What made the difference was soil organisms. While the forest earth was rich in "ecosystem engineers" - earthworms, termites, millipedes and ants - native macro-faunal communities in the pasture land had been all but lost. As this biological diversity disappeared, the soil had been invaded by a single earthworm species, Pontoscolex corethrurus, that literally clogged the soil with its castings.
What is more, soil biota perform functions vital to the environment and, particularly, to agriculture. They range from the regulation of soil structure and groundwater regimes to degradation of pollutants, nutrient cycling, carbon sequestration, plant protection and growth enhancement, and ecosystem purification.
Tea prunings and termites. The report provides other examples of how wise management of soil biodiversity has benefited agricultural productivity and agroecosystem sustainability. To reverse long-term soil degradation on tea estates in Tamil Nadu, India, researchers inoculated trenches dug between rows of tea plants with a mixture of tea prunings, manure and earthworms. They found that the technique was much more effective than organic or inorganic fertilization alone, producing yield increases averaging more than 275%. More than 20 million earthworms are now being produced each year as part of a programme to rehabilitate tea lands in India and other countries.
Earthworms are also being used by farmers in the Peruvian Amazon to grow hybrid tomatoes on local soils that suffer from low nutrient content, high acidity and aluminium toxicity. Soil beds mixed with sawdust discarded by the timber industry are inoculated with the earthworms to produce yields twice the local average and equal to those obtained with mineral fertilizers.
Another soil macroorganism helped farmers in northern Burkina Faso rehabilitate crusted and bare farmland. The addition of mulch to the soil attracted termites that invaded the organic substrate and the topsoil, significantly changing its physical structure. Compaction was reduced and water infiltration and drainage were increased to the point where crops could again be planted. Furthermore, termites enhanced the decomposition and mineralization of the mulch, releasing nutrients and increasing their availability to plants.
Fungi, bacteria and nematodes. No less important to plant productivity are microorganisms, the most abundant of all soil biota and responsible for driving nutrient and organic matter cycling, soil fertility, soil restoration, and plant health and production. More than 90% of the world's plants develop symbiotic association with one of the five main types of Mycorrhizae, a fungus that acts as a natural extension of the plant's root system. This association increases the plants' capacity to take up nutrients, protects them against pathogens, and increases their tolerance to pollutants and to adverse soil conditions, such as water deficiency, low pH and high soil temperature.
The role of the six genera of the Rhizobiaceae bacterial family in legume production is well documented (see box at left). Also widespread in tropical regions, particularly Brazil and Mexico, is use of diazotrophic and endophytic associative bacteria that not only fix atmospheric nitrogen but modify the shape and increase the number of root hairs, helping plants take up more nutrients. The application of these organisms in inoculants (mostly in maize, rice, wheat and sugar cane) has led to yield increases ranging from "negligible to almost 100%".
Several bacterial species and genera have been used to improve plant growth, the most successful being Agrobacterium radiobacter, used to control crown gall in several plant families, Bacilus subtilus, which suppresses cereal root rot, and various Bacilus-based inoculants used throughout China on vegetable crops.
Soil microfauna may even be a useful ally in plant protection. Nematodes have been successfully deployed against a wide range of insect pests, including white grubs, weevils, fruit flies and woodwasps. Greenhouse trials have also shown nematodes to be effective against several root-infecting fungi.
Nevertheless, the report finds that the role of soil microorganisms in sustainable agriculture continues to be undervalued: "Overuse and misuse of external inputs, such as inorganic fertilizers and pesticides - accompanied by crop specialization or monocultures - may lead to considerable increases in overall food production. But they also deplete both soil fertility and the soil's biological components, and degrade the soil's physical condition. Needed is an integrated approach that considers agriculture's potential impacts on soil biodiversity: one that maintains soil fertility, productivity and crop protection by optimizing ecological synergies among biological components of the ecosystem and enhancing the biological efficiency of soil processes. This would be useful in modern commercial agriculture, and especially on marginal lands facing degradation, on degraded lands in need of reclamation and in regions where high external input agriculture is not feasible."
Published November 2000