I enjoyed reading " Seasonal variation and release of soluble reactive phosphorus in an agricultural upland headwater in central Germany" by Rode et al. The authors investigated potential delivery flow paths for P during various baseflow conditions: groundwater (GW) discharge, hyporheic exchange through stream sediments, and soil drainage. This case study is valuable for understanding how P in agricultural catchments is hydrologically delivered to streams and buffered along the way.

I think the manuscript needs at least moderate revision to make the story more effective and clearer. Additionally, I am not so convinced that the GW P delivered to the stream is "geogenic" nor that sediment porewaters are not adding any SRP to the stream -- these points needs more nuance.

Main criticisms

Inconsistencies in methods, results, and conclusions lead to discounting sediment porewaters:

• The radon data are used to support conclusions about GW gains within the stream but much remains unclear. What were the radon concentrations in the GW wells? How was the degassing actually determined? Further, if stream turbulence is the key component behind degassing (L198), then why is the degassing of Rn so much greater in the very slow flow of the summer campaign compared to the winter one?

  • Given the issues noted here by the authors, it may help to consult the review by Raymond et al. 2012 to set an initial estimate of Rn degassing for this system based on basic stream hydrology information.

• My impression of the Rn data is that flow in the summer campaigns was so slow that the gas transfer velocity for Rn was perhaps dominated by gas diffusion. The authors seem to corroborate this point on L313-315.

• Further, the water level must have indeed been very shallow for the September 2020 event (L313). For this event, using the reported velocity of ~0.07 m/s, discharge of 0.51 L/s, assuming the smallest stream width of 0.33 m (derived from 10 m / 30 as mentioned on L174-175), and further assuming a simple rectangular stream channel, I calculate a stream depth of 2.2 cm! (Relaxing the assumptions above may yield an even shallower depth.)

• Given that the stream was moving this slow and the depth was this shallow, I really doubt that sediment porewaters have such apparently little connection to SRP in the water column. Note that the sampled sediment porewaters 7 cm deep (L225) are relevant, but it's likely the uppermost 1 to 2 cm of the sediment that dominate P exchange between sediments and the water column. I suspect that SRP in the porewaters may possibly be lower towards the top of the benthos due to the oxygen gradient (see Palmer-Felgate et al. 2010 for example) but this still may not completely stop a significant flux of SRP to the overlying water column. It's been observed elsewhere that SRP can increase in summer months and that it's likely tied to redox status of benthic sediments (Smolders et al. 2017 cited in text; L75-79) -- why might this argument not apply in this case study?

• Although it's possible that the more oxic sediments at the very surface should have a stronger sorption capacity than the deeper sediments, there's also the possibility of colloidal P -- generated from below -- bypassing the sorption sites and increasing the SRP signal (Gottselig et al. 2017). (I'm assuming filtered water samples [filter size not stated, 0.45 microns?], were still not small enough to prevent these colloids.) There seems to be enough Fe and DOC in the waters to support this.

• It is inappropriate to treat SRP or DP as conservative endmembers mixing in this system, as text on e.g., L331-335 suggest is the basis for arguing that sediment porewaters are insignificant. A more compelling argument is needed. (Further, the data cited on these lines is from 7 cm deep so the DP there may be even less relevant.)

Only geogenic P in GW?

• I find it surprising that GW P in this predominantly agricultural catchment (receiving regular P applications (L104)) is considered geogenic (paragraph starting L401). It seems that these soils have reasonably high P (L120-122). It is even stated on L125 that fluctuations in soil P "suggests a transport of soluble P compounds to deeper layers". [Note that this sentiment reverses later in the paper on L416-418.] So can we really assert here that there's no agricultural P reaching GW and, later on the flowpath, the stream?

• It seems that true natural reference points for GW SRP (i.e. with no history of agriculture) would be needed to support this -- is this available in the Wriedt et al. 2019 reference?

• I take issue here because it is increasingly acknowledged that many of our agricultural catchments are dealing with P "legacies" which will take decades or longer to remediate even with drastic action. Wrongly attributing P sources can lead to even weaker action and longer times for surface waters to recover, if at all.

• The discussion on L415-417 seems to ignore that N and P behave very differently in their transport; seeing faster movement of nitrate than for P is not surprising and doesn't support the statement on L416-417. In fact, the Dupas et al. 2017 paper cited in text argues this very point (and is summed up in their Figure 7).

• I actually concur with what's said on L426-428: SRP is probably well buffered in the catchment but that means that those sorption sites may slowly leak P to the system, maintaining ecologically relevant SRP concentrations in the stream for a long time.

Other general comments:

Stream description:

• The catchment is described quite thoroughly, but what about the stream itself? Slope, width & depth at time of sampling, morphology type, substrate characteristics, light availability, etc. would be quite pertinent to this study.

• Illustrating on Figure 1 or adding a new subplot with focus on the stream itself to show locations of all the measurements (tracers, sediments, etc.) would be great context.

Data visualization:

• Suggest sticking to the red/blue for summer/winter throughout (e.g. avoid the change in Figure 7)

• Please try to fix the x-axes in the figures with stream distance (m) so that they're similar/more comparable across figures. The aspect ratio for Fig 6 is too wide. The changes in figure styles and dimensions when 'distance from upstream' is on the x-axis makes it more difficult to follow the story across figures.

• Consider combining Fig 2 and 3 to save space. Figure 4 could also be combined with 2 and 3 by showing the observed groundwater level for the study period overlaid upon the prior 10 year average -- just an idea.

• Double y axes in general:

  • My preference is to avoid them if possible. It clouds the comparisons between seasons and visual points of data do not immediately map to values on the y-axis (until I re-read the caption to know which is which)

  • In some cases, they're entirely unnecessary, such as in Figure 7. (Also, why not just plot both series on log10 ? Having double axes and different transformations is doubly confusing -- this applies to both radon figures.)

  • If double axes have to be used, I think a simple but very helpful improvement would be to color-code the y-axes text/title with the same red/blue color scheme for the points.

• With all the data available in some of the figures, the summary stats in Tables 1 and 3 aren't necessary and can be removed to reduce clutter; just make references to the stats of interest within text.

• I'm not sure the 3rd subplot in Figure 6 (with net change in SRP flux) is necessary -- it's simple enough to examine the pattern in the actual SRP flux data in the bottom subplot, in my opinion.

• While the log-axes in other figures are more clearly denoted (e.g. Figure 8), it's not very obvious in Figure 9 -- making figure styles more consistent would help here.

Writing and writing style:

• I had forgotten all of Section 3 was results and discussion, as there wasn't too much discussion until much later. (The last few paragraphs felt like a sudden 'dump' of discussion.) Perhaps this paper is better suited to separate R & D sections?

• Please streamline the introduction to build its focus up to the study objectives. I think introducing the main sources in points (a) to (d) on L50-54 and elaborating each is a fair way to structure the intro, but it seems that structure was forgotten after the intro had covered point 'c' (sediments, L71-79); L80 onwards breaks with that structure leaving me wondering where the intro was headed. Further, if the intro is going to focus on "SRP mobilisation ... in various headwater compartments" (L50) as the first paragraph indicated, then lead sentences in paragraphs such as on L63 should more clearly follow that thread. I.e., 'SRP mobilisation' should be the common theme throughout and clearly connected.

• Avoid excessive passive voice throughout. E.g., L336-337 could be rewritten to avoid the "were found" and so make the sentence clearer: "At the outlet, concentrations Fe, DOC, dissolved P, and NH4 were greater in the sediment pore water than in the stream (Table 2),..."

• Proofreading for clearer English would improve the paper. E.g., I'd suggest revising the sentence on L273-274 to: "The spatial pattern of SRP flux largely followed the pattern observed for discharge." This keeps the emphasis on the main topic of this paragraph (SRP flux) rather than on 'spatial pattern of discharge' in the original sentence. Another example sentence that could be made clearer/more effective is L56-59.

• Please break up the paragraph on L401-439 (139 lines!). Further, consider incorporating the discussion with the broader literature done here more evenly throughout Section 3.

Specific comments

• L177-178: I'm confused by this sentence. What is the 'time lapse' issue here and why should that matter if you're moving upstream?

• Section 2.2.1: was there any potential for significant sub-daily variation in discharge or was it stable?

• L199-201: it is stated here that there are multiple ways to estimate the radon degassing from the stream -- which the authors described as a "crucial parameter" just above. So... how was this actually determined here in the present study?

• Section 2.2.2: was there any reason for not measuring Rn in the September 2019 campaign? Also, why were sampling locations different from the salt tracer points?

• L344-350: I don't have any expertise with radiocarbon dating but is it really the case that DOC in the stream is millennia old? Could there be more discussion with literature in this section? Additionally, there's no mention here about the stream metabolism involved. If there's enough light, much of the DOC is likely autochthonous for summer baseflow, especially considering the high nutrient availability.

• L354: Can more concrete evidence be given instead of stating "...probably transported by preferential flow paths."

• L402: where in the paper are the 'oxidised groundwater conditions' established? Could this be included in Table 2?

• L432: Please avoid the "source/sink" dichotomy for P and sediment sorption. It ignores the transient nature of sediment P and how, really, sorptive materials in catchments only buffer P.

Technical comments

• Please superscript all atomic mass numbers: e.g. ²²²Rn and ¹⁴C; fix subscripts too ("Psat" on L59, "NO3" on L69)

• L30: 'SRP losses' here is kind of ambiguous in terms of the direction of the flux.

• L31: 'SRP-fraction for dissolved P' is unclear.

• L59: 'soil P sorption saturation' on its own doesn't mean greater P mobility unless you refer to soils with high P sorption saturation (typically, the saturation is expressed as some sort of degree). Additionally, is "Psat" ever used again in this manuscript? If not, no need for a new acronym.

• L69: "NO3" should be defined.

• L75: don't use "molybdate reactive P" if 'SRP" is used everywhere else

• L89-90: suggest moving the list of pathways out of the parentheses as they're quite important

• L90: "locate" instead of "localise"?

• L92: the "gaining- and losing water fluxes" needs to be reworked, avoid the dangling hyphen if no hyphen is used for 'losing'

• L112-114: this sentence doesn't seem to state anything clearly -- what's the message here?

• L122: move the Kistner et al. 2013 reference up one sentence (to align with the "Previous research has..."). Also, no need to give a new acronym for DPS if that's not used again.

• L150: I think there's a zero instead of "O" in the "NO3" here.

• L150: The Dupas et al. 2017 reference here is out of place; save this for the discussion.

• L213-215: Were the probes calibrated on the day of measurement? Please cite a reference for the 'standard methods' for the P analyses and give some note of accuracy and/or method detection limit.

• L217: how was dissolved iron measured? detection limit?

• L268: 'proportion' not 'proportionate'

• L270: "neutral" is an odd term to use here, perhaps switch for "had little net change in discharge"

• L275: perhaps switch out 'gained' for 'contributed'?

• Figure 7: please also add to the caption or indicate on the figure what the dashed line means (L286). And shouldn't this apply to Figure 8 too? Also, please keep the time labels consistent with other plots (i.e. January 2019 and September 2020 instead of 'winter' / 'summer').

• L310, how reliable are the values given here for groundwater given the text on L296? Is some value representing uncertainty (e.g. confidence interval) possible here?

• Section 3.4: edit the title to include "in summer baseflow (September 2020)" as that's crucial context in this whole section.

• L336: make NH4+ more consistent throughout text (replace "NH4"); consider also including the valence for "NO3-"

• Table 2: why is the Fe in the outlet stream sample "n.d." (unsure whether this is "not determined" or "not detected")? Or was it below detection? If the below detection, what was the MDL?

• Table 2: please add pH, dissolved oxygen (or some indication of redox status), conductivity, and temperature here as that would be very helpful context (and seems to have been measured according to 2.2.3).

• L432: should this be 'House 2003'? This reference is missing

• References -- suggest checking all:

  • Kleinman et al. 2009; van Dael 2020 reference is duplicated

  • also some refs are CAPITALIZED

  • LLFG 2021 reference (L104) is missing

References cited

Gottselig N, Amelung W, Kirchner JW, et al (2017) Elemental composition of natural nanoparticles and fine colloids in European forest stream waters and their role as phosphorus carriers. Global Biogeochem Cycles 31:1592–1607. https://doi.org/10.1002/2017GB005657

Palmer-Felgate EJ, Mortimer RJG, Krom MD, Jarvie HP (2010) Impact of point-source pollution on phosphorus and nitrogen cycling in stream-bed sediments. Environ Sci Technol 44:908–914. https://doi.org/10.1021/es902706r

Raymond PA, Zappa CJ, Butman D, et al (2012) Scaling the gas transfer velocity and hydraulic geometry in streams and small rivers. Limnology and Oceanography: Fluids and Environments 2:41–53. https://doi.org/10.1215/21573689-1597669

Main comments

This research addressed the question of SRP release to surface water during baseflow conditions in different seasons (Winter and Summer). The research combines measurements of SRP in stream water sections, in streambed sediments, sediment pore water and groundwater, as well as tracers’ tests and Rn measurements to localize and quantify GW inflow. This is important to better understand P delivery and release to surface water and the respective role of the different P sources.

I think that it is a manuscript with great potential but with a lot of information that need to be more organised to have a clearer story. There are quite a lot of grammatical issues to correct throughout the manuscript, a thorough proofread is needed. There are some details in the material and methods section to correct/add (measurement methods, uncertainty…). The discussion comes also very late and is too short, without many references to other studies. I would separate the results and discussion sections and would develop the later by discussing YOUR results and the processes that may be involved.

Technical comments

I would improve the resolution of Figure 1 and the y axes (colours when the scales of the 2 y axes are different, labels).

I would add one figure with all the sampling/measurements points, that would make things much clearer.

Check the references to figures and tables (all need to be cited in text and be the right one).

No abbreviations (“SRP”) or numbers (“52%”) at the beginning of a sentence.

Superscript and subscripts to check.

There is a lack of consistency in the terminology (Summer/Winter, September/January).

Specific comments

Line 20: I think there is a mistake in the email address.

Abstract

Line 30: SRP-fraction for DP? It is unclear.

Introduction

Line 50: I would try to improve the transition between the temporal variability of SRP concentrations and the different P sources, it is too quick as it is now.

Lines 56-59: I would suggest separating in two sentences, it is a long sentence that may be difficult to follow and understand.

Lines 63-66: I would also here separate the sentence in two.

Lines 68-69: The sentence about temperature-dependent processes and its link with the redox conditions discussed above are hard to understand, I would clarify this. The transition to temperature-dependant processes would need to be improved.

Lines 71-75: The first part of this paragraph is hard to follow and understand due to grammatical errors, lack of clarity and organisation.

“In situations, …”: I would not use this, but would go directly to the point.

“fed by baseflows”: I would say “fed by groundwater” or “during baseflow conditions”.

“Data suggest…”: which data? Reference needed here.

Line 79: I am missing here a paragraph about geogenic sources of P, as it has nicely been done for the other P sources in the above paragraphs.

Line 88: I am not convinced using the expression “headwater baseflows”, maybe instead “in headwaters during baseflow conditions?”.

Material & Methods

Lines 110 and 111: I would not use “e.g.” when referring to conductivity values, I would give a range or a mean value instead.

Line 112: I think “circa” is commonly used before dates, so I would delete it and just keep “below 0.4 m”.

Line 113: Which “detailed topographic characteristics”? I would give more details here.

Line 121: DPS is Degree of P Saturation.

Line 124: I would specify “declines of WSP in the topsoil…”.

Lines 135: We go from precipitation/Q (Figure 2) to GWL (Figure 4) without mentioning air temperature (Figure 3). Either the data presented in Figure 3 should be presented in the text or Figure 3 should be deleted.

Lines 148 and 149: Change “between X-X” to “between X and X” as before.

Lines 146-150: To which period(s) (e.g. 2010-2020?) do these values refer to?

Line 152: I would write “January 2019 during a period of…” instead of “January 2019 with..”.

Line 153-155: It reads like dilution tests and 222Rn measurements were also used to characterise stream water, groundwater and sediments, which is not true. I would correct this; the sentence can then be used to organise the section.

Here we have: 1) in-stream tracer dilution tests and Rn222 measurements to analyse lateral inflows and 2) what measurements? to characterise stream water, groundwater and sediments properties.

I would slightly change the headings of the section 2.2. to improve its organisation:

2.2.1. Lateral inflows to the stream

2.2.1.1. Water balance of stream sections measured by tracer dilution tests

2.2.1.2. Groundwater discharge investigated by Radon measurements

2.2.2. Stream water, groundwater and sediments chemistry

Lines 169-191: This is a very good section; the methodology is clear.

Lines 203-204: How do these 6 locations relate to the 6 locations used for the tracer dilution tests? Are they different? If so,  why? How far are they from each other?

Lines 208-210: I would develop on the method on how to get from Rn data to the localisation of groundwater discharge and its quantification. How is the rate of radon degassing determined?

Lines 212-228: I would improve the organisation of this section, maybe follow: field instrumentation-field sampling-field measurement-lab analysis? Which method did you use for iron analysis?

Line 215: Please specify which methods you used (and with references) for P analysis, this is important. Include the method detection limit or uncertainty.

Lines 229-235: This part of the section is more organised and easier to understand.

Results & Discussion

Line 240: Change “in…” to “during the two summer campaigns...”.

Lines 241-242: I would not use “highly” and “very”.

Line 241: “Constant” instead of “consistent”?

Lines 241-243: This is a long sentence; I would separate it in two sentences.

Lines 243-244:  I would rather use a factor for comparison instead of a concentration.

Line 250: Should it be summer instead of autumn?

Line 256: I would specify in the heading “along the study reach of the stream”.

Line 267: “proportion” not “proportionate”.

Line 268: Do not start a sentence with number.

Line 270: The term “neutral” cannot be used here, I would rewrite the sentence.

Lines 279-288: I would gather this part into a first paragraph presenting the results: longitudinal patterns and concentrations.

Lines 288-319: Then, in a second paragraph, I would discuss why we see these patterns and concentrations by bringing in the info on groundwater discharge localisation and rate. I think that would improve the organisation of this section.

Lines 296-309: More information in the methods section on the FINIFLUX model would help to better understand the uncertainties related to the modelled results.

Line 312: I think you already said that before (lines 287-288?), in a different way. I would avoid repeating the results.

Line 321: Why are you investigating sources of SRP only in September 2020? Explain why, it is not clear for me.

Line 322-324: It is hard to locate these observation points, a figure showing all sampling/measurement points would help a lot.

Line 356: “are at work”, please rewrite this sentence.

Lines 351-364: There are some good things here but there is no references at all to support your points, and the discussion about the underlying processes is almost absent. The discussion needs to be developed.

Lines 374-386: It is too much focused on presenting the data, and not enough on discussing them. I would discuss briefly how your results compare with long-term data (“our results are consistent with...”) but discuss more about YOUR data and the processes explaining what you observed.

Line 374-375: I would rewrite this sentence, some grammatical issues there.

Line 385: Any reference to support the suggested dilution pattern?

Line 394: I feel like the REAL discussion starts here, so very late. I think separating the results and discussion would be beneficial.

Line 414-426: I really like this part where you discuss your findings, related them to land use and soil type. Some references are missing when you refer to other studies in the same catchment.

Line 418: Please rewrite the beginning of the sentence, it does not seem right.

Figures/Tables:

Figure 1: This figure is hard to read, the resolution needs to be improved. In the legend, I would not use “soil types” since it does not refer to WRB soil types. Maybe hillslope or topographical position?

Figures 2/3/4: I would gather the three figures and use panels. Units of the y axes should be in square brackets.

Figure 4: I would show only the same period as in Figures 2 and 3 in the text, a longer time series can be shown in the Supplement maybe. Is there an issue with where the vertical lines are located? It does not look to be the same dates as in Figures 2 and 3.

Figure 5: Maybe consider colouring the y axes (blue/red) so it is easier to see that they have different scales.

Figure 6: In the caption I would use “during the three sampling campaigns” and not “in”. Should it be “SRP net flux” instead of “SRP net” in the y axis title? Maybe consider colouring the y axes (blue/red) so it is easier to see that they have different scales between January and September. I would also change the order of the panels since Q net is calculated from Q and SRP net flux is calculated from SRP flux: Q, Q net, SRP flux, SRP net flux.

Figure 7: I would add a sentence explaining the dashed line in the caption (even though it is already in the text), so the reader does not have to look for it in the text. In the y axes titles, you use here winter/summer but in Figures 5 and 6 you use January/September, I would stick to one of them and not mix the two, be consistent. I would also change the colours to red and blue to be consistent with the other figures and I would colour the y axes (blue/red).

Figure 8: What is the uncertainty of the modelled Rn concentrations?

Figure 9 : Why are there only 3 points here? And not all the measured concentrations points? Is it the average of each campaign? I am surely missing something here, I would clarify.

Table 2: “nd” refers to “not determined” or “not detected”? If it refers to not detected I would say “< MDL” instead. These MDL need to be given in the method section.

Table 3: Where do these data come from? Any reference?