Nematodes are microscopic worms that are sometimes known as ‘roundworms’ or ‘eelworms’. Those living in soil are generally small (less than 1 mm long and only 15–20 µm wide) and can only be seen with a microscope.
Most grain growers are aware that nematodes cause hundreds of millions of dollars in crop losses every year. What is usually not recognised is that these pest nematodes are only one component of the soil nematode community. A diverse range of beneficial free-living nematodes also occur in soil, and they play a vital role in many important soil processes.
The nematodes found in soil can be sub-divided into several broad groups based on their feeding habits (figure 1).
Plant parasites: These nematodes are armed with a hypodermic-like spear that is used to feed on plant roots. This group includes the three main pests of grain crops in Australia: cereal cyst nematode (Heterodera avenae), lesion nematodes (Pratylenchus) and stunt nematodes (Merlinius).
Bacterial feeders: Bacteria are the most abundant microbes in soil, and nematodes are one of their main predators. One group of bacterial-feeding nematodes suck suspensions of bacteria into their mouths from the water film surrounding soil particles. Others use their lips to scrape bacteria from the surface of soil particles and organic matter.
Fungal feeders: Fungi, often the dominant component of soil microbial biomass, are an important food source for nematodes. Nematodes that feed on fungi are armed with a spear which is similar to the spear of a plant parasite, but smaller. After it is inserted into a fungal hypha or spore, the contents are sucked out.
Omnivores: Omnivorous nematodes are relatively large (1–3 mm long) and have a spear with a wide aperture that is used to feed on algal cells, fungal hyphae, oligochaete eggs and other nematodes. They are termed omnivores because they are able to obtain food from several sources.
Predators: The mouth of some nematodes is an open cavity armed with a tooth, and it is used to capture and consume nematodes and other small animals. Other carnivorous nematodes have a spear that is used to suck out the body contents of the prey.
Figure 1: Examples of different types of soil nematodes (a) plant parasite, (b) bacterial feeder, ( c ) fungal feeder, (d) omnivore, (e) predator. Source: K Linsell
Nematodes that are not parasitic on plants are often referred to as free-living nematodes, and they play many important roles in soil.
Microbial feeding: When nematodes graze on bacteria, new bacteria replace those that have been consumed. Thus, the prey population is continually replenished and microbial turnover increases. This increases nutrient cycling (see below) and ultimately improves root growth and plant performance. Fungal-feeding nematodes are also beneficial to plants, as they help suppress root diseases by feeding on fungal and oomycete pathogens such as Rhizoctonia, Fusarium and Pythium.
Microbial transport: Bacteria are relatively immobile in soil. Free-living nematodes play a crucial role as vectors, dispersing them to new areas. For example, nematodes transfer rhizobia to legume plants, helping to ensure roots are inoculated with these nitrogen-fixing bacteria.
Nutrient cycling: When free-living nematodes consume bacteria and fungi, nutrients held in an organic form within microbial biomass are converted to a mineral form (i.e. mineralised). For example, nitrogen held in proteins and other cell constituents is excreted as NH4+, which can then be utilised by plants. Thus, when populations of free-living nematodes increase, plant growth improves because roots take up the nitrogen released from microbes that were consumed.
A food source for predators: Since free-living nematodes are always present in soil, they provide a continual food source for predators such as nematode-trapping fungi, predatory nematodes and microarthropods. Thus, fungal and bacterial-feeding nematodes help maintain populations of organisms capable of preying on plant-parasitic nematodes.
Studies in Australian grain-growing soils have shown that populations of free-living nematodes vary with soil type and climate. However, there are usually between 5 and 20 free-living nematodes in every gram of soil.
Surveys over many years have shown that the nematode community in grain-growing soils is dominated by plant parasites.Root-lesion nematodes (Pratylenchus thornei, P. neglectus and P. quasitereoides) are widely distributed, and their population densities often range from 5 to 10 nematodes/g soil. However, populations greater than 50 nematodes/g soil have been recorded. Cyst, stunt, pin and spiral nematodes (Heterodera, Merlinius, Paratylenchus and Helicotylenchus, respectively) are also relatively common in some regions.
Studies of healthy soils under grass pasture or natural vegetation have shown that the nematode community has the following characteristics:
In grain-growing soils, plant parasites rather than free-living species tend to dominate the nematode community. Populations of omnivores and predators are low. The overwhelming conclusion from such observations is that the biological status of many of the soils used for grain production is relatively poor.
Further information on nematode community analysis can be found in the Fact Sheet ‘Nematodes as a Biological Indicator’. Details on how results of such analyses can be interpreted and used improve soil and crop management practices are given in the Fact Sheet ‘A Practical Test to Assess the Biological Status of Australian Grain-Growing Soils’.
Authors: Graham Stirling (Biological Crop Protection), Katherine Linsell (South Australian Research and Development Institute)
The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.
The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.