The objective of this study was to improve the comprehensive rate of utilization of rapeseed (Brassica napus subsp. napus L.), Myriophyllum (Myriophyllum spicatum L.) spicatum and alfalfa (Medicago sativa L.), reduce resource waste and environmental pollution. In this experiment, the effects of different proportions of the mixed silage of rapeseed and alfalfa or M. spicatum on the fermentation and nutritional quality were analyzed and further improved the quality of mixed silage using molasses and urea. Rapeseed was separately silaged with alfalfa and M. spicatum based on the ratios of 3:7, 5:5 and 7:3. After 60 days of mixed silage, the fermentation index and nutrient contents were measured to explore the appropriate ratio of mixed silage. The mixing ratio of rapeseed and alfalfa was better at 3:7: The contents of NH₃-N/TN (4.61%), lactic acid (96.46 g·kg⁻¹ dry matter [DM]) were significantly higher (p < 0.05). The crude protein content (118.20 g·kg⁻¹ DM) was the highest (p < 0.05), while the pH (4.56) was the lowest when the mixing ratio of rapeseed and M. spicatum was 7:3. Considering the fermentation and nutrition quality, it is suggested that rapeseed and alfalfa should be mixed as silage at a ratio of 3:7 with 3% molasses and 0.3% urea, and rapeseed and M. spicatum should be mixed as silage at a ratio of 7:3 with 3% molasses.

The resourceful application of biogas slurry liquid (BSL) to farmland promotes circular agriculture, yet nutrient imbalances and potential heavy metal risks restrict its safe application. This study used Chinese cabbage (Brassica rapa L. ssp. pekinensis) to evaluate the agronomic and environmental impacts of BSL applied alone or combined with chemical fertilizers. Heavy metal safety was assessed using the pollution index, geoaccumulation index, and bioaccumulation coefficient. This study demonstrates that BSL application significantly increases soil available nitrogen. The co-application of medium-concentration BSL (0.1 L) with conventional chemical fertilizer effectively alleviates nutrient deficiency, resulting in the highest cabbage yield (e.g., peak total fresh weight of 5.12 g plant⁻¹ and plant height of 20.73 cm) and quality (soluble sugar reaching 4.21–4.31%, vitamin C at 16.3–17.3 μg g⁻¹). Based strictly on this short-term evaluation, no significant anthropogenic enrichment of heavy metals was detected in the soil, and heavy metal concentrations in the edible tissues of Chinese cabbage complied with China’s national food safety standards. While experimental observation of continuous accumulation was outside the scope of this study, predictive mass-balance modeling identifies Cd as the primary limiting factor for the long-term safe use of BSL. Compared with single BSL application, co-application with chemical fertilizers significantly mitigates the projected accumulation of soil environmental hazards (extending the safe utilization period to 27.3–54.6 years), and concurrently enhances both crop yield and quality. This study supports evidence-based decision-making for the safe agronomic reuse of BSL and its ecological risk assessment.