Application of nitrogen and phosphorus improve alfalfa (Medicago sativa L.) yield and nutritional quality under swine wastewater irrigation by regulating shoots osmoregulation substances

Purpose Treated wastewater irrigation has become an effective measure to alleviate the shortage of water resources. To date, there has been no information on the interaction between the application of N and P fertilizers and wastewater. This study aimed to elucidate the regulatory mechanism of N and P on the growth of leguminous plant alfalfa under wastewater irrigation. Methods A pot experiment was carried out with nine treatments of different N and P fertilizers additions during the three crop seasons. All treatments were irrigated with treated swine wastewater. And the effects of N and P addition on alfalfa yield, nutritional quality, antibiotic accumulation, and related stress physiological indicators under swine wastewater irrigation conditions were evaluated. Results The results showed that N addition could significantly increase the total fresh weight, water use efficiency and total crude protein accumulation of alfalfa shoots by 9.89%-15.70%, 5.48%-9.98% and 8.49%-18.30%, respectively. The content of four antibiotics introduced by wastewater irrigation decreased by 18.18%-116.67% after the addition of N and P compared with no fertilizer treatment, and N treatment significantly reduced the total accumulation of antibiotics in each pot by 5.55%-50.00%. More proline and soluble sugar were induced in alfalfa without adding N and P. And the addition of N and P promoted plant growth by reducing the toxicity of Na + and increasing the ratio of K + /Na + and Ca 2+ /Na + . Conclusion N significantly increased the fresh weight, water production efficiency, and crude protein content of alfalfa, and significantly reduced the accumulation of tetracycline antibiotics in alfalfa. N and P can alleviate the salt stress introduced by wastewater through the change of osmotic adjustment substances. Entropy-TOPSIS analysis showed that the high biomass, water use efficiency and quality was obtained with 120 kg N ha -1 and 80 kg P 2 O 5 ha -1 under wastewater irrigation.