Nitrous oxide fluxes and denitrification sensitivity to temperature in Irish pasture soils

Nitrous oxide (N2O) emissions from grazed pastures constitute approximately 28% of total global anthropogenic N2O emissions. The aims of this study were to investigate the effect of inorganic N fertilizer application on fluxes of N2O, 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 N2O fluxes of 67 and 38.7 g N2O-N per hectare per day were associated with times of application of inorganic N fertilizer. Annual fluxes of N2O from control and fertilized treatments were 1 and 2.4 kg N2O-N per hectare, respectively. Approximately 63% of the annual flux was associated with N fertilizer application. Multiple regression analysis revealed that soil nitrate and the interaction between soil nitrate and soil water content were the main factors controlling N2O 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 (r2 = 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 degrees 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 N2O fluxes from soils.