MINERALIZATION OF C AND N AND NITRIFICATION IN SCOTS PINE FOREST SOILTREATED WITH NITROGEN FERTILIZERS CONTAINING DIFFERENT PROPORTIONS OFUREA AND ITS SLOW-RELEASING DERIVATIVE, UREAFORMALDEHYDE

The effects of fast-release urea (U), slow-release ureaformaldehyde (N itroform, NF) and a mixture of the two in different proportions (NF100 %, NF80%, NF50%, NF25%) on soil microbial activities were studied. Ure a fertilization enhances nitrification which can cause nitrogen leachi ng. Our aim was to find whether it is possible to avoid unwanted effec ts of urea by using mixtures of organic N compounds of different solub ility. Net nitrification and mineralization of N and C were determined during aerobic laboratory incubation of soil samples taken 3 months a fter fertilization. The numbers of autotrophic nitrifiers were estimat ed by a Most probable Number (MPN) method and the number of heterotrop hic bacteria by plate count. Ion-exchange resin bags placed in the soi l below the organic horizon were used to estimate movements of nutrien ts in soil. The soil pH and the number of heterotrophic bacteria incre ased in soils receiving high amounts of fast-release N, but CO2 produc tion did not increase. The concentration of exchangeable NH4+ and the number of ammonium oxidizers in the soil were related to the proportio n of urea in the applied mixtures; highest numbers were found in the u rea and lowest numbers in the NF100-treated soils. The low amount of N O3- found in the soil and accumulated in the resins indicates low nitr ification activity in this site. However, the number of ammonium oxidi zers indicates that the increase in nitrifier population correlates wi th the release rate of NH4+. The slow-release NF did not increase nitr ification activity and it had a positive effect on microbial activity even at high doses, hence it seems a possible alternative as a slow-re lease N fertilizer in forestry.