Nitrogen Leaching in Kunigami Mahji Amended with Organic Amendments.

In Okinawa, Japan, due to intense and heavy rainfalls, suspended sediments and nutrient runoff from agricultural fields has been a serious problem causing watershed pollution and destruction of landscape and ecosystem because a regional typical soil called Kunigami mahji has physical characteristics vulnerable to water erosion. There have been many studies of elucidating physical aspects of soil erosion in such a soil, however, few studies on direct measurement of nutrient leaching from agricultural fields in Okinawa soil exist. In addition, effects of application of organic composts derived from sewage sludge to Kunigami mahji on nutrient leaching are not well understood. Therefore, the objective of this study was to clarify the dynamics of nutrient (particularly N) leaching in Kunigami mahji when organic composts derived from sewage sludge were applied. A leaching experiment was performed in triplicate for 12 weeks upon addition of 1.15 L of water each week using columns filled with Kunigami mahji with 3 different types of organic amendments including baggase-derived organic fertilizer, sewage sludge-derived compost, and sewage sludge with baggase-derived composts, and chemical fertilizer and no amendments as control. Over the duration of the experiment, all columns leached almost all NH₄–N by 5 weeks and NO₃-N by 2 weeks. Chemical fertilizer-treated column leached the highest amount of NH₄–N (101.4 mg/column) because the fertilizer was made of ammonium sulfate and the soil had a low CEC to retain NH₄–N. NO₃-N leached was greatest (38.0 mg/column) in column with sewage sludge-derived compost because its raw material contained a high mount of NO₃-N. Total N leached was high in column with chemical fertilizer and low in column with baggase-derived organic fertilizer. The results suggested that N leaching depended on raw materials of organic amendments and soil characteristic.