Nitrogen use by winter wheat and changes in soil nitrate nitrogen levels with supplemental irrigation based on measurement of moisture content in various soil layers

• Optimising supplemental irrigation (SI) regime increased the nitrogen use of wheat.
• Optimising SI regime reduced the leaching of soil nitrate nitrogen.
• Soil relative water content in the 0–40 cm soil layers should be irrigated to 70%.

In northern China, water sources are one of most limiting factors for agricultural development, particularly for winter wheat. Optimizing supplemental irrigation (SI) to achieve the ideal soil moisture content and nutrient uptake by plants is therefore highly important in this region. The present study investigated how various SI regimes affect nitrogen use in winter wheat and alter soil nitrate nitrogen (NO3N) content under field conditions. The soil relative water content (SRWC) of the 0–20 cm (D20), 0–40 cm (D40) and 0–60 cm (D60) soil layers was raised to 70% by supplemental irrigation at jointing and anthesis, and a non-irrigated, rain-fed (R-F) condition served as the control. The highest grain yields of 9648.4 kg ha−1 in 2011–2012 and 10,032.2 kg ha−1 in 2012–2013 were obtained in D40, which had higher nitrogen use efficiency (NUE) than R-F, D20 and D60. NUE was positively correlated with nitrogen translocation efficiency (NtE) and nitrogen fertilizer productivity (NfP) but not with nitrogen uptake efficiency (NupE). These results indicated that optimizing the SI regime improved NUE primarily by increasing nitrogen uptake and translocation and thereby raising NtE and NfP. The soil NO3N content was lower in D40 than in R-F, D20 and D60 in the 0–200 cm soil layers, indicating that NO3N leaching was the lowest in D40. Thus, the optimal SI regime, which consisted of irrigating SRWC in the 0–40 cm soil layer to 70% at jointing and anthesis, reduced NO3N leaching and increased NUE and NfP in winter wheat.