the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Jul 2023
The fate of fixed nitrogen in Santa Barbara Basin sediments during seasonal anoxia
Abstract. Despite long-standing interests in the biogeochemistry of the Santa Barbara Basin (SBB), there are no direct rate measurements of different nitrogen transformation processes. We investigated benthic nitrogen cycling using in-situ incubations with 15NO3- addition and quantified the rates of total nitrate (NO3-) uptake, denitrification, anaerobic ammonia oxidation (anammox), N2O production, and dissimilatory nitrate reduction to ammonia (DNRA). Denitrification was the dominant NO3- reduction process, while anammox contributed 0–27 % to total NO3- reduction. DNRA accounted for less than half of NO3- reduction except at the deepest station at the center of the SBB where NO3- concentration was lowest. NO3- availability and sediment total organic carbon content appeared to be two key controls on the relative importance of DNRA. The negative correlation between NO3- availability and the relative importance of DNRA suggests a negative feedback loop that potentially contributes to stabilizing the fixed N budget in the SBB. Nitrous oxide (N2O) production as a fraction of total NO3- reduction ranged from 0.2 % to 1.5 %, which was higher than previous reports from nearby borderland basins. A large fraction of NO3- uptake was unaccounted for by NO3- reduction processes, suggesting that intracellular storage may play an important role. Our results indicate that the SBB acts as a strong sink for fixed nitrogen and potentially a net source of N2O to the water column.
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Notice on discussion status
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1498', Anonymous Referee #1, 11 Mar 2024
This manuscript presents an interesting and detailed study on the benthic nitrogen cycling in the Santa Barbara Basin. The study is original as the authors used a complex approach measuring in-situ incubation, quantifying benthic rates of nitrate uptake, denitrification, anammox, nitrous oxide production, and DNRA. They found that the sediments in the Santa Barbara Basin acted a sinks for fixed nitrogen with dominating denitrification and as source for nitrous oxide. The data set is well presented and interpreted and the text well written and organized. I only have a few minor comments.
L141-142: How big is the effect of additional substrate added to chamber incubations? In-situ bottom water concentrations ranged from 9.9 µmol L-1 to 27.3 µmol L-1 (Table 1), after adding 15N-labeled nitrate concentration varied between 50 and 100 µmol L-1. Do you suspect to overestimate rates to higher substrate availability?
L256-257: How would rate changes with seasonal altering oxygen concentrations?
L258-259: How representative are the results considering seasonal changes in oxygen and nitrate concentrations?
L297 “However, because the porewater NH4+ concentration was high […]”: Have pore water or bottom water ambient ammonium concentrations been measured? I cannot find any information about porewater sampling in the method section. If you refer to another paper this statement needs a reference. Both anammox and nitrification, which according to the authors contributes at least in part to N2O production (L367), are dependent on available ammonium, it would be interesting to know the in-situ concentrations.
L364-370: Why do you not discuss the potential of DNRA to contribute to N2O production?
Citation: https://doi.org/10.5194/egusphere-2023-1498-RC1 - AC1: 'Reply on RC1', Xuefeng Peng, 29 Mar 2024
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RC2: 'Comment on egusphere-2023-1498', Anonymous Referee #2, 15 Mar 2024
General comment
The authors investigated nitrogen cycling using in-situ incubations with the addition of 15NO3- in the deep Santa Barbara basin, which is mainly anoxic. During incubations, the benthic uptake of total nitrate (NO3-), denitrification, anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA) and N2O production were assessed.
I agree that such incubations are challenging, but they provide new information on rates. In reality, the present contribution provides information about one more new study site using benthic chambers. At the same time, there are also many limitations to using the isotope pairing method during chamber incubations for studying sedimentary dissimilatory pathways. These limitations could be better addressed in the present study.
Regarding benthic dissimilatory pathways, questions still need to be answered about whether the study underestimated 29/30N2 and 15NH4+ production rates since samples were only collected from the water phase. Conventionally, NO3- reduction rates are based on production rates in both the bottom and pore water phases. Additional explanations help address this point. Secondly, I am concerned about the time period that needs to reach diffusion equilibrium after the 15NO3- addition.
Also, I suggest a strongly revised discussion. At the present version of the manuscript, I could not recommend it for publication in BG. My specific and general comments are provided below.Specific comments
Line 29-30: what do you mean here, "feedback loop"?
Line 34-35: this is entirely speculative as the reader can not see by which magnitude benthic processes could affect bottom/water concentrations of NO3 and N2O. The water column is quite high, and what proportion of the standing pool could be affected?
Line 57: what can lead to an increase in oxygen levels during incubations? Please clarify if the C: N ratio provided is molar. Consider adding subscripts for TOC and TON to understand better how they refer to sediments.
Line 85-90: the authors state what they did, which sounds quite descriptive. What was the motivation and aim of this study? I suggest providing questions that the authors aim to answer.
Line 139-144: please provide the final enrichment degree achieved. Maybe I am less familiar with chamber operation, but I need more explanation of how IPT assumptions were achieved during these chamber incubations (for e.g. D14 rates independence of 15N additions (I guess here authors used only D15 rates), equilibrium in diffusion etc.)
Line 169-171: here, the authors state that the production of 29/30N2 was calculated from linear regression; however, looking at the text related to the incubations, it seems they sampled once per incubation. Then, this is quite confusing.
Line 197-232: section 3.1 aims to argue that the technique used did not underestimate measured rates. Overall, this section should be better developed.
Lines 205-207: the authors recognize the weaknesses of the approach used. However, they need to identify what could potentially lead to
underestimation.
Line 207: there appears to be a sudden jump in the logical flow.
Lines 219-223: need more concrete arguments regarding whether rates were underestimated.
Line 222: the authors say that parallel incubations were performed, but I could not find this information in the Materials and Methods.
Line 225: the authors should have discussed intracellular storage in the introduction to clarify their meaning.
Lines 297-299 need to be clarified on where the data presented comes from.
Line 338-340: this is quite a speculative statement.
Line 385-398: this is quite a speculative statement without supportive information.
Line 423-434: in this section, authors should avoid the reption of results.Technical comments
Table 1: The measures in the first column could be clearer, and it is challenging to understand which numbers are for the sediment or water column.Figure 2 is quite complex and needs to be reshaped. I suggest providing the total N2 production and the partition between different dissimilatory pathways. Another option is to provide total nitrate reduction and partitioning between different pathways. I was surprised standard error bars are shown here without clarity on replication. Therefore, Table 2 could be modified.
Citation: https://doi.org/10.5194/egusphere-2023-1498-RC2 - AC2: 'Reply on RC2', Xuefeng Peng, 29 Mar 2024
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1498', Anonymous Referee #1, 11 Mar 2024
This manuscript presents an interesting and detailed study on the benthic nitrogen cycling in the Santa Barbara Basin. The study is original as the authors used a complex approach measuring in-situ incubation, quantifying benthic rates of nitrate uptake, denitrification, anammox, nitrous oxide production, and DNRA. They found that the sediments in the Santa Barbara Basin acted a sinks for fixed nitrogen with dominating denitrification and as source for nitrous oxide. The data set is well presented and interpreted and the text well written and organized. I only have a few minor comments.
L141-142: How big is the effect of additional substrate added to chamber incubations? In-situ bottom water concentrations ranged from 9.9 µmol L-1 to 27.3 µmol L-1 (Table 1), after adding 15N-labeled nitrate concentration varied between 50 and 100 µmol L-1. Do you suspect to overestimate rates to higher substrate availability?
L256-257: How would rate changes with seasonal altering oxygen concentrations?
L258-259: How representative are the results considering seasonal changes in oxygen and nitrate concentrations?
L297 “However, because the porewater NH4+ concentration was high […]”: Have pore water or bottom water ambient ammonium concentrations been measured? I cannot find any information about porewater sampling in the method section. If you refer to another paper this statement needs a reference. Both anammox and nitrification, which according to the authors contributes at least in part to N2O production (L367), are dependent on available ammonium, it would be interesting to know the in-situ concentrations.
L364-370: Why do you not discuss the potential of DNRA to contribute to N2O production?
Citation: https://doi.org/10.5194/egusphere-2023-1498-RC1 - AC1: 'Reply on RC1', Xuefeng Peng, 29 Mar 2024
-
RC2: 'Comment on egusphere-2023-1498', Anonymous Referee #2, 15 Mar 2024
General comment
The authors investigated nitrogen cycling using in-situ incubations with the addition of 15NO3- in the deep Santa Barbara basin, which is mainly anoxic. During incubations, the benthic uptake of total nitrate (NO3-), denitrification, anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA) and N2O production were assessed.
I agree that such incubations are challenging, but they provide new information on rates. In reality, the present contribution provides information about one more new study site using benthic chambers. At the same time, there are also many limitations to using the isotope pairing method during chamber incubations for studying sedimentary dissimilatory pathways. These limitations could be better addressed in the present study.
Regarding benthic dissimilatory pathways, questions still need to be answered about whether the study underestimated 29/30N2 and 15NH4+ production rates since samples were only collected from the water phase. Conventionally, NO3- reduction rates are based on production rates in both the bottom and pore water phases. Additional explanations help address this point. Secondly, I am concerned about the time period that needs to reach diffusion equilibrium after the 15NO3- addition.
Also, I suggest a strongly revised discussion. At the present version of the manuscript, I could not recommend it for publication in BG. My specific and general comments are provided below.Specific comments
Line 29-30: what do you mean here, "feedback loop"?
Line 34-35: this is entirely speculative as the reader can not see by which magnitude benthic processes could affect bottom/water concentrations of NO3 and N2O. The water column is quite high, and what proportion of the standing pool could be affected?
Line 57: what can lead to an increase in oxygen levels during incubations? Please clarify if the C: N ratio provided is molar. Consider adding subscripts for TOC and TON to understand better how they refer to sediments.
Line 85-90: the authors state what they did, which sounds quite descriptive. What was the motivation and aim of this study? I suggest providing questions that the authors aim to answer.
Line 139-144: please provide the final enrichment degree achieved. Maybe I am less familiar with chamber operation, but I need more explanation of how IPT assumptions were achieved during these chamber incubations (for e.g. D14 rates independence of 15N additions (I guess here authors used only D15 rates), equilibrium in diffusion etc.)
Line 169-171: here, the authors state that the production of 29/30N2 was calculated from linear regression; however, looking at the text related to the incubations, it seems they sampled once per incubation. Then, this is quite confusing.
Line 197-232: section 3.1 aims to argue that the technique used did not underestimate measured rates. Overall, this section should be better developed.
Lines 205-207: the authors recognize the weaknesses of the approach used. However, they need to identify what could potentially lead to
underestimation.
Line 207: there appears to be a sudden jump in the logical flow.
Lines 219-223: need more concrete arguments regarding whether rates were underestimated.
Line 222: the authors say that parallel incubations were performed, but I could not find this information in the Materials and Methods.
Line 225: the authors should have discussed intracellular storage in the introduction to clarify their meaning.
Lines 297-299 need to be clarified on where the data presented comes from.
Line 338-340: this is quite a speculative statement.
Line 385-398: this is quite a speculative statement without supportive information.
Line 423-434: in this section, authors should avoid the reption of results.Technical comments
Table 1: The measures in the first column could be clearer, and it is challenging to understand which numbers are for the sediment or water column.Figure 2 is quite complex and needs to be reshaped. I suggest providing the total N2 production and the partition between different dissimilatory pathways. Another option is to provide total nitrate reduction and partitioning between different pathways. I was surprised standard error bars are shown here without clarity on replication. Therefore, Table 2 could be modified.
Citation: https://doi.org/10.5194/egusphere-2023-1498-RC2 - AC2: 'Reply on RC2', Xuefeng Peng, 29 Mar 2024
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David J. Yousavich
Annie Bourbonnais
Frank Wenzhoefer
Felix Janssen
Tina Treude
David L. Valentine
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(816 KB) - Metadata XML
-
Supplement
(301 KB) - BibTeX
- EndNote
- Final revised paper