Can deep fertilizer application enhance maize productivity by delaying leaf senescence and decreasing nitrate residue levels?

Deep fertilizer application is an effective strategy for minimizing gaseous losses and improving the nutrient use efficiency and grain yield. However, it is unclear whether deep fertilizer application can improve the antioxidant defense system in maize (Zea mays L.) leaves and the photosynthetic capacity after silking to delay senescence, and then contribute to improvements in the yield and resource utilization efficiency. The effects on the NO3–-N residue levels also require elucidation. Therefore, we tested different fertilization application depths in the field during 2019–2020, where all of the N-P fertilizer was applied underground at 5 cm (D5), 15 cm (D15), 25 cm (D25), and 35 cm (D35) before sowing spring maize in the Loess Plateau region of China. The results showed that the antioxidant and photosynthetic performance of the leaves followed the order of: D25 > D15 > D5 > D35. The H2O2 and O2– contents decreased by 21.5% and 14.0% under D25 compared with D5, respectively, and the net photosynthetic rate (Pn) and mean grain filling rate (Gmean) increased by 32.9% and 5.0%. The translocation and assimilation of nitrogen under D25 increased by 20.1% and 18.6% compared with D5, respectively, the translocation and assimilation of phosphorus increased by 43.6% and 54.3%, and the biomass and maize yield increased by 12.7% and 13.8%. The water use efficiency (23.5 kg·ha–1·mm–1), water productivity (45.7 kg ha–1 mm–1), nitrogen use efficiency (43.6%), and phosphorus use efficiency (20.3%) were significantly higher under D25 than the other treatments. The accumulated residual nitrate contents in the 0–100 cm soil layer were 6.0% and 8.0% lower under D15 and D25 than D5, respectively (P