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Land use in a catchment area has significant impacts on nitrate eluted fromthe catchment, including atmospheric nitrate deposited onto the catchmentarea and remineralised nitrate produced within the catchment area. Althoughthe stable isotopic compositions of nitrate eluted from a catchment can be auseful tracer to quantify the land use influences on the sources andbehaviour of the nitrate, it is best to determine these for theremineralised portion of the nitrate separately from the unprocessedatmospheric nitrate to obtain a more accurate and precise quantification ofthe land use influences. In this study, we determined the spatialdistribution and seasonal variation of stable isotopic compositions ofnitrate for more than 30 streams within the same watershed, the Lake Biwawatershed in Japan, in order to use 17O excess (Δ17O) ofnitrate as an additional tracer to quantify the mole fraction of atmosphericnitrate accurately and precisely. The stable isotopic compositions,including Δ17O of nitrate, in precipitation (wet deposition;n =  196) sampled at the Sado-seki monitoring station were also determinedfor 3 years. The deposited nitrate showed large 17O excessessimilar to those already reported for midlatitudes: Δ17Ovalues ranged from +18.6 to+32.4 ‰ with a 3-year average of+26.3 ‰. However, nitrate in each inflowstream showed small annual average Δ17O values ranging from+0.5 to +3.1 ‰, whichcorresponds to mole fractions of unprocessed atmospheric nitrate to totalnitrate from (1.8 ± 0.3) to (11.8 ± 1.8) % respectively,with an average for all inflow streams of (5.1 ± 0.5) %. Although theannual average Δ17O values tended to be smaller in accordancewith the increase in annual average stream nitrate concentration from 12.7to 106.2 µmol L−1, the absolute concentrations of unprocessedatmospheric nitrate were almost stable at (2.3 ± 1.1) µmol L−1 irrespective of the changes in population density and land use ineach catchment area. We conclude that changes in population density and landuse between each catchment area had little impact on the concentration ofatmospheric nitrate and that the total nitrate concentration originatedprimarily from additional contributions of remineralised nitrate. By usingthe average stable isotopic compositions of atmospheric nitrate, we excludedthe contribution of atmospheric nitrate from the determined δ15N and δ18O values of total nitrate and estimated theδ15N and δ18O values of the remineralised portionof nitrate in each stream to clarify the sources. We found that theremineralised portion of the nitrate in the streams could be explained bymixing between a natural source with values of (+4.4 ± 1.8) and (−2.3 ± 0.9) ‰ forδ15N and δ18O respectively and an anthropogenicsource with values of (+9.2 ± 1.3) and(−2.2 ± 1.1) ‰ for δ15N and δ18O respectively. In addition, both the uniform absoluteconcentration of atmospheric nitrate and the low and uniform δ18O values of the remineralised portion of nitrate in the streamsimply that in-stream removal of nitrate through assimilation ordenitrification had little impact on the concentrations and stable isotopiccompositions of nitrate in the streams, except for a few streams in summer with catchments of urban/suburban land uses

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