Fate of N-15 labeled nitrate and ammonium salts added to an alpine meadow in the Qinghai-Xizang Plateau, China

To understand the dynamics of added nitrogen (N) in alpine meadow and the role of alpine plants and soil microorganisms in the retention of deposited N, the fate of N-15 labeled nitrate and ammonium salts was determined in an alpine meadow for two months. Two weeks after N-15 application, total recovery of N-15 from NO3--N-15 was 73.5% while it was 78% from NH4+-N-15. More N-15 was recovered in plants than in soil organic matter or in microbial biomass, irrespective of forms of N added. After one month, 70.6% of added NO3--N-15 and 57.4% of NH4+-N-15 were recovered in soils and plants. N-15 recovered in soil organic matter decreased greatly while that recovered in plants varied little, irrespective of the form N. Compared with the results of two weeks after N-15 application, more NO3--N-15 than NH4+-N-15 was recovered in microbial biomass. Total recovery was 58.4% (six weeks) and 67% (eight weeks) from NO3--N-15, and 43.1% and 49% from NH4+-N-15, respectively. Both plants and soil microorganism recovered more NO3--N-15 than NH4+-N-15. But plants recovered more N-15 than soil microorganisms. During the whole experiment plants retained more NO3--N and N-15 than soil microorganisms while N-15 recovered in inorganic N pool did not exceed 1% due to lower amount of inorganic N. This indicates that plants play more important roles in the retention of deposited N although microbial biomass can be an important sink for deposited N in early days after N application