Role Played by Nitrogen Fixation in Global Nitrogen Flows of Plants

The role played by nitrogen fixation in global nitrogen flows has attracted the interest of several workers. Various blue-green algae and symbiotic and asymbiotic bacteria are the chief nitrogen fixers whose contributions to nitrogen fixation in rice fields and leguminous crops have been investigated thoroughly by various workers.

What is less well known, however, is that nitrogen-fixing bacteria are also associated with the rhizosphere of other plants such as sugar cane, rice, sunflower, wheat, maize, some legumes and certain other dicotyle­donous plants. Important bacterial genera involved include Azotohacter, Clostridium, Beijerinckia, Enterobacter, Derxia and Caulobacter, etc.

Unfor­tunately, many strains of diazotrophic bacteria (e.g., Azotobacter) known to be efficient nitrogen fixers are not true rhizosphere bacteria (Brown, 1974) since they cannot thrive well in the rhizosphere; in fact, their decline in the rhizosphere seems to be the chief cause of failure of crop field inoculations with diazotrophs.

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But Dobereiner and Day (1976) have reported a tight plant- diazotroph association between Spirillum lipoferum and the grass Digitaria decumbens (also with Zea mays). In this association, the bacterial cells are localized intercellularly in the cortex of the grass roots.

This interesting nitrogen fixing bacterium (5. lipoferum) seems widely distributed in tropical and subtropical countries as a common inhabitant of rhizosphere and crop field soils. Spirillum is not commonly distributed in tropical forests but is very common in the root zone of forage grasses and tropical grain crops (Dobereiner, 1978). However, little, if any, exploration has been done on the possible occurrence of Spirillum in India, though it has been recorded from Pakistan. This bacterium can not only fix nitrogen in association with grass roots but also in free-living state.

Smith et al. (1978) report higher dry matter yields from tropical grasses inoculated with S. lipoferum than from uninoculated controls. Such increased yields have been found in Panicum maximum, Pennisetum americanum, Digitaria decumbens, and certain other grasses.

Nitrogen fixation depends on the relative requirement for fixed nitrogen in comparison to the total nitrogen flow. In certain open systems character­ized by high losses, e.g., clay desert areas, nitrogen fixation probably contrib­utes significantly to the annual nitrogen flow. But in more closed ecosystems, e.g., grasslands in steady state, the impact of nitrogen fixation to the annual nitrogen flow is considerably smaller.

In many situations nitrogen fixation is carbon limited and primary pro­ductivity in many parts of the world is greatly limited by availability of fixed nitrogen. Rates of nitrogen fixation in tundra, forest, temperate grasslands, and dryland crops such as wheat and millet, are generally quite low. Most grassland seems to have high internal rates of nitrogen cycling with low losses and low requirements for fixed nitrogen for maintenance.

Fogg has estimated that about 100 x 10⁶ tonnes of nitrogen is fixed annually in the oceans. No reliable fixation rates are available for tropical forests and season­al forests which are otherwise highly productive, but it is generally agreed that the nitrogen in a tropical forest occurs in a highly cyclic state. In forest ecosystems of the world, over a range of fixation rates of 0.4 to 12 kg N ha^-1 yr^-1, nitrogen fixation represents up to about 10 per cent of the annual nitrogen cycled by the vegetation. In most such forests, nitrogen fixation is equal to or exceeds the input of nitrogen by precipitation.