Ecophysiology of the internal cycling of nitrogen for tree growth

Internal cycling of nitrogen has been shown to be a major source of nitrogen used for the seasonal growth of both evergreen and deciduous trees providing up to 90% of N used for leaf growth of some species. The processes of internal cycling comprise seasonal nitrogen storage, followed by remobilisation during either periods of growth (e.g. in the spring) or during leaf senescence. The ecophysiology of these processes is reviewed, along with the methods used to quantify their contribution to tree growth. Nitrogen budget studies have been widely used to estimate internal cycling, particularly in relation to soil fertility. These studies have shown that as trees develop their rate of N uptake decreases, but as they grow their storage capacity increases, However, budget studies are imprecise and have not always quantified remobilisation adequately. An alternative approach has been the use of 15 N to quantify N uptake and partitioning, allowing precise measurements of N storage and remobilisation to be made. The use of isotopes has allowed experiments to be run which have shown that environmental factors such as soil fertility influence the amount of N stored, but have no direct influence upon the amount of N remobilised. These methods are discussed in light of recent research on N remobilisation, which has provided an understanding of the processes of storage and remobilisation which potentially allows direct measurements to be made in field grown trees for the first time.