Deep soil organic carbon storage, stability and their responses to human activities

Soil, as one of the largest carbon reservoirs in the ecosystem, holds a substantial storage capacity and significantly impacts global climate change. Currently, most research on soil organic carbon storage and turnover focuses on the surface soil organic carbon (0−1 m) (SSOM), while systematic studies on the storage, stability, and response to human activities of deep soil organic carbon (DSOM) below 1 m are relatively limited. This article offers a comprehensive review of the latest research advancements on the storage, sources, composition, stability, and human activity responses of DSOM. Globally, the storage of deep soil organic carbon ranges from 29.9 to 219.6 t·hm⁻², accounting for 15%−84% of the total soil organic carbon storage. The average age of DSOM lies between 4800 and 28 100 a, with a turnover time spanning from 1000 to 4285 a. The stability of DSOM is significantly higher than that of SSOM, mainly due to the anaerobic environment, mineral protection, and chemical inertness of DSOM. Key factors affecting the stability of DSOM include excessive fertilization, intensified agricultural practices, and fluctuations in groundwater levels. Although the mineralization rate of DSOM is significantly lower than that of SSOM, human activities such as nitrate leaching due to excessive nitrogen fertilizer application, changes in cropping systems, and groundwater level fluctuations have significantly reduced the stability of DSOM in some regions, potentially having a profound impact on global climate change. The substantial storage potential of DSOM for carbon sequestration is compromised by these activities, which may lead to the release of stored carbon back into the atmosphere, thereby exacerbating global warming, Studies indicate that the substantial storage of DSOM has significant potential for carbon sequestration, and human activities may have compromised its stability, potentially leading to profound effects on global climate change. Therefore, it is crucial to investigate DSOM. However, the underlying mechanisms and processes have not been fully explored and require further in-depth research. The future urgently needs to: 1) assess the storage and stability of regional/global deep soil carbon pools; 2) clarify the mechanisms by which deep soil organic carbon pools respond to human activities; 3) quantify the contribution of human activity-induced deep soil organic carbon release to global warming. These research directions are essential to fully understand and leverage the role of DSOM in climate change mitigation and to ensure the sustainability of our planet for future generations. In summation, the study of DSOM is at the vanguard of soil science and global change research, necessitating a multidisciplinary convergence of pedology, geochemistry, microbiology, and environmental science. Advancing our grasp of DSOM is key to harnessing its capacity as a natural climate change mitigation tool and securing the planet’s sustainability for future generations.