Biofertilizers from wastewater treatment as a potential source of mineral nutrients for growth of amaranth plants

Exploring alternative fertilizers is crucial in agriculture due to the cost and environmental impact of inorganic options. This study investigated the potential of biofertilizers derived from sewage treatment on the growth and physiology of Amaranthus cruentus plants. Various treatments were compared, including control treatments with inorganic fertilizer and treatments with biofertilizers composed of microalgae, biosolids and reclaimed water. The following traits were investigated: photosynthetic pigments, gas exchange, growth, and leaf nutrient concentrations. Results show that the concentrations of N, P, Cu, Fe Zn and Na nutrients, in the dry microalgae and biosolids, were quite high for the needs of the plants. The wet microalgae presented high concentration of Cu, Fe and Zn nutrients while reclaimed water contained high concentration of N, K, Ca and S. Na and Zn nutrients increased in the leaf of plants treated with dry microalgae and biosolid, respectively. At the beginning of the flowering phase, total chlorophyll and carotenoids contents were lower for plants grown with wet microalgae while for plants grown with higher doses of biosolid or reclaimed water total chlorophyll was increased, and carotenoids were not affected. Lower photosynthetic pigments under wet microalgae resulted in lower photosynthetic rates. On the other hand, amendments with dry microalgae and biosolid increased photosynthetic rates with the biosolid being the most effective. Higher applications of biosolid, wet and dry microalgae produced a considerable increase in shoot biomass of amaranth, with the dry microalgae being the most effective. Additionally, reclaimed water obtained after tertiary treatment of sewage with microalgae and biosolids applied alone showed promising effects on plant growth. Overall, these findings suggest that organic fertilizers derived from sewage treatment have the potential to enhance plant growth and contribute to sustainable agricultural practices.

Methods
The experiment was carried out in the greenhouse at the Department of Biological Sciences at the Sao Paulo State University/Bauru Campus, under natural photoperiodic conditions and minimum and maximum average temperature of 17 and 33 oC, respectively. The seeds were sown in 4 L plastic pots filled with vermiculite. The microalgae were cultivated in a transparent box with 50 L of anaerobically pre-treated sewage with 5 L of inoculum of native microalgae from sanitary wastewater. After the microalgae cultivation the reclaimed water and the sedimented microalgae were collected. The sludge was obtained in the third chamber of the baffled anaerobic reactor, which had three chambers with a total volume of 597 L and HDT of 8 h.  After collecting, the sludge was oven dried at 60 oC. Then, it was crushed and ground in a mill to facilitate the  absorption by the plants. At approximately 7 day intervals, the height (cm), the diameter (mm) of the stem at insertion of cotyledons and the number of visible leaves were measured. For gas exchange measurements were used an infrared gas analyzer (LCpro Portable Photosynthesis System, ADC Bioscientific Ltd., Hoddesdon, UK). Total chlorophyll and carotenoids were  
expressed on a leaf area basis (g m-2). At the end of the experimental period, plants of each treatment were selected randomly for dry matter determinations.  Chemical analysis of microalge, reclaimed water, biosolid and amaranth leaves were made.

The data were submitted to simple analysis of variance (ANOVA) by using the software SPSS/PC 9.0 for Windows. Statistical analysis was applied to each group of plants separately. Quantitative changes in the different variables were analyzed using a test
of multiple comparison to determine differences between treatments at 5% significance level.