Floristic Composition and Plant Diversity across Land Use Types
Simpson Diversity Index is a measure of diversity that takes into account the number of species present, as well as the relative abundance of each species. As species richness and evenness increase, so will diversity increase. The farmland, secondary regrowth, and wetland had a diversity index of 0.91, 0.94, and 0.93 respectively. An evaluation of the pattern of species richness in a degraded ecosystem gives an insight into the potential of ecosystem recovery . Flora species differ in diversity indices because of changes in land use and soil chemical and physical properties. The four land-use types identified in this study have different floristic compositions probably as a result of persistent anthropogenic activities, the gradient of the land, nearness to the water body, and soil composition. A total of 108 species (trees, grass, shrubs, and herbs) and 38 families were identified in all study areas. There were 35 trees species encountered in the study area, which was lesser than the 57 reported by Ojo . Olajuyigbe and Akwarandu,  and Asinwa et al.  also reported a higher number of tree species in a watershed ecosystems in Nigeria. The variation in plant species diversity has been attributed to the influence of anthropogenic activities such as farming, overgrazing, over-cultivation, and infrastructural development [19-20]. The grassland which had low diversity was dominated by and invasive grass species: Megathyrus maximus.
Soil Properties across the Land Use Types
The soil pH ranged from strongly to slightly acidic, the acidic nature of the soil could be attributed to heavy rainfall which leads to a high rate of leaching of bases and this is prevalent in the humid tropics. The pH of the farmland and grassland were more acidic and this may be due to factors like poorly managed cultivation, inappropriate use of fertilizer, and accelerated erosion because of the site terrain . The pH range recorded in this study were similar to the range (4.9-6.2) reported in some selected soils of South-West Nigeria . The soil organic carbon observed across the land use type can be classified between low to medium class. The low organic carbon observed in the farmland is probably due to the effect of continuous cultivation of crops. It might also be due to the effect of high temperature and relative humidity which favor rapid mineralization of organic matter . The results agree with others studies that the land use type can affect the organic carbon content in the soil (24-26]. Consequently, the report has proven that high organic carbon is related to the rate of vegetation cover and the amount of litter produced on the soil. Total Nitrogen of the soil was low across land use types. The low Nitrogen could be a result of the leaching of the nutrient into the soil profile. Nitrogen level on the farmland soil is relatively lower than other land use types and this could be attributed to the continuous cultivation of agricultural crops (Noma et al., 2011).
Relationship among Soil Properties, Plant Diversity and Land Use type: Implications for management of the Watershed Ecosystem
The results revealed a strong significant relationship between diversity indices and soil physicochemical properties. Positive significant relationships were observed for species dominance and evenness and some soil physical and chemical properties (% sand, Fe, Na, K, Mg, Cu, Exch ALH+). Similar results were recorded by Appiah-Badu et. al. , where land-use effects on tree species diversity and soil properties of Awudua Forest, Ghana were examined. Yaseen, , reported similar results in China, it was observed that plant diversity in the tropical coastal secondary forest was associated with soil variables in the study area. It was reported that soil chemical properties and plant species diversity were strongly correlated along the rainfall gradient in the semi-arid grassland of South Africa . These confirm a strong relationship between soil physical and chemical properties significantly influences species occurrences, abundant and diversity.