the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Oct 2024
Effects of basalt, concrete fines, and steel slag on maize growth and heavy metal accumulation in an enhanced weathering experiment
Abstract. Terrestrial enhanced silicate weathering is a CO2 removal technology involving the application of ground silicate materials to agricultural soils. Next to CO2 sequestration, it can improve soil fertility and crop growth, but silicate materials can also contain toxic trace elements. In a mesocosm experiment, we investigated the effect of basalt, concrete fines and steel slags on biomass, nutrients, and heavy metal concentration of Zea Mays, using a dose-response approach.
Plant biomass increased with basalt, but not with concrete fines and steel slags. Generally, plant Ca, Mg, and corn Si concentrations increased with increasing silicate application amount as a result of increased plant availability. In contrast, plant N, P, and K concentrations were hardly affected by silicate application. Besides increased leaf Pb concentrations with steel slag application, which did not exceed the maximum limit set by the WHO and FAO (0.05 mg Pb kg-1 ww), heavy metal concentrations in aerial plant tissues mostly decreased with increasing silicate application amount, presumably because of an increased soil pH, and accumulation in plant roots. Our study thus indicates mixed effects of silicate application on maize while suggesting that the risk of heavy metal contamination after a one-time application of the tested silicates is limited.
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RC1: 'Comment on egusphere-2024-3022', Anonymous Referee #1, 09 Nov 2024
In their manuscript “Effects of basalt, concrete fines, and steel slag on maize growth and heavy metal accumulation in an enhanced weathering experiment” the authors present a 100 day mesocosm experiment with maize after the application of basalt, steel slag and concrete waste. The authors evaluate the changes in weathering, plant biomass, nutrient and toxic element contents as well es plant update using a dosage approach. The authors report biomass increased with basalt but not with the other amendments. Strong pH effects of the different amendments explain most of the observed effects. Finally, no critical contents of toxic elements but also little changes in nutrients in the plant biomass. The authors conclude that there is now risk of toxic element accumulation in plants and basalt has the most positive effect on biomass. This is an important study and the experimental design (even with only one replicate per treatment, which is due to the high number of replicates) is solid. The study is an important contribution to understand the effects of silicate rich material on soil-plant systems in the framework of ERW. However, the authors miss an important limitation. This is the duration of the experiment. The authors identify that the pH effect is controlling most of the observations. The basalt has the lowest pH effect while the carbonate containing steel sag and concrete waste have stronger effects. As the authors discuss, the nutrient and toxic element availability depends highly on the pH. The authors discuss that the concrete waste and steel sag even overshoot preferential pH conditions for plants (reducing the agronomic benefit of such practices). The long-term effect of such liming effects is not clear. Therefore, the toxicity and bioavailability of the toxic elements might have been undetected given the duration and maintained pH. When the pH decreases with time, this might shift dramatically. The toxic elements might accumulate and at a given pH a much larger load will be bioavailable. This study is still a valuable contribution. But the general conclusion needs to be done with a clear statement of such limitations.
The authors start with the big picture of climatic effects of ERW and thus CDR potential. They never come back to this in the manuscript discussion. However, they argue that concrete waste and steel slag have much higher weathering rates given the DIC compared to basalt. As mentioned before, concrete waste and steel slag contain substantial amounts of CaCO3. Therefore, it is a simple carbonate dissolution for these materials, which has no CDR effect (see also below). This is another strong limitation when thinking about CDR potentials of different materials. The high weathering rates of the steel slag and concrete waste could therefore miss-interpreted as strong CDR potentials. Therefore, the authors should include a critical discussion about the links of their findings to climatic effects.
I include more details and unclear aspects in my specific comments below:
Line 24-25: it should be clear that such practices rely on the transport of the weathering products (e.g. cations and bicarbonate) to the aquatic/oceanic ecosystems where they precipitate as carbonate. This is a large uncertainty. Currently it reads like the CO2 is directly stored on site in the soil.
Line 30: Unfortunately, it is more and more common to name the C capture by weathering "sequestration". Sequestration is in most cases considered to involve the assimilation of atmospheric CO2 by plants/organisms (Don et al 2023). I would recommend to use "atmospheric CO2 removal" here as this is also conform with CDR.
Line 31-37: Here the authors list in several sentence many effects that can improve plant productivity and soil functions. The use of furthermore, moreover and additionally does not make fully sense here as all effects are more or less the same. For example, CEC increase is related to the Ca and Mg, shifts pH and might shift the physical soil properties. This could be rephrased here.
Line 39-40: This can be integrated in the previous paragraph
Line 54-55: Can you provide a reference that most studies have been performed in the tropics. Most of them might not have involved the CDR aspect of ERW but rather the soil re-fertilization.
Line 62-64: This is a very important point here. Thanks for making this.
Line 67-70: Can such by-products contain carbonates? If so, they would weather fast due to the fast dissolution of carbonates. The CDR effect, however, would be low. Considering the IPCC, carbonate dissolution by, e.g. liming, is net zero because of the release of all CO2. The net CO2 effect of carbonate dissolution is also presented in Hartmann et al. (2013)
Hartmann, J., West, A. J., Renforth, P., Köhler, P., De La Rocha, C. L., Wolf-Gladrow, D. A., Dürr, H. H., & Scheffran, J. (2013). Enhanced chemical weathering as a geoengineering strategy to reduce atmospheric carbon dioxide, supply nutrients, and mitigate ocean acidification: ENHANCED WEATHERING. Reviews of Geophysics, 51(2), 113–149. https://doi.org/10.1002/rog.20004
Line 77-78: The three field trials are Branca et al (2014)?
Line 79-80: And the fate of potential toxic elements is unknown.
Line 83: I would prefer "toxic element" rather than "heavy metal" throughout the whole manuscript.
Line 84-86: Also here, please provide evidence that most studies are in tropical systems. Additionally, it might be worth to mention that ERW is already considered for some countries in the temperate regions and thus a better understanding is needed. "justify further research" can be removed here.
Line 91: H1, reads like the authors expect that weathering rates increase. I assume the absolute quantities of weathering products are hypothesized to increase.
Line 122: I assume the five replicates for the 50 t/ha basalt treatment are selected since this is the commonly discussed application rate. The authors do not really advance here on the variability observed between the replicates. This would be helpful.
Table 3: The 5 controls were for each treatment the same correct? it reads here like 15 controls were used with 5 for each treatment
Line 160-161: Why was there one week between aboveground and belowground sampling? This could bias the belowground biomass to some extent.
Line 161-165: The authors considered here only the roots that were in 100 cm^3 cores and extrapolate with the assumptions of 50% surface coverage in the centre of the mesocosm and 25% under the plants. This seems to be potentially highly biased considering the root network of maize. The actual total root biomass by root washing was not determined to prove that such simplification can be made here?
Line 169-174: Was the biomass dried before and how was extracted prior to ICP-OES analysis?
Line 188-189: From which depth were the samples taken?
Line 224: What is the reason for the decrease in soil pH at day 20 for the basalt and the concrete fines?
Figure 2: X axes should be the same everywhere to better follow the different times of sampling. Another visual improvement could be done by using a different symbol for the controls. It is hard to differentiate between treatments and controls. This applies for all other similar figures.
Figure 3: This figure shows that the replicates of the basalt 50 t/ha treatment are in some cases quite variable. Given the fact that the other application rates have no replicates, the variability should receive some attention here.
Figure 5: PC 5 explains only a minor fraction of the variance here.
Line 306-310: Would the authors expect changes in C:N? Given the very high number of figures and presented data in the manuscript, this could be moved to the SI since no important effects are found.
Line 314: the p = 0.08 is not significant here for the tassel Ca. What does "borderline" mean here?
Line 335: Why was Pb in the control?
Line 345: Also here "borderline" is unclear. You could say a non-significant trend for the Ni in stem.
Line 361-362: The steel slag and the concrete fine have substantial amounts of calcite (8.6% and 17.9%, respectively Tab. S1). Thus, it is most likely a dissolution of this carbonate. As mentioned before, carbonate dissolution as no effect on CO2 capture. The CO2 capture is the big picture of the presented manuscript. Therefore, the authors should critically discuss that such strong effects of the sleet slag and concrete waste do not indicate solely the weathering of silicates that would result in a CO2 capturing.
Line 394-395: This is a too simplified conclusion here. The additional root biomass may also induce priming effects. The effects are not clear yet. Therefore, the authors should reconsider this conclusion here. Some literature:
Sokol, N. W., Sohng, J., Moreland, K., Slessarev, E., Goertzen, H., Schmidt, R., Samaddar, S., Holzer, I., Almaraz, M., Geoghegan, E., Houlton, B., Montañez, I., Pett-Ridge, J., & Scow, K. (2024). Reduced accrual of mineral-associated organic matter after two years of enhanced rock weathering in cropland soils, though no net losses of soil organic carbon. Biogeochemistry. https://doi.org/10.1007/s10533-024-01160-0Schiedung, M., Don, A., Beare, M. H., & Abiven, S. (2023). Soil carbon losses due to priming moderated by adaptation and legacy effects. Nature Geoscience. https://doi.org/10.1038/s41561-023-01275-3Line 404-405: It is interesting that for the basalt treatment the pH effect is large in the beginning but even during the short time frame of the experiment, the values are very similar for all application rates and controls at day 100. This indicates that the liming effect of basalt is highly limited. It is in many cases argued that basalt dust could act as a lime substitute. However, if the pH effect is only short this will not be the case. Even for the soil pH, the highest application rate has lower pH values at day 100 than the control. The other two materials that contain carbonates have a much stronger and long-lasting effect on the soil and pore water pH.
Line 458-464: This paragraph is not clear to me. As the authors state, the N effect might be irrelevant as all treatments received similar and high N with fertilization. The interpretation of NUE is a far stretch to me and not convincing here.
Line 464-474: A strong limitation here is the duration of the experiment. The long-term effects remain unknown. It is likely that the release of toxic elements is slower and thus the bioavailability takes longer that captured here.
Line 513-515: Also here the duration of the experiment is important to consider. It is not clear if the bioavailability would be higher with the next season when pH effects diminish and toxic elements will become more bioavailable.
Citation: https://doi.org/10.5194/egusphere-2024-3022-RC1 -
RC2: 'Comment on egusphere-2024-3022', Anonymous Referee #2, 30 Nov 2024
Within the context of enhanced weathering for atmospheric CO2 removal, the authors present an experimental mesocosm study exploring the impact of basalt, concrete fines, and steel slag applications on the growth of maize and potential accumulation of toxic heavy metals. Overall, the authors find evidence of weathering and increased plant growth (with basalt), while they did not observe accumulation of heavy metals in the plants.
I think this is an important study investigating in detail the potential co-benefits and side effects of enhanced weathering, when widely available basalt or industrial materials (concrete, steel slags) are used. I found particularly interesting the dose-response approach, as it may provide more insights into the biochemical mechanisms that make a certain element more or less available. Also, the consideration of concrete fines and steel slags is valuable, as these are being actively considered as potential silicate materials that dissolve relatively fast and favor a circular economy, but there have been concerns for their potential environmental toxicity.
For the reasons mentioned above, I recommend publication after addressing my comments below.
Within the context of enhanced weathering, I think it is important to always consider the CO2 removal aspect, so that co-benefits or side effects can be evaluated relative to the CO2 being removed. This aspect is barely mentioned in the manuscript. In this work, I believe that from the composition of the silicate materials, the authors could obtain an estimate of the potential CO2 removal, calculated based on stoichiometry. In addition, it is not clear whether the authors are collecting and analyzing the leachate, but this could provide an estimate of the actual CO2 being removed and of the weathering rate. If not from the leachate, can the authors estimate the CO2 removal from their measurements? Especially after the first application of silicate materials, the weathering rates are expected to be the highest.
I don’t think this is mentioned explicitly, but the experiment apparently lasted for only 1 season (~100 days). While this may be enough to capture the potential benefits on plant growth, perhaps this is not long enough to capture the accumulation of heavy metals, which is something that occurs over time. In fact, to achieve meaningful CO2 removal, multiple applications may be needed over time. The concern is that heavy metals may cause harm over time, as the soil capacity to retain them is exhausted and they may become more available for plant uptake or even leach with water, potentially affecting water quality. In fact, the problem with heavy metals would not be only for plants, but also for the ecosystem more broadly. I understand the experiment took place in 2021, but I think the authors should discuss these aspects more explicitly.
Additional minor comments:
-The authors should clarify whether the mesocosm includes a mechanism for water drainage from the bottom.
-The font size used in figures should be increased.
-In figure 2, the x axis tick marks should be standardized (for example, all could be 25-50-75). Also, the text on one panel is sometimes overlapping with another panel. Please double check the spacing between panels.
-Line 103-104. I believe there is no need to repeat “to a depth of 40cm in the bottom portion”.
-Line 162. Should the parenthesis be moved to the end of the sentence? Or probably I am not understanding this sentence, as it starts with “one core below the shoot” but then I says “two cores per mesocosm for each layer” in parentheses. Could the authors clarify?
Citation: https://doi.org/10.5194/egusphere-2024-3022-RC2
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