Decomposition of Wolffia arrhiza residues rapidly increases mineral nitrogen and decreases extractable phosphorus in acidic soils.

While nutrient loads from anthropogenic sources upset aquatic ecosystem balance, Wolffia arrhiza (duckweed) has capacity to purge nutrient-rich water if continuously harvested. The nutrients accumulated in biomass have potential as soil fertility amendments. The objective of this study was to determine changes in release of nitrogen (N) and phosphorus (P), and the fate of P in soils after duckweed biomass amendment. An incubation experiment was conducted at 25 °C using three soils amended with proportions equivalent to 501, 1002 and 1503 mg N kg and 62, 124 and 186 mg P kg. Soil samples were collected on 0, 3, 7, 14, 21, 28, 42 and 56 days, for ammonium-N, nitrate-N and extractable-P measurements. At the end of incubation, P pools were determined. At least 25 mg kg of ammonium-N was released on day 0, reaching a peak within the first 2 weeks. Nitrate- and mineral-N increased from 14 to 42 days, with a corresponding decrease in ammonium-N. Relatively fertile soil released more mineral-N at higher applied ratios of duckweed than the less fertile. About 10-80 mg kg of duckweed P was extractable on day 0 and amounts progressively declined over the incubation period. The combined percentage (0.5%) of tissue aluminium (Al) and iron (Fe) facilitated Al and Fe phosphate accumulation as the proportion of duckweed amendment increased. The results suggested that soil type and elemental composition of duckweed are important determinants for N and P release, and liming could improve P availability in soil.