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1970 1974 1978 1982 1986 1990 1994 1998 B Year

Fig. 3.5.4. Changes with time in deposition of A sulfur, B nitrogen, and C deposition of Ca, and leaching of Ca in the groundwater in a forest ecosystem in Soiling, Germany (after Ulrich 1994). Deposition of Ca in dust fell at the beginning of the 1970s; S deposition remained high till the 1980s. Calcium leaching into the groundwater decreased with reduction in S deposition and therefore contributed significantly to the regeneration of forest ecosystems. D Changes with time of the base cations (K, Mg, Ca) to Al ratios in the soil solution. In the 1970s, the cation/AI ratio was >1 due to mobilisation of cations by acid rain. After about 1976, cations were exhausted up to a depth of 90 cm in a soil. The soil was then buffered by Al. Due to cation transport from roots at deeper layers to leaves, followed by leaf fall, the cation/AI ratio increased in the upper surface layer but, despite the small increase, there is still a tendency of a decrease (data: Lower Saxony Forest Institute)

1970 1974 1978 1982 1986 1990 1994 1998 B Year

1970 1974 1978 1982 1986 1990 1994 1998 D Year

Fig. 3.5.4. Changes with time in deposition of A sulfur, B nitrogen, and C deposition of Ca, and leaching of Ca in the groundwater in a forest ecosystem in Soiling, Germany (after Ulrich 1994). Deposition of Ca in dust fell at the beginning of the 1970s; S deposition remained high till the 1980s. Calcium leaching into the groundwater decreased with reduction in S deposition and therefore contributed significantly to the regeneration of forest ecosystems. D Changes with time of the base cations (K, Mg, Ca) to Al ratios in the soil solution. In the 1970s, the cation/AI ratio was >1 due to mobilisation of cations by acid rain. After about 1976, cations were exhausted up to a depth of 90 cm in a soil. The soil was then buffered by Al. Due to cation transport from roots at deeper layers to leaves, followed by leaf fall, the cation/AI ratio increased in the upper surface layer but, despite the small increase, there is still a tendency of a decrease (data: Lower Saxony Forest Institute)

(Schulze 1989). Soil acidification strongly reduced the availability of magnesium (and calcium). This is not only caused by the reduced base saturation of the soil exchanger occurring simultaneously with acidification, but also by competitive inhibition of Mg uptake by ammonium. In addition to ammonifica-tion, ammonium in the soil originates from atmospheric deposition, particularly from animal husbandry. Ammonium causes release of Mg from the exchangers in the soil and stimulates release of Al. Finally, Lange et al. (1989) proved, in a very elegant experiment, that the interaction with growth causes the deficiency. Buds were removed from opposite lateral twigs on some spruce twigs or not removed on opposite twigs along the same branch. Only on that side with twigs of the branch where buds were not removed and where growth occurred was the damage observed (see Chap. 2.3). Obviously, growth of trees is significantly regulated by the N supply (Oren and Schulze 1989). Thus the uptake by the canopy of N from air-borne pollutants (in rain and dew) becomes particularly important, because this additional N supply is not balanced by cation uptake, but leads to increased growth and the observed yellowing. In the case of cation deficiency, growth is not regulated by availability of the limiting nutri-

S04 wet deposition in rain

S04 particles

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