Initial success of microbial biofilmed inocula for forest tree nurseries

In Sri Lanka, forest cover is declining at a rate of 1.5%, annually. In order to sustain the forest cover of 29%, reforestation efforts have been undertaken using native or exotic plant species. However, nursery plants face up to 50% failure in establishment in reforestation. Incapability of exotic species to establish in sufficient numbers on degraded lands has been thought to be due to lack of supporting soil microbial consortia. Therefore, attention has been paid to use native trees, which are however hindered by slow growth rate. Thus, the use of beneficial microbial inocula would facilitate the early survival and rapid growth of seedlings, through biological nitrogen fixation, soil moisture conservation, improved soil-plant nutrient cycling, secretion of growth hormones etc. An inoculum of beneficial microbes was developed in biofilm form, by using bacterial isolates from the rhizosphere of five native tree species namely, Diospyros ebenum (Ebony), Berrya cordifolia (Trincomalee wood), Azadirachta indica (Neem), Cassia fistula (Golden shower tree), Chloroxylon swietenia (Satinwood) and Chukrasia tabularis (Burmese almond wood). Isolated bacteria were screened for their ability of biological nitrogen fixation, organic acid production, solubilization of phosphate and plant growth promoting effect. Nine diazotrophic bacteria were selected and tested for biofilm formation ability with a fungus isolated from the rhizosphere of native tree species. Prepared biofilms were screened for plant growth promoting effect using an indicative plant. Selected best biofilm was applied to four major forest plantation tree species Artocarpus heterophyllus (Jack), Tectonia grandis (Teak), Eucalyptus leucoxylon (Eucalyptus) and Swietenia macrophylla (Mahogany). A. heterophyllus and S. macrophylla, showed improved plant growth, as reflected by increased plant height, leaf number and quick recovery of leaf flush after induced drought. It was also noted that it is important to test species-specific inocula, if their performances are to be further improved. Thus, there is a possibility of using the biofilmed inocula to improve the ecological sustainability of reforestation programs, and further large-scale studies are however, recommended.