4.2 Biological Activity

Soil organisms (biota) carry out a wide range of processes that are important for the maintenance of soil 'health' and fertility in both natural and managed agricultural soils. The total number of organisms, the diversity of species and the activity of the soil biota will fluctuate as the soil environment changes. These changes may be caused by natural or imposed systems. 

Biota Facts

• Soil organisms range in size from microscopic eg bacteria, to centimetres, eg earthworms.

• Their activity is concentrated in the top 10cm of top soil.

• Millions of organisms exist, but only a few have been identified, eg 5% of fungi and 3% of nematodes.

• 80-90% of soil biological activity is carried out by bacteria and fungi.

• Resistance to extreme changes in the soil environment improves as organisms decrease in size.

• The reproductive interval reduces with a decrease in organism size, eg bacteria reproduce themselves in hours, whilst earthworms may take weeks.

• In natural and managed environments a complex food web exists. These 'predator-prey' relationships help control the balance of species present in the soil.

• When these relationships have evolved and a reduced incidence of disease is recorded, such soils are termed 'suppressive'. 

Functions of soil organisms

There is a two-way relationship between soil biota and agricultural production. Plant residues provide sources of energy and nutrients for the biota, which turn over organic matter (OM), improve nutrient availability and soil structure, transmit and prevent disease and degrade pollutants. Agricultural practices can be beneficial and detrimental to the soil biota. Likewise, soil biota can increase or reduce agricultural production. 

How does Agriculture Influence Soil Biota Activity?

Any factor that changes the soil environment will impact on the activity and diversity of soil biota. Different soil environments support different types and numbers of biota, eg soils under a legume have a higher level of rhizobia which fix nitrogen for that legume; after a canola crop soil will have a lower level of root disease fungi because of the fungicidal compounds released by the decomposing canola residues.

Agricultural production can result in increased soil carbon inputs from retained crop residues, root residues and increased nutrient levels from fertilisers. These increase biological activity. Where organic matter declines, biological activity will also decline. Different plant residues will contain varying quantities and availability of carbon (energy), nitrogen and nutrients. This is expressed as a ratio of carbon to nitrogen. The carbon nitrogen ratio (C:N) of biota is 6-10:1. The closer the residue C:N ratio is to the biota C:N ratio, the more readily it is broken down. Consequently, this will also influence the soils biological activity.

Cultivation alters the physical, chemical and biological components of the soil system. No-till/direct-drill systems result in significant differences in soil organism activity compared to conventional tillage (see Figure 1).

Agricultural inputs, such as fertilisers have been shown to have both a positive and negative effect on soil biological activity. High levels of nitrogen or phosphorus reduce the impact of the symbiotic fixing of these nutrients by Rhizobium (nitrogen) and mycorrhiza (phosphorus), but provide nutrients for non-symbiotic organisms.

Herbicides, insecticides and fungicides may be directly toxic to soil organisms or influence the 'predator-prey' interactions. The effect on non-target organisms will depend on whether the product is applied to the bare soil, rate of herbicide decomposition and leaching away from the site of the organisms.

Herbicides applied to stubble cover, as opposed to bare soil, have been shown to persist longer. Continued use of some herbicides, eg paraquat, has been shown to significantly depress some groups of micro-organisms. This is usually a short term effect with levels recovering 20 days after herbicide application. Nitrifying bacteria are the most sensitive to herbicide applications.

The impact of insecticides on soil biota is more questionable than herbicides, as they are designed to kill fauna. However, the majority of insecticides are applied to plants rather than to the soil.

Similarly the concentration of fungicides is generally low in the soil. New products, eg Impact® in furrow, will change this and more research is required to observe the long term impact on the food-web. The frequency of use will also change the balance of the food-web favouring organisms that are able to live by breaking down the chemical residues.

The challenge for agriculture is to minimise nutrient losses and to maximise internal nutrient cycling. Agricultural practices usually alter more than one soil environmental factor making it difficult to isolate which change is the most significant.

A decline in the total and specific population size is considered detrimental to soil 'health,' ie nutrient status, disease resistance, structure and stability and long term productivity.

4.2 Biological Activity

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