|Mineral Fertilizer Use and the Environment (IFA - UNEP, 1998, 52 p.)|
Plant and animal communities may be directly affected by changes in their environment through variations in the quality of water, air and soil and sediments and through disturbance by noise, extraneous light and changes in vegetation cover. Such changes may directly affect the biosphere, for example habitat, food and nutrient supplies, breeding areas, migration routes, vulnerability to predators, or changes in herbivore grazing patterns, which may then have a secondary effects on predators. Soil disturbance and removal of vegetation and secondary effects such as erosion and siltation directly affect communities, and also lead to indirect effects by upsetting nutrient balances and microbial activity in the soil.
A common long-term effect is loss of habitat, which affects both faunal and floral communities, and induces changes in species composition and primary production cycles. For example, in some countries, population pressure is leading to the cultivation of unsuitable, fragile soils. Tropical forests, growing on soils that are usually highly weathered, are being felled on this account. A large proportion of the Amazon forest, for example, grows on poor soils, which deteriorate further and rapidly after deforestation. There is ample scope for improving agricultural productivity on more suitable land elsewhere in Brazil, thus avoiding the opening of new areas of the Amazon forest and even allowing some degraded areas to revert to natural forest.
In Indonesia, land settlement schemes have involved the felling of rain forest, following which soils have deteriorated rapidly. With adequate fertilization and good management practices, it has been shown that this land can be rehabilitated, thus avoiding the necessity of clearing additional rain forest and preventing further soil erosion and degradation.
Over-grazing is one of the major causes of soil erosion and the grazing livestock population is tending to increase. The increased production of fodder, with appropriate fertilization practices, is an excellent means of reducing the pressure of livestock on grazing land.
An indiscriminate reduction in fertilizer use would require farmers to use more crop acres to maintain, or increase, present production levels. This would require the use of less productive, more fragile soils.
Urbanization increases carbon emissions, whereas plants absorb carbon. Mannion (1997) noted, however, mat, with intensification, the agricultural area is tending to decrease in much of the developed world, with corresponding increases for example in the forested area. This represents a net increase in the carbon sink. But in the developing world, the agricultural area is tending to increase, tropical forest is being transformed into agricultural land, and agricultural land is being lost to urbanization. This development clearly reduces the vegetative carbon sink, as well as resulting in a loss of biodiversity and genetic resources.
Currently progress is being made in many regions of the world in implementing diversity-friendly agricultural practices in soil conservation, withdrawing production from marginal areas, mastering chemical and nutrient runoff, breeding crop varieties which are genetically resistant to diseases, pests and abiotic stresses.
In the USA, the 1996 Farm Act created new programs such as the Environmental Quality Incentives Program, the Wildlife Habitat Incentives Program and the Farmland Protection Program. A number of other policy options intended to promote sustainability are in various stages of adoption. In 1996 the Agri-environmental Accompanying Measures of the EU accounted for over 2 billion ECU, or about US$ 1.8 billion.
Until recently, the biology of what happens in the root zone - the rhizosphere - was relatively neglected.
M.J. Swift (1998) writes: Soil management has been dominated by what may be termed an 'environmental management' paradigm. Crop production is seen as being regulated by its physico-chemical environment which can be altered and managed by physical means and the introduction of inorganic chemicals to suit the crop's needs. In recent years an alternative concept of 'biological management' has been emerging which focuses on the manipulation of biological populations and processes in soil as well as on its physico-chemical properties. At no location on the earth's surface has it been possible to assess the full biological diversity of the community of soil organisms. The conventional approach to agricultural management seeks to bypass or even inhibit these biological regulators and often disrupts or destroys ecosystem stability and resilience. A biologically-driven approach provides a broader, ecological concept of soil management which is more readily translated across scales from plot to ecosystem and landscape. It is not only distinct from the green revolution physico-chemical paradigm but also from that of organic agriculture in that it does not eschew petro-chemically derived inputs but rather focuses on the efficiency of their use. Ecosystem science provides a framework which integrates the functional attributes of biological populations with their physical and chemical environments.
It is known that the heavy use of nitrogen fertilizer inhibits the activity of symbiotic nitrogen fixing organisms such as Rhizobium species. If the legume plant is well supplied with nitrogen from the soil and/or mineral fertilizer, it is a less efficient nitrogen fixer, many legumes do not nodulate in the presence of a high soil nitrate level. It has also been contended that fertilizer use, particularly nitrogen application, may inhibit the soil micro-organisms from mineralizing available soil organic matter.
Soil invertebrates (ants, termites, earthworms, spiders, millipedes, centipedes etc) perform an important function in the maintenance of soil fertility. Mineral fertilizers have been accused of having an adverse effect on the earth worm population. It is certainly possible to demonstrate the lethal effects of fertilizer salts and anhydrous ammonia when applied in contact with a living worm. But only a small portion of the soil habitat occupied by worms is in direct contact with applied fertilizers, and consequently, the proportion of the total population detrimentally affected is small. A possible adverse impact on the earthworm population could result from the acidification of soils through the application of certain nitrogen fertilizers not balanced by liming, earthworms are inhibited by soil acidity. However, some researchers have established that the greater supply of fresh organic material obtained through fertilization is of far greater significance to the earthworm. Size and numbers of earthworms invariably increase as soils are brought from a low to high level of fertility through effective fertilization.
The circumstantial evidence of the experiments in which mineral fertilizers have been applied continuously for a very long period of time, in a fully sustainable system, a priori indicates that correct fertilization practices do not harm soil flora and fauna essential for crop production.