Coniferous forests as “secondary agricultural” sources of nitrous oxide

Measurements of the emissions of the trace gas N2O were made for the forest floor of a coniferous plantation down-wind from an intensive animal unit, and receiving an inorganic N deposition in excess of 100 kg N ha-1 a-1. Field measurements were made using a mobile laboratory over two periods in August 1993 and July 1994, with peak flux rates of up to 120 μg N2O---N m-2 h-1 being recorded. These flux rates are very high when compared to published data from other temperate coniferous forests and are related to the high N deposition at this N polluted site. Rainfall incidents were associated with pulses of increased N2O and CO2 production and an increase in the net CH4 consumption rate during the 1994 measurement campaign. It appears that NH3 derived from agriculture is being converted to N2O in the forest floor, with the forest acting as a “secondary agricultural” source of N2O. Despite the high levels of N input to the forest, CH4 consumption was still significant, at mean rates of 17 and 24 μg CH4 m-2 h-1 for the 1993 and 1994 measurement periods, respectively. A marked positive correlation was observed between net CH4 consumption and CO2 flux.