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We hypothesized that the patterns of NO-3 and NH+4 retention are different over short-term scales while they are similar over long-term scales in alpine meadows and that abiotic and biotic factors might be responsible for their different patterns over short-term scales. In order to test the hypotheses, a 15N-labeled experiment was conducted in an alpine meadow in the Qinghai-Tibet Plateau over four years. Our results showed that 15NO3- and 15NH4+ retention was distinctly different within two months, and even one year after tracer additions. The long-term retention of 15N at the whole-plot level did not differ significantly between 15NH4+ and 15NO3- treatments, and averaged 50% after four years. Higher soil temperature or soil organic carbon concentration enhanced 15NH4+ retention, but significantly reduced 15NO3- retention in the soil within two months following tracer additions. Soil moisture significantly affected 15N recovered in soil organic matter and microbial biomass as well as aboveground parts, but had no significant effects on 15N recovered in roots. These findings have important ecological implications with regard to the consequences of deposited nitrogen because of the possible difference in the fate of NH4+ vs. NO3 - in alpine meadow ecosystems. [ABSTRACT FROM AUTHOR]