Quantitative Resolution of Phosphorus Sources in an Agricultural Watershed of Southern China: Application of Phosphate Oxygen Isotopes and Multiple Models
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsManuscript ID: agronomy-3503951-peer-review-v1
Quantitative resolution of phosphorus sources in in an agricultural watershed of southern China: application of phosphate oxygen isotopes and multiple models
Summary
This study investigates the sources of P in the River Xiaodongjiang, in western Guangdong Province, China, which has been showing signs of water quality deterioration and subsequent eutrophication in the last decades. Phosphate oxygen isotopes (δ¹⁸Oₚ) were analysed in water and sediment samples, with the aim of expand the application of δ¹⁸Oₚ as a tracer for source resolution in regions strongly affected by precipitation and storm events. A land-use-based mixing model and the Bayesian MixSIAR model were used to quantify the contributions of different sources of P (fertilisers, domestic sewage and livestock effluents, and endogenous P release from sediments).
The study has the particularity of being spatially developed in a small to medium-sized monsoon-influenced watershed. As well, until now isotope mixing models were not widely tested to characterize phosphorous in terms of δ¹⁸Oₚ. In the end, the authors provide recommendations for phosphate inputs management in catchments with the above mentioned features, and for the River Xiaodongjiang catchment in particular.
Overall opinion
The manuscript contributes with knowledge to phosphorus pollution management in agricultural watersheds, regarding the implications on eutrophication prevention in rivers. The subject addressed in the manuscript is a current (and pertinent) topic, and relevant to the field of of surface water quality impacted by agricultural and livestock units /urban activities. Therefore, the applied methodologies and the results are of potential interest to researchers and decision-makers, regarding the implementation of water management strategies. The manuscript meets the publishing objectives of the journal Agronomy.
The manuscript is structured and organised. Basic background of the research, justification/relevance of the topic under discussion, and the objectives of the work are given in the introduction. The methodology is adequately described. The presentation and discussion of the data are illustrated by appropriate tables and figures. The major findings are discussed in an objective mode.
Minor comments
Introduction: 2nd paragraph – citations need to be revised Wang et al [14] …. Ishida et al. [7]… section 4.2 Pistocchi et al. [34]
The same in Section 2.4.
Section 3.1 – the first sentence is incomplete or needs to be rephrased
wet season - dry season the two sampling campaigns were not mentioned in the methodology section
Section 3.2, first sentence (“Figure S2 presents the δ18OP characteristics of phosphorus sources in the XDJ watershed, as well as related national studies.”) needs to be rephrased; Fig S2 only presents soils data from the present study. As far as it can be understood by the text, the majority of data contained in the table are from other studies.
Section 2.5 should include reference to the data on δ¹⁸Oₚ used in the study which was taken from other studies (Table. S2 - Supplementary information for oxygen isotopes of phosphorus source phosphates), once this information is described in section 3.2, and supported from data presented in Supplementary material.
Section 3.6, first line, were instead of weer; 8th line limitations instead of limtations
Author Response
Comments 1: Introduction: 2nd paragraph – citations need to be revised Wang et al [14] …. Ishida et al. [7]… section 4.2 Pistocchi et al. [34]; The same in Section 2.4.
Response 1: Thank you very much for pointing out this issue. We sincerely appreciate your valuable feedback. In response, we have conducted a thorough search throughout the manuscript for similar issues.
The revised objective now reads: “For example, Wang et al. [14] used δ18OP to track down phosphorus sources in the Yangtze River Basin and found that chemical fertilizers were the dominant phosphorus source. Ishida et al. [7] used δ18OP to characterize the impacts of non-point phosphorus sources in Japan’s Yasu River Basin.” (Page 2, line 19-21); “Inorganic phosphate in water samples was converted to Ag3PO4 following the method of Wang et al. [14], with reagents added stepwisely to remove the organic matter (Figure S1).” (Page 5, line 15-17); “Pistocchi et al. [34] identified a gradual increase in sediment phosphorus concen-tration from upstream to downstream,” (Page12, line: 24-25).
Comments 2: Section 3.1 – the first sentence is incomplete or needs to be rephrased
Response 2: Thank you very much for your careful review. We sincerely apologize for our oversight. Based on your suggestion, we have rewritten this sentence. Please refer to the first paragraph of section 3.1, where the revision is marked in yellow.
The revised objective now reads: “This study investigates the spatial distribution of total phosphorus (TP) and dissolved total phosphorus (DTP) in stream water and sediments, along with the variations in TP concentration in response to precipitation in the XDJ sub-watershed (Figure 2).” (Page6, line: 14-17).
Comments 3: wet season - dry season the two sampling campaigns were not mentioned in the methodology section
Response 3: Thank you very much for your valuable suggestions. Due to the risk of water quality exceeding standards during the wet season in our study area, we only collected samples during this period. In the revised manuscript, we have further clarified the sampling period in section 2.1 (Sample Collection). Please refer to the yellow markings in the revised version for the changes.
The revised objective now reads: “Sampling sites and conditions are presented in Table. S1. 12 sites were selected in July 2023 along a land-use gradient from upstream to downstream, representing the transition from forest to agriculture or artificial areas Figure 1.” (Page4, line: 6-8).
Comments 4: Section 3.2, first sentence (“Figure S2 presents the δ18OP characteristics of phosphorus sources in the XDJ watershed, as well as related national studies.”) needs to be rephrased; Fig S2 only presents soils data from the present study. As far as it can be understood by the text, the majority of data contained in the table are from other studies.
Response 4: Thank you very much for your valuable suggestions. The original description was potentially misleading, so we have rewritten the first sentence. Please refer to the revised manuscript, where the changes are highlighted in yellow.
The revised objective now reads: “Figure S2 illustrates the δ18OP characteristics of soils within the XDJ watershed as well as other sources of phosphorus investigated in other studies.” (Page7, line: 7-8).
Comments 5: Section 2.5 should include reference to the data on δ¹⁸Oₚ used in the study which was taken from other studies (Table. S2 - Supplementary information for oxygen isotopes of phosphorus source phosphates), once this information is described in section 3.2, and supported from data presented in Supplementary material.
Response 5: cThe revised objective now reads: “The phosphate oxygen isotope compositions of the phosphorus sources utilized in this study were sourced from other research (Table S2).” (Page 6, line: 1-2)
Comments 6: Section 3.6, first line, were instead of weer; 8th line limitations instead of limtations.
Response 6: Thank you very much for your careful review. We sincerely apologize for our oversight. We have thoroughly checked the manuscript for similar errors and have made the necessary corrections accordingly. Please refer to the revised manuscript, where the changes are highlighted in yellow.
The revised objective now reads: “Chemical fertilizers were found to be the primary source of exogenous phosphorus to the XDJ watershed and its sub-watersheds by the MixSIAR model, which was consistent with the end-element mixing model results based on land use (Figure 5 and Figure 6) and the large proportion of arable land in these watersheds Figure 1. The phosphorus source analyses for river water and sediments were generally consistent. However, significant differences emerged between the two models regarding phosphorus source contribution analyses for sediments (Figure 5 and Figure 6). The discrepancy was attributed to the model limitations.” (Page 10, line: 4 and 11)
Reviewer 2 Report
Comments and Suggestions for AuthorsDear Authors.
You formulated that the goal of this MS is to expand the application of δ18OP as a tracer and provide theoretical insights for phosphorous management and phosphorus pollution restoration in the XDJ watershed. But it becomes understandable that you are talking about the fact that isotopic data are in accordance with TP, DTP data after reading results and discussion sections only. It is unclear whether the data on phosphorus were obtained by you or from other sources?
You must indicate the statistical significance of the differences when describing data in the Results section. For example, TP concentrations were higher in SSR tributaries than GZR and XDJ-D (One-Way ANOVA, p = 0... p = 0.... respectively), with the highest levels at S-Z3 (0.35 mg/L).
Unnecessary “in” in the title “Quantitative resolution of phosphorus sources in in an”
Page 2, paragraph 2 and in the other places of the MS. You don't need to name all authors from the references. Wang et al. [14] Ishida et al. [7]
For example, Wang, Tian, Zhao, Du, Zhang, Sun, Tekleab, Wei, Fu and Gooddy [14] used δ18OP to track down phosphorus sources in the Yangte River Basin and found that chemical fertilizers were the dominant phosphorus source. Ishida, Uehara, Iwata, Cid-Andres, Asano, Ikeya, Osaka, Ide, Privaldos and Jesus [7]
Page 3, paragraph 1.
(58.63%), artifcial land in the lower reaches (18.29%), - artifcial change on artificial
2.2. Sample Collection
For sediment sampling, three representatives of river sediment
were sampled, including cropland surface soil and forest surface soil (T1, T2 and T3 of Figure 1). T3 absents on the figure 1c.
Page 6. 3. Results
3.1. The spatial distribution of total phosphorus (TP) and dissolved total phosphorus (DTP)…
Any information is not in Methods about how phosphorus data are obtained.
Page 7. Last line. to minimize the water - larget change on largest
Page 10. 3.6. Quantitative Analysis of major Phosphorus Sources in the XDJ Watershed
Chemical fertilizers weer - weer change on were
The top of page 13. The second phrase repeats the previous phrase.
This reflects frequent exchanges at the water-sediment interface, indicating a need for further investigation into phosphorus dynamics at this interface in future research. There is a pressing need for further investigation on phosphorus dynamics at the water-sediment interface in future studies.
Many of the captions under the axes and inside the figures are blurred and need correction.
Comments on the Quality of English LanguageI did not have problems understanding the text of this article.
Several typos found have been noted.
Author Response
Comments 1: You formulated that the goal of this MS is to expand the application of δ18OP as a tracer and provide theoretical insights for phosphorous management and phosphorus pollution restoration in the XDJ watershed. But it becomes understandable that you are talking about the fact that isotopic data are in accordance with TP, DTP data after reading results and discussion sections only. It is unclear whether the data on phosphorus were obtained by you or from other sources?
Response 1: Thank you for pointing out this issue. We apologize for not specifying earlier that the TP and DTP data in this study were collected and analyzed by our team in the laboratory. We have now added this information in the '2.3. Sample Preparation' section on page 4.
The revised objective now reads: “The DTP content was analyzed using the ammonium molybdate spectrophotometric method after filtration (0.45 μm acetate fiber filter membrane, filtered on the day of sampling). The TP content was analyzed using the ammonium molybdate spectrophotometric method without filtration.” (Page 4, line 10-13).
Comments 2: You must indicate the statistical significance of the differences when describing data in the Results section. For example, TP concentrations were higher in SSR tributaries than GZR and XDJ-D (One-Way ANOVA, p = 0... p = 0.... respectively), with the highest levels at S-Z3 (0.35 mg/L).
Response 2: We would like to express our sincere gratitude to the reviewer for your insightful comments and valuable feedback. We acknowledge that we did not perform a significance test for the differences in TP concentrations. Based on your suggestion, we have now included the results of the statistical difference test in Section 3.1 on page 6.
The revised objective now reads: “TP concentrations were higher in SSR tributaries than GZR and XDJ-D (P>0.05), but the difference was not significant (P > 0.05), with the highest levels at S-Z3 (0.35 mg/L).” (Page 6, line 19-21); “During the wet season in May and Jun, upstream SSR and GZR also had higher TP than downstream XDJ-D(P<0.05).” (Page 6, line 28-30).
Comments 3: Unnecessary “in” in the title “Quantitative resolution of phosphorus sources in in an”
Response 3: Thank you very much for your valuable comments. We sincerely apologize for the oversight on our part. After carefully reviewing the manuscript, we have thoroughly checked and revised the entire document to address similar issues.
The revised objective now reads: “Quantitative resolution of phosphorus sources in an agricutural watershed of southern China: application of phosphate oxygen isotopes and multiple models.” (Page 1, line 1-2)
Comments 4: Page 2, paragraph 2 and in the other places of the MS. You don't need to name all authors from the references. Wang et al. [14] Ishida et al. [7]. For example, Wang, Tian, Zhao, Du, Zhang, Sun, Tekleab, Wei, Fu and Gooddy [14] used δ18OP to track down phosphorus sources in the Yangte River Basin and found that chemical fertilizers were the dominant phosphorus source. Ishida, Uehara, Iwata, Cid-Andres, Asano, Ikeya, Osaka, Ide, Privaldos and Jesus [7]
Response 4: Thank you very much for pointing out this issue. We sincerely appreciate your valuable feedback. In response, we have conducted a thorough search throughout the manuscript for similar issues.
The revised objective now reads: “For example, Wang et al. [14] used δ18OP to track down phosphorus sources in the Yangtze River Basin and found that chemical fertilizers were the dominant phosphorus source. Ishida et al. [7] used δ18OP to characterize the impacts of non-point phosphorus sources in Japan’s Yasu River Basin.” (Page 2, line 19-21); “Inorganic phosphate in water samples was converted to Ag3PO4 following the method of Wang et al. [14], with reagents added stepwisely to remove the organic matter (Figure S1).” (Page 5, line 15-17); “Pistocchi et al. [34] identified a gradual increase in sediment phosphorus concen-tration from upstream to downstream,” (Page12, line: 24-25).
Comments 5: Page 3, paragraph 1. (58.63%), artifcial land in the lower reaches (18.29%), - artifcial change on artificial.
Response 5: Thank you very much for your valuable comments. We sincerely apologize for the oversight on our part. After carefully reviewing the manuscript, we have thoroughly checked and revised the entire document to address similar issues.
The revised objective now reads: “Land use in the watershed includes cropland in the middle and lower reaches (58.63%), artificial land in the lower reaches (18.29%), and shrub land in the middle and upper reaches (21.02%).” (Page 3, line 7-9).
Comments 6: 2.2. Sample Collection. For sediment sampling, three representatives of river sedimen were sampled, including cropland surface soil and forest surface soil (T1, T2 and T3 of Figure 1). T3 absents on the figure 1c.
Response 6: We sincerely apologize for our oversight. In the original manuscript, we mistakenly labeled T3 as S3. This error has been corrected in the revised manuscript we have submitted.
The revised objective now reads: “Figure 1. (Page 4).
Comments 7: Page 6. 3. Results. 3.1. The spatial distribution of total phosphorus (TP) and dissolved total phosphorus (DTP)…Any information is not in Methods about how phosphorus data are obtained.
Response 7: Thank you for pointing out this issue. We apologize for not specifying earlier that the TP and DTP data in this study were collected and analyzed by our team in the laboratory. We have now added this information in the '2.3. Sample Preparation' section on page 4.
The revised objective now reads: “The DTP content was analyzed using the ammonium molybdate spectrophotometric method after filtration (0.45 μm acetate fiber filter membrane, filtered on the day of sampling). The TP content was analyzed using the ammonium molybdate spectrophotometric method without filtration.” (Page 4, line 10-13).
Comments 8: Page 7. Last line. to minimize the water - larget change on largest
Response 8: Thank you very much for your valuable comments. We sincerely apologize for the oversight on our part. After carefully reviewing the manuscript, we have thoroughly checked and revised the entire document to address similar issues.
The revised objective now reads: “It is therefore highly recommend that either a larger volume of water should be collected, or an improved extraction technique for δ18OP analysis should be employed to minimize the largest water volume requirements [7,37].” (Page 7 line 21).
Comments 9: Page 10. 3.6. Quantitative Analysis of major Phosphorus Sources in the XDJ Watershed Chemical fertilizers weer - weer change on were
Response 9: Thank you very much for your valuable comments. We sincerely apologize for the oversight on our part. After carefully reviewing the manuscript, we have thoroughly checked and revised the entire document to address similar issues.
The revised objective now reads: “Chemical fertilizers were found to be the primary source of exogenous phosphorus to the XDJ watershed and its sub-watersheds by the MixSIAR model,” (Page 10, )
Comments 10: The top of page 13. The second phrase repeats the previous phrase. This reflects frequent exchanges at the water-sediment interface, indicating a need for further investigation into phosphorus dynamics at this interface in future research. There is a pressing need for further investigation on phosphorus dynamics at the water-sediment interface in future studies.
Response 10: Thank you very much for pointing out this issue.We sincerely apologize for our oversight. In the revised manuscript, we have carefully checked the entire text for similar issues and made the necessary corrections accordingly.
The revised objective now reads: “This reflects frequent exchanges at the water-sediment interface, indicating a need for further investigation into phosphorus dynamics at this interface in future research.” (Page 13, line 1-3).
Comments 11: Many of the captions under the axes and inside the figures are blurred and need correction.
Response 11: Thank you very much for pointing out this issue. Due to system-related reasons, the figures in the previously submitted manuscript were compressed. In the revised version, we have reinserted the figures to ensure better clarity.
The revised objective now reads: “Figure 1., Figure 2., Figure 3., Figure 4., Figure 5. and Figure 6.” (Page 4, 7, 8, 9, 10 and 11).
Reviewer 3 Report
Comments and Suggestions for Authors- Page 3: The region experiences a subtropical monsoon climate with abundant precipitation and heat. It would be beneficial to present the annual precipitation and average air temperature for these three sub-watersheds to provide a clearer understanding of the climatic conditions in the study area.
- Page 4: For sediment sampling, three representatives of river sediment ere sampled, including cropland surface soil and forest surface soil (T1, T2 and T3 of Figure 1). Figure 1 does not include a T3 site but instead shows two S3 sites. Please verify and correct this inconsistency.
- Page 6: TP at all sampling sites exceeded the Chinese national standard for surface water Class Ⅲ (TP < 0.20 mg/L). This finding is intriguing and requires a more detailed explanation of the key factors influencing the results.
- Page 7: In this study, only 10 samples provided sufficient Ag₃PO₄ for δ¹⁸Oₚ analysis due to low phosphate concentrations. Could spatial statistical analysis help address this limitation?
- Page 11: “Both TP and DTP concentrations in this watershed exceeded those of the Yangtze [14] and Yellow River basins [45].” This paragraph should discuss not only the DTP but also the higher TP concentrations. It is important to understand the key factors to affect the higher TP concentrations.
- Page 13: Chemical fertilizers were a major source of phosphorus pollution in the XDJ watershed. Are there any regulations or laws in the study area restricting fertilizer use? Additionally, it would be helpful to provide recommendations for improving fertilizer policies.
Author Response
Comments 1: Page 3: The region experiences a subtropical monsoon climate with abundant precipitation and heat. It would be beneficial to present the annual precipitation and average air temperature for these three sub-watersheds to provide a clearer understanding of the climatic conditions in the study area.
Response 1: Thank you very much for pointing out this issue. We sincerely apologize for any confusion. Since the XDJ watershed selected for this study covers only 1,142 km2, the relatively small area results in minimal variations in rainfall and average temperature within the sub-watersheds. Therefore, we have not made changes to this part in the manuscript.
Comments 2: Page 4: For sediment sampling, three representatives of river sediment ere sampled, including cropland surface soil and forest surface soil (T1, T2 and T3 of Figure 1). Figure 1 does not include a T3 site but instead shows two S3 sites. Please verify and correct this inconsistency.
Response 2: Thank you very much for pointing out this issue. We sincerely apologize for our oversight in mistakenly labeling T3 as S3 in Figure 1. In the revised manuscript, we have corrected this error and thoroughly checked the entire text for similar issues to avoid unnecessary mistakes. Please refer to the green markings in the manuscript for details.
The revised objective now reads: “Figure 1.” (Page 4, line 1).
Comments 3: Page 6: TP at all sampling sites exceeded the Chinese national standard for surface water Class Ⅲ (TP < 0.20 mg/L). This finding is intriguing and requires a more detailed explanation of the key factors influencing the results.
Response 3: Thank you very much for pointing out this issue. We sincerely apologize for not discussing the reasons for the higher total phosphorus concentrations in the discussion section. In the revised manuscript, we have added an analysis of the reasons for the higher phosphorus concentrations in the watershed in the first paragraph of the first part of the discussion. Please refer to the green markings in the manuscript for details.
The revised objective now reads: “Phosphorus concentrations in the XDJ watershed were elevated, aligning with previous studies [30,44]. Both TP and DTP concentrations in this watershed exceeded those of the Yangtze [14] and Yellow River basins [45]. Due to the sampling period coinciding with the wet season, phosphorus-rich substances such as agricultural fertilizers and pesticides from farming areas, mixed with runoff and rainwater, were introduced into the river, leading to the elevated TP concentrations in the XDJ watershed. The high DTP concentrations in the river’s upper reaches indicated substantial anthropogenic phosphorus inputs, particularly in the upstream tributaries [46]. DTP was readily utilized by aquatic organisms, promoting rapid algal blooms [47]. Elevated DTP levels and re-duced flow rates in the middle and lower reaches of the XDJ watershed increased the susceptibility to eutrophication threats in early spring and summer [48]. ” (Page 11, line 6-16).
Comments 4: Page 7: In this study, only 10 samples provided sufficient Ag₃PO₄ for δ¹⁸Oₚ analysis due to low phosphate concentrations. Could spatial statistical analysis help address this limitation?
Response 4: Thank you very much for your valuable suggestion. In our preliminary work, we also considered using spatial statistical analysis to address this issue. However, we found that although spatial statistical analysis can estimate the general conditions of unsampled areas through spatial interpolation, the oxygen isotope abundance of phosphate is primarily influenced by multiple factors, including land use in the upstream watershed, anthropogenic phosphorus inputs, and hydrodynamic conditions of the river. Therefore, spatial statistical analysis may not be suitable for the analysis of isotope data. In future studies, we plan to increase the sampling density to ensure sufficient phosphate for oxygen isotope analysis.
Comments 5: Page 11: “Both TP and DTP concentrations in this watershed exceeded those of the Yangtze [14] and Yellow River basins [45].” This paragraph should discuss not only the DTP but also the higher TP concentrations. It is important to understand the key factors to affect the higher TP concentrations.
Response 5: Thank you very much for your valuable suggestions. We sincerely apologize for not discussing the higher TP concentrations in our initial manuscript. In the revised version, we have added a discussion on the key factors contributing to the higher total phosphorus concentrations in the watershed. Additionally, in the later sections, we have used phosphate isotope methods to quantitatively assess the contributions of agricultural non-point sources and domestic sewage. We also highlight the important contributions of agricultural non-point sources and internal release from sediments to the phosphorus concentrations in the watershed. Please refer to the sections marked in green for these additions.
The revised objective now reads: “Phosphorus concentrations in the XDJ watershed were elevated, aligning with previous studies [30,44]. Both TP and DTP concentrations in this watershed exceeded those of the Yangtze [14] and Yellow River basins [45]. Due to the sampling period coinciding with the wet season, phosphorus-rich substances such as agricultural fertilizers and pesticides from farming areas, mixed with runoff and rainwater, were introduced into the river, leading to the elevated TP concentrations in the XDJ watershed. The high DTP concentrations in the river’s upper reaches indicated substantial anthropogenic phosphorus inputs, particularly in the upstream tributaries [46]. DTP was readily utilized by aquatic organisms, promoting rapid algal blooms [47]. Elevated DTP levels and re-duced flow rates in the middle and lower reaches of the XDJ watershed increased the susceptibility to eutrophication threats in early spring and summer [48]. ” (Page 11, line 6-16).
Comments 6: Page 13: Chemical fertilizers were a major source of phosphorus pollution in the XDJ watershed. Are there any regulations or laws in the study area restricting fertilizer use? Additionally, it would be helpful to provide recommendations for improving fertilizer policies.
Response 6: Thank you very much for your valuable suggestions. Currently, there are no regulations or laws restricting fertilizer use in the XDJ watershed. Based on your valuable advice, we have added recommendations for promoting the formulation of fertilizer use policies in the discussion section. Please refer to the parts marked in blue for these additions.
The revised objective now reads: “Thus, improving the utilization rate of phosphorus fertilizers to enhance the absorption of residual phosphorus in the soil by crops is a key measure to address this issue [14,62]. In addition to improving fertilizer efficiency, regulations should be introduced or strengthened to control fertilizer use in the watershed, including guidelines for application timing, rates, and methods to optimize phosphorus use and reduce runoff. Farmers should be incentivized to adopt precision farming techniques, such as soil testing and variable-rate application, to match fertilizer inputs with crop needs. A watershed-wide nutrient management plan can also help coordinate fertilizer use, reduce over-fertilization, and prevent phosphorus pollution hotspots. Furthermore, field trials in the watershed can help achieve both economic and environmental benefits by re-ducing phosphorus inputs from fertilizers [14,63].” (Page 13, line 33-40).
