Spatial variability of soil organic matter content in an arid desert area

Alxa Left Banner in the west of the Inner Mongolia Autonomous Region, China, is a typical arid desert area in the middle temperate zone between the northern border of China and the eastern Alxa Plateau. Due to overgrazing and human activities, soil degradation (or desertification) in the area is becoming increasingly severe due to loss of soil organic matter (SOM). Low SOM content impacts soil quality directly in this arid desert area. Understanding the spatial variation and distribution pattern of SOM content is needed for sustainable development in the region. In this study, the spatial variation of SOM content in Alxa Left Banner was determined using both traditional statistics and geostatistics to provide information for understanding desertification mechanisms and preventing soil degradation.Based on local landforms, 103 soil samples (0～20 cm) were collected from the area within the Helan Mountain watershed, on the western border of the Wulanbuhe Desert, the northern border of the Tenggeli Desert and the southern border of the Gobi region. The SOM content was analyzed using the Walkley-Black wet combustion method. The geostatistical characteristics, spatial trend and azimuth of anisotropic axle of SOM content were analyzed using the Geostatistical Analyst, ArcGIS8.1. We created a contour map of SOM content for the study area using Kriging interpolation.Traditional statistical analysis (descriptive statistics) showed differences in SOM content among different soil types. The coefficient of variation (CV) of SOM content was greatest in Aeolian sandy soil (CV 1.10) and smallest in Subalpine meadow soil (CV 0.08) among all soil types. For all soil samples (n=103), the mean, minimum, maximum, standard deviation and coefficient of variation of SOM content were 6.65g/kg, 0.13g/kg, 43.05g/kg, 9.42g/kg and 1.42, respectively. The results indicated that SOM content was very low and its distribution was highly variable in this arid desert area.The spatial variation of SOM content in the area was anisotropic, which showed that the SOM semivariogram depended on both the distance and the direction of soil sampling. The spatial correlation distances (ranges) were large (46.6 km on the long axle and 30.0 km on the short axle). The nugget variance (27.4) indicated a large disconnection of the semivariogram value from the origin, and the sill (68.5) showed a large semivariogram value for distance beyond the ranges. The integrative comparison of the prediction errors from the trend effects of 0, 1, and 2 order indicated that the 2-order trend effect was preferable.The Kriging spatial interpolation showed that SOM decreased gradually from the south to the north and also from the southeast to the northwest in the area. The lowest level band of SOM content was in the middle part between the Wulanbuhe Desert and the Tenggeli Desert, running from the northeast to the southwest. In addition, soils at the edge of the two deserts contained very low SOM content. The spatial distribution of SOM content was related to variation in topography, soil type and vegetation cover in the study area.