Possible amelioration of coastal soil salinity using halotolerant nitrogen-fixing cyanobacteria

Abstract

A brackish-water, nitrogen-fixing cyanobacterium, Anabaena torulosa, could successfully grow and fix nitrogen on moderately saline "Kharland" soils (soil conductivity 5 to 8.50 dS m^-1), typical of Indian coastline. During five weeks of growth under laboratory as well as field conditions, the cyanobacterium exhibited high rates of nitrogen fixation and substantially enriched the nitrogen status of saline soils (43-76%), although the fixed nitrogen remained confined to the cyanobacterial biomass. Most (>90%) of the cell-bound Na⁺ remained extracellularly trapped in the mucopolysaccharide sheath of A. torulosa; traces of the cation that permeated cyanobacterial cells were found to exist in an osmotically active, free state. No evidence was found for the incorporation of Na⁺ into any biomolecule, especialty proteins or carbohydrates. Therefore, permanent removal of Na⁺ from saline soils using cyanobacteria may not be possible, since Na⁺ is released back into the soil subsequent to the death and decay of cyanobacteria. Removal of top soil containing cyanobacterial mats significantly decreased the soil salinity (between 26-38%). But such a practice removes all the fixed nitrogen and carbon and also does not seem feasible on a large scale. Amelioration of soil salinity by simultaneous application of A. torulosa during crop growth seems to be an attractive possibility, especially since it can also supplement the nitrogen requirement of the crop.