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J Integr Plant Biol, 2004, 46 (1): 20-28, Research Article
N2O Exchange Within a Soil and Atmosphere Profile in Alpine Grasslands on the Qinghai-Xizang Plateau
PEI Zhi-Yong, OUYANG Hua, ZHOU Cai-Ping, XU Xing-Liang
doi:
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.

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