the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Apr 2025
What if publication bias is the rule and net carbon loss from priming the exception?
Abstract. Priming effects in soil science describe the influence of labile carbon inputs on rates of microbial soil organic matter mineralisation, which can either increase (positive priming) or decrease (negative priming). While both positive and negative priming effects occur in natural ecosystems, the latter is less documented in the peer-reviewed literature and the overall impact of priming effects on the carbon balance of vegetated ecosystems remains elusive. Here, we highlight three aspects which need to be discussed to ensure (rhizosphere) priming effects are correctly perceived in their ecological context and measured at appropriate scales: (i) We emphasize the importance of evaluating net C balances because usually experimental C inputs exceed C losses meaning even positive priming doesn’t cause net C-loss; (ii) We caution against publication bias, which forces overrepresentation of positive priming effects, neglects negative or no priming, and potentially misguides conclusions about C loss; and (iii) We highlight the need to distinguish between general priming effects and rhizosphere- specific priming, which differ in their scale and driving factors, and hence require different methodological approaches. Future research should explore potential discrepancies between laboratory and field studies and examine the role of rhizosphere priming in nutrient cycling and plant nutrition.
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RC1: 'Comment on egusphere-2025-1067', Anonymous Referee #1, 22 Apr 2025
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The paper submitted by Michel et al. explores priming effects in soils, which refer to the influence of labile carbon inputs on the mineralization of organic matter by microbes. These effects can be positive (increased mineralization) or negative (reduced mineralization). The article highlights three key aspects to better understand these effects in their ecological context: (i) Evaluation of net carbon balances (ii) Publication bias and (iii) Difference between general priming effects and rhizosphere-specific priming. The article calls for a more nuanced approach to priming research, encouraging the publication of studies on negative or neutral effects, carefully evaluating publication biases, and distinguishing general priming effects from rhizosphere-specific effects. It also emphasizes the importance of conducting field studies to better understand these phenomena under natural conditions.
I think the points developed by the authors are fair and worth to be published but the current version of the paper is a bit disappointing since it skims over the issues without going into depth. It may come from the format which is not very clear. Is it a review or is it an opinion paper? If this is a review, much more literature must be cited if this an opinion paper, the ideas developed must be more attractive and be inspiring for the community to change their ways of doing priming research.
For instance, the authors wrote that “there is little empirical evidence for net C losses”, I fully agree with the statement but it should be better explained why priming is sometimes see as a mechanism leading to net losses and show some papers that suggest it and explain why they are mistaken. So far the authors mostly cite papers that do not observe any net loss.
The second section is simply a re-analysis of the data from Xu et al, which merely confirms the conclusions of the original paper. In my opinion, this does not add much to the message of Xu et al. More details on the effects of bias should be provided and some ideas to avoid them should be also proposed.
Then the third section is mainly focused on the importance of using intact soils instead of sieved soils whereas the title of the section suggests that RPE and PE should be treated differently. So far this section is not really convincing mostly because it lacks more clear examples and it needs some suggestions to improve the current methods.
Citation: https://doi.org/10.5194/egusphere-2025-1067-RC1 -
RC2: 'Comment on egusphere-2025-1067', Anonymous Referee #2, 24 Apr 2025
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In their manuscript with a promising title “What if publication bias is the rule and net carbon loss from priming the exception?” the authors aim to address an important aspect of the relevance of priming. The main point is that priming (i.e. enhanced or reduced mineralization of native soil organic carbon) occurs as an elemental process but the net carbon balance is in most cases not considers in the literature. I agree that this causes a biased understanding of the priming in relation to total carbon changes of soils. In fact, I agree that priming, especially positive, is not on relevant magnitudes to off-set the actual input that is required to initiate priming in the first place. I also generally agree that our understanding is biased due to studies that used “artificial” organic matter inputs and that we have a limited understanding of the complex effects of the whole rhizosphere effect and associated organic matter inputs but also biogeochemical shifts. Overall, I agree that such statements need to be made clearly in order to facilitate future research focuses. However, I have to be honest that I was a little disappointed that the authors remain on a very broad level with their discussion. In addition, it is not clear if this is supposed to be a review or opinion and I miss any clear statement how we should move on.
The first chapter summarizes very briefly that positive priming is not affecting the net carbon balance.
The second chapter re-analysis the study presented by Qin et al (2024) and Xu et al. (2024). In fact, the authors repeat the statement by Qin et al (2024) who stated that the net balance is mainly positive. Therefor, this is supporting the authors point but not providing new information that is shown in Figure 1a that only shows the included studies in Qin et al. Regarding the Xu et al. (2024) study, the chapter in the manuscript is quite technical how the authors re assessed the data and what funnel plots are (line 93-123, which is a lot regarding the short manuscript). The authors provide an improved interpretation of the same plot that is discussed by Xu et al in the supplement figure 2. The authors apply the standard error on ln transformed the priming effect observed in the studies. Technically this seems correct. However, this seems to be rather a response to the Xu et al paper than making a new point. In fact, the Xu et al in the supplement figure 2 already shows the asymmetry that most studies are reporting positive priming effects, when I visually assess the deviation from x=0 in the plot (see plot below). The authors finally estimate that only six studies with moderate or negative priming effects are need to overcome the publication bias. They state in line 137 “It would be interesting to recalculate a global PE estimate from the primary research data of all underlying meta-analysis corrected for publication bias.”. This raises the question why the authors are not providing this. Finally, the authors just encourage to publish negative and moderate priming effects.
The third chapter claims that rhizosphere priming and general priming are different. Yes, it is important that we include more complexity and understand the rhizosphere effects better. It is a major challenge to study such effects in intact systems that include the whole rhizosphere. This is already discussed in other studies and should be clear. The authors just state that we need to understand rhizosphere priming in natural ecosystems, but they do not provide any way forward to do so. Therefore, I do not see any substantial addition to the discussion based on the very broad and short chapter here.
In addition, the authors do not provide any clear definition of the priming effect in the first place. For example, in line 15 it is stated: “Priming effects in soil science describe the influence of labile carbon inputs on rates of microbial soil organic matter mineralisation, which can either increase (positive priming) or decrease (negative priming).” Therefore, they miss the important aspect that priming is affecting the native or present soil organic matter after the addition of new organic matter.
Funnel plot from Xu et al (2024, supplement) with added x=0 vertical line
Citation: https://doi.org/10.5194/egusphere-2025-1067-RC2
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