As a rule, N-supply of mature Ardenne forest ecosystems is satisfactory. Mineralization rates of soil organic matter are generally high and nitrogen is not a frequent nor an important growth limiting factor. Light N-deficiency is likely to depend on unsatisfactory root absorbing power in very acid soils with dysmoder humus. For other major elements, especially for Mg, Ca and P, near optimal nutrition is rarely observed.During the late sixties, fertilizer experiments have shown that nutrition equilibrium of stands growing on acid soils, poor in exchangeable Mg, is very sensitive to artificial NH4-addition. Mg-deficiency symptoms have been induced. The present continuing atmospheric NH 4 input is believed to produce similar but lasting nutritional stress which might be accentuated by additional acidity generated during nitrification of excess NH 4. Evolving Mg deficiency leads to trees' death and developing forest decline is likely to enhance N-output under NO3-form.Awaiting adequate air purification, fertilizing should restore nutrition equilibrium in order to save damaged stands, to slow down soil acidification and to incorporate excess atmospheric N-input into improved biomass production.
Field observations demonstrated inhibition of root growth by moder humus material of the Of 2 and Oh-layers under beech and spruce. Growth chamber experiments with spruce seedlings showed that root growth on a natural Of2-substrate is ten times lower than on resin-quartz mixtures. Added phenolic acid (protocatechic a.) inhibited root growth only in sterilized substrate. Leachates of Of 2-material inhibited root growth in a mineral substrate devoid of absorption complex (pure quartz sand), but not in a resin-quartz mixture. It is concluded that inhibiting substances are probably ionized molecules that can withstand biodegradation.
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