土壤有機(jī)碳形成機(jī)制的探索歷程

周國逸, 熊鑫,2

土壤有機(jī)碳形成機(jī)制的探索歷程

周國逸1, 熊鑫1,2

(1. 中國科學(xué)院華南植物園, 廣州 510650; 2. 中國科學(xué)院大學(xué), 北京 100049)

土壤有機(jī)碳(SOC)是生態(tài)系統(tǒng)的重要資產(chǎn)，在全球碳平衡中發(fā)揮著關(guān)鍵的作用。2015年巴黎氣候會議以來，促進(jìn)SOC在陸地生態(tài)系統(tǒng)中的積累受到特別重視，被認(rèn)為是有效減緩大氣CO2濃度上升的最重要地表措施。從服務(wù)于這個(gè)目標(biāo)出發(fā)，對過去幾十年全球在探索SOC形成機(jī)制上的歷程進(jìn)行了回顧和總結(jié)，從弄清SOC全球分布規(guī)律，闡明樣地以下尺度的SOC循環(huán)過程及其相應(yīng)的物理、化學(xué)與生物機(jī)理，到樣地及以上尺度的土壤固碳機(jī)制，最后給出了成熟森林SOC積累機(jī)制的實(shí)例。SOC形成機(jī)制的探索歷程就是尋求為促進(jìn)土壤固碳提供理論指導(dǎo)的過程。

土壤有機(jī)碳；形成；固存；機(jī)制

土壤有機(jī)碳(SOC)是土壤的重要組成成分，是生態(tài)系統(tǒng)的重要資產(chǎn)[1]，它不僅是度量土壤肥力的關(guān)鍵因子，更是表征生態(tài)系統(tǒng)服務(wù)功能大小的綜合指標(biāo)[2]。在特定的區(qū)域內(nèi)，SOC含量高往往意味著該生態(tài)系統(tǒng)的服務(wù)功能強(qiáng)，根據(jù)生態(tài)系統(tǒng)SOC儲量的大小并結(jié)合所處的時(shí)空分布特征，可以準(zhǔn)確地量化出該生態(tài)系統(tǒng)的其他服務(wù)功能，諸如固碳釋氧功能[3–4]、水土保持與涵養(yǎng)功能[5]、生物多樣性保育功能[6]等。

探索SOC形成機(jī)制的主要動機(jī)不是因?yàn)樗谛纬缮鷳B(tài)系統(tǒng)服務(wù)功能上的中心地位，而在于SOC對全球碳平衡的貢獻(xiàn)。全球尺度上，SOC儲量是大氣和陸地植被各自碳儲量的3倍以上[1]，中國陸地生態(tài)系統(tǒng)的SOC更是生物量碳的3.9倍[7]。SOC儲量的微小變化將導(dǎo)致全球碳平衡估算的巨大誤差,假設(shè)SOC含量變化0.01% a–1[按百分濃度計(jì)算，即由%變化到(±0.01)%]，在假定土壤容重為1 g cm–3且其年內(nèi)變化可以忽略的情況下，SOC庫的變化將為±1 Mg hm–2a–1，這是一個(gè)很大的值。以這個(gè)值乘以全球森林面積(3.87×109hm2)得到的全球森林土壤碳儲量的年變化量為±3.87 Pg，這個(gè)值甚至比1990年的全球碳“失匯”量還要大。土壤固碳潛力巨大，同樣地，如果SOC的百分濃度增量為0.01% a–1，則凈增加1%需要100 a時(shí)間。Tang等[7]通過分析中國森林生態(tài)系統(tǒng)7 800個(gè)樣地的資料證明，森林生態(tài)系統(tǒng)由建立到成熟，土壤碳庫的增速顯著高于生物量碳庫的增速；且在生物量保持相對穩(wěn)定后，土壤碳庫依然有相對高的增長速率。Zhou等[8]報(bào)道成熟森林土壤可持續(xù)積累有機(jī)碳，且被全球范圍內(nèi)的大量研究結(jié)果所證實(shí)[9–12]，這便為土壤固碳潛力提供了一個(gè)更高的天花板。一個(gè)未經(jīng)證實(shí)的理論認(rèn)為隨著生態(tài)系統(tǒng)的自然成熟，光合作用固定的有機(jī)碳將逐漸被呼吸作用釋放的無機(jī)碳所平衡[13],成熟生態(tài)系統(tǒng)沒有碳匯功能。該理論雖然受到成熟森林土壤持續(xù)固碳和成熟森林生物量增加等現(xiàn)象的挑戰(zhàn)[8,11,14–15]，但長時(shí)間尺度上并不存在異議, 有爭議的是如何定義成熟生態(tài)系統(tǒng)。事實(shí)上，也只有“成熟森林土壤持續(xù)固碳”這個(gè)現(xiàn)象被證明是在生態(tài)系統(tǒng)演替過程中自然發(fā)生的[6]，來源于內(nèi)源驅(qū)動、因而是相對長久的；而成熟森林生物量持續(xù)增加的現(xiàn)象則是全球環(huán)境變化(氣溫升高、CO2濃度上升、氮沉降加劇等)的結(jié)果[16]，來源于外源驅(qū)動、因而是短暫的。相較于生物量碳，SOC平均周轉(zhuǎn)時(shí)間更長[17]、有些甚至能保存上百萬年[1]，因此，在緩解全球大氣CO2濃度上升的舉措中，SOC是更為優(yōu)質(zhì)的有機(jī)碳儲存方式。在全球人工林面積持續(xù)多年增加的基礎(chǔ)上[18]，繼續(xù)依靠植被面積增加從而增加全球陸地生態(tài)系統(tǒng)碳儲量的模式將很快遇到其壁壘，在陸地生態(tài)系統(tǒng)中尋求更優(yōu)質(zhì)的固碳方式，是保障社會經(jīng)濟(jì)持續(xù)發(fā)展的同時(shí)減少碳凈排放的根本途徑。

因此，自2015年巴黎氣候會議以來，促進(jìn)SOC在陸地生態(tài)系統(tǒng)中的積累受到特別的重視，被認(rèn)為是有效減緩大氣CO2濃度上升的最重要地表措施[19–20], 明顯揚(yáng)棄了過去“造林、再造林”的理念。實(shí)現(xiàn)大規(guī)模SOC存貯量的提升并為此規(guī)劃行動方案以及準(zhǔn)確模擬全球碳平衡的先決條件是全面闡明SOC積累的驅(qū)動機(jī)制，為此，全球開展了探索SOC形成機(jī)制的廣泛研究。我們通過Web of Science查找了過去20年來，以“SOC”為主題的論文數(shù)(圖1), 從中可以看出，涉及該內(nèi)容的論文數(shù)一直在持續(xù)增加，特別是2009年以前和近幾年，這反映了探索SOC形成機(jī)制歷程的曲折性、艱難性和前沿性。本文試圖從如下幾個(gè)方面再現(xiàn)這個(gè)歷程。

圖1 通過Web of Science搜索的全球過去20年以“SOC”為主題的論文數(shù)

1 SOC積累現(xiàn)象、空間分布和試驗(yàn)檢驗(yàn)

探索SOC形成機(jī)制的歷程起步于對全球陸地土壤碳庫現(xiàn)狀大小的評估。認(rèn)識到SOC在全球碳平衡中的核心地位后，全球范圍內(nèi)廣泛開展了與SOC在陸地生態(tài)系統(tǒng)分布規(guī)律相關(guān)的大量工作[21–24]。至今，SOC庫在主要生態(tài)系統(tǒng)類型及土壤剖面的分布規(guī)律已基本清晰，不同的研究結(jié)果間沒有數(shù)量級上的差異[7,25–26]?？梢灶A(yù)計(jì)，如果沒有人為的土地利用變化和氣象災(zāi)害的發(fā)生，陸地植被生態(tài)系統(tǒng)碳庫大小將是相對穩(wěn)定且有規(guī)律增長的，具有長時(shí)間尺度上的參照性；但一些特殊生境類型(如：高寒區(qū)、荒漠區(qū)、內(nèi)陸濕地、湖泊、農(nóng)田、大陸架等)下的土壤碳庫狀況尚未完全弄清楚[27–28]。

隨后，重點(diǎn)轉(zhuǎn)移到了在樣地(生態(tài)系統(tǒng))尺度上SOC積累與某些表觀因素間的關(guān)系研究，這類工作占據(jù)了已有研究的大部分，并且還在進(jìn)行中。主要特點(diǎn)是試圖發(fā)現(xiàn)SOC含量隨生態(tài)系統(tǒng)類型和空間位置不同而發(fā)生變化的證據(jù)[29–30]，包括處于不同恢復(fù)階段的人工恢復(fù)生態(tài)系統(tǒng)系列的SOC含量差異證據(jù)[31–32]，以及處于不同演替階段的自然生態(tài)系統(tǒng)系列的SOC含量差異證據(jù)等[33]，都屬于這類研究。同時(shí)，在有長期監(jiān)測數(shù)據(jù)的支持下, 一些報(bào)道闡明了同一生態(tài)系統(tǒng)類型SOC儲存量隨時(shí)間的變化趨勢[7–8]。這些工作不僅開啟了全球范圍內(nèi)探索SOC積累機(jī)制的研究，而且為深入探討該機(jī)制提供了重要的實(shí)驗(yàn)參照和數(shù)據(jù)準(zhǔn)備。隨著探索的不斷深入, 很多研究試圖闡述SOC與植被生態(tài)系統(tǒng)的狀態(tài)參量(如生物多樣性[34]、植被生物量[35]、初級生產(chǎn)力與凋落物量[36–39]、凋落物質(zhì)[40–41]、根系生物量[42]、根系分泌物[43]等)或生態(tài)因子(如光[44]、溫[45]、水[46]、土壤質(zhì)地[47–48]和土壤養(yǎng)分[49]等)的相互關(guān)系，并認(rèn)為這些相互關(guān)系可以推廣到其他區(qū)域或更大尺度上。

對環(huán)境因子和生態(tài)系統(tǒng)本身的控制實(shí)驗(yàn)也在全球范圍內(nèi)廣泛地開展起來，多數(shù)研究者通過FACE (free-air CO2enrichment)試驗(yàn)[50]、人工升溫[51]、改變降水輸入[52]、添加氮磷[53–54]等處理對植被生態(tài)系統(tǒng)施加影響，觀測不同生態(tài)系統(tǒng)的響應(yīng)與適應(yīng)，包括SOC動態(tài)對這些因子變化的響應(yīng)[55–56]；個(gè)別研究者通過人工改變植物物種多樣性(如草原中的物種剔除試驗(yàn)[57]、森林中的不同物種數(shù)混交試驗(yàn)等[58]),以檢驗(yàn)生物多樣性對生態(tài)系統(tǒng)功能包括土壤固碳功能的影響。與此同時(shí)，另一些研究者則通過將整個(gè)生態(tài)系統(tǒng)進(jìn)行移位，即：將處于某一環(huán)境條件下的生態(tài)系統(tǒng)盡可能完整地移位至另一環(huán)境條件下，以研究同一生態(tài)系統(tǒng)在不同環(huán)境條件下的響應(yīng)差異,當(dāng)然也包括SOC動態(tài)響應(yīng)的差異。如: 將亞熱帶與溫帶交界處的土柱整體移位至南亞熱帶地區(qū)[59]；在鼎湖山，將海拔600 m處的山地常綠闊葉林生態(tài)系統(tǒng)整體移位至海拔300和30 m處, 也把海拔300 m處的針闊葉混交林生態(tài)系統(tǒng)整體移位至30 m處, 以檢驗(yàn)不同森林生態(tài)系統(tǒng)對環(huán)境條件改變的響應(yīng)差異[51,60]。一般認(rèn)為，生態(tài)系統(tǒng)整體移位試驗(yàn)比傳統(tǒng)的環(huán)境因子控制試驗(yàn)更加貼近真實(shí)狀況；但如果不在機(jī)理上對這些試驗(yàn)結(jié)果做更深層次的分析，那么所有的控制試驗(yàn)都只是起到了辨識和檢驗(yàn)自然觀測結(jié)果的作用，從機(jī)理探討的角度來說并沒有本質(zhì)的進(jìn)步；而且，由于控制試驗(yàn)的條件或多或少地與自然條件存在差異，所得到的結(jié)果必然與真實(shí)狀況存在偏差，甚至存在錯(cuò)誤的可能。因此，控制試驗(yàn)的主要優(yōu)點(diǎn)在于縮短了研究周期，其最終目的是為深層次的機(jī)理分析提供便利、而不在于觀測到的現(xiàn)象和試驗(yàn)結(jié)果本身。

2 樣地以下尺度的SOC循環(huán)過程及其物理、化學(xué)與生物機(jī)理

2.1 生物量碳的輸入

隨著探索SOC形成機(jī)制的深入，越來越多的研究試圖闡明由植物殘?bào)w向土壤有機(jī)質(zhì)轉(zhuǎn)化的輸入機(jī)制。一些研究指出氮含量豐富、碳氮比低的凋落物不僅分解快，而且分解過程中會產(chǎn)生更高比例的可溶性有機(jī)碳(DOC)和微小植物殘片，有利于傳輸?shù)秸麄€(gè)土層并最終穩(wěn)定存貯[40,61–64]。Sumiyoshi等[41]的試驗(yàn)證實(shí)，木質(zhì)素含量低的凋落物分解速度快的同時(shí)，促進(jìn)了SOC的積累。另一些研究則證實(shí)微生物更能有效地利用氮含量豐富的凋落物以構(gòu)建微生物自身生物量，從而提高碳的利用效率[65]。

包括根系凋落物和根系分泌物在內(nèi)的根系輸入對SOC的貢獻(xiàn)一直是研究者們關(guān)心的問題[42–43,66]。由于物理化學(xué)的保護(hù)機(jī)制、微尺度下的物理保護(hù)機(jī)制、與金屬離子的化學(xué)作用以及根系組織的化學(xué)穩(wěn)定性，來源于根系的SOC比來源于地上部分的平均駐留時(shí)間(residence time)長1.4倍[42]。根系分泌物不僅是SOC的供給者而且給土壤帶來了生命活性物質(zhì), 這些生命活性物質(zhì)在植物與土壤生物之間起著重要的紐帶作用[67]，通過激發(fā)微生物對土壤氮循環(huán)的反饋機(jī)制[68]，調(diào)節(jié)土壤生物化學(xué)過程而間接地影響SOC存貯；而其對SOC存貯的直接作用機(jī)理仍有待更深入的研究。

生物量碳輸入量與SOC積累的關(guān)系一直是研究者關(guān)心的焦點(diǎn)，這是一個(gè)直觀的認(rèn)識，生物量碳輸入量決定著SOC積累的觀點(diǎn)幾乎是全球碳平衡模型的一個(gè)基石[69]。然而，有研究發(fā)現(xiàn)全球生物量分布與SOC分布并沒有關(guān)聯(lián)[35]，間接說明了生物量碳輸入量對SOC積累沒有影響，但對于農(nóng)田土壤，這個(gè)論點(diǎn)似乎并不成立[39]；Zhou等[6]通過對中國森林生態(tài)系統(tǒng)的觀測和全球森林生態(tài)系統(tǒng)的meta數(shù)據(jù)分析，直接證明生物量碳輸入量(包括地上地下所有生物殘?bào)w的輸入)相對于生物殘?bào)w的質(zhì)(以C/N比來表征)來說，對SOC的貢獻(xiàn)是微不足道的。

2.2 SOC的存貯機(jī)制

在SOC的自身形態(tài)上，依據(jù)不同組分的化學(xué)頑抗性水平和周轉(zhuǎn)速率，研究者們往往將SOC劃分為易分解有機(jī)碳和難分解有機(jī)碳[70]，或是活性組分和惰性組分兩大類[71]。不少研究都試圖從SOC形態(tài)組成的角度探索SOC的存貯機(jī)制[1,72]，木質(zhì)素、烷基碳或芳香碳等難分解有機(jī)組分或分子結(jié)構(gòu)的多寡常被用來表征土壤有機(jī)質(zhì)碳的穩(wěn)定性[73–74]。研究者顯然認(rèn)為，易分解有機(jī)碳比例高，則SOC平均存在時(shí)間短，不利于積累；反之則長，利于積累。這看起來似乎是SOC積累的一種機(jī)理，但本質(zhì)上不過是一個(gè)問題的兩個(gè)方面，因?yàn)檠芯空卟]有回答為什么這個(gè)比例在不同的土壤中會存在差異、這個(gè)差異是如何形成的等問題；實(shí)際上，就連“易分解有機(jī)碳比例高，則SOC存在時(shí)間短”的命題也是有條件才成立的，只是從一個(gè)側(cè)面來說明SOC的狀況。

土壤質(zhì)地(soil texture)被認(rèn)為對SOC的存貯起著重要的作用[48]，以至于生物地球化學(xué)模型幾乎無一例外地根據(jù)土壤粘粒的含量來修正土壤有機(jī)質(zhì)的周轉(zhuǎn)速率。然而，Zhou等[6]和Wynn等[47]認(rèn)為大尺度下的SOC積累并未受控于土壤質(zhì)地，也許土壤質(zhì)地只是影響SOC積累的一個(gè)局地因子。土壤結(jié)構(gòu)(soil structure)因?yàn)榭赡苡绊慡OC的存貯而被廣泛地研究[3,75–76]，一般認(rèn)為隨著土壤團(tuán)粒結(jié)構(gòu)的增大，土壤有機(jī)質(zhì)含量增加[77–78]。同時(shí)，土壤團(tuán)粒結(jié)構(gòu)的形成降低了底物的可接觸性，有利于SOC的保存[79]。然而，土壤團(tuán)粒結(jié)構(gòu)的大小與SOC含量的高低可能也只是一個(gè)問題的兩個(gè)方面，分不清哪個(gè)是因、哪個(gè)是果，甚至土壤團(tuán)粒結(jié)構(gòu)的大小更有可能是SOC含量高低的結(jié)果，大量的研究結(jié)果已經(jīng)證實(shí)土壤有機(jī)質(zhì)是大團(tuán)聚體形成的主要膠結(jié)劑[80–81]。

人們還試圖尋求SOC在垂直分布上的差異對其穩(wěn)定性的貢獻(xiàn)[26]，一般認(rèn)為處于缺氧環(huán)境下的深層SOC周轉(zhuǎn)時(shí)間慢，更易于保存[82]。

2.3 環(huán)境因子的調(diào)控機(jī)制

SOC積累涉及一系列連續(xù)的物理、化學(xué)和生物反應(yīng)，這些反應(yīng)可以被粗略地包含在輸入和存貯兩個(gè)緊密聯(lián)系的過程中。僅憑快速的凋落物分解并不一定會帶來SOC的積累，只是為SOC的積累創(chuàng)造了條件；同樣，僅僅依靠良好的存貯環(huán)境，也不會增加SOC。要正確地闡明SOC的積累機(jī)理，必須將這兩個(gè)過程綜合考慮。環(huán)境因子、特別是全球變化因子如何影響這一連串的物理、化學(xué)、生物反應(yīng)從而控制SOC積累是眾多研究者密切關(guān)注的熱點(diǎn)。

氣溫上升和適度的降水將加速植物殘?bào)w的分解[83]，從而控制生物量碳輸入過程，這個(gè)結(jié)論在全球范圍內(nèi)是一致的，不管植物殘?bào)w的類型和性質(zhì)如何變化[84]。盡管如此，卻很少有研究將凋落物分解速率表述為氣溫與降水函數(shù)，可見，這方面的研究至今并不深入，已有的全球和區(qū)域性凋落物交叉分解試驗(yàn)并不多[84–85]。在降水稀少的干旱區(qū)草地，推動凋落物分解的氣候因子是紫外輻射[82]。SOC礦化速率(土壤呼吸速率)明顯與分解者的活性規(guī)律相一致，隨氣溫的上升而加快，雖然不同區(qū)域的Q10值差異很大[86]，但總體上呈指數(shù)增加的模式。與降水相比，土壤水分狀況是影響土壤呼吸更為直接的因子，大多數(shù)相關(guān)研究都將土壤水分而不是降水量作為考量因素，研究發(fā)現(xiàn)土壤呼吸作用與土壤水分呈單峰曲線，即在土壤水分含量低的時(shí)候，土壤呼吸速率隨土壤水分含量增加而單調(diào)地增大，達(dá)到峰值后, 又單調(diào)地減小。土壤水分含量高便于有機(jī)碳向土壤深層運(yùn)輸，增加土壤剖面有機(jī)碳含量的均勻性，同時(shí)維持土壤的厭氧環(huán)境[82]，有利于SOC的保存。

植物殘?bào)w分解速率和土壤呼吸速率對氮沉降的響應(yīng)則比較復(fù)雜，Mo等[53]報(bào)道氮沉降水平的增加延緩了“氮飽和”的成熟森林生態(tài)系統(tǒng)凋落物分解和土壤呼吸、而對未達(dá)到“氮飽和”的先鋒群落和過渡群落則相反；這種現(xiàn)象似乎是由于氮沉降對不同成熟度森林土壤的酸化程度存在差異,土壤酸化增加了土壤陽離子(cation)交換量從而促進(jìn)了SOC的積累[87]；同時(shí)土壤酸化也降低了分解者的活性[88–89]。

一般認(rèn)為，微生物活性在調(diào)控凋落物分解速率上起著重要的作用[83]；但全球尺度上的凋落物分解交互試驗(yàn)結(jié)果表明，與氣候作用比較，分解者的作用是局部的、不會改變凋落物分解速率由熱帶向寒帶逐漸減小的全球格局[84]。有研究表明植物多樣性上升將導(dǎo)致分解者生物量和活性增加[34,90]，從而間接地調(diào)控SOC的積累，因此，植物多樣性也是影響SOC積累的環(huán)境因子之一。

3 樣地及以上尺度的土壤固碳機(jī)制

在樣地及以上尺度檢驗(yàn)和校正樣地以下尺度的個(gè)例研究(case study)所得到的SOC平衡過程及其機(jī)理并歸納出普遍規(guī)律，是探索SOC形成機(jī)制歷程中的一個(gè)關(guān)鍵節(jié)點(diǎn)。如前言中所述，探索SOC形成機(jī)制的根本目的在于建立適合評估區(qū)域或全球碳平衡的模型、促進(jìn)全球陸地生態(tài)系統(tǒng)SOC積累以減緩大氣CO2濃度上升。多年來特別是2015年巴黎氣候會議以來，在這個(gè)目的驅(qū)動下，全球廣泛開展了樣地尺度以上的土壤固碳機(jī)制的探索工作，試圖建立SOC與大尺度環(huán)境因子之間的關(guān)系，諸如SOC儲量與氣候和植被[25–26,91–93]、土壤類型[94]和管理方式[35]的關(guān)系等等。然而，盡管人們對樣地尺度以下的SOC平衡過程如前所述已有了充分的了解，但目前還沒有普遍認(rèn)可的樣地尺度以上的土壤固碳機(jī)制[95]，這種現(xiàn)狀呼吁人們在對不同時(shí)空尺度和土壤類型的SOC進(jìn)行大量測定的基礎(chǔ)上，繼續(xù)開展進(jìn)一步的研究[96]。

基于中國科學(xué)院戰(zhàn)略先導(dǎo)專項(xiàng)課題“中國森林生態(tài)系統(tǒng)固碳現(xiàn)狀、速率、機(jī)制和潛力(森林課題)”在國家尺度上所布置的森林樣地及其調(diào)查數(shù)據(jù)、結(jié)合全球已經(jīng)發(fā)表的研究結(jié)果，Zhou等[6]報(bào)道樣地及以上尺度的SOC動態(tài)與氣候、初級生產(chǎn)者、土壤之間存在如下規(guī)律：(1) 與濕潤指數(shù)P/PET (P-年降水量，PET-年蒸散潛力)呈顯著正相關(guān)關(guān)系；(2) 與凋落物(泛指輸入到土壤的所有初級產(chǎn)品，包括地上地下凋落物及粗死木、根系分泌物)碳氮比(C/N)呈顯著負(fù)相關(guān)關(guān)系；(3) 植物多樣性只直接影響表層SOC動態(tài)，對深層SOC動態(tài)沒有直接影響，但是在自然狀況下，植物多樣性的上升將導(dǎo)致群落凋落物平均C/N比下降，從而間接影響SOC；(4) 與凋落物量沒有顯著關(guān)系；(5) 與土壤質(zhì)地沒有顯著關(guān)系。這些結(jié)果表明，任何能夠引起凋落物C/N比下降的自然過程和人為措施(如自然演替或人工引進(jìn)C/N比低的物種到群落中)、以及能導(dǎo)致濕潤指數(shù)P/PET上升的氣候變化事件都將促進(jìn)SOC的積累。這可以解釋很多先前難以理解的現(xiàn)象。如：FACE試驗(yàn)表明, CO2濃度上升促進(jìn)了生物量的積累且增加了凋落物產(chǎn)量, 卻并不能增加SOC含量[55,97–98]，這顯然是因?yàn)镃O2濃度上升并沒有降低(甚至增加)凋落物C/N比[6]。與此類似，氮肥添加試驗(yàn)雖然增加了初級生產(chǎn)者的生產(chǎn)力和生物量，但能否增加SOC含量則取決于是否降低了凋落物的C/N比[99–100]。營林工作者通過引進(jìn)豆科植物以增加原有林分的土壤肥力[101]，也是因?yàn)槎箍浦参锏募尤虢档土嗽辛址值牡蚵湮顲/N比[6]。

4 成熟森林生態(tài)系統(tǒng)土壤固碳機(jī)制

自發(fā)現(xiàn)成熟森林土壤可持續(xù)固碳現(xiàn)象以來[8],通過13年的研究，其固碳機(jī)理已基本清晰。作為本文主題下的一個(gè)實(shí)例，這里將成熟森林生態(tài)系統(tǒng)土壤固碳機(jī)制總結(jié)如下。

4.1 內(nèi)源驅(qū)動機(jī)制

“內(nèi)源驅(qū)動機(jī)制”是指由生態(tài)系統(tǒng)自身演替所導(dǎo)致的氣候、初級生產(chǎn)者、土壤理化性質(zhì)改變所驅(qū)動的SOC積累。受這種機(jī)制作用，成熟與未成熟生態(tài)系統(tǒng)SOC的反應(yīng)是完全不同的。

對季風(fēng)常綠闊葉林(南亞熱帶地帶性頂級群落)及其演替系列的研究發(fā)現(xiàn)，在先鋒群落向頂級群落自然演替過程中及達(dá)到頂級群落后，雖然凋落物量很快趨向穩(wěn)定，但土壤水分[102]和植物多樣性[103–104]持續(xù)上升，凋落物C/N比和木質(zhì)素含量持續(xù)降低,推動著凋落物分解速率的加快以及流向土壤的有機(jī)碳比例增大[40]，為土壤提供了越來越豐富的有機(jī)碳源。與此同時(shí)，隨著演替的進(jìn)行，土壤有效磷含量的下降和有效氮含量的上升阻礙了SOC的分解從而有利于SOC的保存[53,105–106]；而且，由于土壤水分隨森林成熟度增加而逐步提高，一方面使得成熟森林土壤表層有機(jī)碳較易于向土壤深層運(yùn)輸并在厭氧環(huán)境下得到保存[82]，另一方面森林的順行演替有利于驅(qū)動微生物朝著促進(jìn)SOC積累的方向發(fā)揮功能[102]。

上述成熟森林SOC持續(xù)積累的內(nèi)源驅(qū)動機(jī)制可以被Zhou等[6]近期的研究結(jié)果完美詮釋并相互印證。

4.2 外源驅(qū)動機(jī)制

“外源驅(qū)動機(jī)制”是指生態(tài)系統(tǒng)受外力(人類經(jīng)營管理、自然災(zāi)害和全球及區(qū)域環(huán)境變化)作用所導(dǎo)致的SOC積累。這種機(jī)制對所有生態(tài)系統(tǒng)SOC積累都起作用，但成熟與非成熟生態(tài)系統(tǒng)的反應(yīng)程度可能有所差異。

對季風(fēng)常綠闊葉林及其演替系列的研究發(fā)現(xiàn),該區(qū)域長期受酸沉降和氮沉降上升的脅迫，導(dǎo)致土壤酸化和成熟森林土壤氮含量飽和，酸化土壤和氮飽和土壤都將抑制土壤呼吸[53,107]，增加土壤陽離子交換量(CEC)[87]，有利于SOC的保存，這種效應(yīng)在成熟森林土壤更為顯著[53]。

過去幾十年來，季風(fēng)常綠闊葉林區(qū)域氣溫持續(xù)上升、降水強(qiáng)度兩極化，這在導(dǎo)致土壤水分下降的同時(shí)[104,108]，也增大了成熟與未成熟生態(tài)系統(tǒng)土壤水分的年內(nèi)變幅差異，成熟生態(tài)系統(tǒng)土壤水分的年內(nèi)變幅小于未成熟生態(tài)系統(tǒng)的年內(nèi)變幅，更有利于SOC的積累[102]。

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Exploration History of Soil Organic Carbon Formation Mechanisms

ZHOU Guo-yi1, XIONG Xin1,2

(1. South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China; 2. University of Chinese Academy of Sciences,Beijing 100049, China)

Soil organic carbon (SOC) is an important asset of ecosystems and plays a key role in the global carbon balance. Since the 2015 Paris Climate Conference, promoting SOC accumulation in terrestrial ecosystems has received special attention and is considered to be the most important land-based action of mitigating the rising atmospheric CO2concentration. Starting from serving this goal, this paper reviews and summarizes the history of the global exploration of SOC formation mechanisms over the recent decades. Including the global distribution law of SOC, the SOC cycle processes and the corresponding physical, chemical and biological mechanisms at scales that are smaller than plot, and the mechanisms of soil carbon sequestration at scales that are larger than plot. Finally, an example of the SOC accumulation mechanisms of mature forest is given. So, the exploration process of the SOC formation mechanisms is actually the process of seeking theoretical guidance for promoting soil carbon sequestration.

SOC; Formation; Sequestration; Mechanism

10.11926/jtsb.4094

2019–05–16

2019–07–05

中國科學(xué)院前沿科學(xué)重點(diǎn)研究項(xiàng)目(DYZDJ-SSW-DQC003)；國家自然科學(xué)基金項(xiàng)目(41430529, 41573077)資助

This work was supported by the Key Research Projects in Frontier Science of Chinese Academy of Sciences (Grant No. DYZDJ-SSW-DQC003), and the National Natural Science Foundation of China (Grant No. 41430529, 41573077).

周國逸(1963~ )，男，研究員，博士生導(dǎo)師，研究方向?yàn)樯鷳B(tài)系統(tǒng)生態(tài)學(xué)和生態(tài)水文學(xué)。E-mail: gyzhou@scib.ac.cn