科研之友
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摘要
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 17O excess (Delta O-17) of nitrate as an additional tracer to quantify the mole fraction of atmospheric nitrate accurately and precisely. The stable isotopic compositions, including Delta O-17 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 O-17 excesses similar to those already reported for midlatitudes: Delta O-17 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 Delta O-17 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 Delta O-17 values tended to be smaller in accordance with the increase in annual average stream nitrate concentration from 12.7 to 106.2 mu mol L-1, the absolute concentrations of unprocessed atmospheric nitrate were almost stable at (2.3 +/- 1.1) mu mol 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 delta N-15 and delta O-18 values of total nitrate and estimated the delta N-15 and delta O-18 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 delta N-15 and delta O-18 respectively and an anthropogenic source with values of (+9.2 +/- 1.3) and (-2.2 +/- 1.1)% for delta N-15 and delta O-18 respectively. In addition, both the uniform absolute concentration of atmospheric nitrate and the low and uniform delta O-18 values of the remineralised portion of nitrate in the streams imply that in-stream removal of nitrate through assimilation or denitrification had little impact on the concentrations and stable isotopic compositions of nitrate in the streams, except for a few streams in summer with catchments of urban/suburban land uses.
- 出版日期2016
