Diversity Of Soil Organisms

Microfauna – Nematodes

Part 3


Nematodes are microscopic, wormlike organisms that live in water films and water-filled pore spaces in the soil. Typically, they are most abundant in the upper soil layers where organic matter, plant roots, and other resources are most abundant. Nematode abundance in soils—managed and unmanaged—ranges from 1–10 million individuals/m2

Most research on soil nematodes has focused on the plant-parasitic nematodes that attack the roots of cultivated crops. Less attention has been given to nematodes that are not plant-feeders and play beneficial roles in the soil environment. This nugget describes the important roles played by nematodes in soil ecosystems, as well as their potential to be used as indicators of soil condition in organic farming systems.

Nematode Feeding Habits

Nematodes can be classified into functional groups based on their feeding habits, which can often be deduced from the structure of their mouthparts. In agricultural soils, the most common groups of nematodes are the bacterial-feeders, fungal-feeders, plant parasites, predators, and omnivores. Predatory nematodes feed on protozoa and other soil nematodes. Omnivores feed on different foods depending on environmental conditions and food availability; for example, omnivorous nematodes can be predators, but in the absence of their primary food source, they can feed on fungi or bacteria.

Importance of Nematodes in Agricultural Systems

Nematodes contribute to a variety of functions within the soil system. In agricultural systems, nematodes can enhance nutrient mineralization and act as biological control agents.

Nematodes and Soil Fertility

Soil nematodes, especially bacterial- and fungal-feeding nematodes, can contribute to maintaining adequate levels of plant-available N in farming systems relying on organic sources of fertility. The process of converting nutrients from organic to inorganic form is termed mineralization; mineralization is a critical soil process because plants take up nutrients from the soil primarily in inorganic forms. Nematodes contribute directly to nutrient mineralization through their feeding interactions. For example, bacterial-feeding nematodes consume N in the form of proteins and other N-containing compounds in bacterial tissues and release excess N in the form of ammonium, which is readily available for plant use. Indirectly, nematodes enhance decomposition and nutrient cycling by grazing and rejuvenating old, inactive bacterial and fungal colonies, and by spreading bacteria and fungi to newly available organic residues. In the absence of grazers, such as nematodes and protozoa, nutrients can remain immobilized and unavailable for plant uptake in bacterial and fungal biomass.

Bacterial-feeding nematodes are the most abundant nematode group in agricultural soils. Their abundance closely follows that of bacterial populations, which tend to increase when soil disturbances, such as tillage, increase the availability of readily-decomposable organic matter. Nitrogen mineralization in the soil occurs at a higher rate when bacterial-feeding nematodes are present than when they are absent. The contribution of bacterial-feeding nematodes to soil N supply depends, in part, on the quality and quantity of soil organic matter fuelling the system.

To be continued…


Ref: eOrganic authors: Carmen Ugarte, University of Illinois Ed Zaborski, University of Illinois