Continuous applying N at high rate in crop production in China resulted in increasing basal soil N content and decreasing N fertilizer use efficiency. Moreover, it caused severe underground water pollution in many areas. Therefore, how to optimal use N fertilizer was facing scientists. There were lots of researches on the nitrate dynamics with N fertilizer treatments during winter wheat-summer maize rotation system, especially during summer-maize season. Due to high variation of rainfall and temperature during maize growth season in China, it was difficult to determine the optimal N fertilizer rate applied to maize only by studies on relationship of maize N requirements and soil nitrate dynamics. The objective of this study was to understand the effect of N rates applied to maize on soil nitrate content and crop yield during winter wheat growth season.
A field trial with the summer maize-winter wheat rotation was carried out in Huang-Huai-Hai Plain, North China, in 2002～2003. Five rates of nitrogen fertilizer (i.e. 0, 90, 180, 270 and 360 kg N hm-2) were applied to maize with high-yield hybrid “Zhengdan 958" and the population of 9 plants m-2. 2/3 of the total N fertilizer was applied at sowing stage and 2/3 of that at 10-unfolded leaf stage.After maize harvest, wheat as the following crop was planted in former crop plots with cultivar “97-3" and seed rate of 704 grain m-2. N rate of 144 kg hm-2 was applied at wheat sowing. Except for precipitation of 112.1 mm during wheat season，two times of irrigation with 75 mm at each time were done on April 13, and May 9, 2003, respectively. The trial was completely randomized design with three replicates. Plot size was 6 m by 6 m. The vocation between maize harvest and wheat sowing was 15 days.
The soil in the trial field was light salted loam in 0～130 cm soil layers and loam in 130～200 cm soil layers. There was just ploughed during the vocation of the cropping system. In addition to N fertilizer, P2O5 and K2O were dressed on the day of maize sowing at rate of 105 kg hm^-2and 120 kg hm-2, and on the day of ploughing day at rate of 103 kg hm^-2and 75 kg hm-2, respectively. Soil samples were taken three times (i.e. sowing, shooting and maturity) during wheat season and one time at maize sowing with a soil auger. The samples were divided into different soil layers in 20 cm increment from surface to 100 cm depth, 100～130 cm, 130～160 cm, and 160～200 cm at taking, pretreatment and analysis. Soil nitrate was determined by the Salicylic Acid Colorimetric Method and ammonium by the improved Ninhydrin Colorimetric Method. Considering of the nitrate mobility, we incorporated eight soil layers into three layers (i.e. 0～40 cm, 40～130 cm, and 130～200 cm) according to soil bulk density. At crop maturity, grain yield was measured in field and samples were taken at the same time. Total N content was analyzed with Kjeldahl Digestion Method.
The findings of the trial showed the nitrate content in the 0～200cm soil profile significantly increased during wheat season when N fertilizer applied. In the 0～40cm, 0～130cm and 0～200cm soil profiles, the nitrate content increased with the increment of N rates from wheat shooting to maturity. The higher the soil nitrate content, the more amount soil N increment caused by N fertilizer. The 0～200 cm inorganic amount in zero N fertilizer plots significantly decreased at wheat maturity by comparing to that at maize sowing, while that in the 360 kg hm-2 N fertilizer plots significantly increased. The 0～200 cm soil inorganic amounts in those plots with N rates of 90 kg hm-2 to 270 kg hm-2 little decreased after one maize season and one wheat season. The wheat plant N amount, grain N amount and grain NUE all slightly increased with increment of N rate when the N rate is less than 180 kg hm-2. In comparison with zero N treatment, the 90 kg hm-2 treatment slightly increased the grain yield of winter wheat and that of maize plus wheat, but the 180 kg hm-2 treatment significantly increased it. The grain yields in the 270 kg hm-2 plots and those in the 180 kg hm-2 plots had no significant difference. So, the N fertilizer at rate of 90～180 kg hm-2 was optimal under trial conditions.