A new study reports that the base rate of organic carbon decomposition in soils across the United States can differ by as much as tenfold, a finding with direct implications for climate projections because soil stores more carbon than the atmosphere and vegetation combined. The research team collected soil samples from 20 sites in the National Ecological Observatory Network (NEON) and incubated them under uniform laboratory conditions for 18 months. During that period they measured carbon dioxide emissions and 26 soil properties to estimate decay rates and carbon use efficiency; the study appears in One Earth.
Using machine learning, the authors identified several factors linked to variation in decomposition. Expected variables — soil type, pH and nitrogen — were important, but the analysis also revealed strong connections with fungal abundance and particular forms of iron and aluminum. Those minerals contribute to the stabilization of mineral-associated organic carbon, a pool that can persist for decades or centuries.
The team combined field measurements and base-rate estimates to train AI models that reproduced variation across 156 soil samples and then applied the models to the continental United States, producing maps of decay rates and carbon use efficiency for grid cells about 2.5 miles on a side. The maps show that the Southwest tends to release carbon more rapidly as CO2, the Northwest and East have slower decomposition and more carbon becomes microbial biomass, and the Midwest falls between these extremes.
Chaoqun Lu, associate professor at Iowa State University and the study's corresponding author, noted that models have often assumed similar soils decompose at the same base rate. The new results show wide variation even within the same soil or biome, and the authors argue that Earth system models have underrepresented geochemical and microbial controls. Improving models with these measurements could refine projections of soil carbon feedbacks to climate and help design conservation and carbon market programs that account for regional differences in carbon retention.
- Southwest: faster decomposition, more CO2 released.
- Northwest and East: slower decomposition, more microbial biomass.
- Midwest: values generally between the extremes.
Difficult words
- decomposition — breakdown of organic matter into simpler compounds
- base rate — typical background speed of a natural processbase-rate
- carbon use efficiency — share of carbon converted into microbial biomass
- mineral-associated organic carbon — organic carbon bound to soil minerals
- fungal abundance — amount or presence of fungi in soil
- geochemical — related to the chemical properties of soils
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Discussion questions
- How might regional differences in soil decomposition affect local conservation priorities or land management? Give one or two examples.
- What challenges could arise when designing carbon market programs that account for regional differences in carbon retention?
- What additional soil measurements or data would you collect to improve models of decomposition in your region, and why?
