Abstract:

As the largest organic carbon reservoir in terrestrial ecosystems, soil organic carbon (SOC) comprises approximately 70% of the terrestrial organic carbon pool and stores roughly twice the amount of carbon found in the atmosphere. It plays a pivotal role in regulating atmospheric greenhouse gas concentrations and mitigating global climate warming, thereby serving as a critical and cost-effective natural pathway to global “carbon neutrality” goals. This makes SOC dynamics a core focus of global climate change research and carbon management practices worldwide. However, widespread soil degradation, driven extensively by anthropogenic land-use changes, has led to substantial losses of SOC across key terrestrial ecosystems. Globally, estimates indicate SOC storage reductions ranging from 25% to 75% in vital ecosystems such as forests, croplands, and grasslands, underscoring an urgent need for focused assessment and management of remaining soil carbon stocks. Located in eastern coastal China, Ningbo features a typical subtropical monsoon climate and encompasses three dominant terrestrial ecosystem types: forests, croplands, and urban green spaces. These ecosystems hold unique and irreplaceable value for regional carbon sequestration. Forests, as fundamental components of the world’s largest terrestrial carbon pool, function as essential natural carbon sinks through their extensive above- and below-ground biomass and robust capacity for long-term SOC storage. Croplands, although more intensively anthropogenically managed and generally possessing a smaller inherent carbon pool compared to forests, offer considerable potential for rapid enhancement of carbon sequestration through optimized agricultural practices. Urban green spaces have increasingly emerged as vital carbon sink nodes within densely populated areas where anthropogenic emissions are most concentrated, directly offsetting a portion of local emissions through vegetative carbon uptake and soil carbon storage. Collectively, these three ecosystems synergistically contribute to regional climate change mitigation in Ningbo. Nevertheless, significant knowledge gaps persist regarding the precise spatial heterogeneity of SOC storage and the unquantified carbon sequestration potential across different ecosystems and soil types in the region, hindering a comprehensive assessment of Ningbo’s contribution to China’s national carbon neutrality targets. This study comprehensively evaluates SOC storage and carbon sequestration potential in the top one meter of soil across these three ecosystems. We employed a multi-faceted methodological framework integrating extensive field surveys, detailed laboratory analyses, rigorous numerical calculations, and advanced spatial assessment techniques. A total of 53, 54, and 40 representative sampling points were established in forest, cropland, and urban green space soils, respectively. At each sampling point, soil cores were collected in three layers: topsoil (0-30 cm), middle soil (30-60 cm), and subsoil (60-100 cm). Our findings revealed significant variations in SOC among the three ecosystems, presenting a clear hierarchical order in terms of density: croplands>forests>urban green spaces. Such differences are closely related to the distinct management regimes and natural conditions of each ecosystem type. Soil profiles across all ecosystems exhibited distinct vertical SOC distribution patterns and pronounced spatial heterogeneity, reflecting the compounded influences of natural environmental factors and anthropogenic activities. However, when considering total storage, the order shifted to forests (35.8 Tg)>croplands (29.0 Tg)>urban green spaces (16.6 Tg), highlighting the dominant role of forest coverage in regional-scale carbon pools. Spatially, high-SOC areas in forests were predominantly concentrated in mountainous and hilly regions, including Haishu, Fenghua, and Ninghai. In these forested areas, the topsoil layer showed the highest SOC, with a mean value of 39.0 Mg·hm⁻², underscoring the fundamental role of natural forests as stable carbon sinks. In croplands, high-value SOC areas were primarily distributed across the central plains (e.g., Yinzhou, Haishu) and northern agricultural zones. Notably, the mean topsoil SOC in these croplands reached 46.8 Mg·hm⁻², surpassing that of forest topsoil and clearly reflecting the positive effects of intensive agricultural management practices—such as organic amendments and residue retention—on enhancing topsoil carbon stocks. In contrast, urban green spaces exhibited the lowest mean topsoil SOC at only 25.7 Mg·hm⁻², with high-value areas concentrated in core urban green spaces heavily impacted by urbanization processes including historical land-use conversion, soil compaction, and ongoing physical disturbance. Regarding the inherent soil carbon sequestration potential, defined as the capacity for additional SOC accumulation under improved management, marked differences were observed among the ecosystems. Forest soil exhibited the highest total carbon sequestration potential, estimated at 4804.5 Gg. Within forests, Umbrisols were identified as the most responsive soil type, contributing approximately 64% to the total forest potential. This highlights Umbrisols as a critical target for forest-based carbon storage initiatives in Ningbo. Furthermore, the middle soil layer (30-60 cm) in forests was identified as the core carbon sequestration zone, indicating substantial untapped potential for deep soil carbon stabilization beyond the plow layer. Cropland soils ranked second in total sequestration potential at 2462.1 Gg, with Anthrosols contributing dominantly at 69%. The carbon sequestration potential in croplands was relatively evenly distributed across all sampled soil layers (0-30, 30-60, 60-100 cm), a characteristic that is highly conducive to implementing comprehensive, full-profile carbon management strategies. Urban green space soils demonstrated the lowest total carbon sequestration potential (1363.5 Gg), which was predominantly associated with Technosols. Interestingly, within urban green spaces, the subsoil layer (60-100 cm) showed particularly prominent sequestration potential. From a pedological perspective, the subsoil layers of Umbrisols (forests), Anthrosols (croplands), and Technosols (urban green spaces) all demonstrated notably high carbon sequestration capacities. This commonality is closely linked to their inherent soil properties, such as high clay content, strong organo-mineral complexation, and favorable conditions for organic matter retention, coupled with their specific environmental and management contexts. In conclusion, the soils underlying the forest, cropland, and urban green space ecosystems in Ningbo City each possess considerable, yet distinct, carbon sequestration potentials. These three ecosystems complement one another in terms of carbon pool magnitude, sequestration mechanisms, and feasible management pathways for carbon enhancement, collectively forming the foundational support of Ningbo’s regional terrestrial carbon sink system. Our results provide valuable scientific insights for formulating targeted soil carbon management policies in coastal subtropical regions. Effectively increasing and stabilizing soil carbon storage across these diverse landscapes is not only a vital strategy for contributing to national “carbon neutrality” objectives but also a crucial means for enhancing regional ecological resilience, soil health, and long-term sustainable development capacity.

Key words: soil organic carbon, carbon sequestration potential, terrestrial ecosystem, Ningbo

摘要：

陆地生态系统土壤固碳潜力突出，合理调控土壤碳储量是减缓气候变暖、实现“碳中和”的重要途径。针对宁波市陆地土壤碳库空间异质性大、固碳潜力不明的问题，以该地区森林、农田和城市绿地土壤为研究对象，采用野外调查、室内分析、数值计算等方法评估了1 m深度内不同陆地生态系统土壤有机碳（SOC）储量分布及固碳潜力。结果显示，不同生态系统SOC差异显著，表现为农田>森林>城市绿地；土壤剖面SOC垂直分布特征显著，而且存在较大的空间异质性。1 m深度土壤总碳储量表现为森林（35.8 Tg）>农田（29.0 Tg）>城市绿地（16.6 Tg）。固碳潜力上看，森林土壤固碳潜力最高（4804.5 Gg），其中30－60 cm土层固碳潜力大；农田土壤固碳潜力次之（2462.1 Gg），但不同深度的土壤固碳潜力差异不大；城市绿地土壤固碳潜力最低（1363.5 Gg），其中60－100 cm深度土层固碳潜力突出。从土壤类型上看，森林暗色土、农田人为土和聚铁网纹土以及城市绿地工程土均在60－100 cm土层表现出较高的固碳潜力。宁波市陆地生态系统土壤固碳潜力较大，有效提升该地区土壤碳储量对实现“碳中和”目标具有重要意义。

关键词: 土壤有机碳, 土壤有机碳储量, 固碳潜力, 陆地生态系统, 宁波市