Nitrogen and phosphorus release in soil and fertiliser value of lemna minor biomass relative to chicken litter compost.
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Intensive agricultural production systems produce nutrient-rich wastewaters, which may pollute the environment. High nutrient concentrations on surface water bodies encourages the growth of aquatic plants, and harvesting of these plants could improve water quality and produce an organic fertiliser. The fertiliser value of duckweed may depend on the effluent on which it grows, since it affects its nutrient composition. The aim of this study was to determine the effects of effluent types on duckweeds (lemna species) tissue composition and its influence on (i) mineral nitrogen (N) and phosphorus (P) release in the soil during an incubation and (ii) nutrient uptake and dry-matter yield of spinach (Spinacia oleracea) under greenhouse and field conditions, relative to chicken litter. Duckweeds were sampled from surface water at Ashburton (Lemna AB) enriched with effluent from sewage and cattle manure, at Baynesfield (Lemna BF) on effluent from a piggery, at Wartburg (Lemna WB) enriched with crocodile wastewater (crocodile farm) and chicken litter compost from RGS Drumnadrochit farm, all in the Midland region of Kwa-Zulu Natal. A loam soil was amended with dried Lemna AB, Lemna BF, Lemna WB or chicken litter at a rate of one percent (w/w) in one incubation, while in another incubation 2 and 4% rates were used with an un-amended soil as a control in both. The treatments were adjusted to field capacity moisture and incubated at 25oC for 28 days. Destructive sampling was done after 0, 7, 14, and 28 days of incubation, and analysed for pH, ammonium and nitrate-N, and extractable-P. A leaching experiment was also conducted were 2.0 g of dry matter Lemna AB, Lemna BF and Lemna WB were leached over a period of 0, 6, 12 and 24 hours on sandy soil using deionize water (25 ml). The leachates were analysed for ammonium and nitrate-N and extractable-P. A pot trial in the glasshouse was conducted with the same treatments used in the incubation experiment, at recommended rate of 100 kg N ha-1 replicated three times. Spinach (Spinacia oleracea) seedling were grown for six (6) weeks, and harvested before the determination of dry matter (DM), tissue composition and nutrient uptake. The experiment was repeated under field conditions with Lemna BF, Lemna WB, chicken litter compost, all applied at 100 kg N ha-1, and a negative control (no added N), after a two-week pre-incubation. In the first incubation ammonium-N was higher in the Lemna WB treatments, while nitrate-N was highest on the Lemna AB treatments with the highest peak observed on day 14. In the case of Lemna BF treatments had the highest amount of extractable-P, with the control having less of all determined parameters. Lemna WB rapidly leached higher nutrients (ammonium-N and exchangeable-P) at about 26.47 mg N/kg and 25.59 mg P/kg respectively, while Lemna AB (69.42 mg/kg) was high in nitrate-N within 24 hours in comparison to the other treatments on the leaching experiment. In the second incubation Lemna BF showed higher amounts of ammonium-N (230 mg/kg), nitrate-N (140.83 mg/kg) and extractable-P (10.66 mg/kg), throughout the incubation period than all other treatments, while the control had the least of all determined parameters. Ammonium-N was highest after 7 days of the incubation and declined thereafter while nitrate-N increased. Soil pH was highest in the chicken litter compost treatment, and it declined with incubation period. Spinach dry matter was similar for all duckweed treatments, while the negative control had lower levels in the pot experiment. However, under field conditions the Lemna BF treatment (74.2 g/plant) had higher spinach dry matter (DM) yield than all the other treatments which were similar in DM. The results suggested that duckweed N mineralises rapidly in soil and also has a significant value on spinach yield, both depending on the initial elemental composition of duckweed, which is affected by the effluent on which it grew.