| Although fertilizers are commonly used to increase above ground forest productivity, their effects on below ground microbial biomass and microbially mediated processes have not been studied in depth. The existing information on such effects are conflicting and variable. Therefore, laboratory and field studies were conducted to examine the effects of urea, ammonium nitrate (AN) and triple super phosphate (TSP) on microbial respiration, biomass and several microbially mediated processes.;In the laboratory study (120 day), the addition of AN singly or in combination with TSP to pine FH material decreased microbial respiration, biomass and aspen litter decomposition. Significant increases in microbial respiration were observed following urea addition, with or without TSP, up to 40 days but decreases were observed thereafter. Addition of urea, with or without TSP caused a significant decrease in microbial biomass after 20 days following addition and increased litter decomposition after 20 and 40 days. Inhibitory effects of TSP on microbial respiration and biomass were observed only when added singly and no effect was found on litter decomposition.;Studies on the single and combined effects of NH;When fertilizer effects were studied in the field, no effects of either urea or AN were found on microbial respiration or biomass whereas a positive effect of TSP was found on microbial biomass. Since a significant increase in forest floor organic matter contents were found in the TSP fertilized plots, the enhancement seen in microbial biomass probably was due to this. Fertilization increased net mineralization of N and P. Litter decomposition rates were enhanced with N fertilization and no effects were found on nitrification rates. The fertilizer rates used in the present investigation appeared to have enhancing effects on the biological activities of the forest soil under field conditions. |
