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 and 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 estimated by the fumigation-extraction method. The amounts of nitrogen fertilizer applied in three treatments of the experiment were 0 (control), 100, and 2000 mu g N/g soil. Even in the absence of additional sources of organic carbon, a considerable portion of the added N-15 (up to 74%) was immobilized. No significant increase in the amount of microbial carbon was observed during incubation. The specific growth rate of the microbial community in soil supplemented with glucose decreased twofold after addition of 2000 mu g 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 nitrogen fertilizer. Moreover, considerable changes in the structure of the soil microbial community, such as an increase in the ratio of fungalto 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 mu g 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.