Signs of N Deficiency and N Excess
N deficiency and N excess are shown by the following characteristics:
• N deficiency: Yellowing, reduction of growth of shoots and leaves, high dry weight/leaf area, starch accumulation, premature ageing and shedding of leaves. Note: Yellowing of leaves is not very specific and is often also caused by deficiency of other nutrients and degradation of chlorophyll. The nitrogen cation imbalance determines deficiency and excess.
• N excess: High chlorophyll contents, high water content, low dry weight/area (shade leaf type), nitrate and amino acid storage, long in-ternodes.
Because of the rapid and substantial alterations in N supply with changing conditions in time and location it is very time- and resource-consuming to quantify the N supply. In contrast to nitrate, which is not bound to a soil exchanger and can thus be measured in soil solution, quantifying the exchangeable ammonium in litter is difficult. Ellenberg (1986) suggested, in order to characterise the supply of nitrogen at a particular site, that it would be possible to derive the supply from different sources from the species distribution, i.e. each species is classified by a relative scale, with a range of 1-5 to 1-9 of the so-called indicator values (Box 2.3.9). Conversely, it is possible to draw conclusions about the occurrence of a species from the supply of resources. However, this is only possible within the original areas where the plant occurred, as the interaction with other species changes the occurrence and thus the indicator value of a particular species (see Chap. 4).
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