1 Introduction

Excessive heavy metals in agricultural topsoil have become a widespread environmental problem (Yang et al., 2018; Zhao et al., 2015), mainly due to the discharge of large amounts of contaminated wastewater and runoffs caused by anthropogenic activities (Qin et al., 2021; Yu et al., 2016). Heavy metals can be slowly and consistently taken up into plants during agricultural production, consequently threatening food safety and human health (Rizwan et al., 2016; Shahid et al., 2017). United Nations has set a development target to restore degraded land and contaminated soil by 2030 (Desa, 2016). In China, it is reported that massive agricultural lands are being contaminated with heavy metals, particularly cadmium (Cd) and mainly distributed in South China (MEP, 2014; Zou et al., 2021), which can cause kidney damage and may have serious impacts on the lungs, cardiovascular and musculoskeletal systems (Qu et al., 2022b). To tackle this challenge, the Chinese government has recently launched an action plan for soil pollution prevention and control to remediate contaminated agricultural soil, and to ensure a safe utilization ratio of 93% for agricultural lands by 2030 (Hou and Li, 2017). Consequently, massive resource investment and energy consumption are inevitable to cope with this unprecedented remediation action. In particular, in order to contribute to the global carbon emission reduction (Qi et al., 2020; Tong et al., 2018), the Chinese government has recently framed a strategy of “double carbon” striving to achieve carbon neutrality by 2060, raising new but unanswered scientific issues of sustainable soil remediation in harmony with the carbon strategy.

Carbon-based functional materials are recently regarded as effective and ideal agricultural soils remediation amendments (Liu et al., 2020, 2021; Qiu et al., 2022), which not only reduce the bioavailability of soil Cd but also enhance soil organic carbon benefiting for soil health (Bian et al., 2014; Gao et al., 2022). It also has been suggested that these carbon-based amendments can sequester carbon and mitigate greenhouse gas (e.g. CH4) emissions (Nan et al., 2020), significantly contributing to negative carbon flux in ecosystems (Lehmann et al., 2021). A representative and polular carbon-based material, biochar is produced via biomass pyrolysis, which has been widely used in agricultural soil remediation for mitigating Cd uptake by crops (Lian and Xing, 2017; O'Connor et al., 2018; Qiao et al., 2019b). For example, Qiao and coworkers reported that the contents of Cd in rice grains amended with biochar decreased by 93% noticeably relative to the control (Qiao et al., 2018). Moreover, biochar-based materials can improve the diversity and composition of microbial communities and the relative abundance of heavy metal-resistant bacteria (Qu et al., 2022a, 2022b). In parallel, a type of humic-like carbon material, peat and its based amendments that originate from the degradation of plant residues to coal, has also received extensive attention due to their more abundant functional groups for Cd sequestration and better fertility to soils compared to biochar (Stanislawska-Glubiak et al., 2015; Wang et al., 2019; Yu et al., 2018). In addition to their benefits to soil remediation and carbon sequestration, concerns about their negative impacts such as energy consumption for pyrolyzing biochar and materials transportation also exist and remain unresolved (Song et al., 2018). There is a strong need to take the whole life cycle of remediation action to systematically appraise the sustainability and carbon footprints of the different carbon strategies before the implementation of the large-scale national remediation action. Although we have previously revealed the superiority of biochar relative to lime, including sustainability score and carbon footprint (Liu et al., 2022), and it was based on environmental, social, economic, and agricultural four aspects (Hou et al., 2018). However, a further comparative assessment among diverse carbon-based materials is of high importance for providing a clear guidance on agricultural soil remediation in practice.

In addition, the practice of carbon-based soil remediation strategy also faces inevitable questions of the longevity-associated sustainability of these carbon remediation strategies. For instance, there is limited knowledge available about how the slow decomposition of these carbon materials in soils affects the net ecosystem carbon budget (NECB) at a scale of decades after remediation (De la Rosa et al., 2018; Wang et al., 2016). In particular, peat is considerably less recalcitrant to microbial respiration in the soil compared to biochar, which may be subject to successive decomposition (Qiao et al., 2019a). Therefore, it is necessary to implement an integrated evaluation of the dynamic change of the NECB for these representative carbon materials during their aging in soils. Likewise, different application manners such as one-time and installment applications of these carbon materials can also lead to a distinct endpoint for the NECB. Despite their importance for carbon-based strategies for soil remediation, these issues have never been systematically explored yet.

Herein, the purpose of this study is to assess the sustainability of two representative carbon materials, biochar and peat, for restoring agricultural lands based on a field trial. Both life cycle assessment (LCA) and NECB analyses have been conducted accordingly, to assess the sustainability of biochar and peat for restoring Cd contaminated paddy soil, which contained environmental, social, economic, and agricultural four aspects, as well as their carbon footprints during the life cycle remediation action. The influences of different application dosages and manners on the sustainability of Cd contaminated paddy soil remediation actions and NECB were then scrutinized. In the end, a geographic scale NECB in China based on these two carbon strategies was projected.

2 Materials and methods

2.1 Case description and system boundaries

Paddy fields contaminated with Cd have been selected as study sites, located in Shaoguan City of Guangdong Province, Southern China. This region has a long history of metal mining, with widespread heavy metal contamination in agricultural fields (Zhou et al., 2007). A previous survey has shown that the Cd concentration in the river water near the paddy field of Shaoguan city was up to 2–3 mg/L due to the discharge of wastewater containing heavy metals from mines (Yang et al., 2021). Then, the heavy metal entered the paddy field through irrigation water. In this study site, the total concentration of Cd in two paddy soil were 0.30 mg/kg and 0.33 mg/kg on average, rendering the rice product with Cd concentrations of 0.28 mg/kg and 0.39 mg/kg, respectively, which exceed the Chinese national paddy soil (0.20 mg/L) and rice (0.20 mg/L) thresholds and posed a serious threat to food security and human health.

In this study, two representative carbon-based materials peat and biochar have been used in the two Cd contaminated fields for remediation. The biochar and peat remediation agents selected in this study were already commercial remediation products certified by the Ministry of Agriculture and Rural Affairs of China. Commercially produced biochar and peat were used in this study. In brief, the raw material peat came from a primordial forest in Indonesia. The biochar mainly came from the pyrolysis of coconut shells at 600 ℃, the choice of pyrolysis temperature was to ensure passivation efficiency while reducing economic cost. The carbon contents of biochar and peat were about 80% and 48.5%. Then peat and biochar were mixed with the same iron powder (12%) to further improve the passivation efficiency. During the process of remediation, three treatments were set in the field experiment: peat, biochar, and no action (CK), respectively. As shown in Fig. 1a, in the paddy field, two remediation reagents were fully mixed with the surface contaminated soil (0–20 cm) through machine mixing, respectively. Case 1 dosage was 2.25 t/ha, and it was 15 t/ha in Case 2 with the same conditions. In addition, the cultivation methods of rice were conducted in the same way, including fertilization, irrigation, and management.

Fig. 1
figure 1

Different restoration cases for different dosage: 2.25 t/ha and 15 t/ha (a); different modeled scenarios for application: one-time addition and multiple times addition (b); conceptual model of remediation actions and net ecosystem carbon budget (c)

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In addition, different application manners such as one-time and installment applications are two common methods, which significantly influence the environmental and carbon decomposition. In the present study, for analyzing environmental impacts accurately to make better decisions, we defined two scenarios, which were Scenario 1 and Scenario 2. As shown in Fig. 1b, scenario 1 described a scene where the addition of remediation reagents with 15 t/ha is at one time in the first year. Scenario 2 was also adding the same reagents dosage in total (15 t/ha) as scenario 1, but adding at multiple times. It became adding reagents with 2.25 t/ha at once every year across six years, and adding 1.5 t/ha at once in the seventh year. In addition to the change of adding method, one of the most remarkable things about the two scenarios was that the times of reagent transportation also changed from one time in scenario 1 to seven times in scenario 2.

Significantly, the total Cd content in soil did not change after adding two remediation reagents, but the bioavailability of soil Cd had reduced during the remediation process. The system boundaries included LCA and NECB for the two remediation reagents, which consisted of three stages: the production process of remediation reagents, the transportation process of remediation reagents, and the remediation process of paddy soil (Fig. 1c). The production of biochar contained handling biomass, pyrolysis, and modification, while the peat contained screening, mixture, and modification. System boundaries are shown in Fig. S1, materials, energy, resources, emission, carbon input, and carbon output were involved at all stages.

2.2 Life cycle assessment

2.2.1 Goal, scope, and functional unit

The LCA-based approaches are widely used in the remediation of polluted environments for sustainability assessment in recent years (Jin et al., 2021; Lin et al., 2021; Todaro et al., 2021). The goal of the present research mainly focused on evaluating the potential environmental impacts of using peat and biochar during the remediation process in case 1 and case 2. Meanwhile, the research also evaluated the impacts in scenario 1 and scenario 2. The scope is above the three stages of system boundaries. The functional unit was defined as the remediation of 0.07 ha (1 mu, Chinese area unit) of paddy fields contaminated by Cd with functional life of 30 years.

2.2.2 Life cycle inventory and impact assessment

The data of life cycle inventory were mainly collected following LCA guidance from the three stages of the remediation process, including raw materials, electricity, transport distances, and emissions (SuRF, 2010). The manufacturers provided detailed data on the peat and biochar production process, and the other data, such as the process of transportation and paddy remediation, mainly came from the practical field experiments. Compared with the production data of two remediation reagents, the emission of harmful gas in biochar was higher than that of peat. The complete inventory data are provided in Table 1.

Table 1 Summary of life cycle assessment (LCA) inventory data
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Life cycle impact assessment (LCIA) is an essential step in the LCA, which uses the ReCiPe method to evaluate the significance of potential environmental impacts. The ReCiPe method includes midpoint and endpoint indicators and performs in SimaPro 9.5 software (Song et al., 2018). The endpoint method can describe the integrated damage of impact categories as a single score according to the life cycle inventory, therefore, different scenarios can be compared intuitively in the field of human health, ecosystems, and resources. By contrast, the midpoint method only provides the effects of impact categories (Liu et al., 2022). In the present study, the ReCiPe method was used to assess two remediation reagents in case 1 and case 2 and the different application manners in scenario 1 and scenario 2.

2.3 Sustainability assessment of paddy field remediation

In the previous studies, sustainability assessment has been carried out extensive research in the field of contaminated land, in which the SuRF-UK guideline is widely accepted on sustainability assessment remediation (Bardos et al., 2018). It is mainly composed of three aspects: environmental, social, and economic. However, the indicators in SuRF-UK were mainly applied in industrial sites, which are obviously different from paddy field remediation. In particular, it lacks agricultural indicators. Therefore, a comprehensive framework for sustainability assessment in the paddy field mainly contains four parts: environmental, social, economic, and agricultural sustainability assessment, respectively (Hou et al., 2018). Environmental sustainability assessment is based on LCA, the social mainly contains health and safety and public acceptance, the economic contains cost and benefit, and agricultural sustainability includes soil quality and crop production. The concrete indicators of the four parts can be found in our previous study (Liu et al., 2022). A relative comparison of sustainability assessment indicators for biochar and peat remediation action was based on the input data with different types of qualitative and quantitative. In the relative comparison, the indicator with a high sustainable evaluation is 100%. The overall sustainability assessment uses the multi-criteria analysis (MCA) method to evaluate the remediation options for making decisions by integrating four sustainability assessments into a sustainability score. The sustainability score (ranging from 0 to 100) was calculated by the following function (Deshpande et al., 2020):

$${S}={\sum}\left(\frac H{H_\mathrm{max}}\times{W}\right)\times{100}$$
(1)

where S is the sustainability score, H is the input value of each sustainability assessment, and W is the weight of the sustainability assessment. Normalizing the four parts is adopted by weighing the required input value with the maximum value. Meanwhile, considering that the four parts are integrated, equally important, and inseparable, the weighting is assigned equal to 0.25.

2.4 Net ecosystem carbon budget

The NECB analyzed the carbon change of remediation action concretely within the limits of system boundaries by calculating the difference between the total carbon input and the total output. The detailed method was provided in our previous study (Liu et al., 2022). Therefore, the NECB was calculated as follows:

$${\text{NECB}} \, \text{=} \, E_{\text{input}} \text{- }E_{\text{output}}$$
(2)
$${E}_{\mathrm{input}}=\left({B}_{\mathrm{total}}\times {f}_{1}+{M}_{\mathrm{total}}\right)\times \frac{44}{12}$$
(3)
$${\mathrm{B}}_{\mathrm{total}}={Y}_{\mathrm{g}}\times {f}_{2}$$
(4)
$${E}_{\mathrm{output}}={M}_{\mathrm{output}}\times \frac{44}{12}+{P}_{\mathrm{output}}$$
(5)

where \({E}_{input}\) and \({E}_{output}\) are the total input and the total output (t CO2-eq/ha); \({Y}_{g}\) is the yield of rice (t/ha) and Btotal is the biomass of rice in Case 1 and Case 2 (t/ha); \({f}_{1}\) is the carbon percentage in rice biomass, and f2 is the ratio that rice yield translates into rice biomass; according to the previous study, f1 = 40%, and f1 = 90% (Bi et al., 2009; Dubey and Lal, 2009); Mtotal is the carbon content of remediation materials (t/ha), and Moutput is the carbon emissions of remediation materials production and transportation process (t CO2-eq); Poutput is the carbon emissions produced by rice plant and soil microbial respiration (t CO2-eq/ha) in paddy fields and refers to literature (Wang et al., 2017); the amendment of carbon emissions, which is caused by adding biochar, refers to literature (Liu et al., 2014; Qin et al., 2016).

2.5 Stability prediction of peat and biochar in soils

The stability of remediation reagents is fundamental to assessing their impacts in the process of remediation. It is critical to understand the reagents remaining in the soil through time. To estimate the retention of biochar in Scenario 1 and Scenario 2, a double first-order exponential decay model was used to calculate the remaining amount of biochar (Jones et al., 2011). To validate the accuracy of the model, four groups of experimental data were collected to fit it, which achieved the expected effect.

$${\mathrm{y}}_{\mathrm{bc}}=\mathrm{a}\times \mathrm{exp}\left(-{k}_{1}\times t\right)+\mathrm{b}\times \mathrm{exp}\left(-{k}_{2}\times t\right)$$
(6)

where ybc is the amount of biochar remaining in the soil; a and b are the size of labile and recalcitrant C pools of biochar; k1 and k2 are the decomposition rate of labile and recalcitrant C pools of biochar, and t is time (year); The values of a, b, k1 and k2 came from the literature (Wang et al., 2016). For the same reason, to estimate the retention of peat in Scenario 1 and Scenario 2, a first-decay model was used to calculate the remaining amount of peat (Zimmerman, 2010).

(7)

where yp is the amount of peat remaining in the soil; K = 1.22 × 10–5 and it is the decomposition rate of peat; t is time (year);

Furthermore, the accumulation of carbon emissions after biochar or peat decomposition could be predicted reasonably in 10 years and 40 years by the remaining amount of biochar or peat in Scenario 1:

$${A}_{{\mathrm{s}}_{1}}={M}_{\mathrm{total}}\times \left(1-y\right)\times \frac{44}{12}$$
(8)

where \({\text{A}}_{{\text{s}}_{1}}\) is the accumulation of carbon emissions after biochar or peat decomposition in scenario 1 (t CO2-eq/ha); \({\text{M}}_{\text{total}}\) is the carbon content of remediation reagents (kg/ha); y is the amount of biochar or peat remaining in the soil in the 10th year or 40th year.

In Scenario 2, the adding mode and time of remediation materials had changed, the accumulation of carbon emissions after biochar or peat decomposition in 10 years is as follows:

$${A}_{{\mathrm{s}}_{2}}=\sum {M}_{i}\times \left({1-y}_{11-i}\right)\times \frac{44}{12}$$
(9)

where \({A}_{{s}_{2}}\) is the accumulation of carbon emissions after biochar or peat decomposition in Scenario 2 (t CO2-eq/ha); M is the carbon content of remediation materials (kg/ha); y is the amount of biochar or peat remaining in the soil in different years; i means the i year of adding remediation materials (ranging from 1 to 7).

The accumulation of carbon emissions after biochar or peat decomposition in 40 years is as follows:

$${A}_{{\mathrm{s}}_{2}}=\sum {M}_{\mathrm{i}}\times \left({1-y}_{41-i}\right)\times \frac{44}{12}$$
(10)

where i mean the i year of adding remediation materials (ranging from 1 to 7).

3 Results and discussion

3.1 Overview of LCA results

Figure 2 shows the LCA results by using the ReCiPe endpoint method for peat and biochar remediation action, which mainly contains human health, ecosystems, and resources three aspects (Klöpffer and Grahl, 2014). In case 1, the application of peat for remediation action exhibited the lowest impacts on human health and ecosystems, which were only 27.3% and 17.9% of biochar (Fig. 2a). Resources impacts of both using peat and biochar were lower compared with human health and ecosystems. However, peat (2.1 Pt) had a greater impact on resources compared with biochar (1.4 Pt), it was probably due to peat coming from soil’s natural organic matter, and consuming more natural resources. Subsequently, the total impact of biochar rendered the higher impact at 345.6 Pt (Pt, a normalized indicator that can reveal the overall environmental impacts in the life cycle of remediation), which was 3.8 times that of the peat, it indicating that the addition of biochar in Cd contaminated paddy fields exerted more negative influences on the environment than peat during the life cycle of remediation. Meanwhile, the negative impact of environmental in peat remediation action is less than the lime and other restoration technology such as soil washing, thermal desorption, and phytoextraction (Jin et al., 2021; Liu et al., 2022; Song et al., 2018). The previous mainly focus on the results of remediation, such as the bioavailability of Cd in soil and the content of Cd in rice (Egene et al., 2018; Kumpiene et al., 2007). According to the results of remediation, Cd content in rice grain with peat and biochar remediation was reduced to below 0.2 mg/kg (Fig. S2) and reached the national standard (0.2 mg/kg, GB 2762–2012) (Shao et al., 2014), it was hard for decision making. Compared with the traditional single evaluation standard, the overview of LCA focusing on the production process of remediation reagents to the remediation action provides a more comprehensive evaluation of the environmental impact of different restoration actions (Hou et al., 2016; Jin et al., 2021).

Fig. 2
figure 2

LCA results by endpoint for peat and biochar with different dosage: 2.25 t/ha case1 (a), 15 t/ha case 2 (b)

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In addition, the different dosages of remediation materials were usually considered in the actual remediation action. In this study, we increased the dosage of peat and biochar from 2.25 t/ha (Case 1) to 15 t/ha (Case 2), the result of remediation shows that the concentration of Cd in rice grain was below the national standard (Fig. S2). More importantly, in case 2, the LCA total impact results of two carbon-based materials was significantly improved to 311.5 Pt (Peat) and 2002.6 Pt (Biochar), which were 3.4 and 5.8 times that of 2.25 t/ha (Fig. 2b), respectively. This result suggests that the increase of remediation materials dosage could have a greater negative impact on the environment, especially with the increase of biochar dosage. Meanwhile, the negative impacts on the environment were mainly reflected in human health and ecosystems aspects, which increased by 2.3–2.5 times (peat) and 4.7–5.1 times (biochar). Many previous studies reported that the improvement of dosage is beneficial to the Cd contaminated remediation (Qiao et al., 2018), however, the corresponding negative impact on the environment will also increase. Thus, it is important to choose an appropriate dosage so that both the remediation results and the environmental impact should be considered.

For further detailed analysis, the LCA impact results by midpoint level of peat and biochar remediation action are shown in Table S1, and the relative comparison of peat and biochar was presented in Fig. 3. The results clearly show that the negative impact on the environment of biochar is greater than that of peat in all aspects except for fossil resources scarcity. During the aspects, the application of biochar for Cd remediation brought about 3146.1 kg CO2 eq/ha emission (Table S1), which was about 5.7 times of peat. This indicates that the addition of biochar promotes global warming compared with peat. It has been reported that the reduction of carbon emissions is an important goal for sustainable land management (Sims et al., 2020). The lower carbon emission of the life cycle peat remediation action is more conducive to achieving the national goal of carbon reduction emissions in China (Liu et al., 2015). Furthermore, the relative comparison results clearly exhibit that the application of biochar leads to obvious negative impacts on ozone formation (human health), fine particulate matter formation, ozone formation (terrestrial ecosystems), and terrestrial acidification aspects (Fig. 3), resulting in 204.9 kg ep of NOx, 79.8 kg ep of SO2 emissions (Table S1). This also explains why the application of biochar has a greater negative impact on human health and ecosystems. Therefore, choosing peat as a remediation material both guarantees remediation results and has more beneficial for environmental sustainability.

Fig. 3
figure 3

Relative comparison results (Case 1) of LCIA (midpoint) for remediating of Cd contaminated paddy field

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3.2 Effect of different material application methods on LCA

To reach a better remediation result, there are two material application methods: the addition of remediation materials in large quantities at one-time and addition multiple times, respectively. However, the environmental impact of different application methods is still unclear. Thus, the effect of different application methods on LCA results was studied. As shown in Fig. 4a, scenario 1 (one-time) in peat exhibited the lowest impacts on human health, ecosystems, and resources, which were 22.1%, 22.6%, and 66.1% of scenario 2 (multiple times). Meanwhile, the LCA results of scenario 1 rendered the lowest total impact at 305.5 Pt, which was 22.8% in scenario 2. This result suggests that the addition of peat at one time had a less negative impact on the environment. The main reason was that the addition of peat multiple times would cause multiple transportations and implement processes, which resulted in more energy and resource consumption. Moreover, the LCA impact results by midpoint clearly indicate that the addition of peat multiple times brought 8080.8 kg CO2 eq, 135.8 kg NOx eq, and 33.7 kg SO2 eq emissions (Table S2). In contrast, the addition of peat at one time only led to 1852.8 kg CO2 eq, 26.5 kg NOx eq, and 10.9 kg SO2 eq (Table S2). This further proves that the addition of peat multiple times has a greater negative impact on the environment.

Fig. 4
figure 4

LCA result of peat (a) and biochar (b) for different application methods, one time addition is scenario 1, multiple times addition is scenario 2

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As for biochar remediation action, a similar result could be found that multiple times addition method for paddy fields remediation also resulted in an increase of 54.9%, 33.7%, and 81.1% in the impact on human health, ecosystem, and resources, respectively, (Fig. 4b). Subsequently, the LCA total impact of scenario 2 increased from 2002.7 Pt to 3035.5 Pt (Fig. 4b), and the negative impact on the environment improved by 51.6%. Thus, this again indicates that the addition of biochar at one time is more beneficial to the environment. In addition, the one-time application method was more conducive to reducing the negative impact on the environment in peat (declined 77.2%) by comparing with biochar (declined 34.1%). Therefore, it can conclude that the one-time application has a positive impact on the environment compared with multiple times applications, and peat is more suitable for one-time application compared with biochar.

3.3 Sustainability assessment of remediation actions

The sustainability assessment for paddy field remediations contains environmental, social, economic, and agricultural four aspects during the life cycle (Hou et al., 2018; Liu et al., 2022). Figure 5a exhibits the normalized score results of environmental, social, economic, and agricultural sustainability, the corresponding calculation methods and parameters are provided in Supporting Information (Table S3). The environmental sustainability normalized score is based on the LCA. The results show that the environmental sustainability score of the peat was 89 (Fig. 5a), which was 2 times the biochar remediation action. The dominant reason was that the biochar production process brought more CO2 and toxic and harmful gas emissions compared with peat (Table S1). However, peat is a natural organic matter in the soil and without a complicated production process (Chen et al., 2016; Wang et al., 2019), resulting in the application of peat being more beneficial for environmental sustainability. The social and economic sustainability scores of peat and biochar had no significant difference. The main reason was that the biochar and peat relative comparison results of four indicators in the social sustainability assessment had no significant difference (Fig S3, Table S3). However, the biochar relative comparison results of direct project cost and land value indicators were slightly higher than peat (Fig S4), which led the economic sustainability score of biochar to be slightly higher than peat. A previous study reported that the social and economic benefit was an important factor in choosing a remediation method based on the input and the output (Chen et al., 2020). Such as, Hou et al. study showed that the life cycle costs of the project were the important reason for the selection of excavation, soil flushing, stabilization, and phytoremediation (Hou et al., 2018, 2016). As for paddy field remediations, agricultural sustainability is also the key important aspect. In contrast, the agricultural sustainability score of peat was slightly lower than biochar (Fig. 5a), it was due to the relative comparison results of agricultural indicators in peat were all slightly lower than biochar remediation action (Fig. S5), such as soil quality and crop production indicators, especially the Cd content in rice.

Fig. 5
figure 5

Normalized sustainability assessment scores of environmental, social, economic, and agricultural (a) and overall sustainability assessment scores (b)

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Based on the above analysis, the overall sustainability assessment scores are shown in Fig. 5b. It clearly shows that the overall sustainability assessment score of peat (97.4) was better than biochar (88.4), which suggested that the addition of peat for paddy fields Cd contamination remediation could promote the overall sustainability levels compared with biochar. The domain reason was that the addition of peat was more positive for environmental impact. In summary, it concludes that choosing peat for paddy fields Cd contaminated remediation can promote the overall sustainability levels compared with biochar.

3.4 Net ecosystem carbon budget of remediation actions

To further evaluate the different carbon-based materials effect on the NECB, the difference between carbon input and output was calculated and included the life cycle of remediation actions (Kim et al., 2017; Liu et al., 2022). The carbon input contains material carbon and carbon fixed in the rice plants, the carbon output includes materials production and transportation process and paddy fields (rice plant and soil microbial respiration) (Liu et al., 2022). As shown in Table S4 and Fig. 6a, the amounts of carbon input of peat and biochar remediation action were 11.65 t CO2-eq/ha and 16.46 t CO2-eq/ha, which were 1.3 and 1.8 times the no action (CK). It is mainly because that the peat and biochar are the contained carbon materials (Wang and Wang, 2019). More importantly, the material carbon contained in biochar (80%) was higher than that of peat (48.5%) in this study, which resulted in more carbon input in biochar. Subsequently, the total carbon output of biochar and peat was also higher than that of CK. It could be attributed to the carbon emission of peat and biochar production process and transportation. Meanwhile, biochar production led to more carbon emission than that of peat, which was due to biochar bringing about 3.15 t CO2 eq/ha emission during the life cycle of the remediation action (Table S1), and 5.7 times of peat. However, it is worth noting that the application of biochar significantly decreased paddy fields carbon output compared with peat. A previous study found that the application of biochar reduced CH4 emissions by 14%—52% in three successive years in the paddy fields (Nan et al., 2020). Overall, the total carbon output of biochar (9.75 t CO2-eq/ha) was slightly higher than that of peat (8.48 t CO2-eq/ha).

Fig. 6
figure 6

The input and output of carbon in different remediation materials (a); the NECB of different remediation materials (b), Case 1: 2.25 t/ha; Case 2: 15 t/ha

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After integrating the input and output carbon, the results clearly show that the NECB of the peat and biochar was 3.17 t CO2-eq/ha and 6.70 t CO2-eq/ha in case 1 (Fig. 6b), which were 2.6 and 5.6 times of CK. This result indicates that carbon-based materials can promote a positive carbon budget. It suggests that using carbon-based materials to remediate heavy metal pollution in paddy fields can not only reduce the heavy metal in rice grain but also promote soil carbon sinks. Moreover, the application of biochar can further enhance soil carbon sinks compared with peat. With the dosage of materials increasing to 15t/ha (Case 2), the NECB of the peat and biochar improved to 16.13 t CO2-eq/ha and 33.73 t CO2-eq/ha (Fig. 6b). Qi et al. found that biochar decreased net global warming potential and CO2 emission in the rice season (Qi et al., 2020). Thus, the application of carbon-based materials for paddy field Cd remediation is positive on NECB and achieves carbon sink, especially the biochar.

In addition, according to the area of rice fields in China, the NECB of application peat and biochar for remediation action in China was calculated (Table S5). The NECB distribution of provincials in China is shown in Figs. 7a and b, the high NECB was mainly in distributes in the south and northeast of China, which is also the main rice growing area in China. The highest NECB was in Hunan province. It could bring 69.89 M t CO2-eq/year (peat) and 135.79 Mt CO2-eq/year (biochar) in soils. This result indicates that provinces with a large area of rice cultivation are more conducive to promoting carbon sequestration. Overall, a rough estimation indicates that the application of peat and biochar for restoring the paddy field can bring approximately 527.49 Mt CO2-eq/year and 1023.28 Mt CO2-eq/year of carbon sequestration, which should not be ignored. It is important to promote sustainable agricultural remediation and carbon reduction emissions in China.

Fig. 7
figure 7

the NECB of the application of peat (a) and biochar (b) in China paddy fields with different provinces

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3.5 The stability of carbon-based materials in soils

Biochar and peat are representative carbon-based materials. The stability and decomposition of biochar and peat are fundamental to understanding their persistence and carbon sink in soil. However, biochar and peat show different stability in soil. Different models were selected to evaluate the stability of biochar and peat in soil (Wang et al., 2016; Zimmerman, 2010). Figure 8a illustrates the biochar (Fe-BC) remaining in soil with time, the results clearly show that the biochar decomposed quickly in the first year, and then maintained at a very low level to decompose. Because the unstable carbon on the surface of biochar can be decomposed and released into the soil (Wang et al., 2016). Maestrini reported a 15% increase in soil dissolved organic carbon following biochar addition (Maestrini et al., 2015). Subsequently, most labile biochar compounds are hard to be decomposed, such as aromatic C (Crombie et al., 2013; Lian and Xing, 2017). After 40 years, the biochar remained 90% in soils (Fig. 8a). In contrast, the peat decomposed quickly with time compared with biochar and showed a linear decrease with time. The peat remained about 82% in soils after 40 years (Fig. 8b). Peat was formed by incomplete decay of vegetation (Chen et al., 2016), which is more easily decomposed in soils compared with biochar. Therefore, the application of biochar is more persistent for long-term carbon sinks.

Fig. 8
figure 8

The percentage of biochar change in soil with times (a); the percentage of peat change in soil with times (b); the cumulative decomposition of carbon in materials under different scenarios (application method) after 10 years (c) and 40 years (d)

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Subsequently, two application methods of carbon-based materials were evaluated. As shown in Fig. 8c, the cumulative decomposition of carbon in scenario 1 was greater than in scenario 2 after 10 years, it was mainly due to the materials with one-time application taking longer to decompose in the soil compared with multiple applications. The same results could be found in the peat. In addition, the cumulative decomposition of carbon in biochar was higher than that of peat. According to the previous prediction model (Fig. 8a), biochar decomposed quickly in the first few years. After 40 years (Fig. 8d), the cumulative decomposition carbon of biochar in scenario 1 was similar to that in scenario 2. However, the cumulative decomposition carbon of peat in scenario 1 was higher than that of scenario 2. Moreover, the cumulative decomposition carbon of peat was higher than biochar, which is the opposite of the 10 years result. It suggests that peat is more stable in a short term, and biochar is more stable in a long term.

4 Conclusion

In this study, we evaluated the sustainability and NECB of different carbon-based materials for restoring paddy soils during the life cycle of remediation actions. The results show that the application of biochar for paddy field remediation increases negative impacts on the environment compared with the peat remediation action, especially for the human health and ecosystems. Meanwhile, the one-time addition method had a positive impact on the environment compared with multiple times addition. With integrated social, economic, environmental and agricultural aspects, the overall sustainability score of peat remediation action was higher than that of biochar. In contrast, the biochar remediation action could bring 33.73 t CO2-eq/ha (dosage: 15 t/ha) carbon sequestration, about 2 times of peat. Meanwhile, the application of biochar was more stable for long-term carbon sinks. Thus, the application of peat is more beneficial for the sustainability remediation of heavy metals contaminated agricultural land, and biochar is more positive for carbon sequestration.