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The Battle Below – Saprophytes and Mycorrhizal Fungi Compete for the Same Substrate

Published: 09 June 2016

The soil of the conifer forest is layered; topmost, a carpet of carbon-rich litter, and below, the poorer humus. The litter is dominated by substrate degrading saprophytes, whereas mostly mycorrhizal fungi colonize the humus. But what are the drivers of this distinct fractioning of fungal guilds? This is the topic of a new study from the Department of Forest Mycology and Plant Pathology at the Swedish University of Agricultural Sciences.

When digging a hole in the soil of a coniferous forest, it is hard not to be struck by its characteristically sharp layers. This clear partitioning may seem counter-intuitive to the notion of the soil being a churning, stirring pot of worms, moles and fungi in constant labor. However, such tilling is limited in conifer forests, which is why the layers persist. They consist partly of fallen plant litter in various states of decay and partly of remains from roots and microorganisms. The uppermost phase is mostly freshly shed needles and leaves, twigs, cones et cetera. Below lies the humus layer, which contains old litter in such a late state of decay that no plant structures can be recognized any longer. Deeper down still, another sharp border can be found where the humus is succeeded by mineral soil.

Different fungal guilds dominate different soil types

Just as these layers have significantly varying properties, they are also colonized by different types of fungi. Basically, soil-living fungi can be divided into two guilds. Saprophytes grow freely in the soil and derive carbohydrates by degrading litter, whereas mycorrhizal fungi are involved in a mutualistic symbiosis with the roots of the plants, in which they obtain plant carbon in exchange for nutrients like phosphorous and nitrogen from the soil. These fungal guilds are distinctly separated within the soil, with the saprophytes dominating the litter and the mycorrhizal fungi mainly colonizing the humus. This might sound obvious at first; the rich content of fresh cellulose of the litter layer conveys a competitive advantage to those fungi that excel in quickly degrading such carbon sources, and the lack of accessible carbohydrates in the humus layer benefits fungi that are able to secure its carbon elsewhere, such as through mycorrhizal symbiosis with plant roots. However, some studies indicate that mycorrhizal fungi may also degrade organic substrate, albeit not as effectively as pure saprophytes, and although the humus layer mostly contains carbon sources that are more difficult to degrade, which disfavors saprophytes, they are not completely inaccessible. This makes it tempting to ponder on what is the fundamental driver of this vertical separation of the fungal guilds. Is it solely the difference in accessible substrates of the respective layer, or do other factors also play a role, such as antagonism between the guilds? This is the topic of a new study from the Department of Forest Mycology and Plant Pathology at the Swedish University of Agricultural Sciences.

Karina Clemmensen is one of the scientists behind the study. “If the composition of a fungal community colonizing a given substrate is exclusively controlled by the quality of that substrate, then it should not matter at what depth of the soil it is located”, she says. “We investigated this idea by placing mesh bags containing sterilized litter and humus respectively in both the litter and the humus layer of the forest, and subsequently analyzed the fungal community colonizing them after four and 16 months. If the hypothesis would hold true, the litter bags in either layer should get colonized by saprophytes, and the humus bags by mycorrhizal fungi.”

The DNA analysis of the fungal content of the bags, however, did not support that notion.  After four months, every bag mostly contained the same fungal species as the layer in which had been placed, that is, both litter and humus bags in the humus layer were colonized by this layer’s mycorrhizal fungi, and both bag types in the litter layer were colonized by the saprophytes dominating that layer.

Antagonism an important driver

“We observe that after four months, the force that vertically separates the fungal guilds is strong enough to prevent cross-layer colonization, in spite of the presence of a suitable substrate. We also see that both fungal guilds has the potential to colonize both substrates, at least in the absence of competition”, Karina Clemmensen explains.

After 16 months below ground, the bags were analysed again. This time, the picture had changed. Now, several fungal species had breached the barrier between the layers and colonized “their” substrate, that is, mycorrhizal fungi from the humus layer had migrated to the humus bags and vice versa.

“Considering the whole experiment”, Karina says, “it is clear that the quality of the substrate, either rich litter or poor humus, and their vertical localization, contributes in equal measures to the composition of the fungal community it is eventually colonized by. This means that antagonism between fungal guilds is an important driver in maintaining the partitioning.”

Karina Clemmensen and colleagues also investigated how the decomposition rate differs between bags buried in the respective layer. It became apparent that localization in the soil greatly affected decomposition, as the litter bags in the litter layer were consistently degraded faster than those in the humus layer.

“These analyses show that the fungal community in the respective layer was the sole effector of degradation, not abiotic factors such as moisture or temperature. This means that the mycorrhizal fungi dominating the humus soil are worse degraders than the saprophytes of the litter layer. This is hardly surprising since the mycorrhizal fungi do not need to degrade cell walls to obtain carbohydrates, but derive these from mutualism with plants. On the other hand, they do need nitrogen to offer the plant in return, which they extract from organic substrate. Indeed, we could see that the litter bags in the humus layer quickly lost nitrogen during the first months.”

Favored fungal guilds affect carbon sequestering

“In conclusion”, says Inga Bödeker, the first author of the article, “this project has shown that saprophytes and mycorrhizal fungi has overlapping potential ecological niches, but may use competitive advantages to defend their own turf against the other guild, which creates a vertical partitioning in the soil. We also saw that the fungal community in the litter is more proficient in degrading material with easily accessible carbon, but in poor substrate like humus different mycorrhizal fungi may outcompete these saprophytes, even if the substrate is located within the litter layer, such as with our bags.”

These results lend themselves well to speculation around the effect that different environmental changes may impose on fungal communities and the circulation of carbon. Rising atmospheric levels of carbon dioxide could potentially allow the plants to allocate more carbon to their roots and the mycorrhizal fungi, which would allow those an advantage over saprophytic fungi. This in turn would lead to less degradation of litter and more carbon bound in the soil. Correspondingly, actions that would disfavor mycorrhizal fungi, such as clear cut forests, would benefit saprophytes and subsequently increase litter decomposition rates and decrease soil carbon.