Determination of N mineralization of Tropical soils amended with compost and bokashi generated from water hyacinth and hippo grass

The spread of aquatic weeds, notably water hyacinth and hippo grass, in rivers of tropical regions cause serious problems. Using these weeds as soil amendments could promote their utilization and help reduce their spread. We investigated their use when dried, composted, and fermented (bokashi), to supply nitrogen (N) to tropical soils and to improve sorghum N uptake and dry matter yield (DMY). Our objectives were to determine: (i) the N mineralization rates for the different aquatic weed amendments; (ii) the correlation between N mineralization rates and sorghum N uptake; and (iii) the correlation between N mineralization rate and sorghum DMY. In an incubation greenhouse experiment, we used a Typic Kandiustalf and applied: (i) dried, (ii) fermented, and (iii) composted water hyacinth or hippo grass, (iv) mineral fertilizer, and (v) no amendment. Nitrogen application in treatments i-iv was standardized to rates equivalent to 90 kg N ha^-1. Overall, N mineralization rates were highest for compost (0.412 mg kg⁻¹ day⁻¹), intermediate for bokashi (0.265 mg kg⁻¹ day⁻¹), and lowest for dried weeds (0.211 mg kg⁻¹ day⁻¹). Across amendments, N mineralization rates were strongly correlated to sorghum N uptake (r = 0.808) and to sorghum DMY (r = 0.844). Relative agronomic effectiveness for sorghum biomass production was 163% for compost, 97% for bokashi, and 2% for dried weeds. Both aquatic weeds can supply sufficient N for sorghum when processed into compost or bokashi, with compost providing faster mineral N release.