FLUXES OF NITRIC AND NITROUS OXIDES FROM AGRICULTURAL SOILS IN A COOL TEMPERATE CLIMATE

Fluxes of NO and N2O from sandy loam soils cropped with winter wheat and a clay loam soil under ryegrass, with and without the addition of NH4NO3 fertilizer, were measured using static and dynamic chamber methods. Nitric oxide fluxes ranged from -0.3 (deposition) to 6.9 (emission) ng NO-N m-2s-1. The corresponding N2O flux ranged from 0 to 91 (emission) ng N2O-N m-2 s-1. The NO flux was temperature dependent. Activation energies ranged from 40 to 81 kJ mol-1. Nitric oxide and N2O fluxes increased linearly with soil available nitrogen (NH4+NO3). Emissions of NO and N2O were not detectable from unfertilized ryegrass plots. Instead, nitric oxide was absorbed by the soil and vegetation at a maximum rate of 0.31 ng NO-N m-2 s-1. The aeration state of the soil controlled the relative rates of NO and N2O emission. Nitric oxide was the major gas emitted from well aerated soils, conditions that favour nitrification. The NO/N2O emission ratio was > 100 for the coarse-textured sandy loam soil and the clay loam soil only during low rainfall periods. Nitrous oxide was the major gas emitted from less aerated soils, conditions that allowed denitrification to occur. The NO/N2O emission ratio was < 0.001 for the clay loam soil when rainfall was high and soils were wet. Extrapolation to the U.K. situation showed that agricultural land may account for 2-6% of the total annual NOx emission and for 16-64% of the total annual N2O emission in the U.K.