青藏高原草地地下生物量与环境因子的关系

青藏高原草地生物量大部分分布于地下,地下生物量在其碳循环研究中起着重要的作用.基于大规模野外样地调查数据,分析比较了青藏高原南北和东西样带上草地地下生物量与环境因子的相关关系,探讨了环境因子对地下生物量控制作用的区域差异.研究结果表明:对于所有采样点而言,青藏高原草地地下生物量的环境控制因素主要有土壤含水量、表层土壤有机碳和全氮含量.通过比较南北和东西样带研究结果发现,草地地下生物量与土壤含水量、土壤表层有机碳和全氮含量相关的显著性水平,在东西样带上明显高于南北样带.同时,东西样带上草地地下生物量与降水量有显著正相关关系,这种关系在南北样带上不显著,表明水分对东西样带草地地下生物量的控制作用较强.气温与南北样带草地地下生物量呈显著负相关,但与东西样带草地地下生物量相关不显著,由此说明环境因子对青藏高原草地地下生物量的控制存在显著区域差异.

Belowground biomass in Tibetan grasslands and its environmental control factors

Belowground biomass (BGB), contributing 80% of total biomass to grassland in Tibetan Plateau, plays an important role for grassland carbon cycling. Understanding the relationships between BGB and its environment factors is crucial for grassland resources management and predicting the future influence of climate change on Tibetan grassland ecosystem. Many previous studies have investigated the relationship between BGB and its environment in Tibetan grassland, however large uncertainties still exist, and some research results are even controversial due to limited ground samplings. This study overcomes the previous sampling problem, analyzes the relationship between belowground biomass and its environmental factors based on a large scale of field survey conducted in August 2010. Total 32 field sampling plots were selected along two transects in Tibetan plateau: the North-South transect from the Huangyuan county (36.4°N, 100.96°E), Qinghai province, across 7 latitudes southward to Bomi county(29.64°N, 96.72°E), Tibet Autonomous Region, and the East-West transect mainly located in Xigaze area, Tibet Autonomous Region, along the Yajiang Valley, starting from Saga county(29.33°N, 85.38°E) across 7 longitudes eastward to Gongbujiada county(29.69°N, 92.27°E). Statistical methods were used to analyze the relationships between the observed BGB of grassland in Tibetan Plateau and the environmental factors including observed soil moisture, annual precipitation, annual mean temperature, soil organic carbon and soil nitrogen contents. Research results show that soil moisture is the most critical controlling factor which influences the spatial distribution of grassland BGB in Tibetan Plateau. It is positively correlated with Tibetan grassland BGB, explaining 52.76% of total variation in Tibetan grassland BGB. For these two transects, the grassland BGB is more significantly correlated with soil moisture in the East-West transect (R²=0.6744, P<0.01) than in the North-South transect (R²=0.4081, P<0.01). The connection between grassland BGB and precipitation is complex. For all Tibetan grassland plots, only 7.95% (P<0.01) of the variance in BGB can be explained by precipitation. Grassland BGB increases with increasing precipitation (R²=0.1910, P<0.01) in the East-West transect, but the correlation between the two variables is insignificant in the North-South transect. Such discrepancy between the two transects is partially due to fluctuant topography in this region, causing large differences of evaporation and runoff over a large area, and thus significant differences in grassland BGB. The relationship between grassland BGB and temperature is weak with no significant correlation between temperature and grassland BGB in the East-West transect. Along the North-South transect, the temperature is negatively correlated with grassland BGB, and explains only 11.86% (P=0.0397) of the variation in grassland BGB. Tibetan grassland BGB is positively correlated with surface soil organic carbon (R²=0.3094, P<0.01) and total nitrogen content (R²=0.2081, P<0.01). The positive relationship may be explained by the fact that increased below-ground biomass is related to increased amount of root litter in the soil, thus increased organic carbon and total nitrogen content in the soil. Comparison of the relationships between grassland BGB and the soil organic carbon in the two transects indicates that the correlation coefficient (R²=0.6751) and the confidence level (P<0.01) in the East-West transect are higher and more significant than those (R²=0.1445, P=0.0222) in the North-South transect. The grassland BGB is also closely related to soil total Nitrogen with the R² and confidence level in the East-West transect 0.4593 and less than 0.01, respectively, much higher and more significant than those (0.1050 and 0.0539, respectively) in the North-South transect. The reason for more significant relationships in the East-West transect is still under investigation. Overall, this study demonstrates the strong relationships between BGB of grassland in Tibetan Plateau and its environmental factors, and these relationships vary significantly over large spatial scales.