(1) Background: The escalating issue of soil degradation caused by excessive chemical fertilizer application poses significant threats to the sustainable development of Chinese flowering cabbage (Brassica campestris L. ssp. chinensis (L.) var. utilis Tsen et Lee) production. This research aimed to identify the impacts of reduced chemical fertilizer application integrated with organic amendments on cabbage yield and rhizosphere soil microenvironment characteristics. (2) Methods: A biennial field experiment was conducted during the 2022–2023 growing seasons at Lijun Town, Yinchuan City, Ningxia Hui Autonomous Region. Five treatments were tested: (i) Control (CK, no fertilizer); (ii) Conventional chemical fertilization (CF1, chemical fertilizer only); (iii) Reduced chemical fertilization (CF2, 30% less chemical fertilizer); (iv) CF2 + Well-decomposed chicken manure (FCM, 30% less chemical fertilizer + rotted chicken manure); and (v) CF2 + Vermicompost (FEM, 30% less chemical fertilizer + vermicompost). (3) Results: In 2023, the FCM treatment reduced electrical conductivity (EC) by 24.80% and pH by 2.16%, while the FEM treatment decreased EC by 31.13% and pH by 3.84% compared to controls. The FEM treatment significantly enhanced total nitrogen content by 12.71% and 8.85% relative to CF1 and FCM treatments, respectively. Compared to CF1, FEM increased soil organic matter content by 10.49% in 2022 and 11.24% in 2023. Organic fertilizer amendments elevated available nitrogen, phosphorus, and potassium levels while enhancing sucrase activity: FCM and FEM treatments increased sucrase activity by 23.62% and 32.00%, respectively, in 2022. Organic fertilization improved bacterial diversity and richness, optimized microbial community structure, and increased the relative abundance of Bacillus. It also upregulated microbial metabolic pathways related to carbohydrate and amino acid metabolism. Soil nutrients and bacterial community structure showed positive correlations with yield, whereas soil enzyme activities exhibited negative correlations. Key factors influencing yield were identified as Proteobacteria, Chloroflexi, available potassium, organic matter, available nitrogen, Actinobacteria, Firmicutes, total nitrogen, pH, and sucrase activity. (4) Conclusions: Integrated analysis of yield and soil microenvironmental parameters demonstrates that the fertilization regimen combining 30% chemical fertilizer reduction with vermicompost amendment (FEM) constitutes a more efficient fertilization strategy for Chinese flowering cabbage, making it suitable for regional promotion in the Ningxia area.

Aquaponics is a resource-efficient agricultural system, yet its overall productivity is frequently constrained by micro- and macronutrient deficiencies, particularly iron (Fe) and potassium (K). Currently, the efficacy of combined dietary Fe and K supplementation in optimizing nutrient management in these systems remains unclear. Therefore, a 60-day feeding trial was conducted to evaluate the effects of four dietary Fe and K levels—CK (basal diet without added Fe or K), T1 (Fe 0.1 g/kg + K 2.5 g/kg), T2 (Fe 0.2 g/kg + K 5.0 g/kg), and T3 (Fe 0.3 g/kg + K 7.5 g/kg)—on the growth and quality of lettuce (Lactuca sativa) and the physiological responses of crucian carp (Carassius auratus). The results demonstrated that the T2 treatment was suitable for enhancing system productivity. Compared with the CK group, the lettuce plant height, biomass, and net photosynthetic rate in the T2 group exhibited marked increases of 25.3%, 16.0%, and 26.4%, respectively. Furthermore, the vitamin C content increased by 52.2%, while the nitrate content notably declined by 32.2%. Plant nutrient analysis revealed that the combined Fe and K supplementation markedly promoted the foliar uptake of P, Mn, and Zn (peaking in the T2 group), whereas Cu and S contents increased linearly with the supplementation dose, reaching its maximum in the T3 group. Regarding fish health, the weight gain rate (WGR) of crucian carp in the T2 group peaked at 41.0%, and the feed conversion ratio (FCR) improved to 1.76. Additionally, the dietary supplementation maintained the stability of water quality parameters of the recirculating system. These findings indicate that a combined dietary inclusion of 0.2 g/kg Fe and 5.0 g/kg K can simultaneously enhance the yield and quality of both plants and fish. This approach provides a novel optimization strategy for mitigating acute water quality shocks, while also highlighting the inherent boundaries of competitive ion antagonism in aquaponic nutrient management.