In this study the fixed-N split treatments (N1~N4) indicated that RE, AE and PE of the two cultivars tended to decrease with increased N application, as reported in previous researches (Peng and Cassman, 1998; Timsina et al., 2001; Liu et al., 2004; Jiang et al., 2004; Peng et al., 2006; Mae et al., 2006). High N rate increased post-DMA and post-NR, which theoretically could result in large grain yield and large PE, RE and AE, however, high N rates did reduced them in the two cultivars, In this study, NA at flowering accounted for 99% and 100% of total nitrogen uptake under high N rates in Jinzao22 and Shanyou63 respectively, which resulted in high nitrogen concentration and accumulation in rice plant at flowering stage. It was reported that the decreased accumulation of nitrogen before flowering could be caused by nitrogen volatilization from plant tissues and death of partial leaf and organ at post-flowering stage, and that nitrogen volatilization was the main cause (Huang et al., 2004; Jin and Mian, 2005). In this study high N rate decreased NA at post-flowering in Jinzao22 and increased nitrogen volatilization in Shanyou63. High nitrogen concentration and accumulation at flowering stage was the basis of large post-flowering dry matter production and N redistribution. The negative effect caused by volatilization of large nitrogen concentration and accumulation at flowering stage was larger than the positive effect of post-flowering dry matter production and N redistribution at high N rate, so NUE decreased with increasing N rates. In addition, a high indigenous N supply of soil (INS) will result in higher N concentration in rice straw and produce the phenomenon of luxury consumption of N (Liu et al., 2004; 2005; Feng et al., 2006). In this study, the average grain yield in N omission plots of Jinzao22 and Shanyou63 were higher than 4 t/ha and 6 t/ha, respectively, indicating that the INS of the soil was high.
de Datta (1986) reported that RE is usually 30%~50% in the tropics and PE in the tropics is about 50~70 kg rough rice per kg N absorbed. Yoshida (1981) estimated AE to be 15~25 kg/kg and Cassman and Pingali (1996) reported AE was 15~20 kg/kg in the dry season in the farmers’ fields in the Philippines. NUE of the farmer’s practice in irrigated rice in Zhejiang Province was reported by Wang et al.(2001a) and Feng et al.(2006). Wang et al.(2001a) measured AE in 21 farmers’ fields for four seasons and found that the average AE of the farmers’ practice was 6.4 kg/kg. Feng et al.(2006) reported that N use efficiency was so low that RE was only 21.55% in average in high-fertility paddy field in Northern Zheiiang Province, which was close to 19.9% in Jiangsu Province (Li, 2000). In this study, RE, AE and PE were 16%, 8.02 kg/kg, 51 kg/kg in Jinzao22 and 26%, 3.4 kg/kg, 13 kg/kg in Shanyou63 in FFP. SSNM and RTNM increased RE, AE and PE significantly in contrast to FFP (Table 7). Previous studies reported there was great potential for NUE improvement by changing accumulation and redistribution of dry matter and nitrogen under optimal nitrogen management in different cultivars (Jiang et al., 2003; 2004). We speculate the reasons that may explain the phenomena. In this study, DMA and NA at pre-PI in FFP were significantly higher than SSNM and RTNM for the two cultivars, while DMA at post-flowering was lower. Post-DMR and post-NR in FFP were higher or close to SSNM and RTNM. These results showed accumulation of dry matter and nitrogen before PI in the FFP was excessive, and will cause much invalid tillering and deficient nitrogen supplies in the middle and late rice growth stages. Yet SSNM and RTNM could not improve post-DMR and post-NR. So the disadvantage of FFP in Zhejiang was the improper timing of N applied, and N supply cannot be satisfied with N requirement, as reported in previous research (Zhang and Wang, 2002; Liu et al., 2004; Peng et al., 2006; Lin et al., 2006). Second, overfull N input in FFP was also the important factor causing low NUE of irrigated rice in Zhejiang. Some reporters believe that 30%~50% reduction in N application in most cases would be feasible and necessary in China (Yang et al., 2003; Liu et al., 2004; Feng et al., 2006). In this study, N input can reduce about 15% in Jinzao22 and 45% in Shanyou63; waste of N in single rice was more severe than direct-seeding early rice.
As dominant cultivars in Zhejiang, Shanyou63 and Jinzao22 have shown great differences in plant type, panicle size and grain yield under production practices. There were different NUE in different varieties under the different levels of N fertilizer (Jiang et al., 2003; Luan et al., 2005). In this study, RE of Shanyou63 was higher than that of Jinzao22 while PE was lower than that of Jinzao22 under the same condition (Table 7).
RE, AE and PE of the two cultivars tended to decrease and post-DMR, post-NR increased with increased N application under fixed-N split treatments for two cultivars. SSNM and RTNM increased DMA and NA at middle and late stages of rice, yet reduced DMA and NA before PI. SSNM and RTNM increased NUE significantly in contrast to FFP. The cause of low NUE of irrigated rice in FFP of Zhejiang was the high N input and improper timing of N applied. N input could be reduced about 15% in direct-seeding early rice and 45% in single rice without yield loss. SSNM was superior to other N treatments in two cultivars.
*Project supported by the International Rice Research Institute (IRRI), Swiss Agency for Development and Cooperation (SDC), the Potash & Phosphate Institute and the Potash & Phosphate Institute of Canada (PPI-PPIC), the International Fertilizer Industry Association (IFA), the International Potash Institute (IPI), and 948 Project of the Ministry of Agriculture of China (No. 2003-Z53)