Nitrogen leaching losses from forests in New Zealand

Abstract

Regional governing organisations in New Zealand are developing new policies to assess and manage the effects of land use intensification on water quality. Such policies require an assessment to be made of potential nitrogen leaching losses from different land uses, particularly where land use may be becoming more intensive. This review compiles the information on nitrogen concentrations and fluxes in stream and soil water drainage from indigenous and planted forests in New Zealand. Nitrogen concentrations and losses in streams from undisturbed indigenous forests and from planted forests established on land that has not been developed for agriculture are low. Nitrogen concentrations and losses in streams draining forests established on pasture land are commonly higher than in streams from indigenous forests and from forests planted on non-agricultural land, however most of the values for planted forests on pasture land were influenced by high volcanic nitrogen inputs. The impact of afforestation of pasture land on nitrogen leaching is described, as are the effects of planted forest management practices including fertilisation, harvesting and herbicide use. Nitrogen leaching is generally not greatly affected by forest management practices, however fertilisation can cause large leaching losses in coastal sand forests. While nitrogen leaching from planted forests is normally less than from other major land uses, there are key times during the forest rotation when nitrogen leaching may occur, particularly in forests planted on coastal sands or pasture land. It is advantageous to minimise leaching at these key stages to limit adverse effects on the environment and maintain nitrogen in the soil for the benefit of crop nutrition. Measures to reduce nitrogen leaching losses in forests are discussed, as are practices to minimise nitrogen leaching of forest plantings in catchments with high nitrogen soils, where forests may be planted specifically to reduce nitrogen loss to sensitive waterways.