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The development of ecologically sustainable agricultural systems is essential for maintaining agricultural productivity

By Dr. S.M. ALAM 
and             
A. SHEREEN, NIA, Tandojam.
April 29 - May 05, 2002

Nitrogen is the most abundant element in our atmosphere and is a major constituent of all living organisms. It is an important element supplied from the soil for the production. Compounds of nitrogen useful for the nutrition of living things are made available by several different processes varying greatly in magnitude and importance During thunderstorms small amounts of atmospheric nitrogen gas are converted to nitric acid by electrical discharges and same ammonium nitrogen is reported to be derived from a few types of rocks during the weathering process. An increasing quantity of the nitrogen from the air is industrially reduced to ammonia each by the Haber Process. Some hundred million tons of the N is fixed per year. In many agricultural situations the availability of a suitable source of nitrogen is the major factor limiting crop productivity. Inputs of nitrogen into agricultural systems are primarily from chemical fertilizers and nitrogen derived from atmospheric dinitrogen by the process of biological nitrogen fixation, which is the microbial conversion of atmospheric dinitrogen gas into plant usable ammonia. Indeed, the provision of biologically fixed nitrogen plays a key role in crop production in world agriculture.

Today, there is an increasing awareness in many areas that the development of ecologically sustainable agricultural systems is essential for maintaining agricultural productivity at sufficient levels to meet increasing demands from the growing world population. One of the key factors for sustained agricultural productivity is effective management of nitrogen in the environment. Indeed, the successful manipulation of nitrogen inputs through the application of biologically fixed nitrogen of ten results in farming practices that are economically liable and environmentally prudent. The symbiotic association between leguminous plant and roots nodule bacteria, have been estimated to fix approximately 80 % of the biologically fixed nitrogen in agricultural areas with the remainder being contributed by a diversity of other symbiotic systems, non-symbiotic associations between nitrogen fixing bacteria and roots and their free living microorganisms. Historically, biological N-fixation has been an essential component of many farming systems. During the past fifty years the widespread use of chemical fertilizers to supply nitrogen to plant species has had a substantial impact on food production, and thus nitrogen fertilizer has become a major input in crop production around the world.

Several important objectives have been proposed by the research workers on the need to increase the contribution from the biological nitrogen fixation to N inputs in agricultural systems. These objectives include the need for maximization of the N-fixing potential of the present agricultural systems, the introduction and expansion of the capacity to fix N to new farming system. The precise quantities of nitrogen supplied to the biosphere by all the various processes is not accurately known, but it is generally agreed that the biological fixation of nitrogen gas is of major importance accounting for some hundred million tons of nitrogen fixed per year. The major agronomic benefits from symbiotic biological N-fixation are the direct input of atmospheric nitrogen to the agricultural system and the control of crop diseases as a consequence of involving legumes in crop rotations. "The environmental benefits from using biological N-fixation are seen to be associated with the replacement of chemical based technologies with a biological system. These possible advantages include the decreased inputs of fertilizer N resulting in decreased levels of ground water pollution by nitrates and reduced outputs of greenhouse gas production because the process of fixing nitrogen biologically does not depend on fossil fuel. Biological N-fixation is a process carried out by microorganisms currently approximately 50 genera of bacteria is known to fix atmospheric nitrogen. Many N-fixing bacteria can achieve N-fixation on their own pre-living heterotrophic and autotrophic organisms. The three major groups of microbes are: (i) The root nodule bacteria (Rhizobium, Bradyrhizobium, Sinorhizobium, Azorhizobium), actinonycetes (Frankia) and cyanobacteria and certain species of blue green algae (anabaena, Calothrix, Tolypothrix Tolypothrix, Nostoc). The major amount of fixed N is contributed by legume symbioses and thus providing significant amounts of fixed N to agricultural systems. Nitrogen fixing legumes are significant components of many agricultural systems.

Oilseed legumes occupy about 6 % of land currently under cultivation and the pulse legumes are sown on about 5 % of cultivated land. While, the pastures and fodder crops occupy about 14 % of the world agricultural land. Biological N-fixation has been demonstrated to have many agronomic, economic and environmental benefits for agriculture. These benefits generally include the direct supply of atmospheric N to the legume crops and pastures. Symbiotic legumes play a key role in the maintenance of world crop production through the inputs of biologically fixed nitrogen. The development of the symbiosis between root nodule bacteria and their specific host legume is a continuous process. The root nodule bacteria are common soil bacteria and they live and survive in the soil as free-living heterotophic organisms. Forage legumes are also good source of N-fixer. The contribution of the legume component to the pasture system is important for the inputs of fixed N. In many agricultural systems, farmers mostly depend on legume nitrogen fixation by forage legumes to sustain soil N fertility. Considerable progress is being made in this area recently for the fertility of soils. The temperate grain legumes are also playing an important role in N-fixation. These include broad beans faba beans, common bean, garden beans, field peas, vetch, chickpea red clover, white clover, lentils, cathyrus and lupine. These crops play a significant role in the fixation of N and thus improvement in soil fertility.

Soybean is the most widely grown crop legume with 60 million hectares sown in 1995, producing with an estimated 120 million tons of grain. This is one of the major sources of food and animal feed, which are necessary for human health. The crop can be grown in both temperate and tropical climate and it is capable of producing high yields of grain that contains large amounts of protein and oil. It is an important component of local diets. As a nodulating legume, soybean forms a nitrogen fixing symbiosis with Bradyrhizobium japonium, B. elkarii, B. liaoningense and sinorhizobium fredii. The N requirement of soybean can be met by both assimilation of mineral nitrogen from the soil and symbiotic nitrogen fixation. Recent approaches to improving biological nitrogen fixation by soybean crops include attempts to optimize the numbers the members and effectiveness of rhizobia in the rooting zone, selection of elite strains of bradyzhizobia, improved maculation technique, and improvement in breeding programme for nitrate tolerance in soybean.

Legume pulses and oilseeds are important components of many agricultural systems in tropical regions. Biological nitrogen by these legume crops has a key role in sustaining productivity in tropical agriculture. The important legume crops grown in tropical areas spread throughout the world with comprises of chickpea, black gram, cluster bean, common bean, coarpea, groundnut, lentil, green gram, navy beans, mungbean and snake beans. The main benefit obtained from these crops is their capabilities to fix atmospheric nitrogen resulting in the crop productivity. Flooded rice-based systems also play an important role in the regimen of N-fixation. It has been estimated that greater than 50 % of the world population is dependent on rice grain as the principal component of diet. About 75 % of the rice are grown in wetlands, where the rice grows in flooded fields. Under these conditions, the flooding phenomenon of the rice fields provides for the establishment of a diverse range of nitrogen fixing organisms that make substantial inputs of fixed nitrogen into the rice-based systems. The most important nitrogen fixing organisms present abundantly in the flooded rice-based systems include heterotrophic and autotrophic free-living bacteria photosynthetic bacteria and cyanobacteria, symbiotic cyanobacteria that associate with Azolla a fern and symbiotic bacteria that form root and stem nodules one legume crops. The amounts of nitrogen fixed by these diverse and powerful organisms make substantial contributions to the sustainability of wetland rice cultivation. The genus Anabaena plays an important role in the fixation of atmospheric nitrogen. The fast growing aquatic legumes (sesbania nostrate) play important role in the N-fixation. Many countries and mostly the far-eastern countries use Azolla as a green manure for the cultivation of rice. The use of nitrogen fixing green manures has been documented to improve rice grain yield through effects on physical and chemical soil parameters and reduced losses of nitrogen from the system.

There are many challenges facing research workers in their quest to enhance the role of biological nitrogen fixation in crop production. A considerable international effort will be needed to provide the strong support required to overcome these challenges. The increasing awareness that legumes are an essential component of many farming systems is based on improvements in our understanding of the roles for biological nitrogen fixation in developing sustainable agricultural systems. Considerable progress has been made in recent years in understanding symbiotic nitrogen fixation. Despite the complexities of the symbiosis between Rhizobium species and legumes, it is now possible to macerate nodules and separate the mixture into a bacteroid preparation, soluble proteins and plant cellular material.