Optimizing the Nitrogen Management Strategy for Winter Wheat in the North China Plain Using Rapid Soil and Plant Nitrogen Measurements

Excessive nitrogen (N) fertilizer with improper split-application in small-scale farming is widespread for reducing N use efficiency and polluting the environment. The objective of this study was to develop a strategy for providing winter wheat with twice-topdressing N by quickly measuring the soil and plant N status. During the period 2009–2011, a field experiment was conducted for winter wheat cultivar Zhongmai-175 in the North China Plain. The mineral N (Nmin) pool at a soil depth of 0–90 cm and topdressing N twice, as total N supply, was gradually increased from 0 to 420 kg N ha^–1 to mimic the farmers´ practices. Measurements with the Soil Plant Analysis Development (SPAD) meter were taken on the uppermost fully expanded leaf, and the SPAD index was expressed relative to SPAD readings of sufficiently fertilized plants. Grain yield exhibited linear-plus-plateau responses to total N supply with a significant difference between years, the r² ranged from 0.73 to 0.94. With a basal N application of 30 kg ha^–1, the soil Nmin at 0–90 cm supplemented by twice-topdressing N (1:1 ratio) at Zadoks growth stage (ZGS) 22–23 in early spring and ZGS 47–52 was required at 150–165 kg N ha^–1 to achieve a maximum grain yield of 3.9–5.3 t ha^–1. The SPAD index exhibited a strong exponential response to N supply irrespective of plant growth stage and year (r² = 0.95–0.97); the value of 0.94 was critical in denoting N deficiency from sufficiency status. The N topdressing at ZGS 47–52 could be precisely modified/estimated by the equation y = 161.7–218x^5.16, where x is the SPAD index. Since SPAD readings varied significantly from year to year, our study suggests that it might be difficult to precisely manage field N for winter wheat.