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

Four sites, two of which are dominated by kauri poles and mio dominated by mature kauri trees, were investigated for total nitrogen content, nitrogen fixation, and nitrogen turnover. All sites have low foliage nitrogen concen tration, 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 con tains from 61 kg N ha(-1) in a young pole stand to 900 kg N ha(-1) in a mat ure stand. A large amount of nitrogen is contained in the forest floor, wit h up to 6.5 t N ha(-1) in the mature forest site. Mean residence time for n itrogen 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 free living heterotro phs 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 l inear relationship between both stem productivity and litter production and maximum foliage N in any stand and this has important implications for man agement of kauri forest.