Implications of pre- and post-fertilizing changes in growth and nitrogen pools following multiple applications of nitrogen fertilizer to a Pinus radiata stand over 12 years

Abstract

Around the world large tracts of forest, previously available for production, have been reserved for nature conservation. This means that wood supply must be met from a reduced land base, including land of low productivity. In addition there are likely to be increasing demands on the use of managed forests for sequestering C as one means of reducing the build up of atmospheric CO2. One way for the forest industry to meet the demands of increased production would be through the use of fertilizers. Substantially increased growth from fertilizer N application has been measured in many cases while, in the northern hemisphere, atmospheric N deposition has been associated with increased growth in some forests.

The possibility of using fertilizer N to increase growth, and the effect on the forest and soil, was studied in a research trial area in north-east Tasmania, Australia. Nitrogen was applied for 12 years to a 16 year old P. radiata stand in a low rainfall zone. Growth and foliar nutrient concentrations were measured to age 34 years, to determine change after attainment of steady state growth at age 25 years, and following cessation of fertilizing at age 29 years. Biomass sampling was carried out at ages 25 years, 29 years, and 34 years.

Growth at the steady peak rate achieved in the fertilizer plots, of 31 m3 ha-1 periodic annual increment, was accompanied by changes in the N nutrient pools. Surface applied N built up in the surface litter layer while this layer was increasing, from 15 t ha-1 to nearly 50 t ha-1, and fertilizer was being applied between ages 25 and 29 years. Decline of total N in the soil, between ages 25 and 29 years, indicated continued uptake from that source even though fertilizer was being applied. Subsequent increases in soil N, through leaching from the litter layer, followed cessation of fertilizing.

Following cessation of annual applications of N fertilizer, growth rates declined by less than 15% and remained substantially (>150%) above the control growth rates for 4 years. Over a 3 year period foliar N concentrations declined steadily, to the same as control plots, although mass of needles was not reduced 5 years after cessation of fertilizer applications and remained significantly above that of the control plots.

Without fertilizers, N for new foliage must be supplied from internal recycling and from mineralization of litter and soil organic matter. There were net losses of N from the foliage and the wood, estimated at around 12 kg N ha-1 yr-1, for the fertilized trees for the 5 years following cessation of fertilizing. This indicated retention and recycling within the tree of a considerable proportion of the 104 kg N ha-1 in the foliage, at age 29 years. As rates of mineralization of soil N were likely to be low, this retention of N within the tree was important in maintaining growth rates.

In the forest ecosystems investigated, with low initial content of soil organic matter and N, fertilizer N produced improved tree health and substantially increased growth, thus providing the opportunity to manage this forest for increased wood production or C sequestration. Although there was a substantial build up of N and C in the litter on the fertilized treatment, incorporation of N and C into the mineral soil was slow. Over 12 years 1.34 t N ha-1 had been applied, comparable to 40 years of atmospheric deposition at 30 kg N ha-1. There were no signs of detrimental effects from this application.