Beneath lawns, cornfields, and city parks, soil holds a vast hidden stash of carbon. Scientists estimate that the ground worldwide stores more carbon than the atmosphere and all plants combined, so changes in how fast it breaks down really matter.
A new study finds that this breakdown is far from uniform. Soil samples from across the United States, kept under the same lab conditions, decomposed at rates that differed by up to tenfold, a result that could reshape climate models and future carbon markets.
Why the speed of soil carbon matters
When plants and roots die, their remains become organic carbon in the soil, like deposits in a long-term savings account. Microbes feed on that material and release part of the carbon back into the air as carbon dioxide, while some of it stays locked underground.
How fast this cycle runs helps decide whether soil acts more like a sponge soaking up carbon or a source that leaks it back out. And if we misjudge that speed, especially when soils hold several times more carbon than the atmosphere, how can our climate projections stay on target?
A coast-to-coast soil experiment
To sharpen those estimates, researchers used soil collected from 20 sites in the National Ecological Observatory Network, a federally funded program that tracks ecosystems across the country. They incubated 156 samples for 18 months under identical conditions and regularly measured how much carbon dioxide each sample released.
From those measurements, the team estimated how quickly organic matter decayed and how efficiently microbes kept decomposed carbon instead of releasing it as gas. Ecologist Chaoqun Lu at Iowa State University, working with first author Bo Yi, said the results mean scientists “can no longer assume that similar soils break down carbon at the same basic speed” in their models.
Minerals, microbes, and hidden drivers of decay
Using machine learning tools, they tested which of 26 soil traits lined up with the tenfold spread in decay rates. Factors like soil type, pH, and nitrogen still mattered, but fungi and certain forms of iron and aluminum also emerged as key players that either shield carbon or leave it exposed.
Those minerals are closely tied to mineral associated organic carbon, a pool that can stay in soil for decades or centuries. A second pool, particulate organic carbon from plant pieces, breaks down within years, so each pool’s response to soil chemistry matters for climate forecasts.
Mapping soil carbon behavior across the United States
After spotting the most important predictors, the researchers trained artificial intelligence models to mimic the lab results from the field measurements. They then used those models to map decay rates and carbon use efficiency for land areas about two and a half miles wide across the contiguous United States.
Soils in the Southwest break down organic carbon more quickly and send more of it into the air as carbon dioxide. Soils in the Northwest and the East break carbon down more slowly and keep more of it in microbial biomass.
Rethinking climate models and carbon incentives
For decades, many climate simulations have treated similar soils as sharing a single base decomposition rate when environmental conditions stay the same. The new findings suggest that this shortcut only works in a rough way and that geochemical and microbial details can shift soil behavior far more than many models assumed.
Feeding the new parameter maps into Earth system models should help cut uncertainty around soil carbon feedbacks, the loop where warming speeds up decomposition and extra emissions drive more warming. More realistic numbers will not fix climate change on their own, but they can give policymakers a clearer sense of which choices keep temperatures in check.
The work nudges carbon market programs and conservation planners to think about persistence, not just tons of carbon stored. If one region holds on to new soil carbon much longer than another, the same payment per ton may not reflect its true long-term climate value for landowners already juggling tight margins, rising energy bills, and unpredictable weather.
The study was published in One Earth.
