Nutritional status of KwaZulu-Natal soils affects microbe symbiosis, nitrogen utilization and growth of Vigna radiata (L.) R. Walczak

Abstract

A major driving force behind sustainable agriculture and protection of the environment is effective management of nitrogen (N) in farming. Better understanding on the growth and functional adaptation of legumes under low nutrient savannah and grassland ecosystems remain crucial for food security. Therefore, this study examined if Vigna radiata (L.) R. Wilczek (Mung bean) establishes multiple symbiosis with soil microbes in KwaZulu-Natal (KZN) soils and how that symbiosis affects plant N nutrition and related-carbon growth costs. The study was conducted in the greenhouse using soils from four (Hluhluwe, Izingolweni, Bergville, and Ashburton) different geographical areas of KZN province. The soils were used as a natural inoculum and growth substrate thus treated as four treatments. Soil analysis of the four geographical areas showed marked differences with regard to phoshorus (P) and potassium (K) nutrient concentrations, exchange acidity and arbuscular mycorrhizal (AM) spore counts, while no significant differences were observed among the four treatments in term of N concentration and root colonization. Soils from the four sites were acidic and showed variation in nutrients especially with regard to P and N. Bergville soils had the highest P concentrations, exchange acidity and AM fungi spore counts, while Hluhluwe soils had the highest concentration of K, total cations and lowest spore count. Vigna radiata grown in Hluhluwe soils had higher biomass when compared to Bergville soil-grown plants. All plants nodulated with the exception of plants that were grown with Bergville soil. Despite differences in soil nutrient concentration, V. radiata plants established symbiosis with AM fungi, Rhizobium and numerous Bacillus species. Furthermore, the legume plants including the non-nodulated Bergville-grown V. radiata plants were able to switch N sources (atmospheric and soil N) to maintain their N nutrition and carbon construction costs. This observation suggests that non-nodulated Bergville-grown plants may have established symbiosis with other bacterial taxa such as Bacillus spp. and other endophytic bacterial spp. that fix atmospheric N. Overall, the symbiotic association of V. radiata with Rhizobium, AM fungi and various Bacillus species enhanced N utilization. In addition, V. radiata has the capacity to switch between soil N and atmospheric N while maintaining low carbon construction cost under low soil nutrient conditions. Vigna radiata thus can be adaptable to low soil nutrient savannah and grassland ecosystems in KZN province, South Africa.