Effects of nitrogen fertilization on the activity and biomass of fungi and bacteria in a podzolic soil

The activity and biomass of soil microorganisms were studied in a nitrogen-fertilized Scots pine forest moor in Sweden. The investigation was made five years after the application of 150, 300 and 600 kg N ha⁻¹ of each of urea and ammonium nitrate, respectively. Soil respiration rate and FDA-active fungal biomass were significantly lower in fertilized plots, the decrease in these levels being proportional to the amount applied. A tendency toward lower bacterial biomass after fertilization was also found. Similar results were obtained both in the organic and the mineral soil. There were no significant differences between the two types of fertilizer. Possible explanations for the results are discussed.