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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.

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Spatial heterogeneity of nitrous oxide (N2O) flux was characterized along with various soil chemical, physical, and microtopographical properties to identify those determining flux in fertilized grassland in spring 1993 and in fertilized winter wheat in spring 1994. Measurements were made at random locations within regular grids. Nitrous oxide emission was measured using closed chambers at 84 locations in each grid, spread over 2 d. The ranges of emissions from the grassland and from the winter wheat were 0 to 134 and 0 to 26.4 g N2O‐N ha−1 d−1. Variograms for N2O emission and for concentrations in the soil atmosphere at 100 mm depth indicated that spatial dependence was weak at both sites. Of the other properties, pH and nitrate showed weak autocorrelation but none of the soil physical properties showed any significant spatial dependence. The results of multiple linear regression suggested that denitrification was the main N2O production process at the grassland site, but nitrification may have been equally important at the drier winter wheat site. At both sites, the highest N2O emissions were associated with areas of a few square centimeters to a few square meters, lying below the average slope. Use of a partial least squares regression technique to predict nitrous oxide flux revealed the contribution of air permeability in addition to nitrate, ammonium, and soil water contents. Our analyses suggested that nitrous oxide production, consumption, and transport processes varied markedly with depth (over a few centimeters) near the soil surface.

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Soils are the major source of atmospheric N2O, and better estimates of fluxes are needed to improve the input to climatic general circulation models. We developed a system in a semicontrolled environment to investigate relationships between fluxes of N2O and controlling variables. It consists of 12 soil monoliths (1‐m diam., ≈ 0.6 m deep) in glass fiber casings, the tops of which have been converted into gas flux chambers. These chambers are connected to a gas chromatograph for measurement of N2O and CO2. Gas sampling and analysis is computer controlled and can be done continuously. Temperatures and soil water potential are also recorded continuously. The system has performed reliably since continuous operation began in September 1993. We conducted three experiments, examining the effects of soil water potential, organic matter input, and diurnal temperature variation on N2O fluxes, to illustrate the capabilities of the system. In these experiments, the major emissions of N2O (>800 εg N2O‐N m−2 h−1) occurred when the water potential was above −5 kPa. When plant material was incorporated into the soil, a highly significant correlation was found between N2O and CO2 emissions; the N2O emissions showed pronounced diurnal cycles, with the maxima occurring at night, 4 h after the temperature maxima at 0.1‐m depth. Data interpretation was greatly aided by the frequency and continuity of measurement.