It is estimated that 30% of the world's terrestrial carbon stocks are found in the boreal forest, of which 60% is below ground. Organic soil horizons contain about one-third of the soil carbon. Therefore, the stability of boreal soils is critical for understanding global carbon cycle to climate change and soil management. However, the regulating factors of humus carbon sequestration in the boreal forests globally is still uncertain. 

Dr. SUN Tao and his team from the Institute of Applied Ecology of the Chinese Academy of Sciences have been studying litterfall and soil carbon. They manipulated two independent but complementary experiments and then demonstrated that exchangeable manganese (Mn) is a critical factor regulating carbon accumulation in boreal forests at both regional scales and accross the entire boreal latitudinal range.

Based on a meta-analysis covering the entiring boreal zone, they not only ascertained that exchangeable Mn is an important factor controlling long-term humus carbon sequestration in boreal forest soils, but also provided the first global generalized Mn effect on soil organic carbon stocks for boreal forests.

Sepcifically, in a 14-year fertilization experiment, Mn addition reduced soil carbon stocks, but this effect of Mn addition was only apparent after four years.

The researchers showed that exchangeable Mn is essential for maintaining humus in boreal forests, to the extent that changes in global carbon cycles may be significantly affected over decades. Thus, they suggested that a more accurate representation of Mn-mediated mechanisms affecting carbon storage in biogeochemical models will increase confidence in model structure, parameter estimation, and predictions of soil organic matter production and litter decomposition. 

This study was published in PNAS and it was supported by the National Natural Science Foundation of China, the National Key R&D Program of China, and the International Partnership Program of CAS, etc. 

Relationship between exchangeable Mn concentrations and soil C stocks in humus layers of boreal forests. (A) Area of boreal forest and location of study sites in the database (n = 2,538); (B) Log-log regression between C stocks and exchangeable Mn in the humus layer (n = 2,437). (Image by SUNTao) 

Effect of annual experimental Mn fertilization (MnSO₄) on humus C storage of boreal forest over 14 years. Points show mean soil C stocks in organic horizons (humus) in control and Mn-fertilized plots in a Larix gmelinii stand in boreal China (Huzhong Forest Bureau of Daxing'an Mountains). Bars are means ± SEM. (Image by SUNTao)