Pergamon SoifBiol. Bioehem. Vol. 27, No. 10, 1325-1331, pp. 1995 003fho717(~ Copyright 0 1995 Elscvier Science Ltd Printed in Great Britain. All rights reserved 0038-0717/95 $9.50 + 0.00 MINERALIZATION OF C AND N AND NITRIFICATION IN SCOTS PINE FOREST SOIL TREATED WITH NITROGEN FERTILIZERS CONTAINING DIFFERENT PROPORTIONS OF UREA AND ITS SLOW-RELEASING DERIVATIVE, UREAFORMALDEHYDE T. AARNIO’* and P. J. MARTIKAINEN2 ‘Department of General Microbiology, University of Helsinki, P.O. Box 41, FIN-00014 Helsinki and 2National Public Health Institute, Department of Environmental Microbiology, P.O. Box 95, FIN-70701 Kuopio, Finland (Accepted 24 March 1995) Summary-The effects of fast-release urea (U), slow-release ureaformaldehyde (Nitroform, NF) and a mixture of the two in different proportions (NFlOO%, NF80%, NFSO%, NF25%) on soil microbial activities were studied. Urea fertilization enhances nitrification which can cause nitrogen leaching. Our aim was to find whether it is possible to avoid unwanted effects of urea by using mixtures of organic N compounds of different solubility. Net nitrification and mineralization of N and C were determined during aerobic laboratory incubation of soil samples taken 3 months after fertilization. The numbers of autotrophic nitrifiers were estimated by a Most Probable Number (MPN) method and the number of heterotrophic bacteria by plate count. Ion-exchange resin bags placed in the soil below the organic horizon were used to estimate movements of nutrients in soil. The soil pH and the number of heterotrophic bacteria increased in soils receiving high amounts of fast-release N, but CO, production did not increase. The concentration of exchangeable NH: and the number of ammonium oxidizers in the soil were related to the proportion of urea in the applied mixtures; highest numbers were found in the urea and lowest numbers in the NFlOO-treated soils. The low amount of NO, found in the soil and accumulated in the resins indicates low nitrification activity in this site. However, the number of ammonium oxidizers indicates that the increase in nitrifier population correlates with the release rate of NH,+. The slow-release NF did not increase nitrification activity and it had a positive effect on microbial activity even at high doses, hence it seems a possible alternative as a slow-release N fertilizer in forestry. INTRODUCTION In boreal coniferous forests large amounts of nitro- gen are immobilized, but only about 1% of the total N is in an inorganic form (NBmmik, 1982). Fertiliza- tion is used in order to increase the amount of available N for stand growth. However, there is increasing concern about the harmful environmental effects of traditional fertilizers, such as urea, which are prone to leaching and volatilization. Slow-release fertilizers may be an alternative as suggested by Mahendrappa and Salonius (1974) over 20 years ago. Gradual release of N improves the synchrony be- tween availability and uptake by plants and thus diminishes the loss of N. A slow-release organic-N fertilizer Nitroform (NF, ureaformaldehyde) has mainly been used on golfing greens, lawns and in greenhouses (Kaempffe and Lunt, 1967; Alexander and Helm, 1990). Nitrogen release from NF has been shown to be sometimes even too slow for greens (Will and Relger, 1985). However, it has also been shown that when the *Author for correspondence. proportion of slow-release ureaformaldehyde in the N fertilizer is increased, uptake of N increases and losses of N from the agricultural soil decreases (Carter et al., 1986). It has been observed that NF does not enhance nitrification (Martikainen, 1984, 1985) or depress mineralization (Martikainen et al., 1989). In acid forest soil, NF has not been seen to cause a long-term decrease in CO2 production as has urea (Martikainen et al., 1989). Two years after fertilization Martikainen (1984) observed a much higher accumulation of NH: during laboratory incubation, but no nitrate for- mation, in the NF-treated soils than in the urea- treated soils. The positive effect of NF on the availability of N in the soil was also seen 7 and 14 years after application while there still was no for- mation of nitrate, which indicates that the loss of N from NF-treated forest soils is low (Martikainen et al., 1989). Furthermore, in a laboratory study, it was seen that even quite high NF concentrations do not interfere with the ability of mycorrhizal fungi to infect pine seedlings @en, 1990). Our study was directed towards finding how much of the slow-release NF nitrogen can be substituted 1325