土壤微生物在地球化学、生态系统物质循环和能量流动中扮演着重要角色, 它们通过调控土壤肥力与结构影响植物营养与健康, 进而耦合地下、地上生态过程(Curtis & Sloan, 2005)。城市土壤微生物作为城市生态系统的重要组成部分, 在缓解土壤污染、调控碳氮循环、改善土壤环境等方面发挥着不可替代的作用, 是支撑城市生态系统稳定健康的基础(Delgado-Baquerizo et al, 2016; Bond-Lamberty et al, 2018)。同时, 它们也与人体健康密切相关。例如, 有研究认为减少人与自然环境及生物多样性的接触可能会影响人类共生菌群及其免疫调节能力, 并导致免疫功能障碍和慢性炎症性疾病(von Hertzen et al, 2011; Hanski et al, 2012; Ruokolainen et al, 2015), 而增加与土壤微生物多样性接触可以有效降低过敏人群的患病率(von Hertzen et al, 2011; Hanski et al, 2012), 因此, 城市土壤微生物多样性对人类健康也有重要影响。
当前, 生物多样性受到严重威胁, 尤其是人类活动正使生物多样性加速丧失, 物种消亡速度达到了有史以来的最大值(Pimm et al, 1995)。城市化是所有人类活动中较为剧烈的方式, 被认为是威胁生物多样性的主要因素之一, 也是物种灭绝和同质化最主要的驱动力(Savard et al, 2000; McKinney, 2006; Buczkowski & Richmond, 2012)。同时, 城市化过程中城市扩张所引起的生境丧失, 比其他类型造成的生境丧失持续时间更长且更为严重(McKinney, 2002)。土壤微生物对环境敏感, 易受其变化影响, 城市化过程中的环境变化将导致土壤微生物群落结构和多样性发生显著变化(Yan et al, 2016; Novoa et al, 2020)。然而, 已有关于城市生物多样性的研究和保护多集中于城市植物和动物, 对城市土壤微生物及城市化对土壤微生物的影响等方面认识不足。
随着全球城市化快速发展, 解析城市土壤微生物群落特征及其影响机制, 是当前城市生态学所面临的重要科学问题之一, 也是生物多样性保护面临的新挑战。因此, 本文综述了城市化对土壤微生物多样性的影响, 分析了城市土壤微生物群落特征及城市化对土壤微生物的影响, 探讨了城市土壤微生物多样性的维持策略, 并对未来的研究进行了展望, 旨在为城市土壤微生物多样性的保护、生态系统服务功能的维持及城市生态学研究提供参考。
1 城市化对土壤微生物群落组成特征的影响
城市化的快速推进已经成为全球普遍趋势, 据估计到2050年, 在全球范围内约68%的人口将居住在城市(UN DESA, 2018)。城市化给人类社会带来巨大经济发展和生活便利的同时, 也使原生生态系统遭到毁灭性破坏, 更带来了前所未有的生态环境问题, 如: 土壤污染、水体污染、大气沉降、城市热岛、生境破碎化和土地利用方式转变等, 对城市环境中的生物多样性产生了严重威胁(Grimm et al, 2008; McKinney, 2008)。城市化过程沿中心城区向外扩展形成了城市中心区-城市郊区-自然区域的城市化梯度, 也造就了城市化环境梯度(图1)。城市化梯度上环境类型多样、空间异质性较大, 土壤微生物群落组成和分布也存在差异(Yan et al, 2016; Wang et al, 2017)。城市发展过程中的基础设施建设、城市道路交通等对城市土壤产生了强烈的扰动, 造成土壤孔隙度与渗透性减小、土壤污染、土壤养分流失、土壤酸碱度变化、土壤生物群落变化等(张甘霖等, 2003), 引起城市土壤的理化性质、生物特征等方面的改变(Scharenbroch et al, 2005; Kaye et al, 2006), 进而对土壤微生物群落产生影响。
图1
图1
城市化环境梯度 (改编自网络图片)。
箭头表示城市化梯度方向。
Fig. 1
The environmental gradients of urbanization (adapted from the web images).
Arrow indicates the direction of the urbanization gradient.
1.1 城市化对土壤微生物量的影响
一般来说, 城市土壤的微生物生物量低于农村土壤(Zhu & Carreiro, 2004; Rai et al, 2018), 如符方艳和陆宏芳(2015)基于城区-近郊-远郊的城市化梯度, 发现土壤微生物生物量在远郊最高, 而城区土壤微生物量显著低于远郊。还有研究发现城市化导致土壤中微生物个体总数减少(侯颖等, 2014)。对北京城市土壤微生物量研究也得到相似结论, 即沿城市到自然区域的城市化梯度上土壤微生物量呈现出增加的趋势(Zhao et al, 2012)。但也有学者认为, 城市化并没有对土壤微生物量产生严重影响, 相比于自然土壤, 城市土壤微生物量碳、氮都没有呈现出显著的减少, 反而略有升高(王焕华等, 2005)。Wang等(2017)对厦门城市土壤微生物研究发现, 虽然相比于城区和郊区, 自然区域的土壤微生物量仍为最高, 但城区土壤细菌微生物量略高于郊区。这可能是城市土壤受人类活动频繁影响, 土壤有机碳含量升高, 激发了土壤微生物和酶的活性, 最终使土壤微生物量增大(Madejón et al, 2001)。
1.2 城市化对土壤微生物群落组成的影响
土壤微生物群落组成与土壤环境紧密相关, 城市化发展过程造成土壤环境变化进而对城市土壤微生物群落组成及分布特征产生影响。随着高通量测序技术的广泛应用, 当前我们对城市土壤微生物群落组成已有了深刻的认识。基于已有的研究, 我们发现城市土壤细菌的优势菌门是变形菌门、酸杆菌门、拟杆菌门、放线菌门、芽单胞菌门和疣微菌门, 土壤真菌的优势菌门(相对丰度 > 1%)主要包括, 子囊菌门、担子菌门和接合菌门(表1)。同时, 沿城市化梯度(城市-郊区-农村)土壤细菌群落组成呈现显著差异(Wang et al, 2017)。虽然在门分类水平上城市土壤主要细菌、真菌存在较高的相似性(Ramirez et al, 2014; Huot et al, 2017; Tan et al, 2019), 但属、种组成及其相对丰度因城市化水平不同而存在差异(Xu et al, 2014; Yan et al, 2016; Tan et al, 2019)。笔者发现, 虽然土壤微生物群落组成在城市化过程中发生明显变化, 但其在城市化梯度上的分布规律仍不清楚, 还需进一步研究。
表1 城市土壤微生物群落组成
Table 1
| 研究区域 Study area | 研究对象 Research object | 环境梯度 Environmental gradient | 微生物类群 Microbial taxa | 优势菌门 Dominant phylum | 文献来源 References |
|---|---|---|---|---|---|
| 中国16个城市 16 representative cities, China | 公园土壤 Park soil | - | 细菌 Bacteria | 变形菌门、放线菌门、酸杆菌门、浮霉菌门、绿弯菌门、拟杆菌门 Proteobacteria, Actinobacteria, Acidobacteria, Planctomycetes, Chloroflexi, and Bacteroidetes | Xu et al, 2014 |
| 中国广东东莞 Dongguan, Guangdong, China | 公园土壤 Park soil | 城市-郊区-农村 Urban-Suburban-Rural | 细菌 Bacteria | 变形菌门、酸杆菌门、放线菌门、疣微菌门、WPS-2、浮霉菌门、绿弯菌门、拟杆菌门 Proteobacteria, Acidobacteria, Actinobacteria, Verrucomicrobia, WPS-2, Planctomycetes, Chloroflexi, and Bacteroidetes | Tan et al, 2019 |
| 真菌 Fungi | 子囊菌门、担子菌门、接合菌门 Ascomycota, Basidiomycota, and Zygomycota | ||||
| 美国纽约 New York, USA | 绿色基础设施土壤/生长介质 Green infrastructure soil/substrate | 街边绿地-城市森林 Street green space-Urban forest | 细菌 Bacteria | 变形菌门、酸杆菌门、拟杆菌门、放线菌门、疣微菌门、浮霉菌门、绿弯菌门 Proteobacteria, Acidobacteria, Bacteroidetes, Actinobacteria, Verrucomicrobia, Planctomycetes, and Chloroflexi | Joyner et al, 2019 |
| 西班牙加利西亚 Galicia, Spain | 海滩沙丘 Coastal dune | 城市海滩-自然海滩 Urban beaches-Natural beaches | 细菌 Bacteria | 变形菌门、放线菌门、酸杆菌门、拟杆菌门 Proteobacteria, Actinobacteria, Acidobacteria, and Bacteroidetes | Novoa et al, 2020 |
| 美国纽瓦克市 Newark, USA | 树木根际土壤 Tree rhizosphere soil | 城市-郊区-农村 Urban-Suburban-Rural | 细菌 Bacteria | 浮霉菌门、变形菌门、绿弯菌门、酸杆菌门 Planctomycetes, Proteobacteria, Chloroflexi, and Acidobacteria | Rosier et al, 2021 |
| 中国北京 Beijing, China | 公园土壤 Park soil | 现代公园-古典公园 Young park-Old park | 细菌 Bacteria | 酸杆菌门、变形菌门、绿弯菌门、放线菌门 Acidobacteria, Proteobacteria, Chloroflexi, and Actinobacteria | 张骏达等, 2019 |
| 中国福建 Fujian, China | 草坪土壤 Turfgrass soil | 城市-郊区-自然区域 Urban-Suburban-Natural | 细菌 Bacteria | 变形菌门、酸杆菌门、放线菌门、装甲菌门、厚壁菌门、疣微菌门 Acidobacteria, Acidobacteria, Actinobacteria, Armatimonadetes, Firmicutes, and Verrucomicrobia | Zhang et al, 2020 |
| 美国芝加哥 Chicago, USA | 公园/街道/居民区绿地土壤 Park/Street/Resid-ential soil | 人口密度/绿地类型 Population densities/Greenspace types | 细菌 Bacteria | 变形菌门、酸杆菌门、拟杆菌门、疣微菌门、放线菌门 Proteobacteria, Acidobacteria, Bacteroidetes, Verrucomicrobia, and Actinobacteria | Wang et al, 2018 |
| 芬兰赫尔辛基、拉赫蒂 Helsinki and Lahti, Finland | 公园土壤 Park soil | 公园年龄: 10, 50, >100 Park age: 10, 50, >100 | 细菌 Bacteria | 变形菌门、酸杆菌门、放线菌门、绿弯菌门、芽单胞菌门、硝化螺旋菌门 Proteobacteria, Acidobacteria, Actinobacteria, Chloroflexi, Gemmatimonadetes, and Nitrospirae | Hui et al, 2017 |
| 真菌 Fungi | 子囊菌门、担子菌门、接合菌门、球囊菌门、壶菌门 Ascomycota, Basidiomycota, Zygomycota, Glomeromycota, and Chytridiomycota | ||||
| 中国北京 Beijing, China | 公园/街道/居民区绿地土壤 Park/Street/Resid-ential soil | 城市环路: 2环、2-3环、3-4环、4-5环、5环 Urban ring road: 2H, 2-3H, 3-4H, 4-5H, 5H | 细菌 Bacteria | 变形菌门、酸杆菌门、拟杆菌门、放线菌门、芽单胞菌门、疣微菌门 Proteobacteria, Acidobacteria, Bacteroidetes, Actinobacteria, Gemmatimonadetes, and Verrucomicrobia | Yan et al, 2016 |
| 中国北京 Beijing, China | 公园土壤 Park soil | 公园年龄: 10, 30, >100年 Park age: 10, 30, > 100 | 细菌 Bacteria | 变形菌门、酸杆菌门、拟杆菌门、放线菌门、芽单胞菌门、疣微菌门、浮霉菌门 Proteobacteria, Acidobacteria, Bacteroidetes, Actinobacteria, Gemmatimonadetes, Verrucomicrobia, and Planctomycetes | Yan et al, 2021 |
| 美国纽约 New York, USA | 绿色基础设施土壤/生长介质 Green infrastructure soil/subtrate | 绿色屋顶-街旁洼地-非工程化绿色基础设施 Green roof-Bioswale-Non-engineered Green infrastructure | 细菌 Bacteria | 变形菌门、放线菌门、酸杆菌门、拟杆菌门、浮霉菌门、疣微菌门、厚壁菌门、绿弯菌门、芽单胞菌门 Proteobacteria, Actinobacteria, Acidobacteria, Bacteroidetes, Planctomycetes, Verrucomicrobia, Firmicutes, Chloroflexi, and Gemmatimonadetes | Gill et al, 2020 |
| 真菌 Fungi | 子囊菌门、担子菌门、被孢霉门、球囊菌门、油壶菌门 Ascomycota, Basidiomycota, Mortierellomycota, Glomeromycota, and Olpidiomycota | ||||
| 研究区域 Location | 研究对象 | 环境梯度 Environmental gradient | 微生物类群 Microbial taxa | 优势菌门 Dominant phylum | 文献来源 References |
| 中国北京 Beijing, China | 公园/居民区/街道绿地土壤 Park/Residential/ Street soil | 公园-街道-居民区-工业区绿地 Park-Street-Residential- Industrial green space | 细菌 Bacteria | 变形菌门、酸杆菌门、绿弯菌门、放线菌门、芽单胞菌门、拟杆菌门 Proteobacteria, Acidobacteria, Chloroflexi, Actinobacteria, Gemmatimonadetes, and Bacteroidetes | Zhang et al, 2019 |
| 中国北京 Beijing, China | 公园土壤 Park soil | 公园年龄 Park ages | 真菌 Fungi | 子囊菌门、担子菌门、被孢霉门、壶菌门、隐真菌门 Ascomycota, Basidiomycota, Mortierellomycota, Chytridiomycota, and Cryptomycota | 于天赫等, 2021 |
城市环境中土地覆盖类型多样, 不同土地覆盖类型承受的城市压力和人类活动干扰强度不同(Reese et al, 2016), 进而引起土壤生境差异, 导致土壤微生物群落组成不同。例如, Hu等(2018)对北京5种典型的土地覆盖类型(不透水的表面、可渗透的路面、灌木、草坪和路边的树木)下土壤细菌群落研究发现, 土地覆盖类型改变了细菌群落组成, 在不透水表面之下的土壤细菌群落组成与其他的土地覆盖类型存在差异。在垂直方向上(即不同土层深度)细菌群落组成也呈现出差异。表现为, 城市土壤剖面不同深度细菌群落组成发生变化, 某些细菌门, 如拟杆菌门、浮霉菌门, 在表层土壤中呈现较为丰富的情况, 而在较深的土层中则有另外的一些菌门, 如绿弯菌门、芽单胞菌门、硝化螺旋菌门、酸杆菌门, 丰度更高(Michel & Williams, 2011; Eilers et al, 2012)。
土壤微生物可调控生物地球化学循环, 其群落结构对生态系统功能发挥起决定性作用(Fuhrman, 2009)。沿城市-郊区-农村梯度, 土地利用方式和人口密度呈现明显差异, 土壤细菌群落结构也显示出显著差异(Wang et al, 2017)。一般来说, 在城市建成区, 土壤微生物群落物种和结构不同于城市边缘及周边农耕区, 建成区的细菌和真菌所占的比例高于郊区和农村, 而放线菌占比则是农村要高于城市建成区和郊区(侯颖等, 2014)。杨元根等(2000)研究发现, 城市建成区和邻近农耕区土壤微生物群落结构发生了改变。闫冰等(2016)对北京城市化梯度下土壤微生物的研究发现, 居民区内绿地和公园内绿地中土壤微生物群落结构具有显著的空间分异特征, 而街道边绿地在城市环路梯度上无明显的土壤微生物群落结构分异, 说明不同类型绿地土壤微生物群落结构受城市化影响不同。现有研究表明, 城市化发展过程中土壤微生物群落结构发生明显变化。基于高通量测序分析也发现, 土壤细菌群落结构在不同城市发展梯度下存在显著差异(Yan et al, 2016), 进一步佐证了上述观点。
2 城市化对土壤微生物多样性的影响
土壤中蕴藏着巨大的微生物多样性, 对于土壤功能的发挥(包括碳、氮及其他养分转化、温室气体排放等)及整个生态系统恢复力的维持都极为重要(Allison & Martiny, 2008; Gkorezis et al, 2016; Reese et al, 2016; Neilson et al, 2017)。尽管自2000年以来, 有关土壤生物多样性的知识积累呈指数增长, 但迄今为止, 相关研究工作主要集中在自然或农业环境上(De Kimpe & Morel, 2000), 对城市土壤的调查相对较少(仅约1%的土壤研究涉及城市土壤, 2020年2月Web of Science查询结果)。因此, 人们对城市土壤的微生物多样性特征, 及其在城市生态系统服务功能中扮演的角色, 以及在改善城市居民生活质量上发挥的重要作用等方面所知甚少(McDonnell & Hahs, 2008; Pataki et al, 2011; Morel et al, 2015)。
2.1 城市化梯度上土壤微生物多样性分布特征
城市化过程中, 随着建设时间和人类活动强度的不同形成了城市化环境梯度, 对土壤微生物多样性产生了影响。例如, 沿着北京不同城市环路梯度(2环-5环), 土壤细菌多样性呈先下降后升高的趋势(Yan et al, 2016)。此外, 在城市中心区-近郊-远郊的城市化梯度上, 与远郊乡村或自然生态系统相比, 城市环境受高度干扰和破碎化的生态系统, 其草坪土壤细菌多样性低于乡村农田(Wang et al, 2017), 并且城区森林中通常显示出较低的菌根真菌物种丰富度和多样性(Ochimaru & Fukuda, 2007)。Epp Schmidt等(2017)通过对全球5个城市的土壤微生物研究发现, 城市化导致土壤真菌多样性丧失及群落同质性, 但土壤中古菌和细菌的多样性不易受城市化影响。即使在城市内, 不同类型绿地的土壤微生物多样性也不同。城市绿地中, 城市公园被认为是支撑居民福祉的重要绿地类型。尽管城市公园的主要作用是休闲娱乐, 但它们同时也是城市环境中生物多样性的重要避难所。城市公园相对于其他城市绿地而言生境类型多样, 因此, 公园土壤微生物多样性显著高于居民区绿地(Wang et al, 2018)。同时, 城市公园的历史年限也对土壤微生物群落多样性产生影响, 历史年限长的公园土壤微生物群落功能多样性及α多样性都高于历史年限短的公园(闫冰等, 2019; 张骏达等, 2019)。综上, 城市化改变了土壤微生物多样性, 但其对土壤细菌和真菌的影响不同。
城市化造成生物同质化并将导致全球生物多样性丧失(McKinney & Lockwood, 1999; McKinney et al, 2006; Baiser et al, 2012), 但有研究发现城市化似乎不影响土壤细菌多样性(Epp Schmidt et al, 2017), 意味着城市化并非必然导致土壤微生物多样性的丧失。实际上, 城市化过程中城市环境被人类不断改变, 这种条件更有利于生态幅更宽的物种生存(van Rensburg et al, 2009; Magle et al, 2012; Russo & Ancillotto, 2015)。这些物种能够应对多种多样的食物资源或极端温度条件, 而且它们通常表现出高度的表型(行为、生理或形态)可塑性。Ramirez等(2014)对纽约城市中央公园土壤微生物多样性研究, 发现即使在人工管理的土壤系统中仍然具有丰富的土壤微生物多样性, 但中央公园土壤中主要的细菌、古菌和真核生物的种系型的相对丰度和全球样品数据非常相似。意味着, 当前对于城市土壤微生物多样性分布格局及其形成机制还未能形成统一认识, 具体研究结果因研究尺度、绿地类型、微生物类群的不同而存在较大差异。
2.2 影响城市绿地土壤微生物多样性的主要因素
图2
图2
城市绿地土壤微生物多样性影响因素
Fig. 2
Impacting factors of soil microbial diversity in urban green space
2.2.1 土壤理化性质对土壤微生物多样性的影响
土壤微生物与土壤环境特征密切相关(Singh & Walker, 2006), 土壤pH (Fierer & Jackson, 2006)、土壤含水量(Brockett et al, 2012)、土壤类型(Wu et al, 2008)、土壤粒径(Wang et al, 2018)、盐分(Lozupone & Knight, 2007; Rajaniemi & Allison, 2009)、土壤养分有效性(Jesus et al, 2009)、有机碳和碳氮比(Shen et al, 2013)等均被认为是影响微生物群落的重要因子, 其中土壤pH被认为是微生物多样性变化最好的预测因子之一(Chu et al, 2010; Zhalnina et al, 2015)。城市建设过程中, 人为活动(翻动、压实等)将改变城市土壤pH、容重、孔隙度及含水量和养分等, 进而对土壤微生物多样性产生影响(张甘霖等, 2003)。例如, Xu等(2014)对中国有代表性的16个城市土壤细菌群落的研究发现, pH是与土壤细菌群落变化最相关的因子。于天赫等(2021)通过对北京城市公园乔木土壤真菌的研究发现, 土壤pH、土壤养分和水分影响土壤真菌群落多样性, 城市土壤腐生真菌多样性与土壤pH呈正相关关系。对城市绿地土壤微生物多样性的进一步研究发现, 土壤含水量、土壤含砂量也与土壤微生物多样性密切相关(Wang et al, 2018)。就已有研究而言, 土壤理化性质被认为是影响城市绿地土壤微生物多样性的主要环境因子, 且pH是最重要的因子之一。
2.2.2 土地利用对土壤微生物多样性的影响
快速的城市扩张和人类扰动, 通过改变土地利用对未封闭的城市绿地土壤生物化学过程和土壤质量产生强烈的影响(Betts, 2007)。城市发展过程中, 农田、森林、草地等转换为城市用地, 土地利用类型的转变可能造成土壤微生物多样性变化。对比研究发现, 城市残留森林中土壤细菌的多样性指数(物种丰富度和Shannon指数)显着高于城市公园, 而土壤真菌多样性在两种绿地空间类型中相似(Barrico et al, 2018); 并且研究发现与农村土壤相比, 城市土壤中细菌的多样性有所减少(Huot et al, 2017; Wang et al, 2017; Parajuli et al, 2018)。如前所述, 土壤微生物群落的多样性也受到不同城市土地利用类型的影响。例如, Gill等(2017)对布朗克斯区(纽约州, 美国)的生态湿地、公园和树坑中细菌群落的比较研究发现, 与公园和树坑相比, 重建土壤(生态湿地)中细菌群落的多样性增加了。与此同时, 城市内部,不同功能区绿地土壤真菌α多样性指数存在差异(郭大陆等, 2022)。此外, 对城市内部不同类型绿地之间的土壤微生物多样性比较发现, 细菌和真菌物种丰富度在公园和路边绿岛都没有显著差异(Reese et al, 2016), 且对城市中3种土地利用类型的土壤微生物研究也发现, 城市不同土地利用类型似乎不影响细菌多样性(Epp Schmidt et al, 2017)。究其原因, 笔者认为可能是土地利用往往通过间接作用影响土壤微生物多样性。一方面, 土地利用类型的变化通过改变土壤理化特征及生境作用于土壤微生物; 另一方面, 土地利用类型的转变在一定程度上也涉及到植被类型和植物群落的变化, 这也决定了土地利用变化可能耦合植物群落共同影响土壤微生物多样性。
2.2.3 植物对土壤微生物多样性的影响
植物与土壤微生物关系密切, 不同植物物种的生物化学组成不同, 植物多样性的变化可能会改变养分资源的质量和数量, 进而影响土壤微生物群落(Zak et al, 2003; Neilson et al, 2008); 并且植物物种差异影响土壤养分库和凋落物及根系分泌物化学组成, 从而影响土壤细菌群落多样性(Orwin et al, 2006; Berg & Smalla, 2009)。基于实验研究发现, 增加植物物种丰富度能导致微生物群落功能多样性的增加(Zak et al, 2003; Loranger-Merciris et al, 2006)。此外, 植物根系的分泌物是土壤微生物重要的养分来源, 植物通过根系分泌物与土壤微生物相互之间共同进化, 最终引起土壤微生物群落呈现出多样性。例如, 有研究显示, 即使在土壤特征相同的土地上不同植物根际对应的微生物多样性也存在显著差异(Garbeva et al, 2008; Berg & Smalla, 2009)。这说明植物以凋落物和根系分泌物为纽带, 将植物与土壤微生物密切联系在一起, 进而影响土壤微生物多样性。
然而, 也有研究持相反观点, 认为植物多样性与土壤微生物多样性间无显著相关关系(Meier & Bowman, 2008; McGuire et al, 2013; Ramirez et al, 2014; Prober et al, 2015), 一方面城市绿地受人为管理影响严重, 地上凋落物经常被清扫, 因此不会对土壤微生物产生影响; 另一方面, 植物多样性与凋落物化学成分多样性无显著相关性, 即使物种组成不同但它们凋落物的化学组成成分可能相同(Meier et al, 2008)。同样, 有研究发现, 无论是在城市公园还是在绿地中, 植物多样性似乎都不是土壤微生物多样性的主要决定因素(McGuire et al, 2013; Ramirez et al, 2014), 进一步证实了上述观点。城市生态系统是典型的人工生态系统, 植物物种主要受人为控制, 这一特殊性导致植物对土壤微生物的影响与已知的其他生态系统的结果可能存在差异, 因此, 城市植物对土壤微生物的影响仍需进一步研究。
2.2.4 城市污染对土壤微生物多样性的影响
城市化过程中人类活动剧烈, 导致城市土壤普遍呈现严重污染的状况(De Miguel et al, 1998), 主要包括重金属、持久性有机污染物(POPs)、持久性有毒污染物(PTS) (Perrodin et al, 2011)和抗生素抗性基因(ARGs) (Yan et al, 2019)。人类活动产生的重金属在土壤中富集, 随之而来的城市土壤重金属污染问题日益突出(McBride et al, 2014), 城市土壤中重金属和有机污染物的累积, 以及生活废弃物和废水中污染物直接或间接进入土壤(李有文等, 2017), 对土壤微生物多样性产生了影响(杭小帅等, 2010)。例如, 有研究发现, 土壤微生物多样性随着重金属浓度的增加而降低(Xie et al, 2016); Zhang等(2015)的研究表明, 土壤重金属污染显著改变土壤细菌多样性; Singh等(2019)研究发现, 城市受污染的土壤中细菌群落对重金属污染敏感, 尤其是与Cu、Zn和Pb关系密切。但不同土壤微生物类群受土壤污染的影响不同, 相比于土壤细菌, 土壤真菌群落更易受重金属污染影响(Rex et al, 2018; Abo Shelbaya et al, 2021)。例如, 于天赫等(2021)研究发现, 土壤真菌丰富度指数与重金属铅含量呈负相关关系; 并且郭大陆等(2022)认为, 随重金属铅污染程度加深土壤真菌多样性降低。与城市重金属污染情况类似, 城市绿地土壤中多环芳烃(PHAs)显著高于周边农村, 城市土壤普遍存在有机污染问题(蒋煜峰, 2009; Peng et al, 2016)。相比于重金属, 持久性有机污染物和持久性有毒污染物具有挥发特性, 对土壤微生物更具危害性, 然而相关研究不足。城市化过程中, ARGs丰度和分布特征也受到显著影响, 例如, 工业因素(Yan et al, 2019)和再生水灌溉(Wang et al, 2014)都引起城市绿地ARGs丰度升高。ARGs通过水平基因转移的方式进入微生物体内, 并在微生物间进行扩散传播。这使得某些微生物类群成为特定抗生素抗性基因的宿主, 进而产生多重耐药性(Wichmann et al, 2014)。因此, 当土壤中存在抗生素输入时, 某些类群可能消失, 而具有抗生素抗性的类群则大量繁殖, 进而可能对土壤微生物多样性产生影响。然而, 相关研究当前多关注于土壤微生物群落结构对抗性基因丰度的影响(Forsberg et al, 2014; Dunivin & Shade, 2018), ARGs如何影响土壤微生物多样性仍需进一步研究。
3 城市化对土壤微生物群落功能的影响
土壤微生物是凋落物分解、元素循环转化、气候调节和污染物消纳等土壤及生态系统功能发挥的引擎, 因此城市化对土壤微生物群落功能的影响正在成为城市土壤生态学的研究热点。例如, 研究发现, 空间轴上的不同城市土壤特性或时间轴上的同一城市化历史均导致微生物碳、基础呼吸和微生物代谢熵不同(Yang et al, 2006; Ghosh et al, 2016), 即使是同一城市, 土壤剖面深度也影响其微生物特性, 城市化主要对表层土壤微生物产生影响, 而对下层土壤微生物的影响较小, 如Demina等(2018)研究发现, 表层土壤和下层土壤微生物呼吸存在差异, 并发现土地利用历史也导致城市地区表层土壤微生物呼吸存在高度的空间变异。此外, 城市土地利用类型多样, 即使同一土地利用类型也存在不同植被类型, 如公园内的草坪、疏林和近自然林等, 也造成土壤呼吸速率显著不同, 并且具有显著的季节变化特征(张鸽香等, 2010)。
在城市-郊区-农村梯度上, 与城市和郊区的草地土壤相比, 农村农田土壤N2O排放潜力和反硝化活性更高, 这表明由于城市化导致的土地利用变化可能会减少反硝化过程中N2O的排放(Wang et al, 2017)。进一步对城乡环境梯度森林土壤微生物研究发现, 相比远郊林、城市林, 乡村自然林土壤微生物群落碳源代谢能力最高, 城乡梯度上土壤微生物群落碳源利用能力存在差异(何越等, 2021), 并且城市内不同类型绿地土壤微生物群落偏向于利用不同类型碳源, 但对羧酸类碳源的利用能力都较弱(闫冰等, 2016)。此外, 即使城市内的同一土地利用类型, 不同的利用历史也会导致土壤微生物功能变化(Hermans et al, 2020)。例如, 闫冰等(2019)研究发现, 不同历史年限公园土壤微生物利用碳源能力存在差异, 历史年限较短的公园对多聚物类碳源利用能力强。由此可见, 土壤微生物功能在城市不同生境下具有明显的分异特征, 但已有研究停留在单一功能描述上, 对城市土壤微生物功能的认识不足, 下一步需利用多组学技术深入探究城市化对土壤微生物群落功能的影响。
4 保护和维持城市土壤微生物多样性
4.1 保护城市土壤微生物多样性
土壤微生物具有分解、转化和运输并参与生物地球化学循环以及土壤结构的形成和维持等功能, 对于陆地生态系统服务功能的稳定发挥不可或缺, 因此对于人类社会的发展也必不可少。此外, 最近研究表明人们广泛接触环境中因土地利用类型多样而产生的微生物多样性, 可以有效降低过敏症的患病率(von Hertzen et al, 2011; Hanski et al, 2012), 因此, 这可能是一种潜在的更有益于人类健康的途径。与之相反, 减少人与自然环境及生物多样性的接触可能会对人类共生菌群及免疫调节能力产生不利影响, 并导致免疫功能障碍和慢性炎症性疾病(von Hertzen et al, 2011; Hanski et al, 2012; Ruokolainen et al, 2015)。White等(2019)通过大量调研认为, 每周花费至少2 h亲近自然环境将有助于身心健康, 并可能对控制疾病有积极作用。进一步的研究发现, 广泛接触环境中的土壤微生物多样性, 可改善人体微生物多样性增强免疫系统调节能力(Roslund et al, 2020; Selway et al, 2020)。由此可见, 城市绿地土壤微生物多样性与居民健康密切相关, 城市土壤微生物多样性的维持和保护, 不论是对城市可持续发展还是对城市居民健康, 都显得尤为重要。
4.2 维持城市土壤微生物多样性
土壤微生物与地上植物和其他大型生物不同, 人们很少能直接管理土壤微生物, 因此, 维护土壤生物多样性极具挑战性。一般而言, 增加地上生境的复杂性会促进地下生物多样性(Byrne, 2007; Ossola et al, 2015)。例如, Mills等(2020)认为植被重建可以通过创造更多的野生栖息地条件, 进而改善城市土壤微生物群落多样性。McGuire等(2013)对纽约市绿化屋顶网络研究表明, 虽然屋顶绿地基质人工化程度比较高, 但也为微生物群落提供了栖息地。尽管屋顶绿地土壤微生物量低于同一城市公园绿地, 但仍具有较高的微生物量(McGuire et al, 2013)。屋顶绿地基质也是多种真菌种群的栖息地, 这些真菌种群在污染和高度干扰的环境中具有很强的生存能力(McGuire et al, 2013),说明在这些人类活动强烈的环境中也会形成特殊的微生物群落生态。此外, 具有自然残留植被的城市公园, 通常是生物多样性丰富的区域(Shanahan et al, 2015)。不同种类的城市景观提供了不同水平的生物多样性, 因此, 可以针对不同的需求进行城市景观管理以提高土壤微生物多样性。
人们已经认识到, 维持多样化的土壤微生物群落对于维持城市景观中的土壤长期稳定和人居环境健康至关重要。然而, 当前关于城市环境中土壤微生物多样性维持和保护的文献相对较少, 对于如何调控土壤微生物多样性仍是一个需要深入探究的方向。
5 展望
中国正处于并将持续处于快速城市化进程中。城市生物多样性的研究一直受到大量关注, 城市地上生物多样性的研究已经为人们认识城市化对生物多样性的影响提供了基础。对城市生物多样性的研究, 一方面对生物多样性的认识和保护起到积极作用, 另一方面为城市生态学发展提供理论基础。城市化是生物多样性灭绝和同质化的主要原因之一, 但是由于微生物本身的复杂性及城市环境的复杂性等方面的限制, 关于城市化对土壤微生物多样性的影响仍有待进一步研究。基于对已有研究的梳理, 今后相关研究应注重以下3个方面:
(1)城市化对城市绿地土壤微生物多样性的影响机制。当前, 城市化对土壤微生物多样影响的相关研究仍然不足, 仍存在未知的影响因子, 且已有研究多集中于相关分析, 具有很大的局限性。今后的研究需要进一步解决尺度问题, 在根际、不同大小绿地以及城市不同功能区尺度上开展研究, 并结合地理信息系统技术手段揭示城市景观格局下土壤微生物多样性空间格局特征, 通过多学科交叉融合探究城市化对土壤微生物多样性的驱动机制, 为研究城市土壤微生物多样性维持策略提供基础。
(2)城市土壤微生物多样性变化对生态系统多功能性的影响。城市化引起土壤微生物群落结构和多样性变化, 应进一步运用多组学技术结合生物信息学分析手段, 深入探究土壤微生物群落功能变化特征, 明确土壤微生物多样性变化对土壤碳氮循环、污染物降解、养分维持、气候调节等生态系统功能发挥的影响, 对理解城市土壤微生物如何保障城市生态系统服务功能具有重要意义。
(3)土壤微生物多样性与人类健康的关系。城市绿地中耐药菌和ARGs传播对人类健康存在潜在风险。应进一步研究城市土壤中耐药菌和ARGs的空间分布规律和迁移特征, 明确土壤微生物多样性在ARGs流动中的作用, 探索土壤微生物多样性在改善人居环境中的作用, 并通过维护城市土壤微生物多样性保障人类健康。
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北京城市公园绿地土壤微生物群落碳源代谢活性特征
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苏格兰阿伯丁城市土壤的微生物特性研究
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北京城市公园常见乔木土壤真菌群落特征及影响因素
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城市土壤质量演变及其生态环境效应
Soil respiration under different vegetation types in Nanjing urban green space
南京城市公园绿地不同植被类型土壤呼吸的变化
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基于高通量测序技术的不同年代公园绿地土壤细菌多样性
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