Gabriel A. Oluwatosin

Evaluation of two methods of soil quality assessment as influenced by slash and burn in tropical rainforest ecology of Nigeria

The sustainability of slash-and-burn agriculture for sustainable crop production has been a subject of controversy. The objective of this study was to quantify the effect of slash and burn on soil quality. Two sites, Ibadan (7° 23′ N; 3° 51′ E) and Akure (7° 17′ N; 5° 14′ E), within the tropical rainforest of Nigeria were selected for the study. Burnt and unburnt soils were cropped with maize, melon, and cowpea (in sole and intercrops). Soil and earthworm cast samples were collected and analyzed for physical, chemical, and biological indicators. Integration into soil processes and quality indices involved the transformation of analyzed indicators using Soil Management Assessment Framework (SMAF) and Multiple Variable Indicator Transform (MVIT) techniques. Organic matter, water-stable aggregates, pH, cation exchange capacity, macroporosity, and water infiltration were reduced after burning in both sites. Active carbon and potentially mineralizable nitrogen increased after burning at Ibadan but decreased after burning at Akure. Soil quality decreased after burning by a range of 11.3–24.8% using SMAF and MVIT, respectively, although only MVIT showed significant differences (p ≤ 0.05). Test crops increased in yields on burnt soils due to prompt release of nutrients to the crops; the benefit was dwarfed by the adverse effects of burning on soil quality indices.

Conservation Strategies for Effective Management of Eroded Landform: Soil Structural Quality, Nutrient Enrichment Ratio, and Runoff Water Quality

Water erosion remains a major threat to soil and water conservation in the humid tropics and, thus, requires effective conservation measures to curb it. To quantify the efficacy of vetiver grass (Chrysopogon zizanioides L.) strips and mulch in soil structural quality buildup and in reducing the eutrophic quality index and enrichments ratios of major nutrients, a study was conducted between 2007 and 2009 on eroded landform (8% slope) of a Typic Kanhapludalf in a subhumid region of southwestern Nigeria. Treatments were vetiver buffer strips (10 m vetiver grass strip [VGS]), vetiver mulch (6 Mg ha−1 vetiver grass mulch [VGM]), combined vetiver strips and mulch (10 m VGS + 4 Mg ha−1VGM), and a control (no buffer strips or mulch). Soil structural quality estimated by water-stable aggregates at 0- to 5-cm depth increased by 36%, 73%, and 64% under 10 m VGS, 6 Mg ha−1 VGM, and 10 m VGS + 4 Mg ha−1 VGM, respectively, more than the control. The corresponding increase in mean weight-diameter under the vetiver systems was 30%, 42%, and 47% higher than the control. The water-stable aggregates and mean weight-diameter at the 5- to 15-cm depth followed a similar trend in the 0- to 5-cm layer. The resistive potentials of the vetiver systems in reducing soil loss, sediment-associated nutrients, total suspended solids, and eutrophic quality index were significantly higher than the control because of an increase in surface hydraulic roughness, which were in the order of 6 Mg ha−1 VGM < 10 m VGS < 10 m VGS + 4 Mg ha−1 VGM. Application of 6 Mg ha−1 VGM appear to be better than 10 m VGS and contributed to the reduction of runoff and enrichment ratios, thus, changing the order to 10 m VGS < 6 Mg ha−1 VGM < 10 m VGS + 4 Mg ha−1 VGM. The results indicate that an integration of vetiver buffer strips and mulch (10 m VGS + 4 Mg ha−1 VGM) had a better control of runoff, soil loss, sediment-associated nutrients, and water quality degradation, as well as improving soil structural quality buildup when compared with vetiver buffer strips or mulch alone.

Lead and cadmium contents in a medicinal plant/spice grown in an urban city of Nigeria

Abstract Human exposure to heavy metals is a growing concern across Nigerian urban settings due to potential danger from consuming plants grown on contaminated soils. This study assessed the contents of lead and cadmium in soil and basil (Ocimum basilicum Lamiaceae) grown by vegetable farmers in Ojo Local Government Area of Lagos State, Nigeria. Using grid method, 36 points were located to collect soil samples at 0–15 and 15–30 cm depths, while plant samples were also collected simultaneously. The contents of lead and cadmium in soil and plants were determined and results were subjected to descriptive statistics while the transfer factor (TF) was calculated. Lead ranged from 1.85 mg kg−1 at the topsoil to 2.54 mg kg−1 at subsoil. Cadmium varied from 0.99 mg kg−1 at the topsoil to 1.41 mg kg−1 at subsoil. Average TF were 0.21 for Pb and 0.35 for cadmium. Distribution of lead and cadmium increased in order leaf < stem < root indicating that the root of basil may be useful in bioremediation of metal-contaminated soils. Although, the levels of lead and cadmium contents in the basil leaf are lower than other parts, there could be accumulation of these metals through long-term consumption of the leaf.

Use efficiency of tropical rainforest soils of southwestern Nigeria as affected by agronomic practices

Tropical soils generally have extremely delicate nature and lack resilience to recover once degraded. This is aggravated with increase in demographic pressure on land which has led to continuous and intensive cultivation of the land. With soil quality information, management options can be developed so as to improve the capacity of the soil to function within various ecosystem boundaries. This paper comprises of results of five experiments and it is a review of efforts made by the soil quality team of the Institute of Agricultural Research and Training, Nigeria, to identify physical, chemical and biological soil attributes that can serve as useful indicators in assessing the effects of management on soil quality. Five experiments were conducted by the team to assess effects of continuous cultivation, tillage practices, cropping systems and land use (tree versus arable crops) on soil quality. Our findings show that soil quality is aggraded, degraded or sustained, depending on the type of use and management options. Conventional management methods degraded soil quality indicators while farmers’ traditional practices minimize soil quality degradation. It is recommended that farmers’ practices that will protect sensitive soil quality indicators are panacea to soil quality management in forest ecology of the tropics. Keywords: Soil quality, management options, forest ecology, indicators, degradation

Distributions of cadmium and lead in peri-urban wetlands as influenced by soil organic matter, clay fraction, and moisture content

Abstract Environmental distribution of cadmium (Cd) and lead (Pb) were studied in two wetlands (alluvial plain and inland depression) in Ibadan, Nigeria. Mean and median values of variables measured across the wetlands showed that soil properties varied considerably. Hydraulic conditions, soil organic matter (SOM), and particle size distribution contributed to the distribution of heavy metals recorded. Profile distribution of Cd and Pb maintained two patterns: (1) linear distribution pattern, where concentrations of Cd and Pb increase and decrease respectively, with increase in soil depth and (2) middle depth enrichment. Active carbon (AC), SOM, and clay fraction affected the surface and profile distribution of Cd and Pb. Cadmium showed significant correlation (R2 = −0.61* and 0.64*) with AC and SOM, respectively, while Pb had R2 values of 0.77* and 0.57 with SOM and clay content, respectively. The non-residual fractions of the heavy metals increased with increasing metal loading and SOM. This study is useful in assessing chemical changes for Cd and Pb in different wetlands, and their potential release to the environment.