Abstract
Abstract. Temporal variations in the stable isotopic compositions 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 vegetation, dwarf bamboo (Sasa senanensis), with special emphasis on changes in the fate of atmospheric nitrate that had been deposited onto the watershed based on Δ17O values of nitrate. A significant increase in stream nitrate concentration to 15 μmol L−1 in spring of 2004 was correlated with a significant increase in the Δ17O values of nitrate. Additionally, the high Δ17O values of +14.3‰ suggest that the direct drainage of atmospheric nitrate accounted for more than 50% of total nitrate exported from the forested watershed peaking in spring. Similar increases in both concentrations and Δ17O values were also found in spring of 2005. Conversely, low Δ17O values less than +1.5‰ were observed 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 nitrate 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 important to enhancing biological consumption of atmospheric nitrate.
Highlights
1.1 Effects of clear-cutting and strip-cutting on nitrate in stream waterInvestigation of nitrate in stream water eluted from a forested watershed is important to understanding nitrogen cycles within the watershed
Low 17O values less than +1.5 ‰ were observed 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 been converted to nitrate via microbial nitrification
The low and stable stream nitrate concentration during 2002– 2003 implied that atmospheric nitrate had been effectively removed from the forested watershed, and that rain or snow events had little direct impact on the stream nitrate concentration
Summary
Investigation of nitrate in stream water eluted from a forested watershed is important to understanding nitrogen cycles within the watershed. 15 μmol L−1 and that in soil solution from 30 μmol L−1 to more than 100 μmol L−1 These results implied that nitrogen uptake by the understory vegetation was important to preventing nitrogen leaching after tree-cutting, and that the decline of this nitrogen uptake by removal of understory vegetation led to marked nitrate leaching to stream water (Fukuzawa et al, 2006). Quantitative evaluation of the source of increased nitrate in stream water subsequent to artificial clear-cutting and strip-cutting will improve our understanding of N cycling in forested soils prior to artificial alternations, as well as the mechanisms that regulate the direct discharge of NO−3 atm deposited onto surface ecosystems (Durka et al, 1994; Ohte et al, 2004; Costa et al, 2011; Nakagawa et al, 2013). In this study, we conducted further isotope analysis of archived stream water samples to clarify the source of increased nitrate
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