the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 May 2025
Soil microbial necromass shapes global carbon stocks in agricultural and natural ecosystems
Abstract. Soil carbon (C) plays an essential role in regulating global C cycle and climate. Microbial necromass is an important component of soil C, and yet the relative contribution of microbial necromass in shaping the global C stocks in agricultural and natural ecosystems worldwide remains virtually unknown. In this study, we compiled data on fungal and bacterial necromass along with soil organic carbon (SOC) from the 0–20 cm soil layer across 486 study sites (145 agricultural and 341 natural ecosystems) to evaluate the relative contribution of fungal necromass C (FNC) and bacterial necromass C (BNC) to SOC and the FNC/BNC ratio, after accounting for other biotic and abiotic factors. Our results indicated that, in both agricultural and natural ecosystems, the contribution of FNC to SOC significantly exceeded that of BNC, with FNC contributing approximately twice as much as BNC to SOC. However, the contributions of FNC and BNC to SOC were markedly higher in agricultural ecosystems than those in natural ecosystems, with a contrasting trend in the FNC/BNC ratio. Soil physicochemical properties (C/N and clay) were the most important predictors of the contributions of FNC and BNC to SOC in both ecosystems, while geographical factor (elevation) was the most important predictor of the FNC/BNC ratio. Our study enhances the current level of understanding regarding microbially mediated biogeochemical cycling and SOC dynamics, underscoring the critical role of microbial necromass in the global C cycle.
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Status: open (until 23 Sep 2025)
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RC1: 'Comment on essd-2025-229', Anonymous Referee #1, 18 Aug 2025
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The study worked out the fungal and bacterial necromass contribution to SOC and interpreted the variability with climate, geographical and soil conditions. The data are useful and provide key reference for global ecosystem study of SOC storage. However, as the authors often mentions their work was very similar, or consistent, with others work already published. The shortage may be not robust information about their soil samples, particularly agricultural soils not classified and sampling condition not clearly defined. It could be improved if the authors could add samples from managed grasslands, wetlands and divided agricultural soil into dry land and irrigated as well as submerged paddy. The discussion need rewrite, reorganized and better presented with statistical analysis.
Specific comments as follows:
Title: Suggest to change as "Microbial necromass contribution to topsoil organic carbon storage of natural and agricultural ecosystems".
INTRO
Line 47-51: Soil organic matter, the material containing organic carbon as the core element but preserved in soil matrix, is truly formed based on plant biomass (material), even the organic pollutants in soil based on fossil plant material (coal and petro-oil). It is not contradictory that SOM is composed of plant derived and microbes-derived organics, the letter is formed of metabolic residues upon microbial processing of plant material and preserved through interaction with mostly soil mineral matrix. That is to say, microbial necromass is indirectly derived from plant material, or microbial products via processing plant material. Both plant derived or microbial derived organic carbon could be stable in soil conditions preservation could be allowed. “Stable” here could be changed into “with long turnover time”.
Line 55-58: Microbial biomass carbon generally possessed generally about 2% to SOC, but microbial necromass could predominate (as high as 80% to SOC) in soils low in SOC. For the general estimation (2%) of microbial biomass C percentage to SOC (often termed microbial quotient), maybe cite a recent study(Topsoil microbial biomass carbon pool and the microbial quotient under distinct land use types across China: A data synthesis, Soil Science and Environment, 2:5. Doi:10.48130/SSE-2023-0005)
Line 60-63: many studies…., add description of the study conditions such as land use, management, climate change, regional, etc.
Line 64-65: With the distinct roles of fungi and bacteria in decomposing organic matter and stabilizing organic carbon in soil, the relative contribution to SOC of fungal and bacterial necromass C could be used to track the dynamics of SOC storage (Malik et al., 2016).
Line 66-68: Vague sentence, suggest to delete.
Line 71-74: As bacterial amino-sugars is degradable rather than fungal chitin or β-glucans, fungal necromass existed in soil generally with longer turnover time than bacterial necromass.
Line 74-78: The sentence may not be correct here. By definition, microbial necromass is not microbial biomass.
Line 78-82: Pls delete “Previous studies have also indicated that”.
Line 82-86: May be changed into“ However, few studies on fungal and bacterial necromass carbon and their contribution to SOC has been reported for ecosystems under human interference (Chen et al., 2020)“. The citation of (Chen et al., 2020) may not be appropriate here.
Line88-90：Pls consider to change. The agricultural ecosystems are typical of plant litter derived of single crops under human management (Bohan et al., 2013)
Line 90-92: In contrast, natural ecosystems display greater diversity in plant litter and root deposits (Wu et al., 2019).
Line 93-95: In such ecosystems, fungal mycelial networks and stable soil aggregates are enhanced, leading to higher FNC contributions to SOC (Sanaullah et al., 2020; Sae-Tun et al., 2022).
Line 95-98: in comparison to bacterial?
Line 98-102: These statements may be of questions, or not sufficient. For example, why diverse plant input lead to rich soil cellulose content, why not diverse SOM composition?
Line 102-106: The issue is important, but the rational is not strong as your overview of studies. Could you focus on why FNC and BNC across global ecosystems or why the ratio is important across the global ecosystems?
Line 112-113: delete the sentence as is already ascertained above.
MM
Line 117-118: to be deleted
Line 119: by December 31, 2021?
Line 121-123: why not fungal derived glomalin-related proteins? And, why “fungal necromass, bacterial necromass included in your search engine?
Line 123-130: studies search and screening procedure are not well described. I suppose you first collected all the studies indexed of the keywords, then you made a rough compilation. Secondly, filter the compiled studies with “topsoil”; followed by paired data of fungal and bacterial necromass or so, further divided your filtered studies into ecosystem categories, finally you excluded those potentially disturbed ecosystems from the natural category. Pls do organize clearly your work flow and display in a flow chart or in an order of steps.
A question，here you claim that only data of topsoil (0-20cm) were collected. As I experienced, different depth intervals of topsoil were used for agroecosystems and natural ecosystems, mostly 0-30cm for dry croplands while often undiscerned for natural ecosystems. You may mention these different usages though not critical for your relative contribution and ratio estimation. But this may affect estimation of mass abundance of microbial necromass in soil.
Another key question: did you screen the data for collecting and measurement standard protocol? Is the similar season or growing period among all the samples? All the samples were treated with a consistent procedure (for example sample preparation, shipping and storage, and analysis condition)
Line 130-133: What about the time span of the published studies? How did you check if repeated data in publications of a same study but in different journals and/or years?
Line 134-137: Pls describe the calculation of FNC and BNC respectively, and number the equations.
Line 138-140: Pls clarify the unit of the calculated contents.
Line 141-145: How did you obtain the information of soil temperature. As I know well, soil temperature is not normally recorded while in field sampling. However, soil moisture content data could be available in most sampling procedure or lab measurement before further analysis for specific purposes. In addition, what kind of information for microbial or plant factors.
Line 151: what is the spatial distance of 30 x 30 arc sec ? Is such grid resolution comparable to the site specific climate data?
Line 153-158: Use of data of soil temperature and soil properties digested from the GEO-based data base is questionable for the studied soil in your database.
Line 161-162: data of microbial biomass carbon and nitrogen is not eligible from the geo-database. These varies very much from site to site, or from time to time.
Results
Line 224-226: How did you get these values? Calculation using the numbers you provided in the preceding sentences does not yield the same values (2.23 for agricultural but 2.09 for natural). If the calculation correct, there is significant but slight difference in FNC/BNC ratio between agricultural and natural ecosystems.
The samples of agricultural ecosystem not clearly defined. Dry croplands, irrigated croplands, rain-fed dry lands and waterlogged paddies? Also, the cultivation history is important, at least need to clarify those shortly shifted from natural ecosystem, for example from grassland.
Line 228 the subheading of “ Effects of the driving factors on…” may not be proper for this is a synthesis of data in arbitrary studies without certain treatments. Could be change into “Driving factors of the change in fungal and bacterial necromass contribution to SOC and their ratio”. But this context should be presented in Discussion part, not the direct results presented here.
I suggest you could split your result into two subheadings: 3.1 Fungal and bacterial necromass contribution to SOC; 3.2 Ratio of Fungal and bacterial necromass. In 3.1, you may provide more detailed information of the variation of fungal and bacterial necromass content and the contribution to SOC, among samples, ecosystem types and or other dimension (for example, regionally). In 3.2, provide the ratio variance among the systems, but also digest the relations to SOC level. Possibly, you could align your correlation to these variance to digest the driving factors, respectively.
Discussion
This part not well organized, often repeating the statement of results.
Line 286-296: Not a single independent paragraph.
Line 286-288: Move to INTRO;
Line 288-289: Move to Results part;
Line 289-296: Most are repeated Results context. Delate.
Line 298-299: the subheading is many times repeated in this paper. May use something different, may be like ”Fungal necromass Greater contribution to SOC by fungal necromass than by bacterial one.”
Line 300-302 Should included in INTRO, not repeated here.
Line 302-305: Avoid repeated statement of result. But you need specify the range of the ratio difference among the samples and between your two sets of ecosystems. It may not be true fungal necromass contribution twice as much as bacterial across samples.
Line 305: if this sentence correct, then what is your study’s novelty? If the following discussion about the factors are new, then you may say “ The similar variance feature been reported in previous studies, but the reasons unknown. In this study…….
Line 308-319: Unfortunately, the discussion are weak, just using some knowledge from publications not with your own analysis or statistical attribution.
Line 320-323: If the finding is new, you may rewrite like: In this study we found higher microbial necromss contribution in agricultural system than in natural ecosystems.
Line 324-326: You could use this reason for lower contribution in natural ecosystem but not ending with “potentially resulting in a greater proportion of microbially derived C within SOC (Angst et al., 2021).”.
Line 328-337: The second reason for higher microbial necromass contribution pointed to high quality substrates in agricultural systems, with lower C/N ratio generally. Could you use a correlation respectively of these necromass contribution values to the soils C/N ratio? Lower C/N ratio in agricultural soils is driven by the N fertilization, not necessarily by high quality substrate like legume residue. In fact, agricultural residues are often high C/N ratio, for example wheat straw is over 30.
Line 340-342: may be not the difference between the two microbial groups but the difference in microbial behavior between the two systems, which you mentioned later.
Line 344-349: These are very weak nor robust.
Line 351-359: These are not sound knowledge. Should link the ratio difference to the difference in SOM accumulation between natural and agricultural systems.
The contents in 4.2 should be sued in discussion part 4.1. When the reason of the changes is in discussion, you present these results from statistics to support or to cohere your finding. Not presented separately while leaving your discussion often pale.
Subheading 4.3, statement about limitations are honest. But need to mention that sampling conditions may not be comparable so as to the large variability.
Conclusions
Line 429-430: why not “ FNC two times as much as BNC..”
Line 432-434: significantly but slightly
Line 434-437: no evidence of “consistent trends”, as for the large variability.
Line 437-440: Mention about added value of your study compared to previous study, or future perspectives.
Citation: https://doi.org/10.5194/essd-2025-229-RC1
Data sets
Soil microbial necromass shapes global carbon stocks in agricultural and natural ecosystems Jingli Lu https://dx.doi.org/10.6084/m9.figshare.28827386
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