戈壁灌丛堆周边地表土壤颗粒的空间异质特征

Spatially heterogeneous characteristics of surface soil particles around nebkhas in the Gobi Desert

研究戈壁地区单个灌丛及其下沙堆这一有机整体对周边土壤风蚀的抑制能力,对加强相关地区的植被类型及其空间配置格局的防沙效应研究十分重要,可为荒漠化监测的评价和制定科学的防治措施提供参考。该文利用数字图像处理技术,获取吉兰泰盐湖北部戈壁上单个白刺(Nitraria tangutorum)灌丛沙堆和沙冬青(Ammopiptanthus mongolicus)灌丛沙堆周边地表不同土壤风蚀颗粒的百分含量;并采用经典描述性统计及地统计学方法,对各类土壤风蚀颗粒百分含量的水平空间异质性进行分析。结果表明:(1)灌丛基部和下风向是细物质积累区,以灌丛堆为中心向外,<0.42mm的细颗粒含量呈减少趋势;而且细物质积累的最大值出现在白刺灌丛的迎风侧附近,沙冬青样地则相反,出现在灌丛的背风侧附近。在沙源物质有限的戈壁中,白刺的防风固沙作用集中体现在灌丛附近,其水平空间尺度范围不及沙冬青,这亦是白刺样地粗粒化程度高于沙冬青样地的原因。(2)白刺和沙冬青灌丛附近地表中粒径>0.84mm(不可蚀)、0.84–0.42mm(半可蚀)及<0.42mm(高度可蚀)颗粒的空间异质性尺度分别为17.80m、66.63m、8.41m和9.82m、15.33m、14.91m,均超出了灌丛冠幅覆盖范围,空间自相关部分比例C/(C0+C)在63.40%–99.96%之间,由此推断灌丛沙堆附近的风沙流特征是造成相应尺度内土壤颗粒空间异质性的主要因子。(3)高度可蚀颗粒的空间异质性尺度略大于灌丛平均间距(8.77m包括灌丛半径),从防止土壤风蚀来看,这说明研究区内的建群种灌丛间存在一定程度的相互促进关系,有利于该区植被的稳定与发展。

Aims We studied single desert shrubs and their role in sand accumulation to provide references for monitoring and evaluating soil wind erosion and for scientifically establishing controlling measures. This research is important for strengthening sand fixation effects of vegetation. Methods We obtained percentage composition of soil particles in wind erosion surfaces around single shrubs of Nitraria tangutorum and Ammopiptanthus mongolicus in the Gobi Desert north of Jilantai salt lake with digital image processing technology. We analyzed their horizontal spatial heterogeneities by classic descriptive statistics and geostatistical methods. Important findings Enrichment of soil fine particles appeared on the base of nebkhas and in the downwind direction. The percentage of soil particles （〈0.42 mm） decreased from the center of shrubs to the sample plot. The maximum value of soil particles enrichment was on the windward side around N. tangutorum and on the leeward side of A. mongolicus. In the Gobi Desert where sand material is limited, the effect of wind prevention and sand fixation was concentrated around N. tangutorum, although the spatial scale affected was smaller than with A. mongolicus. This explained that the degree of coarse grains in the N. tangutorum sample plot was higher than that of A. mongolicus. The spatial heterogeneity scales of soil particles （〉0.84 mm, 0.42-0.84 mm and 〈0.42 mm） in wind erosion surface around N. tangutorum and A. mongolicus were 17.80, 66.63 and 8.41 m and 9.82, 15.33 and 14.91 m, respectively, all of which were beyond the shrub canopy cover. The proportion （C/（Co ＋ C）） of structured elements was 63.40% to 99.96%, which indicated that wind-drift sand characteristic around nebkhas was the most important factor in the spatial heterogeneity of soil particles of corresponding scale. The spatial heterogeneity scale of highly erodible soil particles was greater than the average space of nebkhas （8.77 m, which included the semidiameter of nebkhas）. The prevention of soil wind erosion by the relation between nebkhas and species is beneficial for stabilization and development of vegetation in this area.