Nutrient relations in healthy and damaged stands of mature oaks on clayey soils: two case studies in northwestern Germany

Abstract

At two sites in northwestern Germany, which represent the centres of the present oak damage, the nutrient relations in mature oak stands were investigated (site Neuenburg, Quercus robur L.; site Lappwald, Q. petraea [Matt.] Liebl.). In one healthy and one adjacent damaged stand at both sites, the following parameters were determined: element concentrations of soil (including soil solutions) and leaves, element deposition and output, and proton loading and buffering (from input–output budgets). At Neuenburg, leaf Mg concentrations of oaks in the damaged stand 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 ratios in those leaves. NH + 4 deposition exceeded NO − 3 deposition. In the damaged stand, H + from N transformation contributed with 30% to proton loading (as opposed to only 15% in the healthy stand). At Lappwald, where NH + 4 and NO − 3 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 damaged stand. Leaf P concentrations were low, and N/P ratios were elevated. Almost all of the proton loading was generated internally without any H + contribution from N transformation. In none of the stands, indications of soil chemical stress as a cause for oak damage were detected. In all stands investigated, proton buffering was implemented mainly by the release of `base' cations, with Mg 2+ being the predominant cation species. All stands were sinks for N, and nearly all the N taken up by the trees was assimilated. At Neuenburg, the substantial deposition of NH + 4 led, probably due to the combined effect of increased nitrification and reduced N uptake capacity in the damaged stand, to increased NO − 3 output, coupled with the loss of considerable quantities of Mg 2+. However, this seems to be a consequence rather than the cause of oak damage in northwestern Germany.