±0.4% in the 0–20 cm layer while carbon below this upper tilled layer (20–30 cm) ranged from 0.2 to 1.5% with an average of 0.5±0.3%. Total carbon was 2.66 and 3.20 kg m⁻² in the upper 20 cm and upper 30 cm of the upland soils, respectively. Carbon content of the 0–20 cm layer in the riparian system ranged from 1.1 to 67.0% with an average 11.7±17.1%. Carbon content below 20 cm ranged from 1.8 to 79.3% with an average of 18.3±17.5%. Soil carbon in the upper 20 cm of the riparian profile was 10.1 and 15.0 kg m⁻² in the upper 30 cm of the riparian profiles. This is an increase of organic carbon by a factor of 3.8 and 4.7 for the upper 20 cm and upper 30 cm of the riparian profiles, respectively, when compared to the upland soil profiles.     

Methane turnover and temperature response of methane-oxidizing bacteria in permafrost-affected soils of northeast Siberia

The abundance, activity, and temperature response of aerobic methane-oxidizing bacteria were studied in permafrost-affected tundra soils of northeast Siberia. The soils were characterized by both a high accumulation of organic matter at the surface and high methane concentrations in the water-saturated soils. The methane oxidation rates of up to 835 nmol CH₄ h⁻¹ g⁻¹ in the surface soils were similar to the highest values reported so far for natural wetland soils worldwide. The temperature response of methane oxidation was measured during short incubations and revealed maximum rates between 22 °C and 28 °C. The active methanotrophic community was characterized by its phospholipid fatty acid (PLFA) concentrations and with stable isotope probing (SIP). Concentrations of 16:1ω8 and 18:1ω8 PLFAs, specific to methanotrophic bacteria, correlated significantly with the potential methane oxidation rates. In all soils, distinct 16:1 PLFAs were dominant, indicating a predominance of type I methanotrophs. However, long-term incubation of soil samples at 0 °C and 22 °C demonstrated a shift in the composition of the active community with rising temperatures. At 0 °C, only the concentrations of 16:1 PLFAs increased and those of 18:1 PLFAs decreased, whereas the opposite was true at 22 °C. Similarly, SIP with ¹³CH₄ showed a temperature-dependent pattern. When the soils were incubated at 0 °C, most of the incorporated label (83%) was found in 16:1 PLFAs and only 2% in 18:1 PLFAs. In soils incubated at 22 °C, almost equal amounts of ¹³C label were incorporated into 16:1 PLFAs and 18:1 PLFAs (33% and 36%, respectively). We concluded that the highly active methane-oxidizing community in cold permafrost-affected soils was dominated by type I methanotrophs under in situ conditions. However, rising temperatures, as predicted for the future, seem to increase the importance of type II methanotrophs, which may affect methane cycling in northern wetlands.