Review of manuscript egusphere-2025-78 submitted to EGUsphere by Sofía López-Urzúa and colleagues: Quantifying the agricultural footprint on the silicon cycle: Insights from silicon isotopes and Ge/Si ratios

With apologies to the authors and editor for this late review.

López-Urzúa and colleagues present the results of a comprehensive Si (isotope) budget for a small agricultural budget in France. Using different mass-balance approaches to quantify the amount of Si exported from the catchment in harvested crops, they find that it exceeds by a large amount the export of dissolved Si in streamwater, providing a demonstration of anthropogenic impacts on catchment Si cycling.

In general, I find this a solid manuscript worthy of publication after minor revisions. It is well written with clear figures and appropriate referencing, and deals with a topic that I think will be interesting to many in the community. The methods used are appropriate and the data seem of good quality. I have some suggestions or questions the authors may wish to consider in a revised version of the manuscript, that I detail in rough order of appearance.

Perhaps the weakest part of the dataset – as acknowledged by the authors (e.g. around L595) is the small number of total plant and clay samples, and that they are limited to only the leaves and not the full plant biomass. Much of the data interpretation relies on the plant and clay Si isotope fractionations/differences between fractionations for the difference species, but I feel these are not so well constrained. If there is the possibility to provide more data here this would greatly help strengthen the paper.

Related – in some cases the uncertainty propagation seems unrealistically small, in particular for the clay fractionation (Table 4 gives it as ±0.07‰; presumably 1sd?), but I can’t make this fit with the data from table 1. Also, an uncertainty of only 0.01‰ is used for the secondary clay itself, but this is after a series of corrections for the ‘contamination’ of the clay size fraction with primary minerals. How is it possible that this correction process (detailed in appendix B) results in a narrower uncertainty? And is it justifiable that a single clay sample taken at ca. 60cm depth (Fig. 2) is representative of the clay that will eventually be eroded?

The mass-balances approaches detailed here explicitly or implicitly require steady-state, but I wonder how justifiable that is for this heavily anthropgenised catchment. E.g. the Clymans et al. reference that is cited details how the soil pools of Si change over decadal to centennial tiemscales in response to land cover change. This is a bit of an easy criticism to make but perhaps some discussion on how transient increases or decreases in the size of internal soil pools of Si (phytoliths, amorphous Si, clays, …) might impact the interpretation would be warranted?

Regarding the vertical gradients in [Si] and d30Si, there doesn’t seem to be much discussion of a simple mixing between Si-deplete rainwater and Si-rich ‘weathering’ water. Could this be part of the interpretation?

The authors assume that the bedrock is dissolving congruently (e.g. L311, but somewhat contradicted on L541), and that all primary minerals have the same Si isotope signature (e.g. Appendix B, L682). But how justifiable are these assumptions? A growing body of work demonstrates that minerals have specific d30Si signatures. Probably of minor importance here, but perhaps worth considering.

There are three different approaches applied here: 1) a d30Si+Ge/Si mass balance, 2) a mass balance based on river Si fluxes, and 3) a mass balance based on soil geochemistry. Although they are designed to predict slightly different aspects of Si export, I was surprised not to see a more explicit comparison (e.g. in a table or a figure).

The fractional export value for e_Si in approach 2 (stream water + sediment based) is 0.36 (L498). As far as I understand, this includes E_org, E_sec and E_prim - but is this inconsistent with a bedrock dominated by quartz? (which they assume elsewhere to be inert, e.g. 541 – if quartz is not dissolving then a minimum value for e_Si would be the quartz fraction of the bedrock)

Minor comments

L56: Either more recent revisions of the Si budget (e.g. Treguer et al) and/or the ‘original’ river Si flux estimates (e.g. Dürr et al/Beusen et al) might be appropriate here.

L84: To avoid overstating the novelty of this contribution, maybe already mention here that some previous work has identified that plant biomass as a whole doesn’t seem to discriminate against Ge as much as the phytolith-based estimates cited here would suggest.

L158: if the bedrock comprises bedding of different lithologies, is this one sample enough to capture the heterogeneity? Even in plutonic rocks variability in ‘immobile’ element content can be large (which becomes important for e.g. the mass-balances and the ‘tau’ values later).

L180: What is precision/long term reproducibility on the elemental data? Were any secondary reference materials included in the analyses?

Fig 2: presumably cmbs on the y-axis, not mbs. Greek letter mu (not u) on Ge/Si x-axis.

L286: “compared”

345: ‘show a positive correlation’ / ‘are positively correlated’

L415 – also Baronas et al 2020 GBC would be appropriate to cite here?

L483 – actually relatively high?

L519: eSi_sec repeated here – presumably should be eSi_org?

L520: If this is a schist bedrock, how variable is the Ti content, and how are uncertainties propagated?

L530: What is the justification for using stream water rather than soil solutions to define e_prec?

L544: “to be inert”

L567: Why are these values so low compared to previous two estimates?

L595: fractionation factors are not ‘heavy’ or ‘light’; better to talk about magnitude. In general, fractionation factor normally refers to so-called ‘alpha’ notation, and just ‘fractionation’ alone to ‘epsilon’ notation – see Coplen 2011 DOI: 10.1002/rcm.5129.

L634: See also Vandervenne et al 2013 Proc Royal Soc B.

L708: Does the very low number of acceptable iterations (e.g. 0.2% for scenario 2) simply imply that an assumption underpinning the mass-balance or endmember assignments is incorrect?

Peer Review Report

Manuscript Number: EGUSPHERE-2025-78

Title: Quantifying the agricultural footprint on the silicon cycle: Insights from silicon isotopes and Ge/Si ratios

Review of manuscript EGUSPHERE-2025-78 submitted to BG by Sofía López-Urzúa and colleagues:

With apologies to the authors and editor for the delayed review. The manuscript couple silicon isotopes and Ge/Si data of different critical zone compartments to quantify the Si export from the catchment. The authors identify vertical gradient in water pools, with a heavier Si isotopic composition in soil porewater and lighter composition in groundwater interpreted as a result of plant uptake in shallow soil profiles. Using two independent quantitative approach the authors identify plant uptake to be the largest Si export flux from the catchment. The manuscript is generally well written, methodologically sound, and adds valuable insights into terrestrial Si cycling. The results highlighted in the study aligns well with the scope of BG and I recommend the manuscript for publication after considering the following comments.

I have one suggestion regarding the entire section 5.1. One of the key highlights of the manuscript is that the authors have made considerable effort in measuring δ³⁰Si and Ge/Si from different critical zone compartments, with an objective to decipher what controls the Si cycle in the catchment. However, the results are not well depicted in the figures and discussed. I understand the focus is more on the quantification of the Si export, but I would suggest a bit more detail to be included especially in 5.3 about the plant uptake and Si isotopic fractionation pathways linking to vertical gradient (e.g. Appendix C, Fig. C1 nested piezometers and soil solutions).

Minor comments

l83-86: The authors have introduced the potential of Ge/Si ratio in decoupling plant uptake vs. weathering here without commenting on the results from Frings et al., (2021b), which they have discussed in l369-371. I would recommend to introduce the key highlights from Frings et al., (2021b) as well, since the validity of Ge discrimination against Si during plant uptake is under question.

l137: Any irrigation practices?

l124: Please add details of the general climate of the catchment, especially rainfall.

l182: Add the uncertainty and certified reference used for ICP-MS, especially for traces (Al, Fe).

l185-193: The sentences here are not clear here. I suggest you re-write to clarify phases and protocol. If I understand correctly, you target here amorphous aluminosilicates, crystalline Al and Fe oxides? The amorphous aluminosilicates can be clay precursors with different fractionation factors than adsorption onto oxyhydroxides.

l334: Again, here you mention dry and wet season and redox processes but we have no clue about the rainfall variability or seasonality of the study site. Interestingly, I could see that groundwater sampled do not exhibit any significant changes in δ³⁰Si (maybe ±0.2) over the time period of sampling (8 years?)

l380: You mention in results the significant differences in δ³⁰Si between the Gueriniec and Kerroland transect (l285-288) but I don’t see any discussion related to that? What can be the drivers of such differences? I can see it is consistent in shallow as well as deep groundwater, with a higher Si/Al in Kerroland.

l519: Repetition here, please change instead of e^Si_org?

l567-l570: Here, I have a query regarding the assumption of organic matter Si. In Table S2, I can see you mention about the OM and also the Si content associated, which is ~0.1%? Is that the Si bound to organic matter? If yes, could you justify the assumption of 2.3%?

Figures

Fig. 2: Correct the mistake in the depth unit mentioned in y axis, should be cmbs and please expand for reader ease.

Fig. 3: Please include some endmembers from the critical zone here rather than presumed trends perhaps. Are you still certain about the plant uptake trend in Ge/Si vs δ³⁰Si relationship? The plant leaf samples indicate Ge/Si close to or greater than the bedrock/water, pointing to a more no discrimination or selective uptake of Ge relative to Si.