The role of nutritional status of soils from grassland and savanna ecosystems on the biochemical and physiological responses of Vigna unguiculata L. (Walp)
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Date
2022
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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.
Description
Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.