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...

Abstract. To clarify the biological processing of nitrate within temperate forested catchments using unprocessed atmospheric nitrate exported from each catchment as a tracer, we continuously monitored stream nitrate concentrations and stable isotopic compositions, including 17O excess (Δ17O), in three forested catchments in Japan (KJ, IJ1, and IJ2) for more than 2 years. The catchments showed varying flux-weighted average nitrate concentrations of 58.4, 24.4, and 17.1 µmol L−1 in KJ, IJ1, and IJ2, respectively, which correspond to varying export fluxes of nitrate: 76.4, 50.1, and 35.1 mmol m−2 in KJ, IJ1, and IJ2, respectively. In addition to stream nitrate, nitrate concentrations and stable isotopic compositions in soil water were determined for comparison in the most nitrate-enriched catchment (site KJ). While the 17O excess of nitrate in soil water showed significant seasonal variation, ranging from +0.1 ‰ to +5.7 ‰ in KJ, stream nitrate showed small variation, from +0.8 ‰ to +2.0 ‰ in KJ, +0.7 ‰ to +2.8 ‰ in IJ1, and +0.4 ‰ to +2.2 ‰ in IJ2. We conclude that the major source of stream nitrate in each forested catchment is groundwater nitrate. Additionally, the significant seasonal variation found in soil nitrate is buffered by the groundwater nitrate. The estimated annual export flux of unprocessed atmospheric nitrate accounted for 9.4 %±2.6 %, 6.5 %±1.8 %, and 2.6 %±0.6 % of the annual deposition flux of atmospheric nitrate in KJ, IJ1, and IJ2, respectively. The export flux of unprocessed atmospheric nitrate relative to the deposition flux showed a clear normal correlation with the flux-weighted average concentration of stream nitrate, indicating that reductions in the biological assimilation rates of nitrate in forested soils, rather than increased nitrification rates, are likely responsible for the elevated stream nitrate concentration, probably as a result of nitrogen saturation. The export flux of unprocessed atmospheric nitrate relative to the deposition flux in each forest ecosystem is applicable as an index for nitrogen saturation.

position of the inhomogeneity. Margins for range uncertainties due the CT number variations were not considered in this study so as to focus only on the range changes caused by the displacement of inhomogeneity. An expanded or contracted contour of the high density or low density object was created by adding or subtracting 3 mm margin, respectively to the actual contours of the inhomogeneities (blurred image set). A single field optimization plan was created both in the original and blurred image sets for SSPBT. The robustness of these three plans for distal target coverage for up to 3 mm changes in the inhomogeneity position were studied. Results: The treatment plans for the SSPBT using the blurred image set showed similar robustness as the PSPBT plan with compensator smearing for dose coverage of the distal edge of the target. On the other hand, the distal target coverage in the treatment plan for the SSPBT, that used the image set without inhomogeneity blurring, was reduced when the location of the beam isocenter was changed, making the plan less robust compared to the PSPBT plan. Conclusions: The use of an expanded or contracted margin around high or low density objects in the images, respectively for SSPBT planning appears to be equivalent to the compensator smearing effect in PSPBT planning. Results of application of the proposed approach for designing treatment plans for SSPBT for prostate and targets in brain will be presented.

<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>