Over-application of nitrogen fertilizer in intensive rice system in China

摘要

Irrigated rice in China accounts for nearly 30% of global rice production and about 7% of global nitrogen (N) consumption. Low fertilizer-N use efficiency (FNUE) of this system has become a threat to the environment. The objective of this study was to identify the possible causes of low FNUE of irrigated rice in China by comparing farmers' fertilizer practices with other N management strategies such as site-specific N management (SSNM). Field experiments were conducted in farmers' fields in four major rice-growing provinces in China in 2001 and 2002. The same experiment was repeated at the IRRI farm in the dry seasons of 2002 and 2003. Agronomic efficiency of applied N (AE_N, kg grain yield increase per kg N applied) was determined by the "difference method" using N-omission plot. Maximum yield was achieved mostly at 60-120 kg N ha~(-1), which was significantly lower than the 180-240 kg N ha~(-1) applied in farmers' practices at the China sites. With the modified farmers' fertilizer practice, 30% reduction of total N rate during early vegetative stage did not reduce yield but slightly increased yield and doubled AE_N compared with farmers' practices at the China sites. Total N rate in SSNM ranged from 38 to 110 kg ha~(-1) at the China sites, but their yields were similar to or higher than that of farmers' practices. These results suggest that irrigated rice farmers in China could reduce N rate by 30%-50% without significantly reducing crop yield. A quantum leap in AE_N is possible in the intensive rice growing areas in China by simply reducing the current N rate and by allocating less amount of N at the early vegetative stage.