Report: TR05/37

Author: Murray Davis (Centre for Sustainable Forest Management, Ensis)

Abstract

This report provides background information on fertiliser use and nutrient leaching for forests in the Taupo catchment area, with a focus on nitrogen.

Soils of the Taupo catchment are formed dominantly from pumice. Radiata pine forests growing on pumice soils may be marginally deficient in nitrogen (N) and have been shown to respond to nitrogen at rates of up to 200 kg/ha when applied in conjunction with thinning operations. Currently the usual practice is not to apply nitrogen fertiliser to correct such marginal deficiencies, but this may change should economic conditions justify it. Nitrogen also may be applied after planting to promote early growth (at approximately 30 g/tree) or at any stage during the rotation to correct limited areas of severe deficiency.

Nitrogen losses from leaching of undisturbed plantation forests are usually low (of the order of 2 kgN/ha) and similar to native forest, but harvesting can cause limited short-term increases in nitrogen leaching. Available information on nitrogen leaching after fertiliser application to pumice soils in the Taupo catchment is limited. Because of the lack of New Zealand studies, the question "what maximum amount of nitrogen over what period should be specified for forestry operations to prevent ongoing increases to surface or groundwater resources in the Lake Taupo catchment?" cannot be directly answered.

Until additional research is undertaken, and in order to follow a conservative approach to prevent leaching of nitrogen, the following points are suggested as useful guides in terms of increased fertiliser applications in forestry:

• Broadcast applications of nitrogen at any time should not exceed 200 kg/ha.
• Spot applications after planting should not exceed 30 g N/tree.
• Application during winter or other wet periods when soil water drainage is likely should be avoided.
• Except where plantations are severely deficient broadcast applications should be made in conjunction with thinning and pruning operations when N demand is high.
• Application decisions should be based on results of nutrient analysis of foliage.
• Mechanisms to prevent the direct discharge of fertiliser into surface water should be in place.
• Rapid revegetation of clearfelled areas should be promoted to minimise losses after harvest.

Nutrient losses from forestry in the Lake Taupo Catchment [PDF, 126 KB]