Studies On The Capacity Of Soil Nitrogen Supply In The Winter Wheat/Summer Maize Rotation System

A field experiment was conducted from Oct. 1999 to Oct. 2002 in Beijing suburb, in order to study soil N supplying capacity and its effect on N fertilization recommendation in the winter wheat / summer maize rotation system in a fertile soil. Three N fertilization treatments, which were no N fertilization, optimized N fertilization determined by soil Nmin testing and plant testing, and conventional N fertilization according to local farmer's practices respectively, were involved in this experiment. The apparent N mineralization was calculated from the no N fertilization treatment.The results showed that the amount of soil N supplying and apparent N mineralization were 120-160 kg N/ha and 20-104 kg N/ha in three winter wheat growing season respectively. In winter wheat growing season, soil N supplying capacity maintained at a high level from sowing to regreening stage, and declined afterwards, especially from shooting to harvesting stage. Under condition of high soil N supplying capacity, even if the N rate was less than one third of that used at the conventional N fertilization treatment (300 kg N/ha), the same grain yield could be obtained by the optimized and conventional N fertilization treatments. Moreover, soil residual Nmin on optimized N fertilization treatment was quite lower, which could reduce the risk of N loss to the groundwater and atmosphere.In the summer maize growing season, the amount of soil supplying N was 202 kg N/ha in the year 2000. Soil Nmin was higher than 120 kg N/ha before silking in the no N fertilization treatment, which was higher than 80 kg N/ha from silking to harvest. Because of high soil N supplying capacity in the summer maize growing season in the year 2000, the same grain yield was obained by the optimized N fertilization of 42 kg N/ha and the conventional N fertilization of 300 kg N/ha.The soil N-supplying capacity declined with the rotation periods, especially in the third year. However, this experimental fertile soil maintained relatively high N supplying capacity during whole three experimental years, and in summer maize season the soil N supplying capacity was higher than that in the winter wheat season. The total amount of soil N supplying in a rotation period of winter wheat / summer maize was 167-242 kg N/ha. In three winter wheat growing seasons the soil N supplying decreased from 131 kg N/ha in the first year to 75 kg N/ha in the third year, however, in the summer maize growing seasons it still maintained at 125 kg N/ha, a relative higher level in the third year. The calculated apparent N mineralization in each rotation period varied from 184 to 125 kg N/ha in the three experiment years.