A Quantitative Approach for Identifying Nitrogen Sources in Complex Yeongsan River Watershed, Republic of Korea, Based on Dual Nitrogen Isotope Ratios and Hydrological Model
, Won-Seok Lee 2, Taewoo Kang 2,* and Min-Seob Kim 1,*Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe general comments to authors:
Authors provided a quantitative approach for identifying nitrogen sources using dual nitrogen isotopic composition and hydrological model. In Yeongsan River as case study, these approaches applied to determine anthropogenic nitrogen fluxes among different land use types. Based on this investigation, authors provide valuable insights for the formulation of management policies to control inputs nitrogen from point and non-point sources in different land use types for the restoration of water quality and aquatic ecosystems in complex river systems. Given the recent escalation in human activity near aquatic environments, at least, I think that that author's exertion is valuable for estimating the quantitative contribution of individual anthropogenic nitrogen sources transported along riverine systems. Thus, this manuscript can be suitable for the publication via minor revision.
Minor comments:
Line 66; Please clarify this term “ effort” involved in effective water quality management.
Line 202; Please provide analytical precision calculated during isotopic measurement.
Line 472-494; With respect to the estimation of anthropogenic N contributions within water column, I think that some components (e.g., groundwater inflation, air deposition) may be rarely considered for precisely determining N source contribution. Actually, in complex river catchments, these components may be overlooked during the estimation of source contributions. Thus, to resolve these points, authors should be discussed as further study approaches. Please provide some discussion points.
Figure 3; I think that both separated figures (Fig. 3a and 3b) may be integrated as one figure.
Figure 5; Please fit figure’s sizes or replace as more high resolution version
Figure 6; I can not find the explanation related to Fig. 6. Please provide some descriptions involved in Fig. 6.
Comments for author File: 
Comments.pdf
Author Response
Authors provided a quantitative approach for identifying nitrogen sources using dual nitrogen isotopic composition and hydrological model. In Yeongsan River as case study, these approaches applied to determine anthropogenic nitrogen fluxes among different land use types. Based on this investigation, authors provide valuable insights for the formulation of management policies to control inputs nitrogen from point and non-point sources in different land use types for the restoration of water quality and aquatic ecosystems in complex river systems. Given the recent escalation in human activity near aquatic environments, at least, I think that that author's exertion is valuable for estimating the quantitative contribution of individual anthropogenic nitrogen sources transported along riverine systems. Thus, this manuscript can be suitable for the publication via minor revision.
Response: We greatly appreciate Reviewer #1 for the positive evaluation and nice summary of the present manuscript. We also thank for providing us valuable comments. We believe that our manuscript has been improved by the reviewer’s comment. We addressed issues raised by Reviewer #1 below.
Line 66; Please clarify this term “effort” involved in effective water quality management.
Response: We revised the manuscript as you suggested as bellow.
Policies are being made to reduce nitrogen sources and to improve water quality flowing from upstream and downstream through efficient management of point and non-point sources, such as ecological restoration and reservoir water quality management.
Line 202; Please provide analytical precision calculated during isotopic measurement.
Response: We revised the manuscript as you suggested as bellow.
N2 (atmospheric air) was used as the reference standard for nitrogen, and the analytical precision was within 0.5‰. To evaluate the precision and accuracy of Kjeldahl processes, two reference materials (IAEA-NO-3(Potassium Nitrate) and IAEA-N-1(Ammonium Sulfate)) were analyzed repeatedly. The δ15N-NO3 and δ15N-NH4 values of IAEA-NO-3 and IAEA-N-1 were 4.7 ± 0.3‰ and 0.4 ± 0.1‰, respectively, which are within the recommended values of analytical uncertainties.
Line 472-494; With respect to the estimation of anthropogenic N contributions within water column, I think that some components (e.g., groundwater inflation, air deposition) may be rarely considered for precisely determining N source contribution. Actually, in complex river catchments, these components may be overlooked during the estimation of source contributions. Thus, to resolve these points, authors should be discussed as further study approaches. Please provide some discussion points.
Response: Thank you for the valuable comments. In revised version, we have added an explanation of this part to the manuscript as follows.
Excluding the previously mentioned non-point source, the groundwater inflation and air deposition excluding non-point source can led to the increasing amounts of contaminants such as ammonium and nitrate in aqueous water systems (Richa et al., 2022). The nitrogen source in atmospheric deposition comes mainly from the combustion of fossil fuels, such as coal and oil. Atmospheric deposition may be in a dry form, such as droplets and particulates, or wet form, such as fog, rain and snow. Furthermore, ground water is potentially important source of nitrogen, but it is difficult to quantify due to either poorly understood or little data on them exists. It is important to determine the amount of nitrogen contributed by these undocumented sources within watersheds to target those that contribute most to nitrogen-pollution problems. Therefore, we should be considered atmospheric deposition and groundwater inflow to the complex watershed in future study.
Richa, A., Touil, S., Fizir, M. Recent advances in the source identification and remediation techniques of nitrate contaminated groundwater: A review. 2022, J. Environ. Manage. 315, 115265.
Figure 3; I think that both separated figures (Fig. 3a and 3b) may be integrated as one figure.
Response: We revised the Figure 3. as you suggested as bellow.
Figure 5; Please fit figure’s sizes or replace as more high resolution version
Response: We replaced more higher resolution version in the Figure 5. as you suggested as bellow.
Figure 6; I can not find the explanation related to Fig. 6. Please provide some descriptions involved in Fig. 6.
Response: The explanation for Fig. 6 is already at line 435~451.
Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsGeneral comment:
This manuscript entitled “ A quantitative approach for identifying nitrogen source in complex river watershed based on dual nitrogen isotope ratio and hydrological model ” by Hong et al. reported an integrated framework using soluble nitrogen isotope ratio (δ15N-NH4 and δ15N-NO3) and hydrological modeling (hydrological simulation program SPARROW) of the mainstream and tributaries of the Yeongsan River. In general, the research is interesting and can be accepted after moderate revision. My specific comments are:
1. Please name the river and country in the title of the manuscript.
2. The entire manuscript contains too lengthy statements which are hard to understand. Please rephrase such sentences and write in more clear manner.
3. Please recreate all equations and models using the MS Word tool. Also, provide their references.
4. The methodology section is well-structured, but a brief comparison with alternative methods for identifying nitrogen sources could strengthen the manuscript's contribution and contextualize its significance.
5. Please add statistical measures or confidence intervals to quantify the uncertainty associated with the identified nitrogen sources.
6. In conclusion, please add recommendations for future research directions or potential applications of the proposed approach in different geographical contexts.
Author Response
This manuscript entitled “A quantitative approach for identifying nitrogen source in complex river watershed based on dual nitrogen isotope ratio and hydrological model” by Hong et al. reported an integrated framework using soluble nitrogen isotope ratio (δ15N-NH4 and δ15N-NO3) and hydrological modelling (hydrological simulation program SPARROW) of the mainstream and tributaries of the Yeongsan River. In general, the research is interesting and can be accepted after moderate revision. My specific comments are:
Response: We greatly appreciate Reviewer #2 for the positive evaluation and nice summary of the present manuscript. We also thank for providing us valuable comments. We believe that our manuscript has been improved by the reviewer’s comment. We addressed issues raised by Reviewer #1 below.
- Please name the river and country in the title of the manuscript.
Response: We revised title in the manuscript as you suggested as bellow.
The revised title is “A quantitative approach for identifying nitrogen source in complex Yongsan river watershed, Korea based on dual nitrogen isotope ratio and hydrological model”.
- The entire manuscript contains too lengthy statements which are hard to understand. Please rephrase such sentences and write in more clear manner.
Response: This part was reflected through English proofleading.
- Please recreate all equations and models using the MS Word tool. Also, provide their references.
Response: The first equation was recreated using MS tool, but the model equation was difficult to create using MS tool, so the resolution was increased by adding a formula written through the program. Additionally, references were added to each equation.
Page 5, line 204-205
δ = [(Rsample / Rstandard)-1] X 1000
R = 13C/12C, 15N/14N
Page 5, line 234-235
- The methodology section is well-structured, but a brief comparison with alternative methods for identifying nitrogen sources could strengthen the manuscript's contribution and contextualize its significance.
Response: We revised manuscript as you suggested as bellow.
Another technique of double-use of nitrogen (δ15N-NO3) and oxygen (δ18O-NO3) stable isotope analysis of nitrates using denitrification bacterial method has been introduced. Theoretically, nitrate (NO3) formed through nitrification has a total of three oxygen atoms and two of them originate from water (Nitrite, NO2-) and the remaining one originates from nitric oxide (NO). Therefore, the nitrogen and oxygen stable isotope ratio of nitrates is determined by the nitrogen pool characteristics and chemical isotope fractionation that exist at the time of production. The natural ratio of nitrogen and oxygen stable isotope of nitrates has been used to identify various nitrogen pollution sources such as agricultural land, forest reservoir, and lake.
- Please add statistical measures or confidence intervals to quantify the uncertainty associated with the identified nitrogen sources.
Response: We add a standard deviation value in revised manuscript as you suggested.
During the survey period, as for the contribution of nitrogen pollution sources in the mainstream (upper, middle, and lower reaches) and tributaries of the Yeongsan River system, the sewage source accounted for 24±0.7%, the manure source for 31±1.8%, and the soil source for 45±1.6% (Fig. 5). In the upper reaches, the soil source contributed 60±2.5%, higher than the manure source (19±0.2%) and the sewage source (21±0.8%). In the middle reaches, the sewage source (32±3.5%), soil source (32±2.9%), and manure source (36±3.7%) all contributed similarly (Fig. 5). Finally, in the lower reaches, the manure source (50±4.2%) contributed relatively more than the sewage source (18±2.2%) and the soil source (32±1.3%) (Fig. 5).
- In conclusion, please add recommendations for future research directions or potential applications of the proposed approach in different geographical contexts.
Response: We add future research directions in revised manuscript as you suggested.
Excluding the previously mentioned non-point source, the groundwater inflation and air deposition excluding non-point source can led to the increasing amounts of contaminants such as ammonium and nitrate in aqueous water systems. The nitrogen source in atmospheric deposition comes mainly from the combustion of fossil fuels, such as coal and oil. Atmospheric deposition may be in a dry form, such as droplets and particulates, or wet form, such as fog, rain and snow. Furthermore, ground water is potentially important source of nitrogen, but it is difficult to quantify due to either poorly understood or little data on them exists. It is important to determine the amount of nitrogen contributed by these undocumented sources within watersheds to target those that contribute most to nitrogen-pollution problems. Therefore, we should be considered atmospheric deposition and groundwater inflow to the complex watershed in future study.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe manuscript entitled with "A quantitative approach for identifying nitrogen source in com- 2 plex river watershed based on dual nitrogen isotope ratio and 3 hydrological model" is interesting. The introduction needs more introduction of the application of nitrogen isotopic studies. More important, this research needs to the original data to validate their conclusion and increase the impact of this article and journal. Meantime, there are a lot of studies in China and other countries, they need to compare their data with these important studies.
Comments on the Quality of English LanguageDear Editor,
The data of this manuscript is interesting but need to be presented and compared with other studies to give an overall understanding of this topic.
Author Response
The manuscript entitled with "A quantitative approach for identifying nitrogen source in com- plex river watershed based on dual nitrogen isotope ratio and hydrological model" is interesting. The introduction needs more introduction of the application of nitrogen isotopic studies. More important, this research needs to the original data to validate their conclusion and increase the impact of this article and journal. Meantime, there are a lot of studies in China and other countries, they need to compare their data with these important studies.
Response: To the best of our knowledge, the δ15N-NH4 and δ15N-NO3 axes have not yet been applied to identify nitrogen sources in agricultural aquatic environments. There are 1 paper that calculate the contribution rate of nitrogen pollution sources by simultaneously applying δ15N-NH4 and δ15N-NO3 axes in agricultural complex watersheds including sewage treatment plants (Scopus). Additionally, there are 20 papers that simultaneously apply δ15N-NH4 and δ15N-NO3 axes without nitrogen contribution data using Iso-source model (Scopus). The new sentences were added with few studies in China as follows
This study hypothesizes that nitrogen sources can be more effectively distinguished if dual δ15N-NH4 and δ15N-NO3 axes, rather than single isotope approach, are applied in complex river watersheds, especially those with sewage treatment plant. The identification manure and sewage sources based on the δ15N-NO3 and δ18O-NO3 values resulted in overlapping of corresponding values between some sources, leading to lack of identification accuracy (Yang and Toor, 2016; Zhang et al., 2018, Ryu et al., 2021). Additionally, the sensitivity of δ15N-NH4 to NH4+-N concentration is greater than that of δ15N-NO3 to NO3−-N concentration, even when the NH4+-N concentration is low (Lee et al., 2016). Therefore, the applicability of δ15N-NH4 and δ15N-NO3 axes should be evaluated in complex river watershed including agricultural area and intensive livestock farming where nitrogen is predominantly released into aquatic environments in the form of NH4+-N. To the best of our knowledge, the δ15N-NH4 and δ15N-NO3 axes have not yet been applied to identify nitrogen sources in agricultural aquatic environments. This technique should be validated in areas where sewage treatment facility effluent is discharged into rivers or agricultural complex watersheds that face similar problems (Cheng et al., 2020; Su et al., 2020; Wang et al., 2019).
Yang, Y.Y., Toor, G.S., 2016. δ15N and δ18O reveal the sources of nitrate-nitrogen in urban residential stormwater runoff. Environ. Sci. Technol. 50, 2881–2889.
Zhang, M., Zhi, Y., Shi, J., Wu, L., 2018. Apportionment and uncertainty analysis of nitrate sources based on the dual isotope approach and a Bayesian isotope mixing model at the watershed scale. Sci. Total Environ. 639, 1175–1187.
Cheng, H.H., Narindri, B., Chu, H., Whang, L.M., 2020. Recent advancement on biological technologies and strategies for resource recovery from swine wastewater. Bioresour. Technol. 303, 122861–122873.
Su, J.J., Ding, S.T., Chung, H.C., 2020. Establishing a smart farm-scale piggery wastewater
treatment system with the internet of things (IoT) applications. Water. 12, 1654–1666.
Wang, Y., Chen, L., Gao, Y., Chen, S., Chen, W., Hao, Z., Jia, J., Han, N., 2017. Geochemical isotopic composition in the loess plateau and corresponding source analyses: a case study of China’s Yangjuangou catchment. Sci. Total Environ. 581-582, 794–800.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsNo consideration
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript is well revised. I suggest accept.
