The role of nutritional status of soils from grassland and savanna ecosystems on the biochemical and physiological responses of Vigna unguiculata L. (Walp)

Abstract

Most arable soils in sub–Saharan Africa savanna and grassland ecosystems are acidic and nutrient deficient with nitrogen and phosphorus being the most limiting and this poses a huge threat to agricultural productivity. To overcome soil nutrient deficiency and increase crop yields, farmers have resorted to high inputs of synthetic fertilizers, which are expensive and may cause environmental degradation. Use of legumes is an important alternative as they help enhance soil nutrition through biological nitrogen fixation. Vigna unguiculata L. (Walp), a highly nutritious legume crop that could be incorporated in small scale cropping systems to improve soil nutrition. However, there is limited information on the physiological and biochemical strategies enabling the growth of V. unguiculata under acidic and nutrient stress conditions. In this study it was hypothesized that symbiotic association between V. unguiculata and rhizospheric microbes affects the growth, nutrient assimilation and phytochemicals of the grain legume grown in nutrient stressed soils. Firstly, this study evaluated the physicochemical properties, microbial composition and soil enzymes activities of soils from four geographically distinct regions of KwaZulu-Natal representing savanna and grassland ecosystems. Secondly, the study investigated how the tripartite symbiosis of V. unguiculata, arbuscular mycorrhizal fungi and nodulating bacteria affect phosphorus and nitrogen nutrition, and the growth of V. unguiculata grown under acidic and nutrient stress conditions. Then, the study investigated how four V. unguiculata varieties regulated their phenolic acids and antioxidants to enhance their growth in acidic and nutrient stressed soils conditions. The four soil types were acidic with low mineral nutrients, with Bergville being the most acidic. The soils were significantly different in their physicochemical and microbial composition. Most bacterial strains identified in the soils belonged to genera Lysinibacillus, and Bacillus while the most identified fungal strains belonged to Fusarium and Trichoderma genera. There were variations in soil lignin degrading, C, N and P cycling enzyme activities. The identified soil enzymes included β-D Phosphatase, L-asparaginase, β-glucosaminidase, β-cellobioside, catalase and lacasse. The availability of this rich pool of soil microbes and soil enzymes is a great opportunity as these can be used to regulate nutrient cycling and enhance nutrient availability for crop production in the savanna and grassland ecosystems. Four V. unguiculata varieties (IT18, Batch white, Brown mix, Dr Saunders) were grown in these acidic and nutrient poor soils. These V. unguiculata varieties were nodulated by several bacterial strains including those of genera Bradyrhizobium, Rhizobium, Bacillus and Paenibacillus. The V. unguiculata fixed more than 60% of its total nitrogen from the atmosphere across all soil treatments. Interestingly, V. unguiculata plants which were nodulated by non-rhizobial bacteria strains effectively fixed significantly high amounts of atmospheric nitrogen. Vigna unguiculata also developed symbiotic association with arbuscular mycorrhizal fungi (AMF) as evidenced by high root mycorrhizal fungi colonization ranging from 58-100%. Variations were observed on growth kinetics, nutrient assimilation and utilization among the four V. unguiculata varieties. Vigna unguiculata was able to switch N source preferences utilizing both soil and atmospheric nitrogen. These findings revealed that V. unguiculata has the capacity to adapt to nutrient poor ecosystems by establishing symbiotic interaction with naturally occurring soil bacteria and AMF and through its ability to switch N source preferences; by using soil N and atmospheric N2 through biological nitrogen fixation. There were variations in the response of the four V. unguiculata varieties to different levels of soil acidity and nutrient stress with regards to phenolic acid concentration and antioxidant capacities. The most abundant phenolic acids were vanillic acid and protocatechuic acid and these constituted 22.59% and 17.22% respectively of the total phenolic acids in the plants. More so, there were differences in correlations between the phenolic acids and plant biomass, plant nutrition, soil nutrition and AMF infection. There was negative correlation between phenolic acids protocatechuic acid and syringic acid, and concentration of plant nutrients N and P. Varieties IT18 and Batch white had relatively lower concentrations of phenolic acids but these had the highest plant biomass. These results confirm that low phenolic acid concentrations have stimulatory effects on growth and nutrient uptake by plants while high concentrations may inhibit plant growth and development. There were variations among the V. unguiculata varieties with respect to oxygen radical absorbance capacity (ORAC) across the four soil types. Overall, the study demonstrated that V. unguiculata is adaptable to acidic and nutrient poor ecosystems as it has the capacity to regulate its phenolic acids which enhance nutrient uptake, promote legume-microbe symbiosis, and help scavenge radical oxidative species due to their antioxidant properties.