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A new study has challenged the long-standing assumption that global warming will inevitably turn humid subtropical forests into carbon sources, revealing these ecosystems may instead continue accumulating soil carbon under moderate temperature rises. The study was in One Earth on Oct. 6.

Led by Prof. Liu Juxiu from the South China Botanical Garden of the Chinese Academy of Sciences, the team conducted a nine-year ecosystem-level passive warming experiment in a humid subtropical forest. Using an elevational translocation approach, they simulated realistic warming of up to 2.1°C.

The researchers found that soil organic carbon exhibited a two-phase response to warming. In the first four years, they observed carbon loss, driven by a reduction in topsoil mineral-associated organic carbon. However, from years six to nine, soil carbon began accumulating, a shift attributed to sustained plant carbon inputs and microbial adaptation that boosted particulate organic carbon levels.

"Our results show that moderate warming does not necessarily lead to soil carbon loss," said Dr. Liu Xujun, first author of the study. "Sustained plant carbon inputs and microbial thermal adjustments can help maintain—or even enhance—soil carbon storage in some humid subtropical forests."

This contradicts common climate projections, which typically assume warming weakens or reverses the carbon sink capacity of tropical and subtropical forests. Such assumptions have led to overly pessimistic assessments of forest-based climate mitigation in these regions.

The new findings offer a more hopeful outlook, suggesting afforestation in relatively cool, humid subtropical mountainous areas—where conditions favor forest growth—could enhance soil sequestration of atmospheric carbon under modest warming.

The team further identified that warming-induced nutrient competition between plants and microbes accelerated the loss of mineral-associated organic carbon. This underscores the need to place greater emphasis on plant–soil interactions when regulating forest soil carbon dynamics in Earth system models and forest management, as they not only mediate the fate of soil carbon stocks but also influence the accumulation patterns of different soil carbon fractions, the researchers noted.

"Afforestation and forest restoration efforts should avoid excessive use of tree species with inefficient nutrient utilization—particularly of limiting nutrients—or species whose rapid growth may outpace soil nutrient supply," said Prof. Liu Juxiu.

The study provides new insights into how plant–soil interactions regulate long-term soil carbon balance in subtropical forest ecosystems, offering an optimistic perspective for leveraging forests in global climate mitigation efforts.