Soil microbes help plant species divide nutrients and promote community productivity

Shan Luo, Bernhard Schmid, Gerlinde B. De Deyn, Shixiao Yu

A fundamental prediction of ecological theory is that the maximum number of coexisting species cannot exceed the number of limiting resources. Plant species may have only 20 different limiting resources (light, water, CO₂ and the same set of mineral nutrients), and at most three or four resources are limiting in any plant community. However, there are more than 300,000 plant species globally, which challenges our ecological prediction. Therefore, it is important to ask whether plant species divide up limiting resources, and whether soil microbes help plants in this process. And if plant species do divide up resources, will they together be better able to exploit resources when multiple resources are available? First, we conducted a “monoculture experiment” by supplying different chemical forms of nitrogen (ammonium, nitrate or the amino acid glycine) to three tree species associated with different root symbionts (one associated with ectomycorrhizal fungi, one with arbuscular mycorrhizal fungi and the third associated with both arbuscular mycorrhizal fungi and N-fixing bacteria). We found that only in the presence of soil microbes, the three tree species preferred different forms of nitrogen. Then, we conducted a “mixture experiment” by growing the three species at three richness levels (one, two or three species) in soil supplied with a mix of the three nitrogen forms or no added nitrogen, and with or without soil microbes. We found that in soils supplied with the different forms of nitrogen plant mixtures produced more biomass than plant monocultures only in the presence of soil microbes. Our research provides direct evidence that co-existing tree species utilize different chemical forms of nitrogen with the help of soil microbes, which in turn enhances the productivity of diverse plant communities. Our research also enhances our mechanistic understanding of why there are so many plant species, and of the values of plant diversity and soil microbes.

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