Chapter 4: Soil nutritional status drives the co-occurrence of nodular bacterial species and
4.3 Results
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4.3.4 Plant nutrition and arbuscular mycorrhizal fungi root colonization (%)
Percentage nitrogen derived from the atmosphere (%NDFA) was maintained by the four V.
unguiculata varieties across soil types (Table 4.3). Moreso, the NDFA showed no statistical differences across the different V. unguiculata varieties and the four soil types. Similarly, total plant N concentration was maintained across soil types while statistical differences were shown across varieties with IT18 recording the highest total plant N concentration (Table 4.3).
Ashburton soil-grown Batch white, Dr Saunders and IT18 showed a significantly higher nitrogen derived from soil (NDFS), while Brown mix grown in Bergville soil showed a significantly higher concentration of nitrogen derived from soil (Figure 4.1A). Hluhluwe soil- grown plants had a significantly higher SNAR across all the four V. unguiculata varieties while Izingolweni soil-grown plants recorded the least, except for variety Batch white (Figure 4.1B).
Similarly, Hluhluwe soil grown plants had a significantly higher SNUR across the four V.
unguiculata varieties while Bergville soil grown plants had the least SNUR for all varieties except for variety Dr Saunders. Overall, Dr Saunders had distinctly lower SNUR relative to the other three cowpea varieties (Figure 4.1C).
Total plant P concentration of different V. unguiculata varieties showed statistical differences across the four soil types (Figure 4.1D). Overall IT18 had the highest total plant P concentration across all soil types while Dr Saunders recorded the least. Hluhluwe soil-grown plants of varieties Brown mix and IT18 had the highest total plant P concentration. Conversely for Batch white, Ashburton soil grown plants had the highest total plant P concentration. Bergville soil grown plants had the least total plant P concentration for varieties Batch white, Brown mix and IT18 (Figure 4.1D).
The AMF root colonization % showed significant differences for the four V. unguiculata varieties across the four soil types. Hluhluwe soil-grown cowpea varieties had 100% AMF root colonization, and this was closely followed by Izingolweni soil-grown plants while Bergville soil grown plants recorded the least across all V. unguiculata varieties (Figure 4.1E).
There were no marked differences of SPUR across the other three soil types for all the four V.
unguiculata varieties (Figure 4.1F). Hluhluwe soil-grown Brown mix and IT18 plants had the highest SPAR while Ashburton and Bergville soil-grown plants had the highest SPAR for varieties Batch white and Dr Saunders respectively. Overall, Brown mix had the lowest SPAR (Figure 4.1G). Hluhluwe soil-grown plants had the highest SPUR across the four V.
unguiculata varieties.
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4.3.5 Correlations between the physicochemical properties (soil P, N, K, C, exchange acidity, total cations, pH) of four soil types (Bergville, Ashburton, Izingolweni, Hluhluwe) of KwaZulu-Natal
There were distinct variations in the soil physicochemical properties of Hluhluwe and Bergville soils. However, Izingolweni and Ashburton soils had similar properties as shown by the overlap in ellipses (Figure 4.2). The first two principal components of the soil physicochemical properties analysis explained 86.4% of the cumulative variability of the measured components with PCA 1 accounting for 65.1% and PCA 2 accounting for 21.3% of the total variation (Figure 4.2). Principal component (PCA 1) had strong positive loadings for P and exchange acidity, and strong negative loadings for Ca and Mg. The second axis (PCA 2) had strong positive loadings for N and exchange acidity and a strong negative loading for pH. Bergville and Hluhluwe soil properties were separated along PCA 1. Variations in Bergville soils were highly associated with soil P concentrations and exchange acidity values while variations in Hluhluwe soils were highly associated with total cations, and the concentrations of soil Mg, Ca and K.
4.3.6 Correlations between four V. unguiculata varieties grown under different soil types of KwaZulu-Natal
To investigate the correlations among the four V. unguiculata in terms of their growth and nutrition response to different soil properties, the data were subjected to principal component analysis (PCA). The positions of the different varieties grown under the four different soils and the relations among the soil properties and the measured plant parameters are as shown in the PCA biplots (Figure 4.3). There was separation of overall soil type clusters and this clearly shows that the V. unguiculata varieties responded considerably differently to different soil types. The first PCA explained 43.8% of the total variation while PCA2 explained 18.3% of the total variation. The cluster of Hluhluwe soil grown plants was separated from the cluster of Bergville soil grown plants by PCA1 while there was an interaction in terms of response between Ashburton soil grown plants and Izingolweni soil grown V. unguiculata plants as shown by the overlap of the ellipses. Variation within Hluhluwe soil grown plants was mainly associated with soil Mg, Ca and soil pH and to a lesser extent by soil K and soil N. Variation in Bergville soil grown plants was mainly explained by exchange acidity and soil P. Ashburton soil grown IT18 and Batch white were separated from Ashburton soil grown Dr Saunders along PCA1. Variation in Izingolweni soil grown IT18, Ashburton soil grown IT18 and Ashburton soil grown Batch white was explained by NDFS, NDFA, SPAR, total plant N and total plant P. The PCA plot also showed the relationships of the measured response parameters of V.
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unguiculata varieties to different soil properties. The groups closer together showed high correlations.