Different fates and retention of deposited NH4+ and NO3− in an alpine grassland in northwest China: A 15N tracer study

The fates and redistribution of deposited nitrogen (N) determine the influence on N cycling in terrestrial ecosystems. However, the different fates between deposited nitrate nitrogen (NO3−) and ammonium nitrogen (NH4+) as well as the amount of deposited N retained in ecosystems are still not fully understood. We conducted a field experiment to investigate the fates of deposited NO3− and NH4+ by application of stable 15N isotopes over a period of 455 days in an alpine grassland ecosystem and determined the preference of NO3− and NH4+ for different types of plants. Our research showed that at 15 and 455 days after tracer labeling, the total 15N recovery of NO3− and NH4+ declined from 48.48% to 21.99% and 49.31–7.87%, respectively, indicating that the long-term NH4+-N retention capacity of alpine grassland was lower than NO3−-N. This condition indicated that the changes in deposited N composition can affect the N cycling in ecosystems. Most of N in 15N-NO3− and 15N-NH4+ were immobilized in the litter layer from 22.08% to 9.02% and 36.70–4.53%, respectively, suggesting that the litter layer was the major N sink of NH4+and NO3−. In addition, 19.13% and 2.81% of added 15N-NO3− were recovered in plant, which was twice more than those of 15N-NH4+ (8.43% and 1.42%), indicating that the preference of plants in our study area was NO3−-N. Our results demonstrated the great difference of dynamics in the fate and redistribution of deposition N-NO3− and N-NH4+, about half of atmospherically deposited N-NO3− and N-NH4+ were lost in the initial period, and that mere portion retained within the ecosystem consistently decreased, especially for N-NH4+. Our results also show that the N retention capacity of alpine grassland is relatively weak, resulting in a positively but limited effectiveness in growth and carbon sequestration enhancement. These results can help to assess the ecological effects of N deposition.