Symbiotic nitrogen fixation of wild legumes in Tunisia: Soil fertility dynamics, field nodulation and nodules effectiveness

Wild legumes (herbs, shrubs or trees) play a critical role in natural ecosystems, agriculture, and agroforestry, where their ability to fix nitrogen in symbiosis makes them excellent colonizers of low-N environments, and hence an economic and environmentally friendly species. The field natural nodulation of the wild Tunisian legumes, the genetic diversity and symbiotic-efficiency of their root nodulating bacteria (RNB) and the dynamics of soil fertility under symbiotic association in arid regions of Tunisia were investigated during this study. Natural nodulation varied according to the species and site. Morphologically, nodules were spindle-shaped (Trigonella maritima, Hedysarum spinosissimum, Argyrolobium uniflorum, Genista saharae, Retama reteam, Vicia sativa). The colour of the nodules was not related to that of the roots, but depends on soil quality and colour, and on the presence of leghaemoglobin. Based on molecular techniques (16S rRNA sequencing) and phenotypic characteristics, the root-nodule bacteria isolated from the Tunisian wild legumes were classified into four genera (Rhizobium, Mesorhizobium, Sinorhizobium). No Bradyrhizobium genus was recovered in our results. Non-nodulating bacteria considered as endophytic to nodules were also reported in our results and four genera belonging to Phyllobacterium, Starkeya, Pseudomonas, and Kocuria were taxonomically defined, for Lotus sp. and H. spinosissimum. Bacteria belonging to Sinorhizobium genus were widely distributed in area under study. Wild legumes belonging to Lotus spp., Vicia spp. and Hedysarum spp. were nodulated by this genus. Mesorhizobium genus was restricted to Lotus sp., and Rhizobium genus nodulated Vicia and Lotus. Phenotypically, our results showed that the root nodulating bacteria belonging to the genus Sinorhizobium exhibited higher tolerance to salt stress and elevated temperatures. Based on symbiotic properties, our reports indicated that the wild-legume rhizobia formed effective and successful symbioses with their legume hosts. The diversity and effectiveness of the nitrogen-fixing wild legumes are of major significance to soil fertility dynamics in the arid regions. Effects on enzyme activities, microbial biomass and respiration, were evaluated in different agricultural soils and in the presence of different wild legumes such as G. saharae and R. raetam. Results showed that legumes-rhizobia symbiosis improves enzyme activities, microbial biomass and respiration of field soils and regenerate microbiological properties and the microflora activity involved in the decomposition of organic matter. (c) 2012 Elsevier B.V. All rights reserved.