Abstract:

Knowledge of nitrous oxide (N₂O) exchanges through soils and atmosphere in various ecosystems has been of great importance in global climate change studies. However, the relative magnitude of surface and subsurface N₂O production sources from the alpine grassland ecosystem is unclear. In the present study, the N₂O concentration profile from 1.5 m in depth in soil to 32 m in height in air was measured from July 2000 to July 2001 in alpine grassland located in the permafrost area of the Qinghai-Xizang Plateau, which revealed that N₂O concentrations had a distinct variation pattern both in air and in soil during the study period. Mean N₂O concentrations in the atmosphere were significantly lower than those in the soil, which induced the N₂O emission from the alpine steppe soil into the atmosphere. Mean flux of N₂O in this alpine grassland experiment site was 0.05×10^-4 mmol·m^-2·s^-1. But the variation in N₂O emissions did not show any clear trends over the whole-year experiment in our study site. The highest N₂O concentration was found at the depth of 1.5 m in the soil while the lowest N₂O concentration occurred at the height of 8 m in the atmosphere. Mean N₂O concentrations in the soil increased significantly with depth. This was the influence of increasing soil moistures, which induced the increasing denitrification potential with depth. The mean N₂O concentrations at different heights in the air remained a more steady state because of the atmospheric negotiability. Seasonal variations of N₂O concentrations showed significant correlations between the neighbor layers both in the soil and in the atmosphere. The seasonal variations of N₂O concentrations at all horizons in the soil showed very clear patterns, with the highest concentrations occurring from the onset of frost to the freeze-thaw period and lowest concentrations occurring during the spring and the summer. Further analyses showed that the seasonal variations of N₂O concentrations in the soil were hardly explained by soil temperatures at any depth. Temporally, atmospheric N₂O concentrations at all heights exhibited almost the same seasonal pattern with the soil N₂O variations, while soil is believed to be the predominant natural source of atmospheric N₂O near the earth surface in this alpine grassland area. Also, a significant correlation was found between N₂O emissions and soil N₂O concentrations at 0.2 m in depth during the study period. This implied the variation of N₂O concentrations in the soil surface horizon was the most direct driving force of N₂O exchanges between the soil and the atmosphere. Soil atmospheric N₂O at surface layers is the main source of N₂O emissions from the soil surface to the atmosphere. Soil N₂O concentrations at deeper layers were all significantly higher than those at surface layers, which indicated that N₂O was diffused from the deeper layers to the surface layers in the soil, and finally was emitted to the atmosphere.

Key words: N2O, soil, air, alpine grassland, Qinghai-Xizang Plateau