Now researchers in Germany have found that, as well as soil condition, there is an additional species identity and/or diversity effect on soil organic carbon storage. Per-Marten Schleuß and colleagues from the Georg August University in Göttingen and University of Koblenz-Landau have shown that mixed beech forests are better at storing organic carbon in the subsoil than their single-species counterparts.

In an article in Environmental Research Letters (ERL), Schleuß and his colleagues describe how they studied nine plots within a broad-leaved forest in Hainich National Park in Germany. The plots had variable tree species composition and tree diversity with European beech being increasingly diluted by other species.

“The clay and fine silt content was significantly higher in the soils of the plots with mixed tree species when compared with the soil from plots that contained mainly beech,” explained co-author Felix Heitkamp. “This difference in texture will lead to a larger soil organic carbon content because soil with finer texture will have a higher specific surface area, enhancing the association with organic carbon.”

To eliminate the effect of soil texture on the total amount of organic carbon in bulk soil samples, Schleuß and his colleagues compared the amount of carbon in different particle size fractions from the different plots. They found that organic carbon content was higher in those plots with a more diverse range of tree species, indicating that the number of tree species does have an effect on soil organic carbon.

“Soil type is still an important factor in carbon storage and cycling,” said Heitkamp. “But we have shown that tree species also have an effect. The reason for this effect is unclear.”

The group speculates that there are a number of possible mechanisms of tree species effects on soil organic carbon stabilization. These include: higher fine root turnover, less bioturbation and/or less transport of dissolved organic carbon occurs under beech trees; chemical composition of leaf and root litter may differ among the species; or the functional traits of the microbial community vary considerably.

“Whatever the mechanism behind the effect, we believe our results may have important implications for forest management and its role in mitigating climate change,” said Heitkamp. “Germany’s broad-leaf forests, which used to be made up primarily of beech, are becoming more diverse and, in terms of storage of soil organic carbon, this increased diversity seems to be a good thing.”

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