Effects of nitrogen fertilization on grain yield and nitrogen use efficiency of double cropping rice

Field trials with seven treatments of N application rates(N 0, 60, 120, 180, 240, 300 and 360 kg/ha) were set up to determine the optimum N application rate, and to identify the responses of N fertilization contribution rate, soil N dependent rate and N use efficiency to different N application rates in paddy fields with different yield levels(high, middle and low). The results show that the low-, middle-, and high- yield paddy fields have the highest grain yields under the application rates of N 120, 180 and 240 kg/ha, respectively. The nitrogen fertilization contribution rates(NCR) in the low- and middle- yield fields are larger than that in the high-yield field, and the highest NCRs are acquired in the treatments of N 120, 180 and 240 kg/ha for the three fields separately. Soil nitrogen dependent rates(SNPR) are in the order: high-yield field middle-yield field low-yield field, and the SNPR is decreased with the increase of N application rates to some extent. The trend of nitrogen recovery efficiency(NRE) is low-yield field middle-yield field high-yield field, whereas no distinct differences exist for nitrogen agronomic efficiency(NAE), nitrogen physiological efficiency(NPE) and nitrogen partial factor productivity(PFPN) among high, middle-, and low-yield paddy fields. All NAE, NRE and PFPN in high-, middle-, and low-yield paddy fields are decreased with the increase of N application rates, while NPE remains relatively stable among different fertilization treatments. The optimum N application rates for double cropping rice in low-, middle- and high- yield fields are N 120, 180 and 240 kg/ha, respectively.