Conventional fertilizer management can destroy the structure of soil. Replacing chemical fertilizers with organic fertilizers can improve soil quality and nitrogen use efficiency. We aimed to study the effects of organic fertilizer substitutions for chemical nitrogen fertilizer on soil fertility and nitrogen use efficiency in order to clarify the effectiveness of the available nutrient management measures in improving soil quality and increasing foxtail millet yield. A field experiment was carried out over two consecutive years, and a total of six treatments were set up: no fertilizer (CK), chemical nitrogen fertilizer alone (N), the substitution of 25% of chemical nitrogen fertilizer with bio-organic fertilizer (N25A1), the substitution of 25% of chemical nitrogen fertilizer with fermented mealworm manure (N25B1), the substitution of 50% of chemical nitrogen fertilizer with bio-organic fertilizer (N50A2), and the substitution of 50% of chemical nitrogen fertilizer with fermented mealworm manure (N50B2). The results of this study show the following: (1) Compared with chemical nitrogen fertilizer, the substitution of organic fertilizer for nitrogen fertilizer reduced the bulk density and solid phase of the soil, and it increased the total porosity, water content, liquid phase, and gas phase of the soil. (2) Compared with nitrogen fertilizer, the use of an organic fertilizer increased the contents of nitrate nitrogen, ammonium nitrogen, and total nitrogen in the soil by 13.59~52.56%, 4.47~18.27%, and 4.40~12.09%, respectively. The content of alkaline nitrogen increased by 1.70~32.37%, and the contents of soil available potassium, available phosphorus, and organic matter also increased. (3) The activities of sucrase, urease, glutaminase, and asparaginase were improved by replacing chemical nitrogen fertilizer with organic fertilizer. The N25 treatments performed better than the N50 treatments, and fermented mealworm manure performed better than biological organic fertilizer. (4) A moderate application of organic fertilizer (N25) can increase the grain yield, ear weight, grain weight, and 1000-grain weight of foxtail millet, whereas excessive application of organic fertilizer (N50) can reduce foxtail millet yield. (5) Replacing chemical nitrogen fertilizer with organic fertilizer can improve the agronomic use efficiency, physiological efficiency, biased productivity, harvest index, and apparent use efficiency of nitrogen fertilizer. In this study, the substitution of 25% of chemical nitrogen fertilizer with fermented mealworm manure was the best combination for restoring crop productivity and soil quality.

The effects of balanced fertilization with nitrogen, phosphorus, and potassium (NPK) on foxtail millet productivity and the soil environment under the same conditions of total nutrients have received limited research attention. Therefore, in this study, three balanced fertilization patterns of 27-14-10 (T1), 27-17-7 (T2), and 30-10-11 (T3), and one no fertilization treatment (CK), a total of four treatments, were set up through a two-year field experiment to study the effects of balanced fertilization patterns on foxtail millet yield and soil environment. Mantel analysis was conducted to reveal the correlation between soil environmental factors and the community and their contribution to productivity. The results showed that: (1) all balanced fertilization treatments significantly increased foxtail millet yield, with the highest yield in the T1 treatment. (2) The contents of EC, available K, available P, and alkaline-hydrolyzable nitrogen in the soil of the two-year TI treatments were higher than those of the other treatments and increased by 7.20–9.36%, 24.87–52.35%, 55.83–56.38%, and 21.05–43.95%, respectively, compared with CK. (3) Soil urease activity in the T1 treatment increased significantly by 26.67% and 9.00% compared with the control over the two years. Sucrase activity increased by 36.27% and 23.88% in the T1 treatment compared to CK, and glutaminase activity increased by 33.33% and 19.23% in the T1 treatment compared to CK. (4) T1 treatment significantly increased the OUT number and diversity index of the soil bacterial community. (5) Mantel analysis and principal component analysis showed that available soil nutrients and soil enzymes were positively correlated, and soil enzymes and soil nutrients contributed more to foxtail millet productivity. In this study, the 27-14-10 balanced fertilization pattern was more effective, providing a theoretical basis for the research and development of special fertilizers for foxtail millet and offering technical guidance for realizing the light simplified cultivation of foxtail millet and sustainable development of cost–saving and increased efficiency.

The effect of nitrogen and zinc treatment on quality and yield was studied to provide a theoretical basis for generating high-quality and high-yielding foxtail millet. In the years 2021–2022, ‘Jingu 21’ was utilized as the test material in a split-plot design. The main plot was subjected to nitrogen treatment, while the sub-plot was treated with zinc. An arrangement of random blocks was employed. Four levels of nitrogen application (0 kg/hm², 120 kg/hm², 180 kg/hm², 240 kg/hm²) and three levels of zinc application (20 mg/L, 40 mg/L, 80 mg/L foliar spraying) were set, resulting in a total of twelve treatments. Each treatment was replicated three times, with each plot covering an area of 15 m². (1) The findings indicated that the contents of crude fat and crude protein in foxtail millet increased initially and then decreased with the increase in nitrogen application concentration. Additionally, the protein components were found to be in the following order: gliadin > albumin > glutenin > globulin. (2) Adding an appropriate amount of nitrogen fertilizer under the same zinc level promoted the contents of amylose, total carotenoids, and flavonoids in millet to some extent. Over the course of two years, the content of flavonoids in millet was highest when treated with N180Zn40 (nitrogen 180 kg/hm², zinc 40 mg/L), showing an increase of 50.14% and 58.67%, respectively, compared to the treatment of applying zinc fertilizer alone at the same zinc level. (3) The phytic acid content and phytic acid/zinc molar ratio exhibited a pattern of initially decreasing and then increasing with the rise in nitrogen and zinc concentrations. (4) The application of zinc fertilizer and increased nitrogen fertilizer for two consecutive years had a significant impact on the yield of millet. Additionally, the application of zinc fertilizer had a highly significant effect on both the ear weights and thousand-kernel weights of millet (p < 0.001). The results demonstrated a positive synergistic effect when nitrogen fertilizer and zinc fertilizer were applied together. This combination significantly improved millet yield and thousand-kernel weights, enhanced the nutritional quality, and increased the content of functional components. Additionally, it also improved the availability of zinc.