THE NEMATODE COMMUNITY IN AUSTRALIAN GRAIN-GROWING SOILS

Key Points

Pest nematodes can cause damage to plant roots but there is also a diverse range of beneficial free-living nematodes in soil.

The main types of free-living nematodes are bacterial feeders, fungal feeders, omnivores and predators. They help in the cycling of nutrients by feeding on other soil organisms.

A healthy soil will have a high number and diverse range of free-living nematodes and low numbers of pest nematodes.

What are nematodes?

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.

Types of nematodes in soil

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

Ecological role of free-living nematodes

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.

The free-living nematode community in grain-growing soils

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.

A group of nematodes known as fine-tailed tylenchids are the most common nematode in many soils. Some feed on fungi and others on fine root hairs but do not cause economic damage.

A wide range of bacterial and fungal-feeding nematodes also occur, and along with the fine-tailed tylenchids, they make up the microbe-feeding component of the nematode community.

Populations of omnivorous and predatory nematodes are usually low, comprising less than 1% of the free-living nematode community. However, there are situations where populations of these ecologically important nematodes are much higher.

Plant-parasitic nematodes in grain-growing soils

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.

What does the nematode community tell us about the health of grain-growing soils?

Studies of healthy soils under grass pasture or natural vegetation have shown that the nematode community has the following characteristics:

a relatively high population of free-living nematodes and low population of plant parasites

a diverse range of free-living species, and a reasonably high proportion of omnivores and predators.

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.

Fact Sheets The Nematode Community In Australian Grain-Growing Soils