Enhanced rock weathering (ERW) and biochar are potentially effective and scalable options for large-scale carbon dioxide removal (CDR), required to limit global temperature rise to 1.5 degrees C. Here we present experimental data on their co-deployment, an urgent and novel research direction that may render even larger CDR on multiple timescales. Two greenhouse pot experiments were conducted growing maize (Zea mays L) on sandy and clayey soils mixed with various doses of crushed dunite rocks (20-220 t ha-1) and a fixed dose of biochar (20 t ha-1) for two months. Furthermore, through a comparison of multiple soil extraction procedures for mass balance construction, our work supports the development of a standardized quantification method for CDR associated with ERW. Based on these elemental mass balances, dunite weathering was found to sequester between 1.06 f 0.025 and 3.48 f 0.084 t CO2 ha-1 in sandy soils and between 0.28 f 0.015 and 1.60 f 0.051 t CO2 ha-1 in clayey soils, while biochar co-deployment only slightly enhanced dunite weathering in the latter. Soil respiration also significantly increased on both soils, exceeding the achieved inorganic CO2 sequestration in our shortterm experiments. However, we observed significant increases in soil pH and amorphous iron (hydr)oxide minerals, the latter known to be important for long-term organic carbon stabilization. We argue that the reduction in soil carbon due to enhanced soil respiration is only short term and is likely compensated for by the promising potential of ERW and biochar combinations for long-term inorganic carbon sequestration and organic carbon stabilization. The observed effects of ERW and biochar co-deployment on soil chemical properties, most notably increases in reactive (hydr)oxide minerals and soil pH, provide a great opportunity to boost CDR, with important differences between soil types.

Soil is the Earth's largest carbon (C) reservoir, holding nearly twice as much C as the atmosphere. Climate change-induced temperature increases could enhance soil organic matter (SOM) decomposition, potentially releasing carbon dioxide (CO2) into the atmosphere, thus resulting in a positive feedback to climate change. This presents the question: how can we define the response of SOM decomposition rates to short-term changes in temperature? Can we utilize this data to fit mechanistically defined equations in Earth system models for climate predictions? Most research to date has focused on laboratory soil incubations at different discrete temperatures. This, however, does not result in a detailed temperature response curve, which is required to test novel hypotheses imposed by the latest developments in the field, such as the macromolecular rate theory (MMRT). In this paper, we introduce a temperature gradient block, modified and improved from earlier work, detail its construction process, and provide a detailed explanation for its utilization. The temperature gradient block is an aluminum block with four rows of 22 discrete holes, allowing the simultaneous incubation of 88 microcosms (60 mL vials). The block is cooled at one end and heated at the other, creating a linear temperature distribution. It provides 22 discrete temperature points along a user-defined range within limits of 0 to 90 °C. After 3.5 h of incubation, CO2 production is measured from each of the 88 microcosms. Results demonstrate that the block produces the user-defined temperature gradient and facilitates the construction of temperature response curves of SOM decomposition. Additionally, the data collected from this setup can be used to test and develop new hypotheses and theories regarding the impact of temperature changes on SOM decomposition rates, an area of increasing importance in the context of climate change.

Abstract Alkaline industrial by-products have significant potential for permanent CO 2 sequestration, but their reaction kinetics and controlling factors are often overlooked. This study examined the direct aqueous carbonation of basic oxygen furnace (BOF) slag in a slurry setup with continuous mechanical mixing and CO 2 -enriched air bubbling in ultrapure water and natural seawater, at 25% pCO 2 under ambient temperature and 1 atm for 24 h. CO 2 sequestration into carbonate minerals was higher in the ultrapure water (105 ± 5 kg CO 2 t −1 slag; 90% CO 2 capture efficiency) than in seawater (78 ± 4 kg CO 2 t −1 slag; 70% CO 2 capture efficiency). Carbonation proceeded in two distinct stages: an initial rapid carbonation phase (4–12 h), dominated by reactive Ca-(hydr)oxides, silicates and aluminates (e.g. wollastonite), followed by slower reactions involving less-reactive phases as the system gradually approached equilibrium with the pCO 2 in the gas phase. Ionic inhibitors (e.g. Mg 2+ ) affected the overall carbonation rate throughout the process. These findings highlight the importance of considering multi-stage kinetics and the influence of the aqueous matrix on reaction-inhibiting factors when evaluating the achievable extent of CO 2 sequestration potential and the time required to reach it.

Total Alkalinity (TA) is widely used as a proxy for captured CO2 in enhanced weathering (EW) applications. However, organic anions can also contribute to TA. To improve carbon accounting in EW, which is often simplified to that TA equals carbonate alkalinity, their contribution should be taken into account. In this study, we tested how dissolved organic carbon (DOC) contributes to non-carbonate alkalinity (ANC) using microcosm experiments with artificial organo-mineral mixtures. We used different combinations of rock powder with straw, microbes and earthworm additions, under ambient air conditions. The microcosms were flow-through columns placed in a climate chamber at 25 degrees C, which were irrigated with groundwater at rates between 1200 and 3600 mm/yr. The concentrations of several low-molecular-weight organic acids (oxalate, citrate, acetate, gluconate) were quantified to assess which conjugate base anions impact the measured TA. Results revealed a ratio of 3.5 mol DOC per ANC equivalent. In the overall experiment the median contribution of ANC to TA was around 5.5 %. A positive correlation between DOC and charge-balance error suggests that some organic acid anions remained deprotonated during TA titration. Acetate anions found in DOC-rich water samples further support a substantial contribution of organic anions to TA. To investigate the relevance of ANC for natural EW systems, we also quantified ANC contributions in natural waters and leachates from soil EW experiment mesocosms. Because DOC levels were lower, ANC contributions were smaller, ranging from a median of 4.1 % in soil mesocosm leachates down to 0.9 % in Elbe estuary water samples. This ANC contribution, despite seeming small, is relevant for carbon accounting in terrestrial EW practices, where TA is often assumed to be solely carbonate alkalinity.

Background The optimization of enhanced mineral weathering as a carbon dioxide removal technology requires a comprehensive understanding of what drives mineral weathering. These drivers can be abiotic and biotic and can interact with each other. Therefore, in this study, an extensive 8-week column experiment was set up to investigate 30 potential drivers of mineral weathering simultaneously. Methods The setup included various combinations of rock types and surface areas, irrigation settings, biochar and organic amendments, along with various biota and biotic products such as earthworms, fungi, bacteria and enzymes; each varying in type or species and quantity. The resulting changes in dissolved, solid, and total inorganic carbon (∆TIC), and total alkalinity were calculated as indicators of carbon dioxide removal through mineral weathering. Three machine learning models, Least Absolute Shrinkage and Selection Operator (LASSO), Random Forest and eXtreme Gradient Boosting (XGB) regression, were used to predict these indicators. Dominant drivers of the best performing model were investigated using SHapley Additive exPlanations (SHAP). Results SHAP analysis revealed that each CDR indicator was influenced by different factors. However, key drivers were consistently abiotic, though biota also made a significant contribution to the predictions. The most representative CDR indicator, ∆TIC, was predominantly driven by steel slag addition and mixed rock grain sizes but was also substantially impacted by earthworms and microbes. Conclusions These findings provide valuable insights into the complex interplay of numerous abiotic and biotic factors that affect mineral weathering, highlighting the potential of machine learning to unravel complex relationships in biogeochemical systems.

Chemical weathering of silicate rocks redistributes major, minor and trace elements through coupled dissolution–precipitation reactions. These weathering processes drive shifts in ocean acid–base chemistry, modulating atmospheric carbon dioxide levels and providing a stabilizing feedback in the carbon cycle. Silicate weathering occurs in both terrestrial and marine environments, releasing (‘forward’) or consuming alkalinity (‘reverse’), but these have largely been perceived as independent and studied in isolation. However, weathering products are transported downstream across terrestrial and to marine environments, suggesting a dynamic coupling of these weathering processes across scales. Here we propose that the Earth’s silicate weathering occurs along a continuum linking mountains to the deepest sedimentary environments and forward to reverse weathering. In this framework, the magnitude and direction of a local weathering flux depends on the materials’ origin, weathering–erosion history and environmental conditions. Consequently, global silicate weathering fluxes and the long-term carbon cycle feedback may be governed by the dynamic interplay of various environments along the silicate weathering continuum. Chemical weathering of silicate rocks occurs along a continuum from terrestrial to marine environments.

To slow the rise in atmospheric carbon dioxide concentrations, Enhanced Silicate Weathering is emerging as a potentially significant Carbon Dioxide Removal technology. However, the biotic controls on rock weathering are not well understood, particularly for key soil faunal groups such as earthworms. Earthworms have shown to possibly enhance weathering, highlighting their potential to be introduced in controlled or engineered settings, such as reactors, to increase carbon sequestration. Here, we determined the potential for earthworms to thrive and to increase weathering rates in an artificial organo-mineral system simulating a bioreactor. We used two earthworm species (Aporrectodea caliginosa [Savigny] and Allolobophora chlorotica [Savigny]) at four densities (10, 20, 25 and 30 earthworms kg-1 organo-mineral mixture), four silicate rock types (two basanites, dunite and diabase) of two to three grain sizes (d50 between 0.026 and 1.536 mm), two sources of organic materials (straw and co-digestate), two amounts of biochar (0 and 100 g kg-1 organo-mineral mixture) and/or enzyme additions (laccase, urease and carbonic anhydrase at 20, 177 and 1955 units kg-1 organo-mineral mixture, respectively), three water irrigation rates (125, 250 and 375 mL day-1 kg-1 organo-mineral mixture) and three watering frequencies (one, two and five times day-1). The experiment was conducted in eight rounds, each one lasting eight weeks, yielding data for a total of 323 experimental units. We measured earthworm survival and activity, as well as several commonly used weathering indicators in the organo-mineral mixture and in the leachate, as total alkalinity, inorganic carbon, pH, electrical conductivity and major cations. Using random forest regression, we found that earthworm survival and activity mainly depended on variables influencing the structure and drainage potential of the organo-mineral mixture, such as the presence of straw and increasing percentages of coarse grain sizes. Furthermore, we concluded that the effect of earthworms on weathering indicators depended on whether they survived or died by the end of the experimental period. Surviving earthworms had a neutral or negative effect on weathering indicators, likely because the experimental duration was too short to detect an increase in inorganic carbon, or because there was an increase in organic rather than inorganic carbon in the organo-mineral mixture. In contrast, dead earthworms enhanced almost all weathering indicators considered, suggesting that microbial processes associated with decomposing earthworm bodies may play a role in enhancing weathering. Our results also emphasize that the role of earthworms in Enhanced Silicate Weathering within bioreactors might be overestimated if weathering indicators exclusively rely on changes in mineralogy and ions release to quantify earthworm effects on carbon sequestration through weathering.

Carbon dioxide removal technologies are urgently needed to stay within the 2 °C warming limit. This study evaluated Gran Canaria lapilli and a commercially available lava basalt as feedstocks for bio-weathering in flow-through columns inoculated with Aureobasidium pullulans, Suillus variegatus, Bacillus subtilis, and Cupriavidus metallidurans. Weathering rates for biotic lapilli (2.92 ± 0.28 × 10−12 mol m−2 s−1) and lava basalt (2.17 ± 0.15 × 10−12 mol m−2 s−1) are statistically significantly greater than the abiotic controls (lapilli = 1.08 ± 0.01 × 10−12, lava basalt = 0.98 ± 0.02 × 10−12 mol m−2 s−1), with inoculation explaining most of the variance. While lapilli had faster weathering rates and greater phospholipid fatty acid biomasses, lava basalt sequestered more carbon: 1.38 and 0.61 mmol C (biotic and abiotic) compared to 0.56 and −0.25 mmol C for lapilli. Both materials show promise for CDR in bio-weathering systems.

Abstract. Avoiding the most damaging consequences of climate change will almost certainly require pairing rapid emission cuts with large‑scale carbon dioxide removal (CDR). Among the proposed CDR pathways, enhanced weathering (EW) accelerates natural mineral dissolution to convert atmospheric CO₂ into long‑lived bicarbonate and carbonate reservoirs. Despite the many reported data from EW experiments, large uncertainty remains about the realisable CDR potential of applying rock materials to agricultural land. One of the relevant sinks for CO₂ is the transfer to bicarbonate alkalinity, and various EW studies have reported a wide range of results for this process. Intercomparison of these data is problematic due to the different experimental set-ups, environmental conditions as well as combinations of rock materials and soil types. In order to assess and compare the realisable CDR potential of various EW combinations, a large greenhouse experiment was set up in which 4 different soil types (7 different soil batches) were treated with 13 different feedstock materials. The experiment included growing perennial ryegrass (Lolium perenne) and was conducted over two years with high irrigation rates (> 2000 mm a-1) and elevated temperatures (>19 °C) to speed up the weathering process. Alkalinity production was highly variable among the treatments and some even showed a loss of alkalinity compared to their controls. Consistent with expected dissolution kinetics, alkalinity production rates followed the trend: steel slag > limestone / carbonate-rich metabasalt > peridotite > basanite. Matrix analyses of soil properties versus feedstock revealed that alkalinity production from acidic soils was highest. At higher pH-levels (> 7 pH), carbonate mineral saturation likely constrains further dissolution, potentially favouring carbonate formation. Detailed analyses of cation pools (exchangeable, carbonates, oxides and clay) revealed large changes within the first year where 10–50 times more cations were retained than exported via leachate, making the realised CDR potential as alkalinity relatively small compared to the CDR potential of cations retained. Understanding the dynamics of transfers between cation pools and their potential saturation are important to develop models and enable projections. Data reported from EW studies so far are insufficient to enable calibration of models, specifically if projections in CDR-realisations should span decades.

Background The optimization of enhanced mineral weathering as a carbon dioxide removal technology requires a comprehensive understanding of what drives mineral weathering. These drivers can be abiotic and biotic and can interact with each other. Therefore, in this study, an extensive 8-week column experiment was set up to investigate 30 potential drivers of mineral weathering simultaneously. Methods The setup included various combinations of rock types and surface areas, irrigation settings, biochar and organic amendments, along with various biota and biotic products such as earthworms, fungi, bacteria and enzymes; each varying in type or species and quantity. The resulting changes in dissolved, solid, and total inorganic carbon (∆TIC), and total alkalinity were calculated as indicators of carbon dioxide removal through mineral weathering. Three machine learning models, Least Absolute Shrinkage and Selection Operator (LASSO), Random Forest and eXtreme Gradient Boosting (XGB) regression, were used to predict these indicators. Dominant drivers of the best performing model were investigated using SHapley Additive exPlanations (SHAP). Results SHAP analysis revealed that each CDR indicator was influenced by different factors. However, key drivers were consistently abiotic, though biota also made a significant contribution to the predictions. The most representative CDR indicator, ∆TIC, was predominantly driven by steel slag addition and mixed rock grain sizes but was also substantially impacted by earthworms and microbes. Conclusions These findings provide valuable insights into the complex interplay of numerous abiotic and biotic factors that affect mineral weathering, highlighting the potential of machine learning to unravel complex relationships in biogeochemical systems.

Earthworms are among the most important soil fauna. To exert effects on soils, they need to be active. Earthworm presence is often used as an indicator for activity, but this is not always reliable as earthworms may survive long periods of inactivity or even estivation. Other direct, reliable, and objective measurements for earthworm activity are lacking. Here, we present a novel earthworm activity measurement, based on body-density difference between actively feeding and inactive earthworms. The underlying principle is that active earthworms have a higher density as their gut is filled with soil particles, while inactive earthworms generally have an empty gut. The method therefore separates inactive earthworms from active ones by flotation. To achieve separation, a 1.08 g cm(-3) sucrose solution is used. Since it is well established that earthworm activity is reduced in dry soil conditions, we set up a soil moisture gradient experiment to gain a range in earthworm activity. We tested our method on four common European earthworms (endogeic species Allolobophora chlorotica [Savigny] and Aporrectodea caliginosa [Savigny], anecic species Aporrectodea longa [Ude] and epigeic species Lumbricus rubellus [Hoffmeister]) in an experiment with three soil moisture levels (84, 126 and 168 mL kg(-1)) using a sandy topsoil. As additional proxy for earthworm activity, estivation was visually recorded. We found a high inter-method correlation between density-based and visually estimated earthworm activity, regardless of earthworm species and moisture treatment. Furthermore, our novel method detected inactive individuals of L. rubellus, a species without the ability of estivation. We demonstrated that our density-based method allows for easy and quick quantification of active earthworms. This method offers clear advantages over visual assessments of estivation or cast production, in particular objectivity and applicability to a wider range of species, including those that do not enter estivation.

To achieve the 2°C climate goal various carbon dioxide removal (CDR) technologies are being developed. Biochar obtained from biomass pyrolysis contains persistent carbonaceous compounds and offers benefits when applied to soil, including enhanced soil fertility, and improved water retention. Additionally, the application of natural rocks for enhanced rock weathering (ERW) in agricultural soil is gaining attention for its potential to sequester CO2, while increasing soil pH and providing essential nutrients. Given the promising potential of both biochar and ERW as CDR technologies, their combined application could offer synergistic effects, making it crucial to understand their interaction. However, research on their co-application of biochar and rock powder as well as co-pyrolysis of biomass with rock powder (yielding rock enhanced biochar) remains limited.This study quantified alkalinity and ion releaseOxisol soil columns after addition of wood and straw biochar, rock-enhanced biochar or co-application of biochar and rock powder. In total 9 treatments were incubated for 27 weeks under elevated CO2 conditions with 10 leachate sampling events. First results show high initial fluxes of total alkalinity and dissolved inorganic carbon as well as dissolved organic carbon and nutrients, which decrease over time. Notably, pCO2 has minimal impact on the pyrogenic carbon, while it doubles the total alkalinity flux from ERW. While biochar alone creates a larger carbon sink, co-applying rock powder enhances mineral fertilization and increases the weight of biochar pellets. Soil amendments with biochar further prevent a water logging of the clayey Oxisol, enabling rock weathering and alkalinity fluxes to continue.

Background:The optimization of enhanced mineral weathering as a carbon dioxide removal technology requires a comprehensive understanding of what drives mineral weathering. These drivers can be abiotic and biotic and can interact with each other. Therefore, in this study, an extensive 8-week column experiment was set up to investigate 30 potential drivers of mineral weathering simultaneously. Methods:The setup included various combinations of rock types and surface areas, irrigation settings, biochar and organic amendments, along with various biota and biotic products such as earthworms, fungi, bacteria and enzymes; each varying in type or species and quantity. The resulting changes in dissolved, solid, and total inorganic carbon (∆TIC), and total alkalinity were calculated as indicators of carbon dioxide removal through mineral weathering. Three machine learning models, Least Absolute Shrinkage and Selection Operator (LASSO), Random Forest and eXtreme Gradient Boosting (XGB) regression, were used to predict these indicators. Dominant drivers of the best performing model were investigated using SHapley Additive exPlanations (SHAP). Results:SHAP analysis revealed that each CDR indicator was influenced by different factors. However, key drivers were consistently abiotic, though biota also made a significant contribution to the predictions. The most representative CDR indicator, ∆TIC, was predominantly driven by steel slag addition and mixed rock grain sizes but was also substantially impacted by earthworms and microbes. Conclusions:These findings provide valuable insights into the complex interplay of numerous abiotic and biotic factors that affect mineral weathering, highlighting the potential of machine learning to unravel complex relationships in biogeochemical systems.

Enhanced rock weathering (ERW) and pyrogenic carbon capture and storage (PyCCS, or “biochar carbon removal”) are two promising carbon dioxide removal (CDR) techniques that can contribute to soil restoration. These technologies can be combined by co-application of rock powder and biochar or by co-pyrolysis of rock powder with biomass to produce rock-enhanced (RE) biochar. In a 27-week laboratory experiment, we quantified the carbon (C) sink development of co-applications and RE-biochars produced by co-pyrolysis of basanite rock powder with either 50 or 90 wt% willow wood or 90 wt% wheat straw. Incubators featured two elevated soil pCO2 levels (0.012 and 0.062 atm, equivalent to about 1.2 and 6.2 Vol-% CO2) in a clay-rich, nutrient-poor Oxisol, with a simulated annual rainfall of 1,600 mm. Results showed strong initial fluxes of total alkalinity (TA), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and major cations (Mg2+, Ca2+, K+, Na+), which decreased over time. Notably, elevated pCO2 had minimal impact on the release of DOC but doubled the TA flux from ERW. An important observation was the impact of waterlogging on water fluxes in soil columns without biochar, which lowered the amount of leached cations from rock and biochar. We defined the carbon sink (C-Sink) to include all DIC of geogenic and biogenic origin, and pyrogenic carbon from biochar. Biogenic cations were not considered as contributing to additional CO2 sequestration. For a soil application equivalent to application of 12 t ha−1, the total net C-Sink ranged from −0.1 to 30.9 t CO2 ha−1 after 27 weeks under 1.2 Vol-% CO2. We were not able to determine a change in rock weathering rates from co-pyrolysis since biogenic and geogenic cations could not be distinguished. A 20-year forecast suggests net C-Sinks between 0.5 t and 28.7 t CO2 ha−1, driven by increased contributions from weathering, alongside a C-Sink loss of carbon due to biochar mineralization. While biochar alone generally produces a larger C-Sink, co-application with rock powder fosters soil remineralization and provides a higher permanence of the C-Sink. Additionally, biochar increases water-holding capacity, prevents waterlogging of soils and likely improves the retention of organic carbon in soils.

Enhanced rock weathering aims at capturing atmospheric carbon dioxide as inorganic carbon, while potentially stabilizing soil organic carbon. However, the role of soil biota in this process remains underexplored. Earthworms, being key soil engineers, may impact carbon dynamics both when alive, through mineral ingestion and casting activities, and when dead, through microbial processes. Using stable isotope tracing, we investigate how live and dead earthworms affect carbon dynamics during rock weathering. We demonstrate that both living and dead earthworms contributed to carbon capture, albeit through distinct pathways. Live earthworms enhanced the formation of organo-mineral associations via their dejections over time, promoting organic carbon persistence. Dead earthworms boosted microbial abundance and activity, enhancing organo-mineral associations and atmospheric-derived inorganic carbon capture between 60 and 120 days. We show that earthworms influence carbon cycling beyond their lifespan, with contrasting physico-chemical and biological pathways driving carbon capture through rock weathering throughout their life cycle.

Various approaches are currently used to quantify the carbon dioxide removal (CDR) associated with enhanced weathering (EW), which involves amending soils with crushed silicate minerals. We aimed to contribute to the development of a standardized procedure for CDR quantification by complementing the results of a recently published soil column experiment, in which crushed olivine, wollastonite, and albite were added to soils, with total fusion ICP-OES analyses of base cation concentrations. CDR quantified by soil-based mass balance approaches was only comparable to leachate-based total alkalinity measurements after correcting for the weathering products that were retained within the soil profile, which we defined as the retarded fraction. The retarded fraction comprised 92.7–98.3% of the weathered cations, indicating that at least in our short-term study (64 days), the majority of weathering products were retained within the soil. Further investigation of the fate of retarded weathering products showed that small portions precipitated as carbonate minerals (up to 34.0%) or adsorbed to reactive surfaces, such as soil organic matter and clay minerals (up to 32.5%). Hence, a large portion of weathering products may be retained in the soil due to strong adsorption and/or further mineral precipitation reactions (31.6–92.7%), with potentially important implications for the quantification of CDR across time. We conclude that soil-based mass balance approaches are useful in quantifying weathering rates and can infer potential CDR; however, the actual CDR realized for a given time and depth interval can only be constrained after accounting for the retarded fraction.

The development of carbon dioxide removal methods, coupled with decreased CO2 emissions, is fundamental to achieving the targets outlined in the Paris Agreement limiting global warming to 1.5 °C. Here we are investigating the importance of the organic carbon feedstock to support silicate mineral weathering in small-scale flow through bioreactors and subsequent CO2 sequestration. Here, we combine two bacteria and two fungi, widely reported for their weathering potential, in simple flow through bioreactors (columns) consisting of forsterite and widely available, cheap organic carbon sources (wheat straw, bio-waste digestate of pig manure and biowaste, and manure compost), over six weeks. Compared to their corresponding abiotic controls, the inoculated straw and digestate columns release more total alkalinity (~2 times more) and produce greater dissolved and solid inorganic carbon (29% for straw and 13% for digestate), suggesting an increase in CO2 sequestration because of bio-enhanced silicate weathering. Microbial biomass is higher in the straw columns compared to the digestate and manure compost columns, with a phospholipid fatty acid derived total microbial biomass 10 x greater than the other biotic columns. Scanning Electron Microscopy imaging shows the most extensive colonisation and biofilm formation on the mineral surfaces in the straw columns. The biotic straw and digestate columns sequester 50 and 14 mg C more than their abiotic controls respectively, while there is no difference in the manure columns. The selection of organic carbon sources to support microbial communities in the flow through bioreactors controlls the silicate weathering rates and CO2 sequestration.