Grains Research and Development

Date: 01.06.2002

What soil organisms can do for you by Dr Gupta Vadakattu, CSIRO Land and Water

Scanning electron micrograph of amoebae feeding on fungi (thread-like filaments) and bacteria associated with wheat stubble. Predation by soil fauna (e.g. protozoa) reduces the survival and growth of pathogenic fungi in soil.

A DIVERSE, balanced and active soil biota helps to provide the right soil conditions for sustainable crop production with very little negative environmental effects.

Soil biota is involved Fith the increased microbial activity, carbon turnover and the nutrient-supplying potential of soils. Other beneficial functions include preventing aggressive plant pathogens taking hold and improving plants' ability to withstand disease effects; reducing the loss of inorganic fertilisers through leaching and erosion by short-term immobilisation; stabilising soil structure; and reducing the reliance on agrochemicals as well as reducing persistence of pesticides in soil and thus lessening off-site impacts.

A unique balance between the three components of a soil system, i.e. the physical, chemical and biological, is necessary for long-term sustainability of crop production, soil health and other essential ecosystem functions. Microbial activity in most agricultural soils in Australia is mainly regulated by factors such as soil moisture, availability of carbon and nutrients, soil texture and application of agricultural chemicals.

In these agricultural soils, most biota is concentrated in a thin layer of surface soil , which is prone to environmental extremes (lack of moisture and high temperatures) and erosion loss. In addition, the distribution of biological activity in soils is patchy, i.e. concentrated in 'hot spots' such as decomposing crop residues (especially in the absence of plants) and in the soil around plants (the rhizosphere). Crop residues = carbon = available energy = microbial activity

Close-up of all amoeba feedillg on the hyphae of Rhizoctonia solani.Soil microbiota (e.g. fungi, bacteria and protozoa)form a network in carbon-rich microsites such as decomposillg crop residues.Predatioll of bacteria (small rod-like structures) by amoebae (large organisms) in the soil environment

Soil organisms are diverse in terms of physiological nature, size and environmental requirements and are grouped according to their size (e.g. microfiora, microfauna, meso fauna and macrofauna), phenotypic properties, Soil microbiota (e.g. fungi, bacteria and protozoa)form a lIetwork ill carbon-rich microsites such as decomposillg crop residues. Predatioll of bacteria (small rod-like structures) by amoebae (large orgallisms) ill the soil eIIVirOllmellt. function and trophic (food) preference (e.g. bacterial or fungal feeding protozoa and nematodes). Plants are in contact with soil organi sms throughout their growth and the majority of plant- microbe interactions occur in the rhizosphere (soil around roots).

Many biological functions that are essential for plant growth and productivity (for example, nutrient mineralisation and disease suppression) depend upon the interactions between various functional groups of soil microbiota. In a GRDC-supported project at CSIRO Land and Water, researchers are investigating the rhizosphere dynamics of microbiota such as protozoa and their effect on nutrient availabi lity and plant growth, in particular under reduced-till systems.

Protozoa, unicellular eukaryotes, are ubiquitous in most soils and up to 10,000 protozoan species have been estimated to exist. Their populations range from 100 to 1 million per gram of soil.

Bacteria and fungi are their main food source . Results obtained from field experiments at Waikerie in SA (in collaboration with Mallee Sustainable Farming Project) and Harden in NSW (in collaboration with the GRDC research project CSP 329) have shown that populations of protozoa were highest in the rhizosphere of canola plants compared to that of wheat and barley. Amoebae were found to be the largest group (of protozoa), followed by flagellates and ciliates.

Predation of bacteria and fungi by protozoa releases nutrients (nitrogen, sulphur and phosphorus) tied up in bacteria and fungi. Previous work·in South Australia (by Dr Gupta Vadakattu) indicated that increased protozoan predation reduces the survi val and growth of take-all and Rhi zoctonia fungu s. Also the diversity and activity of protozoa were greater in the di sease-suppressive soil compared to a nonsuppressive soil. (See an update in our story below.)

In addition to investigating the role of protozoa in the nitrogen nutrition of plants, this project aims to determine the link between the activity of protozoa and microbial composition that influences early seedling growth under direct-drill systems. (See related research 'On the trail offastgrowing direct-drilled wheat roots ' p11.)

Program 4 Contact: Dr Gupta Vadakattu 08 8303 8579

Region North, South, West