Optimizing urea deep placement to rainfall can maximize crop water-nitrogen productivity and decrease nitrate leaching in winter wheat

The deep placement of urea fertilizer is considered an effective strategy for crop growth and yield formation. How crop root morphology and resource utilization respond to altered rainfall amount and deep urea fertilizer placement is still not fully understood. Thus, we conducted a two-year field experiment in the semi-humid region to assess the effects of different urea fertilizer placement depths of 5 cm (D5, conventional surface fertilization), 15 cm (D15), 25 cm (D25), and 35 cm (D35) under three rainfall conditions (dry year = 125 mm, P125; normal year = 200 mm, P200; wet year = 275 mm, P275) on root distribution, resource utilization and yield of wheat. Deep urea placement significantly increased root length density and root weight density in the 20–60 cm soil layer, which caused a significant increase in wheat nitrogen (N) uptake. Compared with D5, deep urea placement significantly increased the crop N uptake (10.4–27.6 %), grain N content (1.2–18.7 %), N recovery efficiency (NRE, 0.9–16.7 %) and N partial productivity (PFPN, 1.1–20.0 %) in dry and normal years, and D15 promoted those by 0.9 %, 1.3 %, 3.7 % and 1.1 % over D5 under wet year. Additionally, compared with D5, deep urea placement increased crop water productivity (WP) and yield by 9.6–23.9 % and 3.2–20.0 % under dry and normal years, and D15 increased those by 1.1 % and 0.5 % in wet year. The accumulated nitrate contents in the 0–100 cm soil layer was 4.7–14.5 % higher under deep urea placement than D5. Placement of urea at depths of 28.0–30.0 cm, 20.7–21.6 cm and 12.3–13.1 cm in the dry, normal and wet years maximized the wheat yield, grain N content and WP. Moderate deep urea placement according to variable precipitation conditions can be an effective fertilizer management strategy for sustainable wheat development.