Nitrogen fixation associated with roots of Kallar grass (Leptochloa fusca L. Kunth)

Summary

An ecological survey was made to measure the N₂-fixing activity in the rhizosphere of a salt-tolerant grass,Leptochloa fusca (L.) Kunth. Samples were obtained every month at two sites over a period of one year. Soil cores, unwashed, washed and surface-sterilized roots were subjected to acetylene reduction assay (ARA). ARA values up to 50 nmoles h⁻¹ for soil cores, 1095 nmoles g dry root⁻¹ h⁻¹ for unwashed roots, 4929 nmoles g dry root⁻¹ h⁻¹ for washed roots and 2494 nmoles g dry root⁻¹ h⁻¹ for surface-sterilized roots were observed but for most samples the range was 1–200 nmoles g dry root⁻¹ h⁻¹. A lag period of 5–7 h was observed before the onset of N₂-fixing activity by excised roots and O₂ levels had no effect on this lag. Values for roots incubated without preincubation were similar to those for unwashed preincubated roots. Activity was highest in September, October and November when the temperature is not very high and photosynthetic activity is reasonably good. N₂-fixing-bacteria were counted on the same samples by plate count and MPN methods, the latter being estimated on the basis of ARA and pellicle formation. Fairly high numbers of bacteria (10⁴–10⁷) were recorded in the histoplane fraction which indicates the presence of diazotrophs in the inner cells of grass roots.