Reduced and deep application of controlled-release urea maintained yield and improved nitrogen-use efficiency

Reasonable fertilization depth is critical for releasing nitrogen (N), thus improving the fertilization efficiency of controlled-release urea (CRU). However, the fertilization depth of CRU to maximize yield and N-use efficiency (NUE) and its driving mechanism in maize cultivation are still uncertain. Here, we utilized CRU to carry out a field plot experiment along with ordinary urea application at a rate of 250 kg N ha−1 as the control (U100). Three CRU placement depths (6, 12, and 18 cm, designated as C100–6, C100–12, and C100–18, respectively) with an application rate of 250 kg N ha−1 were designed to analyze the effect of fertilization depth on maize yield and NUE. Meanwhile two N-reduction gradients (20% and 40% reduction in N input relative to that of U100, designated as C80–12 and C60–12, respectively) of CRU at a fertilization depth of 12 cm were performed to assess the N-reduction potential of deep application of CRU when maintaining the yield. Furthermore, the mechanism was analyzed based on the spatiotemporal distribution of soil NO3ˉ-N and maize roots, the activity of N-assimilation enzymes in leaves, and N uptake and distribution. The results showed that maize yield was maintained with deep application and N-reduction treatments (except U60–12), whereas NUE was significantly increased with C100–12, C80–12, and C60–12, compared with that of U100. The fertilization depth and N-reduction gradient of CRU significantly affected the spatial distribution of soil NO3ˉ-N and total root length at maize booting stage. Compared with that of U1000, NH3 volatilization decreased significantly in all treatments (except U100–6), while the enzyme activity of glutamine synthetase (GS) and Fd-glutamate synthase (Fd-GOGAT) increased at C100–12 and decreased at C60–12. Structural equation modeling showed that high NO3ˉ-N concentrations in the critical zone stimulated root growth, enhanced N-assimilation enzyme activity, and reduced NH3 volatilization, resulting in increased NUE and yield. Our study identified the key zone of NO3ˉ-N affecting maize yield and NUE and emphasized the importance of matching soil NO3ˉ-N with the maize root system in spatial distribution under CRU application at a depth of 12 cm.