青藏高原高寒草原近地表圈层N2O梯度及交换特征

深入了解N2O在不同生态系统土壤及大气中产生和交换特征对于全球气候变化研究具有重要意义.本研究重点探讨N2O在高寒草原近地表圈层中的产生及迁移过程机制.于2000年7月至2001年7月在青藏高原高寒草原地区从土壤1.5 m深到大气中32 m高度10个层次梯度进行N2O浓度变化的观测.结果显示,土壤和大气中N2O浓度均有明显的变化特征.大气中各个层次N2O的浓度都低于土壤中N2O浓度,此浓度差异直接导致了该地区高寒草原土壤向大气中排放N2O气体,其平均排放通量为0.05×10-4μmo1.m-2.s-1,但是在实验点上全年的观测中,N2O气体排放并没有表现出明显的季节性变化特征.土壤中N2O浓度随深度增加而不断升高,浓度最高值出现在1.5 m深处.进一步的分析表明,N2O浓度随深度递增主要是由环境因子中同样递增的土壤湿度所引起的.大气中不同梯度上N2O气体没有明显的浓度差异.近地表各个圈层中N2O浓度在季节上有非常相似的变化特征,即N2O高浓度均出现在入秋和深冬时节.除了N2O浓度变化在各个圈层之间显著相关以外,表层土壤中N2O浓度也与N2O排放变化有明显的相关关系,这表明浓度的差异是导致N2O气体排放变化的最直接因素.近地表土壤中N2O气体是土壤表层N2O气体排放的直接源泉,并且深层土壤中的N2O气体浓度高于浅层土壤,由此我们可以认定土壤中N2O气体通过微生物作用产生以后,由于浓度差异导致从深层土壤到浅层土壤的逐步扩散,最后经地表排放到大气当中去.

N2O Exchange Within a Soil and Atmosphere Profile in Alpine Grasslands on the Qinghai-Xizang Plateau

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.