The biology of kauri (Agathis australis) in New Zealand 11. Nitrogen cycling in four kauri forest remnants

Abstract

Four sites, two of which are dominated by kauri poles and two dominated by mature kauri trees, were investigated for total nitrogen content, nitrogen fixation, and nitrogen turnover. All sites have low foliage nitrogen concentration, and in most cases this is below the critical deficiency level of 0.81–0.92% N. Leaves may be retained on trees for up to 12 years and contain decreasing amounts of nitrogen as they age. Above-ground kauri biomass contains &om 61 kg N ha-1 in a young pole stand to 900 kg N ha-1 in a mature stand. A large amount of nitrogen is contained in the forest floor, with up to 6.5 t N ha-1 in the mature forest site. Mean residence time for nitrogen ranges from 9 to 192 years. Total ecosystem nitrogen of as much as 14 t ha-1 was recorded in the oldest site, representing an extreme level of N storage and immobilisation. Nitrogen fixation by he living heterotrophs in decaying litter can account for at least 5 kg N ha-1 yr-1, which, when integrated over the life of the forest, accounts for the large build-up in ecosystem N. Nitrogen use efficiency averages 155 g dry matter per g N, which is almost twice that of other forests reported. Overall N-cycling models are produced for the two mature kauri sites and show that only 0.3% and 0.7% of total ecosystem N is mobilised in any year. There is a close linear relationship between both stem productivity and litter production and maximum foliage N in any stand and this has important implications for management of kauri forest. © 2000 Taylor & Francis Group, LLC.