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Abstract
Microbes are the unseen majority in soil and comprise a large portion of life's genetic
diversity. Despite their abundance, the impact of soil microbes on ecosystem processes
is still poorly understood. Here we explore the various roles that soil microbes play
in terrestrial ecosystems with special emphasis on their contribution to plant productivity
and diversity. Soil microbes are important regulators of plant productivity, especially
in nutrient poor ecosystems where plant symbionts are responsible for the acquisition
of limiting nutrients. Mycorrhizal fungi and nitrogen-fixing bacteria are responsible
for c. 5-20% (grassland and savannah) to 80% (temperate and boreal forests) of all
nitrogen, and up to 75% of phosphorus, that is acquired by plants annually. Free-living
microbes also strongly regulate plant productivity, through the mineralization of,
and competition for, nutrients that sustain plant productivity. Soil microbes, including
microbial pathogens, are also important regulators of plant community dynamics and
plant diversity, determining plant abundance and, in some cases, facilitating invasion
by exotic plants. Conservative estimates suggest that c. 20 000 plant species are
completely dependent on microbial symbionts for growth and survival pointing to the
importance of soil microbes as regulators of plant species richness on Earth. Overall,
this review shows that soil microbes must be considered as important drivers of plant
diversity and productivity in terrestrial ecosystems.