Beneficial Role of nematodes in soil ecosystem
Authors: Dr.Gitanjali Devi*, Jisna George

Introduction:

Nematodes have an important function in the economy of the soil as they occupy an important place owing to their great number of individuals and species. There may be 50 different species of nematodes in a handful of soil and millions of individuals can occupy 1m². Nematodes are recognized as a major consumer group in soils, generally grouped into four to five trophic categories based on the nature of their food, feeding apparatus and the method of feeding: bacterial feeders, fungal feeders, predatory, omnivores, and plant feeders. They influence the processes of degradation and mineralization of organic matter mostly due to the trophic relations with fungi and bacteria (Vinciguerra, 1979). Soil organic matter occupies any niche that provides an available source of organic carbon in marine, freshwater and terrestrial environments. Organic matter is very important in the movement of nutrients in the environment and plays a role in water retention on the surface of the planet. Organic matter decomposition serves two functions for the microorganisms, providing energy for growth and supplying carbon for the formation of new cells.

Role of nematodes in decomposition of organic matter and on nutrient cycling:

Bacterivore and fungivore nematode do not feed directly on soil organic matter, but on the bacteria and fungi which decompose organic matter. Their feeding recycles minerals and other nutrients from bacteria, fungi, and other substrates and returns them to the soil where they are accessible to plant roots. Microbial activity increases with any disturbance and enrichment of the soil environment that leads to changes in the proportion of opportunistic bacterial feeders in a community. Over time the enrichment opportunists are followed by more general opportunists including fungal feeders and different genera of bacterial feeders. This succession of nematode species plays a significant role in decomposition of soil organic matter, mineralization of plant nutrients and nutrient cycling. Bacterial feeding nematodes consume nitrogen in the form of proteins and other nitrogen containing compounds in bacterial tissues and release excess nitrogen 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. To assess the potential of nematodes as a sink for the ingested nutrients, the C: N ratio of bacteria and nematodes needs to be considered. Bacteria have a C: N ratio of 5:1.They feed on organic residues which have a higher C: N ratio and respires most of the assimilated carbon but only a small part of the assimilated nitrogen or may immobilize nitrogen. Bacterial feeding nematodes have a C: N ratio is between 5-10. The C: N ratio of fungal feeding nematodes is closer to that of their food source. This explains the enhanced released of CO₂ and NH₄⁺. In the absence of grazers, such as nematodes and protozoa, nutrients can remain immobilized and unavailable for plant uptake in bacterial and fungal biomass. Microbivorous nematodes exhibit a wide range of metabolic rates and behavioral attributes, the contribution of individual species to nitrogen cycling and soil fertility may vary considerably.Diplolaimelloides meyli, D.oschei, Diplolaimella dievengatensis, Panagrolaimus paetzoldi have a singnificant top-down influence on the pool of bacteria growing on cord grass (Mesell et al.,2003)

The bacterial feeding nematode community in the top 15 cm of a field soil mineralized nitrogen at rates increasing to 1.01 µg N per g soil per day in the rhizosphere (Ferris et al., 1997). In microcosm experiments, the rates of nitrogen mineralization by bacterial feeding nematode species of different body size ranged between 0.0012 and 0.0058 mg N per nematode per day, mainly as NH₄⁺. Like bacterial feeding nematodes, fungal feeding nematodes contribute to the process of nutrient mineralization by releasing nitrogen and other plant nutrients from consumed fungal tissue. The amount of nitrogen as NH ₄⁺ released from microcosms in which barley straw was colonized by the fungus Rhizoctonia sp. was always greater in the presence of the fungal feeding nematode Aphelenchus avenae (Ferris et al., 1997).

The contribution of bacterial feeding nematodes to soil nitrogen supply depends on the quality and quantity of soil organic matter. 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. Fungal feeding nematodes are relatively more abundant in less disturbed and perennial systems, where conditions for fungal growth are promoted. However, in agricultural systems, bacterial-feeding nematodes typically release more inorganic nitrogen than fungal-feeding nematodes. Higher temperatures and moisture increase the destruction of soil organic matter by increasing microbial populations in the soil. Organic residues with a low carbon to nitrogen (C: N) ratio (less than 20) are easily decomposed and nutrients are quickly released (4 to 8 weeks), while organic residue with a high C: N ratio (greater than 20) decompose slowly and the microbes will tie up soil nitrogen to decompose the residues. The bacterial feeders prey on bacteria and may ingest up to 5000 cells/minute, or 6.5 times their own weight daily. The annual overall consumption may be as much as 800 kg of bacteria per hectare and the amount of nitrogen turned over in the range of 20-130 kg.

Microbes need regular supplies of active soil organic matter in the soil to survive in the soil. Incorporation of manure, compost, and cover crops with intermediate C: N ratios (ranging from 10 to 18) may stimulate bacterial growth and the abundance of bacterial feeding nematodes, and increase soil nitrogen availability to plants. Under field conditions, bacterivorous and predatory nematodes are estimated to contribute (directly and indirectly) about 8% to 19% of nitrogen mineralization in conventional and integrated farming systems, respectively. Predatory nematodes also regulate nitrogen mineralization by feeding on microbial grazing nematodes, a conduit by which resources pass from bottom to top trophic levels.

The quantity of nitrogen used by nematodes and converted into products of easy utilization, by means of their excretion products, is equal to 10%

of that used by the whole vegetable cover (Ingham

et al., 1985). Thus, it gives a positive effect on the soil, contributing to its fertility and in the long run, to its productivity.

References:

Ferris, H., Venette, R. C. and Lau, S. S. (1997). Population energetics of bacterial-feeding nematodes: carbon and nitrogen budgets. Soil Biol. Biochem. 29: 1183-1194.

Ingham, R. E., Trofymow, J. A., Ingham, E. R. and Coleman, D. C. (1985). Interactions of bacteria, fungi and their nematode grazers: Effects on nutrient cycling and plant growth. Ecological Monographs 55: 119-140.

Mesell, I. De., Derycke, S., Swing, J., Vincx, M., Moens, T. and Mesel, I. De.( 2003). Influence of bacterivorous nematodes on the decomposition of cord grass. J Exp. Marine Biol. Eco.296:227-292.

Vinciguerra, M.T.(1979). Role of nematodes in the biological processes of the soil. Boll. Zool.46:363-374.


About Author / Additional Info:
Junior Scientist, Department of Nematology, Assam Agricultural University.