Abstract
The flux of NO between arable land and atmosphere has been measured with a chamber technique. The net flux from the soil to the atmosphere varied from less than 0.1 up to 62 ng NO-N m-2 s-1 for a fertilized area (200 kg N ha-1 as calcium nitrate) and up to 17 ng NO-N m-2 s-1 for an unfertilized area. The emission was high in the summer when the temperature was high and the soil was dry and decreased to low values when the soil surface was thoroughly wetted by rain. Previously reported findings of equilibrium concentrations of NO (compensation point) have been verified. These concentrations ranged from 2 to more than 75 ppbv. At the rural site where the measurements were made, the atmospheric NO concentration was always below this compensation point and there was consequently a net emission of NO from the soil. Nitrogen gases, measured as the difference between NO and NOx (including NO2 and possibly also HNO3 and PAN), were found to be absorbed on soil and vegetation. The absorption of NO2 was generally smaller than the emission of NO.
The areal variability within an area of 100 m2 was found to be moderate with a standard deviation of 25%, somewhat higher on recently fertilized soil (between 50 and 80%). The temperature dependence of NO emission could be described with an activation energy of 65 to 83 kJ mol-1 (Q10 between 2.7 and 3.6). A more rapid increase of production than that predicted by the temperature increase was observed in morning hours. This is tentatively explained to be caused by nutrient dynamics in the soil.
The yearly emission is estimated to be about 0.6 kg NO-N ha-1 and 0.2 kg NO-N ha-1 for the fertilized and unfertilized areas, respectively. During the vegetation period, NO emission from highly fertilized areas might be of some importance when compared with anthropogenic emission from combustion within Sweden.
