Soil Organisms and their roles in the Rhizophere

[amideutch]

I came across this post in a orchid forum and is the best explanation of soil microbes in a nutshell I have seen. Enjoy, Ami

There is no simple answer to you questions, it is actually incredibly complex. First of all, think of all the organisms that are a part of the soil biological community: bacteria, algae, cyanobacteria, fungi, protozoa, nematodes, and arthropods. Not there that there is not a time and place for us to step in with chemicals to disrupt a plant being over run and killed by runaway pathogen type bacteria and fungi, but the problem is always we also kill everything. Often doing more harm than good. Here is why! A few excerpts from my files. Everything here holds true for that little pot of media we use to grow orchids in. More than you ever wanted to know.

Plants, algae, cyanobacteria, lichens, bryophytes, and photosynthetic bacteria use the sun’s energy to fix atmospheric carbon to sugar form and are the primary producers that fuel the food web. Other soil organisms obtain energy and nutrients by feeding on primary producers and each other. These organisms are the consumers. The mass of dead organisms (or pieces and parts thereof) in the soil is referred to as "soil organic matter." You can think of soil organic matter as a bank, storing currency (nutrients and energy) that help both producers and consumers prosper.

Mycorrhizal fungi colonize the roots of many plants. Mycorrhizal fungi don’t harm the plant; on the contrary, they develop a "symbiotic" relationship that helps the plant be more efficient at obtaining nutrients and water. In return, the plant provides energy to the fungus in the form of sugars.

Here’s how that symbiotic relationship works. The fungus is actually a network of filaments that grow in and around the plant root cells, forming a mass that extends considerably beyond the plant’s root system. This essentially extends the plant’s reach to water and nutrients, allowing it to utilize more of the soil’s resources.

There are two main categories of mycorrhizae

Vesicular-arbuscular mycorrhizae or VAM. VAM is a type of endomycorrhizae (endo = inside), and is the most widespread of the mycorrhizae. These fungi actually reside inside the cells of the plant root. They’re typically found associated with most grasses, forbs, shrubs, and a few trees such as juniper. They are generalists, have only a few species, and are slow to disperse.

Ectomycorrhizae (ecto = outside) grow around the root and between the root cells, but unlike VAM, the fungus doesn’t actually penetrate the root cells. The fungus also forms a considerable mass in the soil surrounding the plant roots. The fruiting, or reproductive bodies, of these fungi are sometimes visible as something we all recognize— mushrooms!

Ectomycorrhizae are commonly associated with forest trees of temperate regions. On rangelands, they are found in riparian areas (the places next to water), open woodlands, and shrub oak communities. They are host-specific, have many species, and can disperse far and quickly.

Not all fungi are mycorrhizal. There are also fungi that help decompose the organic matter in litter and soil.

Why They Are Important

Some plants are "mycorrhizal-obligate," meaning that they can’t survive to maturity without their fungal associate.

Mycorrhizae are particularly important in assisting the host plant with the uptake of phosphorus and nitrogen, two nutrients vital to plant growth.
Mycorrhizae actually increase the surface area associated with the plant root, which allows the plant to reach nutrients and water that might not be available otherwise. Put simply, mycorrhizae extends the plant’s reach, allowing it to get to more of what it needs to survive. That makes the plant stronger, especially during drought periods. Stronger individuals means that the community is more resilient to disturbance. Some mycorrhizae may even protect their host plant against unwanted pathogens.

Bacteria and Actinomycetes ... why are they important?

Bacteria are minuscule, one-celled organisms that can only be seen with a powerful light (1000X) or electron microscope (we're talking TINY). They can be so numerous that a pinch of soil can contain millions of organisms. Bacteria are tough—they occur everywhere on earth and have even been found over a mile down into the core of the earth.

Bacteria can be classified into five functional groups. Autotrophic (literally, self-feeding) bacteria are photosynthetic. They are the primary producers. Decomposers consume soil organic matter, plant litter, and simple carbon compounds, releasing the nutrients in these substances for use by living plants. Mutualists, such as nitrogen-fixing bacteria, form associations with plants and help them absorb nutrients. Pathogens are the bad guys— they cause disease in plants. The last group, the chemolithotrophs (literally, chemical and rock-eating) obtain energy from minerals rather than from carbon compounds.

Bacteria are common throughout the soil, but tend to be most abundant in or adjacent to plant roots, an important food source.

Actinomycetes are a broad group of bacteria that form thread-like filaments in the soil. They are responsible for the distinctive scent of freshly exposed, moist soil.

Bacteria are important in the carbon cycle. They contribute carbon to the system by fixation (photosynthesis) and decomposition. Bacteria are important decomposers in grassland environments. Actinomycetes are particularly effective at breaking down tough substances like cellulose (which makes up the cell walls of plants) and chitin (which makes up the cell walls of fungi) even under harsh conditions, such as high soil pH. Some management activities, particularly those that change nutrient levels in the soil, can shift the dominance of decomposers from bacterial to fungal. When one group becomes dominant where it shouldn't be, there is also a shift in the rest of the system. The shift from bacterial to fungal dominance, for instance, can enhance the conditions.

Bacteria are particularly important in nitrogen cycling. Free-living bacteria fix atmospheric nitrogen, adding it to the soil nitrogen pool. Other nitrogen-fixing bacteria form associations with the roots of leguminous plants such as lupine, clover, alfalfa, and milkvetches. Actinomycetes form associations with some non-leguminous plants and fix nitrogen, which is then available to both the host and other plants in the near vicinity. Some soil nitrogen is unusable by plants until bacteria convert it to forms that can be easily assimilated.

Some bacteria exude a sticky substance that helps bind soil particles into small aggregates. So despite their small size, they help improve water infiltration, water- holding capacity, soil stability, and aeration.

Wait! Aren't there also "bad" bacteria? Yes, there are, but some soil bacteria suppress root-disease in plants by competing with pathenogenic organisms. The key is in maintaining a healthy system so that the good guys can do their work.

Bacteria are becoming increasingly important in bioremediation, meaning that we (people) can use bacteria to help us clean up our messes. Bacteria are capable of filtering and degrading a large variety of human-made pollutants in the soil and groundwater so that they are no longer toxic. The list of materials they can detoxify includes herbicides, heavy metals, and petroleum products.

Soil Protozoa
Protozoa are tiny single-celled animals that mainly feed on bacteria (think of them as little grazers), although some eat other protozoa and organic matter. While protozoa are many times larger than bacteria, you still need a microscope to see them. A pinch of soil can contain thousands.

Protozoa are classified into three groups based on structural peculiarities. Ciliates are the biggest and use many hair-like cilia to scoot through soil and water. Amoebae are small (relatively), amorphous, and use temporary feet to move around. Flagellates are the smallest and swim using a few whip-like flagella. All protozoa need water to move through soil, however, they only need a thin film surrounding the soil particles to get around. Believe it or not, they can be quite active even in very parched desert soils.

Protozoa are found in soils everywhere: even in very dry desert. However, they are most abundant near plant roots, because that's where both bacteria and organic matter (i.e., food) are concentrated in the soil.

Why They Are Important

Protozoa play an important role in the structure of the soil food web. Protozoa eat bacteria and release nitrogen and other nutrients in their waste. Since protozoa are concentrated near plant roots, the plant can benefit from this supply of nutrients. Protozoa can stimulate the rate of decomposition by maximizing bacterial activity.

Protozoa are in turn consumed by nematodes and microarthropods. Remember what we said about missing parts? Think of what would happen if there were no protozoa.

Okay, so they aren't all good guys. Some protozoa attack roots and cause disease in rangeland plants. However, other protozoa feed on root pathogens, thus reducing plant disease.



Soil Nematodes ....What They Are and A Few Interesting Facts

Nematodes are tiny roundworms that are common in soils everywhere, from the freezing Arctic to dry, hot deserts. They are particularly abundant in grassland ecosystems. To give you an idea of exactly how common nematodes are, consider this: one cubic foot of soil can contain millions.

Nematodes can be most easily classified according to their feeding habits. Some graze on bacteria and fungi. Some like plant roots; others prey on other tiny animals. Some aren't fussy at all and will eat any of the above mentioned food items.

Nematodes can't move through the soil unless a film of moisture surrounds the soil particles. Under hot, dry conditions, nematodes can become dormant, allowing them to survive long periods of drought. As soon as water becomes available, they quickly spring back to life.

Why They Are Important

Among the thousands of species that have been identified, many are considered beneficial because they boost the nutritional status of the soil. Nematodes feed on decaying plant material, along with organisms that assist in the decomposition of organic matter (bacteria and fungi). This helps disperse both the organic matter and the decomposers in the soil. Increased organic matter concentration and decomposition boost nitrogen and phosphorus levels.

Because some nematodes prey on other animals, they can be useful for control of pest insects. Nematodes are also being investigated for their potential as biological controls for noxious weeds.

Nematodes aren't all good guys. Some damage the roots of domestic crops, costing U.S. farmers an estimated $8 billion a year. Nematode infestations can be identified by yellowing, stunted plants that grow in sparse stands. Research is underway to develop plants that can resist nematode predation.

Soil food webs are wonderful in their complexity. But like any complex system, the whole is greater than the sum of its parts, and if parts are removed, well, the whole thing disintegrates. Think of your car: you can do without a windshield wiper for a while, or a window, or a glovebox cover. But start removing tires, the steering wheel, the clutch, the brakes, and you won’t be going very far. The soil food web functions in a similar way. Nobody may miss a few ants, a little algae. But if key organisms or multiple groups disappear, you’ve got a problem. And people wonder why their orchids often do better when they are left alone and not given too much human attention. We often do more harm than good being overly attentive.

[matereater]

Great article, thanks Ami!!

[fortyonenorth]

Thanks Ami - quite a nutshell!

[saltmarsh]

Excellent, Ami. Thanks so much.