大气CO2浓度和温度升高对农田土壤碳库及微生物群落结构的影响

2021-09-18房蕊于镇华李彦生谢志煌刘俊杰王光华刘晓冰陈渊刘居东张少庆吴俊江StephenHerbert金剑

中国农业科学 2021年17期

房蕊，于镇华，李彦生，谢志煌,2，刘俊杰，王光华，刘晓冰，陈渊，刘居东，张少庆，吴俊江，Stephen J Herbert，金剑

房蕊1，于镇华1，李彦生1，谢志煌1,2，刘俊杰1，王光华1，刘晓冰1，陈渊1，刘居东1，张少庆1，吴俊江3，Stephen J Herbert4，金剑1

1中国科学院东北地理与农业生态研究所/黑土区农业生态重点实验室，中国哈尔滨 150081；2中国科学院大学，中国北京 100049；3黑龙江省农业科学院大豆研究所/农业农村部大豆栽培重点实验室/黑龙江省大豆栽培重点实验室，中国哈尔滨 150086；4Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA

大气CO2浓度和温度升高会通过影响作物的光合作用，从而影响光合碳向土壤中的输送。输入到土壤中光合碳含量的变化势必会对土壤外源碳的主要分解者--微生物的群落结构产生影响。土壤微生物在土壤有机质的转化过程中发挥着重要的作用，是土壤碳循环的主要驱动者，其群落结构和功能的改变会影响土壤有机质的动态变化，而这些变化会进一步增加或者降低大气中的CO2浓度，从而对气候变化产生反馈作用。未来土壤的碳平衡取决于大气CO2浓度和全球变暖对土壤中碳的输入、输出以及碳在土壤中的驻留时间。因此，只有全面了解大气CO2浓度和温度升高将对土壤碳库及土壤微生物群落结构产生何种影响，才能明确地揭示陆地生态系统对气候变化的反馈机制，对未来农田土壤有机碳库的管理和生产力的维持有重要意义。文章综述了大气CO2浓度和温度升高及其交互作用对土壤碳库和土壤微生物群落结构的影响。主要结论为：（1）大气CO2浓度和温度升高对土壤碳库的影响可以相互抵消，但是土壤碳库是否成为碳“源”与温度升高的幅度密切相关；（2）大气CO2浓度升高增加了光合碳在玉米、小麦等植株各部分的分配，温度升高同样对光合碳的分配规律产生影响，但对不同部位的影响不一致，多呈降低或无显著影响；（3）大气CO2浓度和温度升高可能对土壤微生物活性及其群落结构产生交互影响，且对不同微生物（细菌、真菌和古菌）群落的影响程度不同，进一步对土壤有机碳的转化产生影响。最后提出未来的研究方向：（1）从气候变化影响植物-土壤互作角度解析根系分泌物的转化过程及其对微生物的影响；（2）通过DNA-SIP进一步研究大气CO2浓度和温度升高条件下土壤微生物对不同植物来源碳的选择性利用与碳循环的关系，从而阐明气候变化条件下微生物底物利用策略以及微生物群落结构的变化。

气候变化；土壤有机质；微生物；光合碳；根系分泌物

0 引言

工业革命前，大气中的CO2浓度相对稳定，约为279 μmol·mol-1自工业革命以来，由于人类活动导致温室气体排放急剧增加，目前CO2浓度已突破416 μmol·mol-1，约超出工业化前水平的45%（https://www.co2.earth），预计到2050年CO2浓度将会达到550 μmol·mol-1[1]，到21世纪末，CO2浓度将会达到700 μmol·mol-1[2]。CO2累积排放量的增加是造成全球地表温度变暖的主要原因[3-4]，随着大气CO2浓度的增加，预计到21世纪末地表温度将升高3.7—4.8℃[2]，而我国的平均温度将会上升1—5℃[5]或者更高（3.9— 6.0℃）[6]。

CO2浓度和温度作为影响植物生长的两个关键环境因子，对植物的生长发育和生理功能都会产生影响[7]。CO2是光合作用的底物，植物通过光合作用将大气中的CO2固定到植物体内，又通过根系分泌物、凋落物及根系生物量等将一部分光合碳输入到土壤中[8-9]，为土壤中的微生物提供碳源和能源。陆地生态系统的碳循环通过植物的光合作用和呼吸作用，以及土壤微生物的共同作用影响大气CO2的浓度，使陆地植被系统成为“碳汇”；然而，土壤是否成为碳“源”或者“汇”取决于土壤碳库的平衡[10]。陆地生态系统中的碳储量取决于光合碳的输入和以CO2的形式及甲烷和可溶性有机碳的损失之间的平衡[11]，大气中CO2浓度的增加和由此引发的全球变暖可能会通过改变碳的吸收和释放速率来影响这一平衡。然而，气候变化将对土壤碳库将产生怎样的影响还不清楚。土壤有机碳库是陆地生态系统中最重要的碳储存库，植物是其重要来源，大气CO2浓度日益升高会通过影响光合作用进而改变植物的养分运输和干物质量积累，从而对光合同化物、残体凋落物及根系分泌物产生影响，进而影响土壤碳库的含量及微生物群落结构[12]。而温度升高会刺激土壤微生物活性，加速微生物对新输入到土壤中的有机碳和土壤原有有机碳的分解速率，进一步增加CO2向大气中的排放[13]。

土壤中碳含量取决于土壤中碳的输入、输出以及碳在土壤中的驻留时间。有研究表明，大气CO2浓度升高和温度升高都加速了土壤中有机碳的分解速率，并且两者共同升高时产生的影响更明显，这表明在大气CO2浓度和温度共同升高的作用下可能有更多的有机碳被矿化，从而释放更多的CO2到大气中[14]。然而，目前关于全球气候碳循环模型预测，大气CO2浓度升高将增加土壤有机碳的含量，至少在一定程度上抵消了因温度升高而引起的有机碳分解对碳库所造成的损失[15]。也有研究认为，预计到21世纪末，大气CO2浓度升高和温度升高对陆地碳库的影响会达到平衡，表现为土壤有机碳库的总含量不变[16]。

土壤微生物在土壤有机碳的转化中发挥着重要的作用，作为土壤碳循环的主要驱动者[17]，其群落结构和功能的改变会对土壤有机碳的含量产生影响[18]。由于大气CO2浓度和温度升高的交互影响，根系分泌物的质和量也会相应改变，从而导致利用根际分泌物的根际微生物群落结构发生变化[19]。一般认为，大气CO2浓度升高可能会增加光合同化物的含量，并通过根系分泌物增加不稳定碳（如糖、羧酸及多肽等）的释放，这些可供微生物利用底物的增加刺激了微生物的活性，增加了土壤中有机碳降解酶的活性，从而加速了土壤有机质的分解，不利于土壤碳库的积累[20]。温度升高对土壤碳库的影响主要通过影响参与有机碳分解的微生物来实现，温度升高会迅速刺激土壤微生物的新陈代谢，导致微生物呼吸速率增加[21]。然而，从长远来看，温度升高对微生物的生长和活性的刺激作用会受到可利用底物的限制，从而改变微生物对气候变化的响应[22]。因而，只有全面了解土壤微生物群落结构的变化，才能清楚地揭示陆地生态系统对大气CO2浓度和温度升高的响应和反馈。

1 大气CO2浓度和温度升高对土壤有机碳库的影响

1.1 活性碳库和惰性碳库

植物光合作用是陆地和大气间碳循环的驱动力，植物通过根系分泌物向土壤中输送的光合碳是土壤有机碳的重要来源[12,23]。在陆地生态系统中，土壤有机质是联系土壤物理、化学和生物学性质的重要纽带，它不仅为微生物提供了营养物质及活动场所，适宜的有机质含量还有助于保持土地的可持续利用。因此，土壤有机质是评价土壤质量的重要指标。

根据有机碳周转的时间和在土壤中的存留时间，土壤碳库可划分为活性碳库、缓性碳库和惰性碳库3种[24]。其中活性碳库周转快，由新输入到土壤中且易降解的碳组成；惰性碳库则是长时期内不会有明显变化的碳库，因此是土壤中最稳定的碳，不易被分解和转化[25]。CHENG等[26]研究发现，在大气CO2浓度升高的作用下，植物来源的碳更多的被储存在惰性碳库中，有利于土壤碳的积累。与此相反，GILL等[27]发现大气CO2浓度升高会导致土壤活性碳库含量的增加，惰性碳库含量的损失，这可能是由于土壤中活性碳库增加对土壤原有有机碳的矿化率（-50%至＞300%）产生了影响[28]。土壤中碳的矿化速率对温度的变化很敏感，即使是小幅度的升温也可能促使土壤中的碳被大量释放。CONANT等[17]研究表明，惰性土壤有机碳库的温度敏感性大于活性土壤有机碳库，气候变化引起的土壤温度升高将提高土壤原有有机碳的分解率，从而导致惰性有机碳库含量的减少[29]。

1.2 颗粒有机碳

土壤颗粒物理分级是研究有机碳库的重要手段[30]。根据土壤颗粒的大小，通常将粒径＞250 μm的有机碳称为粗颗粒有机碳（coarse particulate organic carbon，cPOC），粒径在53—250 μm的有机碳称为细颗粒有机碳（fine particulate organic carbon，fPOC），粒径＜53 μm的有机碳称为矿质结合态有机碳（mineral-associated organic carbon，MOC）[31]。MOC能与细土壤微粒（粉粒和黏粒）结合，约占土壤总有机碳库的50%—80%[32]，它的周转时间较长，也更稳定，是最不易被微生物分解的组分，用来表征惰性碳库；而cPOC和fPOC则较容易分解[33]，相当于活性或缓性碳库。大气CO2浓度升高会通过改变植物凋落物[8]和根系分泌物的质和量来间接影响SOC，而这些过程对POC和MOC的影响是不同的。CARDON等[34]研究大气CO2浓度升高对加州草原有机碳含量的影响，结果显示，大气CO2浓度升高对POC和MOC组分产生了相反的影响，MOC周转变慢，而POC周转加速，同位素分析发现MOC中新碳的含量有所减少，土壤中新、旧碳库动态变化的对比效应明显，最终使得土壤碳总量平衡，这种变化对陆地生态系统和大气之间的长期净碳含量将产生重要的影响。与草原中的研究结果不同，在农业生态系统中，连续8年的农田FACE试验表明，小麦和豆科植物轮作体系下土壤的fPOC含量明显下降[35]，说明大气CO2浓度升高促进了土壤碳循环，加快了土壤活性碳库的周转速率，最终对一些农田生态系统的土壤固碳能力产生了限制作用（表1）。

WIESMEIER等[52]研究发现fPOC与温度呈显著负相关关系，说明温度升高更容易分解细颗粒有机碳。BENBI等[53]同样发现了POC比MOC对升温更敏感，这表明升温对MOC的影响较小，而对POC的影响较大。而FANG等[51]关于温度升高对亚热带森林土壤碳库的研究表明，温度升高增加MOC的分解，从而导致亚热带森林的碳损失比之前估计的更大。然而，也有7年升温试验显示，POC和MOC没有发生明显变化，这可能与土壤有机质结构的变化和组分之间的再分配有关[50]。同时升温试验的持续时间也可能导致结果的偏差，因为短期（＜10年）的升温可能不会对MOC产生显著的变化[54]。

表1 大气CO2浓度和温度升高对土壤碳库的影响

MOC代表矿质结合态有机碳；POC代表颗粒有机碳；MBC代表微生物碳；DOC代表水溶性有机碳；SOC代表土壤有机碳

MOC means mineral-associated organic carbon; POC means particulate organic carbon; MBC means microbial carbon; DOC means dissolved organic carbon; TOC means total organic carbon

在农业生态系统中，有关大气CO2浓度和温度同时升高对土壤碳库影响的研究较少。LOISEAU等[55]在气候变化试验中发现，大气CO2浓度升高增加了POC的含量，而温度升高则增加了其周转速率，二者同时升高显著增加了土壤原有有机质的分解速率。房蕊[36]关于气候变化对种植玉米的土壤碳库的影响发现，大气CO2浓度和温度同时升高未对黑土颗粒有机碳含量产生影响。应用稳定同位素技术示踪土壤原有碳库，CARRILLO等[56]在半干旱草原土壤上进行了为期7年的大气CO2浓度升高和升温试验，研究发现，温度单独升高并未对土壤中的碳含量产生影响，但是当温度和CO2浓度同时升高时，造成了土壤中原有有机碳的损失。说明大气CO2浓度和温度升高对碳库产生交互影响，但这种影响可能是叠加的，也可能是拮抗的，这与试验土壤的理化性质、升温的幅度和供试的植物种类有关。

1.3 团聚体

SOC的分解和周转受分解速率以及土壤矿物和团聚体对有机碳的保护程度的共同影响[57]。SIX等[58]研究表明，随着大气CO2浓度的升高，光合碳向土壤中输入的比例增加的同时团聚体也在增大，土壤有机质的周转速度随着团聚体的增大而加快，较小团聚体中的碳更稳定。同样，DORODNIKOV等[40]观察到，在添加葡萄糖后引起CO2浓度升高的条件下，相对微团聚体（＜0.25 mm），大的团聚体（＞2 mm）中的土壤有机碳周转速率明显增加。

温度升高降低了土壤团聚体的稳定性[46]，CHENG等[46]研究表明，随着温度升高，大团聚体（＞2 mm）中碳的分解速率提高。温度升高可能通过影响植物来源碳的输入和官能团结构进而影响土壤团聚体的稳定性和土壤有机碳含量。一方面，升温引起的土壤缺水会降低地上凋落物向土壤中的输入量，抑制了土壤团聚体的形成，增加了土壤侵蚀[59]，从而导致团聚体稳定性下降。另一方面，温度升高会改变团聚体官能团结构。GUAN等[47]指出升温显著减少了疏水性酚官能团，显著增加了亲水性羧基官能团，降低了土壤团聚体的水稳定性。然而，到目前为止有关CO2浓度和温度同时升高对农田土壤团聚体的长期影响还缺乏研究，开展这方面的研究将对发现农田土壤生产力对气候变化的适应性至关重要。

1.4 光合碳在植物-土壤中分配

光合碳在植物-土壤系统间的分配是生态系统中碳循环的重要环节，同大气环境与土壤质量的动态变化过程密切相关[60]。光合碳在植物-土壤中分配也随着生育期和作物种类的不同而有所差异。研究发现，光合碳虽在植物不同器官中的分配不同，但均在茎叶中的分配比例最高，约为40%—93%[36,61-63]，而分配到根系中的光合碳仅占2%—3.5%[62]，分配到根际和非根际土壤中的光合碳分别为9.27%和5.83%[63]，说明植物只有在满足自身生长的需求下，光合碳才会向根系及土壤中输出[64-65]。而HÜTSCH等[65]研究显示一年生植物同化的光合碳约有30%—60%分配到土壤中，这部分碳高达40%—90%以根系沉积物的形式释放到土壤中，但仅有2%—5%的光合碳被固定到土壤中形成了稳定的土壤有机碳。马田等[43]研究发现，大气CO2浓度升高显著增加了小麦生育后期根系中分配的光合碳含量。同样，石元豹等[66]通过13C同位素示踪标记研究了大气CO2浓度升高对枸杞生育期内各部分光合碳累积的影响也得到了相似的规律，即在CO2浓度升高条件下根系13C丰度较高，说明植物向地下分配的光合碳更多。此外，植物通过向根输送更多的碳，以减轻因CO2浓度升高而导致碳水化合物在叶片中的积累对叶片功能造成的不利影响[48]。升温会造成植物早衰，叶片光合能力受限，可能会降低光合碳向根的分配[67]，从而对根系碳净增加量未产生影响[68]。由此可见，气候变化显著影响光合碳在植物-土壤中的分配规律，这可能会对土壤碳循环产生进一步的影响。

1.5 土壤有机碳积累

土壤有机碳含量始终处在外源碳的输入和土壤有机碳的分解输出的动态变化过程中，作物会通过地上凋落物及根系分泌物等形式将碳输入到土壤中，这部分碳通过微生物的作用转化成有机碳固定到土壤中，而另一部分碳则会刺激微生物的活性，短期内会引起激发效应导致土壤有机碳被矿化，从而造成了土壤中原有有机碳的分解[69]，因此土壤中有机碳的消长是不断积累和分解的复杂的动态过程[70]。

陆地生态系统中，气候在很大程度上影响了土壤有机碳储量的平衡。气候变化一方面会对植物的生长产生影响，使得进入到土壤中的凋落物、根系或者分泌物发生变化；另一方面，会对土壤中微生物的活性和生存条件产生影响，从而改变微生物对有机碳的矿化速率[69]。大气CO2浓度和温度升高通过直接影响碳输入和/或土壤有机碳的分解速率来影响土壤碳库的变化[8,20,39]。研究表明，大气CO2浓度升高对多个生态系统中土壤碳库的影响都很小，甚至会造成碳库的损失[37,71-72]。VAN GROENIGEN[20]采用模型分析，表明大气CO2浓度升高对土壤碳的积累会产生不利的影响，主要是因为CO2浓度升高一方面增加了土壤碳的周转速率，另一方面由于激发效应加快了惰性碳库的分解。然而，与定位试验及培养试验的结果不同，对已发表文章的数据进行整合的META（Meta-analysis）分析指出，大气CO2浓度升高使土壤碳含量增加了约6%[70,73]。这些不同的结果可能与CO2浓度升高影响土壤碳输入与输出的平衡有关。GILL等[27]研究发现，经过4年大气CO2升高处理后的草地有机碳含量不变，表明新碳的输入和土壤中原有有机碳的分解之间达到了平衡[74]，但也有可能是土壤碳动态对大气CO2浓度升高的响应太小，难以被测量[71]。近期，KUZYAKOV等[75]总结大气CO2浓度升高对土壤碳库的相关研究，发现大气CO2浓度升高增加了碳向地下生态系统的分配，刺激了微生物的生长，加速了微生物的新陈代谢和呼吸速率，从而提高了酶活性，加速了土壤碳、氮和磷库的循环，从而抵消了植物向土壤中的碳输入。因此大气中的CO2浓度升高对碳库的影响不大，但会强烈加速微生物活性和稳定碳库的通量，从而加速碳、营养物质和非必需元素的生物地球化学循环。

温度升高一方面会通过缩短作物生育期和增强光合生物量的分解来降低作物的生产力，从而减少碳向土壤中的输入[76]；另一方面会通过刺激土壤微生物的活性，增加土壤呼吸而加快对土壤有机碳的分解，从而降低土壤碳库的储存[72,77]，因此温度升高更容易导致土壤有机碳的分解，利用13C同位素示踪技术进一步发现，温度升高会对土壤中原有有机碳造成损失[78]。研究者通常用模型来模拟温度升高对土壤有机碳产生的影响，但近年来模型分析的结果并不一致。META分析发现，气候变暖对土壤碳净储量没有产生影响[13]，而其他研究分析预测指出，全球土壤碳储量将随温度的升高而减少[72]。

在大气CO2浓度和温度同时升高的条件下，通常认为，两者对土壤有机碳的影响可以相互抵消[68]。从植物同化碳的角度分析，许多C3作物都是通过CO2富集而提高碳的同化量，一方面，由于CO2浓度升高增加了羧化作用并抑制了光呼吸释放CO2，促进了碳同化[79]；另一方面，升温会导致光合作用酶的失活，减少Rubisco的特异性，降低光合作用，增加光呼吸，导致作物生物量减少[80]。因此，温度升高可能会抵消CO2浓度升高对作物的促生长作用。当两者同时升高时，高温也会降低CO2浓度升高对产量的积累作用，温度升高一方面会加速植物的衰老从而缩短CO2富集的时间，另一方面可能会增加植物地上和地下部的自养呼吸[29]，因此，二者同时升高时可能不会对土壤碳库产生影响[48]。

全球气候-碳循环的模型预测大气CO2浓度升高能够增加土壤有机碳的积累，最终增加的这部分土壤有机碳能够抵消由温度升高而导致加快土壤有机碳分解速率的损失[15]。CARRILLO等[56]在一项为期7年的全球变化试验中发现，大气CO2浓度和温度同时升高降低了草地土壤的碳含量，而PARTON等[81]报道了大气CO2浓度和温度升高对土壤有机碳分解的影响可以相互抵消。MUELLER等[82]研究发现，随着大气CO2浓度和温度升高的共同作用，植物总生物量增加了约25%，这可能会增加半干旱区草地的有机碳含量。LIN等[38,83]采用模型模拟大气CO2浓度和温度升高对土壤碳库的影响发现，如果大气CO2浓度升高250 μmol·mol-1至少会使土壤有机碳含量增加15%，但当温度同时升高5℃时，土壤有机碳含量则至少降低29%，预示着未来土壤碳库是否成为“碳源”与温度升高的幅度密切相关。

2 土壤微生物群落结构

土壤微生物群落（即细菌、古生菌和真菌）被认为是土壤质量的敏感指标，在调节陆地碳循环及其对气候的反馈方面起着关键作用。土壤中微生物以细菌数量最多，它主要参与小分子有机物的降解，促使碳和营养成分快速循环，有利于无机养分的供应[84]。真菌主要参与难降解有机物质的降解，如真菌分泌酚氧化酶能够降解木质素[45]，且真菌分泌的有机物质能够黏结土壤颗粒，从而促进了土壤团聚体的形成，对土壤有机质起到了保护作用[85]。土壤微生物参与生物化学循环、土壤有机质的分解和土壤结构的形成等过程，对环境因子比较敏感。

大气CO2浓度和温度升高会对输入到土壤中植物源有机质产生影响，进而影响土壤微生物的数量、群落结构和活性，导致土壤有机质矿化和凋落物分解等土壤生化过程的改变[86]。因此了解土壤微生物群落结构如何对特定植物生态系统的气候变化响应也是极其重要的，因为这些响应将影响养分循环动力学，从而可能调控整个生态系统对气候变化的长期响应[87]。

2.1 大气CO2浓度升高对土壤微生物群落结构的影响

植物通过根系向土壤中输入碳，进而影响微生物活性，因此大气CO2浓度升高对土壤微生物的影响主要是通过影响植物生长而间接产生的[88]。一般认为，大气CO2浓度升高会增强植物的光合作用，增加根系分泌物和根系沉积物，从而刺激微生物的生长及活性，改变微生物群落结构和功能[19,89]。然而，有关土壤微生物群落对大气CO2浓度升高响应的研究结果差异较大，大气CO2浓度升高的水平不同、试验地域气候条件、供试作物种类及试验时间等差异可能是主要原因。

在微生物群落结构方面，FACE的研究指出大气CO2浓度升高未对北美枫香根际土壤细菌群落产生影响[90]，然而，长达14年大气CO2浓度升高试验表明，一年生草地土壤微生物群落的分类和功能基因组成均发生改变，表明微生物能够更有效地利用有限的资源维持自身的生存。在CO2浓度升高条件下，黑土中的大豆根际细菌群落结构在门水平上虽未发生变化，但是一些属的OTUs数量发生了显著变化[91]。王艳红[92]进一步利用DNA-SIP技术对大豆根际土壤中应用同化碳的细菌群落进行了区分，研究发现大气CO2浓度升高显著降低了根际土壤中的细菌丰度和多样性，其中快速生长的细菌属如、、和等相对丰度有所降低，而可降解复杂物质的细菌属如、、、和的相对丰度有所增加，大气CO2浓度升高所引起的植物光合同化碳源的改变导致了根际土壤细菌群落结构的演替变化，而这种变化可能会导致未来土壤从潜在的碳汇成为碳源。

在土壤微生物对气候变化响应方面，YU等[49]通过模拟未来大气CO2浓度升高对半干旱草地生态系统的影响发现，大气CO2浓度升高增加了微生物功能多样性，从而对其参与的碳循环产生反馈。研究同样发现，CO2浓度升高增加了不同生态系统中土壤真菌的丰度[93-95]，降解惰性碳库的降解酶（酚氧化酶）的活性较高，导致土壤有机质的矿化速率在高CO2浓度的土壤中更快。LI等[96]研究发现CO2浓度升高增加了分解纤维素的真菌数量，而与之相关的惰性碳的分解速率也随之增加。关于大气CO2浓度升高对草地、农田和森林生态系统中微生物群落的研究发现，参与碳降解和甲烷代谢循环的关键基因被激活[97-98]，参与碳固定的相关基因则基本保持不变[99]，参与合成某些特殊化合物（如谷氨酰胺）的相关基因丰度降低[100]，以上结果表明大气CO2浓度升高可能会刺激微生物碳代谢，加速土壤碳循环。

养分有效性会抑制部分微生物的种群，从而对气候变化产生不同的响应。富营养型微生物和寡养型微生物对CO2浓度升高的差异响应可能会对土壤养分的有效性产生影响，进而影响植物生长，不可避免地影响未来土壤碳的储量[101]。长期的CO2浓度升高试验显示，由于植物对养分的吸收和有机碳分解的增强导致了土壤养分的减少，土壤中微生物更倾向于寡养型微生物的生长，通过有机碳的矿化来获取不稳定的养分，由此CO2浓度升高可能加速微生物对土壤有机碳的分解[102]。由CO2浓度升高所引起的微生物功能变化可能会影响有机碳的稳定性，特别是寡养型微生物群落的富集表明有必要采取相应的对策，以减轻CO2浓度升高对SOC造成的损失，从而提高土壤质量以满足农作物的可持续生产。

2.2 温度升高对土壤微生物群落结构的影响

土壤微生物作为分解者可能通过两套机制应对高温，从而导致长期和短期的温度敏感性的差异[103]。首先，高温促进微生物的活性，使得土壤活性碳库更快地被微生物所利用，而活性碳库的减少将抑制微生物对温度升高的长期反应。升温加快微生物的生长速度，适应更高温度的种群将成为优势种群[87,104]。例如，短期的升温处理导致微生物群落结构的迅速变化，显著增加了放线菌的丰度[105]，降低了真菌总量[106]。在某些时间尺度上，升温可能会增加可利用或活性碳的含量[17]，最终可能导致在某些时间尺度上微生物具有更大的温度敏感性。其次，升温可以改变微生物对温度的长期敏感性[107]。例如，BRADFORD等[104]研究表明，在原位升温超过15年的土壤中，土壤碳矿化的实际速率和潜在速率比对照土壤要低，这表明微生物活性下降，微生物表现出生理适应性。

土壤微生物在调节陆地碳循环及其对气候的反馈方面起着关键作用。全球变化多因子之间的交互作用，主要是通过改变土壤微生物的养分需求和微生物的分解路径来影响微生物的群落结构[108]。温度是影响细菌和真菌丰度的主要因素[109]，温度升高对土壤微生物的影响随气候区域和生态系统类型的不同而产生差异，其不一致的反应主要归因于土壤养分的有效性和土壤的理化性质的差异[110]。已有研究表明，温度升高会减少微生物生物量[111-112]，降低真菌的丰度[113]，促进微生物群落向革兰氏阳性菌和放线菌转移[111]。与革兰氏阴性菌的单层细胞壁相比，革兰氏阳性菌坚固的细胞壁更能抵抗压力[114]，更倾向于利用土壤中的惰性基质[115]。

与细菌相比，真菌更易受到基质质量的影响，而升温加速了土壤有机碳的分解和植物对土壤氮的吸收，导致基质质量下降[116]，真菌的环境适应策略比细菌弱，使得温度升高对真菌生长的抑制作用更为严重，细菌更倾向于在升温的土壤中生长[112]。例如，一年的原位升温降低了温带灌木丛生态系统中土壤真菌的丰度[117]；在农田生态系统中，升温降低了真菌的丰度，增加了革兰氏阳性菌的丰度[118]。CHEN等[119]通过模型分析64篇关于温度升高的研究表明，温度升高显著增加了土壤微生物的丰度（增幅达7.6%），其中温度升高使冻土中细菌和真菌丰度分别增加了37.0%和9.5%，而FREY等[111]研究温度升高（环境温度+5℃）12年后对微生物的影响表明，温度升高显著降低了微生物量碳含量和真菌的丰度。而真菌主导的土壤易于有机碳的积累，因此，未来变暖的气候将导致土壤微生物中碳存储量的减少[117]，从长远来看，土壤中的碳可能会损失掉。

温度升高对细菌的不同分类单元（OTU）的影响也存在差异，放线菌可以降解更多难分解（如纤维素、半纤维素和几丁质）的土壤有机质[120]，而短期的升温会造成放线菌门[87]和厚壁菌门的相对丰度随温度的升高而增加的趋势[121]，而拟杆菌门和变形杆菌则表现出下降的趋势[121]。与惰性碳库分解相关的功能基因（如芳香族、木质素和几丁质多糖）的相对丰度随温度的升高而增加[122]，与碳循环相关的微生物可能会对温度升高产生正反馈从而加速了冻土带土壤有机碳的分解。

2.3 大气CO2浓度和温度升高对土壤微生物群落结构的影响

土壤微生物群落及其活性对温度和大气CO2浓度升高的交互响应可能呈现出强烈的累加效应，产生了显著的碳转化反馈能力[123]。土壤不同微生物类群，如细菌、真菌和古菌对大气CO2浓度升高、温度升高及其交互作用的响应不同。土壤中一些功能微生物群落可能会随着大气CO2和温度的升高而改变，从而改变微生物生理学驱动碳转化过程的速率[124]。HAYDEN等[87]研究温度升高和大气CO2浓度升高对澳大利亚草原土壤中的细菌、真菌和古菌的影响发现，气候变化的交互作用未对真菌的丰度产生显著的影响。LIU等[44]通过模拟大气CO2浓度和温度升高对麦田土壤微生物的影响也得出了相似的结果，但也有研究表明，受大气CO2浓度和温度升高的影响，半干旱草原区微生物群落的组成和结构发生了显著变化[49]。YU等[49]研究发现一些参与惰性碳降解的微生物功能基因在温度和大气CO2浓度升高的交互作用下并未发生显著的变化。OSANAI等[125]研究大气CO2浓度和温度升高对草地土壤碳矿化影响的微生物学机制，研究发现土壤群落的代谢活动受到了大气CO2浓度和温度升高的共同影响，土壤群落对外源有机质的矿化能力均有所增强。大气CO2浓度和温度升高增加了微生物对土壤有机质的矿化作用，可能会造成土壤碳的损失[125]。

3 研究展望

大气CO2浓度和温度升高对光合碳含量产生影响，进而影响光合碳向根系和土壤中的分配。作为连接植物地下-微生物-土壤相互作用的关键组分[126]，根系分泌物不仅是土壤有机质的重要来源[127]，还对土壤养分有效性、微生物活性和土壤有机质分解有重要的影响[128]。气候变化会引起根系分泌物成分的变化，而土壤微生物群落对特定的根系分泌物有不同的反应[45]，进而对碳库产生影响[42]。目前关于根系分泌物与微生物类群区系分布的消长动态变化研究较少，需要进一步探讨其变化。根系分泌物成分的变化可能会对土壤有机碳的动态变化产生影响，因此，未来研究中，明确气候变化下根系分泌物调节碳-营养物质的耦合[41]是至关重要的，根系分泌物转化过程及微生物的响应机制及其所起的生态功能，也有待于进行深入的研究。目前研究多关注气候变化对微生物群落结构产生怎样的影响，而利用光合碳的微生物有何种变化很少被研究，因此应用稳定同位素技术（DNA-SIP）系统研究参与不同植物光合碳转化的微生物群落结构及其生态功能将是未来研究的重要方向。

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Effects of Elevated CO2Concentration and Warming on Soil Carbon Pools and Microbial Community Composition in Farming Soil

FANG Rui1, YU ZhenHua1, LI YanSheng1, XIE ZhiHuang1,2, LIU JunJie1, WANG GuangHua1, LIU XiaoBing1, CHEN Yuan1, LIU JuDong1, ZHANG ShaoQing1, WU JunJiang3, Stephen J Herbert4, JIN Jian1

1Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences/Key Laboratory of Mollisols Agroecology, Harbin 150081, China;2University of Chinese Academy of Sciences, Beijing 100049, China;3Soybean Research Institute of Heilongjiang Academy of Agricultural Sciences/Key Laboratory of Soybean Cultivation, Ministry of Agriculture and Rural Affairs/Heilongjiang Key Laboratory of Soybean Cultivation, Harbin 150086, China;4Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA

Elevated atmospheric CO2concentration (eCO2) and warming may affect the crop photosynthesis, and consequently alter the translocation of photosynthetic carbon to soil. Under climate change, the change of photosynthetic carbon retained in soil may shape the structure of microbial community involved in photosynthetic carbon transformation. As a major driver of soil carbon cycle, soil microorganism plays an important role in the transformation of soil organic matter. The changes of microbial community structure and function under climate change are likely to affect the turnover of soil organic matter, resulting in an increase or decrease in the concentration of atmosphere CO2as a feedback to climate change. Soil carbon balance depends on the input and output of carbon in the soil and its retention in the soil. However, it is unclear that how climate change may affect the stability of the soil carbon pool. Therefore, the change of the soil carbon pool corresponding with soil microbial community structure is the core mechanism of terrestrial ecosystem in response to climate change, which is important to the management of soil organic carbon and the maintenance of soil productivity on farmland in the future. This paper reviewed the responses of soil carbon pool and soil microbial community structure to global climate change (eCO2and warming). The main conclusions were as follows: (1) Elevated CO2and warming exhibited the tradeoff effect on soil carbon pools, but whether soil carbon pool became carbon source depended on the extent of warming; (2) Elevated CO2increased the accumulation of photosynthetic carbon in plant parts of corn and wheat. Warming also posed an impact on the accumulation of photosynthetic carbon, but the impact varied among different parts with negative or no effect; (3) Warming and eCO2showed a cumulative effect on soil microbial activity and community diversity, but different microbial kingdoms (bacteria, fungi and archaea) had different roles to affect carbon turnover. Finally, it was proposed that the future research directions included: (1) in-depth study on the impact of climate change on the turnover of root exudates considering the plant-soil interaction and its influence on microbial properties; (2) DNA-SIP being applied to explore the relationship between different plant-carbon sources utilized by soil microorganisms and carbon cycling under eCO2and warming. Thus, these proposed studies might clarify substrate-utilizing strategies by microbes and the response of microbial community to climate change.

climate change; soil organic matter; microorganism; photosynthetic carbon; root exudates