Long-term excessive nitrogen application decreases spring maize nitrogen use efficiency via suppressing root physiological characteristics

Long-term excessive nitrogen (N) application neither increases nor enhances grain yield and N use efficiency (NUE) of maize, yet the mechanisms involving root morphological and physiological characteristics remain unclear. This study aimed to elucidate the mechanisms underlying stagnant grain yield under excessive N application by examining root morphological and physiological characteristics. A 10-year N fertilizer trial was conducted in Jilin Province, Northeast China, cultivating maize at three N fertilizer levels (zero N, N0; recommended N, N2; and high N level, N4) from 2019 to 2021. Two widely cultivated maize genotypes, ‘Xianyu 335’ (XY335) and ‘Zhengdan 958’ (ZD958), were evaluated. Grain yield, N content, root morphology, and physiological characteristics were analyzed to assess the relationships between N uptake, N utilization, plant growth, and root systems under different N treatments. Compared to N0, root biomass, post-silking N uptake, and grain yield improved significantly with increased N input, while no significant differences emerged between recommended N and high N. High N application enhanced root length and root surface area but decreased root activity (measured by TTC (2,3,5-triphenyltetrazolium chloride) method), nitrate reductase activity, and root activity absorbing area across genotypes. Root length and root to shoot ratio negatively affected N uptake (by –1.2 and –24.6%), while root surface area, root activity, nitrate reductase activity, and root activity absorbing area contributed positively. The interaction between cultivar and N application significantly influenced NUE. XY335 achieved the highest NUE (11.6%) and N recovery efficiency (18.4%) through superior root surface area (23.6%), root activity (12.5%), nitrate reductase activity (8.3%), and root activity absorbing area (6.9%) compared to other treatments. Recommended N application enhanced Post N uptake, NUE, and grain yield through improved root characteristics, while high N application failed to increase or decreased NUE by reducing these parameters. This study demonstrates that root surface area, root activity, nitrate reductase activity, and root activity absorbing area limit NUE increase under high N application.