Nitrous oxide fluxes and denitrification sensitivity to temperature in Irish pasture soils M. Abdalla 1 , M. J ones 1 , P. S mith 2 & M. W illiams 1 1 Department of Botany, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland, and 2 School of Biological Sciences, University of Aberdeen, Cruickshank Building, St Machar Drive, Aberdeen AB24 3UU, UK Abstract Nitrous oxide (N 2 O) emissions from grazed pastures constitute approximately 28% of total global anthropogenic N 2 O emissions. The aims of this study were to investigate the effect of inorganic N fertilizer application on fluxes of N 2 O, quantify the emission factors (EFs) for a sandy loam soil which is typical of large areas in Ireland and to investigate denitrification sensitivity to temperature. Nitrous oxide flux measurements from a cut and grazed pasture field for 1 year and denitrification laboratory incubation were carried out. The soil pH was 7.3 and had a mean organic C and N content at 0–20 cm of 44.1 and 4.4 g ⁄ kg dry weight, respectively. The highest observed peaks of N 2 O fluxes of 67 and 38.7 g N 2 O-N per hectare per day were associated with times of application of inorganic N fertilizer. Annual fluxes of N 2 O from control and fertilized treatments were 1 and 2.4 kg N 2 O-N per hectare, respectively. Approximately 63% of the annual flux was associated with N fertilizer applica- tion. Multiple regression analysis revealed that soil nitrate and the interaction between soil nitrate and soil water content were the main factors controlling N 2 O flux from the soil. The derived EF of 0.83% was approximately 66% of the IPCC default EF value of 1.25% as used by the Irish EPA to estimate greenhouse gases (GHGs) in Ireland. The IPCC-revised EF value is 0.9%. A highly significant exponential regression (r 2 = 0.98) was found between denitrification and incubation temperature. The calculated Q 10 ranged from 4.4 to 6.2 for a temperature range of 10–25 °C and the activation energy was 47 kJ ⁄ mol. Our results show that denitrification is very sensitive to increasing temperature, suggesting that future global warming could lead to a significant increase in soil denitrification and consequently N 2 O fluxes from soils. Keywords: Nitrous oxide, soil denitrification, grazed pasture, acetylene inhibition technique Introduction The increase in concentration of atmospheric N 2 O from 270 to 319 p.p.m. in the last 250 years (IPCC, 2007) has, in part, caused an acceleration in both global warming (IPCC, 2007) and stratospheric ozone depletion (Crutzen, 1981; Cicerone, 1987). According to the IPCC (1996, 2007) N 2 O emission from grazed pasture is about 1600 Gg N 2 O per year which constitutes 28% of the global anthropogenic N 2 O emissions. In Europe, semi-natural and managed grasslands are com- monly used to provide ruminants with fodder either directly in the field during the grazing season or as hay and silage during the winter (Flechard et al., 2007). As N is often limit- ing in grassland ecosystems, mineral or organic fertilizers are frequently applied to increase herbage productivity. How- ever, available N may temporarily exceed plant needs and N losses then occur through nitrate leaching and gaseous emis- sions including N 2 O, even when timing and amounts of N fertilization are optimized. Nitrous oxide emissions from grasslands thus tend to occur in short-lived bursts following the application of fertilizers (Ryden, 1981; Clayton et al., 1997; Leahy et al., 2004). The small-scale spatial variability in N 2 O fluxes is notoriously high (Ambus & Christensen, 1994; Ball et al., 2000; Van den Heuvel et al., 2008) and appears to be controlled by interacting abiotic and biotic factors, such as plants, micro-organisms, precipitation and nutrients. These factors may vary on an annual basis with a significant effect on the magnitude of the N 2 O flux. Further- more, on a temporal basis, the N 2 O flux is also expected to vary depending on the dominant controlling factor (Mum- mey et al., 1997). Thus, besides the technical and analytical Correspondence: M. Abdalla. E-mail: abdallm@tcd.ie Received July 2009; accepted after revision July 2009 Soil Use and Management, December 2009, 25, 376–388 doi: 10.1111/j.1475-2743.2009.00237.x 376 ª 2009 The Authors. Journal compilation ª 2009 British Society of Soil Science Soil Use and Management