Effects of fertilizer-N application for double early rice on the yield,nitrogen use efficiency and soil nitrogen content of double late rice

To explore the effects of fertilizer-N application for early cropping rice on rice yield,nitrogen use efficiency and soil nitrogen content in late cropping rice.Long-term experiment with different N treatments in double rice cropping system was conducted on the farm of Hunan agricultural university by using random block design with 4 replications from 2002 to 2005.There were four N treatments: N fertilizer applied in early season but not in late season(ENL0),N fertilizer applied in both early and late seasons(ENLN),no N fertilizer applied in either early or late seasons(EOL0),and N fertilizer applied in late season but not in early season(EOLN).The results showed that when N fertilizer was applied for both early and late rice,the average yield,dry matter production and N uptake of late rice reached 6.45 t/ha,12.13 t/ha and 183.6 kg/ha,respectively,which were 28.0%,35.1% and 103.5% higher than those of ENL0,respectively.The average grain yield,dry matter production and N uptake of the EOLN treatment reached 6.61 t/ha,12.14 t/ha and 165.6 kg/ha,respectively,which were 33.4%,37.6% and 95.6% higher than those of EOL0 treatment,respectively.The agronomic efficiency,physiological efficiency and partial factor productivity of fertilizer N of EOLN increased by 18.1%,37.8% and 2.7%,respectively,but the recovery efficiency decreased by 6.8%,compared with those of ENLNThe alkali-hydrolyzed soil N content in plots without N fertilizer in either early or late season rice was significantly lower than that in plots with N fertilizer in either season,but did not decrease continuously over years.No significant difference of the soil N content was observed among treatments receiving N application in one or both seasons.The results indicated that the grain yield of double late rice was significantly affected by N fertilizer applied in late season but not by that in early season.The agronomic efficiency,physiological efficiency of N fertilizer in late cropping rice was higher without N fertilizer in early season than those with N fertilizer in early season.To some extend,reduce N fertilizer rate would not significantly reduce soil indigenous N content.