The rhizosphere in the soil zone is directly influenced by the plant roots. Plant roots can also effect rhizosphere solution composition through their effect on rhizosphere microorganisms. The chemical composition of rhizosphere can be very different from those of bulk soil due to root exudation and microbial activity, which has been observed significantly.
A higher microbial population can increase decomposition of organic matter and therefore increase nutrient transformation in the rhizosphere. Root-induced changes in the rhizosphere may affect mineral nutrition of plants in various ways. Soil nutrients are taken up by plant roots via the rhizosphere, which is the key zone of interaction between plants and soil. Therefore, root growth and rhizosphere processes have a great influence on soil nutrient transformation, mobilisation, and efficient use by plants.
Plant roots cannot only highly regulate morphological traits to adapt to soil environmental conditions, but also significantly modify rhizosphere processes through their physiological activities, particularly the exudation of organic acids, phosphatases and some signalling substances, proton release and redox changes. Rhizospheric processes are the linkages between plant processes and soil processes to some extent determining the bio-availability of soil nutrient and thus affect the crop production.
The rhizosphere management strategies lay emphasis on maximising the efficiency of root rhizosphere in nutrient acquisition towards high yield of fruit crops by optimising nutrient input in rooting zone, regulating root growth and manipulating rhizospheric interactions.
Soil microbial diversity is considered as one of the most sensitive tools of soil fertility evaluation.The rhizosphere is an area enriched by many microbes as plants release different types of exudates in this region thus boosting the soil microbial diversity. Microorganisms play an important role in soil fertility because they oxidise organic matter and promote bio-mediated cycling of nitrogen (N), phosphorus (P) and sulphur (S).
The root exudates play a key role in the selective stimulation of microorganisms. Plants have an important effect on soil microbiological properties due to release of different nutrients and organic compounds into the soil. The microbial diversity has been found to be more in rhizosphere than in the non rhizosphere soil. Soil microorganisms help in maintaining soil quality and have a great influence on soil hydrolytic reactions which are reflected in the natural cycles of N,P and S, 80-90 per cent of the processes in the soil are reactions mediated by microbes.
The plants alter rhizosphere populations through root exudation and the sloughing of root cells. Most plants also interact with specific fungi to form associations known as mycorrhizae and these have considerable effects on populations of rhizospheric microorganisms.Many bacteria are intimately associated with plant roots, the microbial population in the rhizosphere differs from those in the surrounding soil both in total number and in species distribution.
Microorganisms in the soil maintain bio-geochemical cycles in the soil by virtually degrading organic compounds sooner or later. Total microbial counts were commonly found to be increased 10-50 folds in the rhizosphere. The population of bacteria around the roots of plants in the rhizosphere is generally much greater than the bacterial density in the bulk soil. Microbes produce extracellular enzymes to acquire energy, carbon (C) and nutrients for growth and metabolisms.
It is generally accepted that microbial extracellular enzyme-production abides by a cost-effective rule of economy, i.e., microbes will maximise the energy, C and nutrient returns by minimising resource utilisation.The rhizosphere harbours a large and diverse community of prokaryotic and eukaryotic microbes that interact and compete with each other and with the plant root. There are various techniques that can be used to quantify the microbial diversity in the soil such as plate count technique and the most probable number technique. Bacteria were the most dominant followed by fungi and actinomycetes.
Many microbial interactions which are regulated by specific molecules/signals are responsible for key environmental processes such as bio-mediated cycling of nutrients, improvement of soil quality and maintenance of soil health. The influence of NPK fertilisers on the population of bacteria and fungi in the rhizosphere brings an increase in the microbial population with the application of fertiliser to the soil.
The organic amendments stimulate the microbial population and enhances the soil microbiological activity in the rhizosphere. Microbial population enhanced with the addition of organic and inorganic fertilisers and are reported to have high bacterial population and fungal population.
Microbial activity in the rhizosphere affects rooting patterns and supply of available nutrients to the plants thereby modifying the quality and quantity of root exudates. Rhizospheric microorganisms can both mobilise and immobilise plant nutrients and can produce growth promoting substances such as phytohormones as well as phytotoxins. The presence of certain antimicrobial compounds in the roots of plant tissues may have antagonistic effect on the soil microbial diversity in the rhizosphere.
(The writer is Dr Rehana Akhtar Bijili, a doctorate in Soil Science and is presently serving as J&K Administrative Service (KAS) Officer.)