Quantifying nitrate dynamics in an oligotrophic lake using Δ<suP>17</sup>O

Abstract

Abstract. The stable isotopic compositions of nitrate, including the 17 O anomalies ( 17 O), were determined twice in 1 yr (June and August 2007) in the oligotrophic water column of Lake Mashu, Japan. These data were then used to quantify the geochemical dynamics of nitrate in the lake, by using the deposition rate of the atmospheric nitrate onto the entire catchment area of the lake. The total amount of nitrate in the lake water decreased from 4.2 to 2.1 Mmol during the period between the observations, while the average 17 O values remained uniform at +2.5‰. The 17 O values corresponded to an small and uniform mixing ratio of atmospheric nitrate to total nitrate of 9.7 ± 0.8%. These results indicate that 0.52 ± 0.34 Mmol of the remineralized nitrate was fed into the water column through nitrification, while 2.6 ± 0.4 Mmol of nitrate was simultaneously removed from the water column by assimilation, during the period between the observations. The lake water dissolved nitrate was characterized by rapid removal through assimilation during summer until it was almost completely removed from the euphotic layer, as well as continuous feeding into the lake through nitrification (3.2 ± 0.3 Mmol a −1 ) and deposition (0.35 ± 0.2 Mmol a −1 ), regardless of the seasons. The 15 N-depleted nitrogen isotopic compositions of nitrate were as low as −6.5‰ in June, which also indicates that in-lake nitrification is the major source of nitrate in the lake and suggests that there is low potential for denitrification in and around the lake. Atmospheric nitrate deposited into the lake will be assimilated quickly, having a mean residence time of 1.2 ± 0.1 yr. In addition, more than 90% of the assimilated nitrate will be remineralized to nitrate and re-assimilated via active nitrogen cycling in the lake.

Microbial denitrification dominates nitrate losses from forest ecosystems

“…These results, in combination with the changes in NO 3 − concentration observed (Figs. 1C and 2), demonstrate that NO 3 − was both consumed and elevated in 15 N/ 14 N and 18 O/ 16 O as it moved from environments of NO 3 − production in surface soils to deeper soils and streams.…”

“…This method has been used to evaluate gross nitrification rates in a lake system (16) and recently in soils (17), and it requires determination of Δ 17 O of NO 3 − in atmospheric deposition and stream water, as well as NO 3 − deposition mass fluxes. In addition to NO 3 − source partitioning via Δ 17 O, we partitioned NO 3 − losses to streams (F L ) and denitrification (F D ), and gross biological uptake by microbes and plants (F U ) using δ 15 N and the corresponding isotope effects of these processes on 15 N/ 14 N. According to isotope balance, we thereby derive Eqs.…”

“…This approach takes advantage of the biological imprint of soil denitrification on the N cycle and the tendency for kinetic isotope effects to elevate 15 N/ 14 N and 18 O/ 16 O of NO 3 − systematically in forests (8). Here, we extend on this approach by developing a new way to estimate NO 3 − supply rates to denitrification, which involves a combined Δ 17 O, δ 15 N, and δ 18 O analysis.…”

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

“…The natural stable isotopic composition of nitrate has been widely applied in the determination of the sources of nitrate in natural freshwater systems (Wada et al, 1975;Durka et al, 1994;Williard et al, 2001;Burns and Kendall, 2002;Campbell et al, 2002;Michalski et al, 2004;Ohte et al, 2004;Hales et al, 2007;Barnes et al, 2008;Burns et al, 2009;Tsunogai et al, 2010;Tobari et al, 2010;Ohte et al, 2010;Barnes and Raymond, 2010;Tsunogai et al, 2011;Nestler et al, 2011;Curtis et al, 2011;Costa et al, 2011;Pellerin et al, 2012;Dejwakh et al, 2012;Yue et al, 2013;Ohte, 2013;Lohse et al, 2013;Thibodeau et al, 2013;Nakagawa et al, 2013 (Michalski et al, 2003;Morin et al, 2008;Alexander et al, 2009). Using the 17 O signature defined by the following equation (Miller, 2002;Kaiser et al, 2007) …”

<sup>17</sup>O excess traces atmospheric nitrate in paleo-groundwater of the Saharan desert

“…3 in modern groundwater, lakes, and wet depositions (Darrouzet-Nardi et al, 2012;Dejwakh et al, 2012;Li et al, 2010;Michalski et al, 2005;Michalski et al, 2004b;Tsunogai et al, 2011;Tsunogai et al, 2010;Nakagawa et al, 2013).…”