关键词: 黄土高原, 样带, 土壤有机碳, 空间变异, 环境因子

Abstract: The China’s Loess Plateau is characterized by severe water shortage,thick loess deposits and intensive soil erosion,and it is a typical ecological fragile region in China and even around the world.Serious soil erosion and long-term irrational land reclamation in this region leads to a decline of soil organic carbon content(SOC).As an important component in the terrestrial ecosystem,the SOC which is closely related to soil physical,chemical and biological properties is a key factor affecting soil fertility and productivity of ecosystem.Slight fluctuation of soil organic carbon pool,which is caused by the changes of land use,land management measures or soil erosion rate,affects strongly the carbon dioxide in atmosphere and thus the global climate change.The spatial distribution of SOC has generally a high heterogeneity due to the influence of climate(i.e.temperature and precipitation),soil texture,soil type,vegetation type,terrain conditions(such as elevation,slope aspect and slope gradient)and land use type.However,the SOC was focused on mainly on the small spatial scales (generally<2 km²)in previous studies,which is insufficient for understanding the spatial characteristics of SOC on regional and/or global scale.Besides,the available researches overlooked the amount and spatial pattern of SOC in deeper soil layers (deeper than 1 m),which failed to provide a reliable evidence for precise regional soil carbon stock evaluation.In this study,an 860-km south-north transect was designed,and the values of SOC at 500 cm depth were measured at an interval of 10 km(n=86) along the transect.Thus,the objectives of the study were to investigate the spatial variation of SOC on regional scale and to derive the primary factors dominating the spatial distribution of SOC in different soil layers by using the classical statistics and geostatistics.The results showed that the mean SOC of the 0-500 cm soil profile decreased generally from the south to the north of the plateau along the horizontal direction.Along the vertical direction,SOC decreased gradually with the increase of soil depth above 100 cm soil layer,while it tended to be stable in the 100-500 cm layer.Under different land use types,SOC was generally in an order of cropland(5.79 g·kg^-1)>forestland (3.34 g·kg^-1)>grassland(2.20 g·kg^-1).A moderate variation of SOC occurred in various soil layers along a 0-500 cm profile,and the best fitting semivariogram models for SOC in the 0-150 cm and 150-500 cm soil layers were Gaussian and Exponential.The controlling factors of SOC were different from different soil layers:SOC in the top 40 cm soil layer was mainly influenced by mean annual precipitation and slope gradient,while that in the 40-500 cm soil layer was controlled by soil texture,mean annual precipitation and temperature.Results suggested that the regional distribution of SOC in shallow soil layers(<40 cm in depth) was determined by climate and topographic conditions,while that in deep soil layers (40-500 cm in depth)tended to be determined by soil texture and climate.The results could provide a scientific basis for ecological regeneration and regional soil carbon stock evaluation.

Key words: Loess Plateau, transect, soil organic carbon content, spatial variation, environmental factor