the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Tracing the source of nitrate enriched in a forested stream during storm events
- 1Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusaku, Nagoya 464-8601, Japan
- 2Asia Center for Air Pollution Research, 1182 Sowa, Nishi-ku, Niigata-shi, Niigata 950-2144, Japan
- 1Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusaku, Nagoya 464-8601, Japan
- 2Asia Center for Air Pollution Research, 1182 Sowa, Nishi-ku, Niigata-shi, Niigata 950-2144, Japan
Abstract. To clarify the source of nitrate increased during storm events in temperate forested streams, we monitored temporal variation in the concentrations and stable isotopic compositions including Δ17O of stream nitrate in a forested catchment (KJ catchment, Japan) during three storm events I, II, and III. The stream showed significant temporal variation in nitrate concentration, from 24.7 µM to 122.6 µM, from 28.7 µM to 134.1 µM, and from 46.6 µM to 114.5 µM during the storm events I, II, and III, respectively. On the other hand, the isotopic compositions (δ15N, δ18O, and Δ17O) of stream nitrate showed a decrease in accordance with the increase in the stream nitrate concentration, from +2.5 ‰ to −0.1 ‰, from +3.0 ‰ to −0.5 ‰, and from +3.5 ‰ to −0.1 ‰ for δ15N, from +3.1 ‰ to −3.4 ‰, from +2.9 ‰ to −2.5 ‰, and from +2.1 ‰ to −2.3 ‰ for δ18O, and from +1.6 ‰ to +0.3 ‰, from +1.4 ‰ to +0.3 ‰, and from +1.2 ‰ to +0.5 ‰ for Δ17O during the storm events I, II, and III, respectively. Besides, we found strong linear relationships between the isotopic compositions (δ15N, δ18O, and Δ17O) of stream nitrate and the reciprocal of stream nitrate concentrations during each storm event, implying that the temporal variation in the stream nitrate can be explained by simple mixing between two distinctive endmembers of nitrate having different isotopic compositions. Furthermore, we found that both concentrations and the isotopic compositions of soil nitrate obtained in the riparian zone of the stream were plotted on the nitrate-enriched extension of the linear relationship. We conclude that the soil nitrate in the riparian zone was responsible for the increase in stream nitrate during the storm events. In addition, we found that the concentration of unprocessed atmospheric nitrate in the stream was stable at 1.6 ± 0.4 µM, 1.8 ± 0.4 µM, and 2.1 ± 0.4 µM during the storm events I, II, and III, respectively, irrespective to the significant variations in the total nitrate concentration. We conclude that the storm events have little impacts on the concentration of unprocessed atmospheric nitrate in the stream and thus the annual export flux of unprocessed atmospheric nitrate relative to the annual deposition flux can be a robust index to evaluate nitrogen saturation in forested catchments, irrespective to the variation in the number of storm events and/or the variation in the elapsed time from storm events to sampling.
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Weitian Ding et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2022-30', Anonymous Referee #1, 21 Mar 2022
This manuscript investigated the source of nitrate exported from a forested watershed in Japan during 3 summer storm events using hydrologic, nitrate concentration, and nitrate isotope data. The results indicate that nitrate concentrations increase during storm events as the result of the flushing of soil nitrate that accumulates in soils near the stream between storm events. The manuscript is solid; the writing is generally clear and the interpretations are generally supported by the data. My main suggestion for improvement is for the authors to do a more thorough job of putting their results in the context of prior studies on this topic. For example, Sebestyen et al. 2019 (ES&T) and the references it contains address a similar issue as this manuscript (e.g. Buda et al. 2009 and Sabo et al. 2016 both sampled storm events), and this manuscript could do a more throughout job of using those studies to help justify this study (in the Introduction) and then comparing/contrasting the results of this study to those studies in the Discussion. Similarly, oher studies (Burns et al. 2009; Barnes et al. 2010; Bostic et al. 2021) have addressed similar questions in non-forested systems and could be useful for helping to provide a broader context for the results that are presented in this manuscript.
Specific comments
Title: “Enriched” is a word that is often used incorrectly in the isotope literature to refer to increased values of the heavier isotope. Here I believe the authors use “enriched” to mean increased nitrate concentrations, which is might cause confusion given that this paper also talks about isotopic enrichment (e.g lines 66 and 303). One solution might be to simply delete “enriched” from the title and another solution might be replace “nitrate enriched” in the title with something like “elevated nitrate concentrations”.
Lines 2-3: This sentence implies that nitrate concentrations always increase in temperate forest streams everywhere. Is that true? If not, perhaps slightly adjust this sentence. For example, do some severely nitrogen saturated forests that show higher NO3 concentrations in baseflow than stormflow?
Line 5: Please tell the reader what time of year (winter, spring, summer, autumn) these storm events occurred.
Line 6: It might be helpful to insert “increasing” before “from” to help the reader understand that the “variation” nitrate concentration that was observed was primarily an increase in concentrations.
Line 14: I believe “(d15N, d18O, and C17O)” can be deleted without sacrificing meaning.
Line 26-27: Could the authors support this claim by calculating annual export of NO3-atm (and NO3-terr) using their concentration and flow data?
Lines 26-30: Is this conclusion specific to the author’s study site (or certain types of forests) or are they suggesting that is a more broad/general conclusion that applies to forested catchments everywhere?
Line 33: “representative” of what? Please clarify.
Line 50: First, how are the authors using “overland flow” here and elsewhere (e.g. line 463) in the manuscript? My understanding is that overland flow is unlikely in areas that are not near channels or stream/riparian areas in forests except for unique situations, such as intense rain events or rain that occurs on frozen soils. Second, I don’t believe either of the cited studies suggest that overland flow is a mechanism for direct suppler of atmospheric nitrate to stream water. As far as I recall, Kaushal et al. didn’t show overland flow for their forested site and Sebestyen et al. talked about routing of NO3-atm along flow paths that allowed NO3-atm to bypass uptake/processing (but not specifically about overland flow).
Line 72: Is beta completely constant or can it exhibit some variation around 0.5279? If so, does the variation affect the authors data analyses or interpretations?
Lines 162-164: It seems like there would be potential for microbial alteration of the samples during the 1-2 weeks that they stayed in the field before being returned to the lab. Did the authors assess this?
Lines 184-185: How many “local laboratory nitrate standards” were used and what are their isotope values?
Lines 205-206: What data were used to calculate the reported standard error of the mean for each isotope? For example, was precision determined from the lab standards, replicate samples, or something else?
Line 226: How was the error range “allowed”?
Line 279: I believe “events” should be singular.
Line 353: I suggest inserting “primarily” or “likely” before “responsible” here and elsewhere that this conclusion is presented. The soil and stream data the authors are using come from different years as they describe on lines 318-342, so I think the conclusion on lines 351-354 should be considered tentative.
Lines 389-390: Please indicate which symbols indicate upland samples and which indicate riparian samples.
- AC1: 'Reply on RC1', Weitian Ding, 24 Apr 2022
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RC2: 'Comment on bg-2022-30', Anonymous Referee #2, 28 Mar 2022
Review of the manuscript entitled “Tracing the source of nitrate enriched in a forested stream during storm events” (MS No: bg-2022-30) by Ding et al.
The study provides a rather comprehensive analysis of nitrate dynamics over a temperate forest-stream system, with emphasis on sources of stream nitrate during storms. The results are potentially publishable. However, the way the results are presented stops me from recommendation at the present form. Furthermore, it is unclear to me to what extent the results of the concluded nitrate saturation/release in temperate forests can impact our knowledge of nitrate dynamics. Major comments follow.
- Abstract: it is not clear to me how the “stable” “unprocessed” atmospheric nitrate can be used to evaluate nitrogen saturation in forested catchments. I’m also not able to follow why the conclusion of “the storm events have little impacts on the concentration of unprocessed atmospheric nitrate in the stream” is important and how the conclusion is arrived. Overall, I’m not able to follow why “unprocessed atmospheric nitrate fraction” in river water is so important that the authors have to repeat and emphasize many times in the manuscript. My understanding is that with finite fraction of atmospheric nitrate, one can utilize the unique triple oxygen isotope composition in atmospheric nitrate for riverine nitrogen dynamics study, which is what the group did in the past years. The fraction of “unprocessed atmospheric nitrate” represents a balance of release of soil nitrate and atmospheric deposition.
- Line 25-30: no flux estimation is provided, and so it is not clear how the statement of “the annual export flux of unprocessed atmospheric nitrate relative to the annual deposition flux” is obtained.
- Overall, from my understanding, the value of NO3_atm is quite stable. The values of the 3 storms are 1.6+/-0.4, 1.8+/-0.4, and 2.1+/-0.4 uM, while that during non-storm time is 2.2+/-0.6 uM. Isn’t it more valuable to discuss storm and non-storm samples in the same context of nitrogen saturation and dynamics?
Specific comments
- The term “enriched” may cause confusion. In isotope community, often the term is used for indicating an increase in isotope values, i.e., increase in the abundance of heavier isotopic compounds.
- Line 121: M_atm, D_atm are not defined till much later in section 4.3. Even in section 4.3, the two variables are not clearly defined and explained. Instead, the authors referred to their earlier paper (Nakagawa et al., 2018). The authors are fine to have the details in their previous paper but the authors have to at least explain the meaning of the two.
- M_atm (or NO3_atm) is obtained by assuming a certain number of D17O_atm, which is not measured in this work. And so, D_atm is not known. Please elaborate and explain why M_atm/D_atm is little affected by storms and how this conclusion is arrived.
- Line 163: Please discuss whether 1-2 weeks of storage would affect the sample nitrate concentration and isotope compositions.
- Line 428, enhancement of D17O on 2019/1/31: I did a simple estimate by assuming that the snow nitrate has the same D17O value as the atmospheric at 26 per mil and took 2018/12/28 as an initial state before snow melting. From 2018/12/28 to 2019/1/31, the D17O value increases by 7 per mil, implying ~30% (=7 per mil/26 per mil) of stream nitrate is from snow melting. This increase however is not reflected in the water flow rate (from 110.0 to 117.3 L/min only). Please elaborate and provide a more quantitative explanation.
- To be more complete, for routine sampling analysis and discussion, please include precipitation and do the same analysis as the storm events.
- Fig 4: it seems there are two groups (one having smaller slope and one steeper) of D17O vs. 1/[NO3-] in the storm event II. Any reason for that?
- AC2: 'Reply on RC2', Weitian Ding, 24 Apr 2022
Weitian Ding et al.
Weitian Ding et al.
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