Fertilizer‐Nitrogen Use Efficiency of Irrigated Wheat: II. Partitioning Efficiency of Preplant versus Late‐Season Application

Abstract Partitioning of N to grain is an important determinant of N‐use efficiency in wheat ( Triticum aestivum L.). This study evaluated the effects of N‐fertilizer treatments on N partitioning, and compared the partitioning of labeled N applied at planting with labeled N applied at anthesis. A 2‐yr field experiment was conducted with applications of 120, 180, or 240 kg N ha −1 at planting and 0, 30, or 60 kg N ha −1 at anthesis. The mean N harvest index (NHI) of N acquired from fertilizer applied at planting was 0.70, compared to 0.89 for that N acquired from fertilizer applied at anthesis. Mean NHI of total plant N derived from all sources (fertilizer and soil) was 0.74. The amount of N applied at anthesis had little effect on N partitioning, but increasing rates of preplant N from 120 to 240 kg N ha −1 caused a decrease in NHI averaging 0.10 the first yr and 0.04 the second yr. A second 2‐yr experiment, using similar N treatments and a small dose (<1 kg ha −1 ) of NH 4 −15 N injected into the soil at anthesis verified these results. In the second experiment the effects of preplant N on NHI differed between cultivars; Yecora Rojo was considerably more efficient than Anza at high preplant N rates. Fertilizer N applied at anthesis was found in leaf, stem, and spike tissue 14 days after anthesis and contributed 4.2 to 16.8% of the N in each tissue at that time. Anthesis N fertilizer was also well distributed among structural, soluble, and ribulose‐1,5‐diphosphate carboxylase N feactions of flag leaves. This study demonstrated that the early season N environment has a large influence on N partitioning at maturity, whereas N applied at anthesis has little effect on N partitioning and is allocated more efficiently to grain.

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