Improving nitrogen management and mitigating pollution in semi-arid areas of Western Inner Mongolia, China: Insights from sugar beet cultivation

Optimizing nitrogen (N) management mitigates fertilization pollution in arid and semi-arid areas. This study used sugar beet in Western Inner Mongolia as a case, integrating ¹³C/¹⁵N tracing, metabolomics, and enzyme analysis to quantify carbon-nitrogen partitioning, yield, and fertilizer fate under different N regimes. In this region, the optimized N rate (180 kg ha⁻¹, N180) significantly enhanced nitrate reductase activity during early to middle growth stages, and synergistically improved ¹⁵N partitioning to taproots and late-phase ¹³C assimilation. This synergy achieved the highest ¹⁵N recovery (40.73 %) during root and sugar accumulation and a maximal sugar yield of 14.55 Mg ha⁻¹, exceeding N60 (60 kg ha⁻¹) and N300 (300 kg ha⁻¹) yields by 20.9 % and 12.7 %, respectively. Critically, under N300, shoot δ¹⁵N decreased least during root and sugar accumulation, with higher ¹⁵N accumulation and a 53.0 % greater partial N balance than N180, indicating severe N remobilization impairment and more residual N prone to environmental losses. Under N60, leaves maintained energy via glycolysis-pentose phosphate pathway-tricarboxylic acid cycle, while taproots adopted low-energy state via nucleotide salvage, a potential N-limited adaptation for this crop. This study demonstrates that 180 kg ha⁻¹ precision N management is effective for sugar beet production in Western Inner Mongolia, China, synchronizing high productivity and fertilizer-N recovery to reduce soil N residue and non-point source pollution. While nitrogen management practices have crop-specific and region-specific limitations, the integrated method identifies optimal regional N strategies by dissecting crop physiology, yield, and N use efficiency, providing insights for precision N management in other crops.