Effects of Nitrogen Fertilizer Application on Sugar Beet Yield and Sugar Content: A Meta-Analysis

Sugar beet is widely cultivated and highly valued in regions with temperate climates. Nitrogen is an essential nutrient for crop growth. It has been generally accepted that nitrogen fertilization increases sugar beet yield and improves quality. However, a systematic analysis investigating the effects of nitrogen fertilization on sugar beet yield and sugar content is currently lacking. In this study, we analyzed 814 data pairs from 42 studies to make a global meta-analysis of nitrogen fertilization on yield and sugar content. Under most climatic conditions, nitrogen application showed a positive effect on root yield, with the maximum effect size when mean annual average precipitation (MAP) ≤ 300 mm, means mean annual air temperature (MAT) > 10 °C, and means mean annual sunshine (MAS) > 2500 h. While nitrogen application basically showed a negative effect on sugar content, especially when MAP > 300 mm, MAT > 10 °C, and MAS > 2500 h. Similar trend was found in terms of soil conditions, with an exception in clay soil. Nitrogen application had a positive and significant effect on root yield under all the management practices, with the maximum effect size when nitrogen application frequency ≥ 5, 150 kg ha⁻¹ ≤ nitrogen application rate ≤ 300 kg ha⁻¹, and 1 × 10⁵ plants·ha⁻¹ ≤ plant density ≤ 1 × 10⁵ plants·ha⁻¹. The effect of nitrogen application on sugar content was insignificant and more complicated. There was an insignificant linear and negative relationship between root yield and sugar content effect. It can be concluded that nitrogen application led to a positive and significant response on root yield, yet a negative and insignificant response on sugar content. And the optimal nitrogen application rate should be determined by local climatic factors, soil conditions, and management practices. This meta-analysis improved our understanding of crucial factors influencing the effects of nitrogen fertilization on root yield and sugar content and provided guidance for optimizing nitrogen fertilizer application.