Growth response of Pinus radiata to multiple applications of nitrogen fertilizer and evaluation of the quantity of added nitrogen remaining in the forest system

Abstract

Over a period of nine years, 922 kg ha-1 of N was added in eight applications to a 16 year old P. radiata stand in a low rainfall area in north-east Tasmania, Australia. Fertilizing lifted current annual increment from 8.5 m3ha-1 for the unfertilized plots, to 31 m3ha-1. Increased growth was associated with improved health of the trees. Biomass measurements showed that there was a large increase in needle retention as well as needle mass on the fertilized trees. Concentration of N was also higher in fertilized trees. Fertilized plots contained 467 kg ha-1 more N than unfertilized plots. This represented about 50 percent of the N applied. Most of this extra N was in the forest crop and in the surface 10 cm of the soil. There was no increase in organic carbon in the surface soil with the result that the C/N ratio was reduced from a very high 28 to 17. Despite the high growth rates attained in the N fertilized plots, the failure to increase soil organic matter and the loss from the site of much of the applied N indicated that long term improvement of growth of these low rainfall sites was unlikely. Growth would only be maintained with continued N fertilizer additions.