A symphony orchestra in the soil
Wim van der Putten's first article as 'reviewing editor' for Science stresses the importance of fungi and other soil organisms for aboveground plant diversity. Just think of a symphony orchestra: "you have to listen to all the musicians together to understand the tune".
"A handful of soil can contain more than 5000 species", writes Wim van der Putten in his 'Perspectives' piece in the 13 January issue of Science.
Together, these species "operate like a gigantic symphony orchestra. Groups of players can be classified according to their instruments, but the overall tune can be understood only when listening to all musicians together."
Results in the field
The soil equivalent of the symphony orchestra's 'tune' is plant-soil feedback, which ecologists measure by comparing plant performance in soil previously exposed to the same or other plant species. Such comparisons have been done mostly under greenhouse conditions until now.
But in his 'Perspectives' article, Van der Putten reviews two studies published elsewhere in the same issue of Science that specifically assess the role of plant-soil feedback in the field, by Jonathan Bennett (et al) and by François Teste (et al).
Van der Putten: "the two research groups have linked their results so strongly to the field that it's now evident such processes do play an important role in nature."
Protection vs diversity
Bennett and his team looked at two types of tree species in real forest settings in the United States. The first type of tree is associated with ectomycorrhizal fungi, which form a dense network around plant roots. The second type with arbuscular mycorrhizal fungi, which penetrate plant roots.
After planting tree seedlings with or without mycorrhizal fungi in the forest, the researchers found that arbuscular mycorrhizal fungi protect tree offspring less well against pathogen effects than ectomycorrhizal fungi.
But that also means arbuscular mycorrhizal trees are better promotors of forest diversity, as more seedlings from other tree species could be found under arbuscular mycorrhizal trees than under ectomycorrhizal trees.
For Van Der putten, it proves that you have to look beyond just the positive symbiotic effects that many studies focus on: "we can only understand plant species diversity when combining the positive and negative interactions between plants and their soil organisms."
First violin
The other study under review, which François Teste carried out in Australian shrublands, showed that plant-soil feedback effects of arbuscular mycorrhizal plant species were in fact neutral rather than positive or negative.
To Wim van der Putten, this "suggests that the effects are context-dependent". Together, the two studies nevertheless open up 'new avenues: "what role do the symbiotic soil microbes play in the soil symphony orchestra: do they play the first violin, or are they led by the soil pathogens?"
Further field studies will be crucial, the new reviewing editor concludes, "for understanding context dependence and inclusion of plant-soil feedback effects in predictions of ecosystem responses to global changes."
