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.
Shen et al. (Tue,) studied this question.