黄土高原地区位于我国中北部(N32°—41°,E107°—114°),总面积约64.87万km2。黄土高原地形地貌复杂多样,沟壑纵横,黄土物质疏松,易遭受侵蚀,是典型的生态脆弱敏感区和世界上水土流失最严重的地区,也是我国生态恢复和生态文明建设的重点区域[1-3]。黄土高原在退耕还林后,植被多是以柠条、沙柳、小叶杨为主的人工植被(图 1a,图 1b),在黄土高原许多地区植被不连续的空地上,普遍存在生物结皮(图 1c),其生长和发育有效地改变了土壤结构[4]。
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图 1 黄土高原主要植被(a,b)和生物结皮(c)景观图 Fig. 1 Main vegetation(a, b)and landscape of biological soil crust(c)on the Loess Plateau |
生物结皮是干旱半干旱地区景观的重要组成之一,它是由隐花植物如细菌、蓝藻、真核藻类、真菌、地衣、苔藓植物与土壤表层颗粒胶结形成的复杂的有机复合体,其发育过程改变了土壤的物理、化学和生物学特性,进而对径流入渗、土壤侵蚀、土壤水分蒸发、地球化学循环和生物多样性等均具有独特的生态功能[5-6]。黄土高原的生物结皮与世界荒漠区的生物结皮生物组成、胶结方式相同,它们均是由微生物类群和地衣、苔藓孢子植物类型与土壤颗粒结合成的有机复合体[5, 7-8];在生态功能上,生物结皮在不同区域均能维持地表稳定性和增加土壤养分[4, 6, 9]。然而,黄土高原生物结皮在生态功能上与荒漠区生物结皮又存在明显的区别,在黄土高原上,在不同生境中广泛发育着不同类型的生物结皮(图 2),它们能有效地消减降雨动能和径流的冲刷,减少甚至完全控制土壤流失[10-11];而在荒漠区,生物结皮则在防治风蚀、维持地表稳定性上发挥着重要功能[12-14]。本文对生物结皮类型与演替、黄土高原生物结皮的物种多样性、生态功能以及生物结皮的人工培养与生态恢复问题进行了综述,旨在对未来研究提供参考。
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a:藻结皮Algal crust;b:地衣结皮Lichen crust;c:混生结皮Mixed crust;d:苔藓结皮Moss crust. 图 2 黄土高原生物结皮不同演替阶段 Fig. 2 Different successional stages in BSCs in Loess Plateau |
在干旱半干旱区,生物结皮在裸地上开始形成后,在适宜条件下会进一步地发育和演替,一般而言,根据生物结皮的优势物种组成,可将生物结皮分为藻结皮、地衣结皮、苔藓结皮,在发育过程中也有一些混生结皮存在,如藻-地衣混生结皮、地衣-藓混生结皮[15-18]。生物结皮未形成的阶段是裸沙或裸土阶段,此阶段土壤表面沙粒松散,藻类和微生物量均很低[19]。一些耐贫瘠的细菌类群如古菌门、蓝藻门、放线菌在裸沙阶段占较高比重,同时,一些耐极端环境的丝状蓝藻如具鞘微鞘藻(Microcoleus vaginatus)在裸土中逐渐殖入,一些丝状蓝藻开始和土壤颗粒捆绑粘结在一起,形成浅色藻结皮,厚度约为1~3 mm,在荒漠区域浅色藻结皮的主要物种以微鞘藻(Microcoleus spp.)占优势的丝状蓝藻等占优势[20-21]。当结皮进一步发育,颜色逐渐变深,呈深色或黑色,结皮增厚,形成深色藻结皮,此阶段具异形胞的固氮蓝藻种类和数量增多,如念珠藻(Nostoc spp.)、伪枝藻(Scytonema spp.)、单岐藻(Tolypothrix spp.),另外一些球形蓝藻和真核藻类逐渐增多[5]。深色藻结皮藻类物种多样性明显增多,在发育过程中蓝藻或绿藻与真菌结合形成蓝藻地衣或绿藻地衣,逐渐发育成地衣结皮[22-23]。藻结皮和地衣结皮在发育过程中,土壤肥力和持水能力进一步增强,为苔藓植物的生长提供了适宜的生境条件,苔藓植物逐渐殖入、扩繁并最终占据优势,进而演替成为藓结皮[24]。在荒漠区域,生物结皮演替速率很慢,从结皮开始形成到发育为成熟结皮需几十年时间[6, 15],而在水分条件相对较好的半干旱区,如黄土高原,生物结皮演替速度显著加快,整个演替一般在一年或几年即可完成[25]。
研究者在不同研究区开展了生物结皮演替机制及微生物多样性的研究,研究普遍认为,生物结皮发育影响了土壤理化性质,尤其是显著提高了土壤养分,改善了土壤结构[6],从而使生物结皮的不同演替阶段具明显的生境异质性,导致微生物群落多样性的差异[22-24],如蓝藻和古菌群落在生物结皮的早期演替中发挥着重要的生态功能,而在后期演替过程中,异养细菌和真菌多样性和生物量更丰富[15, 22-23, 25-26]。
2 黄土高原生物结皮的物种多样性在黄土高原,在生物结皮物种多样性的早期研究中多关注藓类植物[28]。苔藓结皮在黄土高原陕北水蚀风蚀交错区占绝对优势,藓结皮主要分布在干扰少、侵蚀弱、水分好的梁峁坡或梁峁顶上[27-28]。流域内藓类共有2科8属13种,狭网真藓(Bryum algovicum)、真藓(Bryum argenteum)、尖叶对齿藓(Didymodon constrictus)分布最广[28]。微环境对苔藓结皮的发育有显著影响,藓结皮在沙地呈连片分布,藓结皮在黄土的覆盖度相对低于沙地;因藓类植物喜阴湿的环境,灌丛下有利于藓结皮的发育,在阴坡藓结皮的覆盖度和厚度均高于阳坡[28]。
随着研究的深入,研究者开始关注微生物多样性在黄土高原生物结皮的重要作用,并进行了积极的探讨。蓝藻是生物结皮中的重要光合微生物类群,主要存在生物结皮的早期阶段,在生物结皮的形成和发育中发挥着重要功能。通过培养手段,杨丽娜[29]对黄土高原生物结皮中的蓝藻进行了初步研究,发现蓝藻5目5科13属76种,其中多数种类是以颤藻科占优势的丝状颤藻,约占87%。通过对黄土高原水蚀区、水蚀风蚀交错区、风蚀区生物结皮蓝藻的进一步研究,发现蓝藻Shannon-Weiner多样性指数和丰富度均表现为水蚀风蚀交错区>水蚀区>风蚀区,水蚀风蚀交错区、水蚀区和风蚀区的优势物种分别是阿氏鞘丝藻(Lyngbya allorgei)、含钙席藻(Phormidium calciola)和颗粒颤藻(Oscillatoria granulata),与其他荒漠区域类似,黄土高原生物结皮的蓝藻多样性与土壤质地、土壤pH、气候等环境因子密切相关[30-31]。在微生物的培养中发现,黄土高原生物结皮对微生物数量具有显著的影响,生物结皮中土壤微生物数量显著高于无结皮,在垂直分布上,结皮层的细菌和微生物总数量显著高于结皮下不同土壤层次,放线菌和真菌也呈明显下降趋势[32]。随着实验手段和技术的快速发展,研究者进一步利用高通量技术对生物结皮微生物多样性进行进一步探讨,发现黄土高原藓结皮中的细菌群落以酸杆菌门(24.3%)、变形杆菌门(23.8%)、绿弯菌(15.8%)、放线菌门(14.5%)为主。真菌群落则以子囊菌门(68.0%)和担子菌门(23.8%)为主[33]。藓结皮细菌菌落数量远高于无结皮土壤,分别为固定沙地和流动沙地的2.29倍和4.41倍,真菌密度分别为固定沙地和流动沙地的2.76倍和6.41倍。细菌和真菌的物种丰富度也显著高于沙地。藓结皮群落中细菌和真菌数量多、多样性高,细菌和真菌通过提高土壤肥力而在生态系统功能中发挥着重要作用[33],这与荒漠区的研究结果相类似[23, 25]。磷脂脂肪酸的证据表明生物结皮演替提高微生物群落的丰度,藓结皮阶段微生物群落具最高的丰度和结构复杂性。在结皮演替过程中,微生物多样性升高,微生物群落结构发生明显变化,细菌/真菌和G+/G-比例在结皮层均呈降低趋势,这表明真菌和革兰氏阴性菌在结皮演替过程中呈升高趋势,而细菌和革兰氏阳性菌比例下降[34]。生物结皮微生物的生物量和多样性受干扰强度影响,中等强度的干扰有利于提高土壤养分和微生物量和微生物(细菌、AM真菌和总微生物)多样性,而严重的干扰将显著降低土壤养分和微生物丰度[35-36]。
近年来,生物结皮微生物多样性的研究逐渐从描述阶段转向生态过程和多样性形成机制的探讨[37-39];同时,在土壤关键元素循环过程发挥重要作用的微生物功能群已成为微生物生态学的研究热点[40-42],并且微生物多样性逐渐和生态功能结合在一起。在黄土高原,生物结皮还缺乏调控关键元素转化过程的微生物功能群的研究,微生物多样性的形成机制也有待于深入探究。
3 生物结皮生态功能生物结皮在干旱半干旱地区发挥着极为重要的生态功能,国内外学者在研究生物结皮物种多样性的同时,对生物结皮的生态功能或多功能属性的关注越来越多[43]。在黄土高原,主要集中在生物结皮的土壤肥力和水土保持效应上[27, 44]。黄土高原在退耕还林还草恢复措施实施后,生物结皮广泛发育,其存在对降雨入渗、水蚀风蚀以及植物生长等具极大的影响,尤其对黄土高原的水土保持发挥着重要功能[7, 34, 44]。
3.1 提高土壤肥力生物结皮通过其光合生物和固氮微生物对碳氮的固定作用、以及对大气颗粒和降雨养分的捕获作用显著提高土壤养分含量已成为研究共识[5, 45]。在黄土高原的水蚀风蚀交错区,研究发现自然和人工生物结皮的发育对土壤有机质、全氮和速效氮、全磷和有效磷、速效钾均有显著的贡献,在退耕地中也发现生物结皮的发育迅速提高了土壤全磷、有效磷和速效钾含量,藓结皮的C、N、P含量较藻结皮分别增加了161%、127%和9%[4, 44, 46]。姚春竹等[48]将黄土高原生物结皮土壤有机质和全氮的积累过程以发育13年为界限分为快速增长和趋于稳定两个阶段,结皮发育13年后土壤有机质和全氮含量分别为25.41 g·kg-1和1.34 g·kg-1,是一年以内撂荒地的5.05倍和3.72倍;生物结皮层明显增加了土壤碳氮比,结皮层发育15年其土壤碳氮比最高(11.8),表明生物结皮对有机碳固定的速率高于氮固定速率。在荒漠生态系统中,随着结皮的演替,其固碳和固氮能力急剧增加,演替后期的藻类-地衣混生结皮的年固碳量和固氮量较以具鞘微鞘藻为优势的藻结皮分别高20倍~30倍和3倍~4倍[5, 46],生物结皮在荒漠生态系统净初级生产力的贡献高达9%,其固氮量约占全球陆地生物氮固定的27%~53%[41, 49]。
3.2 促进养分代谢研究者在研究生物结皮养分特征的同时,对在土壤元素生物化学循环中发挥着重要作用的土壤酶有较多的关注。研究表明,生物结皮不仅能显著提高表层土壤有机质和土壤养分的含量,同时也显著提升了碳、氮、磷相关的土壤酶活性[19, 49-50]。在黄土高原水蚀风蚀交错区,生物结皮层的土壤碱性磷酸酶、脲酶和过氧化氢酶活性明显高于结皮下层[46];黄绵土藓结皮中的蔗糖酶、碱性磷酸酶、脲酶和蛋白酶活性是无结皮土壤的20.7倍、7.6倍、2.4倍、2.4倍,而风沙土藓结皮这四种酶活性较无结皮土壤提高了22.3倍、22.2倍、3.5倍、2.0倍,藓结皮中蔗糖酶和碱性磷酸酶表现出更快的增长速度[51]。
在土壤生态系统中,土壤微生物驱动了碳氮循环多个重要代谢过程,如碳循环主要有碳固定、甲烷代谢和碳降解过程[42],而氮循环则包括固氮、硝化、反硝化、厌氧氨氧化、氨同化/异化还原、氨化和同化作用等过程[52]。同时,土壤功能微生物在磷元素循环过程(有机磷矿化、无机磷溶解、磷同化)中也扮演着重要角色[53]。研究者在黄土高原中已开展了部分生物结皮微生物多样性的相关研究[29, 33-34],但在生物结皮功能微生物多样性还有待于深入研究,以期解读黄土高原生物结皮生态过程的微生物学机理。
3.3 生物结皮抗侵蚀功能土壤侵蚀是黄土高原最严重的环境问题,生物结皮不仅对土壤养分和酶活性具有显著影响,而且在增加土壤结构的稳定性、有效地减少或避免土壤侵蚀的发生与发展均起着关键作用[54-56]。随着研究的深入,对生物结皮抗侵蚀机理的认识越来越清楚。研究表明,生物结皮通过生物覆盖层和生物量形成的直接物理保护以及通过间接改变土壤性质来提高土壤的抗侵蚀性[57]。藓结皮抗侵蚀效果优于混合结皮;随着结皮盖度的增加,土壤抗蚀性能增强[58]。苔藓覆盖度或生物量为36% 和1.22 g·dm-2,是生物结皮抵抗径流侵蚀的重要临界值,在苔藓覆盖量小于36%的生物结皮发育早期,蓝藻在抵抗径流侵蚀方面发挥主要作用,其抗性与蓝藻生物量正相关;当苔藓覆盖度或生物量超过36% 和1.22 g·dm-2,生物结皮可以完全保护土壤免受径流侵蚀[57, 59]。此外,土壤有机质和土壤凝聚力的提高对减少侵蚀也非常重要,如以藓结皮和蓝藻结皮的生物有机质含量分别为裸地的4倍,生物结皮较裸地含有更多的细颗粒(< 0.01 mm)和更少的粗颗粒(0.05~0.25 mm),土壤粒径变细,土壤黏结力进一步增大,同时对 > 5 mm的水稳性团聚体的形成具有促进作用,使土壤水稳性团聚体由小向大转变,土壤抗剪切强度明显加强,从而提高了土壤抗侵蚀能力[55, 60]。生物结皮的演替在很大程度上增加了地表粗糙度,在降雨过程中生物结皮增加入渗,减少径流[56];室内模拟实验表明,生物结皮在坡度5°、雨强46.8 mm·h-1、历时1 h的模拟降雨下可减少49%~64%的径流、消除土壤侵蚀[61];相对于裸土坡面,野外人工培育的生物结皮在降雨时能够显著延长坡面初始产生径流时间,抑制坡面产流产沙,可降低21%~78%的坡面径流量和77%~95%的产沙量[11, 44]。在农田中,农业措施也影响生物结皮的入渗和径流,如轻度和中度翻耙生物结皮可以促进入渗,减少径流,而中度翻耙生物结皮入渗和减少径流能力明显下降[61]。
4 人工生物结皮与黄土高原生态恢复近年来,研究者在脆弱生态系统开展了大量土壤侵蚀与生态恢复的相关研究工作[62-64]。在黄土高原,对生物结皮的人工培养和生态恢复开展了大量工作,研究主要以生物结皮中藓类植物为培养材料,在黄土高原水蚀风蚀交错区和风蚀区通过施加人工措施(如遮阳、浇水、覆膜、添加菌液或营养液)在野外对苔藓进行小规模的接种,结果发现水分对藓结皮的生长限制作用最明显,添加混合藻液(小球藻和硅藻)和芽孢杆菌对人工生物结皮的形成和发育具积极的影响,使藓结皮的盖度和厚度增加[65-67]。藓结皮盖度、植株密度和生物量随表层土壤含水量和接种量的提高逐渐增加,藓类植物的营养液(如Hoagland,改良Knop、Part、MS等营养液)和糖类(蔗糖、葡萄糖、甘露醇)均能在不同程度上提高藓结皮的盖度、植株密度和生物量,藓结皮的发育速度与光照强度呈负相关关系[68]。研究者对人工生物结皮开展了生理学和生态学研究,如生物结皮对失水-再水合和热胁迫的生理响应和水土保持效应。肖波等[45]通过撒播粉碎后的藓结皮,培育1个雨季即可形成盖度高达30%~60%的藓结皮。藓结皮的室内人工培养发现,结皮团块粉碎接种更有利于藓结皮盖度的形成;藓结皮在500~750 g·m2的接种量时,其盖度和藓类植株密度较高;17 ℃是藓类植物的生长和藓结皮恢复的最适温度;土壤田间持水量大于60%时利于藓结皮的形成和发育[69]。与荒漠区相比,黄土高原相对较多的降雨条件为生物结皮的恢复提供了有利条件,生物结皮的人工培养亟需在黄土高原更大范围内推广,如矿山开采后的土地修复或退耕地。然而,依靠利用自然生物结皮粉碎后直接接种将会破坏大量的生物结皮,形成新的生态脆弱区,不利于生态恢复。藓结皮在小规模的室内和野外培养中取得了一定的成功,但多处于方法探索阶段,人工培养的藓类植物与不同微生物类群和非生物因子的相互作用关系有待于进一步深入探讨,以期获得突破性进展。
5 研究展望生物结皮是黄土高原的重要组成和典型景观,在生态系统稳定性和水土保持功能中发挥着重要作用。近20年来,国内学者系统研究了黄土高原生物结皮土壤养分、抗侵蚀性和藓结皮的人工培育,近几年对生物结皮中的微生物多样性亦开展了详细的研究,为后期深入研究奠定了坚实基础。然而,在生物结皮的演替机制、生物结皮微生物功能群与养分转化规律、生物结皮不同生物类群之间以及生物结皮与维管植物的相互作用关系还缺乏深入研究。未来的研究在以下方面仍需进一步加强:1)加强黄土高原生物结皮微生物多样性的系统研究,微生物多样性由描述性工作向机理性研究转变,如对生物结皮演替过程的微生物群落结构、群落物种共存和构建机制可进一步深入研究;2)深入开展生物结皮的功能微生物多样性及其对碳、氮、磷关键元素生物地球化学循环的驱动作用的相关研究;3)进一步研究生物结皮中不同生物类群的种间关系,如微生物(自养微生物-异养微生物)-孢子植物(地衣、苔藓)-种子植物之间的相互作用,在区域内筛选出适宜的乔-灌-草-生物结皮相结合的组合模式;4)深入研究生物结皮蓝藻、真核藻类和苔藓对培养基质、水分、温度、关键养分(N、P、K)、pH等关键环境因素的响应机制,在实验室或野外建立起生物结皮培养技术体系,为更好地利用人工生物结皮开展生态修复奠定理论基础。
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