Forests Retain More Carbon in Nutrient-Rich Soil

Forests growing in nutrient-rich soils are able to absorb five times as much carbon from the atmosphere as those in nutrient-poor soils. A new study from a team of international researchers shows that 30 percent of carbon taken up during photosynthesis can be sequestered in forests with ample nutrients in the soil. By contrast, poor-soil forests retain only 6 percent. The rest returns to the atmosphere as respiration.

"This paper produces the first evidence that to really understand the carbon cycle, you have to look into issues of nutrient cycling within the soil," said International Institute for Applied Systems Analysis Ecosystems Services and Management Program Director Michael Obersteiner, who worked on the study, which was originally published in Nature Climate Change. "When plants are in nutrient poor conditions, they send out more roots and produce chemicals that can help dissolve nutrients from the soil. This takes energy, though, and so the plants produce less biomass."

Up to this point, scientific models to predict how much carbon forests sequester on a global scale had only considered nitrogen in the soil without accounting for other constraints such as phosphorus or the pH of the soil, which is an indicator of available nutrients.

The new study includes both those factors in addition to nitrogen availability, combining data from 92 forests in different climate zones on the planet. Tropical rainforests had the poorest nutrient availability and were least efficient at sequestering carbon, the researchers found.

They believe the difference in efficiency of carbon absorption could be due to several factors, including the fact that plants in nutrient-poor soils devote more energy to locating nutrients.

"In general, nutrient-poor forests spend a lot of energy - carbon - through mechanisms to acquire nutrients from the soil, whereas nutrient-rich forests can use that carbon to enhance biomass production," said Marcos Fernandez-Martinez, first author of the paper and researcher at the Center for Ecological Research and Forestry Applications.

Furthermore, the study showed that nutrient-rich ecosystems also generally have more stable ground organic material, which is not easily degraded, and thus retains more carbon.