鄂尔多斯草地退化过程中个体分布格局与土壤元素异质性

应用复合个体-距离法分析了鄂尔多斯草地退化过程中多种群和优势种个体分布格局，应用统计方法分析了土壤有机碳和氮元素的异质化过程。结果表明：在鄂尔多斯草地退化过程中，多种群和优势种个体分布格局大多表现出不同尺度下的随机分布和聚集分布，但多种群的个体分布格局在本氏针茅(Stipa bungeana) 油蒿(Artemisia ordosica)群落中出现均匀分布；优势种油蒿和牛心朴子(Cymanchum komarovii)在油蒿 牛心朴子群落中均出现了均匀分布。土壤元素在单一优势种群落中存在不同程度的异质性，油蒿群落中异质化程度最强；在共优势种本氏针茅 油蒿群落中，出现了土壤元素相对均质化，油蒿 牛心朴子群落中，异质化程度相对减弱。由于对土壤水分和养分的竞争，使共优势种群落出现植物的均匀分布，土壤元素相对均质化。土壤元素先后异质化过程首先表现为土壤有机质的异质化过程，然后才表现为土壤氮的异质化过程。土壤元素先后异质化过程表明异质化过程首先表现为植被异质化，植被的异质化导致土壤某些元素的异质化。

THE INDIVIDUAL DISTRIBUTION PATTERNS AND SOIL ELEMENTS HETERO-GENEITY DURING THE DEGRADATION OF GRASSLAND IN ORDOS

We analyze the distribution patterns of the populations and the dominants of Ordos grassland in its degradation, using Combined Count-distance Analysis Method. 'Statistica' was used to analyze soil elements (total organic carbon, total nitrogen and dissolve nitrogen) in root-sphere and bare openings to show the process of soil element redistribution with changes of plant life-forms. The Stipa bungeana community, S. bungeana + Artemisia ordosica community, A. ordosica community, A. ordosica + Cynanchum komaravii community and C. komarovii community that presented successionally degraded stages in desertification were chosen for the study. The results show that, during the degradation course of the grassland, most of the distribution patterns of populations are clump and random distribution at different scales, but multi-population distribution in the S. bungeana + A. ordosica community and in the A. ordosica + C. komarovii community was even with A. ordosica and C. komarovii as the dominant species. There was soil element heterogeneity at different degrees in single-dominant communities. The soil elements had the strongest heterogeneity in A. ordosica community. The heterogeneity in co-dominant communities was inferior to single-dominant communities. At the beginning of degradation, invasion of the S. bungeana grassland by the shrub A. ordosica does not lead to the immediate development of " islands of fertility". Competition for soil elements between 5. bungeana and A. ordosica makes the S. bungeana + A. ordosica community temporarily homogeneous. Similarly, the soil elements have weaker heterogeneity in A. ordosica + C. komarovii community. The competition for soil water and soil resources leads to plant spatially homogeneous distributions and spatial homogeneity of soil elements in co-dominant communities. The heterogeneity process of the soil chemistry was characterized first by total organic carbon (TOC) heterogeneity and later by total nitrogen (TN) heterogeneity. Soil TOC is derived mainly from decomposed plant material; soil TN is provided by nitrogen-plant detritus, either plant-fixed or gas-fixed nitrogen. Thus, it is tentatively concluded from the results that the heterogeneity pro- cess is initially characterized by plant spatially heterogeneity; then the plant spatial heterogeneity leads to the spatial heterogeneity of soil elements.