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

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 17 O 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 17 O values of nitrate. Additionally, the high 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 watershed peaking in spring. Similar increases in both concentrations and 17 O values were also found in spring of 2005. Conversely, low 17 O 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.

Export flux of unprocessed atmospheric nitrate from temperate forested catchments: A possible new index for nitrogen saturation

“…In 5 addition to these traditional isotopes, 17 O-excess (Δ 17 O; the definition will be presented in section 1.2) of nitrate has been used as an additional, more robust tracer for unprocessed NO 3 − atm (nitrate that had been supplied via atmospheric deposition and have survived the biological processing of nitrate, such as assimilation and denitrification, within surface ecosystems) in stream water in recent years (Bourgeois et al, 2018;Michalski et al, 2004;Riha et al, 2014;Sabo et al, 2016;Tsunogai 10 et al, 2010;Tsunogai et al, 2014;Tsunogai et al, 2016). By determining both concentration and the 17 O-excess of stream nitrate, we can quantify unprocessed NO 3 − atm in stream water accurately and precisely.…”

“…By determining both concentration and the 17 O-excess of stream nitrate, we can quantify unprocessed NO 3 − atm in stream water accurately and precisely. Recent studies on nitrate including unprocessed NO 3 − atm exported from forested catchments via streams during base flow period have revealed that the export flux of unprocessed NO 3 − atm increases in accordance with increases in the stream nitrate concentration (Rose et al, 15 2015a; Rose et al, 2015b;Tsunogai et al, 2014). These results implied that unprocessed NO 3 − atm exported from forested catchments can be used as a tracer to identify the biological processing of nitrate in each catchment area and to clarify the processes regulating nitrate concentrations in stream water.…”

“…To quantify the export flux of unprocessed NO 3 -atm from each catchment, the daily export flux of 15 unprocessed NO 3 -atm per unit area of the catchment (F atm ) was calculated for each day on which the Δ 17 O value of nitrate was determined, by applying equation (3) (Tsunogai et al, 2014):…”

“…6(c)) in IJ2. (Bourgeois et al, 2018;Nakagawa et al, 2013;Riha et al, 2014;Sabo et al, 2016;Tsunogai et al, 2014;Tsunogai et al, 2016).…”

“…Therefore, while the δ 15 N or δ 18 O signature of NO 3 − atm can be overprinted by biological processes subsequent to deposition, Δ 17 O can be used as a robust tracer of 20 unprocessed NO 3 − atm to reflect its accurate mole fraction within total NO 3 − regardless of partial metabolism through denitrification and assimilation subsequent to deposition (Michalski et al, 2004;Tsunogai et al, 2011;Tsunogai et al, 2014), using the following equation: (Dejwakh et al, 2012;Liu et al, 2013;Riha et al, 2014;Tsunogai et al, 2011;Tsunogai et al, 2010;Tsunogai et al, 2014;Tsunogai et al, 2016).…”