Whispers in the dark - do earthworms talk to plants?

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Earthworms are widely recognised as being beneficial to ecosystems. They are responsible for many of the soil processes that give rise to so called "ecosystem services", that is the things that ecosystems do and provide for humans, such as food production and recreational opportunities. One commonly observed benefit that earthworms have on ecosystems is to increase plant growth. Plants tend to grow larger and more rapidly when earthworms are in the soil. With increasing concerns about food security and sustainability anything that can increase plant productivity in a "natural" way warrants investigation. There are a number of mechanisms by which earthworms could increase plant growth. They might alter the micro-organism population in the soil, either increasing numbers of beneficial micro-organisms or reducing numbers of detrimental ones; ingestion of seeds by earthworms may increase germination success rates; earthworm consumption and digestion of organic matter may increase nutrient availability; burrowing activity may increase the aeration of the soil or the amount of water the soil can hold. All the above mechanisms undoubtedly play a part in the way that earthworms boost plant growth. Another, less well understood mechanism by which earthworms might affect plant growth is through the production of plant growth promoting hormones.Plants, just like animals, respond to hormones. Soil micro-organisms exploit this fact to get food. Plant roots release organic compounds into the soil; soil bacteria feed on these compounds. Other soil micro-organisms feed on the bacteria and this releases nutrients that help the plants to grow. The soil micro-organisms release plant growth promoting hormones which leads to more plant growth. This increase in plant growth leads to the release of more organic compounds by the plants into the soil and thus an increase in the amount of food available for micro-organisms.Recently it has been suggested that earthworms might also increase the concentration of plant growth promoting compounds in soils. They might do this for similar reasons to the bacteria, to increase the amount of food that is available to them. Earthworms feed on root debris and soil micro-organisms. By increasing levels of plant growth promoting compounds in the soil, earthworms could stimulate the production of more plant roots and also more soil micro-organisms feeding on the increased levels of plant secreted organic compounds that come from them. How earthworms increase the concentration of plant growth promoting compounds is not known and that is what we will seek to discover in this project.Earthworms could release plant growth promoting hormones themselves, the hormones could be produced by bacterial populations that are boosted by earthworm activity, or the hormones could be produced by the digestion of organic matter by earthworms. Plants may also have a role, stimulating earthworms to produce the hormones. Our study will consist of a series of experiments in which we test a variety of earthworm - bacteria - plant - soil present / absent combinations to allow us to state which of the above mechanisms are important.Our results could help lead to a change in land management practises to maximise the natural boost that earthworms give to plant growth.