Exploring the potential of invasive aquatic weeds to supply N to tropical soils

Water hyacinth and hippo grass are aquatic weeds that negatively impact freshwater bodies in (sub) tropical regions. Using these weeds as soil amendments can help reduce their spread and enhance soil fertility. Materials used as soil amendments must contain high nutrient levels and low contaminants. Understanding how water and sediment properties influence the chemical makeup of these weeds is crucial for selecting weeds with high nutrient and low heavy metal levels. In a field survey study, we examined the variation in chemical parameters of these weeds across different rivers and explored the relationship between their chemical makeup and that of water and sediments in those rivers. Our results indicated that the levels of Phosphorus (P) and nitrogen (N) in water hyacinth were positively correlated with their concentrations in water. In hippo grass, N and P levels showed strong, positive correlations with sediment concentrations but only weak correlations with water concentrations. The results also indicated that the weeds found in water bodies passing through populated residential areas contained significantly higher levels of N and P. In contrast, weeds in waterbodies flowing through industrial and mining regions showed higher concentrations of heavy metals. After the field survey, we set up an incubation and greenhouse experiment to investigate the effect of organic amendments made from these weeds on N mineralization of tropical soils. We applied: (i) dried, (ii) fermented, and (iii) composted water hyacinth or hippo grass, (iv) mineral fertilizer, and (v) no addition, and investigated their effect on sorghum. We standardized N application in treatments i-iv to an equivalent of 90 kg N/ha. Overall, k values were highest for compost (0.0116 day-1), intermediate for bokashi (0.0088 day-1), and lowest for dried weeds (0.0075 day-1). For compost and bokashi, k values were comparable between water hyacinth and hippo grass, but slower for dried hippo grass (k = 0.0054 day-1) than water hyacinth (k = 0.0096 day-1). Across different amendments, k related strongly to sorghum N uptake (r = 0.854) and sorghum DMY (r = 0.854). We later set up a field experiment to investigate the effect of aquatic weed amendments on the yield and grain quality of sorghum. The results indicated that the sorghum grain yield for treatments with bokashi and compost was comparable to that of mineral fertilizer, improving yield by 21% and 12%, respectively, while the dried weeds decreased yield by 6% relative to the unfertilized control. Sorghum grain N uptake was comparable for aquatic weed compost (65.9 kg/ha) and bokashi (66.4 kg/ha), and lower for dried weeds (48.6 kg/ha), mineral fertilizer (43.1 kg/ha), and the control (35.2 kg/ha). Crude protein content in sorghum grains was highest in sorghum grown in soil treated with compost (17.6%), followed by bokashi (16.4%), dried weeds (15.4%), mineral fertilizer (12.2%), and the control (10.5%). Relative agronomic efficiency for sorghum grain N uptake was significantly higher for the organic amendments compared to the mineral fertilizer (RAE 100 %); it was comparable for bokashi (391%) and compost (385%), but significantly lower for dried weeds (169%).