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

The effects of fast-release urea (U), slow-release ureaformaldehyde (Nitroform, NF) and a mixture of the two in different proportions (NF100%, NF80%, NF50%, 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 CO2 production did not increase. The concentration of exchangeable NH4+ 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 NF100-treated soils. The low amount of NO3− 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 NH4+. 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.