不同质地土壤中荒漠灌木梭梭“肥岛”的初步探讨

为检验荒漠灌木"肥岛"是否受土壤质地调节,本研究选取了中亚干旱区梭梭(Haloxylon ammodendron(C.A.Mey.))的两种原生生境:古尔班通古特沙漠的风沙土和绿洲区的灰漠土。两种土壤的质地有显著差异,风沙土中砂粒含量是灰漠土的2.1倍~2.4倍,而灰漠土中粉粒和黏粒含量分别是风沙土的3.0倍~3.6倍、2.9倍~5.1倍。通过对比分析两种土壤中梭梭冠下水分和养分的空间变异特征,获得了如下结果:两种土壤中均存在灌木"肥岛"效应,且随深度增加而减弱;在0~20 cm土层,与灰漠土相比,风沙土中冠下水分和养分的空间异质性更强,富集程度更高,风沙土中水分、有机质、有效氮和有效磷的富集率分别是灰漠土的4.6倍~12.3倍、3.4倍~8.6倍、3.0倍~4.9倍和2.1倍~2.6倍。本研究表明,两种土壤均存在灌木"肥岛"效应,而土壤质地可能决定了"肥岛"效应的强弱程度。

Preliminary Studies onHaloxylon Ammodendron'Fertile Islands' in Desert Soils Different in Texture

In arid and semi-arid ecosystems,shrubs often create patches of soil underneath spatially heterogenous in distribution of soil water and nutrients,which are called"fertile islands"or"resource islands". Although the so-called"fertile island"is a phenomenon of spatial heterogeneity in distribution of soil water and nutrients at a plant scale,it may have some great impacts on vegetation distribution, productivity and evolution processes(like desertification)of the ecosystem. Therefore,the phenomenon of"fertile islands"has attracted more and more attention from more and more researchers. So far,researches have demonstrated that distribution pattern of the soil resources underneath shrub canopies was related to soil depth,plant species,plant development stage and spatial scales(e.g.,rhizosphere,individual, population,geomorphology,and region). However,little has been reported on spatial pattern of soil resources relative to soil textured soil,especially in desert soils. This calls for further attention. In this study,two different types of native habitats ofHaloxylon ammodendronin the Junggar Basin were selected:the Gurbantonggut Desert and nearby oasis.Haloxylon ammodendronis a dominant species in arid regions of Central Asian,playing a key role in maintaining structure and functions of these desert ecosystems. Aeolian sandy soil and grey desert soil are the zonal soils of the two habitats and have developed under the same climatic conditions and experienced the same weather process,but are markedly different in mechanical composition even though they are only 8 km apart. Specifically,the aeolian sandy soil is of loamy fine sand soil,and the grey desert soil is silt loam soil. The former is 2.1~2.4 times as high as the latter in sand content,while the latter is 3.0~3.6 and 2.9~5.1 times as high as the former in silt and clay content, respectively. This provides an ideal environment for determining whether the effect of individual plants on spatial variability of soil properties under their canopies might be mediated by soil texture. In each of the two native habitats,eightHaloxylon ammodendronplants similar in size(about 200 cm in plant height,about 10 cm in basal diameter and about 100 cm crown radius)were selected. For soil sampling,the soil under each plant was divided vertically into three soil layers,0~20,20~60,and 60~100 cm. Then in each soil layer,four sampling sites were set representing four micro-habitats,that is, near the taproot,in the center of the shrub canopy,at the edge of the canopy,and in the space between two shrubs,or at 5,50,100,and 200 cm away from the taproot,respectively. At each sampling site, soil samples were collected at four directions of the site,with an angle of 90°apart and then mixed into one for analysis of soil properties,soil texture,water,organic carbon,available nitrogen,and available phosphorus with a laser particle analyzer,the oven-drying and weighing method,the K2Cr2O7-H2SO4 oxidation method,the alkalysis diffusion method,and NaHCO3 extraction-Mo-Sb colorimetric method. Through comparative analysis of the soils under the canopies ofHaloxylon ammodendronin the two habitats in spatial distribution of soil water and nutrients,attempts were made to characterize"fertile islands"in the two soils sharply different in texture. Results showed that(1)spatial heterogeneity of soil water and nutrients existed in both soils under the canopies of the shrubs,and weakened with soil depth;and(2)at the soil depth of 0~20 cm,the aeolian sandy soil was more obvious in spatial heterogeneity of the distribution of soil water and nutrients than the grey desert soil under the canopy ofHaloxylon ammodendron,and higher in enrichment level of soil water and nutrients,too. The former was 4.6~12.3, 3.4~8.6,3.0~4.9 and 2.1~2.6 times as high as the latter in enrichment rate of soil water,organic matter, available nitrogen and available phosphorus,respectively. All these findings indicated that the phenomenon of"fertile islands"caused by shrubs exists in both soils,and soil texture may determine intensity of the"fertile island"effect. Knowledge of the spatial variability pattern of plant resources at individual scale may help better understand spatial distribution pattern of the resources at the population,community,and ecosystem scales,as well as relationships between soils and plants. Further studies need to be done on such a phenomenon in different ecosystems.