When a plant dies, its leaves and branches fall to the ground. Decomposition of soil organic matter is then mainly carried out by fungi and bacteria, which convert dead plant materials into carbon dioxide and mineral nutrients.

Until now, scientists have thought that high quality organic materials, such as leaves that are rich in soluble sugars, are mainly decomposed by bacteria. Lower quality materials, such as cellulose and lignin that are found in wood, are mainly broken down by fungi.

Previous research has also shown that organic material that is broken down by fungi results in a reduced leakage of carbon dioxide and nutrients compared to material decomposed by bacteria.

This has consequences for climate models, since more loss of carbon dioxide and mineral nitrogen would have a direct bearing on the soil’s contribution to greenhouse gases and eutrophication.

We used a field experiment, the Detritus Input and Removal Treatments, or DIRT, experiment (Harvard Forest Long-Term Ecological Research Site, USA) where litter and root inputs (control, no litter, double litter, or no tree roots) have been experimentally manipulated during 23 years, generating differences in soil C quality. We hypothesized (1) that δ¹³C enrichment would decrease with higher soil C quality and that a higher C quality would favor bacterial decomposers, (2) that the C mineralized in fungal-dominated treatments would be of lower quality and also depleted in δ¹³C relative to bacterial-dominated high-quality soil C treatments, and (3) that higher C mineralization along with higher gross N mineralization rates would occur in bacterial-dominated treatments compared with more fungal-dominated treatments. The DIRT treatments resulted in a gradient of soil C quality, as shown by up to 4.5-fold differences between the respiration per soil C between treatments. High-quality C benefited fungal dominance, in direct contrast with our hypothesis. Further, there was no difference between the δ¹³CO₂ produced by a fungal compared with a bacterial-dominated decomposer community.

—Rousk and Frey (2015)

In contrast with expectations, there was no evidence that high quality organic material was mainly broken down by bacteria. In fact, the data strongly suggested the contrary. There was also no evidence to suggest that organic material broken down by fungi reduced the leakage of carbon dioxide into the atmosphere, or the leakage of nutrients. Once again, the results tended to suggest the contrary.

—Johannes Rousk, researcher in Microbial Ecology at Lund University

The results could have consequences not only for future climate models, but may also impact current policies on land use intended to promote fungi. This may be based on flawed assumptions regarding the fungal role in reducing negative environmental effects.

Resources

• Johannes Rousk and Serita D. Frey (2015) “Revisiting the hypothesis that fungal-to-bacterial dominance characterizes turnover of soil organic matter and nutrients” Ecological Monographs 85:3, 457-472 doi: 10.1890/14-1796.1