Plantand Soil 168-169: 75-81, 1995. © 1995 Kluwer AcademicPublishers.Printed in the Netherlands. Effects of forest fertilization on nitrogen leaching and soil microbial properties in the Northern Calcareous Alps of Austria H. Insam and A. Paloj~vi Department of Microbiology, University of lnnsbruck, A-6020 Innsbruck, Austria and GSF-lnstitute of Soil Ecology, D-85758 Oberschleissheim, Germany Key words: forest fertilization, lysimeter, microbial biomass, mineralization, nitrogen leaching, slow-release fertilizer, soil Abstract Several boreal and alpine forests are depleted in nutrients due to acidification. Fertilization may be a remedy, but rapidly-soluble salts (N, P, K, Mg) may pose nitrate problems for the groundwater or decrease microbial activity. With the aim to investigate potential nitrogen leaching after fertilization we set up an experiment employing intact soil cores (11 cm diameter, 20--40 cm long) from a mixed forest and a Picea abies stand (soil type Rendsina) in the Northern Calcareous Alps of Austria. The cores were fertilized with a commercial NPK fertilizer or a methylene-urea-apatite-biotite (MuAB) fertilizer at a rate corresponding to 300 kg N ha-l and incubated for 28 weeks together with unfertilized controls. Both soil water (retrieved 5 cm below the soil surface) and leachate were analyzed for nitrate and ammonium in regular intervals. After the incubation, soil microbial biomass and basal repiration were determined and a nitrogen mineralization assay was performed. For the control, in the soil water and leachate maximum NH + and NO~- concentrations of 5 and 11 mg N L- 1, respectively, were found. Compared to that, MuAB fertilizer resulted in a slow increase of NH + and NO 3 in the soil water (up to 11 and 35 mg N L-1 respectively) and in the leachate (4 mg NH+-N L-l and 44 mg NO 3 -N L- 1). Highest nitrogen loads were found for the fast release NPK fertilizer, with NH + and NO~- concentrations up to 170 and 270 mg N L -1 , respectively, in the soil water. NH + -N levels in the leachate remained below 5, while NO3-N levels were up to 190 mg L -1. Fast- release NPK caused a significant decrease of microbial biomass and basal respiration. These parameters were not affected by MuAB fertilizer. The results suggest that the MuAB fertilizer may be an ecologically appropriate alternative to fast-release mineral fertilizers for improving forest soils. Introduction Anthropogenic influences may lead both to an increase or a decrease of soil nutrient contents. On the one hand, atmospheric depositions are considered a possible dan- ger for forest ecosystems through acidification. This may lead to cation losses and N saturation (Bredemeier and Ulrich, 1992). On the other hand,in some alpine forests the site history (litter harvesting) or present- day practices like cattle grazing and full-tree harvest are responsible for nitrogen exports (Glatzel, 1991). Together with increased leaching and erosion this may lead to seasonal N deficits (Liu, pers. commun.). Fertilization of such stands is considered a pos- sible remedy but some rapid soluble N forms are likely to cause nitrate problems for the groundwater (Katzensteiner et al., 1995). Since many alpine forests are water supply areas this deserves special attention. Fertilization with slow-release N fertilizers has been shown to increase net mineralization and N availabili- ty without losses through nitrification and leaching in a number of investigations both in boreal and alpine forests (Katzensteiner et al., 1995; Martikainen et al., 1989). Aarnio et al. (1993) also found long-lasting positive effects on tree growth of slow-release nitro- gen fertilizer. Besides N, other nutrients often are deficient in alpine forests. However, fertilization with rapidly sol- uble salts of P, K and Mg have been shown to decrease microbial activities (S6derstr6m et al., 1983; Van