Associative N2-fixation in plants growing in saline sodic soils and its relative quantification based on15N natural abundance

Abstract

Saline-sodie soils are characterized by a very low nitrogen and organic matter content and thus are practically non fertile. However under these conditions, certain plants have been found to grow luxuriantly. One of such plants,Leptochloa fusca (Kallar grass) has exhibited nitrogenase activity associated with its roots as determined by acetylene reduction assay (ARA). Quantification of such nitrogen fixation was also carried out using15N isotope dilution technique.

In addition to Kallar grass, other plant species growing in saline sodic soils namelyAtriplex amnicola, A. lentiformis, Sporobolus sp.,Kochia indica, Desmostachya bipinnata, Cynodon dactylon, Suaeda fruiticosa andPolypogon monspilensis have been screened for the presence of root associated nitrogenase activity. Some of the plant species tested showed high excised root acetylene reduction activity (ERARA). Isolation of diazotrophs from various fractions of the rhizosphere has also been carried out.Azospirillum was the dominant organism in niches closer to the roots, whereas there was a preponderance of the members of the family Enterobacteriaceae in general.

In order to have a relative estimate of the nitrogen fixing ability of different plant species screened, the delta15N values of plant tops were estimated and were correlated with their ARA values. The delta13C values of these plants were also determined which indicated that all the plants tested exceptP. monspilensis had the C-4 photosynthetic pathway.

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