大氣CO2濃度和溫度升高對農(nóng)田土壤碳庫及微生物群落結(jié)構(gòu)的影響

房蕊，于鎮(zhèn)華，李彥生，謝志煌,2，劉俊杰，王光華，劉曉冰，陳淵，劉居?xùn)|，張少慶，吳俊江，Stephen J Herbert，金劍

大氣CO2濃度和溫度升高對農(nóng)田土壤碳庫及微生物群落結(jié)構(gòu)的影響

房蕊1，于鎮(zhèn)華1，李彥生1，謝志煌1,2，劉俊杰1，王光華1，劉曉冰1，陳淵1，劉居?xùn)|1，張少慶1，吳俊江3，Stephen J Herbert4，金劍1

1中國科學(xué)院東北地理與農(nóng)業(yè)生態(tài)研究所/黑土區(qū)農(nóng)業(yè)生態(tài)重點實驗室，中國哈爾濱 150081；2中國科學(xué)院大學(xué)，中國北京 100049；3黑龍江省農(nóng)業(yè)科學(xué)院大豆研究所/農(nóng)業(yè)農(nóng)村部大豆栽培重點實驗室/黑龍江省大豆栽培重點實驗室，中國哈爾濱 150086；4Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA

大氣CO2濃度和溫度升高會通過影響作物的光合作用，從而影響光合碳向土壤中的輸送。輸入到土壤中光合碳含量的變化勢必會對土壤外源碳的主要分解者--微生物的群落結(jié)構(gòu)產(chǎn)生影響。土壤微生物在土壤有機(jī)質(zhì)的轉(zhuǎn)化過程中發(fā)揮著重要的作用，是土壤碳循環(huán)的主要驅(qū)動者，其群落結(jié)構(gòu)和功能的改變會影響土壤有機(jī)質(zhì)的動態(tài)變化，而這些變化會進(jìn)一步增加或者降低大氣中的CO2濃度，從而對氣候變化產(chǎn)生反饋作用。未來土壤的碳平衡取決于大氣CO2濃度和全球變暖對土壤中碳的輸入、輸出以及碳在土壤中的駐留時間。因此，只有全面了解大氣CO2濃度和溫度升高將對土壤碳庫及土壤微生物群落結(jié)構(gòu)產(chǎn)生何種影響，才能明確地揭示陸地生態(tài)系統(tǒng)對氣候變化的反饋機(jī)制，對未來農(nóng)田土壤有機(jī)碳庫的管理和生產(chǎn)力的維持有重要意義。文章綜述了大氣CO2濃度和溫度升高及其交互作用對土壤碳庫和土壤微生物群落結(jié)構(gòu)的影響。主要結(jié)論為：（1）大氣CO2濃度和溫度升高對土壤碳庫的影響可以相互抵消，但是土壤碳庫是否成為碳“源”與溫度升高的幅度密切相關(guān)；（2）大氣CO2濃度升高增加了光合碳在玉米、小麥等植株各部分的分配，溫度升高同樣對光合碳的分配規(guī)律產(chǎn)生影響，但對不同部位的影響不一致，多呈降低或無顯著影響；（3）大氣CO2濃度和溫度升高可能對土壤微生物活性及其群落結(jié)構(gòu)產(chǎn)生交互影響，且對不同微生物（細(xì)菌、真菌和古菌）群落的影響程度不同，進(jìn)一步對土壤有機(jī)碳的轉(zhuǎn)化產(chǎn)生影響。最后提出未來的研究方向：（1）從氣候變化影響植物-土壤互作角度解析根系分泌物的轉(zhuǎn)化過程及其對微生物的影響；（2）通過DNA-SIP進(jìn)一步研究大氣CO2濃度和溫度升高條件下土壤微生物對不同植物來源碳的選擇性利用與碳循環(huán)的關(guān)系，從而闡明氣候變化條件下微生物底物利用策略以及微生物群落結(jié)構(gòu)的變化。

氣候變化；土壤有機(jī)質(zhì)；微生物；光合碳；根系分泌物

0 引言

工業(yè)革命前，大氣中的CO2濃度相對穩(wěn)定，約為279 μmol·mol-1自工業(yè)革命以來，由于人類活動導(dǎo)致溫室氣體排放急劇增加，目前CO2濃度已突破416 μmol·mol-1，約超出工業(yè)化前水平的45%（https://www.co2.earth），預(yù)計到2050年CO2濃度將會達(dá)到550 μmol·mol-1[1]，到21世紀(jì)末，CO2濃度將會達(dá)到700 μmol·mol-1[2]。CO2累積排放量的增加是造成全球地表溫度變暖的主要原因[3-4]，隨著大氣CO2濃度的增加，預(yù)計到21世紀(jì)末地表溫度將升高3.7—4.8℃[2]，而我國的平均溫度將會上升1—5℃[5]或者更高（3.9— 6.0℃）[6]。

CO2濃度和溫度作為影響植物生長的兩個關(guān)鍵環(huán)境因子，對植物的生長發(fā)育和生理功能都會產(chǎn)生影響[7]。CO2是光合作用的底物，植物通過光合作用將大氣中的CO2固定到植物體內(nèi)，又通過根系分泌物、凋落物及根系生物量等將一部分光合碳輸入到土壤中[8-9]，為土壤中的微生物提供碳源和能源。陸地生態(tài)系統(tǒng)的碳循環(huán)通過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響大氣CO2的濃度，使陸地植被系統(tǒng)成為“碳匯”；然而，土壤是否成為碳“源”或者“匯”取決于土壤碳庫的平衡[10]。陸地生態(tài)系統(tǒng)中的碳儲量取決于光合碳的輸入和以CO2的形式及甲烷和可溶性有機(jī)碳的損失之間的平衡[11]，大氣中CO2濃度的增加和由此引發(fā)的全球變暖可能會通過改變碳的吸收和釋放速率來影響這一平衡。然而，氣候變化將對土壤碳庫將產(chǎn)生怎樣的影響還不清楚。土壤有機(jī)碳庫是陸地生態(tài)系統(tǒng)中最重要的碳儲存庫，植物是其重要來源，大氣CO2濃度日益升高會通過影響光合作用進(jìn)而改變植物的養(yǎng)分運輸和干物質(zhì)量積累，從而對光合同化物、殘體凋落物及根系分泌物產(chǎn)生影響，進(jìn)而影響土壤碳庫的含量及微生物群落結(jié)構(gòu)[12]。而溫度升高會刺激土壤微生物活性，加速微生物對新輸入到土壤中的有機(jī)碳和土壤原有有機(jī)碳的分解速率，進(jìn)一步增加CO2向大氣中的排放[13]。

土壤中碳含量取決于土壤中碳的輸入、輸出以及碳在土壤中的駐留時間。有研究表明，大氣CO2濃度升高和溫度升高都加速了土壤中有機(jī)碳的分解速率，并且兩者共同升高時產(chǎn)生的影響更明顯，這表明在大氣CO2濃度和溫度共同升高的作用下可能有更多的有機(jī)碳被礦化，從而釋放更多的CO2到大氣中[14]。然而，目前關(guān)于全球氣候碳循環(huán)模型預(yù)測，大氣CO2濃度升高將增加土壤有機(jī)碳的含量，至少在一定程度上抵消了因溫度升高而引起的有機(jī)碳分解對碳庫所造成的損失[15]。也有研究認(rèn)為，預(yù)計到21世紀(jì)末，大氣CO2濃度升高和溫度升高對陸地碳庫的影響會達(dá)到平衡，表現(xiàn)為土壤有機(jī)碳庫的總含量不變[16]。

土壤微生物在土壤有機(jī)碳的轉(zhuǎn)化中發(fā)揮著重要的作用，作為土壤碳循環(huán)的主要驅(qū)動者[17]，其群落結(jié)構(gòu)和功能的改變會對土壤有機(jī)碳的含量產(chǎn)生影響[18]。由于大氣CO2濃度和溫度升高的交互影響，根系分泌物的質(zhì)和量也會相應(yīng)改變，從而導(dǎo)致利用根際分泌物的根際微生物群落結(jié)構(gòu)發(fā)生變化[19]。一般認(rèn)為，大氣CO2濃度升高可能會增加光合同化物的含量，并通過根系分泌物增加不穩(wěn)定碳（如糖、羧酸及多肽等）的釋放，這些可供微生物利用底物的增加刺激了微生物的活性，增加了土壤中有機(jī)碳降解酶的活性，從而加速了土壤有機(jī)質(zhì)的分解，不利于土壤碳庫的積累[20]。溫度升高對土壤碳庫的影響主要通過影響參與有機(jī)碳分解的微生物來實現(xiàn)，溫度升高會迅速刺激土壤微生物的新陳代謝，導(dǎo)致微生物呼吸速率增加[21]。然而，從長遠(yuǎn)來看，溫度升高對微生物的生長和活性的刺激作用會受到可利用底物的限制，從而改變微生物對氣候變化的響應(yīng)[22]。因而，只有全面了解土壤微生物群落結(jié)構(gòu)的變化，才能清楚地揭示陸地生態(tài)系統(tǒng)對大氣CO2濃度和溫度升高的響應(yīng)和反饋。

1 大氣CO2濃度和溫度升高對土壤有機(jī)碳庫的影響

1.1 活性碳庫和惰性碳庫

植物光合作用是陸地和大氣間碳循環(huán)的驅(qū)動力，植物通過根系分泌物向土壤中輸送的光合碳是土壤有機(jī)碳的重要來源[12,23]。在陸地生態(tài)系統(tǒng)中，土壤有機(jī)質(zhì)是聯(lián)系土壤物理、化學(xué)和生物學(xué)性質(zhì)的重要紐帶，它不僅為微生物提供了營養(yǎng)物質(zhì)及活動場所，適宜的有機(jī)質(zhì)含量還有助于保持土地的可持續(xù)利用。因此，土壤有機(jī)質(zhì)是評價土壤質(zhì)量的重要指標(biāo)。

根據(jù)有機(jī)碳周轉(zhuǎn)的時間和在土壤中的存留時間，土壤碳庫可劃分為活性碳庫、緩性碳庫和惰性碳庫3種[24]。其中活性碳庫周轉(zhuǎn)快，由新輸入到土壤中且易降解的碳組成；惰性碳庫則是長時期內(nèi)不會有明顯變化的碳庫，因此是土壤中最穩(wěn)定的碳，不易被分解和轉(zhuǎn)化[25]。CHENG等[26]研究發(fā)現(xiàn)，在大氣CO2濃度升高的作用下，植物來源的碳更多的被儲存在惰性碳庫中，有利于土壤碳的積累。與此相反，GILL等[27]發(fā)現(xiàn)大氣CO2濃度升高會導(dǎo)致土壤活性碳庫含量的增加，惰性碳庫含量的損失，這可能是由于土壤中活性碳庫增加對土壤原有有機(jī)碳的礦化率（-50%至＞300%）產(chǎn)生了影響[28]。土壤中碳的礦化速率對溫度的變化很敏感，即使是小幅度的升溫也可能促使土壤中的碳被大量釋放。CONANT等[17]研究表明，惰性土壤有機(jī)碳庫的溫度敏感性大于活性土壤有機(jī)碳庫，氣候變化引起的土壤溫度升高將提高土壤原有有機(jī)碳的分解率，從而導(dǎo)致惰性有機(jī)碳庫含量的減少[29]。

1.2 顆粒有機(jī)碳

土壤顆粒物理分級是研究有機(jī)碳庫的重要手段[30]。根據(jù)土壤顆粒的大小，通常將粒徑＞250 μm的有機(jī)碳稱為粗顆粒有機(jī)碳（coarse particulate organic carbon，cPOC），粒徑在53—250 μm的有機(jī)碳稱為細(xì)顆粒有機(jī)碳（fine particulate organic carbon，fPOC），粒徑＜53 μm的有機(jī)碳稱為礦質(zhì)結(jié)合態(tài)有機(jī)碳（mineral-associated organic carbon，MOC）[31]。MOC能與細(xì)土壤微粒（粉粒和黏粒）結(jié)合，約占土壤總有機(jī)碳庫的50%—80%[32]，它的周轉(zhuǎn)時間較長，也更穩(wěn)定，是最不易被微生物分解的組分，用來表征惰性碳庫；而cPOC和fPOC則較容易分解[33]，相當(dāng)于活性或緩性碳庫。大氣CO2濃度升高會通過改變植物凋落物[8]和根系分泌物的質(zhì)和量來間接影響SOC，而這些過程對POC和MOC的影響是不同的。CARDON等[34]研究大氣CO2濃度升高對加州草原有機(jī)碳含量的影響，結(jié)果顯示，大氣CO2濃度升高對POC和MOC組分產(chǎn)生了相反的影響，MOC周轉(zhuǎn)變慢，而POC周轉(zhuǎn)加速，同位素分析發(fā)現(xiàn)MOC中新碳的含量有所減少，土壤中新、舊碳庫動態(tài)變化的對比效應(yīng)明顯，最終使得土壤碳總量平衡，這種變化對陸地生態(tài)系統(tǒng)和大氣之間的長期凈碳含量將產(chǎn)生重要的影響。與草原中的研究結(jié)果不同，在農(nóng)業(yè)生態(tài)系統(tǒng)中，連續(xù)8年的農(nóng)田FACE試驗表明，小麥和豆科植物輪作體系下土壤的fPOC含量明顯下降[35]，說明大氣CO2濃度升高促進(jìn)了土壤碳循環(huán)，加快了土壤活性碳庫的周轉(zhuǎn)速率，最終對一些農(nóng)田生態(tài)系統(tǒng)的土壤固碳能力產(chǎn)生了限制作用（表1）。

WIESMEIER等[52]研究發(fā)現(xiàn)fPOC與溫度呈顯著負(fù)相關(guān)關(guān)系，說明溫度升高更容易分解細(xì)顆粒有機(jī)碳。BENBI等[53]同樣發(fā)現(xiàn)了POC比MOC對升溫更敏感，這表明升溫對MOC的影響較小，而對POC的影響較大。而FANG等[51]關(guān)于溫度升高對亞熱帶森林土壤碳庫的研究表明，溫度升高增加MOC的分解，從而導(dǎo)致亞熱帶森林的碳損失比之前估計的更大。然而，也有7年升溫試驗顯示，POC和MOC沒有發(fā)生明顯變化，這可能與土壤有機(jī)質(zhì)結(jié)構(gòu)的變化和組分之間的再分配有關(guān)[50]。同時升溫試驗的持續(xù)時間也可能導(dǎo)致結(jié)果的偏差，因為短期（＜10年）的升溫可能不會對MOC產(chǎn)生顯著的變化[54]。

表1 大氣CO2濃度和溫度升高對土壤碳庫的影響

MOC代表礦質(zhì)結(jié)合態(tài)有機(jī)碳；POC代表顆粒有機(jī)碳；MBC代表微生物碳；DOC代表水溶性有機(jī)碳；SOC代表土壤有機(jī)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

在農(nóng)業(yè)生態(tài)系統(tǒng)中，有關(guān)大氣CO2濃度和溫度同時升高對土壤碳庫影響的研究較少。LOISEAU等[55]在氣候變化試驗中發(fā)現(xiàn)，大氣CO2濃度升高增加了POC的含量，而溫度升高則增加了其周轉(zhuǎn)速率，二者同時升高顯著增加了土壤原有有機(jī)質(zhì)的分解速率。房蕊[36]關(guān)于氣候變化對種植玉米的土壤碳庫的影響發(fā)現(xiàn)，大氣CO2濃度和溫度同時升高未對黑土顆粒有機(jī)碳含量產(chǎn)生影響。應(yīng)用穩(wěn)定同位素技術(shù)示蹤土壤原有碳庫，CARRILLO等[56]在半干旱草原土壤上進(jìn)行了為期7年的大氣CO2濃度升高和升溫試驗，研究發(fā)現(xiàn)，溫度單獨升高并未對土壤中的碳含量產(chǎn)生影響，但是當(dāng)溫度和CO2濃度同時升高時，造成了土壤中原有有機(jī)碳的損失。說明大氣CO2濃度和溫度升高對碳庫產(chǎn)生交互影響，但這種影響可能是疊加的，也可能是拮抗的，這與試驗土壤的理化性質(zhì)、升溫的幅度和供試的植物種類有關(guān)。

1.3 團(tuán)聚體

SOC的分解和周轉(zhuǎn)受分解速率以及土壤礦物和團(tuán)聚體對有機(jī)碳的保護(hù)程度的共同影響[57]。SIX等[58]研究表明，隨著大氣CO2濃度的升高，光合碳向土壤中輸入的比例增加的同時團(tuán)聚體也在增大，土壤有機(jī)質(zhì)的周轉(zhuǎn)速度隨著團(tuán)聚體的增大而加快，較小團(tuán)聚體中的碳更穩(wěn)定。同樣，DORODNIKOV等[40]觀察到，在添加葡萄糖后引起CO2濃度升高的條件下，相對微團(tuán)聚體（＜0.25 mm），大的團(tuán)聚體（＞2 mm）中的土壤有機(jī)碳周轉(zhuǎn)速率明顯增加。

溫度升高降低了土壤團(tuán)聚體的穩(wěn)定性[46]，CHENG等[46]研究表明，隨著溫度升高，大團(tuán)聚體（＞2 mm）中碳的分解速率提高。溫度升高可能通過影響植物來源碳的輸入和官能團(tuán)結(jié)構(gòu)進(jìn)而影響土壤團(tuán)聚體的穩(wěn)定性和土壤有機(jī)碳含量。一方面，升溫引起的土壤缺水會降低地上凋落物向土壤中的輸入量，抑制了土壤團(tuán)聚體的形成，增加了土壤侵蝕[59]，從而導(dǎo)致團(tuán)聚體穩(wěn)定性下降。另一方面，溫度升高會改變團(tuán)聚體官能團(tuán)結(jié)構(gòu)。GUAN等[47]指出升溫顯著減少了疏水性酚官能團(tuán)，顯著增加了親水性羧基官能團(tuán)，降低了土壤團(tuán)聚體的水穩(wěn)定性。然而，到目前為止有關(guān)CO2濃度和溫度同時升高對農(nóng)田土壤團(tuán)聚體的長期影響還缺乏研究，開展這方面的研究將對發(fā)現(xiàn)農(nóng)田土壤生產(chǎn)力對氣候變化的適應(yīng)性至關(guān)重要。

1.4 光合碳在植物-土壤中分配

光合碳在植物-土壤系統(tǒng)間的分配是生態(tài)系統(tǒng)中碳循環(huán)的重要環(huán)節(jié)，同大氣環(huán)境與土壤質(zhì)量的動態(tài)變化過程密切相關(guān)[60]。光合碳在植物-土壤中分配也隨著生育期和作物種類的不同而有所差異。研究發(fā)現(xiàn)，光合碳雖在植物不同器官中的分配不同，但均在莖葉中的分配比例最高，約為40%—93%[36,61-63]，而分配到根系中的光合碳僅占2%—3.5%[62]，分配到根際和非根際土壤中的光合碳分別為9.27%和5.83%[63]，說明植物只有在滿足自身生長的需求下，光合碳才會向根系及土壤中輸出[64-65]。而HüTSCH等[65]研究顯示一年生植物同化的光合碳約有30%—60%分配到土壤中，這部分碳高達(dá)40%—90%以根系沉積物的形式釋放到土壤中，但僅有2%—5%的光合碳被固定到土壤中形成了穩(wěn)定的土壤有機(jī)碳。馬田等[43]研究發(fā)現(xiàn)，大氣CO2濃度升高顯著增加了小麥生育后期根系中分配的光合碳含量。同樣，石元豹等[66]通過13C同位素示蹤標(biāo)記研究了大氣CO2濃度升高對枸杞生育期內(nèi)各部分光合碳累積的影響也得到了相似的規(guī)律，即在CO2濃度升高條件下根系13C豐度較高，說明植物向地下分配的光合碳更多。此外，植物通過向根輸送更多的碳，以減輕因CO2濃度升高而導(dǎo)致碳水化合物在葉片中的積累對葉片功能造成的不利影響[48]。升溫會造成植物早衰，葉片光合能力受限，可能會降低光合碳向根的分配[67]，從而對根系碳凈增加量未產(chǎn)生影響[68]。由此可見，氣候變化顯著影響光合碳在植物-土壤中的分配規(guī)律，這可能會對土壤碳循環(huán)產(chǎn)生進(jìn)一步的影響。

1.5 土壤有機(jī)碳積累

土壤有機(jī)碳含量始終處在外源碳的輸入和土壤有機(jī)碳的分解輸出的動態(tài)變化過程中，作物會通過地上凋落物及根系分泌物等形式將碳輸入到土壤中，這部分碳通過微生物的作用轉(zhuǎn)化成有機(jī)碳固定到土壤中，而另一部分碳則會刺激微生物的活性，短期內(nèi)會引起激發(fā)效應(yīng)導(dǎo)致土壤有機(jī)碳被礦化，從而造成了土壤中原有有機(jī)碳的分解[69]，因此土壤中有機(jī)碳的消長是不斷積累和分解的復(fù)雜的動態(tài)過程[70]。

陸地生態(tài)系統(tǒng)中，氣候在很大程度上影響了土壤有機(jī)碳儲量的平衡。氣候變化一方面會對植物的生長產(chǎn)生影響，使得進(jìn)入到土壤中的凋落物、根系或者分泌物發(fā)生變化；另一方面，會對土壤中微生物的活性和生存條件產(chǎn)生影響，從而改變微生物對有機(jī)碳的礦化速率[69]。大氣CO2濃度和溫度升高通過直接影響碳輸入和/或土壤有機(jī)碳的分解速率來影響土壤碳庫的變化[8,20,39]。研究表明，大氣CO2濃度升高對多個生態(tài)系統(tǒng)中土壤碳庫的影響都很小，甚至?xí)斐商紟斓膿p失[37,71-72]。VAN GROENIGEN[20]采用模型分析，表明大氣CO2濃度升高對土壤碳的積累會產(chǎn)生不利的影響，主要是因為CO2濃度升高一方面增加了土壤碳的周轉(zhuǎn)速率，另一方面由于激發(fā)效應(yīng)加快了惰性碳庫的分解。然而，與定位試驗及培養(yǎng)試驗的結(jié)果不同，對已發(fā)表文章的數(shù)據(jù)進(jìn)行整合的META（Meta-analysis）分析指出，大氣CO2濃度升高使土壤碳含量增加了約6%[70,73]。這些不同的結(jié)果可能與CO2濃度升高影響土壤碳輸入與輸出的平衡有關(guān)。GILL等[27]研究發(fā)現(xiàn)，經(jīng)過4年大氣CO2升高處理后的草地有機(jī)碳含量不變，表明新碳的輸入和土壤中原有有機(jī)碳的分解之間達(dá)到了平衡[74]，但也有可能是土壤碳動態(tài)對大氣CO2濃度升高的響應(yīng)太小，難以被測量[71]。近期，KUZYAKOV等[75]總結(jié)大氣CO2濃度升高對土壤碳庫的相關(guān)研究，發(fā)現(xiàn)大氣CO2濃度升高增加了碳向地下生態(tài)系統(tǒng)的分配，刺激了微生物的生長，加速了微生物的新陳代謝和呼吸速率，從而提高了酶活性，加速了土壤碳、氮和磷庫的循環(huán)，從而抵消了植物向土壤中的碳輸入。因此大氣中的CO2濃度升高對碳庫的影響不大，但會強(qiáng)烈加速微生物活性和穩(wěn)定碳庫的通量，從而加速碳、營養(yǎng)物質(zhì)和非必需元素的生物地球化學(xué)循環(huán)。

溫度升高一方面會通過縮短作物生育期和增強(qiáng)光合生物量的分解來降低作物的生產(chǎn)力，從而減少碳向土壤中的輸入[76]；另一方面會通過刺激土壤微生物的活性，增加土壤呼吸而加快對土壤有機(jī)碳的分解，從而降低土壤碳庫的儲存[72,77]，因此溫度升高更容易導(dǎo)致土壤有機(jī)碳的分解，利用13C同位素示蹤技術(shù)進(jìn)一步發(fā)現(xiàn)，溫度升高會對土壤中原有有機(jī)碳造成損失[78]。研究者通常用模型來模擬溫度升高對土壤有機(jī)碳產(chǎn)生的影響，但近年來模型分析的結(jié)果并不一致。META分析發(fā)現(xiàn)，氣候變暖對土壤碳凈儲量沒有產(chǎn)生影響[13]，而其他研究分析預(yù)測指出，全球土壤碳儲量將隨溫度的升高而減少[72]。

在大氣CO2濃度和溫度同時升高的條件下，通常認(rèn)為，兩者對土壤有機(jī)碳的影響可以相互抵消[68]。從植物同化碳的角度分析，許多C3作物都是通過CO2富集而提高碳的同化量，一方面，由于CO2濃度升高增加了羧化作用并抑制了光呼吸釋放CO2，促進(jìn)了碳同化[79]；另一方面，升溫會導(dǎo)致光合作用酶的失活，減少Rubisco的特異性，降低光合作用，增加光呼吸，導(dǎo)致作物生物量減少[80]。因此，溫度升高可能會抵消CO2濃度升高對作物的促生長作用。當(dāng)兩者同時升高時，高溫也會降低CO2濃度升高對產(chǎn)量的積累作用，溫度升高一方面會加速植物的衰老從而縮短CO2富集的時間，另一方面可能會增加植物地上和地下部的自養(yǎng)呼吸[29]，因此，二者同時升高時可能不會對土壤碳庫產(chǎn)生影響[48]。

全球氣候-碳循環(huán)的模型預(yù)測大氣CO2濃度升高能夠增加土壤有機(jī)碳的積累，最終增加的這部分土壤有機(jī)碳能夠抵消由溫度升高而導(dǎo)致加快土壤有機(jī)碳分解速率的損失[15]。CARRILLO等[56]在一項為期7年的全球變化試驗中發(fā)現(xiàn)，大氣CO2濃度和溫度同時升高降低了草地土壤的碳含量，而PARTON等[81]報道了大氣CO2濃度和溫度升高對土壤有機(jī)碳分解的影響可以相互抵消。MUELLER等[82]研究發(fā)現(xiàn)，隨著大氣CO2濃度和溫度升高的共同作用，植物總生物量增加了約25%，這可能會增加半干旱區(qū)草地的有機(jī)碳含量。LIN等[38,83]采用模型模擬大氣CO2濃度和溫度升高對土壤碳庫的影響發(fā)現(xiàn)，如果大氣CO2濃度升高250 μmol·mol-1至少會使土壤有機(jī)碳含量增加15%，但當(dāng)溫度同時升高5℃時，土壤有機(jī)碳含量則至少降低29%，預(yù)示著未來土壤碳庫是否成為“碳源”與溫度升高的幅度密切相關(guān)。

2 土壤微生物群落結(jié)構(gòu)

土壤微生物群落（即細(xì)菌、古生菌和真菌）被認(rèn)為是土壤質(zhì)量的敏感指標(biāo)，在調(diào)節(jié)陸地碳循環(huán)及其對氣候的反饋方面起著關(guān)鍵作用。土壤中微生物以細(xì)菌數(shù)量最多，它主要參與小分子有機(jī)物的降解，促使碳和營養(yǎng)成分快速循環(huán)，有利于無機(jī)養(yǎng)分的供應(yīng)[84]。真菌主要參與難降解有機(jī)物質(zhì)的降解，如真菌分泌酚氧化酶能夠降解木質(zhì)素[45]，且真菌分泌的有機(jī)物質(zhì)能夠黏結(jié)土壤顆粒，從而促進(jìn)了土壤團(tuán)聚體的形成，對土壤有機(jī)質(zhì)起到了保護(hù)作用[85]。土壤微生物參與生物化學(xué)循環(huán)、土壤有機(jī)質(zhì)的分解和土壤結(jié)構(gòu)的形成等過程，對環(huán)境因子比較敏感。

大氣CO2濃度和溫度升高會對輸入到土壤中植物源有機(jī)質(zhì)產(chǎn)生影響，進(jìn)而影響土壤微生物的數(shù)量、群落結(jié)構(gòu)和活性，導(dǎo)致土壤有機(jī)質(zhì)礦化和凋落物分解等土壤生化過程的改變[86]。因此了解土壤微生物群落結(jié)構(gòu)如何對特定植物生態(tài)系統(tǒng)的氣候變化響應(yīng)也是極其重要的，因為這些響應(yīng)將影響?zhàn)B分循環(huán)動力學(xué)，從而可能調(diào)控整個生態(tài)系統(tǒng)對氣候變化的長期響應(yīng)[87]。

2.1 大氣CO2濃度升高對土壤微生物群落結(jié)構(gòu)的影響

植物通過根系向土壤中輸入碳，進(jìn)而影響微生物活性，因此大氣CO2濃度升高對土壤微生物的影響主要是通過影響植物生長而間接產(chǎn)生的[88]。一般認(rèn)為，大氣CO2濃度升高會增強(qiáng)植物的光合作用，增加根系分泌物和根系沉積物，從而刺激微生物的生長及活性，改變微生物群落結(jié)構(gòu)和功能[19,89]。然而，有關(guān)土壤微生物群落對大氣CO2濃度升高響應(yīng)的研究結(jié)果差異較大，大氣CO2濃度升高的水平不同、試驗地域氣候條件、供試作物種類及試驗時間等差異可能是主要原因。

在微生物群落結(jié)構(gòu)方面，F(xiàn)ACE的研究指出大氣CO2濃度升高未對北美楓香根際土壤細(xì)菌群落產(chǎn)生影響[90]，然而，長達(dá)14年大氣CO2濃度升高試驗表明，一年生草地土壤微生物群落的分類和功能基因組成均發(fā)生改變，表明微生物能夠更有效地利用有限的資源維持自身的生存。在CO2濃度升高條件下，黑土中的大豆根際細(xì)菌群落結(jié)構(gòu)在門水平上雖未發(fā)生變化，但是一些屬的OTUs數(shù)量發(fā)生了顯著變化[91]。王艷紅[92]進(jìn)一步利用DNA-SIP技術(shù)對大豆根際土壤中應(yīng)用同化碳的細(xì)菌群落進(jìn)行了區(qū)分，研究發(fā)現(xiàn)大氣CO2濃度升高顯著降低了根際土壤中的細(xì)菌豐度和多樣性，其中快速生長的細(xì)菌屬如、、和等相對豐度有所降低，而可降解復(fù)雜物質(zhì)的細(xì)菌屬如、、、和的相對豐度有所增加，大氣CO2濃度升高所引起的植物光合同化碳源的改變導(dǎo)致了根際土壤細(xì)菌群落結(jié)構(gòu)的演替變化，而這種變化可能會導(dǎo)致未來土壤從潛在的碳匯成為碳源。

在土壤微生物對氣候變化響應(yīng)方面，YU等[49]通過模擬未來大氣CO2濃度升高對半干旱草地生態(tài)系統(tǒng)的影響發(fā)現(xiàn)，大氣CO2濃度升高增加了微生物功能多樣性，從而對其參與的碳循環(huán)產(chǎn)生反饋。研究同樣發(fā)現(xiàn)，CO2濃度升高增加了不同生態(tài)系統(tǒng)中土壤真菌的豐度[93-95]，降解惰性碳庫的降解酶（酚氧化酶）的活性較高，導(dǎo)致土壤有機(jī)質(zhì)的礦化速率在高CO2濃度的土壤中更快。LI等[96]研究發(fā)現(xiàn)CO2濃度升高增加了分解纖維素的真菌數(shù)量，而與之相關(guān)的惰性碳的分解速率也隨之增加。關(guān)于大氣CO2濃度升高對草地、農(nóng)田和森林生態(tài)系統(tǒng)中微生物群落的研究發(fā)現(xiàn)，參與碳降解和甲烷代謝循環(huán)的關(guān)鍵基因被激活[97-98]，參與碳固定的相關(guān)基因則基本保持不變[99]，參與合成某些特殊化合物（如谷氨酰胺）的相關(guān)基因豐度降低[100]，以上結(jié)果表明大氣CO2濃度升高可能會刺激微生物碳代謝，加速土壤碳循環(huán)。

養(yǎng)分有效性會抑制部分微生物的種群，從而對氣候變化產(chǎn)生不同的響應(yīng)。富營養(yǎng)型微生物和寡養(yǎng)型微生物對CO2濃度升高的差異響應(yīng)可能會對土壤養(yǎng)分的有效性產(chǎn)生影響，進(jìn)而影響植物生長，不可避免地影響未來土壤碳的儲量[101]。長期的CO2濃度升高試驗顯示，由于植物對養(yǎng)分的吸收和有機(jī)碳分解的增強(qiáng)導(dǎo)致了土壤養(yǎng)分的減少，土壤中微生物更傾向于寡養(yǎng)型微生物的生長，通過有機(jī)碳的礦化來獲取不穩(wěn)定的養(yǎng)分，由此CO2濃度升高可能加速微生物對土壤有機(jī)碳的分解[102]。由CO2濃度升高所引起的微生物功能變化可能會影響有機(jī)碳的穩(wěn)定性，特別是寡養(yǎng)型微生物群落的富集表明有必要采取相應(yīng)的對策，以減輕CO2濃度升高對SOC造成的損失，從而提高土壤質(zhì)量以滿足農(nóng)作物的可持續(xù)生產(chǎn)。

2.2 溫度升高對土壤微生物群落結(jié)構(gòu)的影響

土壤微生物作為分解者可能通過兩套機(jī)制應(yīng)對高溫，從而導(dǎo)致長期和短期的溫度敏感性的差異[103]。首先，高溫促進(jìn)微生物的活性，使得土壤活性碳庫更快地被微生物所利用，而活性碳庫的減少將抑制微生物對溫度升高的長期反應(yīng)。升溫加快微生物的生長速度，適應(yīng)更高溫度的種群將成為優(yōu)勢種群[87,104]。例如，短期的升溫處理導(dǎo)致微生物群落結(jié)構(gòu)的迅速變化，顯著增加了放線菌的豐度[105]，降低了真菌總量[106]。在某些時間尺度上，升溫可能會增加可利用或活性碳的含量[17]，最終可能導(dǎo)致在某些時間尺度上微生物具有更大的溫度敏感性。其次，升溫可以改變微生物對溫度的長期敏感性[107]。例如，BRADFORD等[104]研究表明，在原位升溫超過15年的土壤中，土壤碳礦化的實際速率和潛在速率比對照土壤要低，這表明微生物活性下降，微生物表現(xiàn)出生理適應(yīng)性。

土壤微生物在調(diào)節(jié)陸地碳循環(huán)及其對氣候的反饋方面起著關(guān)鍵作用。全球變化多因子之間的交互作用，主要是通過改變土壤微生物的養(yǎng)分需求和微生物的分解路徑來影響微生物的群落結(jié)構(gòu)[108]。溫度是影響細(xì)菌和真菌豐度的主要因素[109]，溫度升高對土壤微生物的影響隨氣候區(qū)域和生態(tài)系統(tǒng)類型的不同而產(chǎn)生差異，其不一致的反應(yīng)主要歸因于土壤養(yǎng)分的有效性和土壤的理化性質(zhì)的差異[110]。已有研究表明，溫度升高會減少微生物生物量[111-112]，降低真菌的豐度[113]，促進(jìn)微生物群落向革蘭氏陽性菌和放線菌轉(zhuǎn)移[111]。與革蘭氏陰性菌的單層細(xì)胞壁相比，革蘭氏陽性菌堅固的細(xì)胞壁更能抵抗壓力[114]，更傾向于利用土壤中的惰性基質(zhì)[115]。

與細(xì)菌相比，真菌更易受到基質(zhì)質(zhì)量的影響，而升溫加速了土壤有機(jī)碳的分解和植物對土壤氮的吸收，導(dǎo)致基質(zhì)質(zhì)量下降[116]，真菌的環(huán)境適應(yīng)策略比細(xì)菌弱，使得溫度升高對真菌生長的抑制作用更為嚴(yán)重，細(xì)菌更傾向于在升溫的土壤中生長[112]。例如，一年的原位升溫降低了溫帶灌木叢生態(tài)系統(tǒng)中土壤真菌的豐度[117]；在農(nóng)田生態(tài)系統(tǒng)中，升溫降低了真菌的豐度，增加了革蘭氏陽性菌的豐度[118]。CHEN等[119]通過模型分析64篇關(guān)于溫度升高的研究表明，溫度升高顯著增加了土壤微生物的豐度（增幅達(dá)7.6%），其中溫度升高使凍土中細(xì)菌和真菌豐度分別增加了37.0%和9.5%，而FREY等[111]研究溫度升高（環(huán)境溫度+5℃）12年后對微生物的影響表明，溫度升高顯著降低了微生物量碳含量和真菌的豐度。而真菌主導(dǎo)的土壤易于有機(jī)碳的積累，因此，未來變暖的氣候?qū)?dǎo)致土壤微生物中碳存儲量的減少[117]，從長遠(yuǎn)來看，土壤中的碳可能會損失掉。

溫度升高對細(xì)菌的不同分類單元（OTU）的影響也存在差異，放線菌可以降解更多難分解（如纖維素、半纖維素和幾丁質(zhì)）的土壤有機(jī)質(zhì)[120]，而短期的升溫會造成放線菌門[87]和厚壁菌門的相對豐度隨溫度的升高而增加的趨勢[121]，而擬桿菌門和變形桿菌則表現(xiàn)出下降的趨勢[121]。與惰性碳庫分解相關(guān)的功能基因（如芳香族、木質(zhì)素和幾丁質(zhì)多糖）的相對豐度隨溫度的升高而增加[122]，與碳循環(huán)相關(guān)的微生物可能會對溫度升高產(chǎn)生正反饋從而加速了凍土帶土壤有機(jī)碳的分解。

2.3 大氣CO2濃度和溫度升高對土壤微生物群落結(jié)構(gòu)的影響

土壤微生物群落及其活性對溫度和大氣CO2濃度升高的交互響應(yīng)可能呈現(xiàn)出強(qiáng)烈的累加效應(yīng)，產(chǎn)生了顯著的碳轉(zhuǎn)化反饋能力[123]。土壤不同微生物類群，如細(xì)菌、真菌和古菌對大氣CO2濃度升高、溫度升高及其交互作用的響應(yīng)不同。土壤中一些功能微生物群落可能會隨著大氣CO2和溫度的升高而改變，從而改變微生物生理學(xué)驅(qū)動碳轉(zhuǎn)化過程的速率[124]。HAYDEN等[87]研究溫度升高和大氣CO2濃度升高對澳大利亞草原土壤中的細(xì)菌、真菌和古菌的影響發(fā)現(xiàn)，氣候變化的交互作用未對真菌的豐度產(chǎn)生顯著的影響。LIU等[44]通過模擬大氣CO2濃度和溫度升高對麥田土壤微生物的影響也得出了相似的結(jié)果，但也有研究表明，受大氣CO2濃度和溫度升高的影響，半干旱草原區(qū)微生物群落的組成和結(jié)構(gòu)發(fā)生了顯著變化[49]。YU等[49]研究發(fā)現(xiàn)一些參與惰性碳降解的微生物功能基因在溫度和大氣CO2濃度升高的交互作用下并未發(fā)生顯著的變化。OSANAI等[125]研究大氣CO2濃度和溫度升高對草地土壤碳礦化影響的微生物學(xué)機(jī)制，研究發(fā)現(xiàn)土壤群落的代謝活動受到了大氣CO2濃度和溫度升高的共同影響，土壤群落對外源有機(jī)質(zhì)的礦化能力均有所增強(qiáng)。大氣CO2濃度和溫度升高增加了微生物對土壤有機(jī)質(zhì)的礦化作用，可能會造成土壤碳的損失[125]。

3 研究展望

大氣CO2濃度和溫度升高對光合碳含量產(chǎn)生影響，進(jìn)而影響光合碳向根系和土壤中的分配。作為連接植物地下-微生物-土壤相互作用的關(guān)鍵組分[126]，根系分泌物不僅是土壤有機(jī)質(zhì)的重要來源[127]，還對土壤養(yǎng)分有效性、微生物活性和土壤有機(jī)質(zhì)分解有重要的影響[128]。氣候變化會引起根系分泌物成分的變化，而土壤微生物群落對特定的根系分泌物有不同的反應(yīng)[45]，進(jìn)而對碳庫產(chǎn)生影響[42]。目前關(guān)于根系分泌物與微生物類群區(qū)系分布的消長動態(tài)變化研究較少，需要進(jìn)一步探討其變化。根系分泌物成分的變化可能會對土壤有機(jī)碳的動態(tài)變化產(chǎn)生影響，因此，未來研究中，明確氣候變化下根系分泌物調(diào)節(jié)碳-營養(yǎng)物質(zhì)的耦合[41]是至關(guān)重要的，根系分泌物轉(zhuǎn)化過程及微生物的響應(yīng)機(jī)制及其所起的生態(tài)功能，也有待于進(jìn)行深入的研究。目前研究多關(guān)注氣候變化對微生物群落結(jié)構(gòu)產(chǎn)生怎樣的影響，而利用光合碳的微生物有何種變化很少被研究，因此應(yīng)用穩(wěn)定同位素技術(shù)（DNA-SIP）系統(tǒng)研究參與不同植物光合碳轉(zhuǎn)化的微生物群落結(jié)構(gòu)及其生態(tài)功能將是未來研究的重要方向。

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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

10.3864/j.issn.0578-1752.2021.17.009

2020-07-16；

2020-11-23

國家重點研發(fā)計劃項目（2017YFD0300300）、黑龍江省自然科學(xué)重點項目（ZD2021D001）、國家自然科學(xué)基金（41771326）

房蕊，E-mail：fangrui@iga.ac.cn。通信作者金劍，E-mail：jinjian@iga.ac.cn

（責(zé)任編輯 李云霞）