A new study reports that climate warming can increase soil carbon accumulation in boreal Sphagnum peatlands by boosting plant productivity, protecting iron, and inhibiting microbial decomposition. These responses contrast sharply with warming-enhanced soil carbon mineralization—the process by which carbon is released as CO₂—in boreal forests and tundra. Together, these contrasting processes highlight the vital yet often overlooked role of Sphagnum peatlands in counteracting boreal carbon loss under future warming.

The study, led by Prof. FENG Xiaojuan from the Institute of Botany of the Chinese Academy of Sciences, in collaboration with researchers from the University of Helsinki and the Finnish Meteorological Institute, was published in Nature Ecology & Evolution on February 9.

Boreal ecosystems store twice as much carbon as the atmosphere and are warming at a rate faster than the global average. The current paradigm, based on observations from boreal forests and tundra, posits that warming will accelerate boreal carbon loss by activating microbial degraders. However, these regions also harbor extensive Sphagnum peatlands, which constitute ~20% of the boreal biome and store ~40% of its carbon stocks. Characterized by high moisture content and acidic, antimicrobial environments, Sphagnum peatlands may respond differently to warming than other, drier boreal ecosystems.

To better understand the soil carbon cycle in boreal regions, the researchers synthesized 735 paired observations from 93 studies based on field warming experiments across boreal regions. They compared the responses of Sphagnum peatlands and vascular plant-dominated wetlands, boreal forests, and tundra to warming. Additionally, the researchers leveraged two long-term open-top chamber warming experiments (six and 16 years) in Finland for more detailed mechanistic investigations.

Their analysis shows that warming consistently promotes soil carbon accumulation in boreal Sphagnum peatlands via three pathways. First, warming stimulates Sphagnum growth and ecosystem productivity in boreal peatlands with sufficient moisture. Second, it increases Sphagnum synthesis of antimicrobial secondary metabolites, which have negative cascading effects on the capacity of soil microbes to decompose. Third, warming promotes the protection of soil carbon by iron via the accumulation of reactive iron (hydr) oxides, which is driven by elevated growth of the "rust engineer" Sphagnum.

Based on these mechanisms, researchers suggested that the increase in soil carbon induced by warming in boreal Sphagnum peatlands may offset nearly half of the decline in the boreal forest carbon sink or the increase in heterotrophic respiration in Arctic tundra under warming.

Overall, this study highlights the unique metabolic and ecosystem responses of Sphagnum peatlands to warming and advances our understanding of the mechanisms that govern soil carbon dynamics in these ecosystems under warming conditions.

"Our study reveals the crucial yet overlooked role of Sphagnum peatlands in buffering boreal ecosystem carbon emissions under future climate warming. This information is important for understanding how boreal ecosystems respond to climate change," said Prof. FENG. "Incorporating the warming response of Sphagnum peatlands into Earth system models would greatly improve future predictions of boreal carbon-climate feedbacks."

Conceptual framework illustrating contrasting carbon dynamics under warming for different boreal ecosystems (Image by ZHAO Yunpeng)

Sphagnum moss, the flagship species of boreal peatlands (Image by by ZHAO Yunpeng)