May 19, 2021
Hear the word nematodes and you might immediately think soybean cyst. But while some nematodes can cause harm in plants and animals, non-parasitic nematodes have many beneficial roles.
"It might be hard to believe, but you may never have seen the most abundant animal on Earth: soil nematodes," Ashley Shaw from the University of Oregon writes in the Soil Matters blog. "They represent 80% of animal life by number and live in nearly every habitat. They are hard-working and important organisms."
Soil-dwelling nematodes are usually between 1/500th to 1/20th of an inch in size. By contrast, there is a nematode that lives inside sperm whales that is nearly 30 feet long.
Some of the best-known nematodes are parasites and there are different nematode parasites of plants and animals. Some nematodes live in or on a plant or animal and cannot survive without them. Some kill their host and then move on. Others are free-living. In soils, nematodes live in water films that surround soil particles. Both plant root parasitic and free-living nematodes play an important role in plant health and plant feedback to soil carbon.
An incredible variety of soil nematodes exist at all levels of the soil food web. At the base of the food web, some feed on plants and algae, others graze on bacteria and fungi. At higher levels in the food web, nematodes that are predators and omnivores eat other invertebrates, protists, and even other nematodes. In some cases, “predatory” nematodes are the “good guys,” keeping populations of parasitic nematodes in check.
This food web is important to plant health and soil carbon storage. For example, by feeding on bacteria and fungi, microbial grazing nematodes help return nitrogen to the soil through their waste. This makes the nitrogen available again for plant use, improving plant growth.
Nematodes bring other species into the soil food web, too. Some bacteria survive the nematode gut and are deposited along with nematodes’ waste products. Still more hitch a ride on the outside of nematodes’ bodies. As nematodes move around in soil, they deposit bacteria in new places, spreading them around. The bacteria can contribute to and speed the process of decomposition, returning carbon to the soil for storage.
But most good things have a limit: at very high populations, nematodes that feed on bacteria and fungi can reduce their populations. This can lead to lower decomposition and nutrient turnover rates by bacteria and fungi, even lowering plant growth.
Plant parasitic nematodes attack roots using a piercing tool in their mouth. This “stylet” punctures plant cells so it can suck its carbon-rich juices. Some nematodes release chemicals that cause lesions or tumor-like growths on roots. They drain the plant's strength above- and belowground.
In small populations, plant parasitic nematodes can stimulate root growth, but in high numbers they destroy roots, stunt aboveground growth, and cause disease. Lower plant growth (of both roots and shoots) leads to lower return of organic material to soil and eventually, lower soil carbon.
While the nematode species responsible for plant diseases have received a lot of attention, far less is known about the non-parasitic part of the soil nematode community, which plays mostly beneficial roles in soil. Ensuring a balance between beneficial and plant parasitic nematode groups is important for plant health and its contributions to soil carbon.
Source: The Soil Science Society of America, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.
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