Soil organic carbon (SOC) is a critical component of the Earth's carbon cycle, while iron oxides (Fed, Feo, Fep) play a crucial role in its accumulation and storage.

To better understand the dynamics of iron-bound organic carbon (OC-Fe) in terrestrial ecosystems, a research team led by Prof. ZENG Fanjiang from the Xinjiang Institute of Ecology and Geography (XIEG) of the Chinese Academy of Sciences, has conducted an extensive study and investigated the global distribution, formation mechanisms, and saturation states of OC-Fe in terrestrial ecosystems.

Their findings were recently published in Nature Communications.

The researchers compiled a global dataset based on 3,395 soil profiles and used machine learning methods to quantify OC-Fe stocks, assess their saturation levels, and identify the primary formation pathways of OC-Fe.

Their study revealed that the global OC-Fe stock stands at approximately 232.9 petagrams (Pg), accounting for 33.5% of the SOC and 52.3% of the mineral-associated organic carbon (MAOC) stock. Moreover, wetlands exhibited the highest saturation of OC-Fe, while forests showed considerable potential for increased OC-Fe storage in the future.

In addition, the researchers identified co-precipitation as the main pathway for the formation of OC-Fe in terrestrial soils, rather than previously considered mechanisms such as adsorption or the promotion of soil aggregate formation.

"These findings can help us understand more about the mechanisms of soil carbon stability, and predict global carbon stocks," said LI Lei, corresponding author of this study.

This study underscored the importance of OC-Fe in stabilizing soil organic carbon, and provides essential data support for carbon cycle models which incorporate mineral factors.

Modelled global patterns of OC-Fe at soil depths ranging from 0 to 30 cm. (Image by XIEG)