Nodulation behaviour of Acacia mangium wikd.in response to rhizobium inoculation

Abstract

An investigation was carried out during April 1996 to December 1997 at the College of Forestry, Kerala Agricultural University, Vellanikkara to study the nodulation characteristics of Acacia mangium Willd. plantation and to isolate and compare the efficiency of rhizobia from different agroclimatic regions of Kerala. The efficiency of these rhizobial isolates under varying conditions of soil fertility and acidity were also evaluated. Survey conducted throughout the state showed rhizobial association and nodulation in all mangium plantations. The soil of these plantations were acidic with pH values ranging from 4.9 to 6.2. Experiments conducted to study the rhizobial characteristics indicated that, the fast growers (Rhizobium spp.) as well as the slow growers tBradyrhizobium spp.) are involved in nodulation of A. mangium in Kerala. The efficiencies of different rhizobial isolates were studied in polypots using sterilized and unsterilized soil - sand mixture. Two other polypot experiments were conducted to study the response of the rhizboial isolates to liming and Nand P application. The experiments were laid out in C.R.D with three replications. Seedlings raised in unsterilized soil performed better as compared to seedlings in sterilized soil. However, the response to inoculation was more prominent in sterilized soil. In sterilized soil, seedlings inoculated with Rhizobium isolates from mangium ;MRh-3 (Mananthavadi), MRh- 2(Perambra), MRh-13 (Trivandrum), MRh-6 (Vellanikkara) and MRh-lO (Mavelikkara) and commercial mangium culture (" Agroforester - Group A") showed superiority in nodulation and growth characteristics while the isolates from A. auriculijonnis and commercial culture of A. auriculijonnis, ("Agroforester - Group C") were not much effective.

Seedling - response to inoculation varied with rhizobial isolates, and mangium showed its specificity in Rhiozbium requirement for nitrogen fixation. The application of lime adversely affected nodulation capacity of the rhizobial isolates of mangium from Kerala, indicating the adaptations of these isolates to acidic soils. The nodulation efficiency of commercial culture "Agroforester - Group A" was improved by lime application, indicating that this strain is adapted to soils of higher pH. The nodulation and nitrogen fixing efficiency of the isolates were improved by supplementating fertilized N and P. The need for a starter dose of Nand P for effective nodulation and efficient Ny-fixation in mangium seedlings. MRh-6 (Vellanikkara) performed poorly in unfertilized soil, but showed superiority when applied with N and P fertilizers, indicating the sensitivity of this isolate to site conditions. As the rhizobial isolates from different agroclimatic regions of Kerala responded differently under varying soil pH and doses of Nand P, inoculation of selected strains of rhizobia depending on site conditions may be needed for successful establishment of mangium seedlings.