Articles | Volume 11, issue 2
https://doi.org/10.5194/soil-11-735-2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.Aeration and mineral composition of soil mediate microbial CUE
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- Final revised paper (published on 01 Oct 2025)
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- Preprint (discussion started on 21 Feb 2025)
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Interactive discussion
Status: closed
Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
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RC1: 'Comment on egusphere-2025-481', Wolfgang Wanek, 02 Apr 2025
- AC1: 'Reply on RC1', Roey Angel, 16 May 2025
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RC2: 'Comment on egusphere-2025-481', Anonymous Referee #2, 08 Apr 2025
- AC2: 'Reply on RC2', Roey Angel, 16 May 2025
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RC3: 'Comment on egusphere-2025-481', Anonymous Referee #3, 06 May 2025
- AC3: 'Reply on RC3', Roey Angel, 16 May 2025
Peer review completion
AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
ED: Publish subject to minor revisions (review by editor) (30 May 2025) by Ashish Malik

AR by Roey Angel on behalf of the Authors (06 Jun 2025)
Author's response
Author's tracked changes
Manuscript
ED: Publish subject to minor revisions (review by editor) (20 Jun 2025) by Ashish Malik

AR by Roey Angel on behalf of the Authors (24 Jun 2025)
Author's response
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ED: Publish as is (03 Jul 2025) by Ashish Malik
ED: Publish as is (17 Jul 2025) by Raphael Viscarra Rossel (Executive editor)

AR by Roey Angel on behalf of the Authors (21 Jul 2025)
Post-review adjustments
AA: Author's adjustment | EA: Editor approval
AA by Roey Angel on behalf of the Authors (23 Sep 2025)
Author's adjustment
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EA: Adjustments approved (23 Sep 2025) by Ashish Malik
Review of EGUSPHERE-2025-481
Aeration and mineral composition of soil determine microbial CUE
The authors report on a study of substrate-carbon allocation in soil microbial communities, depending on aerobic respiration versus anaerobic metabolism (iron respiration, fermentation), using two soils differing in Fe content, and performing 13C-glucose tracing measurements under aerobic and anoxic conditions. Going beyond just measuring 13C in microbial biomass and in respiration the authors also tried to measure substrate consumption (glucose depletion) and exometabolites secretion (organic acid accumulation). Using these data the authors calculate different proxies of microbial carbon use efficiency i.e. ambient CUEa, CUE, CSE, and TCSE. And they coupled these to the measurement of active microbes incorporating 13C-glucose in their RNA by RNA-SIP. A very nice move.
My main issue with the current paper is the following ( would suggest to discuss this at least in a revised version of this manuscript, if the authors do not confer): In their C allocation scheme the authors ‘mix’ low- and high-molecular weight (LMW and HMW) metabolites into one class, exudates or extracellular metabolites (see Line 31-49). However, I would criticize this strongly as polymeric forms of exudates such as exopolysaccharides and exoproteins likely contribute to non-growth anabolic production, accumulating in soil through sorption (they would need to be added to 13C in biomass or 13C-growth but they were not measured here) while LMW exometabolites either do almost not sorb and therefore do not or negligibly contribute to SOC formation (sugars), or rather mobilize SOC and desorb it (organic acids) or only some/slightly sorb (amino acids, pH dependent). While HMW-C exudates therefore clearly are anabolic outlets of microbial metabolism and contribute to SOC formation together with microbial growth and biomass formation, LMW-C exudates - if formed by fermentation - are catabolites (acetate, formate, butyrate) and should therefore rather be classified as such (catabolites) to be summed with CO2 and CH4. However, adding catabolites to the anabolic processes and to the growth plus non-growth anabolic component does not make sense here. Organic acids per se likely do not contribute to SOC formation but rather decrease it by mobilizing and solubilizing SOC. To my mind, therefore, the exometabolites that the authors measured (organic acids) should not be added to the growth side but to the catabolic side. Unless, under changing conditions those fermentation products which can constitute a major fraction of consumed sugar (glucose) under anaerobic conditions to recycle NAD+ from NADH+, but under oxic conditions they might be used by aerobic microbes and utilized for growth and energy formation (mentioned finally in Lines 609-612). But this is a different situation than tested and discussed here, either anoxic or oxic, no switches tested. Acetate and formate – if further metabolized under anoxic conditions – are largely ending up in CH4 and CO2 and little in biomass, so they remain catabolites.
My overall conclusion on this is therefore that just measuring 13C-respiration and 13C-microbial biomass as many researchers have done under aerobic conditions and summing them for microbial uptake to estimate CUE as biomass/uptake would underestimate total uptake and therefore overestimate CUE under anaerobic/fermenting conditions. Adding organic acid formation as catabolites would increase microbial uptake, would not affect growth estimates and overall considering fermentation products would decrease CUE (CUEa to CUE conversion) estimates under anaerobic conditions. Beautifully in this work the authors assessed uptake not by summing respiration and growth (and other products such as exometabolites) but by measuring the depletion of added substrate (glucose). So, in this paper uptake would not have changed considering exometabolites or not, and growth efficiency as growth/uptake would not be affected by any of those considerations. In contrast to this, including extracellular polymeric outputs (EPS-protein and -sugars) not related to growth would increase anabolic outputs as a fraction of C uptake and therefore increase CUE, under both aerobic and anaerobic conditions.
More detailed comments:
Line 11: some sentences are unclear, imprecise and might be improved, e.g. this one “Microbial carbon use efficiency (CUE) in soils is used to estimate the balance of CO2 respired by heterotrophs versus the accumulation of organic carbon (C).”
Line 13: biomass growth was not assessed here, only 13C immobilization (uptake and retention) in microbial biomass, which can be stored or used in cell size and cell number growth.
Line 18-19: due to the reasons discussed in the paragraph above I do not copy the conclusion in the abstract, neither the “short-term C preservation” by exudation f organic acids (!), nor the “underestimation of apparent CUE” - “Our findings confirm that anoxia in soils enhances short-term C preservation. Accordingly, excluding exudates in mass flux calculations would underestimate apparent CUE values.”
Line 24: rewrite “soil C storage”
Line 64: EPS is mostly polysaccharides and polypeptides (instead of amino acids)
Line 69-70: this sentence is completely unclear….
Line 111-112: Fe(tot) is given in mM kg-1 i.e. mmol per liter of volume (usually solution) per kg dry soil, I guess this should be mmol kg-1?
Line 294, 298: CSE (carbon storage efficiency) – this largely comprises 13C incorporated in microbial organic C products (including biomass, necromass, and secreted products), TCSE (carbon stabilization efficiency) – this largely comprises 13C incorporated into microbial necromass, excluding biomass and secreted products. If both are true this might be additionally mentioned to make it easier for the readers to follow.
Line 368+: In part the paper is super detailed e.g. in section 3.4 which might be shortened for readability.
Line 386: you mention that in anoxic soils TCSE was larger than 1, but from the equation used to calculate this CSE-CUE this is simply impossible. Please check.
Line 505: the reason that CUE is higher in anoxic than in oxic conditions is that catabolite secretion is counted to growth and anabolism instead, which increases CUE. See my critique previously.
Line 517-528: this is a simply list of potential 13C sinks. My “feeling” says that a large fraction of the unaccounted, non-extractable 13C ion the soils is in microbial necromass, given the fast metabolism and turnover of microbial biomass. Fast microbial turnover (70-170 hrs) means that a large part of that biomass 13C in polymeric forms is transformed into necromass, remaining adsorbed, unextractable, HMW. Having microbial turnover and microbial biomass would even allow to calculate the fraction of 13C in necromass.
Line 541: acetate is not a tricarboxylic acid but a di-
Line 572: please rephrase “much-extended growth” which I do not grasp in terms of meaning.
Line 593: though it has been discussed or claimed that copiotrophs are less energy and carbon efficient that oligotrophs I have rarely seen convincing data for this. Actually copiotrophs which grow fast should waste less energy in respiration, have lower maintenance respiration, have less costs for exoenzyme production and therefore allocate more anabolic output into growth, and therefore have higher CUE. Moreover their genomes are often more simple and smaller, consuming less resources in replication.