Delaying application time of slow-release fertilizer increases soil rhizosphere nitrogen content, root activity, and grain yield of spring maize

Application of slow-release fertilizer (SF) is a nutrient-management measure aimed at improving maize nutrient use and yield and saving labor cost. One-time application of SF at sowing usually results in nutrient deficiency during the post-silking stage, owing to the long growth period of spring maize. This study was conducted to investigate the effects on spring maize of SF application stage (zero, three-, and six-leaf stages, designated as SF0, SF3, and SF6, respectively) on grain yield, total soil rhizosphere nitrogen (N) content, and root activity, in comparison with the conventional fertilization mode (CF, application of compound fertilizer at sowing time, and topdressing urea at six-leaf and tasseling stages) at the same fertilization level as the control. Compared with no fertilization (F0) and CF, SF increased grain number and weight. The maize cultivars Suyu 30 (SY30) and Jiangyu 877 (JY877) produced the highest grain yield and net return under SF6 treatment over the three years. SF6 increased enzymatic activities including oxidoreductase, hydrolase, transferase, and lyase in rhizosphere soil at silking (R1) and milking stages (R3). SF6 increased the total N contents of rhizosphere soil by 7.1% at R1 and 9.2% R3 stages compared with SF0. The activities of antioxidant enzymes in roots were increased under SF6 treatments at R1 and R3. The mean root activities of SF0, SF3, and SF6 increased by 7.1%, 12.8%, and 20.5% compared with CF at R1 and by 8.8%, 13.0%, and 23.5% at R3. Delaying the application time of SF could increase grain yield by increasing total N content of rhizosphere soil, delaying root senescence, and increasing root activity at the late reproductive stage. Applying SF at the six-leaf stage is recommended as an effective fertilization strategy for the sustainable production of spring maize in southern China.