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

[摘要] Temporal variations in the stable isotopic compositions of nitrate dissolvedin stream water eluted from a cool–temperate forested watershed (8 ha) weremeasured to quantify the biogeochemical effects of clear-cutting of treesand subsequent strip-cutting of the understory vegetation, dwarf bamboo(Sasa senanensis), with special emphasis on changes in the fate of atmospheric nitrate thathad been deposited onto the watershed based on Δ¹⁷O values ofnitrate. A significant increase in stream nitrate concentration to 15 μmol L⁻¹ in spring of 2004 was correlated with a significantincrease in the Δ¹⁷O values of nitrate. Additionally, the highΔ¹⁷O values of +14.3‰ suggest that thedirect drainage of atmospheric nitrate accounted for more than 50% oftotal nitrate exported from the forested watershed peaking in spring.Similar increases in both concentrations and Δ¹⁷O values werealso found in spring of 2005. Conversely, low Δ¹⁷O values lessthan +1.5‰ were observed in other seasons, regardlessof increases in stream nitrate concentration, indicating that the majorityof nitrate exported from the forested watershed during seasons other thanspring was remineralized nitrate: those retained in the forested ecosystemas either organic N or ammonium and then been converted to nitrate viamicrobial nitrification. When compared with the values prior tostrip-cutting, the annual export of atmospheric nitrate and remineralizednitrate increased more than 16-fold and fourfold, respectively, in 2004, andmore than 13-fold and fivefold, respectively, in 2005. The understoryvegetation (Sasa) was particularly important to enhancing biological consumptionof atmospheric nitrate.

[学科分类] 地球化学与岩石