There is a paucity of information on nitrogen (N) and phosphorus (P) mineralization in humic soils, which are highly weathered and have high carbon (C) (>1.8%). This study was to determine effects of liming on N and P mineralization in humic soils. Lime was applied to reduce acid saturation to 20% of the 0–10 and 10–20 cm depths of soils from Eston and Eshowe. Soils were incubated at field capacity moisture and 25 °C temperature, with destructive sampling after 0, 7, 14, 21, 28, 56, 84 and 112 days. Samples were analysed for pH, ammonium- and nitrate-N and extractable P. Phosphorus pools and soil microbial biomass C and N (SMBC and N) were analysed after 112 days only. Soil pH increased up to day 7 and decreased thereafter in Eston soil but decreased throughout the incubation in Eshowe soil. Ammonium- and nitrate-N increased with lime rate, with ammonium-N peaking after 7 and 14 days for Eston and Eshowe soils, respectively. The 0–10 cm depth had higher ammonium-N than 10–20 cm for both soils. Nitrate-N increased with corresponding decrease in ammonium-N. Extractable P decreased till day 21 and increased thereafter in Eston soil, with slight changes in Eshowe. Higher lime rate decreased Al-P, Fe-P and CBD-P and increased soluble-P, Ca-P, and SMB-C and N for both soils. The findings imply that liming humic soils increase nitrate-N and, to a lesser extent, extractable P, possibly improving productivity and exposing N to leaching.

Urban soils in Africa are increasingly contaminated by toxic heavy metals due to rapid urban expansion, industrialisation, traffic emissions, and inadequate waste management. Toxic heavy metals such as lead (Pb), mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), and nickel (Ni) are of major concern because of their persistence, bioaccumulation potential, and ecological risks for the environment and humans. This systematic review synthesises evidence from 38 peer-reviewed studies published between 2010 and 2024 on the concentrations and sources of toxic heavy metals in urban soils of Africa. The results demonstrated an uneven research distribution on toxic heavy metals across different countries of Africa, with Nigeria and Ghana most represented, while large regions, including countries such as Egypt and Ethiopia, remain poorly studied. Industrial activities (27%) and traffic emissions (26%) accounted for more than 50% of the reported contamination sources, followed by domestic waste disposal (18%) and agricultural inputs (10%). Weighted mean concentrations of Pb, Cd, and Cr in many countries exceeded FAO permissible limits, indicating a significant threat to humans and the environment. Microwave digestion was the most commonly used extraction method, while X-ray fluorescence has gained increasing application. The findings demonstrate widespread contamination from rapid urbanisation and industrialisation but highlight limited research done on toxic heavy metals in urban areas of most African countries. Future research should focus on quantifying the metal concentration in African countries, where limited work has been done; the bioavailable fractions of toxic heavy metals and possible remediation strategies to improve soil quality in urban areas.

South Africa produces approximately 122 million tons of solid waste annually, which poses a significant environmental concern. The decomposition of solid waste in landfill sites results in the release of toxic heavy metals into the soil, posing a threat to human health and biodiversity. This study evaluated the impact of solid waste disposal in the distribution of heavy metals, that is, lead (Pb), zinc (Zn), cadmium (Cd), and arsenic (As), in landfill site soils. Soil samples collected (0-20 cm depth) from five dumping cells, two non-dumping zones, and a wetland were analyzed for soil chemical properties (exchangeable bases, extractable phosphorus (P), and micronutrients) and heavy metal concentrations. The results showed clear spatial variation in nutrient concentration across the landfill, with boundaries exhibiting lower fertility than dumping cells. The results indicated that cell 5 had higher organic matter concentration compared to all the treatments, except for cell 4. Heavy metal concentration results showed that Cd was below detection (< 1 mg kg ⁻¹ ) in the boundaries, wetlands, and cell 5 but exceeded the permissible levels (17.5-32.8 mg kg ⁻¹ ) in cells 1 to 4. Pb exceeded the permissible limit (50 mg kg ⁻¹ ) across all zones, with cell 5 showing concentrations almost double the ones in boundaries and wetland. Zinc concentrations were extremely high in cell 5 (1795 mg kg ⁻¹ ), > 100% higher than other treatments, while As remained relatively uniform across the treatments. These findings demonstrate the importance of natural remediation in reducing contaminant mobility and provide landfill sites management insights.