Ecophysiology of the internal cycling of nitrogen for tree growth

Abstract

Internal cycling of nitrogen has been shown to be a major source of nitrogen used for the seasonal growrh 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, panicularly in relation to soil fertility. These studies have shown that as trees develop their rate of N uptlke decreases, but as they grow their storage capacity incnsxs. However, budget studies are imprecise and have not always quantified rcmobilisation adequately. An alternative approach has been the use of I5N 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 6uch as soil fertility influence the amount of N stored, but have no direct influence upon the amount of N nmobiliscd. These methods are &scussed 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 p w n trees for tht first time.

Use of stable isotopes to quantify nitrogen, potassium and magnesium dynamics in young Scots pine (Pinus sylvestris)

“…Millard (1996) reported that 18-93% of the nitrogen *Author for correspondence (tel j44 1224 318611 ; fax j44 1224 311556 ; e-mail m.proe!mluri.sari.ac.uk).…”

“…Studies of remobilization in trees have often suffered from flawed or intractable methodologies (Millard, 1996). Deriving whole-tree nutrient budgets is time-consuming, and such budgets are often difficult to close because of problems associated with the recovery of root systems.…”

Soluble N compounds in trees exposed to high loads of N: a comparison between the roots of Norway spruce (Picea abies) and beech (Fagus sylvatica) trees grown under field conditions

“…Therefore, the differences in TSNN contents in root xylem and phloem (Figs 2, 3) between spruce and beech during the development of new leaf tissues may be attributed to differences in N mobilization. Since N is stored in conifers mainly in last year's needles (Nambiar & Fife, 1991 ;Millard & Proe, 1993 ;Millard, 1996), the roots might not contribute as much to the transport of mobilized N to the new developing leaves during bud break as in deciduous trees where N is stored in the wood and bark of the trunk and the roots (Millard, 1996). Therefore, the high amount of TSNN in the transport systems of underground parts of beech during bud break might originate from N mobilized from storage tissues, whereas the lower amounts in pruce indicate that root-borne N is less important for new developing needles.…”

“…4). In spring during bud break, N uptake by roots is supposed to be negligible compared with mobilization of N from storage tissues (Millard, 1996 ;Geßler et al, 1998). The uptake of inorganic N and subsequent assimilation of Asn and Arg in the roots might be important after N pools of the storage tissues are exhausted.…”

The Effect of Weed Competition on Apple Fruit Quality

“…The practices increasing cell number in fruit may promote firmness. During the cell division period, the most notably determining factor is the N content of trees accumulated in the previous year (Millard, 1996;Neilsen et al, 2001).…”