Experimentelle Mo-Düngung eines sauren Waldbodens: Molybdän-Dynamik im Boden und Auswirkungen auf die Ernährung von Fichten

Anthropogenic immissions might cause Mo deficiency at acid forest sites. Thus, Mo fertilization could be necessary to compensate for the deposition effects. Because at date there is no experience concerning the Mo-dynamics in acid forest soils, risk assessment of Mo-fertilization is not yet possible. The aim of our study was to investigate the fate of Mo applied to forest sites and to test whether the Mo plant availability could be increased by Mo-fertilization of the soils. We applied 4 kg Mo ha^-1 to a 30 year old spruce stand on acid sandy soil located in the Black Forest (SW Germany). The Mo input into the mineral soil was determined at fertilized and untreated plots (8 x 10 m). During a period of 3 years we analysed the Mo contents of the soils, the Mo mobilization rate and the depth distribution of Mo. In addition, Mo and N-contents of the spruce needles were determined. Immediately after fertilization 50% of the applied Mo was leached from the organic layer into the mineral soil. Six months later Mo fluxes decreased and Mo contents in the O-layer remained constant. Leaching of Mo within the mineral soil was considerably fast, as compared to arable soils. One year after fertilization 75% of the Mo that had reached the mineral soil was leached deeper than 15 cm. Molybdenum remaining in the mineral soil is fixed by iron oxides, within the three years of observation the extractability of Mo by oxalate decreased from 65% to 20% of total Mo. Spruce needles sampled from the seventh whorl showed no fertilization effect. Our results show that the forest floor has a large Mo storing capacity. We conclude that the long-term efficiency of Mo-fertilization at forest sites mainly depends on plant availability of Mo bound in the O-layer. Plant availability of Mo transfered into the mineral soil is low, due to fixation to iron oxides.