4.6: Biological Diversity, Abundance, Activity and Balance

Figure 4.4. Close-up view of a plant root: (a) The mucigel layer is shown containing some bacteria and clay particles on the outside of the root. Also shown is a mycorrhizal fungus sending out its rootlike hyphae into the soil. (b) Soil aggregates are surrounded by thin films of water. Plant roots take water and nutrients from these films. Also shown is a larger aggregate made up of smaller aggregates pressed together and held in place by the root and hyphae.

A diverse biological community in soils is essential to maintaining a healthy environment for plant roots. There may be over 100,000 different types of organisms living in soils. Most are providing numerous functions that assist plants, such as making nutrients more available, producing growth-stimulating chemicals and helping form soil aggregates. In a teaspoon of agricultural soils it is estimated that there are from 100 million to 1 billion bacteria, several yards of fungi and several thousand protozoa. It may hold 10 to 20 bacterial-feeding nematodes and a few fungal-feeding and plant parasitic nematodes. Arthropods can number up to 100 per square foot, and earthworms from 5 to 30 per square foot.

Soil organisms are not evenly distributed through the soil, and even when present, organisms may be in a resting state. On the other hand, there are a number of zones of high amounts of active organisms in soil that are taking in food sources, interacting with other organisms, growing and reproducing. The zone immediately surrounding roots contains a very large population of diverse organisms (the root microbiome), stimulated by the continuous leakage (exuding) of energy sources from the roots as well as sloughed off root cells. Other locations of high activity of organisms are around particles of decaying organic matter, on or near aggregate surfaces, and inside earthworm channels and old root channels.

Of all the organisms in soils, only a small number of bacteria, fungi, insects and nematodes might harm plants in any given year. Their negative impact is reduced in a more diverse soil biome. Diverse populations of soil organisms maintain a system of checks and balances that can keep disease organisms or parasites from becoming major plant problems. Some fungi kill nematodes, and others kill insects. Still others produce antibiotics that kill bacteria. Protozoa feed on bacteria and may attack fungi. Some bacteria kill harmful insects. Many protozoa, springtails and mites feed on disease-causing fungi and bacteria.

Figure 4.5. Management practices that influence soil life. Modified from Kennedy, Stubbs and Schiller (2004).

Beneficial organisms, such as the fungus Trichoderma and the bacteria Pseudomonas fluorescens, colonize plant roots and protect them from attack by harmful organisms. Some of these organisms or their byproducts, such as the insect-attacking chemical produced by Bacillus thuringiensis (BT), are now sold commercially as biological control agents. (Plants have also been genetically engineered to produce the toxin that BT produces in order to control crop-eating insects.) The effects of bacteria and fungi that suppress plant disease organisms are thought to arise from competition for nutrients, production of antagonistic substances, and/or direct parasitism. In addition, a number of beneficial soil organisms induce the immune systems of plants to defend the plants (systemic acquired resistance; see discussion in Chapter 8). Also, roots of agronomic crops usually have their own characteristic microbial communities with numerous interactions.

Soil management can have dramatic effects on soil biological composition (see Figure 4.5 for management effects on organisms). For example, the less a soil is disturbed by tillage, the greater the importance of fungi relative to bacteria. Cropping practices that encourage abundance and diversity of soil organisms encourage a healthy soil. Crop rotations of plants from different families are recommended to keep microbial diversity at its maximum and to break up any potential damaging pest cycles such as the soybean cyst nematode. Crop rotations that include perennial crops, usually grass and legume forages, can also reduce annual weeds. Additional practices that promote the diversity and activity of soil organisms include low amounts of soil disturbance, use of cover crops, maintaining pH close to neutral and routine use of organic sources of slow-release fertility.

It is believed that more is unknown about soil life than what is known. New methodologies like microbial community analysis use DNA sequencing and advanced computational methods to help us understand the makeup of soil life. The next step is to use this technology to enhance the plant-soil microbiome and increase our capacity to grow more food, more sustainably.