Fate of urea and ureaformaldehyde nitrogen in a one-year laboratory incubation with Douglas-fir forest floor

In a 1 y laboratory incubation of a Douglas-fir forest floor (FH) the effects of two different kinds of organic N compounds, fast-release urea (U) and slow-release ureaformaldehyde (UF), on N transformations were studied. Compounds labelled with 15N were used to follow the mineralization and distribution of added N in the following pools: extractable NH4+N, (NO2− + NO3−)N, soluble organic N, microbial biomass N and total N in the soil residue. The effects of U and UF on microbial activity (CO2 production), microbial biomass (FE and SIR) and on the numbers of autotrophic nitrifiers (MPN) were also studied. The pattern of transformation of N was quite different. In the U-treated soils the added N contributed mostly to the exchangeable NH4+ pool, whereas in the UF-treated soils the highest amount of the added N was found in the soil residue. In the U-treated soils the amount of NH4+ was constant throughout the experiment, but the 15N in it was diluted by mineralization of native organic N. In teh UF-treated soils the accumulation of exchangeable NH4+ started slowly and increased steadily. However, the atom%15N excess in the NH4+ pool stayed constant, as it did in the soil residue. This unchanged 15N enrichment of NH4+ indicates formation of a UF-humic complex. Higher atom%15N excess in the UF-treated soils in the exchangeable NH4+ pool (2.8%) than in the soil residue (1.5%) suggests also that the UF-N entered the active organic N pool in the soil. The results presented here help to explain earlier field observations, where UF was shown to improve the N status of forest soil, and the applied N was retained in an available N pool.