Potential Biofertilizer Application for the Enhancement of Tree Legumes

Nitrogen is one of the major elements available to plants through biological nitrogen fixation, which has received much attention in recent years. The present study aims at improving tree legumes viz., Albizzia lebbeck, Pithecolobium dulce, Sesbania grandiflora, Albizzia amara, Enterolobium saman, Erythrina indica, Leucaena leucocephala, Acacia mellifera, Pongamia glabra and Acacia auriculiformis through dual inoculation with arbuscular mycorrhizal fungi and suitable rhizobial isolates. Rhizobial isolates viz., AcM05 (from Acacia mellifera), AlL01 (from Albizzia lebbeck), EnS08 (from (Enterolobium saman), LeL02 (from Leucaena leucocephala) and ErI06 (from Erythirina indica) were characterized. Based on the biochemical characterization of rhizobial isolates and dry matter yield of test tree legume species, strain AcM05 and Glomus fasciculatum were scored as the most efficient strain and these two were used to inoculate Acacia mellifera. Dual inoculation with Rhizobium and Glomus fasciculatum increased the nodule nitrogenase activity (36-213%), dry matter yield (156-279%), total nitrogen content (12-159%) and total chlorophyll content of leaves of Acacia mellifera (125-395%) compared to the uninoculated control or single inoculation with either Rhizobium or Glomus fasciculatum alone. The impact of dual inoculation on soil enzymes viz., amylase, chitinase, protease and phosphatase was also studied. Initially the soil had high potassium content (115mg/kg soil) and low nitrogen (14mg/kg soil) and phosphorous (1.1mg/kg soil) content. In contrast, dual inoculation significantly enhanced the nitrogen (60.0mg/Kg soil) and phosphorous (24.3mg/Kg soil) contents of soil, but to a lesser extent with respect to potassium content (155mg/Kg soil). The per cent increase of the activity of soil enzymes amylase, protease, phosphatase and chitinase was more upon dual inoculation. Higher percent mycorrhizal root colonization was found in Acacia mellifera with dual inoculation than compared with plants inoculated with either Rhizobium or Glomus fasciculatum alone. In conclusion, it may be stated that tripartite association not only enhances plant growth but also improves the nutrient status and activities of soil enzymes.