Nitrogen leaching from agricultural fields is a major source of nutrient load into groundwater on tropical and sub-tropical islands, where limestone is widely distributed. To clarify the effects of reducing the nitrogen application rate on the growth of sugarcane, a major crop on Okinawa, and the leaching of nitrate nitrogen, we carried out a sugarcane cultivation experiment and monitored the leaching using a lysimeter. The experimental design was a randomized block in a 3 × 2 factorial design and control (without nitrogen) with two replications, including the nitrogen rate at the 1st application (0, 35 or 70 kg ha-1) and the amount of the 2nd and 3rd nitrogen rates (80 or 160 kg ha-1). Different rates did not affect crop growth in the early growth stages and leaching of nitrate nitrogen occurred mainly at that time. These results suggest that the current standard of nitrogen application for sugarcane is excessive in the early growth stages. When the first nitrogen application was reduced by 50%, the cane yield did not differ significantly from that with the recommended nitrogen application rate. Therefore, to decrease the nitrogen load in groundwater while maintaining the current level of sugarcane yield, the amount of the first fertilizer application could be reduced in order to decrease the leaching of nitrate nitrogen in the early growth stages.
