A. O. Nengi-Benwari’s scientific contributions

Studies have shown that microbial community structure is influenced by tree species identity thus the objective of this study was to determine microbial community structure in the rhizosphere of five selected tropical trees. This research was conducted at the Forestry Arboretum of the Faculty of Agriculture, University of Port Harcourt. Random sampling was used to collect soil samples from the rhizosphere of selected trees at a depth of 0-30cm for microbial analysis and identification in the laboratory using standard procedures. Results showed significant difference in microbial population across all trees with the highest microbial population observed under Tectona grandis (4.6x106 cfu/g) and the lowest microbial population recorded under Irvingia gabonensis. Ranges of microbial population were, total heterotrophic bacteria 0.88x106cfu/g – 4.6x106 cfu/g, and total heterotrophic fungi 3.0x103 cfu/g – 15.0 x103 cfu/g. Microbial diversity showed variation across all tree species, a total of 11 Bacteria and 9 fungi species were isolated and identified. Bacillus spp was the most predominant bacterium; Aspergillus spp was the most predominant fungus, highest microbial diversity was observed under Tectona grandis and Gmelina arborea, and lowest microbial diversity was found under Nauclea diderrichii. All five selected tropical tree species had effect on the microbial community structure but Tectona grandis exerted the most effect. This tree can therefore be utilized in agroforestry to boast nutrient availability and sustainable agriculture.

In order to maintain soil fertility and quality, sustainable agriculture, and ecosystem processes, the population and diversity of soil organisms are crucial. This study was carried out at the Arboretum of the Faculty of Agriculture, University of Port Harcourt, and examined the diversity and population of nematodes and earthworms under a few chosen trees. Samples of soil, roots, and earthworms were randomly taken from the rhizosphere of trees at a depth of 0–30 cm using a spade and hand trowel. The extraction tray method was used to extract nematodes, the hand sorting and ethological method was used to collect and count earthworms, and the analysis of variance was performed on the data collected, and least significant difference was used to separate the means. Results showed significant difference in earthworm population across all trees with the highest earthworm population observed under Annona muricata (25.8), while lowest earthworm population was recorded under Irvingia gabonensis. Nematode population ranged from 3.0 – 53.0 while earthworm population ranged from 7 – 25.8 under the various tree species. Four (4) nematode species were identified, and Dorylaimida spp was the most predominant nematode. All five selected tropical tree species had effect on nematode community structure and earthworm population but Tectona grandis exerted the greatest effect. These impacts may be due to passive byproducts of nutrient intake, root shape, shearing habitat preference by trees and soil organisms, or active selection for soil microbes by plants through root exudates. These trees could be used in sustainable agriculture and agroforestry.

Compost plays major role in the quality of soil parameters and the microbiological diversity. The effect of compost rates on microorganisms and growth parameters of Capsicum chinense was investigated. Organic compost with concentrations: 0, 200, 400, 600, 800 and 1000g was transferred into separate polyethene bags containing 15kg soil and kept at the greenhouse of the Faculty of Agriculture, University of Port Harcourt. The bacterial and fungal parameters were determined using standard plate count on Nutrient and Sbouraud Dextrose agar plates while the physicochemical and growth parameters were measured using standard method. Results of the total heterotrophic bacteria ranged from 2.96x106– 6.42x106 CFU/g, total heterotrophic fungi ranged from 1.41x103– 4.56x103 SFU/g, respectively. A total of 11 Bacteria and 10 fungi species were isolated and identified. Bacillus spp and Pseudomonas spp was the most predominant bacterium; Penicillium spp was the most predominant fungus in the samples. Highest microbial diversity was observed under the 1000 g. Conversely, lowest microbial diversity was found under 0 g compost rates. All six compost rates had effect on the microbial community structure and plant height.

The population and diversity of soil organisms are important factors in maintaining soil fertility and quality. The aim of this study was to determine the impact of varied compost rates on Arbuscular Mycorrhiza Fungi, rhizosphere soils of Capiscum chinense. Soil samples were treated with various concentrations of compost (0 g, 200 g, 400 g, 600 g. 800 g and 1000 g). Systematic method of random sampling was used to collect soil samples from the rhizosphere of the various soils of different compost rates using a spade and hand trowel at a depth of 0-10cm. Results showed significant difference (P<0.05) in the Arbuscular Mycorrhiza Fungi (AMF) across all compost rates. AMF population ranged from 67 – 100 g/dwt respectively with the highest AMF population observed under 400 g(100 g/dwt, while the lowest AMF population was recorded under 600 g/dwt. Results for microbial diversity showed variation across all compost rates. A total of four AMF species were extracted from the rhizosphere of the soils. Acaulospora spp was the most predominant AMF in the studied soils. Highest microbial diversity was observed under the 1000 g. Conversely, lowest microbial diversity was found under 0 g compost rates. All six compost rates had effect on the AMF population but 1000 g exerted the most effect. These effects could be as a result of compost passive by-product of nutrient uptake, Root structure and soil organisms.

Soils are intricately linked to the atmospheric and climatic system, and soil chemical properties are strongly influenced by seasonal variations which in turn affect agricultural productivity and food security. The aim of this study therefore was to investigate the effect of season on the chemical properties of two vegetation soil types. Samples of surface (0-15cm) and subsurface (15-30cm) soils were collected from June 2018 to May 2019 at Mangrove and Rainforest vegetation types in Port Harcourt during dry and wet seasons. Sampling months were categorized into Peak of Rainy season, Rainy season, Peak of Dry season and Dry season according to rainfall data, relative humidity, atmospheric temperature and soil temperature recorded throughout the sampling period. Soil chemical properties were analyzed using standard analytical methods and the data obtained were subjected to statistical analysis using analysis of variance (ANOVA). Results showed that soil chemical properties were generally higher in topsoil than in subsoil except for available phosphorus which recorded higher concentrations in subsoil than at topsoil. Soils in the mangrove vegetation were strongly acidic (pH-3.1), while soils in the rainforest vegetation were slightly acidic (pH 5.0). Values of available phosphorus followed the trend DS>PDS>PRS>RS, while magnesium and percentage base saturation had similar trend of DS>RS>PDS>PRS. Values of sodium and potassium were similar in all seasons, while effective cation exchange capacity and exchangeable acidity followed the trend PDS>PRS>RS>DS and PRS>PDS>RS>DS respectively. Generally, concentrations of soil chemical properties tend to be higher in the drier seasons than during the wet seasons. This investigation showed that the chemical properties of the soils were influenced by seasonal changes in the region which could in turn influence agricultural production.

Water relations of arbuscular mycorrhiza fungi (AMF) soils have been compared with the non-AMF soils, and often found that AMF increased soil water retention in cultivated soils. However, little knowledge is available about the influence of AMF on water retention characteristics of rainforest soils. In this study, we evaluated the population and the relationship between AMF of a tropical rainforest soil and water retention characteristics in the dry and rainy seasons. Results showed that AMF population and species abundance increased significantly in the dry season than rainy season. The AMF population was significantly higher (p<0.05) at 392.5 spores 100 g-1 soil in the dry season, than 297.7 spores 100 g-1 soil found in the rainy season. Acaulospora and Glumos sp were dominant in the dry season soils with 106.1 and 78.5 spores 100 g-1 soil, respectively, in comparison with 92.3 and 67.7 spores 100 g-1 soil in the rainy season soils, respectively. Micro-porosity was significantly higher (p<0.01) in the rainy season than dry season, indicating that AMF aided more water retained in the soil during the rainy season. Water retention at field capacity (FC) was significantly higher (p<0.05) at 20.3% in dry season. Available water content (AWC) of 9.2% in dry season, indicated tendency of the soil to take in water in dry season than in rainy season. Macro-porosity favoured increase in AMF, while micro-porosity decreased AMF population in the soil. Water content at field capacity (FC) and available water content (AWC) showed significant positive relationships with AMF (r = 0.741, p<0.05 and r = 0.814, p<0.01), (respectively). Therefore, AMF could be effective in the maintenance of soil water during the dry and rainy seasons and potentially increased water retention at permanent wilting point for crop use.