Changes in the structure and activity of a soil microbial community caused by inorganic nitrogen fertilization.

The changes in the structure and activity of a soil microbial community caused by addition of moderate andd high rates of the mineral nitrogen fertilizer (KNO3) were studied in a laboratory incubation experiment. The structure of the microbial community was evaluated from the phospholipid fatty acid (PLFA) profile; specific growth rate of the microorganisms was determined by the method of the kinetics of substrate-induced respiration; the total pool of microbial carbon was estimate by the fumigation-extraction method. The amounts of nitrogen fertilizer applied in three treatments of the experiment were 0 (contol), 100, and 2000 μg N/g soil. Even in the absence of assitional sources of organi carbon, a considerable portion of the added 15N (up to 74%) was immobilized. No significant inrease in the amount of microbial carbon was observed during incubation. The specific growth rate of the microbial community in soil supplemented with glucose deccreased twofold after addition of 2000 μg N/g soil. In this treatment, the ratio of cyclic fatty acids to their monoenoic precursors also increased, indicating the adaptation of microbial cells to extremely high amounts of nitrogfen fertilizer. Moreover, consierable changes in the structure of the soil microbial community, such as an increase in the ratio of fungal- to bacterial markers and a decrease in the ratio between PLFA of gram-positive and gram-negative bacteria, were observed in the treatment with addition of 2000 μg N/g soil. Our data clearly indicate that mineral nitrogen fertilization of soil under carbon limitation has a pronounced impact on the structure and activity of soil microbial communities