Nutrition Diagnosis for Nitrogen in Winter Wheat Based on Critical Nitrogen Dilution Curves

Accurate and timely diagnosing of nitrogen (N) requirements for crop growth is vital for improving the efficiency of N use and reducing the risk of N losses. This study conducted a comparison of N dilution curves with urea and controlled-release N fertilizer (CRN) to explore the applicability of critical N curve and to determine the appropriateness of CRN. Experiments on winter wheat (Triticum aestivum L.) with different N applications were conducted during 2013 to 2015 in Shaanxi Province, China. The N applications consisted of urea at 80 (U80), 160 (U160), and 240 kg N ha(-1) (U240), with 40% applied as a basal fertilizer and 60% topdressed at the jointing stage, and CRN at 60 (C60), 120 (C120), 180 (C180), and 240 kg N ha(-1) (C240), all applied as a basal fertilizer. A treatment with no N application served as a control (CK). The critical N concentration and shoot dry matter of winter wheat fitted a power function for both CRN and urea. The curves were validated with data from another season, and their performances were proved to be stable. The estimated N nutrition index and accumulated N deficit under each N application enabled crop N condition to be well differentiated and suggested that a suitable N application rate for winter wheat was 120 to 180 kg CRN ha(-1) and 160 to 240 kg urea ha(-1). Moreover, the C180 treatment produced a similar yield as the U240 treatment, but its N physiological efficiency, N agronomic efficiency, partial N productivity, and apparent N recovery were on average 5.87, 35.93, 34.12, and 28.39% higher than the U240 treatment, respectively, in the two seasons. These results are useful for N application management and can offer a theoretical reference for CRN application.