Nitrogen retention patterns and their controlling factors in an alpine meadow: implications for carbon sequestration

Abstract. We hypothesized that the patterns of NO₃⁻ and NH₄⁺ 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.