Abstract. Grazing animals on managed pastures and rangelands have been identified recently as significant contributors to the global NzO budget. This paper summarizes relevant literature data on N20 emissions from dung, urine and grazed grassland, and provides an estimate of the contribution of grazing animals to the global NzO budget.The effects of grazing animals on N2O emission are brought about by the concentration of herbage N in urine and dung patches, and by the compaction of the soil due to treading and trampling. The limited amount of experimental data indicates that 0.1 to 0.7% of the N in dung and 0.1 to 3.8% of the N in urine is emitted to the atmosphere as N2O.There are no pertinent data about the effects of compaction by treading cattle on N2O emission yet. Integral effects of grazing animals have been obtained by comparing grazed pastures with mown-only grassland. Grazing derived emissions, expressed as per cent of the amount of N excreted by grazing animals in dung and urine, range from 0.2 to 9.9%, with an overall mean of 2%. Using this emission factor and data statistics from FA0 for numbers of animals, the global contribution of grazing animals was estimated at 1.55 Tg N20-N per year.This is slightly more than 10% of the global budget.

There is an urgent need to provide an accurate, up-to-date estimate of N(2)O fluxes in order that national policies can be developed to reduce emissions of N(2)O from soils. There are only limited data on temporal and diurnal patterns of N(2)O fluxes to the atmosphere, mainly due to constraints in the measurement techniques. In this paper we present the first terrestrial source values of N(2)O isotopomers and have measured and quantified the temporal and diurnal variability in N(2)O fluxes following urine addition to a grassland system in the UK. The experiment was carried out over a 2-week period on an artificially drained grassland system at the Institute of Grassland and Environmental Research (IGER), North Wyke, UK. Duplicate samples of urine, each of 2 L, were collected from dairy cows and applied to chambers (of area 0.16 m(2)). The N(2)O diurnal fluxes from urine and control (no urine) plots were measured by an automatic closed chamber technique. The isotopomers of N(2)O were obtained by analysing the gas samples collected during a peak emission phase. Soil and meteorological data were also collected. The results showed strong diurnal variations in N(2)O fluxes with minimum fluxes generally occurring between 7:00 and 14:00 hrs. The total cumulative flux of N(2)O for the whole experimental period was higher by a factor of >2 compared with estimates based on the daytime (between 10.00-16.00 hrs) measurements only. Therefore, measurements of N(2)O fluxes based on daily single exposure and expressed on a 24-h basis could impose a considerable bias and inaccuracy to the emission estimates, depending on when it was taken. The measured site preference values (difference between the centre (delta(15)Nalpha) and the end (delta(15)Nbeta) N atom of the N(2)O molecule) for soil-emitted N(2)O measured during our study were always lower than the tropospheric value. This work confirms that the enhanced tropospheric N(2)O site preference value could be the result of the back injection from the stratosphere. The intramolecular isotope ratios of nitrogen (delta(15)N) and oxygen (delta(18)O) and the site preference of the emitted N(2)O indicated that there was a shift of processes during the measurement period.

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In Sweden, 90% of ammonia (NH 3 ) emissions to the atmosphere originate from agriculture, predominantly from animal manure handling. It is well known that incorporation of manure into soil can reduce NH 3 emissions after spreading. However, there is a risk of increased nitrous oxide (N 2 O) and methane (CH 4 ) emissions caused by bacterial activity and limited oxygen availability under these conditions. A full-scale injector was developed and evaluated in a field experiment on grassland. Cattle slurry was either injected in closed slots 5 cm below ground or band spread on the soil surface above the crop canopy at a rate of 25 t ha )1 . In a control treatment, no slurry was applied. During a 5-day period after application, NH 3 emissions were measured using an equilibrium concentration method. Gas samples for estimating CH 4 and N 2 O emissions were also collected during 7 weeks following slurry application. Injection in closed slots resulted in no detectable NH 3 emissions. After band spreading, however, NH 3 emissions corresponded to nearly 40% of the total ammoniacal nitrogen in the applied slurry. The injection of slurry gave rise to a broad peak of N 2 O emissions during the first 3 weeks after application. In total, for the measuring period, N 2 O emissions corresponded to 0.75 kg N ha )1 . Band spreading resulted in only a very small N 2 O release of about 0.2 kg N ha )1 during the same period. Except for the first sampling occasion, the soil was predominantly a sink for CH 4 in all the treatments. The use of the injector without slurry application reduced grass yield during unfavourable growing conditions. In conclusion, shallow injection in closed slots seems to be a promising technique to reduce negative environmental impacts from NH 3 emissions with a limited release of N 2 O and CH 4 .

Nitrifiers and denitrifiers are the main producers of the greenhouse gas nitrous oxide (N(2)O). Knowledge of the respective contributions of each of these microbial groups to N(2)O production is a prerequisite for the development of effective mitigation strategies for N(2)O. Often, the differentiation is made by the use of inhibitors. Measurements of the natural abundance of the stable isotopes of N and O in N(2)O have been suggested as an alternative for the often unreliable inhibition studies. Here, we tested the natural abundance incubation method developed by Tilsner et al.1 with soils from four European grasslands differing in long-term management practices. Emission rates of N(2)O and stable isotope natural abundance of N(2)O and mineral N were measured in four different soil incubations: a control with 60% water-filled pore space (WFPS), a treatment with 60% WFPS and added ammonium (NH(4) (+)) to support nitrifiers, a control with 80% WFPS and a treatment with 80% WFPS and added nitrate (NO(3) (-)) to support denitrifiers. Decreases in NH(4) (+) concentrations, linked with relative (15)N-enrichment of residual NH(4) (+) and production of (15)N-depleted NO(3) (-), showed that nitrification was the main process for mineral N conversions. The N(2)O production, however, was generally dominated by reduction processes, as indicated by the up to 20 times larger N(2)O production under conditions favouring denitrification than under conditions favouring nitrification. Interestingly, the N(2)O concentration in the incubation atmospheres often levelled off or even decreased, accompanied by increases in delta(15)N and delta(18)O values of N(2)O. This points to uptake and further reduction of N(2)O to N(2), even under conditions with small concentrations of N(2)O in the atmosphere. The measurements of the natural abundances of (15)N and (18)O proved to be a valuable integral part of the natural abundance incubation method. Without these measurements, nitrification would not have been identified as essential for mineral N conversions and N(2)O consumption could not have been detected.

The N2O and N2 fluxes emitted from a temperate UK grassland soil after fertiliser application (equivalent to 25 and 75 kg N ha(-1)) were simultaneously measured, using a new automated soil incubation system, which replaces soil atmosphere (N2 dominated) with a He+O2 mixture. Dual isotope and isotopomer ratios of the emitted N2O were also determined. Total N2O and N2 fluxes were significantly lower (P<0.001) in the control (0 kg N) than in the 25 and 75 kg N treatments. The total N2O flux was significantly higher (P<0.001) in the 75 kg N than in the 25 kg N treatment. The general patterns of N2O and N2 fluxes were similar for both fertiliser treatments. The total gaseous N loss in the control treatment was nearly all N2, whereas in the fertiliser treatment more N2O than N2 was emitted from the soil. The ratio N2O/N2 fluxes as measured during the experiment suggested three phases in N2O production, in phase 1 nitrification>denitrification, in phase 2 denitrification>nitrification, and in phase 3 denitrification (and total denitrification)>>nitrification. Dual delta15N and delta18O isotope and isotopomer (delta15Nalpha and delta15Nbeta) value ratios of emitted N2O also pointed towards an increasing dominance of the production of N2O by denitrification and total denitrification. The site preference value from the soil-emitted N2O was lower than the troposphere value. This confirmed that the enhanced troposphere N2O site preference could result from back injection of N2O from the stratosphere. The measurements of N2O/N2 flux ratio and the isotopic content of emitted N2O pointed, independently, to similar temporal trends in N2O production processes after fertiliser application to grassland soil. This confirmed that both measurements are suitable diagnostic tools to study the N2O production process in soils.

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Nitrous oxide emission factors (EFs) were calculated from measurements of emissions from UK wheat crops and grassland, that were part of a wider research programme on N loss pathways and crop responses. Field studies were undertaken in 2003, 2004 and 2005-a total of 12 site-seasons. Nitrous oxide emissions were measured by the closed static chamber method, following the application of various N fertilizer forms (ammonium nitrate (AN), calcium ammonium nitrate (CAN), urea (UR), urea ammonium sulphate and urea ammonium nitrate) at the recommended rates. Emission factors for the growing season (March-September) ranged from less than 0.1-3.9 %. In the 2nd year, measurements continued at three sites until the following February; the resulting annual EFs were one-third greater, on average, than those for the growing season. There was some evidence that N 2 O emissions from UR were smaller than from AN or CAN, but when this was adjusted for loss of ammonia by volatilization, there was generally little difference between different forms of N. Emissions from UR modified by the addition of the urease inhibitor nBTPT (UR ? UI) were lower than corresponding emissions from nitrate forms, except under conditions where Electronic supplementary material The online version of this article (emissions were generally low, even allowing for indirect emissions, suggesting that the use of a urease inhibitor can provide some mitigation of N 2 O, as well as NH 3 , emissions. The emission data broadly bear out the relationships obtained in earlier UK studies, showing a strong dependence of N 2 O emission on soil wetness, temperature and the presence of sufficient mineral N in the soil, which decreases rapidly after N application mainly as a result of plant uptake. Overall net mean EFs for the whole season (after subtracting background emissions from unfertilized controls) covered a range wider than the 0.3-3.0 % range of IPCC (2006).