Nematodes as biological indicators of soil health

Soil animals at higher trophic levels, particularly nematodes, are generally considered better biological indicators than soil bacteria and fungi.

Nematodes are commonly used as biological indicators. They occur in all soils and, even in relatively poor soils, there are millions in every square metre.

Nematodes feed on plant roots and on all the organisms that live in soil.  The size of their population can also give information about the organisms that they eat (e.g. bacteria, fungi, algae, protists, springtails, and nematodes).

Nematode numbers fluctuate in response to the population dynamics of the organisms they consume. They are also influenced by the soil physical and chemical environment.

Nematodes are readily extracted from soil, and their food sources can be determined by looking at their mouthparts under a microscope:

• A high ratio of bacterial- to fungal-feeding nematodes indicates nutrient cycling is occurring rapidly through bacterial decomposition. It is also indicative of high nitrogen inputs or recent tillage
• A predominance of fungal-feeding nematodes indicates that the food web is dominated by fungi, and that biological nutrient cycling will be relatively slow, and soils are relatively undisturbed
• Low populations of omnivorous nematodes indicate that the soil biology is likely to be affected by pollutants or excessive fertiliser inputs, but also disturbance through practices such as tillage
• A high population of omnivorous and predatory nematodes indicates that the soil is biologically complex and has some capacity to suppress populations of parasitic nematodes and other soilborne pathogens

Ecological indices are calculated from the species and numbers of nematodes present in a soil sample.

Enrichment index:

• Reflects the presence of those nematodes that reproduce quickly and respond to high levels of nutrients, generally bacterial-feeders that indicate rapid-nutrient cycling and a surplus of available nutrients

Structure index:

• Reflects the stability and undisturbed nature of the food web as it depends more on the sensitive and relatively long-lived predatory and omnivorous species

A healthy soil is likely to have a nematode community with a low enrichment index and a high structural index.

Manual nematode community analysis is not something that is currently done on a routine basis. The methods used to extract nematodes from soil are laborious, and expertise are required for identification.

DNA-based tests (i.e. PCR-based methods) are mainly applied to single organisms and are used to measure populations of plant-parasitic nematodes.

Recent advances in rapid DNA sequencing (metabarcoding) could provide a practical and affordable way for laboratories to determine the composition of the soil nematode community.

Soil management that supports a healthy and diverse soil biological community will give more effective and resilient regulation of soil function within cropping systems.

The physical structure of soils (air/water balance) and the chemical environment (pH, nutrients) provide the habitats for roots and soil organisms to interact. Therefore, biological indicators of soil health, such as nematodes, need to be considered together with indicators of the physical and chemical properties of soils and interpreted carefully for each soil type and cropping system.