Producing microalgae with agricultural drainage water (ADW) allows recycling water and nutrients, with the production of a biofertilizer, avoiding receiving waters' contamination. Chlorella vulgaris and Scenedesmus obliquus were cultivated using ADW and standard media supplementation and presented higher productivities, relatively to the control industrial growth medium (using freshwater). Selected strains were grown outdoors in pilot flat panel photobioreactors, reaching 2.20 g L-1 for S. obliquus and 1.15 g L-1 for C. vulgaris, and degrading herbicides in the ADW to non-quantifiable concentrations. The potential of the C. vulgaris and S. obliquus suspensions to replace 50% of nitrogen (N) mineral fertilization of lettuce (0.5 g pot-1) was evaluated through a pot trial, also using a 2-times (1.0 g pot-1) and 5-times (2.5 g pot-1) higher dose, applied 31 days before lettuce transplanting. Even the lower dose of N, applied via C. vulgaris or S. obliquus suspensions, was able to provide significantly higher lettuce fresh matter yield, relatively to the mineral fertilized control. Soil enzymatic activities were improved, with significantly higher dehydrogenase, β-glucosidase, and acid phosphatase activities for the 2.5 g pot-1 dose, more marked for S. obliquus, which was also able to increase soil organic matter content. Both the non-fertilized control and microalgae fertilized pots led to similar soil electrical conductivities, 3-fold lower than in the N-mineral fertilized pots, evidencing the capacity of microalgae fertilizers to avoid soil secondary salinization. Results suggest benefits from using ADW from maize cultivation to produce C. vulgaris or S. obliquus suspensions, that can be further used as liquid organic slow-release fertilizer.
Keywords: Herbicides; Lettuce (Lactuca sativa); Microalgae; Nutrients; Organic slow-release fertilizer; Subsurface agricultural drainage.
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