Nitrogen Fixation: Meeting Crop Needs
Setatou, Helen; Simonis, Asterios
Soil Science Institute, 541 10 Thessaloniki, Greece
|Biological nitrogen fixation has much to
contribute to the sustainability of agriculture: the promise of »free« N from the
atmosphere to enhance productivity, the virtual elimination of risk of groundwater
contamination by nitrate, the supplemental of human and animal diets with better-quality
protein and the provision of vegetative cover to reduce the susceptibility of soils
Competitive indigenous strains of Rhizobium japonicum occupy many soils of Greece, but they are generally considered to be less efficient in fixing N2 than many strains added as inoculants. The research undertaken included (a) isolation of local Rhizobium japonicum strains well adapted in Greek soils (distribution and effectiveness of rhizobia) and (b) effectiveness of known rhizobia strains, in pot and field experiments.
Rhizobium strains isolated from Greek soils varied in their effectiveness in symbiosis with soybean, the least effective fixing one fifth of the N2 of the most effective strains. Average strain recoveries when introduced in soil, ranged from 0 to 59 percent. A significant response to inoculation was observed in soils not previously used to grow soybean, even in the presence of higher indigenous population. Increased nodule occupancy by introduced strains did not result in increased plant growth or yield.
Careful selection of strains can increase nodule occupancy when added to soils containing indigenous rhizobium at rates used for commercial inoculants. The task of choosing a competitive strain that has wide application will not be easy because competitiveness varied with soil conditions and from year to year. Even though little production advantage occurred with inoculation, responses to N fertilizer suggest that improved grain yields should be possible with better Rhizobium strains. The strains that were found to be competitive in these studies may be suitable receptors of improved N2-fixation genes for better inoculants.
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