Experimental method
A 10-L plastic bucket was filled with 7.5 kg air-dry soil. Urea, NaH2PO4 and KCl were used as N,P and K fertilizers, respectively. Four N fertilizer levels, i.e. 0 kg N/ha (Control), 80 kg N/ha (Low N), 160 kg N/ha (Middle N), and 320 kg N/ha (High N), were arranged. Each treatment supplied with P2O5 of 120 kg/ha and K2O of 240 kg/ha. Seventy percent of N fertilizer and all P and K fertilizers were evenly blended with soil before rice transplanting, and the remaining 30% of N fertilizer was applied at the tillering stage.