Turning Livestock Waste into Nutrient Resources: Advances in Crop–Livestock Circular Agriculture for Sustainable Farming Systems

The increasing specialization of crop and livestock production has disrupted traditional nutrient cycling in agricultural systems, creating a dual challenge of excessive dependence on synthetic fertilizers and inefficient utilization of livestock manure. Crop–livestock circular agriculture offers a promising pathway to address these interconnected problems by recoupling animal production, manure management, and crop cultivation within an integrated nutrient-recycling framework. This review critically synthesizes recent advances in crop–livestock circular agriculture, with particular emphasis on manure valorization, nutrient recovery and reuse, enabling treatment technologies, crop–livestock nutrient matching, and regionally adapted implementation models. Current evidence demonstrates that appropriately managed manure recycling can partially substitute synthetic fertilizers, improve soil fertility and structure, enhance nutrient-use efficiency, and reduce nutrient losses and associated environmental pressures. Technologies including solid–liquid separation, aerobic composting, anaerobic digestion, and emerging resource-recovery approaches further expand the potential for converting livestock waste into fertilizers, energy, and other value-added agricultural inputs. However, the environmental and agronomic benefits of these systems depend strongly on balancing manure-derived nutrient supply with crop demand and local land carrying capacity. This requirement is particularly important in the hilly agricultural regions of Southwest China, where fragmented farmland, dispersed livestock production, complex terrain, and high transportation costs constrain the direct adoption of large-scale centralized models. Locally adapted strategies integrating decentralized manure treatment, nearby land application, and coordinated regional nutrient allocation may therefore provide more practical solutions. Despite substantial progress, broader implementation remains limited by spatial mismatches between manure production and cropland demand, insufficient technological adaptation, economic constraints, and a lack of long-term system-level assessments. Future research should move beyond individual waste-treatment technologies toward integrated crop–livestock management that combines nutrient budgeting, precision manure application, resource recovery, digital decision support, and region-specific governance. Such advances are essential for transforming livestock manure from an environmental liability into a strategic nutrient resource and for accelerating the transition toward resource-efficient, low-impact, and resilient agricultural systems.