Effect of experimental nitrogen load on methane and nitrous oxide fluxes on ombrotrophic boreal peatland

Methane (CH₄) and nitrous oxide (N₂O) dynamics were studied in a boreal Sphagnum fuscum pine bog receiving annually (from 1991 to 1996) 30 or 100 kg NH₄NO₃-N ha^–1. The gas emissions were measured during the last three growing seasons of the experiment. Nitrogen treatment did not affect the CH₄ fluxes in the microsites where S. fuscum and S. angustifolium dominated. However, addition of 100 kg NH₄NO₃-N ha^–1 yr^–1 increased the CH₄ emission from those microsites dominated by S. fuscum. This increase was associated with the increase in coverage of cotton grass (Eriophorum vaginatum) induced by the nitrogen treatment. The differences in the CH₄ emissions were not related to the CH₄ oxidation and production potentials in the peat profiles. The N₂O fluxes were negligible from all microsites. Only minor short-term increases occurred after the nitrogen addition.     

Oxidation of methane in boreal forest soils: a comparison of seven measures

Methane oxidation rates were measured in boreal forest soils using seven techniques that provide a range of information on soil CH₄ oxidation. These include: (a) short-term static chamber experiments with a free-air (1.7 ppm CH₄) headspace, (b) estimating CH₄ oxidation rates from soil CH₄ distributions and (c)²²²Rn-calibrated flux measurements, (d) day-long static chamber experiments with free-air and amended (+20 to 2000 PPM CH₄) headspaces, (e) jar experiments on soil core sections using free-air and (f) amended (+500 ppm CH₄) headspaces, and (g) jar experiments on core sections involving tracer additions of¹⁴CH₄. Short-term unamended chamber measurements,²²²Rn-calibrated flux measurements, and soil CH₄ distributions show independently that the soils are capable of oxidizing atmospheric CH₄ at rates ranging to < 2 mg m^–2 d^–1. Jar experiments with free-air headspaces and soil CH₄ profiles show that CH₄ oxidation occurs to a soil depth of 60 cm and is maximum in the 10 to 20 cm zone. Jar experiments and chamber measurements with free-air headspaces show that CH₄ oxidation occurs at low (< 0.9 ppm) thresholds. The¹⁴CH₄-amended jar experiments show the distribution of end products of CH₄ oxidation; 60% is transformed to CO₂ and the remainder is incorporated in biomass. Chamber and jar experiments under amended atmospheres show that these soils have a high capacity for CH₄ oxidation and indicate potential CH₄ oxidation rates as high as 867 mg m^–2 d^–1. Methane oxidation in moist soils modulates CH₄ emission and can serve as a negative feedback on atmospheric CH₄ increases.     

Methane and nitrous oxide exchange in differently fertilised grassland in southern Germany

We examined the effect of fertilisation (200 kg cattle slurry-N ha^–1 year^–1) on the exchange of N₂O and CH₄ in the soil–plant system of meadow agroecosystems in southern Germany. From 1996 to 1998, we regularly determined the gas fluxes (closed chamber method) and associated environmental parameters. N₂O and CH₄ fluxes were not significantly affected by fertilisation. N₂O fluxes at the unfertilised and fertilised plots were small, generally between 50 and –20 g N m^–2 h^–1. We identified some incidents of N₂O uptake. CH₄-C fluxes ranged from 1.3 to –0.2 mg m^–2 h^–1 and were not significantly different from 0 at both plots. We budgeted an annual net emission of 15.5 and 29.6 mg m^–2 N₂O-N and an annual CH₄-C net emission of 184.2 and 122.7 mg m^–2 at the unfertilised and fertilised plots, respectively. Apparently, rapid N mineralization and uptake in the densely rooted topsoil prevents N losses and the inhibition of CH₄ oxidation.     

Three-year measurement of methane emission from an Indonesian paddy field

Yearly and seasonal (rainy and dry seasons) variations of CH₄ emission from a Sumatra paddy field were measured for 3 years. The mean CH₄ emission rates during the growth period were in the range of 16.0–26.1 mg CH₄ m^-2 h^-1 for the chemical fertilizer plots and 23.3–34.9 mg CH₄ m^-2 h^-1 for the plots with rice straw application, respectively. The increase in the amounts of CH₄ emission by rice straw application were from 1.3 to 1.6 times. There was no significant difference in the mean CH₄ emission rates between rainy and dry seasons.Total amounts of CH₄ emitted during the period of rice growth were in the ranges of 29.5–48.2 and 43.0–64.6 g CH₄ m^-2 for the plots applied with chemical fertilizer and those with rice straw application, respectively. Nearly the same amounts of CH₄ were emitted in the first and second half of the growth period, irrespective of rice straw application.     

Methane emissions from fen,bog and swamp peatlands in Quebec

A static chamber technique was used weekly from spring thaw to winter freezing to measure methane emissions from 10 sites representing subarctic fens and temperate swamps and bogs. Rates of < 200 mg CH₄ m^–2 d^–1 were recorded in subarctic fens: within-site emissions were primarily controlled by the evolution of the peat thermal regime, though significant releases during spring thaw were recorded at some sites. Between subarctic fens, topography and water table elevation were important controls on methane emissions, with the general sequence: pool = horizontal fen> string. Emission rates from the 2 swamp sites were lower (< 20 mg CH₄ m^–2 d^–1 ), except during the spring thaw and when the sites were saturated. The low water table ( < 80 cm depth) in abnormally dry years reduced emission rates; rates were also low from a swamp site which had been drained and cleared of vegetation for horticulture. Methane emission rates were also low (< 5 mg CH₄ m^–2 d^–1) from 2 ombrotrophic bog sites. Laboratory measurements of rates of methane production under anaerobic conditions and methane consumption under aerobic conditions revealed that production rates were generally highest in the surface layers (0 to 2.5 cm depth); production was high in the fens and very low in the bogs. The swamp samples were able to produce methane under anaerobic conditions, but were also able to consume methane under aerobic conditions. Annual methane emission rates are estimated to be 1 to 10 g CH₄ m^–2 from the fens, 1 to 4 g CH4 m^–2 from the swamps and <0.2 g CH₄ m^–2 from the bogs and drained swamp.     

模拟SO42-沉降对闽江口淡水感潮野慈姑湿地甲烷排放通量的影响

2015年12月—2016年10月,每月小潮日原位定期向闽江口塔礁洲淡水感潮野慈姑(Sagittaria trifolia L.)湿地施加剂量为60、120 kg S hm~(-2)a~(-1)的K_2SO_4溶液(分别记做S-60和S-120),探讨模拟硫酸根(SO_4~(2-))沉降对河口淡水感潮湿地甲烷(CH4)排放通量及间隙水SO_4~(2-)浓度的影响。对照、S-60和S-120处理组CH_4排放通量年均值分别为(7.88±1.00)mg h~(-1)m~(-2)、(6.55±0.97)mg h~(-1)m~(-2)和(6.66±1.49)mg h~(-1)m~(-2)。在年尺度上,两个高强度模拟SO_4~(2-)沉降处理组均未显著降低闽江口淡水感潮野慈姑湿地CH_4排放通量(P0.05),即高强度SO_4~(2-)沉降不会对河口淡水感潮湿地CH_4排放通量产生类似于其对泥炭湿地和水稻田的显著抑制效应。在年尺度以及秋、冬季,两个施加K_2SO_4溶液处理显著增加了野慈姑湿地10 cm深度土壤间隙水SO_4~(2-)浓度。对于各个处理组,温度较高的夏、秋季CH_4排放通量均显著高于温度相对较低的冬、春季(P0.05)。不同处理组CH_4排放通量均与土壤温度呈显著正相关关系,温度仍然是影响亚热带河口淡水感潮湿地CH_4排放通量的重要环境因子。     

Simulated Nitrogen Deposition Reduces CH4 Uptake and Increases N2O Emission from a Subtropical Plantation Forest Soil in Southern China

To date, few studies are conducted to quantify the effects of reduced ammonium (NH₄ ⁺) and oxidized nitrate (NO₃ ⁻) on soil CH₄ uptake and N₂O emission in the subtropical forests. In this study, NH₄Cl and NaNO₃ fertilizers were applied at three rates: 0, 40 and 120 kg N ha⁻¹ yr⁻¹. Soil CH₄ and N₂O fluxes were determined twice a week using the static chamber technique and gas chromatography. Soil temperature and moisture were simultaneously measured. Soil dissolved N concentration in 0–20 cm depth was measured weekly to examine the regulation to soil CH₄ and N₂O fluxes. Our results showed that one year of N addition did not affect soil temperature, soil moisture, soil total dissolved N (TDN) and NH₄ ⁺-N concentrations, but high levels of applied NH₄Cl and NaNO₃ fertilizers significantly increased soil NO₃ ⁻-N concentration by 124% and 157%, respectively. Nitrogen addition tended to inhibit soil CH₄ uptake, but significantly promoted soil N₂O emission by 403% to 762%. Furthermore, NH₄ ⁺-N fertilizer application had a stronger inhibition to soil CH₄ uptake and a stronger promotion to soil N₂O emission than NO₃ ⁻-N application. Also, both soil CH₄ and N₂O fluxes were driven by soil temperature and moisture, but soil inorganic N availability was a key integrator of soil CH₄ uptake and N₂O emission. These results suggest that the subtropical plantation soil sensitively responses to atmospheric N deposition, and inorganic N rather than organic N is the regulator to soil CH₄ uptake and N₂O emission.     

Landscape patterns of CH4 fluxes in an alpine tundra ecosystem

We measured CH₄ fluxes from three major plant communities characteristic of alpine tundra in the Colorado Front Range. Plant communities in this ecosystem are determined by soil moisture regimes induced by winter snowpack distribution. Spatial patterns of CH₄ flux during the snow-free season corresponded roughly with these plant communities. InCarex-dominated meadows, which receive the most moisture from snowmelt, net CH₄ production occurred. However, CH₄ production in oneCarex site (seasonal mean=+8.45 mg CH₄ m^–2 d^–1) was significantly larger than in the otherCarex sites (seasonal means=–0.06 and +0.05 mg CH₄ m^–2 d^–1). This high CH₄ flux may have resulted from shallower snowpack during the winter. InAcomastylis meadows, which have an intermediate moisture regime, CH₄ oxidation dominated (seasonal mean=–0.43 mg CH₄ m^–2 d^–1). In the windsweptKobresia meadow plant community, which receive the least amount of moisture from snowmelt, only CH₄ oxidation was observed (seasonal mean=–0.77 mg CH₄ m^–2 d^–1). Methane fluxes correlated with a different set of environmental factors within each plant community. In theCarex plant community, CH₄ emission was limited by soil temperature. In theAcomastylis meadows, CH₄ oxidation rates correlated positively with soil temperature and negatively with soil moisture. In theKobresia community, CH₄ oxidation was stimulated by precipitation. Thus, both snow-free season CH₄ fluxes and the controls on those CH₄ fluxes were related to the plant communities determined by winter snowpack.     

Fluxes of methane and carbon dioxide from a small productive lake to the atmosphere

The fluxes of CH₄ and CO₂ to the atmosphere, and the relative contributions of ebullition and molecular diffusion, were determined for a small hypertrophic freshwater lake (Priest Pot, UK) over the period May to October 1997. The average total flux of CH₄ and CO₂ (estimated from 7 sites on the lake) was approximately 52 mmol m^–2 d^–1 and was apportioned 12 and 40 mmol m^–2 d^–1 toCH₄ and CO₂ respectively. Diffusion across the air-water interface accounted for the loss of 0.4and 40 mmol m^–2 d^–1 of CH₄ and CO₂ respectively whilst the corresponding figures for ebullition losses were 12.0 (CH₄) and 0.23 (CO₂) mmol m^–2 d^–1. Most CH₄ (96%) was lost by ebullition, and most CO₂ (99%) by diffusive processes. The ebullition of gas, measured at weekly intervals along a transect of the lake, showed high spatial and temporal variation. The CH₄ content of the trapped gas varied between 44 and 88% (by volume) and was highest at the deepest points. Pulses of gas ebullition were detected during periods of rapidly falling barometric pressure. Therelevance of the measurements to global estimates ofcarbon emission from freshwaters are discussed.     

Effects of parathion and malathion on transformations of urea and ammonium sulfate nitrogen in soils

Summary A study of the effects of malathion and parathion applied at 10 and 50 g/g of soil on transformations of urea and (NH₄)₂SO₄–N in a sandy loam showed that the insecticides retarded urea hydrolysis as well as nitrification of urea and (NH₄)₂SO₄–N. At 50 parts/10⁶ rate of the insecticides, inhibition of urea hydrolysis ranged from 44 to 61% after 0.5 week and from 7 to 21% after 3 weeks of application. The insecticides inhibited the conversion of NH₄ ⁺ to NO₂ ^– without appreciably affecting the subsequent oxidation of NO₂ ^– to NO₃ ^– –N. This resulted in accumulation of higher amounts of NH₄ ⁺–N in soil samples treated with ammonium sulfate or urea N.The results suggest that transformations of urea and NH₄ ⁺ fertilizers in soils may be influenced by the amount of organophosphorus insecticide present and this may affect plant nutrition and fertilizer use.     

Seasonal variation of methane emissions from a temperate swamp

Methane flux measurements were made at four sites in a freshwater temperate swamp over the 13 month period of April 1985 through May 1986. Emissions were highly variable both between sites and over time at any one site. Ebullition from sediments was an important component of methane release. Although release of methane through bubbling occurred in only 19% of the measurements made between April and June 1985, when instrumentation allowed us to separate diffusive and bubble fluxes, ebullition accounted for 34% of the total flux during this period. Methane release rates showed a strong seasonal variation, with highest emission rates observed in early spring and again in late summer, which was associated with changes in plant growth and physiology. Emission rates were partially correlated with sediment temperature, but the relationship was not straightforward, and resembled a step function. Emissions responded strongly to temperature change through the range of 10–16°C. At winter sediment temperatures between 4–9°C, CH₄ flux continued at low rates (0–28 mg CH₄ m^–2d^–1; average = 7.9 mg CH₄m^–2d^–1) and appeared insensitive to changes in sediment temperature. Annual methane emission from three constantly flooded sites (mean water depth = 35 cm) was 43.7 +/- 7.8 gm^–2 (standard error); annual flux from a bank site was 41.4 +/- 20.5 gm^–2. A comparison of flux measurements from fresh and saline wetlands in the immediate area of Newport News Swamp emphasizes the importance of edaphic factors in controlling flux.     

Quantification of methane emissions from Chinese rice fields (Zhejiang Province) as influenced by fertilizer treatment

Methane emissions from rice paddies were quantified by using an automatic field system stationed in Zhejiang Province, one of the centres for rice cultivation in China. The data set showed pronouned interannual variations over 5 consecutive vegetation periods; by computing average values of all experimental plots the annual emissions were 177 g CH₄ m⁻² yr⁻¹ in 1987, 50 g CH₄ m⁻² yr⁻¹ in 1988, and 187 g CH₄ m⁻² yr⁻¹ in 1989. The field preparations encompassed 4 different treatments: (1) no fertilizers, (2) mineral fertilizer (KCl, K₂SO₄), (3) organic manure (rape seeed cake, animal manure), (4) mineral fertilizer plus organic manure. The methane emission rates of the different fertilizer treatments did not show significant differences. The mean emission rates, calculated over the entire observation period of 5 seasons, were 30.4 mg CH₄ m⁻² h⁻¹ (non-fertilized plot) and 28.3 mg CH₄ m⁻² h⁻¹ (mineral fertilizers). These values indicate a high level of methane production even without additional input of organic material into the rice-soils. In the other plots, the organic fertilizers were added once per vegetation period at app. 1 t fresh weight per ha, a relatively low application rate by agronomical standards. The mean emission rates were 35.1 mg CH₄ m⁻² h⁻¹ when manure was applied as sole fertilizer and 27.5 mg CH₄ m⁻² h⁻¹ when applied jointly with potassium fertilizers. Based on the results of this study we estimate a range of 18–28 Tg CH₄ yr⁻¹ as the total methane emission from Chinese rice fields. However, more field data from representative sites in China are needed to reduce the uncertainties in this estimate.     

Microbial population dynamics in an extreme environment: controlling factors in talus soils at 3750 m in the Colorado Rocky Mountains

Variation of CH₄ emissions over a three-year period was studied in a reed-dominated (Phragmites australis) littoral transect of a boreal lake undergoing shoreline displacement due to postglacial rebound. The seasonal variation in plant-mediated CH₄ emissions during open-water periods was significantly correlated with sediment temperature. The highest plant-mediated emission rates (up to 2050 mg CH₄ m^–2 d^–1) were found in the outermost reed zone, where culms of the previous growing seasons had accumulated and free-floating plants grew on the decomposing culms. In reed zones closer to the shoreline as well as in mixed stands of reed and cattail, the maximum daily rates were usually > 500 mg CH₄ m^–2 d^–1. The total plant-mediated CH₄ emission during the open-water period was significantly correlated with the seasonal maximum of green shoot biomass. This relationship was strongest in the continuously flooded (water depth > 25 cm) outermost zones. In this area, emissions through ebullition were of greatest importance and could exceed plant-mediated emissions. In general, total emissions of the open-water periods varied from ca. 20 to 50 g CH₄ m^–2 a^–1, but in the outermost reed zone, the plant-mediated emissions could be as high as 123 g CH₄ m^–2 a^–1; ebullition emissions from this zone reached > 100 g CH₄ m^–2 a^–1. The proportion of CH₄ released in winter was usually < 10% of annual emissions. Emissions of CH₄ were higher in this flooded transgression shore the than those measured in boreal peatlands, but the role of ancient carbon stores as a substrate supply compared with recent anthropogenic eutrophication is unknown.     

Mineral nitrogen and microbial dynamics in the forest floor of clearcut or partially harvested successional boreal forest stands

The effects of clearcut and partial harvesting of early-seral trembling aspen plots were compared to conventional clearcut harvesting in mid-seral mixedwood and late-seral conifer plots. Twice a year, for three consecutive years, we assessed mineral N and microbial dynamics in the forest floor of these plots to test three hypotheses related to the higher litter quality of aspen leaves and to the sustained inputs of available C on partially harvested plots: (1) the post-clearcutting mineral N flush and the net [(NO₃^–): (NO₃^– + NH₄⁺)] production ratio (RNI) are higher in aspen plots than in black spruce plots, with intermediate values occurring in mixedwood plots; (2) net N mineralization rates in aspen plots are higher in spring than in autumn; and (3) compared to clearcutting, partial harvesting reduces potential ammonification and nitrification rates. Initial NH₄⁺ and NO₃^– concentrations respectively ranged between 1.7–4.4 and 0.2–1.5 g N kg^–1 N_total, net ammonification and nitrification rates (30 d incubations) respectively ranged between 5.3–17.8 and 0.1–27.6 g N kg^–1 N_total, basal respiration ranged between 20.9–38.9 mg CO₂-C kg^–1 h^–1, and microbial biomass ranged between 6.1–8.7 g C_mic kg^–1. Although clearcutting increased NO₃^– concentrations in aspen plots, the balance of our results did not support our first hypothesis, because NH₄⁺ concentrations increased in conifer plots only, potential ammonification was unaffected by clearcutting, potential nitrification increased in mixedwood plots only, and RNI increased in all plots. In each seral stage, basal respiration, microbial biomass, and metabolic quotient either increased or were unaffected by clearcutting, suggesting that increases in RNI after disturbance were not related to lower microbial immobilisation of NO₃^– due to lower available C. Forest floors in mid-seral mixedwood plots exhibited a distinct combination of mineral N and microbial properties, suggesting that the functional richness of the forest is enhanced not only by the number of species, but also by the diversity of assemblages that are present. Results supported our second hypothesis and showed, furthermore, that net N mineralization in conifer stands is greater in autumn than in spring. Partial harvesting in aspen stands resulted in lower potential mineralization of N and lower RNI, compared to clearcutting. Further lysimetry studies are needed to confirm whether partial harvesting mitigates NO₃^– leaching following disturbance.     

Effects of Winter Cover Crops Straws Incorporation on CH4 and N2O Emission from Double-Cropping Paddy Fields in Southern China

Residue management in cropping systems is believed to improve soil quality. However, the effects of residue management on methane (CH₄) and nitrous oxide (N₂O) emissions from paddy field in Southern China have not been well researched. The emissions of CH₄ and N₂O were investigated in double cropping rice (Oryza sativa L.) systems with straw returning of different winter cover crops by using the static chamber-gas chromatography technique. A randomized block experiment with three replications was established in 2004 in Hunan Province, China, including rice–rice–ryegrass (Lolium multiflorum L.) (Ry-R-R), rice–rice–Chinese milk vetch (Astragalus sinicus L.) (Mv-R-R) and rice–rice with winter fallow (Fa-R-R). The results showed that straw returning of winter crops significantly increased the CH₄ emission during both rice growing seasons when compared with Fa-R-R. Ry-R-R plots had the largest CH₄ emissions during the early rice growing season with 14.235 and 15.906 g m⁻² in 2012 and 2013, respectively, when Ry-R-R plots had the largest CH₄ emission during the later rice growing season with 35.673 and 38.606 g m⁻² in 2012 and 2013, respectively. The Ry-R-R and Mv-R-R also had larger N₂O emissions than Fa-R-R in both rice seasons. When compared to Fa-R-R, total N₂O emissions in the early rice growing season were increased by 0.05 g m⁻² in Ry-R-R and 0.063 g m⁻² in Mv-R-R in 2012, and by 0.058 g m⁻² in Ry-R-R and 0.068 g m⁻² in Mv-R-R in 2013, respectively. Similar result were obtained in the late rice growing season, and the total N₂O emissions were increased by 0.104 g m⁻² in Ry-R-R and 0.073 g m⁻² in Mv-R-R in 2012, and by 0.108 g m⁻² in Ry-R-R and 0.076 g m⁻² in Mv-R-R in 2013, respectively. The global warming potentials (GWPs) from paddy fields were ranked as Ry-R-R>Mv-R-R>Fa-R-R. As a result, straw returning of winter cover crops has significant effects on increase of CH₄ and N₂O emission from paddy field in double cropping rice system.     

Losses of inorganic carbon and nitrous oxide from a temperate freshwater wetland in relation to nitrate loading

We studied the export of inorganic carbon and nitrous oxide (N₂O) from a Danish freshwater wetland. The wetland is situated in an agricultural catchment area and is recharged by groundwater enriched with nitrate (NO₃ ^–) (1000 M). NO₃ ^– in recharging groundwater was reduced (57.5 mol NO₃ ^– m^–2 yr^–) within a narrow zone of the wetland. Congruently, the annual efflux of carbon dioxide (CO₂) from the sediment was 19.1 mol C m^–2 when estimated from monthly in situ measurements. In comparison the CO₂ efflux was 4.8 mol C m^–2 yr^–1 further out in the wetland, where no NO₃ ^– reduction occurred. Annual exports of inorganic carbon in groundwater and surface water was 78.4 mol C m^–2 and 6.1 mol C m^–2 at the two sites, respectively. N₂O efflux from the sedimenst was detectable on five out of twelve sampling dates and was significantly (P < 0.0001) higher in the NO₃ ^– reduction zone (0.35–9.40 mol m^–2 h^–1, range of monthly means) than in the zone without NO₃ ^– reduction (0.21–0.41 mol m^–2 h^–1). No loss of dissolved N₂O could be measured. Total annual export of N₂O was not estimated. The reduction of oxygen (O₂) in groundwater was minor throughout the wetland and did not exceed 0.2 mol 0₂ m^–2yr^–1. Sulfate (SO₄ ^––) was reduced in groundwater (2.1 mol SO₄ ^–– m^–2 yr^–1) in the zone without NO₃ ^– reduction. Although the NO₃ ^– in our wetland can be reduced along several pathways our results strongly suggest that NO₃ ^– loading of freshwater wetlands disturb the carbon balance of such areas, resulting in an accelerated loss of inorganic carbon in gaseous and dissolved forms.     

Persistence and competitiveness of threeBradyrhizobium japonicum inoculant strains in clay loam Nile valley soil

The nitrogen contribution from the shoot and root system of symbiotically grown leucaena was evaluated in a field experiment on an Alfisol at IITA in Southern Nigeria. Maize in plots that received prunings from inoculated leucaena contained more N and grain yield was increased by 1.9 t.ha.^–1. Large quantities of nitrogen were harvested with leucaena prunings (300 kg N ha^–1 in six months) but the efficiency of utilization of this nitrogen by maize was low compared to inorganic N fertilizer (ammonium sulphate) at 80 kg N ha^–1. Maize yield data indicated that nitrogen in leucaena prunigs was 34 and 45% as efficient as 80 kg N ha^–1 of (NH₄)₂SO₄ for uninoculated and inoculated plants with Rhizobium IRc 1045, respectively. In plots where the prunings were removed, the leaf litter and decaying roots and nodules contributed N equivalent of 32 kg ha^–1. Twenty-five kg ha^–1 was the inorganic N equivalent from nitrogen fixed symbiotically by leucaena when inoculated with Rhizobium strain IRc 1045. Application of prunings from inoculated leucaena resulted in higher soil ogranic C, total N, pH and available NO₃.     

Determinants of spatial variability of methane emissions from wet grasslands on peat soil

Methane (CH₄) emissions from soils, representing the consequence of CH₄ production, CH₄ consumption and CH₄ transport, are poorly characterised and show a large spatial variability. This study aimed to assess the determinants of field-scale spatial variability of CH₄ emissions from wet grasslands on peat soil. Mean CH₄ emission rates of a three-year experiment at 18 plots distributed over three sites in the nature preserve Nieuwkoopse Plassen on peat soil in the Netherlands were related to CH₄ production and CH₄ consumption capacities of soil layers, and to soil and vegetation characteristics. Spatial variability of CH₄ emissions and possible determining factors was high. Annual CH₄ emissions ranged from 3 to 37 g CH₄ m^–2 yr^–1. Coefficients of variation (CV) of CH₄ emissions were on average 37% among sites and 83% within sites. Most important determinants of spatial variability were CH₄ production capacity (average: 211 ng CH₄ g^–1 dry soil h^–1; CV: 131%) and aboveground biomass of sedges (Carex spp.) (average: 0.45 g dm^–2; CV: 127%) (P<0.01). Sedges may affect CH₄ emissions by stimulating CH₄ transport from anaerobic layers to the surface via their vascular system and/or by serving as substrate for methanogens. For extrapolation of CH₄ emissions to larger areas, best results will be obtained by using factors that are easy to determine, like vegetation.     

Influence of soil temperature on methane emission from rice paddy fields

Methane emission rates from an Italian rice paddy field showed diel and seasonal variations. The seasonal variations were not closely related to soil temperatures. However, the dieL changes of CH₄ fluxes were significantly correlated with the diel changes of the temperature in a particular soil depth. The soil depths with the best correlations between CH4 flux and temperature were shallow (1–5cm) in May and June, deep (10–15cm) in June and July, and again shallow (1–5 cm) in August. Apparent activation energies (E_a) calculated from these correlations using the Arrhenius model were relatively low (50–150 kJ mol^–1) in May and June, but increased to higher values (80–450 kJ mol^–1) in August. In the laboratory, CH₄ emission from two rice cultures incubated at temperatures between 20 and 38°C showed E . values of 41 and 53 kJ mol^–1) Methane production in anoxic paddy soil suspensions incubated between 7 and 43°C showed E values between 53 and 132 kJ mol^–1 with an average value of 85 kJ mol^–1) and in pure cultures of hydrogenotrophic methanogenic bacteria E _a values between 77 and 173 (average 126) kJ mol^–1. It is suggested that diel changes of soil properties other than temperature affect CH₄ emission rates, e.g. diel changes in root exudation or in efficiency of CH⁴ oxidation in the rhizosphere.     

Measured and modelled effects of temperature,dissolved oxygen and nutrient concentration on sediment-water nutrient exchange

Empirical models of sediment-water fluxes of NH₄ ⁺, NO₃ ^– were and PO₄ ^3– were formed based on published reports. The models were revised and parameters evaluated based on laboratory incubations of sediments collected from Gunston Cove, VA. Observed fluxes ranged from — 18 (sediments uptake) to 276 (sediment release) mg NH₄ ⁺ m^–2 day^–1, –17 to –509 mg NO₃ ^– m^–2 day^–1, and –16.4 to 8.9 mg PO₄ ^3– m^–2 day^–1. The model and observations indicated release of NH₄ ⁺ was enhanced by high temperature and by low DO. Uptake of NO₃ ^– was enhanced primarily by high NO₃ ^– concentration and to a lesser extent by high temperature and by low DO. Direction of PO₄ ^3– flux depended on concentration in the water. Release was enhanced by low DO. No effect of temperature on PO₄ ^3– flux was observed.