In a new study, researcher Nico Eisenhauer and his team examine how plants influence communities of bacteria and fungi beneath the forest floor. From allocating carbohydrates to soil horizons, to structuring communities of soil fauna, plant diversity drives a broad range of species interactions. These photosynthetic organisms have a leading impact on these soil inhabitants, that in turn, promote further ecological feedbacks. Again, research like this hints at how complex life is beneath the forest floor.

A reoccurring theme I keep reiterating here on FFN is that the forest floor is not a place of benevolence. Yes, mutualisms occur, but if one partner isn’t pulling their weight, the relationship dissolves and both parties must fend for themselves or find a new partner. Competition is king, and what we see in natural communities today is really just the tip of the iceberg from both a temporal and spatial scale. Temporally, these communities represent the descendants of the organisms that competed the best over millennia. Spatially, we only can see aboveground biomass, with most biophysical interactions occurring in the dark soil depths. 

I’m telling you, there’s a war going on in your own backyard. Plants constantly tussle for nutrients, light, pollinators and suitable communities of bacteria and fungi. In many cases, they do this through chemical means, pumping secondary compounds into the soil called allelochemicals. These allelochemicals can directly reduce competitor fitness by slowing neighboring plant growth, or halting seed germination. What’s most interesting to me is that these compounds can also indirectly reduce the fitness of neighboring plants, by promoting the soil community to shift towards species assemblages that benefit their own fitness.

The fungi and bacteria living within plant rhizospheres have been a driving force in plant interactions since plants first made their way to terrestrial habitats. The most common misconception about plants, is that some see them as steady state organisms. Because they are sessile, one may falsely perceive them to interact with their environment in a continuous, predictable manner. However, this is not the case, and depending on the abundance and diversity of competitor plants present, a single plant organism may employ a diverse array of strategies. 

To better understand this dynamism, these scientists studied the role that plant richness has on their microbial communities by using microcosm experiments. In each microcosm, either 1, 3 or 6 plant species where planted in the same soil type, containing the same microbial community. Interestingly, like they predicted, these plants interacted with their soil environment more robustly when they shared the soil with more plant species. These plants growing in microcosms with the most diversity had significantly greater root biomass, shoot biomass, and produced the most allelochemicals. In the face of competition, nutrients from the soil and fixed carbon from photosynthesis is quickly assimilated into below/aboveground structures as well as allelochemicals. These dynamic strategies radiate throughout the microbial community living around plant roots.

With more plant diversity, the bacteria and fungi living in the rhizosphere also grew in biomass. Along with more allelochemicals, more sugar rich photosynthate is pumped into the soil. This entices the microbiome to mobilize more nutrients that the plant can access. The increased root biomass also enhances microbial growth with the soil itself becoming more structured, allowing the communities along with the plant exudates to congregate rather than dissipate throughout the soil horizons. Although the soil fungi and bacteria both had a positive response to plant diversity, the fungi had a slightly greater biomass increase. This resulted in the marginal community shift towards more fungal dominance. 

These interactions are diverse and complicated. A ton of factors are manipulated in the face of competition and it was not my goal here to explain them all as it is nearly impossible to do so. There is however an ecological feedback beckoning. Diverse plant assemblages drive competitive responses like enhanced root biomass, and increases in allelochemicals and plant photosynthate entering the soil. Together these responses enhance microbial growth and can shift these communities living in the rhizosphere to be slightly more fungal dominant. More microbial growth relates to plant enhancement through the increases in mobilized nutrients. Simply put, the more exudates reaching the soil, the more microbial growth occurs. With more microbial biomass, the greater the plants can grow and allocate even more resources to the rhizosphere. We take for granted the functionality associated with diverse plant communities. It is these species assemblages that can get the most out of their environment by promoting microbial growth and the services they provide.

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