1 Introduction

Biochar is a kind of porous and carbon-rich organic material derived from the pyrolysis of biomass, including all kinds of crop straws, forestry residues, livestock manure and other wastes, under relatively low temperatures (300–700 °C) and restricted oxygen conditions. Applying biochar to pollution abatement on soil or water (Kim et al. 2021; Zhang et al. 2019), carbon (C) emission reduction (Qian et al. 2015; Woolf et al. 2010), also C capture and sequestration (Singh et al. 2019) has exhibited a great development potential.

The whole process from production to utilization in the biochar-soil system (biochar production and its storage in soil) is regarded as a typical carbon-negative emission activity (Lehmann 2007). Biochar returning to the field is beneficial to agricultural production and climate change mitigation, which has attracted more and more attention in the international community of climate change. It is estimated that biochar can offer carbon dioxide (CO2) removal potential of 0.3 to 2 Gt per year (Hepburn et al. 2019). The world-renowned scientific journals such as Nature and Science have published feature articles emphasizing the combination of plant carbon fixation and biochar production for filed return, calling for increased research on the environmental behavior and environmental impact of biochar-soil system (Paustian et al. 2016; Sohi 2012), to provide important support for global climate change.

The stability of biochar, such as the persistence of biochar in the environment, can reflect the resistance to biotic and abiotic decomposition. The biotic stability of biochar is mainly affected by microorganism -induced the oxidative decomposition process (Ameloot et al. 2013). And the abiotic stability of biochar is mainly affected by dissolution (Lian and Xing 2017) and oxidation (chemical oxidations, oxygen and temperature) (Wang et al. 2017) of biochar in the environment. Microorganisms in soil can cause the biotic consumption of C fixed by biochar, however, the abiotic factors can influence the rate of biochar decomposition, leading overestimating the potential of C sequestration (Yu et al. 2020).

Previous studies have shown that the abiotic stability of biochar is closely related to its composition properties and environmental applications (De la Rosa et al. 2018; Leng et al. 2019). Pyrolysis temperature and feedstock have a direct influence on the composition properties of biochar (Das et al. 2021; Hassan et al. 2020; Kim et al. 2020; Wu et al. 2022; Xu et al. 2021; Zhang et al. 2020). Biochar applied into soil will be affected by severval abiotic factors, such as rainfall (Wang et al. 2022), temperature changes (Liu and Chen 2022), and environmental oxidation (Wang et al. 2021a). For example, rainfall can cause biochar to release soluble organic matter or mineral components by dissolution (Wang et al. 2021b). Dissolved organic carbon (DOC) is an important component of soluble organic matter, which will flow into the water environment through surface runoff (Jaffe et al. 2013) or infiltrate into the soil and then be used by microorganisms (Quan et al. 2020), decreasing the capacity of C sequestration. Furthermore, the surface properties of biochar will undergo chemical oxidation under soil oxides or light conditions, which will make the C in biochar thermodynamically unstable under aerobic conditions (Wang et al. 2021a).

The abiotic stability of biochar has been evaluated by extensive studies (Liu et al. 2020a, 2020b; Pariyar et al. 2020; Wei et al. 2019; Zornoza et al. 2016). Most studies only focused on the single or multi-composition changes related to the above influencing factors, however, universal methods that can directly or indirectly evaluate the abiotic stability of biochar are missing (Leng and Huang 2018; Leng et al. 2019), causing difficulties in the practical evaluation of biochar stability. For instance, the ratio of volatile matter (VM) to fixed carbon (FC) (Aller et al. 2017) and the thermal stability index R50 (calculated as the ratio of temperatures at which half weight loss occurs during calcination of biochar and graphite, respectively) (Gómez et al. 2016) can be used to evaluate the thermal stability of biochar. International Biochar Initiative (IBI) and European Biochar Foundation (EBC) recommended using the atomic ratios of hydrogen (H) and organic carbon (Corg) or oxygen (O) and Corg to assess the stability of the carbon structure in biochar (Leng et al. 2019). DOC was used to evaluate the dissolution stability of biochar (Han et al. 2020); the chemical oxidation-resistant carbon (Coxidation) could reflect the oxidation-resistance of biochar (Liu et al. 2020a, b), and the ratio of Coxidation and C can reflect the chemical oxidation-resistant stability of biochar (Calvelo-Pereira et al. 2011).

Given the above mentioned examples, the abiotic stability of biochar is not only affected by feedstocks, process parameters, and composition characteristics, but is also closely related to the methods for stability evaluation. Therefore, four kinds of straws (wheat straw, corn straw, rape straw and rice straw) were used because of their large production and wide distribution. The objective of this study is to systematically illustrate the effects of pyrolysis temperature on composition, carbon fraction and the abiotic stability of straw biochar, and to establish the relationship between pyrolysis temperature, composition, carbon fraction and abiotic stability of straw biochar. The results are expected to be valuable for a scinetific comprehension of the inherent properties of straw biochar, and thus help simplify the screening of appropriate indicators for evaluating the properties and abiotic stability of biochar.

2 Materials and methods

2.1 Biochar samples

Four kinds of straws (wheat straw, corn straw, rape straw, and rice straw) were used to produce biochar, and the detailed information about the biochar preparation was presented in author’s previous research (Zhang et al. 2020). In brief, the feedstocks were crushed and dried, and then pyrolyzed in a tube furnace (GSL-1100X, Hefei Kejing Materials Technology Co. Ltd., China) under nitrogen (N2) atmosphere (all straw samples without pyrolysis treatment were marked as CK). Since 300–600 °C was an important temperature range for weight loss of straw (Zhang et al. 2020), four pyrolysis temperatures, 300 °C, 400 °C, 500 °C, and 600 °C were chosen for biochar preparation. The heating rate was 10 °C/min and the pyrolysis process was held for 1 h at the final temperature.

2.2 Lab analysis of biochar characteristics

The elemental composition of straw feedstocks and biochar samples were analyzed by the elemental analyzer (Vario Macro Elementar, Germany).

The proximate composition was determined by a thermogravimetric analyzer (SDTQ600, TA Instruments, USA) using the method described in ref. (Crombie et al. 2013). The sample placed into the crucible was heated to 105 °C under N2 for 10 min to obtain the moisture (MC) content, and then was heated at 25 °C/min to 900 °C where kept for a further 10 min to remove VM. Introduced the air into test system, the sample was combusted at 750 °C (the test temperature was set according to ASTM D5142–09) for 15 min, and the residual weight was ash content. The content of FC was calculated by difference (FC = 100-MC-VM-ash).

Corg was measured by total organic C analyzer (Elemental Vario TOC select, Germany). The sample was weighted into a silver boat and acidified using 1 M HCl solution; the acidified sample was dried at 100 °C for 1 h to remove inorganic C such as carbonate in biochar, and then were packed for analysis (Enders et al. 2012).

DOC was also determined by total organic C analyzer (Elemental Vario TOC select, Germany). The sample and deionized water were mixed at 1:20 (wt/v) and after shaking (150 rpm) for 2 hours, the treated sample was centrifuged at 10000 rpm for 10 min (Wu et al. 2019). Then, the filtrate used for final DOC analysis was obtained by supernatants through a 0.45 μm filter.

Potassium dichromate (K2Cr2O7) has strong oxidation ability which can be used to determine carbon components that are not easy to decompose and oxidate potentially decomposable carbon structures (Calvelo-Pereira et al. 2011). The K2Cr2O7 oxidation method was used for Coxidation determination. The sample containing 0.1 g C (±0.0001 g) was weighted into a 50 ml centrifuge tube with 40 ml of 0.1 M K2Cr2O7/2 M H2SO4 solution, and the chemical oxidation was performed at 55 °C for heating for 60 h (Yang et al. 2018). In order to ensure complete reaction, the oxidation solution was replaced for once after 30 h-reaction during the oxidation process. After the test, the solid phase was separated by centrifugation and then weighted after drying. The C content remained in solid phase was determined by the elemental analyzer (Vario Macro Elementar, Germany). The content of Coxidation in sample was calculated as follows:

$$C_{\mathit o\mathit x\mathit i\mathit d\mathit a\mathit t\mathit i\mathit o\mathit n}\mathit{\left(\%\right)}\mathit=\mathit{\left({C_{after}\times W_{after}}\right)}\mathit/W_{\mathit b\mathit e\mathit f\mathit o\mathit r\mathit e}$$
(1)

Where Wbefore and Wafter were the mass (g) of the sample before and after oxidation, and Cafter was the C content (%) of the sample after oxidation.

The temperature programmed oxidation (TPO) curves of samples, were measured by a thermogravimetric analyzer (SDTQ600, TA Instruments, USA). Samples were placed in an aluminum crucible and heated to 1000 °C at a heating rate of 10 °C/min under air atmosphere. Then, the TPO curves were corrected following the method described in previous study (Harvey et al. 2012).

2.3 Calculation for the abiotic stability evaluation of biochar

VM/FC and atomic ratios (H/Corg and O/Corg) were calculated from the results of chemical analysis to evaluate the thermal stability of straw biochar.

R 50 is also used for the evaluation of thermal stability. It was calculated as follows (Harvey et al. 2012):

$$R_{\mathit{50}}\mathit=T_{\mathit{50}\mathit x}\mathit/T_{\mathit{50}\mathit\;\mathit g\mathit r\mathit a\mathit p\mathit h\mathit i\mathit t\mathit e}$$
(2)

Where, T50x and T50 graphite were the temperature (°C) of biochar and graphite where the mass loss was 50%. T50x was obtained from the corrected TPO curves of samples, and T50 graphite was 886 °C, which was taken from Harvey et al. (2012).

The dissolution stability was calculated by the percentage of DOC content in C content, expressed as DOC/C (%).

The oxidation resistant stability was calculated by the percentage of Coxidation content in C content expressed as Coxidation/C (%).

All tests were replicated three times.

2.4 Statistical analysis

A one-way analysis of variance (ANOVA) and Kendall correlation analysis were conducted using the software SPSS (IBM SPSS statistics 25), and the Turkey’s HSD post-hoc tests (p < 0.01) (Jing et al. 2022) were used to identify significant differences among different biochars. All characteristics of samples were analyzed by SPSS using the factor analysis. Unitary and binary regression methods were performed with the software Origin (Origin lab 2018) and the correlation coefficient (R2) was chosen to evaluate the goodness of fit.

3 Results and discussion

3.1 Effects of pyrolysis temperature on biochar composition and carbon fraction

The composition and carbon fraction related to abiotic stability of four kinds of straw feedstocks and their corresponding biochars at different pyrolysis temperatures are shown in Table 1.

Table 1 Ultimate analysis, proximate analysis and carbon fractions of straw biochars produced at different pyrolysis temperatures
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Table 1 shows that the feedstock had no significant influence on H, FC, VM and Coxidation (p ≥ 0.01), but had a significant influence on C, DOC, Corg, and ash (p < 0.01). Wheat straw had the highest content of Corg. Corn straw and rape straw had the highest content of C and the lowest content of ash. In contrast, the rice straw had the lowest ash content and the highest C content.

The pyrolysis temperature had a similar effect on the composition and carbon fraction of different straw biochars. As the pyrolysis temperature increased, the content of H, O and VM significantly decreased (p < 0.01) and the content of FC and ash increased. The content of C and Coxidation significantly increased (p < 0.01) and the Corg content increased with increases in temperature, but the DOC content significantly decreased (p < 0.01). At 500–600 °C, the FC, VM and DOC content became steady (p ≥ 0.01), probably due to the lignocellulosic components’ decomposition and volatilization at this temperature range (Zhang et al. 2020). Significant differences were found among the content of ash, C and Coxidation of different straw biochars (p < 0.01). At the same pyrolysis temperature, rice straw biochar showed the lowest ash content and the highest C content, which may be related to the content of intrinsic compositions in rice straw (Table 1). The content of Coxidation in wheat straw biochar was much higher than that in other straw biochars.

Therefore, compared with feedstock, pyrolysis temperature may have a more important effect on compositions and carbon fractions of straw biochar, as verified by the results of Fourier transform infrared spectroscopy of straw biochars (Zhang et al. 2020).

3.2 Effects of pyrolysis temperature on biochar abiotic stability

The results of the abiotic stability evaluation indicators of straw biochar derived at different pyrolysis temperatures are shown in Table 2. It can be seen from Table 2 that the pyrolysis temperature had a significant effect on the abiotic stability evaluation indicators of straw biochar (p < 0.01). With the increase of pyrolysis temperature, VM/FC, H/Corg, O/Corg and DOC/C ratios decreased, however, R50 and Coxidation/C ratio increased. Although there were differences among the compositions and carbon fractions of different straw feedstocks, the results of the abiotic stability evaluation indicators were similar among different straw biochars above 500 °C. Especially for H/Corg, O/Corg and DOC/C ratios, there was no significant difference among different straw biochars (p ≥ 0.01). This indicated that pyrolysis temperature is the main factor affecting the abiotic stability of straw biochars.

Table 2 Effects of pyrolysis temperature on the abiotic stability evaluation indicators of different straw biochars
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The fitting results of the abiotic stability of straw biochars prepared at different pyrolysis temperatures are shown in Fig. 1. As can be seen in Fig. 1, all abiotic stability evaluation indicators increased exponentially with the increasing temperature. However, different stability evaluation indicators showed different exponential function changes, which can be expressed as following formulas:

$$\mathrm{O}/{\mathrm{C}}_{\mathrm{org}}=-0.72\exp \left(-\mathrm{T}/290.93\right)+0.07\kern0.5em {R}^2=0.97$$
$$\mathrm{H}/{\mathrm{C}}_{\mathrm{org}}=1.42\exp \left(-\mathrm{T}/312.08\right)\ {R}^2=0.89$$
$${R}_{50}=-0.10\exp \left(-\mathrm{T}/133.26\right)+0.45\ {R}^2=0.93$$
$$\mathrm{DOC}/\mathrm{C}=6.43\exp \left(-\mathrm{T}/274.28\right)\ {R}^2=0.78$$
$${\mathrm{C}}_{oxidation}/\mathrm{C}=-100\exp \left[\left(300-\mathrm{T}\right)/137.84\right]+100\ {R}^2=0.78$$
Fig. 1
figure 1

Numerical relationship between pyrolysis temperature and abiotic stability evaluation indicator of straw biochar

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As depicted in Fig. 1 (a, b, and c), the VM/FC, H/Corg and O/Corg ratios exponentially decreased as the pyrolysis temperature increased and became steady over 500 °C. This might due to the breakage of weaker chemical bonds in feedstock at low temperatures (Imam and Capareda 2012), leading to the depolymerization of lignocellulose. However, the R50 exponentially increased as pyrolysis temperature increased from 300 to 500 °C, and reached the maximum value at 500 °C (Wang et al. 2021b). Straw biochar derived at 500 and 600 °C had the similar R50 value, which was consistent with the change rule of corrected TPO curves measured (Fig. 2).

Fig. 2
figure 2

The corrected temperature programmed oxidation (TPO) curves of straw biochar produced at different temperatures

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The pyrolysis temperature had an obvious exponential function relationship with DOC/C ratio (Fig. 1e). When the pyrolysis temperature was higher than 500 °C, the DOC/C ratio was close to zero.

The straw and biochar samples at 300 °C had no chemical oxidation-resistance, indicating that the stable C structure may have not formed. When the pyrolysis temperature exceeded 300 °C, the Coxidation/C ratio had an exponential function relationship with the pyrolysis temperature. With the increase in the pyrolysis temperature, the chemical oxidation-resistant stability increased, which was consistent with the results of previous study (Chen et al. 2016). The Coxidation /C ratio gradually stabilized over 500 °C, which might due to the disappearance of unstable components (aliphatic containing C and H, alkane groups) and the formation of chemical oxidation-resistant C structures which were not easily oxidized by chemical reagents (Han et al. 2018; Zhang et al. 2020).

The above results showed that the pyrolysis temperature was an important factor affecting the abiotic stability of straw biochar. The higher the pyrolysis temperature is, the better the abiotic stability of straw biochar is. The abiotic stability of straw biochar tended to be steady over 500 °C.

3.3 Correlations among composition, carbon fraction and abiotic stability evaluation indicator of straw biochar

3.3.1 Kendall correlation analysis and factor analysis

The correlation analysis showed that the composition, carbon fraction and abiotic stability of straw biochar were significantly correlated with the pyrolysis temperature (p < 0.01), but had no significant correlations with the types of feedstocks (p ≥ 0.01) (Table 3). This was consistent with the analysis results in 3.1 and 3.2. Furthermore, there were significant correlations among different stability evaluation indicators (p < 0.01).

Table 3 The Kendall correlation analysis results of the pyrolysis temperature, feedstock, composition, carbon fraction and abiotic stability evaluation indicator of straw biochar
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For further analysis, factor analysis was performed based on the varimax-rotation method (Fig. 3). All characteristics of straw biochars can be divided into two groups. The first group was H, O, VM, DOC, H/Corg, O/Corg, DOC/C, and VM/FC, and the second group was C, FC, Corg, Coxidation, ash, R50 and Coxidation/C. Within the same group, there was a positive relationship between any two characteristics. And there was a negative correlation between any two characteristics from different groups. Except for R50, the rest of thermal stability evaluation indicators were clustered into the same group with dissolution stability evaluation indicators, and the others including R50 were clustered into another group. This indicated the close relationship among H/Corg, O/Corg, DOC/C, and VM/FC, as well as R50 and Coxidation/C. Thus, it is necessary to further study the quantitative relationships among different abiotic stability evaluation indicators of straw biochar.

Fig. 3
figure 3

Factor analysis results of the composition, carbon fraction and abiotic stability of straw biochar

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3.3.2 Quantitative relationships of abiotic stability evaluation indicators of straw biochar

The linear correlations among different abiotic stability evaluation indicators of straw biochar are shown in Fig. 4. The significance of model test was 0.000, indicating that the results of fitting analysis were effective. The linearity of different abiotic stability evaluation indicators varied.

Fig. 4
figure 4

Unitary linear regression analysis of different thermal evaluation indicators (a), dissolution stability evaluation indicator and the evaluation indicators of thermal stability and chemical oxidation-resistant stability (b), and chemical oxidation-resistant stability evaluation indicator and thermal stability evaluation indicator (c)

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Among the evaluation indicators of thermal stability, VM/FC and R50, O/Corg and R50, as well as VM/FC and O/Corg, showed a strong linear relationship (R2 > 0.90). Therefore, VM/FC, O/Corg and R50 can replace each other in evaluating the thermal stability of biochar. In addition, there was a high linear relationship between H/Corg and O/Corg (R2 = 0.81), which may be related to the high linear relationship between H and O in straw biochar (R2 = 0.83) (Zhang et al. 2020). There was a high correlation between H/Corg and DOC/C (R2 = 0.81) (Fig. 4b). Both VM/FC and O/Corg showed a linear correlation with DOC/C (R2 = 0.74 and R2 = 0.77, respectively). There was no linear relationship between the indicators of dissolution stability and chemical oxidation-resistant stability, indicating that there were differences between them. In Fig. 4c, Coxidation/C showed a high linear correlation with H/Corg (R2 = 0.81). It is suggested that H/Corg can be selected as an alternative evaluation indicator for the evaluation of dissolution stability and chemical oxidation-resistant stability. In view of these results, it can be considered that the thermal stability evaluation indicators, especially H/Corg, are of great significance for the evaluation of abiotic stability.

3.3.3 Multivariate analysis of composition, carbon fraction and abiotic stability of straw biochar

A multivariate analysis of the composition, carbon fraction and abiotic stability of straw biochar is needed to simplify the evaluation of abiotic stability for practical application. The results of binary linear regression analysis are shown in Fig. 5. Both VM/FC and DOC/C were affected by the interactive effects of H and O. Comparing with DOC/C. There was a better binary linear relationship among VM/FC, H and O, which indicated that the VM/FC ratio could be calculated by H and O content. C, Corg, and FC were respectively selected to fit R50 with ash. Compared with FC and Corg, there was a better fitting effect of C and ash on R50 (R2 = 0.90), which demonstrated that the content of C and ash can be used for evaluating R50. And compared with R50, there were worse results when Coxidation/C was fitted with C and ash, Corg and ash, as well as FC and ash. This demonstrated that chemical oxidation-resistant stability may be affected by other factors, in addition to these interactions.

Fig. 5
figure 5

Binary linear regression analysis of compositions, carbon fractions and abiotic stability evaluation indicators of straw biochar

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4 Conclusion

It can be concluded that pyrolysis temperature influenced the compositions and carbon fractions directly, which affected the abiotic stability of biochar (p < 0.01). The higher the pyrolysis temperature (up to 500 °C), the higher the abiotic stability of biochar.

The exponential functions of stability indicators with pyrolysis temperature are as follows:

$$\mathrm{O}/{\mathrm{C}}_{\mathrm{org}}=-0.72\exp \left(-\mathrm{T}/290.93\right)+0.07\kern0.75em {R}^2=0.97$$
$$\mathrm{H}/{\mathrm{C}}_{\mathrm{org}}=1.42\exp \left(-\mathrm{T}/312.08\right)\kern0.75em {R}^2=0.89$$
$${R}_{50}=-0.10\exp \left(-\mathrm{T}/133.26\right)+0.45\kern0.75em {R}^2=0.93$$
$$\mathrm{DOC}/\mathrm{C}=6.43\exp \left(-\mathrm{T}/274.28\right)\ {R}_2=0.78$$
$${\mathrm C}_{oxidation}/\mathrm C=-100\exp\left[\left(300-\mathrm T\right)/137.84\right]+100\;R^2=0.78$$

The established unitary and binary linear regressions equations among compositions, carbon fractions and the abiotic stability indicators are valuable for simplifying the screening of appropriate indicators for evaluating the properties and abiotic stability of biochar, which will be beneficial to the effective utilization of straw biochar, especially in the scope of carbon capturing and sequestering application.