Microbial responses to nitrogen additions in alpine tundra soil

Soil nitrogen transformations were measured the year following nitrogen fertilization of alpine Kobresia myosuroides meadows to determine the influence of greater plant production and N content on net N mineralization and the microbial N pool. Previously fertilized soils contained substantially greater amounts of organic N than control soils. The average increase in soil organic N accounted for 75% of total added N and, although variable, this quantity suggests a large capacity for retention of added N in these soils. Nitrogen transformations and more active pools also responded to fertilization. Net N mineralization, nitrification and soil inorganic N concentrations clearly were higher in fertilized than in control plots throughout the snow-free season. The most pronounced increase in mineralization in fertilized relative to control soils occurred during the second half of the snow-free period (mid-July-October), primarily after the short alpine growing season (June-mid-August). Although the soil microbial N pool was not affected by fertilization during the growing season, microbial N did increase in the fall in fertilized compared to control soils, coinciding with the time of greatest N mineralization. The late season net uptake of N into the microbial pool exceeded by several times present rates of anthropogenic N inputs to these soils, and the microbial biomass may act as an increasingly important short-term sink for available N if N deposition to these areas continues to increase.