Soil is essential for the maintenance of biodiversity above and below ground. The wealth of biodiversity below ground is vast and unappreciated: millions of microorganism s live and reproduce in a few grams of topsoil, an ecosystem essential for life on earth…’
Living organisms both plants and animals, constitute an important component of soil. The soil is now believed to be a dynamic or rather a living system, containing a dynamic population of organisms/microorganisms. Cultivated soil has relatively more population of microorganisms than the fallow land, and the soils rich in organic matter contain much more population than sandy and eroded soils.
Microbes in the soil are important in maintaining soil fertility / productivity, cycling of nutrient elements in the biosphere and sources of industrial products such as enzymes, antibiotics, vitamins, hormones, organic acids etc. At the same time certain soil microbes are the causal agents of human and plant diseases.Soil microbes play a crucial role in returning nutrients to their mineral forms, which plants can take up again. This process is known as mineralization.. Some soil microbes secrete polysaccharides, gums and glycoproteins, which glue soil minerals together, forming the basis for soil structure. Fungal hyphae and plant roots further bind soil aggregates together. Soil structure is essential to good plant growth.
Soil microbes play both beneficial (decomposition and nutrient cycling) and detrimental roles as pathogens and contributors to soil environmental problems such as global warming and groundwater contamination. The physical, chemical, and biological soil properties and their interactions with the resident community of soil microorganisms have a profound impact on growth and activity of microorganisms. As our understanding of these complex relationships develops, we should be able to develop soil management practices that are sustainable and that lead to maintenance and improvement of soil quality.
Plants associate with other life forms (animals, bacteria or fungi) to complete their life cycle, to fight against pathogens or to thrive in adverse environments. The plant root and its associated living organisms are together called ‘rhizosphere’, the region of mycorrhizal association. Mycorrhiza is one of the best examples of symbiotic1 association between plants and fungi. The term ‘mycorrhiza’ comes from Greek – mycos meaning fungus and rhiza meaning roots. In nature, more than eighty percent of angiosperms, and almost all gymnosperms are known to have mycorrhizal associations.
Mycorrhizae form a network of filaments that associate with plant roots and draw nutrients from the soil that the root system would not be able to access otherwise. This fungus-plant alliance stimulates plant growth and accelerates root development. In return the plant provides carbohydrates and other nutrients to the fungi. They utilize these carbohydrates for their growth and to synthesize and excrete molecules like glomalin (glycoprotein). The release of glomalin in the soil environment results in better soil structure and higher organic matter content.
Mycorrhizal fungi allow plants to draw more nutrients and water from the soil. They also increase plant tolerance to different environmental stresses. Moreover, these fungi play a major role in soil aggregation process and stimulate microbial activity. According to the plant species and to the growing practices and conditions, mycorrhizae provide different benefits to the plants and to the environment:
There are two major groups of mycorrhizal fungi: ectomycorrhizal and endomycorrhizal fungi. Members of the former group develop exclusively on the exterior of root cells, whereas those of the latter penetrate the plant cells where direct metabolic exchanges can occur. Ectomycorrhizae are essentially found on trees and form visible structures whereas endomycorrhizal fungi colonize trees as well as shrubs and most herbaceous plants and do not form visible structures.
Soil structure refers to soil particle aggregation as well as pore spaces. Maintenance of soil structure is of critical importance to the preservation of soil functions and fertility. Mycorrhizal fungi play a major role in soil aggregation through hyphae networking and glomalin (biological glue) production. Therefore, their presence in the soil is essential to maintain physical soil properties. Better soil structure results in: