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Fractions of soil organic matter (SOM) were obtained from three soils using alternative physical fractionation procedures, and evaluated against the requirements of model pools. We compared two‐stage density fractionation (isolating free and intra‐aggregate fractions, before and after dispersion, respectively) with particle‐size separation of dispersed soil. For full comparison, the organomineral fraction residual from density fractionation was also size separated. In standardizing the density‐based method, we found recovery of intra‐aggregate organic matter highly sensitive to separation density as compared with the free. Recovery of the intra‐aggregate was also influenced by dispersion energy. The greatest amount was obtained using a combination of the highest density (1.80 g cm−3) and dispersion energy (1500 J g−1). Analysis by 13C nuclear magnetic resonance (NMR) showed O‐alkyl/alkyl‐C ratios 1.38 to 2.30 times greater in intra‐aggregate organic matter than in the free. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) also indicated a greater proportion of aliphatic hydrocarbon, carboxylic anions, and aromatic C in intra‐aggregate organic matter. The findings suggest this fraction comprises more decomposed and transformed organic matter relative to the free. Higher signal/noise ratios in NMR spectra of particle‐size fractions (compared with their organomineral equivalents) were attributed to C in particulate SOM, not removed by prior density separation. Whilst particle‐size fractions confuse particulate SOM with that attached to mineral surfaces, fractions isolated by two‐stage density separation are small in number and display distinct chemical properties. We suggest they provide a sound basis for a model of SOM turnover based on measurable pools.

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The aims of this study were to use closed chambers to improve estimates of N2O‐N losses from intensively managed grassland on poorly drained soils and to provide measurements for comparison with fluxes determined simultaneously using micrometeorological methods. A 10‐ha field on clay soil in central Scotland received 185 kg NH4NO3‐N ha−1 on April 3, 1992. Twenty‐four closed chambers were installed, six in a 2–3‐ha area grazed by cattle the previous summer, the remainder in an ungrazed area. Fluxes were measured regularly for 3 weeks. Nitrous oxide accumulation in the chambers was determined by gas chromatography. No flux was detected before fertilization. After fertilization, fluxes from the ungrazed and grazed areas were 153±9 and 557±107 g N2O‐N ha−1 d−1, respectively (means and standard errors of all measurements). The individual fluxes ranged from 8 to 712 and 6 to 1519 g N2O‐N ha−1 d−1, respectively, showing marked temporal and spatial variability and lognormal distributions. Fluxes peaked five days after fertilization and were one sixth of their maxima by April 24. Spatial differences observed initially were generally maintained. Incubation of cores with 10% acetylene suggested that the N2O was produced by denitrification in the top 5 cm of soil, and in situ soil N2O measurements confirmed that the concentration was highest close to the surface. In a regression model of the flux from the ungrazed area (including the pre‐ to postfertilization transition), air temperature, recent rainfall, and NO3−‐N could account for 52% of the temporal variability. The higher flux from the grazed area may have resulted from greater local heterogeneity of the surface soil in that area, arising from uneven compaction due to treading by livestock. The total N2O‐N losses (1.7 and 5.1% of applied N from the ungrazed and grazed areas, respectively) confirm that fertilized grassland can contribute substantially to global N2O emissions.

Nitrous oxide (N,O) emissions and concentrations in the soil atmosphere were measured at a number of sites of differing soil type in south-east Scotland between 1985 and 1988. Concentrations followed log-normal distributions and were significantly affected by soil type, tillage treatment, and nitrate application rate. The shape of the profiles suggested significant consumption in the upper 5 cm, making calculations of emission rates using Fick's Law unsatisfactory. Emission rates measured using closed flux chambers were at least one order of magnitude smaller from heavier-textured arable soils than from lighter ones.Denitrification fluxes measured by field application of the acetylene inhibition technique were lowest in a clay loam, and highest in an alluvial sandy loam; this was attributed to a failure to achieve a satisfactory distribution of acetylene in the heavier soil. Denitrification rates in soil cores generally exceeded measured surface fluxes; incubation at decreased oxygen concentrations typical of those measured in the field produced a further significant increase. Core incubation should be used as an alternative to in situ field measurement only if the oxygen concentration in the incubation vessels is adjusted to mimic that in the field; otherwise denitrification rates may be significantly underestimated.

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Measurements of methane emission rates and concentrations in the soil were made during four growing seasons at the International Rice Research Institute in the Philippines, on plots receiving different levels of organic input. Fluxes were measured using the automated closed chambers system (total emission) and small chambers installed between plants (water surface flux). Concentrations of methane in the soil were measured by collecting soil cores including the gas phase (soil-entrapped methane) and by sampling soil solution in situ (dissolved methane). There was much variability between seasons, but total fluxes from plots receiving high organic inputs (16-24 g CH4 m(-2)) always exceeded those from the low input plots (3-9 g CH4 m(-2)). The fraction of the total emission emerging from the surface water (presumably dominated by ebullition) was greater during the first part of the season, and greater from the high organic input plots (35-62%) than from the low input plots (15-23%). Concentrations of dissolved and entrapped methane in the low organic input plots increased gradually throughout the season; in the high input plots there was an early-season peak which was also seen in emissions. On both treatments, periods of high methane concentrations in the soil coincided with high rates of water surface flux whereas low concentrations of methane were generally associated with low flux rates.

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