NUTRIENT RELATIONS IN HEALTHY AND DAMAGED STANDS OF MATURE OAKS ON CLAYEY SOILS - 2 CASE-STUDIES IN NORTHWESTERN GERMANY

At two sites in northwestern Germany, which represent the centres of t he present oak damage, the nutrient relations in mature oak stands wer e investigated (site Neuenburg, Quercus robur L.; site Lappwald, Q. pe traea [Matt.] Liebl.). In one healthy and one adjacent damaged stand a t both sites, the following parameters were determined: element concen trations of soil (including soil solutions) and leaves, element deposi tion and output, and proton loading and buffering (from input-output b udgets). At Neuenburg, leaf Mg concentrations of oaks in the damaged s tand were at the threshold of deficiency, although Mg reserves in the soil, in contrast to the healthy stand, were high. This, together with the generally high leaf N concentrations, resulted in elevated N/Mg r atios in those leaves. NH4+ deposition exceeded NO3- deposition. In th e damaged stand, H+ from N transformation contributed with 30% to prot on loading (as opposed to only 15% in the healthy stand). At Lappwald, where NH4+ and NO3- were deposited at nearly the same rates, nutrient pools were moderate to high, and significant differences in soil and plant parameters could hardly be found between the healthy and the dam aged stand. Leaf P concentrations were low, and N/P ratios were elevat ed. Almost all of the proton loading was generated internally without any H+ contribution from N transformation. In none of the stands, indi cations of soil chemical stress as a cause for oak damage were detecte d. In all stands investigated, proton buffering was implemented mainly by the release of 'base' cations, with Mg2+ being the predominant cat ion species. All stands were sinks for N, and nearly all the N taken u p by the trees was assimilated. At Neuenburg, the substantial depositi on of NH4+ led, probably due to the combined effect of increased nitri fication and reduced N uptake capacity in the damaged stand, to increa sed NO3- output, coupled with the loss of considerable quantities of M g2+. However, this seems to be a consequence rather than the cause of oak damage in northwestern Germany. (C) 1998 Elsevier Science B.V.