Introduction

Atmospheric carbon dioxide (CO2) concentration has increased from 280 ppm during pre-industrial times to 425 ppm currently, which intensified global warming1,2. Soil is the large terrestrial surface carbon pool on the earth, accounting for 25% of the global CO2 exchange, which is of great significance in the global CO2 cycle3. Soil receives carbon through plant root growth and litter from the canopy and also achieve CO2 sequestration through inorganic abiotic processes4. When CO2 is dissolved in soil solution, the concentration of soil gas changes, leading to a change in the atmospheric pressure gradient within the soil, which, in turn, pumps atmospheric CO2 into the soil5. Xie et al. (2009)6 showed that alkaline soil on land absorbs CO2 at a rate of 0.3–3.0 μmol m−2 s−1 with an inorganic, non-biological process. Four possible inorganic abiotic processes has been proposed: (1) variation in volume of gases caused by changes in pressure and temperature governed by the ideal gas law; (2) change in solubility of CO2 governed by Henry’s Law; (3) pH-mediated CO2 dissolution chemistry; and (4) surface adhesion of CO2 onto soil minerals3,7. In these processes, mineral carbonatization, the conversion of CO2 to carbonate minerals, via CO2-fluid-mineral reactions could minimize the risk of re-leakage and thus facilitates long-term and safe carbon storage8. However, the potential for carbonation in partial CO2 reservoirs is limited due to the lack of minerals rich in calcium, magnesium, etc., which are required for the formation of carbonate minerals1,9,10.

Biochar technology based on pyrolysis of biomass followed by storage of the formed biochar in soil has been suggested as one notable carbon sink channel for reducing the atmospheric CO211,12. It has been reported that the biochar technology could deliver emission reductions of 3.4~6.3 Pg CO2 annually and persist in soil for hundreds to thousands of years11,13. This biochar carbon sequestration technology also features multiple non-climate benefits, such as crop yield enhancement, soil amelioration, and contamination control13,14. Hence, such a measure has aroused intense attention over the past two decades since it was brought up by Lehmann in 200715. Biochar is characteristic of abundant minerals, high pores, and high alkalinity in addition to rich carbon16,17,18. When the biochar is introduced into soil for carbon sequestration, the soil properties such as pH, CEC, and minerals could be changed, which may affect the sorption of CO2 by soil. However, whether the biochar-amended soil can further sorb CO2 remain greatly unknown.

Biochar is made from various of biomass sources such as agricultural and forestry residues, livestock manure, and municipal solid waste19. Among them, agricultural and forestry waste derived biochar has the highest carbon content and relatively greater potential for carbon stored in the soil20. As one of the agricultural wastes, wheat straw has a high yield and sustainable production. Turning these large amounts of wheat straw into biochar can achieve greater benefits for soil carbon sequestration. The carbon sequestration potential of biochar can be further improved through functionalization21. Mineral salts (Ca, Mg, Fe, etc.) have been shown to enhance carbon retention during biomass pyrolysis and can also form mineral protection on biochar surfaces against biological/abiotic mineralized decomposition for long term carbon sequestration in soil22. More importantly, such functional modification can also play a full role in the sorption properties of biochar23,24, and may further affect CO2 sorption of soil. However, the potential of CO2 sorption depends not only on the characteristics of biochar, but also on the soil properties. The soil acidity is one of the major properties that regulates various biogeochemical processes in soil25, and also seriously affects the dissolution and subsequent reaction of CO2 at normal temperature and pressure. Therefore, it is necessary to study the CO2 sorption after biochar is amended into soil with different acidities.

Hence, this study was aimed to investigate the potential of biochar-C sequestration in soil and the ability of the biochar-amended soil for further sorption of atmospheric CO2. Four biochar samples, original wheat straw biochar (BC) and modified by calcium (CBC), magnesium (MBC), and iron (FBC) were considered. Two typical soils, acid clay loam soil (CL) and alkaline sandy loam soil (SL), were tested. The overall objective of this work was to elucidate the influence of biochar amended on soil CO2 sorption capacity and the underlying CO2 sorption mechanism. It is expected that this study would provide supporting information for a comprehensive evaluation of the carbon sequestration potential of biochar stored in soil and give some recommendations to soil or geological storage of atmospheric CO2.

Results and Discussion

Kinetics of CO2 sorption by biochar-amended soils

The sorption of CO2 by clay loam (CL) soil and sandy loam (SL) soil alone were fast within the first 20 min and further reached the maximum sorption plateau at about 9.54 mg g−1 and 4.80 mg g−1, respectively (Fig. 1a, b). Theoretically, alkaline environment in SL soil (pH = 9.33) could be more favorable to acidic CO2 molecule sorption than acidic condition in CL soil (pH = 4.81), via chemical carbonate formation. However, the contrary results were observed, which was ascribed to the difference in specific surface area (SSA) of soils. The CL soil had higher SSA than SL soil (Table 1), providing more sorption sites than SL soil, which determined the higher CO2 sorption capacity by CL soil than that by SL soil26.

Fig. 1: Sorption kinetics and total experimental/calculated values of CO2 by biochar amended soils.
figure 1

a Sorption kinetics of CO2 by biochar amended clay loam (CL) soil. b Sorption kinetics of CO2 by biochar amended sandy loam (SL) soil. c Experimental values and calculated values of CO2 sorption by different biochar-amended clay loam (CL) soil. d Experimental values and calculated values of CO2 sorption by different biochar-amended sandy loam (SL) soil. The black virtual slash lines in (c, d) are 1:1 line. The experimental value was the real CO2 sorption capacity after 100 min, the calculated value was the sum of single biochar and single soil alone, correspondingly, according to the mixture ratio. BC, CBC, MBC, and FBC represent pristine and Ca-, Mg-, and Fe-enriched wheat straw biochar, respectively. The error bars are standard deviations of three replicates.

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Table 1 Basic chemical and physical properties of two soils and the biochar-amended soils
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In the biochar-amended CL and SL soil, the CO2 sorption greatly increased and reached to 11.8–16.7 mg g−1 and 5.80–12.7 mg g−1, respectively, with 20.8%–75.0% and 23.6%–164% increase relative to CL and SL soils alone (Fig. 1a, b). The results suggested that the input of biochar promoted the soils for CO2 sorption. Meanwhile, the more increase of CO2 sorption capacity by SL after biochar application was due to the high pH and more increased SSA (Table 1) in the SL soils, which help more CO2 dissolution and sorption in soil. Further observation found that the CO2 sorption capacity in the biochar-amended soils varied with the different biochar, Ca-rich biochar (CBC)-amended soil demonstrated the highest sorption followed by Mg-rich biochar (MBC)>Fe-rich biochar (FBC) ~ BC alone. The similar trends for the CO2 sorption capacity were observed in biochar alone (Supplementary Fig. 1). It means that biochar played a crucial role in determining the sorption of CO2 by biochar-amended soil. Obviously, the SSA is not a key factor for CO2 sorption since the soil amended by the MBC with the highest SSA did not show the highest sorption of CO2 (Table 1 and Fig. 1a, b). The CBC contained high Ca-bearing minerals (Supplementary Fig. 2), which was conducive to the surface adhesion of CO2 after its amendment in soil. Parsons et al. (2004)27 also noted that the Ca-mineral-rich basalt in dry valley soils could contribute to the high uptake of CO2 through the Ca carbonate process. Although the FBC has a larger SSA and micropore volume than pristine BC, preferring to sorption of CO2 (Supplementary Table 1), pH decrease in the FBC amended soil (Table 1) is not favorable for the sorption and dissolution of CO218, therefore, the CO2 sorption by FBC amended soil was comparable to the BC amended soil (Fig. 1).

Further, we compared the actual experimental values with the calculation results. The calculation results were obtained by summing the CO2 sorption capacity of biochar alone (Supplementary Fig. 1) and soil alone (Fig. 1) according to the mixing ratio (5% wbiochar:wsoil) of biochar and soil. The experimental results agreed well with the calculated one for most soils, except for CBC-amended soil (Fig. 1c, d). The sorption capacity of CO2 by CBC-amended soil was much lower in the experiment than that in the calculation. This is because CBC not only has the strongest CO2 sorption capacity, but also can continuously increase in a relatively long time, compared with other biochar (Supplementary Fig. 1). However, after biochar enters the soil, it can quickly interact with soil components28, fill or wrap on the surface of biochar (Fig. 2), thereby inhibiting the continuous increase of CO2 sorption by biochar, making the experimental value of CO2 sorption by CBC-amended soil smaller than the calculated value (Fig. 1). Despite the influence of soil minerals, CBC-amended soil can still have the highest CO2 sorption capacity (Fig. 1). Of course, the application of other biochar also affected by the wrapping of soil minerals (Fig. 2), but their sorption can quickly reach a plateau (Supplementary Fig. 1), so the influence of soil mineral wrapping may not be significant for total CO2 sorption. The differences between calculated and experimental results suggested the occurrence of interactions between biochar and soil, which affected CO2 adsorption. We further performed batch equilibrium experiments to explore the mechanism for CO2 sorption.

Fig. 2: Surface morphology and elements distribution of biochar particles extracted from the amended soils after CO2 sorption.
figure 2

a Ca-enriched biochar (CBC) particle amended in clay loam soil (CL). b Ca-enriched biochar (CBC) particle amended in sandy loam soil (SL). c Mg-enriched biochar (MBC) particle amended in clay loam soil (CL). d Mg-enriched biochar (MBC) particle amended in sandy loam soil (SL). The white short lines in (a, c) are on a 50μm scale, and the white short lines in (b, d) are on a 25μm scale. C, O, Ca, Mg, Si, Al, and Fe diagrams represent the distribution of carbon, oxygen, calcium, magnesium, silicon, aluminum, and iron on the surface of biochar particles, respectively.

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Change in carbon species after CO2 sorption by biochar-amended soils

Input of biochar rapidly increased the total carbon (TC) in both CL and SL soils within 24 h from 1.28% to 3.46–5.00% and from 1. 52% to 3.93–4.71%, respectively (Fig. 3a, b). Weng et al. (2022)29 showed that the application of Eucalyptus saligna biochar raised the soil C storage by 9.3 Mg new C ha−1 after 8.2 years, and the second application raised this by another 2.3 Mg new C ha−1 after 1.3 years. Therefore, our results again demonstrated biochar input could achieve carbon sequestration. It should be noted that under the same amendment ratio, the BC-amended soil had higher TC content than the mineral-rich biochar (CBC, MBC, FBC) amended soil, which was related to the TC content of biochar itself. The mineral-rich biochar has less carbon than the pristine biochar (Supplementary Table 1).

Fig. 3: Total carbon, dissolved organic and inorganic carbon contents in biochar-amended soils before and after CO2 sorption.
figure 3

a Total carbon contents of biochar amended in clay loam soil (CL). b Total carbon contents of biochar amended in sandy loam soil (SL). c Dissolved organic and inorganic carbon contents of biochar amended in clay loam soil (CL). d Dissolved organic and inorganic carbon contents of biochar amended in sandy loam soil (SL). In all the columns, the left column shows the total carbon or dissolved carbon content before CO2 sorption, and the right column shows the content after CO2 sorption. BC, CBC, MBC, and FBC represent pristine and Ca-, Mg-, and Fe-enriched wheat straw biochar, respectively. The error bars are standard deviations of three replicates.

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The changes in TC of biochar-amended soil before and after CO2 sorption were not obvious due to the relatively low ratio of sorbed CO2-C (0.27~0.44%) compared with total carbon contents (1.35~5.00%) (Fig. 3a, b). The obvious increase in the soluble inorganic carbon in CBC and MBC-amended soil (Fig. 3c, d) confirmed the carbonation process during CO2 sorption. The soluble inorganic carbon is an essential parameter of the carbon exchange process involving CO2 flux in soils, especially in saline soil5. Fa et al. (2016)26 also found that a large amount of atmospheric CO2 absorbed by the soil was converted into soluble inorganic carbon in the soil liquid phase used 13C isotope tracing technology.

Overall, biochar sequestration could increase soil carbon storage while enhancing the capture of atmospheric CO2, especially in CBC and MBC-soils. To explore the possible interaction among CO2, soil, and biochar after CO2 sorption, we extracted the biochar from the amended soil for characterization. The FTIR spectra showed appearance of several functional groups of soil minerals in the extracted biochar, compared to biochar itself that was not co-cultured with soil (Fig. 4). Among them, O-Si-O stretching of silicates at 1100–1000 cm−1 30,31, AlMgOH groups related to smectite minerals at 900–800 cm−1 32, and Al–O–Si or Si–O–Si bending vibrations at 600–500 cm−1 30 were detected in extracted biochar, which were not observed in the biochar itself. The presence of these soil mineral components indicated that they could exist on the biochar surface or enter inside of the biochar pores through functional group bonding or pore blockage, and thus be detected by FTIR. Results from SEM-EDS diagrams also displayed that many aluminosilicates (Si-O-Al) occupied the surface and inside of both CBC and MBC (Fig. 2), confirming the soil mineral diffusion to biochar. Yang et al. (2016)33 indicated that biochar could form organic-mineral complexes with soil minerals, especially with the soluble inorganic salts and/or aluminosilicates, through functional group bonding or pore blockage. These mineral complexes are favorable for biochar stability and thus for long-term carbon sequestration22,33.

Fig. 4: Fourier Transform Infrared Spectroscopy (FTIR) spectra of biochar alone and the biochar extracted from the amended soils after CO2 sorption.
figure 4

a–d represent FTIR spectra of pristine biochar (BC), Ca-enriched biochar (CBC), Mg-enriched biochar (MBC), and Fe-enriched biochar (FBC) extracted from the amended clay loam soil (CL) and sandy loam soil (SL), respectively. The black line at the top of each chart is the control biochar that was not placed in the soil.

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In particular, a strong carbonate peak at 1440 cm−1 30,34 was observed in the biochar from the amended soils after CO2 sorption (Fig. 4). Apparent CO32− peaks in CBC and MBC from the amended soil could be likely resulted from carbonate minerals the reaction of CO2 with Ca or Mg in biochar. More C-Ca and C-Mg associations were observed in the CBC (Fig. 2a, b) and MBC (Fig. 2c, d) extracted from soil, which further confirmed the carbonation reaction after CO2 sorption.

Mechanisms of CO2 sorption by biochar-amended soils

The results and discussion shown above indicated that the biochar itself in the biochar-amended soil played a key role in the sorption of CO2. Therefore, a pure sorption experiment was conducted on the biochar alone to further explore the process mechanism of CO2 sorption after its biochar is stored in soil.

Total C in biochar increased after CO2 sorption, especially in CBC (p = 0.007) and MBC (p = 0.031) (Supplementary Table 1). The XRD patterns of BC and FBC were basically unchanged after CO2 sorption (Supplementary Fig. 2), further indicating that the main CO2 sorption mechanism in the two biochar was physical adsorption. Although iron minerals were rich in FBC, the more acidic environmental condition (Supplementary Table 1) was not favorable for the CO2 chemisorption. Therefore, BC and FBC sequestration could not improve soil CO2 sorption capacity as much as other biochar.

However, CBC and MBC biochar not only provided more microzone alkaline environments, but also carried much Ca and Mg minerals into soil (Table 1), which might enhance the rapid formation of carbonate nodules5,10. Researches indicated that “alkaline” silicate minerals containing higher levels of Ca and Mg, such as olivine, serpentine, pyroxene, and plagioclase, have the greatest potential for fixing CO2 into carbonate minerals due to their high molar ratio of divalent cations, allowing them to rapidly react and form carbonate minerals1. In this study, the CBC after sorption of CO2 showed the formation of carbonate minerals (Supplementary Fig. 2). It could be that CaCl2·Ca(OH)2·H2O and CaClOH originally in CBC were dissolved and reacted with CO2 to form K2Ca(CO3)2 and poorly soluble mineral (Ca6(CO3)2(OH)7Cl) following Eqs. (1 and 2) 35. Therefore, after CBC sorbed CO2, it was mainly reflected in increase in the form of soluble and insoluble inorganic carbon (Supplementary Table 1), which was also revealed in CBC-amended soils (Fig. 3).

$$2{{\rm{CaClOH}}}+2{{{{\rm{HCO}}}}_{3}}^{-}+2{{{\rm{K}}}}^{+}\to {{{\rm{K}}}}_{2}{{\rm{Ca}}}{({{{\rm{CO}}}}_{3})}_{2}+{{{\rm{CaCl}}}}_{2}\cdot 2{{{\rm{H}}}}_{2}{{\rm{O}}}$$
(1)
$$3({{{\rm{CaCl}}}}_{2}\cdot {{\rm{Ca}}}{({{\rm{OH}}})}_{2}\,{{{\rm{H}}}}_{2}{{\rm{O}}})+2{{{{\rm{HCO}}}}_{3}}^{-}\to {{{\rm{Ca}}}}_{6}{({{{\rm{CO}}}}_{3})}_{2}{({{\rm{OH}}})}_{7}{{\rm{Cl}}}$$
(2)

MgO and Mg(OH)2 minerals in MBC after CO2 sorption were transformed into MgCO3·3H2O with a small amount of Mg0.03Ca0.97CO3, which might follow Eqs. (3–5) 36,37. Meanwhile, the generation of inorganic carbonate, such as HCO3− and CO32−, following Eqs. (3–5) resulted in a large increase of inorganic carbon after CO2 sorption by MBC (Supplementary Fig. 2b–d).

$${{\rm{MgO}}}+{{{\rm{CO}}}}_{2}\to {{{\rm{MgCO}}}}_{3}$$
(3)
$${{\rm{Mg}}}{({{\rm{OH}}})}_{2}+{{{\rm{CO}}}}_{2}+{{{\rm{H}}}}_{2}{{\rm{O}}}\to {{{\rm{MgCO}}}}_{3}\cdot 3{{{\rm{H}}}}_{2}{{\rm{O}}}$$
(4)
$${{{\rm{Mg}}}}^{2+}+{{{\rm{CaCO}}}}_{3}\to ({{{\rm{Mg}}}}_{0.03}{{{\rm{Ca}}}}_{0.97})({{{\rm{CO}}}}_{3})$$
(5)

FTIR analysis showed the obvious formation of CO32− antisymmetric stretching absorption in the range of 1400–1500 cm−1 38 and CO32− out-of-plane bending vibration at 874 cm−1 39 in CBC and MBC after CO2 sorption, especially in MBC (Supplementary Fig. 3). It further evidenced the carbonation reaction after CO2 sorption. The TGA-MS analysis showed that more CO2 was released from CBC after adsorbing CO2 in the range of 600–950°C (Supplementary Fig. 4a). Such temperature range was considered to be the decomposition of calcium carbonate and calcium magnesium carbonate40, which also indirectly proved that there was the production of calcium carbonate-like minerals after CBC adsorbed CO2. Similarly, more CO2 was released at about 350°C from MBC adsorbed CO2 (Supplementary Fig. 4b), and this temperature was considered as the decomposition temperature of magnesium carbonate41, which was consistent with the XRD results of MBC (Fig. 4c).

Therefore, the sequestration of biochar can increase soil CO2 sorption and convert it into carbonate minerals by providing a micro-domain alkaline environment and introducing more alkaline minerals such as Ca/Mg, thus sequestering more atmospheric CO2.

Country-level calculation of CO2 storage by biochar-amended soils

The two soils of CL and SL can absorb CO2 9.54 mgCO2 gsoil−1 and 4.80 mgCO2 gsoil−1, respectively (Fig. 1). After biochar application (5% mass ratio), BC, CBC, MBC, and FBC could further increase soil CO2 sorption capacities by 1.00–2.26, 7.16–7.90, 2.16–4.26, and 1.94–2.36 mgCO2 gsoil−1, respectively (Fig. 1). Importantly, biochar sequestration also increased soil carbon stocks by 32.0–37.2, 21.9–24.2, 26.8–32.7, and 29.6–34.1 mgC gsoil−1 by BC, CBC, MBC, and FBC, respectively (Fig. 3). Based on the average soil density of 1.32 g cm−3 42 and typical application depth (0–20 cm)43 of biochar, the input of biochar in cultivated land (1,276,000 square kilometers, China Statistical Yearbook) in China countrywide could increase the C sequestration by 7.38–12.5 billion tons (Fig. 5), which is equivalent to the reduction of atmospheric CO2 by 27.1–45.8 billion tons. In addition, the application of biochar could increase soil CO2 sorption by about 0.34–2.66 billion tons (Fig. 5). The sum of the biochar sequestration and extra CO2 sorption could reduce 27.4–48.5 billion tons CO2 from atmosphere. It should be pointed out that when we measured the CO2 sorption capacity of biochar-amended soil, the selection of dry soil may expose more pores to facilitate CO2 sorption. However, in the actual process of agricultural practice, frequent tillage will also repeatedly expose the pores of the topsoil in cultivated land. In addition, if there was a certain amount of water, it would also change after agricultural activities such as tilling. Therefore, the selection of dry soils allows us to better assess the maximum potential of biochar-amended soil for CO2 sorption. Besides, due to the diversity of soil and biochar properties, there are certain limitations in our estimation of carbon sequestration in biochar-amended soil. Our results illustrated that biochar could rapidly increase soil carbon storage while achieve more CO2 uptake in short-term. Over time, biochar carbon also decomposes under the action of microorganisms and produces a certain loss, but this loss rate is very slow28. At the same time, long-term practice results also found that although there is a certain amount of biochar carbon decomposition, this is also accompanied by the stabilizing of rhizodeposits carbon to enhance soil organic carbon ceiling and offset the loss of biochar carbon29. Studies have also shown that biochar practice could help the demand of the negative CO2 emission in most mitigation scenarios compatible with China’s target of carbon neutrality by 206044. Hence, our results could still provide support for a comprehensive assessment of the carbon sequestration potential of biochar in soil. At the same time, it can also provide suggestions and references for the formulation of national carbon sequestration measures.

Fig. 5: Carbon sequestration and enhanced CO2 sorption potential of biochar in cultivated soil in China country wide.
figure 5

BC, CBC, MBC, and FBC represent pristine and Ca-, Mg-, and Fe-enriched wheat straw biochar, respectively.

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Conclusions

This study revealed that biochar input increased soil carbon storage while enhancing CO2 capture. The minerals enriched in biochar play an important role in the chemical sorption of CO2 in the soil. The input of Mg-biochar resulted in the high specific surface area of soil, which enhanced its physical adsorption. Besides, MgO and Mg(OH)2 present in Mg-biochar could also reacted with CO2 and eventually transformed into carbonate minerals in the Mg-biochar sequestrated soil. The Ca-biochar amended soil did not increase specific surface area like MBC. However, Ca(OH)2 and CaClOH in Ca-biochar can react with CO2 via carbonation process, and were mainly retained as inorganic carbonate minerals. The results from this study indicated that biochar-amended soil could allow extra sorption of atmospheric CO2 and Ca or Mg alkaline-earth metal mineral in biochar production significantly improved the CO2 sorption capacity of soil. Therefore, biochar input increased soil C storage and meanwhile further sorb atmospheric CO2. If such extra CO2 uptake is not considered, then the carbon sequestration potential of soil biochar will be underestimated. Therefore, both biochar C sequestration and extra atmospheric CO2 sorption should be taken into consideration for evaluating overall carbon sequestration potential of biochar-amended soil. By the national prediction, this biochar practice could help in the mitigation of CO2 emission, making contribution to the implementation of China’s target of carbon neutrality by 2060.

Materials and methods

Collection and characterization of soil

Two typical soils, clay loam (CL) soil and sandy loam (SL) soil, were sampled from surface 0–20 cm soil layer in Hainan Province and Shaanxi Province, China, respectively. The collected soil was air-dried, crushed, and passed through a 2-mm sieve. Then, the basic physiochemical properties such as pH, soil texture, surface area, and major metal compositions of soils were characterized according to the previous study45 and results were displayed in Table 1.

Preparation and characterization of biochar

Wheat straw was collected from a farm field in Anqing, Anhui province, China. The straw was cut into a small segment of about 1 cm, washed 3 times with ultra-pure water and then dried at 105 °C. Later, the wheat straw was impregnated into H2O (control), CaCl2, MgCl2, and FeCl3 solution with a metallic element/biomass ratio of 1:5 (w/w), respectively, and then the mixture samples were stirred for 4 h and dried at 60 °C to remove the moisture46,47. Referring to the pyrolysis method of Xu et al. (2016)18, the above-dried samples were placed in a tube furnace for pyrolysis at 500 °C for 4 h under N2 conditions. After cooling, the solid products were taken out and stored in a glove box with N2 conditions for subsequent experiments. The obtained solid products were biochar and named as BC, CBC, MBC, and FBC, respectively. Then, the basic physiochemical properties of biochar, including pH, total carbon, dissolved inorganic carbon, undissolved inorganic carbon, specific surface area, and pore size were characterized according to the previous study18 and results are displayed in Supplementary Table 1.

Sorption of CO2 by biochar amended soil

The biochar was incubated in the sterilized CL and SL soils at a 5% ratio (wbiochar:wsoil), with three replicates for each treatment. The incubation was kept for 24 h with 50% moisture content. Under the same conditions, individual biochar or soil was also treated and used as control groups. Then, about 20 mg of freeze-dried samples of the soils were taken out and placed in the Thermogravimetric Mass Spectrometry (TGA/DSC1, Netzsch, Germany) to perform the CO2 sorption experiments48,49. Briefly, the sample was heated to 125 °C under 50 mL min−1 N2 condition for 100 min to remove the already adsorbed gases. After the system was cooled to 25 °C, the injected gas was switched to pure CO2 at a flow rate of 50 mL min−1 for 100 min. Here, we used a high concentration of CO2 to study the sorption potential of biochar-amended soil for CO2. Normally, the atmospheric CO2 concentration is about 430 ppm, but in the soil environment, the CO2 concentration in the soil pores or soil surface is 45–50 times that of the atmosphere, due to soil respiration50. Therefore, we selected a high concentration of CO2 and carried out the experiment at normal temperature and pressure. In addition, the use of high CO2 is also conducive to better assessing the maximum CO2 sorption potential of biochar-amended soil. The sorption kinetics of CO2 by the soil samples were calculated from the later weight change information using the sample weight at the moment when the injected gas was switched to CO2 as the initial value.

Transformation of CO2 in the biochar-amended soil during sorption process

In order to better understand the mechanism of CO2 sorption and transformation, we performed batch equilibrium experiments, which can provide sufficient samples for mechanism analysis18. The sterilized soil (10 g) and biochar + soil mixtures (5% wbiochar:wsoil) (10 g) had been separately pre-incubated in a 20 mL glass bottle for 24 h with 50% moisture content. Three replicates were performed for each treatment group. After vacuuming, the glass bottles were supplemented with pure CO2 gas and placed on an inverted shaker for 24 h. This operation was repeated three times, and the control treatment groups without CO2 supplementation were set. All samples were freeze-dried at the end of the reaction and subjected to subsequent analyses. The pH values of the samples (1:20, wsample:vCO2-free water) were tested by a pH meter (pH 510, Alalis, USA). The total carbon content was determined by an elemental analyzer (Vario EL Cube, Germany). Total organic carbon (TOC) was measured by oxidizing with potassium dichromate (HJ615-2011). Dissolved organic carbon and inorganic carbon were determined using TOC analyzer (multi 3100, Analytik Jena, Germany). Biochar particles were selected and separated from biochar+soil mixture groups through the flotation method (1.6 g cm−3 NaI) according to the method of Liu et al. (2022)51. The method raised by Kemp et al. (2022)40 was used to test the weight loss curve (30~1000 °C) of biochar samples after CO2 sorption, and the CO2 escape during the heating process was analyzed with TG-MS (TGA/DSC1, Netzsch, Germany). The chemical structure of biochar was characterized by Fourier transform infrared spectroscopy (FTIR, Thermo Fisher Scientific, Waltham, MA). The surface morphology and elements distribution of biochar particle was measured using the scanning electron microscope coupled with X-ray energy dispersive spectroscopy (SEM-EDS, Zeiss Ultra Plus). Further, the selected biochar particles were dispersed in water and settled 24 h after ultrasonic treatment. The heavy components were separated by centrifugation and the mineral composition was tested by X-ray diffraction (XRD, D8-Focus, Bruker AXS Co., Ltd., Germany). A one-sided t test was used to examine the effect of measured parameters before or after CO2 sorption of biochar using Origin 2022 software.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.