Long-term partitioning of ammonium and nitrate among different components in an alpine meadow ecosystem

The fate of N-15-labeled ammonium and nitrate was determined in a Kobresia pygaea C. B. Clarke meadow after 11-13 months following N-15 additions in order to understand the role of alpine meadows in retention of deposited nitrogen X. It was shown that the fate of NO3--N-15 or NH4+-N-15 was distinctly different. In 11-13 months after N-15 additions, total N-15 recovery of NO3--N-15 was 92.83%, 92.64% and 79.96% while N-15 recovery of NH4+-N-15 was 49.6%, 63.33% and 66.22%. Their different fates were obviously shown on partitioning of N-15 among different components in alpine meadows. After 11-13 months following N-15 additions, more NO3--N-15 was recovered in plants than in soil organic matter or in soil microorganisms. N-15 retained in soil organic matter was also high and increased with time. N-15 immobilized in soil microorganisms was close to that retained in soil organic matter within 11-12 months while it decreased significantly after 13 months. When NH4+-N-15 was added, more N-15 was retained in soil organic matter than in plants or in soil microorganisms. N-15 taken up by plants was constant while that immobilized in soil microorganisms altered greatly. Eleven to thirteen months after N-15 additions, plants and soil microorganisms took up more NO3--N-15 than NH4+-N-15 while soil organic matter recovered more NH4+-N-15 than NO3--N-15. It indicates that alpine meadows play different roles in retention of deposited NO3- and NH4+ after a long time