毛乌素沙地南缘沙漠化临界区域土壤养分的空间异质性

毛乌素沙地南缘沙漠化临界区域是沙地-草地景观界面的关键部位,研究该区域土壤养分的空间格局和生态学过程,对于土地沙漠化的机理研究具有重要的意义。采用经典统计与地统计学相结合的方法,通过半变异函数及其模型、克里格局部插值估计、空间分布图等研究了毛乌素沙地南缘沙漠化临界区域土壤养分的空间异质性特征。结果表明:研究区土壤速效钾含量符合球状模型,全氮和速效磷含量符合指数模型;速效钾含量具有强空间自相关性,其结构方差比为0.882,而全氮和速效磷含量表现为中等程度的空间自相关性,其结构方差比分别为0.501和0.514;3种土壤养分空间自相关距离存在差异,其中全氮和速效钾的变程均为511m,而速效磷的变程为143m;3种土壤养分的分布格局呈现明显的空间规律性,从牛枝子群落到黑沙蒿群落,速效钾和速效磷含量先降低后升高,与研究区的界面变化过程一致,而全氮含量逐渐降低,与研究区的植被覆盖度变化一致;3种土壤养分的标准差都较小,Kriging插值结果比较可靠。

Spatial heterogeneity of soil nutrients in the critical area of desertification in Southern Mu Us Sandy Land

The Southern edge of Mu Us Sandy Land is the frontier of land desertification. Conservation and sustainable use of the vegetation in this sandy land-grassland transition area is crucial for preventing vegetation deterioration in the sandy land and its bordering areas. Investigating spatial heterogeneity of soil nutrients and relevant ecological processes in this critical area is useful to understand the processes and mechanisms of land desertification, and helpful for sustainable land use planning. Based on the field survey along a sandy land-grassland transect, we analyzed the heterogeneity of soil nutrients using the geostatistical techniques such as semivariogram, Kriging interpolation, and spatial distribution maps. Results showed that (1) The semivariogram of soil available potassium (K) was best described by the spherical model and that of soil total nitrogen (N) and soil available phosphorus (P) were best described by the exponential model. Soil available K showed a characteristic of strong spatial autocorrelation, and 88.2% of the variation was found within the distance of 511m. Soil total N and available P showed a modest degree of autocorrelation; and 50.1% and 51.4% of the variation were found within the distance of 511m and 143 m respectively. Geostatistical analysis indicated that spatial variation in soil available P occurred in a small scale; whereas ecological processes determining soil total N and available K contents exerted their effects in a larger scale. (2) Regular spatial patterns of all the 3 soil nutrients were identified. The content of soil available K and P decreased at first and then increased along the transect from Lespedeza potaninii community to Artemisia ordosica community; and this trend was approximately consistent with the changing process of the ecological boundary in the study area. While the change of soil total N along the transect was quite different, decreasing gradually from Lespedeza potaninii community to Artemisia ordosica community, which corresponded with the decrease in vegetation cover. (3) The spatial distribution of the standard deviations of the contents of all these soil nutrients showed very lower values, confirmed the accuracy of Kriging interpolation in this study. (4) The spatial pattern of soil nutrients was closely related with distribution of vegetation types. Distribution of soil nutrients was also related with soil type, micro-topography, plant litter composition, soil animal activity and grazing. Existence of aeolian sandy soil and arenosol in the study area resulted in some variation in soil nutrients content. The wavy topography also affected the movement of soil nutrients and consequently their distribution. Vegetation changes along the sandy land-grassland transect had different species composition and productivity, and different plant litter composition and activities of soil invertebrates and microbes. Also, grazing had profound effects of soil nutrients through excreta deposition. More intense grazing in the area of Lespedeza potaninii community and L. potaninii-Artemisia ordosica community boundary area (good quality forages) than in the area of Artemisia ordosica community (which contained poisonous plants such as Cynanchum komarovii) caused different nutrient cycling rate in these areas. (5)The strong wind in the study area also had a big effect on the process of desertification. The strong wind, combined with the effect of "islands of fertility" led to the rapid process of desertification of the semiarid grassland. These research findings provided the basis for understanding vegetation degradation and desertification processes and useful for land use planning in this fragile transitional region of the sandy land and grassland.