Rotational Tillage and Nitrogen Rate Affect Maize Yield Through Regulations on Deep Root Morphology and Physiology

In the maize systems of Liaonan, China, soil compaction and inefficient nitrogen use are key constraints to sustainable productivity. To enhance nitrogen (N) use efficiency and sustainable productivity in the maize systems of Liaonan, China, a field split-plot trial was conducted from 2018 to 2022 to investigate the synergistic effects of rotational tillage and N rates on root physiology and yield. Three straw return practices were tested as follows: NT (1 year no-tillage + 1 year subsoiling), PT (continuous subsoiling), and RT (continuous rotary tillage), each under three nitrogen levels: 150 (N150), 210 (N210), and 240 kg ha−1 (N240). Root length density (RLD) and root surface area density (RSD) were monitored in situ, while root protein content, cellulose/lignin composition, root activity, and photosynthesis were analyzed at the tasseling (VT) and milk stage (R3). The results showed that NT-N210 treatment maximized deep root (30–50 cm) growth, increasing RLD by 54.5% compared to PT-N150 and RSD by 62.0% compared to RT-N150. NT was also associated with a stronger protein-associated FTIR signal and greater lignin accumulation, collectively correlating with delayed senescence. Photosynthesis and yield were strongly correlated with deep RLD (*r* = 0.82, p < 0.01). NT-N210 achieved the highest yield (12,896 kg ha−1, 38.0% higher than PT-N150) with 12.5% less N than conventional practice. These findings indicate that combining the NT rotation with moderate N (210 kg ha−1) optimizes deep root functionality and delays senescence. This improvement was correlated with shifts in protein-associated FTIR signals and cell wall composition (e.g., lignin accumulation), which collectively contributed to significantly improved resource use efficiency and yield. Therefore, adopting a biennial no-tillage/subsoiling rotation combined with moderate nitrogen application (210 kg ha−1) is recommended as an effective strategy to alleviate soil compaction, enhance deep root growth, delay senescence, and achieve high maize yield with improved nitrogen use efficiency in similar agricultural systems.