Nematodes are the most abundant multicellular animals on earth, present in just about every underground habitat. They are small, non-segmented worm-like animals about less than 5 mm long.
Most nematode species have a beneficial role in the soil, but a few are pests that feed on plant roots, damage roots directly or make them more susceptible to fungal diseases. Beneficial nematodes are important in controlling pest species as well as improving overall soil health.
As with most soil organisms, nematodes are concentrated in the top few centimeters of soil and in the rhizosphere around plant roots. They live in the thin films of water surrounding soil particles, which they use to move. Nematodes are found in well-structured soils with large pore spaces like those that are sandy or loam-based.
Nematodes can be divided into FOUR groups based on their diet.
Bacterial-feeders consume bacteria.
Fungal-feeders feed by puncturing the cell wall of fungi and sucking out the internal contents.
Predatory nematodes eat all types of nematodes, microbes, and protozoa. They eat smaller organisms whole, or attach themselves to the cuticle of larger nematodes, scraping away until the prey’s internal body parts can be extracted.
Omnivores eat a variety of organisms or may have a different diet at each life stage. Root-feeders are plant parasites, and thus are not free-living in the soil.
Backyard Nematode Ecology
Nutrients, including nitrogen and phosphorus, stored in the bodies of bacteria, fungi, and other organisms are released when nematodes eat and digest them. The bacteria and fungi contain more nutrients than the nematodes need so the excess is released into the soil in a form that can be used directly by plants.
Some beneficial nematodes physically break down organic matter, which makes it easier for other organisms to break it down further. This promotes the cycling of carbon and nutrients in the soil.
Dispersal of microbes
Bacteria and fungi cannot move around in the soil without relying on nematodes for transportation, either by riding on or in bodies. Nematodes are parasitised by some bacteria and fungi, which helps their dispersal as nematodes move in the soil.
Some predator nematodes attack and kill a range of pests such as tree borers, grubs, thrips and beetles with little effects on other organisms. These nematodes use two strategies to find their prey. Some species wait for their prey to move past them in the soil and locate them by direct contact through ambush. They function at the soil surface where they attack highly mobile pests such as cutworms and armyworms. Others actively search out their prey using a searching strategy. They function at various depths in the soil and prey on slow moving targets such as grubs and other insect larvae. These insect eaters are known as entopathogenic nematodes.
Agricultural cultivation tends to encourage an increase in plant parasitic nematodes over other species. This is because cultivation causes disruption that impacts more larger soil organisms than small ones, and so causes the smaller plant parasitic nematodes to flourish. Cultivation also disrupts soil fungi, which are the food source for many of the beneficial nematodes described above. Nematicides (chemical controls) are broad spectrum and when applied can also reduce the population levels of beneficial nematodes. Other agricultural chemicals for reducing weeds or insect pests can also reduce beneficial nematode populations by reducing their food sources.
To ensure beneficial nematodes remain abundant in the soil, or to allow damaged populations to recover, they need food in the form of organic matter and other soil organisms, water, and not tilling the soil. Management options that support beneficial nematodes in soil also inevitably support good soil microfauna in general.
Analysis of the diversity of nematode communities in the soil is a valuable indicator of soil biological fertility (soil health). The ratios of bacterial, fungal feeders, and omnivores can indicate the type of healthy soil functions going on beneath our feet. Varying numbers can indicate the types of food web (carbon or nitrogen based) and the status of disturbance amount, maturity of the soil, and potential degradation of soil.
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