Effects of clear-cutting on nitrogen leaching and fine root dynamics in a cool-temperate forested watershed in northern Japan

Stream and soil solution chemistry, fine root biomass and soil nitrogen processing before and after clear-cutting of trees and subsequent strip-cutting of understory vegetation, dwarf bamboo (Sasa senanensis), were investigated to understand the effect of these disturbances on biogeochemical processes on forested watershed in northern Japan. Tree-cutting did not cause a significant increase of nitrate (NO3−) concentration in stream water during the growing season after the cutting. Subsequent Sasa-cutting caused significant increase of stream NO3− concentration to ca. 15 μmol L−1. There was no significant change of stream pH following both cuttings. NO3− concentration in soil solution increased after both cutting, but the change of concentration was higher (>100 μmol L−1) after the Sasa-cutting than after the tree-cutting. In a riparian conserved area, on the other hand, NO3− in soil solution remained low after tree-cutting, suggesting the riparian area acted as a NO3− sink after the tree-cutting. There was no significant change in total biomass of fine roots after the tree-cutting because of an increase in Sasa root biomass despite a decrease in tree roots. The subsequent Sasa-cutting caused a 50% decrease of fine root biomass compared to that in the un-cut Sasa site. These results suggested that nitrogen uptake by Sasa was very important in preventing nitrogen leaching after tree-cutting, and decline of this nitrogen uptake after Sasa-cutting lead to marked NO3− leaching to the stream.