| Nowadays, about 30% of world nitrogen (N) fertilizer is consumed by China. Average rate of N application for rice production in China is higher and fertilizer-N use efficiency (NUE) is lower compared with that in other major rice growing countries. Therefore, it is profitable not only on the profit of rice production, but also on the help to release farming environmental stress. The field experiments were conducted in 2001 and 2002 at Hunan site in China, and in 2002 dry season (DS) and 2003DS at IRRI farm in Philippines. The objectives are: (1) to determine the effect on rice growth by fertilizer-N application timing, rates and methods; (2) to identify the hypothesis on the causes of low NUE of irrigated rice in China; (3) to compare the recovery efficiency estimating by 15N dilution method and 'difference method' through frequently sampling; and (4) to test the effect on improving NUE of irrigated rice in China through real-time nitrogen management and Site-Specific Nutrient Management (SSNM). The results showed as followings:1. At rice earlier growing season, fertilizer-N was the most important contribution to the rice nitrogen, it accounted for 60% from total nitrogen (TN) from transplanting to panicle initiation (PI); At maturity, 40% of nitrogen derived from fertilizer, among them, fertilizer-N (FN) uptake at PI contributed 20%, and mid-tillering (MT) or basal contributed 10% respectively;2. The 15N uptake rate was greatest while FN applied at PI, and absorbed the approximate maximum amount in 9 days after fertilization; The highest uptake rate based on total N accumulation was 9.5 kgN hm-2 d-1, and 5.4~6.4 kgN hm-2 d-1 from 15N-fertilizer at PI. The rice plant reached the maximum 15N accumulation in 14 days after FN topdressing at MT, but 30 days after FN applied as basal;3. The difference of nitrogen recovery use efficiency (RE) was significant among various FN application stages. RE were 59%, 26%, and 24% determined by 15N dilution method when FN applied at PI, MT and basal, and which were 67%, 54%, and 45% estimating by 'difference method' respectively;4. RE determined by 'difference method' were always overestimated due to priming effect if using the blank plots as control. And it was found that if set a proper control for 'difference method', the measurement value is highly close to the determination value that is estimated by I5N dilution method when fertilizer was topdressing at MT or PI.5. It was found that, rice plant existed a pathway to lose nitrogen by volatilization from plant tissues at later growing season. According to the I5N balance at maturity, the nitrogen loss proportion was 16% from 15N uptake at MT, and 13% from I5N uptake at PI;6. The differences in SPAD readings were statistically significant among the six meters and the difference between two meters was as large as 2.7 units. Such magnitude of discrepancy should be considered when different SPAD meters are used in a same study;7. It was found that the difference of biomass production and nitrogen uptake by rice was insignificant between Chinese hybrid rice Shanyou63 and tropical indica rice IR72 under tropical ecosystem conditions. Therefore, it was to say, variety is not the main cause to contribute the low RE of irrigated rice in China;8. Comparison was done to the RE, agronomic use efficiency (AE), and physiological use efficiency (PE) between tropical ecosystem at IRRI farm, in Philippines, and in sub-tropical ecosystem at Hunan site, in China. The results showed that RE was 65% at Hunan site, higher than that at IRRI site. But the AE and PE in China was significantly lower than that at IRRI site due to poor yield response to fertilizer application at Hunan, China;9. Among different nitrogen management strategies, real-time nitrogen management and SSNM could reduce N application rate but maintain the grain yield or increased the grain yield in some cases, compared with that under fanner's practice at Hunan, China. Therefore, the AE and PE were significantly improved simultaneously u...
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