The Paris Agreement's climate goals necessitate large-scale deployment of negative emission technologies (NETs). For China to achieve carbon neutrality by 2060, NETs are projected to provide substantial negative emissions (0.01–2.91 billion tons of CO₂ per year). BECCS and DACCS, while potential solutions, face significant economic and technological hurdles, high costs (exceeding US$100 per ton of CO₂ removed for BECCS and US$200 for DACCS), and potential conflicts with other sustainable development goals (e.g., food security). Biochar, a proven technology with co-benefits for soil quality and crop yield, offers a potentially viable alternative. However, uncertainties regarding its potential, cost, and national-level deployment strategies hinder its widespread adoption in China. This study addresses these gaps by conducting a spatially explicit analysis to quantify biochar's negative emission potential, evaluate its economic viability, and identify priority deployment sites across China, considering multiple feedstocks.

Previous studies have explored biochar's potential for carbon sequestration and its co-benefits for agriculture. However, these studies often focus narrowly on specific biochar types or lack granular estimations of negative emission potential and economic impact at regional levels. Global evaluations have highlighted biochar's crucial role as a NET, but regional-level analyses are needed to guide deployment strategies. Existing estimates in China mainly focus on agricultural residues, neglecting the contributions of forestry and grass residues, and potential energy crops, leading to an underestimation of the country's biochar potential. Furthermore, variations in biochar properties from different feedstocks and spatial factors (soil texture, pH) are often overlooked, leading to inaccurate economic and emissions assessments. This study leverages recent advancements in data availability, assessment methodologies, and spatial data resolution to address these limitations.

The study employs a three-step approach. First, it evaluates the potential biomass feedstocks (agricultural, forestry, grass residues, and energy crops) available for biochar production in China. Three scenarios are developed: the ‘Maximum Theoretical Potential’, ‘Sustainable Technical Potential’, and ‘Current Technical Potential’, each reflecting different assumptions about biomass availability and considering current and future technological and policy advancements. Second, it quantifies the negative emission potential of biochar using a unified empirical framework that accounts for biochar properties, pyrolysis parameters, and biomass type variations. Third, it conducts a cost-benefit analysis incorporating data from the literature and pilot projects to construct supply curves for negative emissions from multiple feedstocks. The analysis considers the revenue from by-product sales (syngas) and co-benefits from improved crop yields. Finally, a spatially explicit analysis is performed to identify the most cost-effective biomass types and deployment locations, taking into account factors such as soil texture and pH. The study uses high-resolution spatial data, including crop yields, land use data from multiple sources, national statistics, and environmental factors to estimate biomass availability, carbon sequestration potential, and the economic costs and benefits at a 0.5° x 0.5° grid resolution. The accounting framework adopted for the calculation of negative emission potentials is detailed, including consideration of the pyrolysis process, carbon content, and permanence of biochar in the soil. The economic analysis includes detailed breakdown of costs (feedstock purchasing, transportation, pyrolysis, biochar application, etc.) and revenues (syngas sales and yield improvements) to calculate net negative emission costs and supply curves. The Monte Carlo analysis assesses the uncertainty associated with these estimations, taking into account variations in biomass resource availability, biochar properties, pyrolysis conditions, and crop yield responses.

The study finds that biochar possesses a significant negative emission potential in China. Under the maximum theoretical scenario, the total biomass feedstock could reach 2.43 Gt yr⁻¹, with agricultural residues, energy crops, forestry, and grass residues contributing 0.99 Gt yr⁻¹, 0.86 Gt yr⁻¹, 0.29 Gt yr⁻¹, and 0.29 Gt yr⁻¹, respectively. The sustainable technical potential scenario, considering current biomass use patterns and ecological constraints, reduces this to 1.73 Gt yr⁻¹. This translates to a negative emission potential of up to 1.29 GtCO₂ yr⁻¹ (maximum theoretical) and 0.92 GtCO₂ yr⁻¹ (sustainable technical) respectively. The current technical potential scenario yields 0.43 GtCO₂ yr⁻¹ of negative emission potential. These potentials are comparable to or exceed the negative emission demands projected in various climate mitigation scenarios for China. The average net cost of negative emissions from biochar is estimated at US$90 per ton of CO₂, ranging from US$60–96 for biochar from agricultural and forestry residues to US$101–144 for biochar from energy crops and grass residues. This is generally lower than the estimated costs for BECCS and other NETs. The relatively low net cost is attributed to the sale of syngas by-products and co-benefits from increased crop yields, with syngas sales playing a much larger role in reducing costs than yield improvements. The spatially explicit analysis reveals that regions with high negative emission potential and low costs largely overlap, primarily in Central and South China, and East China. Some areas demonstrate the potential to achieve positive returns due to high crop yields and crop values. The uncertainty analysis reveals a mean estimate of 1.07 GtCO₂ yr⁻¹ for negative emission potential under the sustainable technical scenario, ranging from 0.68 to 1.46 GtCO₂ yr⁻¹, and a mean negative emission cost of US$92/tCO₂, ranging from −13 to 197 $/tCO₂.

The findings demonstrate biochar's considerable potential as a cost-effective NET for achieving China's carbon neutrality goals. The high negative emission potential, coupled with relatively low costs, suggests biochar's strong competitiveness compared to other NETs. The spatially explicit analysis offers valuable insights for targeted deployment, prioritizing regions with high potential and low costs. However, the study highlights that current carbon prices in the Chinese national carbon market are not high enough to promote biochar adoption via offset mechanisms, necessitating further policy support and the exploration of voluntary carbon removal platforms. The results suggest the need for integrated strategies combining biochar deployment with infrastructure development to enhance syngas utilization and further reduce costs. Although the study provides a robust framework, several factors need further investigation, including the potential trade-offs between maximizing negative emissions and bioenergy production. The study also acknowledges the importance of addressing potential limitations, such as competition for biomass resources and the environmental and socioeconomic impacts of large-scale biochar deployment.

This study provides compelling evidence for biochar as a viable and cost-effective NET for China. The significant negative emission potential, coupled with relatively low costs and identified high-potential regions, demonstrates its suitability for inclusion in China's climate change mitigation strategies. The results highlight the importance of comprehensive policy support, infrastructure development, and further research into optimizing biochar production and deployment strategies. Future research should focus on refining the quantification of co-benefits, evaluating the life cycle emissions more comprehensively, and integrating biochar into national emission trading schemes.

The study's limitations include the exclusion of some biomass resources (e.g., livestock manures), the focus on a specific co-production technology that doesn't prioritize biochar yield maximization, and the absence of a full accounting for dynamic factors (technological advancements, economies of scale, evolving carbon prices). The analysis also does not fully capture the environmental and socioeconomic trade-offs associated with large-scale biochar deployment, such as land-use change emissions and competition with food production. The uncertainty analysis, while valuable, does not capture all potential uncertainties associated with biochar implementation.