Coniferous forests as “secondary agricultural” sources of nitrous oxide

Abstract

Measurements of the emissions of the trace gas N 2O 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 N 2O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 N 2O and CO 2 production and an increase in the net CH 4 consumption rate during the 1994 measurement campaign. It appears that NH 3 derived from agriculture is being converted to N 2O in the forest floor, with the forest acting as a “secondary agricultural” source of N 2O. Despite the high levels of N input to the forest, CH 4 consumption was still significant, at mean rates of 17 and 24 μg CH 4 m -2 h -1 for the 1993 and 1994 measurement periods, respectively. A marked positive correlation was observed between net CH 4 consumption and CO 2 flux.