荒漠地表生物土壤结皮形成与演替特征概述

Summary on formation and developmental characteristics of biological soil crusts in desert areas

土壤表面结皮是世界范围内干旱沙漠地区土壤表面广泛存在的自然现象,包括物理结皮和生物土壤结皮两大类型。其中,生物土壤结皮作为干旱沙漠地区特殊环境的产物,是由细菌、真菌、蓝绿藻、地衣和苔藓植物与土壤形成的有机复合体。它的形成使土壤表面在物理、化学和生物学特性上均明显不同于松散沙土,具有较强的抗风蚀功能和重要的生态效应,成为干旱沙漠地区植被演替的重要基础。随着形成生物土壤结皮的物种更替,维持结皮结构的主要胶结方式亦随之发生变化,即由胞外多糖的粘结作用逐渐转变为蓝藻和荒漠藻的藻丝体、地衣菌丝体以及苔藓植物假根的缠绕和捆绑作用,物种更替是结皮微结构和胶结方式转化的生物基础。生物土壤结皮的形成通常可以分为以下几个阶段:生物土壤结皮的早期阶段(土壤酶和土壤微生物),藻结皮阶段、地衣结皮阶段和苔藓结皮阶段。即随着土壤微生物在沙土表面的生长,随后出现丝状蓝藻和荒漠藻类植物,形成以藻类植物为主体的荒漠藻结皮;当土壤表面得到一定固定后,便开始出现地衣和苔藓植物,形成以地衣和苔藓植物为优势的生物结皮类型。其中,前一阶段的完成又为后一阶段的开始提供良好的环境条件。当环境条件适宜时,生物土壤结皮也可以不经历其中某个阶段而直接发育到更高级的阶段。

In arid and semi-arid lands,the vegetation cover is usually sparse or absent.Nevertheless,in open spaces among higher plants,the soil surface is generally covered by a community of highly specialized organisms,such as mosses, lichens,liverworts,algae,fungi,cyanobacteria,and bacteria.These communities are usually referred to biological soil crusts,or cryptogamic,cryptobiotic,microbiotic,microphytic soil crusts.Biological soil crusts,given their extraordinary abilities to survive desiccation and extreme high temperatures,high pH and high salinity,have been found in desert areas all over the world and may constitute as high as 70% of the living cover in some plant communities.They play a significant role in ensuring the proper functioning of desert ecosystem,such as involvement in the process of formation,stability and fertility of soil,prevention of soil erosion caused by wind or water,augment of vascular plant colonization,and stabilization of sand dunes.The biological soil crust resulting from the colonization of soil surface by communities of filamentous cyanobacteria were mainly dominated by Microcoleus,which occurs as a cluster of filaments surrounded by a gelatinous sheath.Other important taxa are Lyngbya,Anabaena and Xenococcus lyngbyge.At this developmental stage,the main contributors for sand fixation were changed from bacteria to filamentous cyanobacteria.Microscopic examination of this kind of crust revealed an intricate network of filamentous cyanobacteria and extracellular polymer secretions,which binds and entraps mineral particles and finer particles stick on the filament surface.These effects enhance soil cohesion and resistance to erosion.Two major mechanisms are suggested for maintaining sand surface stabilities：（ 1 ） the ability of exopolysaccharides from some microorganisms （ mainly bacteria） to cohere sand particles,and （ 2） formation of network by the filamentous microbes and cryptogam （ cyanobacteria,algae,lichen and moss） .With the alternation of crust-related species from cyanobacteria,algae and lichen to moss,the main agents maintaining microstructure of biological soil crust changed accordingly from glutinous exopolysaccharides to filamentous algae and hyphae of lichen and moss.Generally,the development of biological soil crust can be divided into three phases：original succession phase （ including soil microorganisms and soil enzymes） ,algae crust phase and lichen-bryophyte crust phase.The establishment of former phase crust serves as a basis for the next phase of crust succession.Under drought conditions,soil crust is brittle,and can be crushed easily when subjected to compressional or shear forces.When the surface crust is broken,the unconsolidated loose sand grains below the crust are exposed to wind,resulting in severe soil erosion.Under favorable environmental conditions, such as sufficient water supply and moderate temperature,the less developed biological soil crust may surpass the intermediate phase and develop into higher level biological soil crust.