Soils hold the largest supply of organic carbon on Earth, triple the amount contained in the atmosphere. As the climate warms, scientists expect that microbes and roots will break down soil organic carbon (SOC) more quickly and release carbon dioxide emissions to the atmosphere at a faster rate. The process, known as soil respiration, is a crucial element in climate models. Yet global SOC databases are out of sync with field measurements, hindering efforts to model Earth’s future climate, a new study finds.
Organic carbon enters the soil as plants and animals decompose, and it’s exuded by living and dead microorganisms as well. The amount of SOC in any given patch of soil varies according to climate, time, and soil texture—clay-rich soil, for example, is better at retaining carbon. To estimate the distribution of SOC around the globe, scientists have created databases like SoilGrids, which predicts soil properties down to 200 centimeters on the basis of 110,000 soil profiles from around the world; the Harmonized World Soil Database (HWSD), a collection of geographic information on soil properties from inventories all over the world, down to 100 centimeters; and the Northern Circumpolar Soil Carbon Database (NCSCD), which quantifies organic carbon in soils in the permafrost around the North Pole to a depth of 300 centimeters.
Past studies have revealed that these databases often don’t agree with one another or with field measurements. A new study by Tifafi et al. underscores that uncertainty by comparing these three databases (SoilGrids, HWSD, and NCSD) to field measurements collected from North America, France, England, and Wales. They calculated SOC concentrations down to 1 meter; soils at this depth and below are important to global climate modeling as they often hold the greatest concentrations of SOC. They are also expected to warm at roughly the same rate as surface soils and the atmosphere.
The team found that most data sets underestimate SOC by more than 40% compared to field data, and some underestimated stocks by 80%–90%. In the Arctic regions, where SOC is rapidly being released from melting glaciers, the discrepancy between the databases and field data was even worse. Scientists urgently need to refine the statistical methods they use to predict soil composition to improve global climate models, the authors conclude. (Global Biogeochemical Cycles, https://doi.org/10.1002/2017GB005678, 2018)
—Emily Underwood, Freelance Writer