Understanding nitrous oxide (N 2 O) emissions from agricultural soils in semi-arid regions is required to better understand global terrestrial N 2 O losses. Nitrous oxide emissions were measured from a rain-fed, cropped soil in a semi-arid region of southwestern Australia for one year on a sub-daily basis. The site included N-fertilized (100 kg N ha À1 yr À1 ) and nonfertilized plots. Emissions were measured using soil chambers connected to a fully automated system that measured N 2 O using gas chromatography. Daily N 2 O emissions were low (À1.8 to 7.3 g N 2 O-N ha À1 day À1 ) and culminated in an annual loss of 0.11 kg N 2 O-N ha À1 from N-fertilized soil and 0.09 kg N 2 O-N ha À1 from nonfertilized soil. Over half (55%) the annual N 2 O emission occurred from both N treatments when the soil was fallow, following a series of summer rainfall events. At this time of the year, conditions were conducive for soil microbial N 2 O production: elevated soil water content, available N, soil temperatures generally 425 1C and no active plant growth. The proportion of N fertilizer emitted as N 2 O in 1 year, after correction for the 'background' emission (no N fertilizer applied), was 0.02%. The emission factor reported in this study was 60 times lower than the IPCC default value for the application of synthetic fertilizers to land (1.25%), suggesting that the default may not be suitable for cropped soils in semi-arid regions. Applying N fertilizer did not significantly increase the annual N 2 O emission, demonstrating that a proportion of N 2 O emitted from agricultural soils may not be directly derived from the application of N fertilizer. 'Background' emissions, resulting from other agricultural practices, need to be accounted for if we are to fully assess the impact of agriculture in semi-arid regions on global terrestrial N 2 O emissions.

Denitrification is an important soil process for assessing nitrogen cycling and controlling nitrogen pollution in the environment. Numerous studies of denitrification rates in soils have been reported over the last decade, many with sampling protocols that are more reliable than in the past. In this paper, we review denitrification rates for agricultural and forest soils that have been reported in the literature, discuss factors that appear to be important in controlling the amount of denitrification that occurs in these soils, and summarise modelling approaches that have been used to predict annual denitrification rates. Most studies of in situ denitrification in upland soils have been conducted in agricultural grassland and forest ecosystems, with a paucity of studies reported from other ecosystems. A large range of annual, in situ, denitrification rates have been reported (0–239 kg N/ha.year), with the highest rates typically occurring in irrigated, nitrogen-fertilised soils. However, most annual denitrification rates reported in the literature appear to be fairly low, with over half of the rates in forest soils being <1 kg N/ha.year (mean of 1.9 kg N/ha.year). Rates of denitrification in agricultural soils tend to be higher than in forest soils, with 85% of rates reported being >1 kg N/ha.year, and a mean rate of 13 kg N/ha.year. Numerous soil, site, and management factors have been reported to affect the denitrification process in situ. The literature indicates that the highest rates of denitrification can be expected in nitrogen-fertilised soils, or where site management increases soil nitrate availability. Where nitrate is non-limiting, denitrification rates appear to be highest in irrigated loam soils. The review suggests that it is difficult to predict denitrification rates based on our current understanding, and that pilot studies should still be conducted if soil nitrogen balances are required.

Many recent reviews and meta-analyses of N2O emissions do not include data from Mediterranean studies. In this paper we present a meta-analysis of the N2O emissions from Mediterranean cropping systems, and propose a more robust and reliable regional emission factor (EF) for N2O, distinguishing the effects of water management, crop type, and fertilizer management. The average overall EF for Mediterranean agriculture (EFMed) is 0.5%, which is substantially lower than the IPCC default value of 1%. Soil properties had no significant effect on EFs for N2O. Increasing the nitrogen fertilizer rate led to higher EFs; when N was applied at rates greater than 400 kg N ha-1, the EF did not significantly differ from the 1% default value (EF: 0.82%). Liquid slurries led to emissions that did not significantly differ from 1%; the other fertilizer types were lower than 1% but did not significantly differ from each other. Rain-fed crops in Mediterranean regions have lower EFs (EF: 0.27%) than irrigated crops (EF: 0.63%). Drip irrigation systems (EF: 0.51%) had 44% lower EF than sprinkler irrigation methods (EF: 0.91%). Extensive crops, such as winter cereals (wheat, oat and barley), had lower EFs (EF: 0.26%) than intensive crops such as maize (EF: 0.83%). For flooded rice, the inundated environment favored anaerobic conditions leading to complete denitrification and low EFs (EF: 0.19%). Our results indicate that N2O emissions from Mediterranean agriculture are overestimated in current national greenhouse gas inventories and that, with the new EF determined from this study, the effect of mitigation strategies such as drip irrigation or the use of nitrification inhibitors, even if highly significant, may be smaller in absolute terms.

The Intergovernmental Panel on Climate Change methodology estimates that over 50% of total nitrous oxide (N 2 O) emissions in New Zealand derive from animal excreta-N deposited during grazing. The emission factor for excreta-N as used by this methodology has an important impact on New Zealand's total N 2 O inventory. The objectives of this study were to refine the N 2 O emission factor for urine by simultaneously measuring N 2 O emissions from 5 pastoral soils of different drainage class, in 3 different regions in New Zealand following a single application of urine; plus test various aspects of the soil cover method for determining emission factors. Cow urine and synthetic urine was applied to pastoral soils in autumn 2000 and N 2 O emissions were measured using closed flux chambers at regular intervals for 4-18 months following application. The N 2 O emission factors for cow urine estimated for the first 4 months after urine application varied greatly depending on rainfall and soil drainage class, and ranged from 0.3 to 2.5% of the urine-N applied, suggesting that adopting a single emission factor for New Zealand may be inappropriate. The largest emission factor was found in a poorly drained soil, and the lowest emission factor was found in a well-drained stony soil. Ongoing measurements on one of the soils resulted in an increase in emission factors as the N 2 O emissions had not reached background levels 4 months after urine application. To characterise urine-induced N 2 O emissions, we recommend measurements continue until N 2 O emissions from urine-amended soil return to background levels. Furthermore, we recommend using real animal urine rather than synthetic urine in studies when determining the N 2 O emission factor for urine. S R 0 2 1 2 8 N i t r o u s o x i d e e m i s s i o n f a c t o r C e c i l e A . M . d e K l e i n e t a l .

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