Optimal rates of nitrogen fertilization for a winter wheat-corn cropping system in northern China

The sequential growth of winter wheat and corn within one year is the main cropping rotation system in northern China, excess nitrogen (N) fertilizer application to this system is a common problem. Four field experiments with four or five N rates were conducted in Beijing City and Henan province to study the optimal N rate for this system. The results of this study clearly indicate that the high rates of fertilization recommended by the quadratic model were less profitable than the lower rates recommended by the LRP and QRP models. The amounts of N in above-ground plant tissue tended to increase with increase in rate of N application but the percentage recovery of N in each increment decreased with each additional increment of fertilizer applied, and higher percentage recoveries were observed in wheat than in corn. Soil nitrate concentrations increased with increasing rates of N application. Average of 30% of the soil mineral N deficit can be accounted for uptake of N by wheat, but linear regression analysis showed no significant relationship between soil mineral N deficit and uptake N by corn. The relationships between soil mineral N deficit and mineral N in the soil before planting indicated that soil mineral N deficits were directly proportional to the. amounts of mineral N initially in the soil. Good relationships were observed between relative yields and amounts of mineral N in the soil before planting, and the r(2) values for the relationships with nitrate to 40 cm were similar to those observed with nitrate to 100 cm. The need to sample no deeper than 40 cm increases the practicality of soil testing and adjusting rates of N fertilization.