Abstract
Sugarcane has been cultivated in large areas on Negros Island, the Philippines. Farmers are used to apply nitrogen fertilizer two times. The first application is immediately after planting and the second is three months later. Sugarcane might not absorb much nitrogen during the early growth stage. If a large proportion of the first applied nitrogen cannot be absorbed by sugarcane due to early growth, high risk of groundwater pollution should occur. Since the quality of groundwater is important for the resident people, it is imperative to reduce the nitrogen load from sugarcane cultivation. The objectives of the present study were 1) to clarify the first applied nitrogen-fertilizer use efficiency using the 15N tracer technique, 2) to measure root extension, and 3) to observe nitrate-nitrogen leaching by using the porous cup and the wick sampler methods. The experiment consisted of several treatments including a sugarcane planting or no planting condition with or without application of nitrogenous fertilizer in the field. The 15N tracer experiment revealed that fertilizer use efficiency was 3.8% and the proportion of nitrogen derived from nitrogen fertilizer was 18.1%. Based on the root extension survey, sugarcane roots hardly extended deeper than 30 cm in the soil until three months after planting. High concentration of nitrate-nitrogen was observed in soil layers under the root extension depth, suggesting the leaching of nitrogen. To decrease the nitrogen load in groundwater while maintaining the current sugarcane yield level, it is important to increase fertilizer use efficiency of the first applied nitrogen in the early growth stage of sugarcane by adjusting the timing and the amount of applied fertilizer.
