Quantifying the effects of clear-cutting and strip-cutting on nitrate dynamics in a forested watershed using triple oxygen isotopes as tracers

Temporal variations in the stable isotopic com- positions of nitrate dissolved in stream water eluted from a cool-temperate forested watershed (8 ha) were measured to quantify the biogeochemical effects of clear-cutting of trees and subsequent strip-cutting of the understory vegeta- tion, dwarf bamboo (Sasa senanensis), with special empha- sis on changes in the fate of atmospheric nitrate that had been deposited onto the watershed based on 1 17 O values of nitrate. A significant increase in stream nitrate concentra- tion to 15 µmol L 1 in spring of 2004 was correlated with a significant increase in the 1 17 O values of nitrate. Addi- tionally, the high 1 17 O values of +14.3 ‰ suggest that the direct drainage of atmospheric nitrate accounted for more than 50 % of total nitrate exported from the forested water- shed peaking in spring. Similar increases in both concentra- tions and 1 17 O values were also found in spring of 2005. Conversely, low 1 17 O values less than +1.5 ‰ were ob- served in other seasons, regardless of increases in stream nitrate concentration, indicating that the majority of nitrate exported from the forested watershed during seasons other than spring was remineralized nitrate: those retained in the forested ecosystem as either organic N or ammonium and then been converted to nitrate via microbial nitrification. When compared with the values prior to strip-cutting, the annual export of atmospheric nitrate and remineralized ni- trate increased more than 16-fold and fourfold, respectively, in 2004, and more than 13-fold and fivefold, respectively, in 2005. The understory vegetation (Sasa) was particularly im- portant to enhancing biological consumption of atmospheric nitrate.