Influence of Liming and Kiesèrite Fertilization on the Molybdenum Dynamics of Forest Sites | Einfluß von Kalkung und Kieserit-Düngung auf die Molybdän-Dynamik von Waldstandorten

Liming of forest soils induces nitrate mobilization. Plants can use nitrate only if there is a sufficient amount of molybdenum (Mo) to reduce the nitrate. Mo availability could therefore be crucial for tree nutrition, especially in limed forest stands.Very few studies deal with the Mo dynamics of forest sites and to our knowledge there are no experiments on the influence of liming on Mo availability in forest ecosystems, although liming effects are to be expected due to the pH dependency of Mo mobility and the increase in Mo demand in the presence of nitrate. In the present study the Mo dynamics were determined on three limed forest sites (15t dolomite per ha applied in 1988), one stocked with pinus sylvestris, the other two with picea abies. Another plot fertilized with (K,Mg)SO₄ (0.6t „Forstkieserit“ per ha applied in 1991) was also included in our investigation. The Mo contents of needles and topsoils (extracted with ascorbic acid, ammonium oxalate and an ion exchange resin) as well as the Mo fluxes from the organic layers into the mineral soil were determined.On all sites liming causes elevated Mo needle contents. Resin-extractable Mo contents indicate improved Mo plant availability as a consequence of increased pH values in the limed plots. This leads to increased Mo needle contents and Mo fluxes. Sulphate induced Mo mobilization in the organic layer, causing a raise in Mo contents in the mineral soil of the plots fertilized with kieserite. However, Mo mobility decreased following Forstkieserit fertilization due to Mo fixation. Together with Mo/S uptake antagonism this process reduces plant Mo availability.Our study shows that liming improves the Mo availability of forest soils provided the pH of the mineral soil increases. The Mo dynamics on the kieserite plots suggest that So₄ depositions may decrease Mo uptake in forest stands.