Soil is a huge component of the global carbon cycle. Organic material in soil, stabilized by interactions with mineral particles, contains about twice as much carbon as the atmosphere. An accurate treatment of soil’s capacity to sequester carbon is thus an essential ingredient in climate models. It’s long been assumed that all mineral surfaces are equally good at stabilizing carbon, so a soil’s carbon-storage capacity is determined by its total surface area. But Ingrid Kögel-Knabner and colleagues at the Technical University of Munich have now laid that assumption to rest. Using nanoscale secondary-ion mass spectrometry (nanoSIMS), a technique for mapping chemical species on a surface, they’ve shown conclusively that organic matter binds to just 20% of the mineral surface area. The researchers mixed natural topsoil with an organic litter isotopically enriched in carbon-13 and nitrogen-15, and incubated it for six weeks. Several times during the experiment, they took a sample of the mixture and imaged the mineral particles with nanoSIMS to map the distributions of all organic matter (yellow regions in the left panel) and isotopically enriched organic matter (pink and green regions in the right panel). The mineral particles themselves are shown in blue. Through statistical analysis of images taken at different times, the researchers concluded that the newly incorporated organic material was binding only to surfaces where old organic material was already present. As they extend their work to other types of soil, they hope to paint a clearer picture of how much carbon soil can hold. (C. Vogel et al., Nat. Commun. 5, 2947, 2014.)—Johanna L. Miller