Environmental impact of genetically modified crops.

Chapter 10 (Page no: 225)

Impact of genetically modified crops on soil and water ecology.

Soil ecosystem functioning is vital to the sustained functioning of the biosphere. Many processes occur in soils. These include the recycling of nutrients from previous crop residues to maintain primary production, and the consequent introduction of the sun's energy into the biosphere. Other processes that enhance environmental quality such as the suppression of plant pathogens, carbon sequestration and bioremediation also occur. Soils support one of the most diverse ecosystems in the biosphere, with a population consisting of many thousands of species of bacteria, protozoa, fungi and micro-, meso- and macro-fauna, which are responsible for a large number of functional processes. The soil population varies greatly in types, numbers and functional expression both temporally and spatially, and is responsive to a variety of inputs and physical conditions. These are dynamic systems that are constantly evolving and changing, in which many components are intrinsically interconnected, and conversely others are completely autonomous. Plant inputs are the major drivers of soil ecosystem functioning, and responses to these inputs vary according to both the quantity of these inputs and the constituent chemicals. These inputs come from roots, root exudates and debris, and residues from the aerial parts during active plant growth and from senescing material after the plant dies. They provide energy from the breakdown of the carbon fixed during primary production in the plant, and frequently, although consequentially, nitrogen, in many forms. Inputs differ in both amounts and types of compounds according to plant species. Hence, different plants can have differing effects on soil process dynamics. Soil organisms are also involved in other nutrient acquisitions such as phosphorous mobilization and metal chelation to enable plant uptake, as well as mycorrhizal associations. Their activities also impact on soil health and quality, with subsequent impacts on water quality and supply. Without these soil-ecosystem-driven phenomena primary production would cease. Here the possible impacts of genetically modified (GM) crops on agricultural and natural systems are considered. However, it should be borne in mind that all the topics considered will apply equally to the introduction of new, or indeed different, cultivars or crops, since it is the consequences of changed inputs from these that are relevant. Similarly, many changes in agronomic practice, such as cultivation technique and timing, planting and sowing rates, pesticide usage, etc., must also be considered. The environmental impacts of these should receive equal attention as these large perturbations that are routinely applied to soil ecosystems during normal agricultural management practices have significant impacts on soil communities and functions (Buckley and Schmidt, 2001). These and other factors such as plant type being cultivated, season, climate and geography may well be much greater drivers of community structure in the rhizosphere than any changes in inputs resulting from GM of the crop plant (Donegan et al., 1999; Griffiths et al., 2000; Lukow et al., 2000; Dunfield and Germida, 2001; Hopkins et al., 2001). When the environmental impacts of GM plants are being assessed, comparisons must be drawn against the background of such 'normal' baseline variations, in both structure and function.

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