Knowledge of key sources and biogeochemical processes that affect the transport of nitrate (NO(3)(-)) in streams can inform watershed management strategies for controlling downstream eutrophication. We applied dual isotope analysis of NO(3)(-) to determine the dominant sources and processes that affect NO(3)(-) concentrations in six stream/river watersheds of different land uses. Samples were collected monthly at a range of flow conditions for 15 mo during 2004-05 and analyzed for NO(3)(-) concentrations, delta(15)N(NO3), and delta(18)O(NO3). Samples from two forested watersheds indicated that NO(3)(-) derived from nitrification was dominant at baseflow. A watershed dominated by suburban land use had three delta(18)O(NO3) values greater than +25 per thousand, indicating a large direct contribution of atmospheric NO(3)(-) transported to the stream during some high flows. Two watersheds with large proportions of agricultural land use had many delta(15)N(NO3) values greater than +9 per thousand, suggesting an animal waste source consistent with regional dairy farming practices. These data showed a linear seasonal pattern with a delta(18)O(NO3):delta (15)N(NO3) of 1:2, consistent with seasonally varying denitrification that peaked in late summer to early fall with the warmest temperatures and lowest annual streamflow. The large range of delta (15)N(NO3) values (10 per thousand) indicates that NO(3)(-) supply was likely not limiting the rate of denitrification, consistent with ground water and/or in-stream denitrification. Mixing of two or more distinct sources may have affected the seasonal isotope patterns observed in these two agricultural streams. In a mixed land use watershed of large drainage area, none of the source and process patterns observed in the small streams were evident. These results emphasize that observations at watersheds of a few to a few hundred km(2) may be necessary to adequately quantify the relative roles of various NO(3)(-) transport and process patterns that contribute to streamflow in large basins.

Sources and Transformations of Nitrate from Streams Draining Varying Land Uses: Evidence from Dual Isotope Analysis

Rainfall and conduit drainage combine to accelerate nitrate loss from a karst agroecosystem: Insights from stable isotope tracing and high-frequency nitrate sensing

. 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 (Δ17O) of nitrate as an additional tracer to quantify the mole fraction of atmospheric nitrate 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 determined for 3 years. The deposited nitrate showed large 17O excesses similar to those already reported for midlatitudes: Δ17O 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 Δ17O values ranging from +0.5 to +3.1 ‰, which corresponds 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 Δ17O 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 δ15N and δ18O values of total nitrate and estimated the δ15N and δ18O 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 δ15N and δ18O respectively and an anthropogenic source with values of (+9.2 ± 1.3) and (−2.2 ± 1.1) ‰ for δ15N and δ18O respectively. In addition, both the uniform absolute concentration of atmospheric nitrate and the low and uniform δ18O 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.

/pdf/accurate-and-precise-quantification-of-atmospheric-nitrate-h5bie1jql0.pdf

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

. 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.

/pdf/export-flux-of-unprocessed-atmospheric-nitrate-from-452grj6ehu.pdf

Export flux of unprocessed atmospheric nitrate from temperate forested catchments: a possible new index for nitrogen saturation

Isotopic evidence for seasonality of microbial internal nitrogen cycles in a temperate forested catchment with heavy snowfall.

Temporal variations in the stable isotopic com- positions of nitrate dissolved in stream water eluted from a cool-temperate forested watershed (8 ha) were measured to quantify the biogeochemical effects of clear-cutting of trees and subsequent strip-cutting of the understory vegeta- tion, dwarf bamboo (Sasa senanensis), with special empha- sis on changes in the fate of atmospheric nitrate that had been deposited onto the watershed based on 1 17 O values of nitrate. A significant increase in stream nitrate concentra- tion to 15 µmol L 1 in spring of 2004 was correlated with a significant increase in the 1 17 O values of nitrate. Addi- tionally, the high 1 17 O values of +14.3 ‰ suggest that the direct drainage of atmospheric nitrate accounted for more than 50 % of total nitrate exported from the forested water- shed peaking in spring. Similar increases in both concentra- tions and 1 17 O values were also found in spring of 2005. Conversely, low 1 17 O values less than +1.5 ‰ were ob- served in other seasons, regardless of increases in stream nitrate concentration, indicating that the majority of nitrate exported from the forested watershed during seasons other than spring was remineralized nitrate: those retained in the forested ecosystem as either organic N or ammonium and then been converted to nitrate via microbial nitrification. When compared with the values prior to strip-cutting, the annual export of atmospheric nitrate and remineralized ni- trate increased more than 16-fold and fourfold, respectively, in 2004, and more than 13-fold and fivefold, respectively, in 2005. The understory vegetation (Sasa) was particularly im- portant to enhancing biological consumption of atmospheric nitrate.

/pdf/quantifying-the-effects-of-clear-cutting-and-strip-cutting-2klmjcofk9.pdf

Quantifying the effects of clear-cutting and strip-cutting on nitrate dynamics in a forested watershed using triple oxygen isotopes as tracers

. The stable isotopic compositions of nitrate dissolved in 49 brands of bottled drinking water collected worldwide were measured, to trace the fate of atmospheric nitrate (NO3− atm) that had been deposited into subaerial ecosystems, using the 17O anomalies (Δ17O) of nitrate as tracers. The use of bottled water enables collection of groundwater recharged at natural, background watersheds. The nitrate in groundwater had small Δ17O values ranging from −0.2‰ to +4.5‰ n = 49). The average Δ17O value and average mixing ratio of atmospheric nitrate to total nitrate in the groundwater samples were estimated to be 0.8‰ and 3.1%, respectively. These findings indicated that the majority of atmospheric nitrate had undergone biological processing before being exported from the surface ecosystem to the groundwater. Moreover, the concentrations of atmospheric nitrate were estimated to range from less than 0.1 μmol L−1 to 8.5 μmol L−1 with higher NO3−atm concentrations being obtained for those recharged in rocky, arid or elevated areas with little vegetation and lower NO3−atm concentrations being obtained for those recharged in forested areas with high levels of vegetation. Additionally, many of the NO3−atm-depleted samples were characterized by elevated δ15N values of more than +10‰. Uptake by plants and/or microbes in forested soils subsequent to deposition and the progress of denitrification within groundwater likely plays a significant role in the removal of NO3−atm.

/pdf/tracing-atmospheric-nitrate-in-groundwater-using-triple-2pkxqtgvzm.pdf

Tracing atmospheric nitrate in groundwater using triple oxygen isotopes: evaluation based on bottled drinking water

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Quantifying the effects of clear-cutting and strip-cutting on nitrate dynamics in a forested watershed using triple oxygen isotopes as tracers

Tracing atmospheric nitrate in groundwater using triple oxygen isotopes: evaluation based on bottled drinking water