We conducted field experiments in which nitrogen (N) was applied to summer maize at different rates and different basal/topdressing ratios. The experiments were carried out in 2009 in Hengshui and Xinji, Hebei province, China. The results showed that basal application of N was necessary for maize growth in early summer and for high grain yields. For the Hengshui and Xinji sites, 30 and 57？kg N？ha？1, respectively, would meet the N demands of maize before 7-leaf stage. The total rates of 120 and 180？kg N？ha？1, respectively, would maximize grain yields, and in-season N management based on crop N demands and soil N supply could reduce N inputs by more than 50% in Hengshui and 25% in Xinji, respectively, in one maize growth season, compared with farmers' practice, but the sustainability of the optimum N rates for maximum grain yield of next seasons crop needs to be further studied. Optimum N management should take into account the existing nutrient conditions at each site, soil fertility and texture, and crop demands. 1. Introduction The North China Plain (NCP) is one of the most important areas for cereal production in China. To meet the high demands for food, excessive amounts of nitrogen (N) fertilizer have been applied in this intensive agricultural region during the last two decades [1–3]. Summer maize (Zea mays. L.) is one of the staple grain crops in the NCP, and its planting area and total grain production accounted for 32 and 31% of the China maize crop in 2002 [4]. In the Shandong province, one of the provinces in the NCP, the amount of mineral N fertilizer applied to summer maize ranges from 50 to 600？kg N？ha？1, and the average application rate is 240？kg N？ha？1 [5]. These rates of application exceed the requirements of the crop to achieve maximum grain yield [6]. Overapplication of N fertilizer results in accumulation of N as nitrate-N in the soil, leading to groundwater pollution. Residual soil nitrate-N after winter wheat and summer maize harvest was reported to be 316 and 271？kg N？ha？1 in the top 100-cm soil profile when N was applied at rates of 300 and 240？kg N？ha？1, which are the rates used in normal farmers’ practice in this region [7, 8]. In 2001, the groundwater nitrate-N concentration exceeded 45？mg N O 3 ？ ？L？1 in 49% of samples from this area [9]. High accumulation of soil nitrate-N and excessive rates of N application result in low nitrogen use efficiency (NUE), and the NUE of summer maize crops in this area has decreased from 30–35% in the 1980s to less than 20% in the 2000’s [10, 11]. At present, normal farmers’ N practice is to
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