Nitrogen immobilization by soil microorganisms depending on nitrogen application rates

Nitrogen immobilization in gray forest soil sampled from under a forest and from a cropland was studied in an incubation experiment with different rates of mineral N-15 fertilizer (from 50 to 2000 mg N/kg soil). The rates of N immobilization reached 143 mg N/kg of soil per day, and the portion of immobilized N was up to 72% even under the highest rates of mineral nitrogen application. The intensity of N immobilization in the soil sampled under the forest was much higher than that in the arable soil. The nitrogen immobilization was not accompanied by growth of the soil microbial biomass.