Optimizing nitrogen fertilizer application for achieving high yield with low environmental risks in apple orchard

The great challenges of food security, climate change and environmental degradation resulting from unreasonable nitrogen fertilizer (NF) inputs necessitate the optimization of fertilizer application to ensure fruit production and environmental sustainability. However, there is a lack of systematic analysis on the effects of NF inputs on apple yield and reactive nitrogen losses (N2O emission, NH3 volatilization, and nitrate leaching) at the global scale. Therefore, we used a meta-analysis of 159 observations from 31 published studies to evaluate the response of apple yield and reactive nitrogen losses to NF. We aim to identify suitable NF rates, environmental conditions, and planting factors that improve apple yield while minimizing reactive nitrogen losses. Our results showed that NF significantly increased apple yield, N2O emission, NH3 volatilization, and nitrate leaching by 17.1%, 255.7%, 236.4%, and 68.7% compared with no nitrogen fertilizer (NNF), respectively. The effects of NF rates and environmental factors on the response of N2O emission, NH3 volatilization, and nitrate leaching to NF were prior to those of planting factors, but the result of apple yield was the opposite. Apple production under NF in regions with MAT (mean annual air temperature) ≥ 10 °C and MAP (mean annual precipitation) < 600 mm declined the risk of reactive nitrogen losses and significantly increased apple yield by 24.4%−28%. NF application was more beneficial in soil with pH ≥ 7 and SOM < 15 g kg−1 for apple production. We found that the suitable NF rate was 250–450 kg ha−1, which can increase apple yield by 0.8%−21.3% while significantly reducing reactive nitrogen losses compared with lower NF rate (< 250 kg ha−1) and higher NF rate (> 450 kg ha−1). Our findings provide guidance for optimizing NF management in apple orchard to achieve high crop yields, reduce emissions, and mitigate pollution.