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Abstract Forest soils store about one‐fifth of the global terrestrial biosphere carbon stock. However, our understanding of how soil geochemical, plant and microbial factors regulate forest soil organic carbon (SOC) storage, stability, and saturation levels remains limited. Here, we conducted a sampling campaign across a 5000‐km natural forest transect in China, measuring climate, geochemical factors and SOC fractions with varying stability. Additionally, we compiled a global data set of SOC fractions in major forest biomes. Our field survey and global synthesis consistently demonstrate that warmer climates not only reduce the content of labile particle organic matter (POM), but also decrease the typically stable mineral‐associated organic matter (MAOM), leading to a significant decline in total soil carbon storage. Additionally, warmer climates promote the crystallization of Fe/Al oxides, which decreases the formation efficiency of Fe/Al oxide associated organic complexes. Consequently, the mineralogical carbon saturation level declines from boreal forests (37%) to tropical forests (25%). Our findings underscore that, beyond the well‐established climate impacts, soil geochemical properties play a pivotal role in shaping forest SOC composition and saturation levels across latitudes. This highlights that colder regions harbor larger and more stable carbon pools, and that ongoing climate warming and associated soil geochemical properties shift could potentially lead to a decline in soil carbon storage and its capacity to mitigate climate change.
Wu et al. (Thu,) studied this question.