The influence of extreme storms on nitrogen export from mountain forest watershed

Abstract:

To determine the influence of extreme storms on nitrogen export from forested watersheds, we quantitatively evaluated nitrate, dissolved organic nitrogen and particle nitrogen export at 22 rainfall events (from 3.0 to 403.5 mm) in Aburahi-S experimental forested watershed (Ab-S, 3.34 ha) located in central Japan. We also evaluated the atmospheric NO₃^- export at 9 rainfall events (from 3 mm to 144mm) by measuring oxygen isotope of nitrate.

The atmospheric nitrate export depended on streamwater discharge rate in many rainfall events, however, the lowest atmospheric nitrate export was recorded in rainfall event of 82.5 mm, which occurred 5 days after a previous large rainfall event (144 mm), even though streamwater discharge rate was considerable. On the other hand, nitrified nitrate export depended on streamwater discharge rate in almost all rainfall events. These results suggest that the pool of both nitrate in watershed strongly influence nitrate export pattern in each rainfall event.

Total nitrate export did not peak out in the largest rainfall (403.5 mm) event, but total nitrate export was larger at two largest rainfall evets (172.6 mm and 403.5 mm) against the amount of rainfall. Dissolved organic nitrogen export depended on the amount of rainfall, particle nitrogen export started to increase from rainfall event larger than 70 mm. As the results, total nitrogen export in two largest rainfall event (403.5mm and 172.6 mm) were 2.2 and 2.3 times larger than calculated values from liner regression between the amount of rainfall and total nitrogen export. These results suggest that increase of large storm lead to increase annual total nitrogen export from forested watersheds.