Land use in a catchment area has significant impacts on nitrate eluted from the catchment, including atmospheric nitrate deposited onto the catchment area and remineralised nitrate produced within the catchment area. Although the stable isotopic compositions of nitrate eluted from a catchment can be a useful tracer to quantify the land use influences on the sources and behaviour of the nitrate, it is best to determine these for the remineralised portion of the nitrate separately from the unprocessed atmospheric nitrate to obtain a more accurate and precise quantification of the land use influences. In this study, we determined the spatial distribution and seasonal variation of stable isotopic compositions of nitrate for more than 30 streams within the same watershed, the Lake Biwa watershed in Japan, in order to use 17 O excess ( 17 O) of nitrate as an additional tracer to quantify the mole fraction of atmospheric nitrate accurately and precisely. The stable isotopic compositions, including 17 O of nitrate, in precipitation (wet deposition; n = 196) sampled at the Sadoseki monitoring station were also determined for 3 years. The deposited nitrate showed large 17 O excesses similar to those already reported for midlatitudes: 17 O values ranged from +18.6 to +32.4 ‰ with a 3-year average of +26.3 ‰. However, nitrate in each inflow stream showed small annual average 17 O values ranging from +0.5 to +3.1 ‰, which cor-responds to mole fractions of unprocessed atmospheric nitrate to total nitrate from (1.8 ± 0.3) to (11.8 ± 1.8) % respectively, with an average for all inflow streams of (5.1 ± 0.5) %. Although the annual average 17 O values tended to be smaller in accordance with the increase in annual average stream nitrate concentration from 12.7 to 106.2 µmol L −1 , the absolute concentrations of unprocessed atmospheric nitrate were almost stable at (2.3 ± 1.1) µmol L −1 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 and that the total nitrate concentration originated primarily from additional contributions of remineralised nitrate. By using the average stable isotopic compositions of atmospheric nitrate, we excluded the contribution of atmospheric nitrate from the determined δ 15 N and δ 18 O values of total nitrate and estimated the δ 15 N and δ 18 O values of the remineralised portion of nitrate in each stream to clarify the sources. We found that the remineralised portion of the nitrate in the streams could be explained by mixing between a natural source with values of (+4.4 ± 1.8) and (−2.3 ± 0.9) ‰ for δ 15 N and δ 18 O respectively and an anthropogenic source with values of (+9.2 ± 1.3) and (−2.2 ± 1.1) ‰ for δ 15 N and δ 18 O respec-Published by Copernicus Publications on behalf of the European Geosciences Union. 3442 U. Tsunogai et al.: Accurate and precise quantification of atmospheric nitratetively. In addition, both th...

Vertical distributions of both concentrations and stable isotopic compositions of nitrate, including the 17 O-excesses (D 17 O), were determined four times during 1 yr within the mesotrophic water column of Lake Biwa in Japan. By using both the deposition rate of atmospheric nitrate onto the entire surface of the lake and the influx/efflux of both atmospheric and remineralized nitrate via streams reported in the literature, we quantified the annual dynamics of nitrate (gross production rate of nitrate through nitrification and gross metabolic rate of nitrate through assimilation and denitrification), together with their seasonal variations, based on the D 17 O method. The results revealed that 642 6 113 Mmol (Mmol 5 10 6 mol) of the remineralized nitrate was supplied into the water column through nitrification in the lake on an annual basis, while 810 6 120 Mmol of nitrate was metabolized in the lake through assimilation and denitrification. In addition, it turns out that nitrification was active, not only in the hypolimnion, but also in the epilimnion and upper thermocline in this lake. Furthermore, the total metabolic rates of nitrate varied seasonally, with the highest rates in summer and the lowest in winter. Because the difference between the annual metabolic rate of nitrate estimated based on the D 17 O method and the annual assimilation rate of nitrate estimated based on the traditional 15 N incubation method was only 20%, we concluded that the D 17 O method reliably estimates the dynamics of nitrate in mesotrophic lakes. Nitrate (NO 23 ) is a key nutrient in aquatic environments that often limits primary production. Nitrate dynamics in an aquatic environment, i.e., gross production rate of nitrate through nitrification (F nit ), gross metabolic rate of nitrate through assimilation (F assim ), and gross metabolic rate of nitrate through denitrification (F denit ), are important parameters to be quantified when evaluating both the present and future state of the aquatic environment. In most studies that have been conducted to date, nitrate dynamics has been estimated via incubation experiments using 15 N tracer techniques. To quantify F assim , for instance, 15 N-labeled NO 2 3 is added into bottles that simulate in situ conditions of the aquatic environment studied, which leads to the production of particulate organic-15 N (PO 15 N) through assimilation over a known incubation period of several hours to several days (Dugdale and Goering 1967;Knap et al. 1996). The PO 15 N is then gathered and quantified using mass spectrometry.The experimental procedures using 15 N tracer, however, are generally costly and complicated. Besides, while the obtained nitrate dynamics is an instantaneous assimilation rate at the point of observation, such a value may be temporally variable in response to various factors such as changes in temperature, light intensity, nutrients, and community structure. As a result, tedious and time-consuming time series observations are needed to estimate long-term nitrate dynamics (s...

<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>