Losses of nitrogen fertiliser under oil palm in Papua New Guinea: 1. Water balance, and nitrogen in soil solution and runoff

Nitrogen (N) fertiliser is an important and expensive input to oil palm in Papua New Guinea. Of about 3000mm/ year of rainfall, about 1300mm is lost as evaporation. This leaves an excess of > 1000 mm/year lost as surface runoff and/or deep drainage, and with it the potential for N loss. Approximately 11% of rainfall reached the ground as stem flow. Throughfall was generally lowest near the trunk and highest where canopies overlapped, but random spatial variability was large. The difference between the measured rainfall and stem flow plus throughfall was 6%, indicating relatively little interception. Surface runoff from the volcanic ash soils was 6% of rainfall at one site, but only 1.4% at the other. Less than 2% of the applied N was lost in the surface runoff after an ammonium chloride application. Calculations of N leaching losses made using suction cup data and the water balance indicated that significant losses occur, but the estimates were not reliable due to the huge spatial variability in the suction cup and throughfall data. Therefore, another technique is needed to study N leaching in oil palm plantations.