Nitrogen balance and potential reduction of nitrogen fertilizer input in Beijing-Tianjin-Hebei crop-livestock system based on Nufer model

Objectives  This research aims to examine the nitrogen (N) flow and balance characteristics of crop-livestock system in Beijing-Tianjin-Hebei (BTH) region, and analyze the potential reduction of chemical N fertilizer application. This study could provide a theoretical basis for zero growth of chemical fertilizer inputs and the sustainable agricultural development in the region.

Methods  Using literature data and statistical data of 2017, the N flow characteristics and N-use efficiency (NUE) of crop-livestock system in the BTH region was quantified through Nufer (nutrient flows in food chain, environment and resources use) model. The reduction potential of chemical N fertilizer use in this region was evaluated by scenario analysis via two measures of N balance and manure substitution.

Results  The N input in crop and livestock systems of BTH region were 2961 and 1336 thousand tons, respectively. The total N input of the crop-livestock system in BTH region was 3066 thousand tons and chemical N fertilizer was the highest contributor (62.5%). The NUE in crop and livestock systems were 40.6% and 25.0%, respectively, while the overall NUE of the crop-livestock system was 33.8%. The N recycling rate (NRR) of crop-livestock system was 55.2%, while cost of N for producing food containing a unit N (NCf) was 3.0 g/kg. The soil N accumulation was 51.2 × 10⁴ t (16.7% of the total N input), and the total N emission was 140.4 × 10⁴ t (45.8% of the total N input). The two main pathways of the total N emission included ammonia volatilization in field and livestock manure, and the N loss via water and livestock manure stacking, and accounted for 21.0% and 9.5% of the total N input, respectively. These two pathways were mostly managed for N emission reduction. By balancing N input and output to decrease N accumulation in farmland, there would be a 40% potential reduction, total N input in crop-livestock system would be decreased to 230.0 × 10⁴ t, and NUEs of crop and crop-livestock systems would increase by 14.1 and 11.3 percentage points, respectively. By further optimizing N management via increase in the N input from livestock manure, which reduced N loss via water and manure stacking at the same time, the potential reduction could be as high as 50%. Thus, the total N input of crop-livestock system would be 31.2% lower than the current level; the NUEs of crop and crop-livestock systems would increase by 15.3% and 15.4%, respectively; the total N emission would decrease by 36.2%; the N recycling rate would increase by 20.0%, and the NCf would decrease by 33.3%.

Conclusions  The heavy application of chemical N fertilizers and the separation of crop and livestock systems are the main causes of the relatively low NUE and high N emission in Beijing-Tianjin-Hebei region. By balancing the N input and output in crop system to reduce soil N accumulation, the potential reduction of N input in crop-livestock system would be 40%. Further optimization of N management by increasing the rate of returning livestock manure to the field would raise the potential reduction of N input to 50%, increase NUE, and decrease total N emission in the whole region.