Enhancing Phosphorus and Nitrogen Uptake in Maize Crops with Food Industry Biosolids and <i>Azotobacter nigricans</i>

The problem of phosphorus and nitrogen deficiency in agricultural soils has been solved by adding chemical fertilizers. However, their excessive use and their accumulation have only contributed to environmental contamination. Given the high content of nutrients in biosolids collected from a food industry waste treatment plant, their use as fertilizers was investigated in <i>Zea mays</i> plants grown in sandy loam soil collected from a semi-desert area. These biosolids contained insoluble phosphorus sources; therefore, given the ability of <i>Azotobacter nigricans</i> to solubilize phosphates, this strain was incorporated into the study. In vitro, the suitable conditions for the growth of <i>Z. mays</i> plants were determined by using biosolids as a fertilizer and <i>A. nigricans</i> as a plant-growth-promoting microorganism; in vitro, the ability of <i>A. nigricans</i> to solubilize phosphates, fix nitrogen, and produce indole acetic acid, a phytohormone that promotes root formation, was also evaluated. At the greenhouse stage, the <i>Z. mays</i> plants fertilized with biosolids at concentrations of 15 and 20% (<i>v</i>/<i>w</i>) and inoculated with <i>A. nigricans</i> favored the development of bending strength plants, which was observed on the increased stem diameter (>13.5% compared with the negative control and >7.4% compared with the positive control), as well as a better absorption of phosphorus and nitrogen, the concentration of which increased up to 62.8% when compared with that in the control treatments. The interactions between plants and <i>A. nigricans</i> were observed via scanning electron microscopy. The application of biosolids and <i>A. nigricans</i> in <i>Z. mays</i> plants grown in greenhouses presented better development than when <i>Z. mays</i> plants were treated with a chemical fertilizer. The enhanced plant growth was attributed to the increase in root surface area.