Do degree and rate of silicate weathering depend on plant productivity?

Oeser, Ralf A.; von Blanckenburg, Friedhelm

doi:https://doi.org/10.5194/bg-17-4883-2020

Articles | Volume 17, issue 19

https://doi.org/10.5194/bg-17-4883-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Earth surface shaping by biota (ESurf/BG/ESD/ESSD/SOIL inter-journal...

https://doi.org/10.5194/bg-17-4883-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 19

Research article

|

14 Oct 2020

Research article |

| 14 Oct 2020

Do degree and rate of silicate weathering depend on plant productivity?

Ralf A. Oeser and Friedhelm von Blanckenburg

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Final revised paper (published on 14 Oct 2020)
Preprint (discussion started on 09 Mar 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Interesting hypothesis but I'm not convinced the data point to the conclusions drawn.', Anonymous Referee #1, 04 Apr 2020
- AC1: 'Response to RC1', Ralf Oeser, 05 May 2020
SC1: 'Interactive comment on “Decoupling silicate weathering from primary productivity – how ecosystems regulate nutrient uptake along a climate and vegetation gradient” by Ralf A. Oeser and Friedhelm von Blanckenburg', Marijn Van de Broek, 09 Apr 2020
- AC2: 'Response to SC1', Ralf Oeser, 08 May 2020
RC2: 'Review of the manuscript "Decoupling silicate weathering from primary productivity – how ecosystems regulate nutrient uptake along a climate and vegetation gradient"?', Anonymous Referee #2, 11 May 2020
- AC3: 'Response to RC2', Ralf Oeser, 17 May 2020
RC3: 'Comment on “Decoupling silicate weathering from primary productivity – how ecosystems regulate nutrient uptake along a climate and vegetation gradient” by Ralf A. Oeser and Friedhelm von Blanckenburg', Anonymous Referee #3, 14 May 2020
- AC4: 'Response to RC3', Ralf Oeser, 19 May 2020
- AC5: 'Final response to all referee comments and short comments', Ralf Oeser, 28 May 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (28 May 2020) by Jack Middelburg

AR by Ralf Oeser on behalf of the Authors (28 Jun 2020) Manuscript

ED: Referee Nomination & Report Request started (06 Jul 2020) by Jack Middelburg

RR by Anonymous Referee #2 (22 Jul 2020)

Suggestions for revision or reasons for rejection

Review of the manuscript “Do degree and rate of silicate weathering depend on plant productivity?“ by Oeser and von Blanckenburg submitted to Biogeosciences
July 2020

I had already reviewed the first version of the manuscript and appreciate the complete overhaul, which rendered the paper much clearer. However, I have still concerns with respect to (i) the unusual use of soil scientific terms, (ii) partly sloppy and even wrong use of plant nutritional terminology, (iii) the statistical data treatment, and (iv) the general structure of the paper, which is still overly long for its message. I regret that the authors seem to be resistant against some of the reviewer comments without presenting arguments that I find convincing.

i. I think that the use of saprolite is not consistent with its usual meaning in Soil Science. Saprolite is the lowermost part of the deep, strongly chemically weathered soil in the humid inner tropics where desilication prevails. Saprolites are characterized by their intensive degree of weathering while at the same time the original structure is maintained because of the location under tens of meters of weathered material. Saprolites surround Woolsacks and occur as relics of the Tertiary at erosion-protected locations in temperate soils but according to the soil description at your study sites in Chile, there are no saprolites. As a consequence, the use of the term regolith does not seem to be necessary, because there is no part of the weathering mantle which is free of living organisms. What you are studying is the soil with A, B, C and perhaps even R horizons. I understand what you mean by “weathering front” when referring to deep cracks in the granitoid rocks, where initial weathering indeed takes place but in Soil Science the weathering front is usually seen at the contact interface between soil and bedrock, i.e. between the Cw and the (nearly) unweathered rock (i.e. R horizon). The latter does not need to be changed but it should be clear what you mean.

ii. Although the authors removed the part that I most criticized in my past review, there are still several issues with the plant nutritional terminology. First of all, it must be clearly recognized that the most limiting nutrient is N, which is not rock-derived. The demand for the rock-derived nutrients follow the N supply in little variable stoichiometric ratios. All essential nutrients are required at the same time. A common differentiation would be to distinguish plant mineral macronutrients (N, P, K, Ca, Mg, S) from micronutrients (Fe, B, Cl, Mn, Zn, Cu, Mo, Ni). It is problematic to generally include Al in the list of plant beneficial elements because for most plants Al is not beneficial but even more likely toxic. Moreover, as I already stated in my previous review, nutrient concentrations are mostly regulated in plants so that their concentrations vary by much less than a factor of 10. One of the most extreme cases is the comparison of straw with grain of wheat where the straw contains 5% N and the grain 20%, i.e. a variation by a factor of four. If the whole plant is considered, the variation of nutrient concentrations is usually even smaller. The same is true for element ratios. Consequently, if there is a deviation of a factor of 10 of element ratios between the so-called plant-available nutrient pool in soil and the plants, then this is clearly dissimilar. Moreover, it is not easily possible to characterize plant availability with a single extract, because a single extract does not consider the kinetic replenishment of nutrients after plants have taken up a nutrient and because the extract cannot mimick nutrient acquisition strategies like enhanced mineralization of organic matter by exoenzymes or local acidification by roots and mycorrhizae. This is particularly true for P.

iii. In principle, I find the approach to single out biological weathering with a statistical approach great. However, I see problems in the small number of independent data you have, i.e. n = 4. You are not transparent about the number of data considered in your analysis and I suspect that you have included more the four data pairs. If one considered the two soil profiles on the different slopes as independent from each other – which is debatable – you had n = 8. (But I would not recommend to do this.)

iv. I think that the conceptual perspectives should be included into the introduction to avoid references to later sections such as in l. 78-79 and 91-94 and because you present your methods before you justify them. I also think that the introduction would become more concise (alone deleting the references to later section saves 5 lines). I therefore strongly suggest to combine and shorten Sections 1 and 2. Moreover, all results are still presented threefold, i.e. in tables, figures, and as numbers in the text. I had already previously suggested that the tables should be moved to the appendix and the numbers in the text deleted, while the figures should be kept and I still insist on this. This could be a really interesting paper. However, it is still cumbersome to read, because it is not sufficiently concise.

Minor comments:
l. 11, 13, 167: Delete “substantial”. If the work was not substantial, it should not be published.
l. 82-83: Move to acknowledgments.
l. 90: Replace “nutritive element” by “mineral nutrient”.
l. 97-98: Delete. At this point, this is an unsubstantiated claim and moreover a repetition from the abstract.
l. 125-127: This is not true. In many terrestrial ecosystem, biomass production is limited by N which is rarely (only in bituminous sedimentites) rock-derived but acquired by biological N2 fixation from the air.
l. 127-129: I think that this is a too far-reaching generalization. There are e.g., biodiversity hotspots in mountainous areas, which are not particularly nutrient-poor.
l. 156: The “geogenic nutrient pathway” rarely influences nutrient limitation in native ecosystems. I can only imagine that this would be the case if K was growth limiting. Even the P supply is governed by mineral dissolution and sorption-desorption equilibria, organic matter mineralization and biological acquisition strategies such as local acidification by plant roots and mycorrhiza. Of these processes, only mineral dissolution could be counted as weathering process but mostly of secondary minerals. In strongly weathered humid inner tropical ecosystems it is thought that the small losses of nutrients escaping from the close ecosystem cycling are replenished by deposition, while weathering does not play any role.
l. 166: Start a new sentence after the references.
l. 171: Mediterranean (with upper scale M).
l. 178 (and in the whole paper): Replace “gC” by g m-2 yr-1 C.
l. 229-230: Umbric Podzols, Orthodystric Umbrisols (mind the spelling including upper case letters).
l. 230: Do you mean an organic layer? Usually, there are several organic horizons (Oi, Oe, Oa).
l. 260: Delete “As is commonly the case in field studies”, because this is not true. There are many field studies on root distribution even in forests.
l. 262: What is the “litter layer”? Only the Oi horizon, i.e. the freshly fallen litter of the same or perhaps two years? Or the whole organic layer? Use consistent terminology.
l. 281: Where are the results of these quality controls?
l. 286 (and in the whole paper): The correct SI unit is mL (with upper case L).
l. 287 (and in the whole paper): Milli-Q is a brand name, which should not be used. Replace by “deionized water”.
l. 290 and l. 303: “rounds per minute” are meaningless without the knowledge of the geometry of your centrifuge or shaker. Delete or replace by g.
l. 351: P and K are not the “most important nutrients” in a N-limited system like yours. It is N, and N supply does not depend on weathering at your study sites. This must be acknowledged and discussed.
l. 354: It is wrong to consider Al as a generally beneficial element for plants, only for a few and only at low concentrations. In Marschner (2012) it is stated: “Aluminum is beneficial to some plants, such as tea, and may alleviate proton toxicity and increase the activity of antioxidant enzymes.”
l. 355: It is wrong to include the micronutrient Fe into the group of beneficial elements. It does not make sense to refer to an arbitrary selection of nutrients as “plant-essential elements”.
l. 356: It is wrong to consider Sr as a nutrient. It is at best a surrogate. However, its metabolism in plants differs from that of Ca for which it is used as surrogate (see e.g., Blum et al., 2012, Plant Soil 356, 303-314).
l. 364: I do not understand what you mean by “the shallowest mineral soil”?
l. 382: element-specific
l. 395: The total stock of elements would include the organic layer, which plays a particular important role for plant nutrition, where present.
l. 403: If you just sum up the stocks of macro and micronutrients the result will be (almost) identical to that for the macronutrients. I do hard in seeing an additional value in this metric.
l. 433-435: This is an example where numbers are mentioned in the text, which are additionally shown in Table 3 and Figure 4. It is sufficient to show the data once (preferably in the figure).
l. 448: Is such a small difference really significant?
l. 481: I believe that your estimate of the contribution of sea spray is not correct. At the short distance of 80 km to the ocean there must be a visible input of sea spray and this input should increase with increasing surface area of the canopy, i.e. from N to S. Couldn’t the deviation of the Sr isotope ratio in the so-called bioavailable pool be caused by incorporation of Sr deposited from the sea? What is the Sr isotope ratio of sea spray? – I mentioned this already in my previous review and am not satisfied by the answer.
l. 489: This is pure speculation.
l. 495: The most-demanded (in terms of limitation and quantitatively) is N, not P.
l. 501: One order of magnitude is more than nutrient concentrations and ratios in plants vary.
l. 523: A large part of the nutrient recycling occurs via the organic layer which is not considered here. This could at least be acknowledged in the discussion.
l. 524-525: The plants take up nutrients from greater depth, primarily because they only find water there. Without water, there is no nutrient uptake via the roots.
l. 561: Organo-Al complexes are not bioavailable (and non-toxic).
l. 567: This “dilution effect” is called “leaching” in Soil Science.
l. 570: It is the pH value and leaching.
l. 595: Weathering only needs to replace the loss from plant nutrient cycling, not supply the whole required plant nutrients.
l. 636-641: What is the number of replicates? You need to mind that these replicates need to be statistically independent from each other. You need to add this number to Table A1. The maximum number of such data you have is in my view n = 4 (see above).
l. 667-669: Given the low number of independent correlation pairs, I would refrain from such a quantitative comparison of r values.
l. 685: How was this tested? Again, what was your number of replicates per site? Two?
l. 754-756: This is typical for the weathering regime of temperate soils where Si released from weathering is not leached but forms secondary minerals such as clay minerals. Thus nothing new.
l. 769: Even though/Although
Table 2, Eq. (5): I think that you need to exchange numerator and denominator.

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RR by Anonymous Referee #3 (06 Aug 2020)

ED: Publish subject to minor revisions (review by editor) (17 Aug 2020) by Jack Middelburg

AR by Ralf Oeser on behalf of the Authors (28 Aug 2020) Author's response Manuscript

ED: Publish subject to technical corrections (02 Sep 2020) by Jack Middelburg

AR by Ralf Oeser on behalf of the Authors (03 Sep 2020) Manuscript

Short summary

We present a novel strategy to decipher the relative impact of biogenic and abiotic drivers of weathering. We parameterized the nutrient fluxes in four ecosystems along a climate and vegetation gradient situated on the Chilean Coastal Cordillera. We investigated how nutrient demand by plants drives weathering. We found that the increase in biomass nutrient demand is accommodated by faster nutrient recycling rather than an increase in the weathering–release rates.