Soil stores more carbon than the atmosphere and all vegetation combined. Yet, how quickly that carbon decomposes—and returns to the air as CO₂—remains one of the largest uncertainties in climate projections.
A new study published in One Earth, reveals that the rate of soil organic carbon decomposition across the United States can vary by up to tenfold, largely due to overlooked geochemical and microbial factors such as reactive iron, aluminium, and fungi.
Mapping Decomposability Across the U.S.
To uncover why soils behave so differently, the research team incubated soils from 20 National Ecological Observatory Network sites for more than 18 months, monitoring CO₂ emissions and 26 physical, chemical, and biological properties.
Machine-learning analyses identified soil metals and fungal abundance as dominant predictors of decomposition, in addition to well-known factors like pH and nitrogen. These findings challenge the long-held modeling assumption that soils of similar type and climate decompose at comparable rates.
From Laboratory Data to Continental Maps
Using the incubation results, we built a predictive framework and upscaled it across the contiguous U.S., generating high-resolution maps of decay rate and carbon-use efficiency—two key parameters controlling soil carbon turnover.
Areas with the most vulnerable soil carbon showed decomposition rates up to ten times faster than the most stable regions. The new maps provide an essential benchmark for improving Earth system models and understanding soil-carbon–climate feedbacks.
Implications for Climate Modeling and Policy
By incorporating soil-metal and microbial information, climate models can better estimate how carbon responds to environmental change. The study also highlights regional contrasts in soil-carbon stability: rapid loss potential in the arid Southwest versus long persistence in the humid Northwest and Eastern U.S.
Such differences have practical meaning for carbon-sequestration programs, which could reward regions where carbon is more likely to remain stored for centuries.