What is biological nutrient cycling?

Biological nutrient cycling is the process where soil microorganisms (microbes) consume soil nutrients and organic matter, absorbing these nutrients into their bodies then releasing them either voluntarily (i.e. mychorrizal fungi directly exchanging nutrients for plant root exudates) or involuntarily through predation (i.e. protozoa eating bacteria and excreting the excess/waste). This nutrient rich ‘dead microbe waste’ can then be consumed by other microbes or taken up by plant roots, completing the cycle.

Some bacteria and fungi are especially adapted to ‘unlock’ nutrients and minerals that are tightly bound to soil particles and/or parent material, providing a natural ‘input’ into the nutrient pool.

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Farmers can manage this biological nutrient cycling process to:

• Increase the pool of nutrients cycling in the soil and therefore available to plants.

• Reduce the amount of nutrients being lost via leaching or lock up, by having them bound in organic, microbial forms

The key driver and foundation of biological nutrient cycling is a diversity of healthy, green and growing plants.

Why? Because:

• Plants supply root exudates to soil microbes - this is the primary source of energy for this underground workforce.

• Different microbes partner with different plant species - more plant diversity means more microbe diversity and diversity creates resilience.

How nutrients are cycled - mutualistic relationships

Mutualistic relationships in the soil are those where organisms help each other.

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Rhizosheathing/Rastafarian roots as a result of mutualistic relationship between plant and soil microbes.

The key mutualistic relationship is between plants and microbes, specifically bacteria and fungi. Without them, plants’ access to nutrients and water and protection from pests and diseases is significantly reduced, causing reliance on external inputs.

First and foremost, plants capture the sun’s energy through photosynthesis to create sugars, fats, proteins, and other biochemicals.

Plants secrete these substances (root exudates) that attract and feed bacteria and fungi in the rhizosphere.

In exchange for the plant’s exudates, bacteria and fungi provide the plant with water, inorganic nutrients (e.g. phosphate), and protection from pests and diseases.

Mycorrhizae are mutualist fungi that grow into plant roots, allowing them to exchange these substances more efficiently. Mycorrhizal fungi are smaller than plant roots, therefore they can access parts of soil aggregates that plant roots can’t, and their extensive hyphae networks dramatically expand the area of soil that the plant root can access for water and nutrients - up to 100X!

Most plants are able to form these mycorrhizal relationships, except for the brassica family - the ability has been bred out of these species over time.

Mutualistic relationships do not just occur between one plant and one species or group of microbes. Mycorrhizal fungi also have mutual relationships with other organisms such as bacteria; exchanging organic (carbon) compounds for nutrients and protection. And, when there is a diversity of plant species, there are many microbes forming many mutualistic relationships with many plants; through the network of soil microbes, a diversity of plants can support each other in what is known as a facilitative effect - sharing water, nutrients, and protection with each other.

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How nutrients are cycled - predatory relationships

In the soil food web, there are many predatory links between all soil organisms that keep nutrients cycling.

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Plants supply this food (i.e. organic matter) in other ways too:

• as their roots and shoots grow and then die off

• as the whole plant dies

• as the living plant is eaten

Fungi and bacteria then decompose (i.e. eat) these forms of organic matter, releasing inorganic nutrients (e.g. phosphate, ammonia) into the soil for plants to take up.

Fungi and bacteria are then eaten by protozoa, nematodes, and arthropods, again releasing inorganic nutrients to the soil in the process. The same applies when nematodes, protozoa, and arthropods are eaten by organisms such as worms and vertebrates.

Then, when these larger organisms (e.g. earthworms) naturally die, the process of feeding and releasing nutrients happens in reverse.

The cycle continues, and nutrients can be cycling in both directions at the same time!

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