Objectives Focusing on the nitrogen (N) and phosphorus (P) nutrient cycles, this study comprehensively evaluated the current status of the crop and livestock industries, the land carrying capacity for livestock and poultry manure, the N and P loads from livestock and poultry manure on cropland, and the associated environmental risks in Zhouqu County. The study aimed to explore pathways for optimizing the crop-livestock balance in this ecologically fragile plateau area, thereby providing a theoretical basis for the rational utilization of livestock and poultry manure resources, optimization of the spatial layout of crop and livestock production, and the development of integrated crop-livestock systems and ecological circular agriculture.

Methods Based on statistical data and field surveys, the quantity and spatial distribution characteristics of livestock and poultry manure resources in Zhouqu County were determined, and the nutrient requirements of major crops were estimated. Nutrient demand of crops and nutrient supply from manure were analyzed under different manure nutrient substitution ratios. Different livestock and poultry species were converted into pig equivalents (PE), and the land carrying capacity index (I), based on N and P₂O₅ were calculated for each township. Furthermore, the environmental loading and pollution risk levels under the existing scale of livestock and poultry production were evaluated.

Results 1) Livestock and poultry manure resources in Zhouqu County were abundant but unevenly distributed. The amounts of available N and P₂O₅ supplied by manure were 713.2 t and 236.8 t, respectively, accounting for 79.3% and 62.6% of the total nutrient requirements of major crops. Therefore, manure alone could not fully replace chemical fertilizers, and supplemental fertilizers should be applied precisely according to crop nutrient requirements. 2) In 2023, the actual livestock and poultry production in Zhouqu County was equivalent to 1.6×10⁵ pigs. With N as the limiting nutrient and a manure nutrient substitution ratio of 60%, the maximum carrying capacity was 1.4×10⁵ PE, and the land carrying capacity index (I) was approximately 1.1, indicating slight overloading. With P₂O₅ as the limiting nutrient and a manure nutrient substitution ratio of 50%, the maximum carrying capacity was 1.7×10⁵ PE, with a land carrying capacity index (I) of 1.0, indicating a balance between crop production and livestock and poultry production. 3) In terms of the environmental risk associated with manure application to cropland, the overall risk in Zhouqu County was controllable, although locally high-risk areas existed. The average manure loading on cropland was 22.1 t/hm², while the average cropland loadings of total nitrogen (TN) and total phosphorus (TP) were 54.3 kg/hm² and 8.3 kg/hm², respectively. The cropland risk alarm value was 0.7, corresponding to Grade Ⅲ, indicating a potential threat of environmental pollution.

Conclusions Under the current scale of livestock and poultry production in Zhouqu County, the overall environmental risk associated with manure application to cropland was controllable, although some areas were at high risk. Nevertheless, livestock and poultry manure alone could not fully meet the nutrient requirements of crop production. Based on N balance, the manure nutrient substitution ratio could reach 60%, but this would entail a slight risk of overloading. Considering the ecological fragility of the plateau ecosystem, it is recommended that P balance be adopted as the benchmark (I=1.0) and that the manure nutrient substitution ratio be controlled at 50% to maintain environmental safety and crop–livestock balance.