الصفحة 1
الصفحة 1
Nutrient Cycling in Terrestrial Ecosystems
The first part of the book presents the fundamentals of nutrient cycling. Topics included are cycling of carbon, nitrogen, phosphorus, sulphur and micronutrients. The second part discusses nutrient cycling at an ecosystem scale, covering cropping systems, pastures, natural grasslands, arid lands, tundras and forests. The final chapter reviews current models of nutrient cycling.
Microbial Activity in the Rhizosphere
The rhizosphere is a very complex environment in which the effects of the plant on soil microorganisms and the effects of the microorganisms on the plant are interacting and are interdependent. Plant root exudates and breakdown products attract microbes and feed them and, in turn, the plants often benefit from the microbes. Interactions among microorganisms and plant roots are essential for nutritional requirements of the plant. Plant growth, development and productivity are largely dependent on the soil environment in the root region rhizosphere. The new techniques of studying the rhizosphere enables us to get a much better understanding of the dynamics of the rhizosphere population, such rhizosphere studies being of interest to agriculturists, soilbiologists, chemists, microbiologists and molecular biologists. The rhizosphere microbes in?uence the root environment in several ways. They may change the oxidation-reduction potential, influence the availability of moisture and nutrients, produce growth inhibiting or growth promoting substances in the form of exudates, provide competition and possibly induce many other effects. My corrhizal associations are beneficial in mineral uptake and in increasing root surface area for effective ion absorption. Antagonism, ompetition and synergism in soil and the rhizoplane (rhizosphere) are the most important microbial interactions to consider in the study of rhizosphere biology. With the growing information on the production of growth regulators, competitiveness of the microbes in the rhizosphere, microsymbionts, and other factors, their effect upon plant growth will become more evident. Experiments on the introduction of microbes or their products in the rhizosphere will help to improve our understandingofthebiologyoftherhizosphere.
