Accurate and precise quantification of atmospheric nitrate in streams draining land of various uses by using triple oxygen isotopes as tracers

Abstract

<p><strong>Abstract.</strong> <sup>17</sup>O anomalies were used to quantify the influence of changes in land use and population density between each catchment area on the fate of atmospheric nitrate by determining the areal distribution and seasonal variation in stable isotopic compositions including the <sup>17</sup>O anomalies (Δ<sup>17</sup>O) of nitrate for more than 30 streams within the same watershed. Those in precipitation (wet deposition; <i>n</i> = 213) sampled at Sado-seki monitoring station were determined for three years as well. The deposited nitrate showed similar large <sup>17</sup>O anomalies with those already reported for mid-latitudes: Δ<sup>17</sup>O values ranged from +18.6‰ to +32.4‰ with a three-year average of +26.3‰. However, nitrate in each inflow stream showed small annual average Δ<sup>17</sup>O values ranging from +0.5‰ to +3.1‰, which corresponds to the mixing ratios of unprocessed atmospheric nitrate to total nitrate from 1.8 ± 0.3% to 11.8 ± 1.8%, with 5.1 ± 0.5% as the average of all inflow streams. Although the annual average Δ<sup>17</sup>O values tended to be smaller in accordance with the increase in annual average nitrate concentration from 12.7 to 106.2 μmol L<sup>−1</sup>, the absolute concentrations of unprocessed atmospheric nitrate in the streams were almost stable at 2.3 ± 1.1 μmol L<sup>−1</sup> irrespective of the changes in population density and land use in each catchment area. We conclude that changes in population density and land use between each catchment area had little impact on the concentration of atmospheric nitrate. Thus, the total nitrate concentration originated primarily from additional contribution of remineralized nitrate from both natural sources, having values of +4.4 ± 1.8‰ and −2.3 ± 0.9‰ for δ<sup>15</sup>N and δ<sup>18</sup>O, respectively, and anthropogenic sources having values of +9.2 ± 1.3‰ and −2.2 ± 1.1‰ for δ<sup>15</sup>N and δ<sup>18</sup>O, respectively. In addition, both the uniform absolute concentration of atmospheric nitrate and the low and uniform δ<sup>18</sup>O values of the remineralized portion of nitrate in the streams imply that in-stream removal of nitrate through assimilation or denitrification had small impact on the concentrations and stable isotopic compositions of nitrate in the streams, except for a few streams in summer having catchments of urban/suburban land uses. Additional measurements of the Δ<sup>17</sup>O values of nitrate together with δ<sup>15</sup>N and δ<sup>18</sup>O enabled us to exclude the contribution of unprocessed atmospheric nitrate from the determined δ<sup>15</sup>N and δ<sup>18</sup>O values of total nitrate and to use the corrected δ<sup>15</sup>N and δ<sup>18</sup>O values to evaluate the source and behaviour of the remineralized portion of nitrate in each stream.</p>

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.…”

“…Biogeosciences Discuss., https://doi.org /10.5194/bg-2018- , which is produced from organic nitrogen through general chemical reactions including microbial N mineralization and microbial nitrification in the biosphere (Alexander et al, 2009;Michalski et al, 2003;Morin et al, 2008;Tsunogai et al, 2010;Tsunogai et al, 2016). By using the Δ 17 O signature 10 (the magnitude of 17 O-excess) defined by the following equation (Kaiser et al, 2007;Miller, 2002),…”

“…(Alexander et al, 2009;Kaiser et al, 2007;Michalski et al, 2003;Nelson et al, 2018;Tsunogai et al, 2010;Tsunogai et al, 2016). In addition,…”

“…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).…”