Potential for large-scale CO2 removal via enhanced rock weathering with croplands

Exogenous calcium-induced carbonate formation to increase carbon sequestration in coastal saline-alkali soil

Promoting soil carbon sequestration is a possible way to mitigate global warming. To investigate the effects of exogenous calcium on soil carbon sequestration during the application of organic matter to improve coastal saline-alkali soil. In this study, a 30-day incubation experiment was based on the application of corn straw biochar + chicken manure (BM) and rice straw + chicken manure (SM). Usages of exogenous calcium in each treatment under each organic matter combination as follow: CK (No exogenous calcium), CaSi1 (1.24 g CaSiO3, i.e. 4.28 g Ca kg-1 soil), CaSi2 (2.48 g CaSiO3, i.e. 8.56 g Ca kg-1 soil), CaOH1 (0.79 g Ca(OH)2, i.e. 4.28 g Ca kg-1 soil), CaOH2 (1.58 g Ca(OH)2, i.e. 8.56 g Ca kg-1 soil), CaSiOH (1.24 g CaSiO3 + 0.79 g Ca(OH)2, i.e. 8.56 g Ca kg-1 soil). Results showed that exogenous calcium significantly reduced CO2 emission. Organic matter addition promoted the loss of SOC, and exogenous did not significantly affect the mineralization of SOC albeit strongly increased SIC, making up for the loss of SOC, increasing soil total carbon and realizing soil carbon fixation. Soil carbon fixation was mainly realized by the reaction of exogenous calcium with CO2 generated by mineralization and converting it into calcium carbonate. pH and soil CO2 emission are the major controlling factors for soil inorganic carbon sequestration. Therefore, applying organic matter with exogenous calcium can realize soil carbon fixation by generation of calcium carbonate.

Scaling of development indicators in countries and its origin

Population-normalized indicators (e.g., GDP per capita), under the assumption of the indicators scaling linearly with population, are ubiquitously used in national development performance comparison. This assumption, however, is not valid because it may ignore agglomeration effect resulting from nonlinear interactions in socioeconomic systems. Here, we present extensive empirical evidence showing the sub-linear scaling rather than the presumed linear scaling between population and multiple indicators of national development performance. We then develop a theoretical framework based on the scaling rule observed in cities to explore the origin of scaling in countries. Finally, we demonstrate that urbanization plays a pivotal role in transforming national development from limited sub-linear growth to unlimited super-linear growth. This underscores the significance of urbanization in achieving sustained growth and elevating human living standards at the national level. Our findings have the potential to inform policies aimed at promoting equitable inter-country comparison and achieving sustainable development in countries.

Iron Chelation in Soil: Scalable Biotechnology for Accelerating Carbon Dioxide Removal by Enhanced Rock Weathering

Enhanced rock weathering (EW) is an emerging atmospheric carbon dioxide removal (CDR) strategy being scaled up by the commercial sector. Here, we combine multiomics analyses of belowground microbiomes, laboratory-based dissolution studies, and incubation investigations of soils from field EW trials to build the case for manipulating iron chelators in soil to increase EW efficiency and lower costs. Microbial siderophores are high-affinity, highly selective iron (Fe) chelators that enhance the uptake of Fe from soil minerals into cells. Applying RNA-seq metatranscriptomics and shotgun metagenomics to soils and basalt grains from EW field trials revealed that microbial communities on basalt grains significantly upregulate siderophore biosynthesis gene expression relative to microbiomes of the surrounding soil. Separate in vitro laboratory incubation studies showed that micromolar solutions of siderophores and high-affinity synthetic chelator (ethylenediamine-N,N'-bis-2-hydroxyphenylacetic acid, EDDHA) accelerate EW to increase CDR rates. Building on these findings, we develop a potential biotechnology pathway for accelerating EW using the synthetic Fe-chelator EDDHA that is commonly used in agronomy to alleviate the Fe deficiency in high pH soils. Incubation of EW field trial soils with potassium-EDDHA solutions increased potential CDR rates by up to 2.5-fold by promoting the abiotic dissolution of basalt and upregulating microbial siderophore production to further accelerate weathering reactions. Moreover, EDDHA may alleviate potential Fe limitation of crops due to rising soil pH with EW over time. Initial cost-benefit analysis suggests potassium-EDDHA could lower EW-CDR costs by up to U.S. $77 t CO2 ha-1 to improve EW's competitiveness relative to other CDR strategies.

Impact of Lime Saturation Factor on Alite-Ye'Elimite Cement Synthesis and Hydration

Alite(C3S)-Ye'elimite(C4A3$) cement is a high cementitious material that incorporates a precise proportion of ye'elimite into the ordinary Portland cement. The synthesis and hydration behavior of Alite-Ye'elimite clinker with different lime saturation factors were investigated. The clinkers were synthesized using a secondary thermal treatment process, and their compositions were characterized. The hydrated pastes were analyzed for their hydration products, pore structure, mechanical strength, and microstructure. The clinkers and hydration products were characterized using XRD, TG-DSC, SEM, and MIP analysis. The results showed that the Alite-Ye'elimite cement clinker with a lime saturation factor (KH) of 0.93, prepared through secondary heat treatment, contained 64.88% C3S and 2.06% C4A3$. At this composition, the Alite-Ye'elimite cement clinker demonstrated the highest 28-day strength. The addition of SO3 to the clinkers decreased the content of tricalcium aluminate (C3A) and the ratio of Alite/Belite (C3S/C2S), resulting in a preference for belite formation. The pore structure of the hydrated pastes was also investigated, revealing a distribution of pore sizes ranging from 0.01 to 10 μm, with two peaks on each differential distribution curve corresponding to micron and sub-micron pores. The pore volume decreased from 0.22 ± 0.03 to 0.15 ± 0.18 cm3 g-1, and the main peak of pore distribution shifted towards smaller sizes with increasing hydration time.

The potential for carbon dioxide removal by enhanced rock weathering in the tropics: An evaluation of Costa Rica

Tropical environments show great potential to sequester CO2 by enhanced rock weathering (ERW) of powdered mafic rocks applied to agricultural fields. This study seeks to assess carbon dioxide reduction (CDR) potential in the humid tropics (1) by experimental weathering of mafic rock powders in conditions simulating humid tropical soils, and (2) from weathering rates determined from a Holocene tropical soil chronosequence where parent material is andesitic sediments. Experimentally determined weathering rates by leaching of basaltic andesites from Costa Rica (Arenal and Barva) for 50 t ha-1 applications indicate potential sequestration of 2.4 to 4.5 t CO2 ha-1 yr-1, whereas the USGS basalt standard BHVO-1 yields a rate of 11.9 t ha-1 yr-1 (influenced by more mafic composition and finer particle size). The chronosequence indicates a rate of 1.7 t CO2 ha-1 yr-1. The weathering experiment consisted of 0.6 mm of powdered rock applied atop 12 mm of Ultisol at 35 °C. To simulate a tropical soil solution, 100-mL aliquots of a dilute solution of oxalic acid in carbonated DI water were rained onto soils over a 14-day period to simulate soil moisture in the humid tropics. Solutions were collected and analyzed by ICPMS for concentrations of leached cations. A potential ERW scenario for Costa Rica was assessed assuming that one-half of lowland agricultural kaolinitic soils (mainly Ultisols, common crop and pasture soils, excluding protected areas) were to receive 50 t ha-1 of annual or biennial applications of powdered mafic rock. With an experimentally determined humid tropical CDR rate for basaltic andesite (3.5 t ha-1 yr-1) and allowances for carbon costs (e.g. emissions from processing and delivery) that reduce CDR to a net 3.2 t ha-1 yr-1, potential annual CDR of this tropical nation is ∼2-4 million tons, amounting to ∼25-50 % of annual CO2 emissions (mainly from transportation in Costa Rica).

'Climate Healing Stones': Common Minerals Offer Substantial Climate Change Mitigation Potential

This review proposes that mineral-based greenhouse gas (GHG) mitigation could be developed into a substantial climate change abatement tool. This proposal was evaluated via three objectives: (1) synthesise literature studies documenting the effectiveness of geological minerals at mitigating GHG emissions; (2) quantify, via meta-analysis, GHG magnitudes that could be abated by minerals factoring-in the carbon footprint of the approach; and (3) estimate the global availability of relevant minerals. Several minerals have been effectively harnessed across multiple sectors-including agriculture, waste management and coal mining-to mitigate carbon dioxide/CO2 (e.g., olivine), methane/CH4 (e.g., allophane, gypsum) and nitrous oxide/N2O (e.g., vermiculite) emissions. High surface area minerals offer substantial promise to protect soil carbon, albeit their potential impact here is difficult to quantify. Although mineral-based N2O reduction strategies can achieve gross emission reduction, their application generates a net carbon emission due to prohibitively large mineral quantities needed. By contrast, mineral-based technologies could abate ~9% and 11% of global CO2 and CH4 anthropogenic emissions, respectively. These estimates conservatively only consider options which offer additional benefits to climate change mitigation (e.g., nutrient supply to agricultural landscapes, and safety controls in landfill operations). This multi-benefit aspect is important due to the reluctance to invest in stand-alone GHG mitigation technologies. Minerals that exhibit high GHG mitigation potential are globally abundant. However, their application towards a dedicated global GHG mitigation initiative would entail significant escalation of their current production rates. A detailed cost-benefit analysis and environmental and social footprint assessment is needed to ascertain the strategy's scale-up potential.

Metal bioaccumulation and effects of olivine sand exposure on benthic marine invertebrates

Due to the anthropogenic increase of atmospheric CO2 emissions, humanity is facing the negative effects of rapid global climate change. Both active emission reduction and carbon dioxide removal (CDR) technologies are needed to meet the Paris Agreement and limit global warming to 1.5 °C by 2050. One promising CDR approach is coastal enhanced weathering (CEW), which involves the placement of sand composed of (ultra)mafic minerals like olivine in coastal zones. Although the large-scale placement of olivine sand could beneficially impact the planet through the consumption of atmospheric CO2 and reduction in ocean acidification, it may also have physical and geochemical impacts on benthic communities. The dissolution of olivine can release dissolved constituents such as trace metals that may affect marine organisms. Here we tested acute and chronic responses of marine invertebrates to olivine sand exposure, as well as examined metal accumulation in invertebrate tissue resulting from olivine dissolution. Two different ecotoxicological experiments were performed on a range of benthic marine invertebrates (amphipod, polychaete, bivalve). The first experiment included acute and chronic survival and growth tests (10 and 20 days, respectively) of olivine exposure while the second had longer (28 day) exposures to measure chronic survival and bioaccumulation of trace metals (e.g. Ni, Cr, Co) released during olivine sand dissolution. Across all fauna we observed no negative effects on acute survival or chronic growth resulting solely from olivine exposure. However, over 28 days of exposure, the bent-nosed clam Macoma nasuta experienced reduced burrowing and accumulated 4.2 ± 0.7 μg g ww-1 of Ni while the polychaete Alitta virens accumulated 3.5 ± 0.9 μg g ww-1 of Ni. No significant accumulation of any other metals was observed. Future work should include longer-term laboratory studies as well as CEW field studies to validate these findings under real-world scenarios.

Silicon: A valuable soil element for improving plant growth and CO2 sequestration

BACKGROUND

Silicon (Si), the second most abundant and quasi-essential soil element, is locked as a recalcitrant silicate mineral in the Earth's crust. The physical abundance of silicates can play an essential role in increasing plant productivity. Plants store Si as biogenic silica (phytoliths), which is mobilized through a chemical weathering process in the soil.

AIM OF REVIEW

Although Si is a critical element for plant growth, there is still a considerable need to understand its dissolution, uptake, and translocation in agroecosystems. Here, we show recent progress in understanding the interactome of Si, CO2, the microbiome, and soil chemistry, which can sustainably govern silicate dissolution and cycling in agriculture.

KEY SCIENTIFIC CONCEPTS OF THIS REVIEW

Si cycling is directly related to carbon cycling, and the resulting climate stability can be enhanced by negative feedback between atmospheric CO2 and the silicate uptake process. Improved Si mobilization in the rhizosphere by the presence of reactive elements (for example, Ca, Na, Al, Zn, and Fe) and Si uptake through genetic transporters in plants are crucial to achieving the dual objectives of (i) enhancing crop productivity and (ii) abiotic stress tolerance. Furthermore, the microbiome is a symbiotic partner of plants. Bacterial and fungal microbiomes can solubilize silicate minerals through intriguingly complex bioweathering mechanisms by producing beneficial metabolites and enzymes. However, the interaction of Si with CO2 and the microbiome's function in mobilization have been understudied. This review shows that enhancing our understanding of Si, CO2, the microbiome, and soil chemistry can help in sustainable crop production during climatic stress events.

Synergistic advantages of volcanic ash weathering in saline soils: CO2 sequestration and enhancement of plant growth

Carbon dioxide (CO2) is a primary greenhouse gas that has experienced a surge in atmospheric concentration due to human activities and lifestyles. It is imperative to curtail atmospheric CO2 levels promptly to alleviate the multifaceted impacts of climate warming. The soil serves as a natural reservoir for CO2 sequestration. The scientific premise of this study is that CO2 sequestration in agriculturally relevant, organically-deficient saline soil can be achieved by incorporating alkaline earth silicates. Volcanic ash (VA) was used as a soil amendment for CO2 removal from saline soil by leveraging enhanced silicate rock weathering (ERW). The study pursued two primary objectives: first, we aimed to evaluate the impact of various doses of VA, employed as an amendment for organically-deficient soil, on the growth performance of key cultivated crops (sorghum and mung bean) in inland saline-alkaline agricultural regions of northeastern China. Second, we aimed to assess alterations in the physical properties of the amended soil through mineralogical examinations, utilizing X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) analyses, quantifying the increase in inorganic carbon content within the soil. In the potting tests, mung bean plant height exhibited a noteworthy increase of approximately 41 % with the addition of 10 % VA. Sorghum plant height and aboveground and belowground biomass dry weights increased with VA application across all tested doses. At the optimal VA application rate (20 %), the sorghum achieved a CO2 sequestration rate of 0.14 kg CO2·m-2·month-1. XRD and SEM-EDS analyses confirmed that the augmented inorganic carbon in the VA-amended soils stemmed primarily from calcite accumulation. These findings contribute to elucidating the mechanism underlying VA as an amendment for organically-deficient soils and provide an effective approach for enhancing the carbon sink capacity of saline soils.

Acidification of European croplands by nitrogen fertilization: Consequences for carbonate losses, and soil health

Soil acidification is an ongoing problem in intensively cultivated croplands due to inefficient and excessive nitrogen (N) fertilization. We collected high-resolution data comprising 19,969 topsoil (0-20 cm) samples from the Land Use and Coverage Area frame Survey (LUCAS) of the European commission in 2009 to assess the impact of N fertilization on buffering substances such as carbonates and base cations. We have only considered the impacts of mineral fertilizers from the total added N, and a N use efficiency of 60 %. Nitrogen fertilization adds annually 6.1 × 107 kmol H+ to European croplands, leading to annual loss of 6.1 × 109 kg CaCO3. Assuming similar acidification during the next 50 years, soil carbonates will be completely removed from 3.4 × 106 ha of European croplands. In carbonate-free soils, annual loss of 2.1 × 107 kmol of basic cations will lead to strong acidification of at least 2.6 million ha of European croplands within the next 50 years. Inorganic carbon and basic cation losses at such rapid scale tremendously drop the nutrient status and production potential of croplands. Soil liming to ameliorate acidity increases pH only temporarily and with additional financial and environmental costs. Only the direct loss of soil carbonate stocks and compensation of carbonate-related CO2 correspond to about 1.5 % of the proposed budget of the European commission for 2023. Thus, controlling and decreasing soil acidification is crucial to avoid degradation of agricultural soils, which can be done by adopting best management practices and increasing nutrient use efficiency. Regular screening or monitoring of carbonate and base cations contents, especially for soils, where the carbonate stocks are at critical levels, are urgently necessary.

Zero-carbon inertization processes of hazardous mine tailings: Mineral physicochemical properties, transformation mechanism, and long-term stability

Hazardous mine tailings (HMTs) dam failures can cause devastation to the ecology environment, people's lives and property, which require expensive and complicated remediation engineering systematacially. A cheap and sustainable inertization disposal is proposed for de-risking HMTs without any carbon emissions, stabilizing hazardous heavy metal cations within safety minerals and also sequestering CO2 in the process, simultaneously. Herein, lead-zinc tailings as target HMTs were inertized by using waste rice husk ashes (RHAs) and carbide slag (CS) with a certain ratio, and lead-zinc tailings hardened pastes (LZTHPs) were investigated based on the experimental performance, analytical characteristics, and simulation diffusion methods, to deeply unveil the minerals transformation mechanisms and long-term stability from the cation perspectives. Results revealed that LZTHPs' compressive strength ranged from 1.04-4.73 MPa and leaching toxicity concentrations of Pb, Zn, Cr, and Cd reached 0.03 mg/L, 1.78 mg/L, 0.01 mg/L, and 0.01 mg/L, respectively. C-S-H gels (Type I and II), cation hydroxides and CO2 mineralization carbonates were the hydrates in LZTHPs. Pb (86%), Zn (78%), Cr (76%), and Cd (65%) were immobilized as residual state, and CO2 mineralization capacity was 0.16 kg/kg. The diffusion coefficient of Pb, Zn, Cr, and Cd below 4.48 × 10-10 cm2/s, 1.39 × 10-10 cm2/s, 4.72 × 10-10 cm2/s, and 0.30 × 10-12 cm2/s, which would be sufficient in most scenarios to adequately stabilize tailings. Diffusion control is the leaching mechanism of cations. After 100 years of simulation diffusion, the diffusion areas of Pb, Zn, Cr, and Cd are 1.33 × 10-3∼1.49 cm2, 2.47 × 10-4∼0.48 cm2, 2.47-8.61 × 10-4 cm2, and 1.49 cm2, respectively, and the environmental impact of LZTHPs was negligible. This study provides promising solutions for alleviating hazardous tailings dangerous, achieving sustainable development with zero-carbon emission, implying the concept of eliminating waste by waste, synchronously.

Microbial Catalysis for CO2 Sequestration: A Geobiological Approach

One of the greatest threats facing the planet is the continued increase in excess greenhouse gasses, with CO2 being the primary driver due to its rapid increase in only a century. Excess CO2 is exacerbating known climate tipping points that will have cascading local and global effects including loss of biodiversity, global warming, and climate migration. However, global reduction of CO2 emissions is not enough. Carbon dioxide removal (CDR) will also be needed to avoid the catastrophic effects of global warming. Although the drawdown and storage of CO2 occur naturally via the coupling of the silicate and carbonate cycles, they operate over geological timescales (thousands of years). Here, we suggest that microbes can be used to accelerate this process, perhaps by orders of magnitude, while simultaneously producing potentially valuable by-products. This could provide both a sustainable pathway for global drawdown of CO2 and an environmentally benign biosynthesis of materials. We discuss several different approaches, all of which involve enhancing the rate of silicate weathering. We use the silicate mineral olivine as a case study because of its favorable weathering properties, global abundance, and growing interest in CDR applications. Extensive research is needed to determine both the upper limit of the rate of silicate dissolution and its potential to economically scale to draw down significant amounts (Mt/Gt) of CO2 Other industrial processes have successfully cultivated microbial consortia to provide valuable services at scale (e.g., wastewater treatment, anaerobic digestion, fermentation), and we argue that similar economies of scale could be achieved from this research.

Ectomycorrhizal fungi integrate nitrogen mobilisation and mineral weathering in boreal forest soil

Tree growth in boreal forests is driven by ectomycorrhizal fungal mobilisation of organic nitrogen and mineral nutrients in soils with discrete organic and mineral horizons. However, there are no studies of how ectomycorrhizal mineral weathering and organic nitrogen mobilisation processes are integrated across the soil profile. We studied effects of organic matter (OM) availability on ectomycorrhizal functioning by altering the proportions of natural organic and mineral soil in reconstructed podzol profiles containing Pinus sylvestris plants, using 13CO2 pulse labelling, patterns of naturally occurring stable isotopes (26Mg and 15N) and high-throughput DNA sequencing of fungal amplicons. Reduction in OM resulted in nitrogen limitation of plant growth and decreased allocation of photosynthetically derived carbon and mycelial growth in mineral horizons. Fractionation patterns of 26Mg indicated that magnesium mobilisation and uptake occurred primarily in the deeper mineral horizon and was driven by carbon allocation to ectomycorrhizal mycelium. In this horizon, relative abundance of ectomycorrhizal fungi, carbon allocation and base cation mobilisation all increased with increased OM availability. Allocation of carbon through ectomycorrhizal fungi integrates organic nitrogen mobilisation and mineral weathering across soil horizons, improving the efficiency of plant nutrient acquisition. Our findings have fundamental implications for sustainable forest management and belowground carbon sequestration.

Size, distribution, and vulnerability of the global soil inorganic carbon

Global estimates of the size, distribution, and vulnerability of soil inorganic carbon (SIC) remain largely unquantified. By compiling 223,593 field-based measurements and developing machine-learning models, we report that global soils store 2305 ± 636 (±1 SD) billion tonnes of carbon as SIC over the top 2-meter depth. Under future scenarios, soil acidification associated with nitrogen additions to terrestrial ecosystems will reduce global SIC (0.3 meters) up to 23 billion tonnes of carbon over the next 30 years, with India and China being the most affected. Our synthesis of present-day land-water carbon inventories and inland-water carbonate chemistry reveals that at least 1.13 ± 0.33 billion tonnes of inorganic carbon is lost to inland-waters through soils annually, resulting in large but overlooked impacts on atmospheric and hydrospheric carbon dynamics.

A state of the art of review on factors affecting the enhanced weathering in agricultural soil: strategies for carbon sequestration and climate mitigation

As the urgency to address climate change intensifies, the exploration of sustainable negative emission technologies becomes imperative. Enhanced weathering (EW) represents an approach by leveraging the natural process of rock weathering to sequester atmospheric carbon dioxide (CO2) in agricultural lands. This review synthesizes current research on EW, focusing on its mechanisms, influencing factors, and pathways for successful integration into agricultural practices. It evaluates key factors such as material suitability, particle size, application rates, soil properties, and climate, which are crucial for optimizing EW's efficacy. The study highlights the multifaceted benefits of EW, including soil fertility improvement, pH regulation, and enhanced water retention, which collectively contribute to increased agricultural productivity and climate change mitigation. Furthermore, the review introduces Monitoring, Reporting, and Verification (MRV) and Carbon Dioxide Removal (CDR) verification frameworks as essential components for assessing and enhancing EW's effectiveness and credibility. By examining the current state of research and proposing avenues for future investigation, this review aims to deepen the understanding of EW's role in sustainable agriculture and climate change mitigation strategies.

Enhanced weathering in the US Corn Belt delivers carbon removal with agronomic benefits

Terrestrial enhanced weathering (EW) of silicate rocks, such as crushed basalt, on farmlands is a promising scalable atmospheric carbon dioxide removal (CDR) strategy that urgently requires performance assessment with commercial farming practices. We report findings from a large-scale replicated EW field trial across a typical maize-soybean rotation on an experimental farm in the heart of the United Sates Corn Belt over 4 y (2016 to 2020). We show an average combined loss of major cations (Ca2+ and Mg2+) from crushed basalt applied each fall over 4 y (50 t ha-1 y-1) gave a conservative time-integrated cumulative CDR potential of 10.5 ± 3.8 t CO2 ha-1. Maize and soybean yields increased significantly (P < 0.05) by 12 to 16% with EW following improved soil fertility, decreased soil acidification, and upregulation of root nutrient transport genes. Yield enhancements with EW were achieved with significantly (P < 0.05) increased key micro- and macronutrient concentrations (including potassium, magnesium, manganese, phosphorus, and zinc), thus improving or maintaining crop nutritional status. We observed no significant increase in the content of trace metals in grains of maize or soybean or soil exchangeable pools relative to controls. Our findings suggest that widespread adoption of EW across farming sectors has the potential to contribute significantly to net-zero greenhouse gas emissions goals while simultaneously improving food and soil security.

Microbial weathering of montmorillonite and its implication for Cd(II) immobilization

Interactions between silicate bacteria and silicates are very common in nature and hold great potential in altering their mutual physicochemical properties. But their interactions in regulating contaminants remediation involving performance and mechanisms are often overlooked. Here, we focused on the interactions between silicate bacteria (Paenibacillus polymyxa, PP; Bacillus circulans, BC) and a soil silicate montmorillonite (Mt), and their impact on Cd(II) immobilization. The obtained results showed that Mt greatly promoted the growth of the bacteria, resulting in a maximum 10.31 times increase in biomass production. In return, the bacteria strongly enhanced the Cd(II) adsorption on Mt, with adsorption capacities increased by 80.61%-104.45% in comparison to the raw Mt. Additionally, the bacteria-Mt interaction changed Cd(II) to a more stabilized state with a maximum reduction of 38.90%/g Mt in bioavailability. The enhancement of Cd(II) adsorption and immobilization on the bacterial modified Mt was caused by the following aspects: (1) the bacteria activities altered the aggregation state of Mt and made it better dispersed, thus more active sites were exposed; (2) the microbial activities brought about more rough and crumpled surface, as well as smaller Mt fragments; (3) a variety of microbial-derived functional groups were introduced onto the Mt surface, increasing its affinity for heavy metals; (4) the main Cd(II) immobilization mechanism was changed from ion exchange to the combination of ion exchange and functional groups induced adsorption. This work elucidates the potential ecological and evolutionary processes of silicate bacteria-soil clay mineral interactions, and bears direct implications for the clay-mediated bioremediation of heavy metals in natural environments.

The Mining Industry's Role in Enhanced Weathering and Mineralization for CO2 Removal

Enhanced weathering and mineralization (EWM) aim to remove carbon dioxide (CO2) from the atmosphere by accelerating the reaction of this greenhouse gas with alkaline minerals. This suite of geochemical negative emissions technologies has the potential to achieve CO2 removal rates of >1 gigatonne per year, yet will require gigatonnes of suitable rock. As a supplier of rock powder, the mining industry will be at the epicenter of the global implementation of EWM. Certain alkaline mine wastes sequester CO2 under conventional mining conditions, which should be quantified across the industry. Furthermore, mines are ideal locations for testing acceleration strategies since tailings impoundments are contained and highly monitored. While some environmentally benign mine wastes may be repurposed for off-site use─reducing costs and risks associated with their storage─numerous new mines will be needed to supply rock powders to reach the gigatonne scale. Large-scale EWM pilots with mining companies are required to progress technology readiness, including carbon verification approaches. With its knowledge of geological formations and ore processing, the mining industry can play an essential role in extracting the most reactive rocks with the greatest CO2 removal capacities, creating supply chains, and participating in life-cycle assessments. The motivations for mining companies to develop EWM include reputational benefits and carbon offsets needed to achieve carbon neutrality.

Diversifying crop rotation increases food production, reduces net greenhouse gas emissions and improves soil health

Global food production faces challenges in balancing the need for increased yields with environmental sustainability. This study presents a six-year field experiment in the North China Plain, demonstrating the benefits of diversifying traditional cereal monoculture (wheat-maize) with cash crops (sweet potato) and legumes (peanut and soybean). The diversified rotations increase equivalent yield by up to 38%, reduce N2O emissions by 39%, and improve the system's greenhouse gas balance by 88%. Furthermore, including legumes in crop rotations stimulates soil microbial activities, increases soil organic carbon stocks by 8%, and enhances soil health (indexed with the selected soil physiochemical and biological properties) by 45%. The large-scale adoption of diversified cropping systems in the North China Plain could increase cereal production by 32% when wheat-maize follows alternative crops in rotation and farmer income by 20% while benefiting the environment. This study provides an example of sustainable food production practices, emphasizing the significance of crop diversification for long-term agricultural resilience and soil health.

Stimulating and toxic effect of chromium on growth and photosynthesis of a marine chlorophyte

Marine phytoplankton can interchange trace metals in various biochemical functions, particularly under metal-limiting conditions. Here, we investigate the stimulating and toxicity effect of chromium (Cr) on a marine Chlorophyceae Osetreococcus tauri under Fe-replete and Fe-deficient conditions. We determined the growth, photosynthesis, and proteome expressions of Osetreococcus tauri cultured under different Cr and Fe concentrations. In Fe-replete conditions, the presence of Cr(VI) stimulated significantly the growth rate and the maximum yield of photochemistry of photosystem II (Fv /Fm ) of the phytoplankton, while the functional absorption cross-section of photosystem II (σPSII ) did not change. Minor additions of Cr(VI) partially rescued phytoplankton growth under Fe-limited conditions. Proteomic analysis of this alga grown in Fe-replete normal and Fe-replete with Cr addition media (10 μM Cr) showed that the presence of Cr significantly decreased the expression of phosphate-transporting proteins and photosynthetic proteins, while increasing the expression of proteins related to carbon assimilation. Cr can stimulate the growth and photosynthesis of O. tauri, but the effects are dependent on both the Cr(VI) concentration and the availability of Fe. The proteomic results further suggest that Cr(VI) addition might significantly increase starch production and carbon fixation.

A horizon scan of global biological conservation issues for 2024

We present the results of our 15th horizon scan of novel issues that could influence biological conservation in the future. From an initial list of 96 issues, our international panel of scientists and practitioners identified 15 that we consider important for societies worldwide to track and potentially respond to. Issues are novel within conservation or represent a substantial positive or negative step-change with global or regional extents. For example, new sources of hydrogen fuel and changes in deep-sea currents may have profound impacts on marine and terrestrial ecosystems. Technological advances that may be positive include benchtop DNA printers and the industrialisation of approaches that can create high-protein food from air, potentially reducing the pressure on land for food production.

Reduced Carbon Dioxide by Overexpressing EPSPS Transgene in Arabidopsis and Rice: Implications in Carbon Neutrality through Genetically Engineered Plants

With the increasing challenges of climate change caused by global warming, the effective reduction of carbon dioxide (CO2) becomes an urgent environmental issue for the sustainable development of human society. Previous reports indicated increased biomass in genetically engineered (GE) Arabidopsis and rice overexpressing the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene, suggesting the possibility of consuming more carbon by GE plants. However, whether overexpressing the EPSPS gene in GE plants consumes more CO2 remains a question. To address this question, we measured expression of the EPSPS gene, intercellular CO2 concentration, photosynthetic ratios, and gene expression (RNA-seq and RT-qPCR) in GE (overexpression) and non-GE (normal expression) Arabidopsis and rice plants. Results showed substantially increased EPSPS expression accompanied with CO2 consumption in the GE Arabidopsis and rice plants. Furthermore, overexpressing the EPSPS gene affected carbon-fixation related biological pathways. We also confirmed significant upregulation of four key carbon-fixation associated genes, in addition to increased photosynthetic ratios, in all GE plants. Our finding of significantly enhanced carbon fixation in GE plants overexpressing the EPSPS transgene provides a novel strategy to reduce global CO2 for carbon neutrality by genetic engineering of plant species, in addition to increased plant production by enhanced photosynthesis.

India's biogeochemical capacity to attain food security and remediate climate

In order to supply wholesome food and slow down climate change, this paper covers India's agrogeological resources. The soils are the result of the weathering of rocks with ages ranging from more than a billion years to the most recent Holocene. Because they are severely deficient in vital minerals, many soils have low agricultural production. In addition to helping to fertilise soils, reduce atmospheric carbon dioxide levels, and stop the acidification of the Indian Ocean, rock powder weathering and biochar have significant positive effects on the productivity of Indian soils. The nutrient density of food is also increased which improves health and lowers the demand for and cost of medical treatment. Remineralization may help to solve Indian soil issues including soil infertility and texture. To improve soil and plant nutrition, dusts of carbonate, basic, and ultrabasic rocks are readily available at mining sites in India combined with biochar. Adding different grain sizes to the soil helps improve the texture of the soil. Silicate and carbonate rock powders enhance soil structure by promoting the creation of soil organic matter and fostering the growth of advantageous microbial communities. These processes offer a low-cost method of remineralizing soils with important macro- and micronutrients. For each significant soil/crop/climate system, an optimised application of India's rock powder resources must be determined through a national research and development programme. India's capacity to adapt to the mounting challenges of population expansion and climate change would be significantly improved by the findings of this study programme.

Decarbonization of Agriculture: The Greenhouse Gas Impacts and Economics of Existing and Emerging Climate-Smart Practices

The worldwide emphasis on reducing greenhouse gas (GHG) emissions has increased focus on the potential to mitigate emissions through climate-smart agricultural practices, including regenerative, digital, and controlled environment farming systems. The effectiveness of these solutions largely depends on their ability to address environmental concerns, generate economic returns, and meet supply chain needs. In this Review, we summarize the state of knowledge on the GHG impacts and profitability of these three existing and emerging farming systems. Although we find potential for CO2 mitigation in all three approaches (depending on site-specific and climatic factors), we point to the greater level of research covering the efficacy of regenerative and digital agriculture in tackling non-CO2 emissions (i.e., N2O and CH4), which account for the majority of agriculture's GHG footprint. Despite this greater research coverage, we still find significant methodological and data limitations in accounting for the major GHG fluxes of these practices, especially the lifetime CH4 footprint of more nascent climate-smart regenerative agriculture practices. Across the approaches explored, uncertainties remain about the overall efficacy and persistence of mitigation-particularly with respect to the offsetting of soil carbon sequestration gains by N2O emissions and the lifecycle emissions of controlled environment agriculture systems compared to traditional systems. We find that the economic feasibility of these practices is also system-specific, although regenerative agriculture is generally the most accessible climate-smart approach. Robust incentives (including carbon credit considerations), investments, and policy changes would make these practices more financially accessible to farmers.

Improving food security and farmland carbon sequestration in China through enhanced rock weathering: Field evidence and potential assessment in different humid regions

Enhanced rock weathering (ERW) in farmland is an emerging carbon dioxide removal technology with crushed silicate rocks for soil improvement. However, due to climatic variability and field data limitations, uncertainties remain regarding the influence of ERW on food security and soil carbon pools in temperate regions. This study focused to evaluate the crop productivity and carbon sequestration potential of farmland ERW in China by conducting field monitoring in different humid regions and ERW performance model. Additionally, the contribution of climate, soil, and management factors to ERW-mediated yield and carbon sequestration changes was explored using random forest and correlation networks. Field monitoring indicated that farmland ERW significantly improved crop yield in humid region (13.5 ± 5.2 %), along with notable improvements in soil pH and available nutrients. Precipitation (10.4-16.7 %) and soil pH (9.7-16.8 %) had the highest contribution on ERW mediated yield and carbon sequestration changes, but the contribution of management factors (24-26.2 %), especially N input (2.7-7.0 %), should not be disregarded. The model evaluation demonstrated that the carbon sequestration rate of farmland ERW in China can reach 0.28-0.40 Gt yr-1, thereby presenting an opportunity to expand and accelerate the nationally determined contributions of China. The mean sequestration cost of farmland ERW was 633 ± 161 CNY ¥ t-CO2-1, which was an attractive sequestration price considering the positive benefits of rock powder on soil pH and nutrients. Deploying ERW in acidified and mineral nutrient deficient regions was able to serve as an alternative to lime and part chemical fertilizers to improve yield and maximize agricultural sustainability and resource co-benefits. Farmland ERW also has the potential to resource silicate waste to assist traditional, difficult-to-decarbonize industries to reduce carbon emissions. As a result, a comprehensive assessment of existing artificial silicate waste materials could further expand the application of farmland ERW.

Initial Validation of a Soil-Based Mass-Balance Approach for Empirical Monitoring of Enhanced Rock Weathering Rates

Enhanced rock weathering (ERW) is a promising scalable and cost-effective carbon dioxide removal (CDR) strategy with significant environmental and agronomic co-benefits. A major barrier to large-scale implementation of ERW is a robust monitoring, reporting, and verification (MRV) framework. To successfully quantify the amount of carbon dioxide removed by ERW, MRV must be accurate, precise, and cost-effective. Here, we outline a mass-balance-based method in which analysis of the chemical composition of soil samples is used to track in situ silicate rock weathering. We show that signal-to-noise issues of in situ soil analysis can be mitigated by using isotope-dilution mass spectrometry to reduce analytical error. We implement a proof-of-concept experiment demonstrating the method in controlled mesocosms. In our experiment, a basalt rock feedstock is added to soil columns containing the cereal crop Sorghum bicolor at a rate equivalent to 50 t ha-1. Using our approach, we calculate rock weathering corresponding to an average initial CDR value of 1.44 ± 0.27 tCO2eq ha-1 from our experiments after 235 days, within error of an independent estimate calculated using conventional elemental budgeting of reaction products. Our method provides a robust time-integrated estimate of initial CDR, to feed into models that track and validate large-scale carbon removal through ERW.

Experimentally-calibrated estimation of CO2 removal potentials of enhanced weathering

Enhanced weathering (EW) of (ultra)mafic rocks is widely considered as a promising option for carbon dioxide removal (CDR). However, accurately measuring its CDR potential remains unavailable due to sluggish weathering process. Previous models have estimated annual CDR potentials ranging from 1 to 95 Gt by 2100, with the maximum significantly exceeding the anthropogenic CO2 emissions in 2021 (approximately 41 Gt). This raises concerns that a misconception may arise, suggesting active mitigations of CO2 emissions might not be necessary. Herein, we address this issue by partitioning the CDR potential of EW into two components, flow-through and non-flow-through processes, and develop an experimentally-calibrated model to reduce discrepancies between previous theoretical and experimental weathering rates. Our model estimates the upper bound of CDR potentials to be 0.22 (±0.16) Gt annually and 17 (±13) Gt cumulatively by 2100, thereby emphasizing the significance and urgency to advance ultra-enhanced weathering strategies.

Soil carbon sequestration in global working lands as a gateway for negative emission technologies

The ongoing climate crisis merits an urgent need to devise management approaches and new technologies to reduce atmospheric greenhouse gas concentrations (GHG) in the near term. However, each year that GHG concentrations continue to rise, pressure mounts to develop and deploy atmospheric CO2 removal pathways as a complement to, and not replacement for, emissions reductions. Soil carbon sequestration (SCS) practices in working lands provide a low-tech and cost-effective means for removing CO2 from the atmosphere while also delivering co-benefits to people and ecosystems. Our model estimates suggest that, assuming additive effects, the technical potential of combined SCS practices can provide 30%-70% of the carbon removal required by the Paris Climate Agreement if applied to 25%-50% of the available global land area, respectively. Atmospheric CO2 drawdown via SCS has the potential to last decades to centuries, although more research is needed to determine the long-term viability at scale and the durability of the carbon stored. Regardless of these research needs, we argue that SCS can at least serve as a bridging technology, reducing atmospheric CO2 in the short term while energy and transportation systems adapt to a low-C economy. Soil C sequestration in working lands holds promise as a climate change mitigation tool, but the current rate of implementation remains too slow to make significant progress toward global emissions goals by 2050. Outreach and education, methodology development for C offset registries, improved access to materials and supplies, and improved research networks are needed to accelerate the rate of SCS practice implementation. Herein, we present an argument for the immediate adoption of SCS practices in working lands and recommendations for improved implementation.

Potential contribution of chlorella vulgaris to carbon-nitrogen turnover in freshwater ecosystems after a great sandstorm event

Urban lakes represent important land-water and nature-human dual interfaces that promote the cycling of elements from terrestrials to sediments and consequently modulating the stabilization of regional climate. However, whether disturbances caused by extreme weather events can have substantial effects on carbon-nitrogen (C-N) cycling in these ecosystems are vague. To explore the impact of phytoplankton on the ecological retention time of C-N, two kinds of freshwater (natural and landscape) were collected and conducted a microcosm experiment using a freshwater algal species Chlorella vulgaris. Sandstorm events increased dissolved inorganic carbon in freshwater (65.55 ± 3.09 and 39.46 ± 2.51 mg·L-1 for samples from Jinyang and Nankai, respectively) and significantly affected the relevant pathways of photosynthesis in Chlorella vulgaris, including enhancing chlorophyll fluorescence (The effective quantum yield of PSII at the fifth day of incubation was 0.34 and 0.35 for Nankai and Jinyang, respectively), promoting the synthesis of sugars and inhibiting the synthesis of glycine and serine related proteins. Besides, carbon from plant biomass accumulation and cellular metabolism (fulvic acid-like, polyaromatic-type humic acid and polycarboxylate-type humic acid, etc.) was enriched into residues and become a kind of energy source for the decomposer (TC mass increased by 1.63-2.13 times after 21 days of incubation). This means that the accumulation and consumption of carbon and nitrogen in the residue can be used to track the processes controlling the long-term C-N cycle. Our findings shed light on the plant residues were key factors contributing to the formation of water carbon pool, breaks the traditional theory that dissolved carbonates cannot produce carbon sinks.

Techno-Economic and Life Cycle Assessment of Enhanced Rock Weathering: A Case Study from the Midwestern United States

Enhanced rock weathering (ERW) is a carbon dioxide removal (CDR) strategy for combating climate change. The CDR potentials of ERW have been assessed at the process and national/global levels, but the environmental and economic implications of ERW have not been fully quantified for U.S. applications with real-world supply chain considerations. This study develops an optimization-based, integrated life cycle assessment and techno-economic analysis framework for ERW, which is demonstrated by a case study applying mining waste to croplands in the Midwestern U.S. The case study explores maximum transportation distances for intermodal transportation at varied mineral CDR yields and costs, informing supply chain design for economically viable ERW. ERW costs (US$45 to 472/tonne of net CO2e captured) and cradle-to-farm gate GHG emissions (41 to 359 kg CO2e/tonne of CO2e captured) are estimated based on a range of CDR yields and by transportation distances to and from two Midwest port destinations: Chicago and Duluth. Our sensitivity analysis identifies CDR yields, and transportation modes and distances as driving factors for result variations. Our study reveals the importance of ERW supply chain design and provides an example of U.S. CDR implementation. Our framework and findings can be applied to other regional ERW projects.

A sustainable food future

The adverse environmental impacts of food production, the ill-health resulting from excess consumption and malnutrition, and the lack of resilience to the increasing number of threats to food availability show that the global system of food provision is not fit for purpose. Here, the causative flaws in the food system are identified and a framework presented for discovering the best ways to eliminate them. This framework is based upon an integrated view of the food system and the socio-economic systems in which it functions. The framework comprises an eight-point plan to describe the structure and functioning of the food system and to discover the optimum ways to bring about the changes needed to deliver a sustainable food future. The plan includes: priorities for research needed to provide options for change; an inclusive analytical methodology that uses the results of this research and incorporates acquisition, sharing and analysis of data; the need for actions at the local and national levels; and the requirements to overcome the barriers to change through education and international cooperation. The prospects for implementation of the plan and the required changes in the outcomes of the food system are discussed.

Constraining the Capacity of Global Croplands to CO2 Drawdown via Mineral Weathering

Terrestrial enhanced weathering of alkaline silicate minerals is a promising climate change mitigation strategy with the potential to limit the global temperature rise. The formation and accumulation of pedogenic carbonate and bicarbonate in soils/subsoils and groundwater offers a large sink for C storage; the amount of soil inorganic carbon (SIC) presently held within soils has been estimated to be 720-950 Gt of C. These values can be augmented by the addition of a variety of calcium and magnesium silicates via enhanced weathering. While the concept of the application of finely milled silicate rocks for faster weathering rates is well established, there has been limited discussion on the role of local climate, natural SIC content (i.e., the SIC innately present in the soil), and soil pH (among other important agronomic factors) on silicate weathering when applied to croplands, especially in view that the aim is to establish terrestrial enhanced weathering as a carbon dioxide removal (CDR) strategy on a global scale. In this work, we emphasized the importance of soil pH and soil temperature on silicate weathering and looked to estimate an upper limit of (i.e., constrain) the global capacity until the year 2100 for enhanced rock weathering (ERW) to draw down CO2 in the form of accumulated pedogenic carbonate or soluble bicarbonate. We assessed the global spatial distribution of cropland soil pH, which serves as a proxy for local innate SIC; annual rate of pluvial (rainfall) precipitation; and soil temperature, and found that the potential CO2 drawdown difference between faster and slower weathering silicates is narrower in Asia, Africa, and South America, while the gap is larger for Europe, North America, and Oceania.

Effects of nitrogen fertilizers on the bacterial community diversity and the weathering of purple mudstone in Southwest China

Introduction

Rock weathering is crucial in the development of soil. Yet the role of bacteria in the fine particle-forming process of purple mudstone is not fully understood, especially under nitrogen fertilization.

Methods

In this study, the particles (0.25 mm to 1 mm) of purple mudstone from Penglai Group (J₃p) were selected as the test material. Two nitrogen fertilizers, i.e., urea (U) and ammonium bicarbonate (AB), and four application levels (0, 280, 560, and 840 N kg∙ha^-1) with 18 replications were designed in an incubation experiment. The weathering indices and bacterial community structure of the purple mudstone particles were investigated after 120 days of incubation.

Results

The results showed that the weathering indices of purple mudstone particles in the AB treatment were higher than that in the U treatment at the same fertilization levels and a reducing trend was observed with increasing nitrogen fertilizer levels under the same nitrogen fertilizer application types. The diversities of the bacterial community were extremely significantly altered by nitrogen fertilizer application (p < 0.01). The effect of the nitrogen fertilizer application level on the beta diversity of the bacterial community (R² = 0.34) was greater than that of the nitrogen fertilizer application type (R² = 0.20). Through stepwise regression analysis, the positive effects of nitrification of Nitrobacter (Nitrolancea) (R² = 0.36), the Phosphorous-dissolving bacteria (Massilia) (R² = 0.12), and N-NO₃^- (R² = 0.35) on the weathering indices of J₃p purple mudstone particles could be observed. Structural equation modelling indicated that nitrogen fertilizer application level affects the abundance of the dominant species at the genus level (Nitrolancea and Massilia), and key environmental factor (N-NO₃^-), which in turn accelerated the weathering indices (59%).

Discussion and Conclusion

Our findings imply that the enhancements of nitrification of Nitrobacter (Nitrolancea) and of phosphorus solubilization of Phosphorous-dissolving bacteria (Massilia) by nitrogen fertilization are the key factors affecting the weathering indices of J₃p purple mudstone particles.

Do Southeast Asia's paleo-Antarctic trees cool the planet?

Many tree genera in the Malesian uplands have Southern Hemisphere origins, often supported by austral fossil records. Weathering the vast bedrock exposures in the everwet Malesian tropics may have consumed sufficient atmospheric CO₂ to contribute significantly to global cooling over the past 15 Myr. However, there has been no discussion of how the distinctive regional tree assemblages may have enhanced weathering and contributed to this process. We postulate that Gondwanan-sourced tree lineages that can dominate higher-elevation forests played an overlooked role in the Neogene CO₂ drawdown that led to the Ice Ages and the current, now-precarious climate state. Moreover, several historically abundant conifers in Araucariaceae and Podocarpaceae are likely to have made an outsized contribution through soil acidification that increases weathering. If the widespread destruction of Malesian lowland forests continues to spread into the uplands, the losses will threaten unique austral plant assemblages and, if our hypothesis is correct, a carbon sequestration engine that could contribute to cooler planetary conditions far into the future. Immediate effects include the spread of heat islands, significant losses of biomass carbon and forest-dependent biodiversity, erosion of watershed values, and the destruction of tens of millions of years of evolutionary history.

Will reshoring manufacturing of advanced electric vehicle battery support renewable energy transition and climate targets?

Recent global logistics and geopolitical challenges draw attention to the potential raw material shortages for electric vehicle (EV) batteries. Here, we analyze the long-term energy and sustainability prospects to ensure a secure and resilient midstream and downstream value chain for the U.S. EV battery market amid uncertain market expansion and evolving battery technologies. With current battery technologies, reshoring and ally-shoring the midstream and downstream EV battery manufacturing will reduce the carbon footprint by 15% and energy use by 5 to 7%. While next-generation cobalt-free battery technologies will achieve up to 27% carbon emission reduction, transitioning to 54% less carbon-intensive blade lithium iron phosphate may diminish the mitigation benefits of supply chain restructuring. Our findings underscore the importance of adopting nickel from secondary sources and nickel-rich ores. However, the advantages of restructuring the U.S. EV battery supply chain depend on projected battery technology advancements.

The environmental controls on efficiency of enhanced rock weathering in soils

Enhanced rock weathering (ERW) in soils is a promising carbon removal technology, but the realistically achievable efficiency, controlled primarily by in situ weathering rates of the applied rocks, is highly uncertain. Here we explored the impacts of coupled biogeochemical and transport processes and a set of primary environmental and operational controls, using forsterite as a proxy mineral in soils and a multiphase multi-component reactive transport model considering microbe-mediated reactions. For a onetime forsterite application of ~ 16 kg/m2, complete weathering within five years can be achieved, giving an equivalent carbon removal rate of ~ 2.3 kgCO2/m2/yr. However, the rate is highly variable based on site-specific conditions. We showed that the in situ weathering rate can be enhanced by conditions and operations that maintain high CO2 availability via effective transport of atmospheric CO2 (e.g. in well-drained soils) and/or sufficient biogenic CO2 supply (e.g. stimulated plant-microbe processes). Our results further highlight that the effect of increasing surface area on weathering rate can be significant-so that the energy penalty of reducing the grain size may be justified-only when CO2 supply is nonlimiting. Therefore, for ERW practices to be effective, siting and engineering design (e.g. optimal grain size) need to be co-optimized.

Geochemical carbon dioxide removal potential of Spain

Many countries have made pledges to reduce CO₂ emissions over the upcoming decades to meet the Paris Agreement targets of limiting warming to no >1.5 °C, aiming for net zero by mid-century. To achieve national reduction targets, there is a further need for CO₂ removal (CDR) approaches on a scale of millions of tonnes, necessitating a better understanding of feasible methods. One approach that is gaining attention is geochemical CDR, encompassing (1) in-situ injection of CO₂-rich gases into Ca and Mg-rich rocks for geological storage by mineral carbonation, (2) ex-situ ocean alkalinity enhancement, enhanced weathering and mineral carbonation of alkaline-rich materials, and (3) electrochemical separation processes. In this context, Spain may host a notionally high geochemical CDR capacity thanks to its varied geological setting, including extensive mafic-ultramafic and carbonate rocks. However, pilot schemes and large-scale strategies for CDR implementation are presently absent in-country, partly due to gaps in current knowledge and lack of attention paid by regulatory bodies. Here, we identify possible materials, localities and avenues for future geochemical CDR research and implementation strategies within Spain. This study highlights the kilotonne to million tonne scale CDR options for Spain over the rest of the century, with attention paid to chemically and mineralogically appropriate materials, suitable implementation sites and potential strategies that could be followed. Mafic, ultramafic and carbonate rocks, mine tailings, fly ashes, slag by-products, desalination brines and ceramic wastes hosted and produced in Spain are of key interest, with industrial, agricultural and coastal areas providing opportunities to launch pilot schemes. Though there are obstacles to reaching the maximum CDR potential, this study helps to identify focused targets that will facilitate overcoming such barriers. The CDR potential of Spain warrants dedicated investigations to achieve the highest possible CDR to make valuable contributions to national reduction targets.

New estimates of the storage permanence and ocean co-benefits of enhanced rock weathering

Avoiding many of the most severe consequences of anthropogenic climate change in the coming century will very likely require the development of "negative emissions technologies"-practices that lead to net carbon dioxide removal (CDR) from Earth's atmosphere. However, feedbacks within the carbon cycle place intrinsic limits on the long-term impact of CDR on atmospheric CO₂ that are likely to vary across CDR technologies in ways that are poorly constrained. Here, we use an ensemble of Earth system models to provide new insights into the efficiency of CDR through enhanced rock weathering (ERW) by explicitly quantifying long-term storage of carbon in the ocean during ERW relative to an equivalent modulated emissions scenario. We find that although the backflux of CO₂ to the atmosphere in the face of CDR is in all cases significant and time-varying, even for direct removal and underground storage, the leakage of initially captured carbon associated with ERW is well below that currently assumed. In addition, net alkalinity addition to the surface ocean from ERW leads to significant increases in seawater carbonate mineral saturation state relative to an equivalent emissions trajectory, a co-benefit for calcifying marine organisms. These results suggest that potential carbon leakage from the oceans during ERW is a small component of the overall ERW life cycle and that it can be rigorously quantified and incorporated into technoeconomic assessments of ERW at scale.

Inducing Inorganic Carbon Accrual in Subsoil through Biochar Application on Calcareous Topsoil

Biochar amendments add persistent organic carbon to soil and can stabilize rhizodeposits and existing soil organic carbon (SOC), but effects of biochar on subsoil carbon stocks have been overlooked. We quantified changes in soil inorganic carbon (SIC) and SOC to 2 m depth 10 years after biochar application to calcareous soil. The total soil carbon (i.e., existing SOC, SIC, and biochar-C) increased by 71, 182, and 210% for B30, B60, and B90, respectively. Biochar application at 30, 60, and 90 t ha^-1 rates significantly increased SIC by 10, 38, and 68 t ha^-1, respectively, with accumulation mainly occurring in the subsoil (below 1 m). This huge increase of SIC (mainly CaCO₃) is ∼100 times larger than the inorganic carbon present in the added biochar (0.3, 0.6, or 0.9 t ha^-1). The benzene polycarboxylic acid method showed that the biochar-amended soil contained more black carbon particles (6.8 times higher than control soil) in the depth of 1.4-1.6 m, which provided the direct quantitative evidence for biochar migration into subsoil after a decade. Spectral and energy spectrum analysis also showed an obvious biochar structure in the biochar-amended subsoil, accompanied by a Ca/Mg carbonate cluster, which provided further evidence for downward migration of biochar after a decade. To explain SIC accumulation in subsoil with biochar amendment, the interacting mechanisms are proposed: (1) biochar amendment significantly increases subsoil pH (0.3-0.5 units) 10 years after biochar application, thus forming a favorable pH environment in the subsoil to precipitate HCO₃^-; and (2) the transported biochar in subsoil can act as nuclei to precipitate SIC. Biochar amendment enhanced SIC by up to 80%; thus, the effects on carbon stocks in subsoil must be understood to inform strategies for carbon dioxide removal through biochar application. Our study provided critical knowledge on the impact of biochar application to topsoil on carbon stocks in subsoil in the long term.

How temperature-dependent silicate weathering acts as Earth's geological thermostat

Earth's climate may be stabilized over millennia by solubilization of atmospheric carbon dioxide (CO₂) as minerals weather, but the temperature sensitivity of this thermostat is poorly understood. We discovered that the temperature dependence of weathering expressed as an activation energy increases from laboratory to watershed as transport, clay precipitation, disaggregation, and fracturing increasingly couple to dissolution. A simple upscaling to the global system indicates that the temperature dependence decreases to ~22 kilojoules per mole because (i) the lack of runoff limits weathering and retains base metal cations on half the land surface and (ii) other landscapes are regolith-shielded and show little weathering response to temperature. By comparing weathering from laboratory to globe, we reconcile some aspects of kinetic and thermodynamic controls on CO₂ drawdown by natural or enhanced weathering.

Specific PhytOC fractions in rice straw and consequent implications for potential of phytolith carbon sequestration in global paddy fields

Phytoliths are silica biomineralization products within plants and have been considered as a promising material to sequester carbon (C). However, there is considerable uncertainty and controversy regarding the C content in phytoliths due to the lack of detailed information on variation of C under different extraction procedures. Herein, we established a series of batch digestion experimental procedures coupled with analyses of phytoliths using Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy to divide phytoliths into three fractions. We then reported an approach for standardizing across hundreds of values found in the literature. Combining this standardized approach with C contents in phytoliths extracted from different digestion degrees, we revaluated the potential production rates of phytolith-occluded carbon (PhytOC) input globally in rice paddy fields. The results showed that the C content in recovered phytoliths exhibited a significantly fitting exponential relationship (p < 0.01) with digestion degrees and decreased from 30 to 75 g kg^-1 under moderate digestion to <5 g kg^-1 under over digestion. On a global scale, the production of total PhytOC in the world paddy fields reached up to (2.71 ± 0.85) × 10⁶ t year^-1. Therein, the contribution of sub-stable PhytOC fraction, stable PhytOC fraction, and recalcitrant PhytOC fraction was 63 %, 28 %, and 9 %, respectively. Our results imply that the estimation of phytolith C sequestration potential across the global paddy fields is associated with specific PhytOC fractions. Therefore, further determining the storage time limits of these specific PhytOC fractions after returning to soil will be vital for predicting terrestrial biogeochemical C sequestration potentials of phytoliths.

Nano- to Global-Scale Uncertainties in Terrestrial Enhanced Weathering

Enhanced weathering (EW) is one of the most promising negative emissions technologies urgently needed to limit global warming to at least below 2 °C, a goal recently reaffirmed at the UN Global Climate Change conference (i.e., COP26). EW relies on the accelerated dissolution of crushed silicate rocks applied to soils and is considered a sustainable solution requiring limited technology. While EW has a high theoretical potential of sequestering CO₂, research is still needed to provide accurate estimates of carbon (C) sequestration when applying different silicate materials across distinct climates and major soil types in combination with a variety of plants. Here we elaborate on fundamental advances that must be addressed before EW can be extensively adopted. These include identifying the most suitable environmental conditions, improving estimates of field dissolution rates and efficacy of CO₂ removal, and identifying alternative sources of silicate materials to meet future EW demands. We conclude with considerations on the necessity of integrated modeling-experimental approaches to better coordinate future field experiments and measurements of CO₂ removal, as well as on the importance of seamlessly coordinating EW with cropland and forest management.

Deliberating enhanced weathering: Public frames, iconic ecosystems and the governance of carbon removal at scale

Meeting goals for 'net zero' emissions may require the removal of previously emitted carbon dioxide from the atmosphere. One proposal, enhanced rock weathering, aims to speed up the weathering processes of rocks by crushing them finely and spreading them on agricultural land. Public perceptions of enhanced rock weathering and its wider social and environmental implications will be a critical factor determining its potential; we use six 2-day deliberative workshops in England, Wales and Illinois to understand public views. Consideration of enhanced rock weathering deployment in tropical countries led participants to frame it from a social justice perspective, which had been much less prevalent when considering Western agricultural contexts, and generated assumptions of increased scale, which heightened concerns about detrimental social and environmental impacts. Risk perceptions relating to 'messing with nature' became amplified when participants considered enhanced rock weathering in relation to 'iconic' environments such as the oceans and rainforest.

Dissolution-After-Precipitation (DAP): a simple microfluidic approach for studying carbonate rock dissolution and multiphase reactive transport mechanisms

We propose a simple microfluidic approach: Dissolution-After-Precipitation (DAP), to investigate regimes of carbonate rock dissolution and multiphase reactive transport. In this method, a carbonate porous medium is created in a glass microchannel via calcium carbonate precipitation, after which an acid is injected into the channel to dissolve the precipitated porous medium. Utilizing the DAP method, for the first time we realized all five classical single-phase carbonate rock dissolution regimes (uniform, compact, conical, wormhole, ramified wormholes) in a microfluidic chip. The results are validated against the established theoretical dissolution diagram, which shows good agreement. Detailed analysis of these single-phase dissolutions suggests that the heterogeneity of the porous medium may significantly impact how the dissolution patterns evolve over time. Furthermore, DAP is utilized to investigate multiphase dissolution. As examples we tested the cases of an oleic phase (tetradecane) and a gaseous phase (CO₂). Results show that the presence of a nonaqueous phase in pore spaces induces the formation of wormholes despite weak advection, and these wormholes ultimately become pathways for nonaqueous phase transport. However, the transport of tetradecane in the wormhole is very slow, causing acid breakthrough into neighboring regions. This mechanism enhances lateral connectivity between wormholes and may lead to a wormhole network. In contrast, CO₂ moves rapidly and continuously seeks to enter a widening wormhole from a narrower wormhole or the porous regions, generating phenomena such as ganglia redistribution and counterflow (flow of gas opposite to acid flow). Extensive independent experiments are conducted to verify the reproducibility of the observed phenomena/mechanisms and further analyze them. Real-time monitoring of fluid pressure drop during dissolution is implemented to complement microscopy image analysis. Our method can be implemented repeatedly on the same chip, which offers a convenient and inexpensive option to study pore-scale reactive transport mechanisms.

The Effects of High CO2 and Strigolactones on Shoot Branching and Aphid-Plant Compatibility Control in Pea

Elevated atmospheric CO2 concentrations (eCO2) regulate plant architecture and susceptibility to insects. We explored the mechanisms underpinning these responses in wild type (WT) peas and mutants defective in either strigolactone (SL) synthesis or signaling. All genotypes had increased shoot height and branching, dry weights and carbohydrate levels under eCO2, demonstrating that SLs are not required for shoot acclimation to eCO2. Since shoot levels of jasmonic acid (JA) and salicylic acid (SA) tended to be lower in SL signaling mutants than the WT under ambient conditions, we compared pea aphid performance on these lines under both CO2 conditions. Aphid fecundity was increased in the SL mutants compared to the WT under both ambient and eCO2 conditions. Aphid infestation significantly decreased levels of JA, isopentenyladenine, trans-zeatin and gibberellin A4 and increased ethylene precursor ACC, gibberellin A1, gibberellic acid (GA3) and SA accumulation in all lines. However, GA3 levels were increased less in the SL signaling mutants than the WT. These studies provide new insights into phytohormone responses in this specific aphid/host interaction and suggest that SLs and gibberellins are part of the network of phytohormones that participate in host susceptibility.

The Carbon-Capture Efficiency of Natural Water Alkalinization: Implications For Enhanced weathering

Enhanced weathering (EW) is a promising negative-emission technology that artificially accelerates the dissolution of natural minerals, promotes biomass growth, and alleviates the acidification of soils and natural waters. EW aims to increase the alkalinity of natural waters (alkalinization) to promote a transfer of CO₂ from the atmosphere to the water. Here we provide a quantification of the alkalinization carbon-capture efficiency (ACE) as a function of the water chemistry. ACE can be used for any alkaline mineral in various natural waters. We show that ACE strongly depends on the water pH, with a sharp transition from minimum to maximum in a narrow interval of pH values. We also quantify ACE in three compartments of the land-to-ocean aquatic continuum: the world topsoils, the lakes of an acid-sensitive area, and the global surface ocean. The results reveal that the efficiency of terrestrial EW varies markedly, from 0 to 100 %, with a significant trade-off in acidic conditions between carbon-capture efficiency and enhanced chemical dissolution. The efficiency is more stable in the ocean, with a typical value of around 80 % and a latitudinal pattern driven by differences in seawater temperature and salinity. Our results point to the importance of an integrated hydrological and biogeochemical theory to assess the fate of the weathering products across the aquatic continuum from land to ocean.

Environmental Galenics: large-scale fortification of extant microbiomes with engineered bioremediation agents

Contemporary synthetic biology-based biotechnologies are generating tools and strategies for reprogramming genomes for specific purposes, including improvement and/or creation of microbial processes for tackling climate change. While such activities typically work well at a laboratory or bioreactor scale, the challenge of their extensive delivery to multiple spatio-temporal dimensions has hardly been tackled thus far. This state of affairs creates a research niche for what could be called Environmental Galenics (EG), i.e. the science and technology of releasing designed biological agents into deteriorated ecosystems for the sake of their safe and effective recovery. Such endeavour asks not just for an optimal performance of the biological activity at stake, but also the material form and formulation of the agents, their propagation and their interplay with the physico-chemical scenario where they are expected to perform. EG also encompasses adopting available physical carriers of microorganisms and channels of horizontal gene transfer as potential paths for spreading beneficial activities through environmental microbiomes. While some of these propositions may sound unsettling to anti-genetically modified organisms sensitivities, they may also fall under the tag of TINA (there is no alternative) technologies in the cases where a mere reduction of emissions will not help the revitalization of irreversibly lost ecosystems.

This article is part of the theme issue ‘Ecological complexity and the biosphere: the next 30 years’.

Lake metabolic processes and their effects on the carbonate weathering CO2 sink: Insights from diel variations in the hydrochemistry of a typical karst lake in SW China

The precipitation of carbonate minerals does not invariably result in CO₂ emission to the atmosphere, because dissolved inorganic carbon (DIC) can be partially utilized by terrestrial aquatic phototrophs, thus generating an autochthonous organic carbon (AOC) sink. However, little is known about the potential effects of this mechanism on carbon cycles in DIC-rich lakes, mainly due to the lack of detailed documentation of the related processes, which limits our ability to accurately evaluate and predict the magnitude of this carbon sink. We conducted field observations in Fuxian Lake, a large and representative karst lake in the Yunnan-Guizhou Plateau, SW China. Continuous diel monitoring was conducted to quantitatively assess the coupled relationship between lake metabolism and DIC cycling and its influence on the carbonate weathering-related CO₂ sink. We found that the diel physicochemical variations and isotopic characteristics were mainly controlled by the metabolism of aquatic phototrophs, evidenced by a significant relationship between net ecosystem production and diel DIC cycling, and demonstrating the significance of DIC fertilization in supporting high primary production in karst lakes. The data showed that a reduction in photosynthesis occurred in the afternoon of almost every day, which can be explained by the lower CO₂/O₂ ratio that increased the potential for the photorespiration of aquatic plants, thus reducing photosynthesis. We found that a net autotrophic ecosystem prevailed in Fuxian Lake, suggesting that the lake functions more as a sink than a source of atmospheric CO₂. Considering carbonate weathering, the estimated AOC sink amounted to 650-704 t C km^-2 yr^-1, demonstrating both the potentially significant role of metabolism in lacustrine carbon cycling and the potential of the combination of photosynthesis and carbonate weathering for carbon sequestration. Our findings may help to quantitatively estimate the future impact of lake metabolism on carbon cycling, with implications for formulating management policies needed to regulate the magnitude of this carbon sink.

Carbon mineralization with concurrent critical metal recovery from olivine

Carbon dioxide utilization for enhanced metal recovery (EMR) during mineralization has been recently developed as part of CCUS (carbon capture, utilization, and storage). This paper describes fundamental studies on integrating CO₂ mineralization and concurrent selective metal extraction from natural olivine. Nearly 90% of nickel and cobalt extraction and mineral carbonation efficiency are achieved in a highly selective, single-step process. Direct aqueous mineral carbonation releases Ni²⁺ and Co²⁺ into aqueous solution for subsequent recovery, while Mg²⁺ and Fe²⁺ simultaneously convert to stable mineral carbonates for permanent CO₂ storage. This integrated process can be completed in neutral aqueous solution. Introduction of a metal-complexing ligand during mineral carbonation aids the highly selective extraction of Ni and Co over Fe and Mg. The ligand must have higher stability for Ni-/Co- complex ions compared with the Fe(II)-/Mg- complex ions and divalent metal carbonates. This single-step process with a suitable metal-complexing ligand is robust and utilizes carbonation processes under various kinetic regimes. This fundamental study provides a framework for further development and successful application of direct aqueous mineral carbonation with concurrent EMR. The enhanced metal extraction and CO₂ mineralization process may have implications for the clean energy transition, CO₂ storage and utilization, and development of new critical metal resources.