Plant and Soil 252: 227–239, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 227 Long-term availability of nutients in forest soil derived from fast- and slow-release fertilizers T. Aarnio 1,3 , M. Räty 1 & P.J. Martikainen 2 1 Vantaa Research Centre, Finnish Forest Research Institute, P.O.Box 18, FIN-01301 Vantaa, Finland. 2 Department of Environmental Sciences, University of Kuopio, P.O.Box 1627, Bioteknia 2, FIN-70211 Kuopio, Finland. 3 Corresponding author ∗ Received 7 March 2002. Accepted in revised form 26 November 2002 Key words: apatite, biotite, forest soil, long-term nutrient availability, mineralization Abstract The availability of P, K and Mg was studied in boreal forest soil treated 10 years earlier with slow- and fast-release fertilizers. Fast release superphosphate, potassium chloride and magnesium sulphate and slow-release apatite (P) and biotite (K, Mg) were applied alone or together with urea or urea+limestone. The concentrations of total and exchangeable nutrients in the organic horizon and the concentration of exchangeable nutrients in the uppermost mineral horizon were measured. CO 2 production during aerobic laboratory incubation was used to estimate the microbial activity and substrate-induced respiration to determine the microbial biomass C in soil. Biotite caused a moderate but persistent increase in pH in the organic horizon, but this increase was smaller than with lime. The fast-release fertilizers had no effects on the nutrient status of the soil 10 years after the fertilization. However, apatite and biotite still increased the total content of Mg, K and P and the concentrations of exchangeable Mg and soluble P in soil. On the other hand, simultaneous addition of lime and biotite reduced the release of soluble P from apatite. The reduction in soil microbial activity found with urea and the fast-release salts soon after application was no longer evident 10 years later. There was no increase in nitrification in the fertilized soils, not even with the urea+lime treatment. The previous results right after the application and the results presented here do not indicate major leaching of nutrients from the slow-release fertilizers to the deeper soil profiles. Introduction The fertility of forest soils, and hence tree growth, is constantly affected by human activities. Acidification of soils and subsequent cation leaching due to high at- mospheric deposition can lead to nutrient imbalances (Alewell et al., 2000; Krajick, 2001; Schulze, 1989; Thimonier et al., 2000). Deficiencies of magnesium and potassium have been shown to be the main cause of damage in nitrogen-induced disturbances of tree growth (Hüttl, 1990; Van den Driessche and Ponsford, 1995), and deficiency of phosphorus due to depos- ition of atmospheric nitrogen has also been reported (Houdijk and Roelofs, 1993; Stevens et al., 1993). ∗ FAX No: 358 9 857 2575. E-mail: Tuula.Aarnio@metla.fi Calcium depletion in soil can lead to reduced growth, especially in areas where the rate of weathering from primary minerals is insufficient to compensate for the ion loss (Huntington et al., 2000). One of the major environmental challenges is how to sustain nutrient status and vitality of forests in areas with high at- mospheric deposition. Furthermore, reclamation of large areas of destroyed land, such as mine spoils or areas deprived of nutrients due to intensive contamin- ation derived from industry, places great demands on strategies for restoring and maintaining nutrients for new vegetation. Use of fast-release salts for amelioration of nu- trient deficiency provides only a short-term solution while simultaneously increasing the risk of nutri- ent leaching (Aarnio et al., 1995; Ingerslev, 1997; Matzner et al., 1985). In some cases, fast-release salts