Potential mineralizable N and P mineralization of local organic materials in tantalite mine soils

Nitrogen (N) and phosphorus (P) remain the most limiting plant nutrients in most mine soils, particularly in the tropics. We estimated the potential mineralizable N of the biomass of Canavalia brasiliensis and goat manure and P mineralization of Tithonia diversifolia biomass and goat manure in tantalite mine soils of western Rwanda. The mineralizable N was estimated through a four-week incubation of amended soil-sand mixtures followed by fortnightly leaching with 200 ml of 0.01 M CaCl₂. Conversely, Phosphorus mineralization was estimated from four-week incubation of amended soil-resin mixtures. The resins were separated from the soils and eluted with 0.5 M HCl to estimate phosphate content. Among the treatments, Canavalia had the highest mineralizable N (130 μg g⁻¹ soil, P < 0.01) in the Kavumu Technosol while the native forest soil had the lowest (–20 μg g⁻¹ soil). Goat manure had negative mineralizable N in all soils but was 30–70% less severe in the “cultivated soils”. P mineralization in goat manure was triple that of Tithonia, constituting 61–71% of total P applied. Each amendment uniquely contributed to N and P supply, suggesting optimum benefits of N and nutrient release as well as soil organic matter accrual through combined application in specific ratios or co-composting.