Variations in methane and nitrous oxide mixing ratios at the southern boundary of a Canadian boreal forest

Diurnal and seasonal variations in methane (CH4) and nitrous oxide (N2O) mixing ratios were measured above a boreal aspen stand at the southern boundary of the Canadian boreal forest, about 5 km north of agricultural land. The research was conducted between 16 April and 16 September 1994, in the Prince Albert National Park, Saskatchewan, to better understand patterns of CH4 and N2O cycling in boreal ecosystems. The research also presents a method for detecting the long-range transport of trace gases using a micrometeorological, laser-based gas monitoring system. Both CH4 and N2O featured diurnal cycles consistent with a pattern of net emission for each trace gas. The CH4 mixing ratio displayed a seasonal variation that was strongly related to soil temperature, with measured values roughly 30 ppb higher in the late summer than in spring. During the latter half of the experiment, the CH4 mixing ratios varied with wind direction and suggested areas of higher emission to the northeast and east of the measurement tower. The N2O fluxes also showed favoured directions, although in this case the highest mixing ratios were measured during the springtime in air masses originating south and southwest of the tower. The high springtime values coincided with spring thaw emissions of N2O from agricultural fields to the south, and the results suggest that the trace gas analysis system detected the long-range transport of N2O from the agricultural land. Ammonia and ammonium likewise may be transported to the southern boreal forest from agricultural land, and a future investigation at this site could seek to determine the effect of their long-range transport on the southern boreal forest