the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Jul 2025
Increased soil CO2 emissions after basalt amendment were partly offset by biochar addition in an urban field experiment
Abstract. Enhanced weathering (EW) and biochar amendment are proposed carbon dioxide removal (CDR) techniques with potential co-benefits for soil health and plant productivity. However, knowledge gaps remain regarding their impacts on soil carbon dynamics and heavy metal mobility. This study investigates the effects of basalt and biochar amendments on soil CO2 efflux (SCE), soil base cation dynamics, biomass yield and heavy metal uptake in clover (Trifolium pratense) and mustard (Brassica juncea) field plots. Despite potential CO2 uptake through weathering, we found that basalt increased SCE in both crops, suggesting increases in soil organic matter (SOM) decomposition and/or rhizosphere respiration. Biochar enhanced plant biomass and reduced plant uptake of several trace metals in both mustard and clover plants, while basalt did not affect any of the 33 assessed elements in aboveground plant biomass. While basalt alone increased CO2 efflux, co-application with biochar tempered this response, potentially mitigating a basalt-induced priming of soil organic matter decomposition. Hence co-application of biochar with basalt countered the basalt-induced rise in soil CO2 emissions, while biochar also reduced plant trace metal uptake, highlighting biochar’s potential to mitigate both environmental and food safety risks.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Biogeosciences.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3232', Anonymous Referee #1, 18 Aug 2025
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RC2: 'Comment on egusphere-2025-3232', Anonymous Referee #2, 29 Aug 2025
This study compares the impacts of basalt amendment for enhanced weathering (EW) and biochar addition on soil cation release, dissolved inorganic carbon, soil organic matter, plant biomass, and heavy metals released into soil and plant biomass. It is a timely study, as we need more information on these two carbon dioxide removal (CDR) strategies, and whether their co-application supports the conjectured synergistic effects. However, in its current form, I cannot recommend this manuscript for publication and recommend a major revision.
Major concerns and comments:
- The main limitation of this study, in my view, is that it does not estimate the carbon sequestered by EW. One could argue that this was not an objective of the paper, but without knowing how much EW actually took place, it is difficult to interpret whether the magnitude of the CO₂ efflux under basalt application has any real significance, or what the heavy metal results mean. Even better would have been to present a carbon budget for the duration of the experiment. Furthermore, the way soil CO₂ fluxes are presented makes interpretation difficult. They are referred to as “soil CO₂ emissions,” but in fact the measurements include both autotrophic root respiration and heterotrophic respiration. As a result, the fluxes alone do not tell us much, especially since no information on root biomass is provided. What if basalt stimulated root growth and the observed efflux is mainly plant-driven? As it stands, we are left with multiple possible explanations and a great deal of speculation.
- The overall presentation of results and graphs needs significant improvement. The choice of overlapping bar graphs is uninspired and difficult to follow. Units on the axes are not properly formatted. Table and figure captions are not self-explanatory (e.g. the abbreviation SCE in Fig. 7 is not explained). The results section also needs more detail and context. For example, instead of simply stating at line 187 that “Basalt significantly increased soil exchangeable Ca,” it would be far more informative to give the percentage increase and the p-value. That way readers can understand the magnitude of effects without constantly having to check the figures. This should be done consistently throughout the results section.
- Data analysis and presentation are inconsistent across figures. For some variables, results are shown as differences of basalt, biochar, and their combination relative to the control (Figs. 3–4). For others, data for all four treatment combinations are presented. Why this difference? It is not explained in the methods, and it is not clear what statistical analysis was applied in these cases. It does not appear to match what is described in the statistics section.
Other comments:
- Basalt particle size distribution should at least be briefly described in the main methods. The fact that the basalt used was relatively coarse is important, as it could explain the weak effects on plant growth and trace metal release. It also deserves a short mention in the discussion.
- Soil texture is also important. Simply describing it as a “clayey sand” is not enough to allow proper interpretation of results.
- Introduction: synergies have indeed been postulated, but the introduction should give a clear picture of the state of the art. There is already at least one study (Honvault et al. 2025) that looked at this and generally found additive rather than synergistic effects. This work should be mentioned.
- Line 81: “Because the SCE model had a positive skew” — what exactly does this mean? Do you mean the data were heteroscedastic?
- Lines 82–83: you tested the main effects of two categorical variables and their interaction. This is not multiple regression. Even if you used lm in R to extract fitted coefficients of categorical/factor variables, what you actually ran is a two-way ANOVA, right?
- It is not very clear when the basalt and biochar amendments were added relative to the onset of the cultures. Please clarify.
- Soil CO₂ flux measurements require a more thorough methodological description. I assume this was done by accumulation rather than open flow? How was the 1-hour duration chosen? This is quite long, and longer is not necessarily better. If accumulation continues too long, the increase becomes non-linear due to CO₂ gradient limitation, which can bias flux estimates depending on how the curve is fitted.
Citation: https://doi.org/10.5194/egusphere-2025-3232-RC2
Data sets
datasets for the basalt-biochar experiment Arthur Vienne https://zenodo.org/records/15001309
Model code and software
R scripts for data analyses Arthur Vienne https://zenodo.org/records/15001309
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This study conducted an interactive experiment to figure out the effects of basalt, biochar, and basalt * biochar on clover and mustard yield, soil chemistry and trace metals. They found most significant effect was caused by biochar addition rather than basalt, which differentiates with some results reported in some previous studies (Beerling et al., 2018, nature plants; Beerling et al., 2024, PNAS). The result that increased soil CO2 emissions with basalt addition were partly offset by biochar addition found in the present study is interesting. But the deep analysis and discussion on results are scarce in this version, and the making of figures also needs to be improved. In figures, vertical lines overlapped for tree treatments, which is not easily recognized well.
In the main context, most results associated with trace metals, but the title of this manuscript only refer to the soil CO2 emissions. It is not very integrated.
Introduction
Lines 65 Why does biochar increase weathering rates of EW?
Methods
Lines 95 How did authors decide the dosage of basalt and biochar?
Lines 120 Please briefly introduce how to distinguish cations in four different consecutive fractions using the sequential extraction scheme of Tessier et al. (1979).
Results
Line 235 delete “observed”
Discussion
Figure 3, I do not understand the decreased ΔCa in four soil fractions with basalt addition after 550 days compared with the initial sampling time. If the basalt is weathered, Ca should be released, resulting in an increase of ΔCa over time.
Lines 260 With basalt addition, Ni showed significant accumulation in both the reducible fraction and the oxidizable fraction associated with soil organic matter (SOM), while Cr increases were confined to the reducible fraction. Why do Ni and Cr have different response to basalt application?
For plant biomass, why do plants have different response to basalt and biochar addition? Biochar application increased plant biomass, why? Are there any correlations between plant biomass and nutrient contents? Why do harvest 1 and 2 of mustard have different responses to biochar addition?
For SCE, for acidic soils, EW may increase pH, and thus increase SOC decomposition, but the soil pH in this study is 8.22, which is quite alkaline. Basalt addition significantly increased SCE, why?