Deep nitrogen application improves wheat yield, agronomic nitrogen use efficiency and water productivity in China: A meta-analysis

Achieving synergistic improvements in wheat productivity and resource efficiency is critical for sustainable agricultural production. Deep nitrogen (N) application is an effective strategy to enhance crop resource efficiency; however, its impacts on wheat yield, agronomic N use efficiency (AE) and water productivity (WP) have received limited systematic analysis in China. The meta-analysis synthesizes 385 paired observations from 42 studies to quantify the effects of deep N application versus surface application on wheat performance, as well as identifying the main factors that influenced the effectiveness of deep N application. Compared with surface N application, deep N application significantly increased grain yield by 5.7%, AE by 42.9%, and WP by 4.5%. The effectiveness of deep N application was critically dependent on N application depth and N application rate. The optimal improvements in AE (62.4%) and WP (5.4%) were achieved at a depth of 5–15 cm, whereas the greatest yield increase (8.3%) was observed at 15–25 cm. Regarding N rate, the highest yield increase (9.4%) was achieved at > 250 kg/ha, while the optimal enhancing effects on AE (67.7%) and WP (6.0%) were observed at 150–250 kg/ha. Random forest regression analysis further revealed that soil available phosphorus (AP), exchangeable potassium (AK), and total N (TN) were the main drivers of comprehensive wheat productivity under deep N placement. These findings suggest that integrating a moderate N rate of 150–250 kg/ha with an optimal placement depth of 5–15 cm can synergistically enhance grain yield, AE, and WP under specific soil conditions in China. This study provides an evidence-based foundation for optimizing N management strategies to support sustainable wheat intensification.