Filmed on location in the Rhizosphere              

Root Exudates

Bacteria

Fungi

Protozoa

Nematodes

Can you feel the rhythm of the rhizosphere beneath your feet? Or chuckle as busy bacteria and fungi feast at the root exudates buffet until their tummies are plump with nitrogen fertilizer? Or stand horrified as large hungry protozoa, and even larger nematodes, ambush these over-stuffed microbes, eat them by the thousands, and excrete the excess carbon and other nutrients into the rhizosphere?

Don’t worry if you missed the gory story! The setting and cast of characters are microscopic.

So grab your electron microscope. Let’s dive into the top six-inches of soil. Here you will meet a few microbe characters who live in this mysterious silent kingdom known as the rhizosphere.

The Rhizosphere

Welcome to this narrow and very sticky region. It is home to many different soil organisms. Its borders extend about one tenth of an inch from the plant’s roots. Within this rhizosphere, there is a constant mix of microbe characters, each of whom plays a unique role: bacteria, fungi, protozoa, nematodes, and even larger organisms.

Root Exudates

The director of ‘The Silent Kingdom’ has just walked on the set. So, pop your head through the soil again, and let’s go meet him.

Sunny Photosynthesis, an intensely focused director, sits among the leaves. He senses that his cast of microbes is hungry. Sunny wipes the perspiration from his brow, and directs the roots to feed the hungry microbes. At his command, a chemical secretion of root exudates, rich in carbohydrates and protein, starts to ooze out of the roots into the rhizosphere. Lunch has arrived!

The tiniest members of the cast are bacteria and fungi. They are the first to arrive at the root exudates buffet table in spite of being on the lowest level of the food chain. They stuff their small bodies with nitrogen and other nutrients from the exudates. They are like little bags of fertilizer eager to provide the plant with its vital seasonal nutrients. Bacteria are so small that they produce a slime that sticks to things, otherwise they would wash away. This slime is useful because it traps pathogens before they reach the root, and it helps soil particles to stick together. Bacteria produce streptomycin which helps the plant maintain good health.

Look at the bacteria sitting on the fungal hypha strands.

Fungi, sometimes introduced as mycorrhizal fungi, are skilled protectors of the roots. If foraging root-eating nematodes try to penetrate a root, then their access is instantly barred by the swashbuckling flamboyance of a fungal hypha strand. These courageous fungi provide water, phosphorous, and other necessary nutrients to the roots. Sometimes they deliver vitamins and antibiotics to the roots in the form of penicillin.

Protozoa

A story isn’t complete without the all-important protozoa villains – all 60,000 species--the majority of which live in the soil. Their main role is to monitor  population-control. They make sure that bacteria don’t get out of hand, so they eat them—about 10,000 per day, each.

Their secondary role is ‘intense adaptability'. Protozoa need moisture to live, travel, and reproduce. If there’s a dry spell, then the protozoa stop feeding, go dormant, and encase themselves in a cyst – sometimes for several years.

Here is the largest protozoa called Ciliates.  They move rapidly through the soil using fine oar-like cilia surrounding their bodies. (Photo Credit: Elaine R. Ingham, Oregon State University, Corvallis)

The good news is that protozoa’s waste contains carbon and nitrogen compounds including ammonium which the plants need. Nearly 80% of the nitrogen a plant needs comes from the wastes produced by bacteria-eating and fungi-eating protozoa.

Nematodes

This long, thread-like microbe varies in length from 2 millimeters to a whopping 30 feet! Complete books could be written about this member of the cast. No worries soil diggers! Your electron microscope will be focused on the 2 millimeter variety found in soil. To ease your curiosity, the 30-foot nematode is found in the placenta of sperm whales. We’ll save that information for another time.

These blind roundworms mineralize nutrients when they ingest bacteria and fungi—just like protozoa. There are 20,000 known species. Scientists estimate that there could be as many as  one million species.

This cast member is unfairly criticized by gardeners as a predator of plant roots. This is somewhat true when soil is unhealthy. However, when the rhizosphere is intact and the microbial cast members work well together, then the predator root-eating nematodes are kept under control.

Actually, the majority of nematodes are high-maintenance and very picky. A teaspoon of healthy soil usually contains 20 bacteria-eating nematodes, 20 fungus-eating nematodes, and very few predators. They require porous soil in which to travel. If the soil is too dense and their search for food is hindered, then they will die. Consequently, the nitrogen in the soil will be greatly reduced, too.

So, it is in the best interest of the cast of characters to cater to the needs of picky nematodes as they search for food. They must be given space to blindly ‘feel’ minute changes in temperature in order to locate their preferred food source – bacteria and fungi.

We hope you enjoyed your visit to the land of rhizosphere, where the rhythm below your feet is alive. There are many more actors in this soil story, but we’ll save them for another time.

Written by Linda Rivers 

May 20th 2018

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