宋 敏, 劉銀占, 井水水

河南大學(xué)生命科學(xué)學(xué)院, 開封 475004

土壤線蟲對(duì)氣候變化的響應(yīng)研究進(jìn)展

宋 敏*, 劉銀占, 井水水

河南大學(xué)生命科學(xué)學(xué)院, 開封 475004

全球變化對(duì)陸地生態(tài)系統(tǒng)功能具有重要而深遠(yuǎn)的影響。陸地生態(tài)系統(tǒng)地下部分具有重要的生態(tài)功能，其組成及結(jié)構(gòu)對(duì)氣候變化的響應(yīng)將進(jìn)一步減緩或加劇全球化進(jìn)程。土壤線蟲在各類生態(tài)系統(tǒng)中分布十分廣泛，是地下食物網(wǎng)的重要組分，在維持土壤生物多樣性及營(yíng)養(yǎng)物質(zhì)循環(huán)過程中發(fā)揮重要作用，其組成及結(jié)構(gòu)對(duì)不同氣候變化驅(qū)動(dòng)因子的響應(yīng)機(jī)制與模式不盡相同。增溫及降水格局變化主要是通過改變線蟲生境而直接影響其種群密度與結(jié)構(gòu)，兩者通常表現(xiàn)為正效應(yīng)且作用效果隨處理時(shí)間的延長(zhǎng)而增強(qiáng)。CO2與大氣氮沉降主要是通過影響地上植被，凋落物質(zhì)量，土壤理化性質(zhì)等間接過程影響土壤線蟲。同時(shí)，不同的全球變化因子之間存在著復(fù)雜的交互作用，深入理解這些因子之間交互作用對(duì)線蟲群落的影響模式與機(jī)制對(duì)于探討未來氣候變化情景下生態(tài)統(tǒng)生物多樣性及養(yǎng)分循環(huán)過程具有重要的理論指導(dǎo)意義。

氣候變暖; 降水格局變化; 大氣CO2濃度升高; 氮沉降;線蟲群落

工業(yè)革命以來，伴隨著化石燃料的燃燒及土地利用方式的轉(zhuǎn)變，地球生態(tài)系統(tǒng)的物質(zhì)循環(huán)也在不斷加速。以大氣二氧化碳濃度升高，氣候變暖，降水格局變化及大氣氮沉降為主要特征的全球變化正在對(duì)人們賴以生存的陸地生態(tài)系統(tǒng)產(chǎn)生重要而深遠(yuǎn)的影響，如物種滅絕，多樣性喪失及由此引發(fā)的生態(tài)系統(tǒng)功能的改變[1-3]。

陸地生態(tài)系統(tǒng)由地上、地下兩部分組成，兩者相互依存，不可分割。一方面，地上植物通過凋落物及根際為地下生物提供碳源，另一方面，地下生物通過礦化作用將儲(chǔ)存于有機(jī)質(zhì)中的養(yǎng)分釋放，用于植物生長(zhǎng)。然而，由于研究方法等因素限制，過去對(duì)于全球變化對(duì)陸地生態(tài)系統(tǒng)影響的研究主要集中于地上部分，而對(duì)于地下生態(tài)系統(tǒng)研究相對(duì)較少[4-5]。土壤中含有植物根系、凋落物、微生物及土壤動(dòng)物等眾多組分，其中土壤動(dòng)物通過對(duì)凋落物進(jìn)行破碎及取食微生物等方式影響土壤有機(jī)質(zhì)的礦化過程[6-7]。對(duì)維持陸地生態(tài)系統(tǒng)的生物地球化學(xué)循環(huán)和生態(tài)系統(tǒng)功能至關(guān)重要。

線蟲是土壤動(dòng)物的主要功能類群之一,分布廣泛，數(shù)量巨大，種類繁多[8-9]，結(jié)構(gòu)簡(jiǎn)單，易于鑒定，處于食物網(wǎng)的多個(gè)營(yíng)養(yǎng)級(jí)。同時(shí)，線蟲對(duì)環(huán)境變化十分敏感，是土壤營(yíng)養(yǎng)元素循環(huán)過程的重要參與者與執(zhí)行者[10-11]，其群落組成及結(jié)構(gòu)的變化將對(duì)生態(tài)系統(tǒng)服務(wù)及功能產(chǎn)生重要影響，進(jìn)而減緩或加劇氣候變化進(jìn)程[12-14]。土壤線蟲群落動(dòng)態(tài)主要受資源有效性及所處生境微環(huán)境變化的調(diào)控[15-16]。然而，不同全球變化驅(qū)動(dòng)因子對(duì)土壤養(yǎng)分及環(huán)境影響的機(jī)制及幅度不同[17-19]。同時(shí)，線蟲的不同營(yíng)養(yǎng)類群及種屬對(duì)氣候變化的敏感性也不盡相同[18,20-22](根據(jù)線蟲食物來源的不同，可分為食細(xì)菌性線蟲、食真菌線蟲、植物寄生線蟲、雜食性線蟲及捕食性線蟲)。線蟲群落動(dòng)態(tài)調(diào)控因子的復(fù)雜性及其不同類群間的差異性導(dǎo)致目前對(duì)于土壤線蟲群落對(duì)不同氣候變化因子的響應(yīng)模式與機(jī)制依然缺乏規(guī)律性認(rèn)識(shí)。

本文通過歸納總結(jié)近年發(fā)表的土壤線蟲對(duì)氣候變化響應(yīng)的研究結(jié)果，論述了土壤線蟲在群落水平、營(yíng)養(yǎng)類群水平及種屬水平上對(duì)不同氣候變化驅(qū)動(dòng)因子及其交互作用的響應(yīng)。進(jìn)一步綜合探討了這些響應(yīng)模式在時(shí)間尺度及空間尺度上的差異，旨在加深全球變化對(duì)線蟲群落結(jié)構(gòu)及功能影響的理解和認(rèn)識(shí)，為未來全球變化情景下，土壤生物多樣性保護(hù)及土壤碳動(dòng)態(tài)研究提供參考。

1 二氧化碳濃度升高對(duì)土壤線蟲的影響

1.1 二氧化碳濃度升高在群落水平上對(duì)土壤線蟲的影響

2011年，大氣中CO2濃度達(dá)到391 μL/L，比工業(yè)化前的1750年高了40%[1]。二氧化碳濃度升高可以通過改善地下資源有效性及微環(huán)境進(jìn)而對(duì)土壤線蟲種群密度產(chǎn)生有利影響[23-29]，例如，Yeates通過在一個(gè)放牧草地進(jìn)行大氣CO2富集實(shí)驗(yàn)(FACE)，發(fā)現(xiàn)大氣CO2濃度增加能顯著提高表層土壤中線蟲數(shù)量[]30]。究其原因，一方面，CO2濃度升高可以促進(jìn)植物的光合作用，提高植物凈初級(jí)生產(chǎn)力(NPP)并增加光合產(chǎn)物向地下的分配[31]，促進(jìn)細(xì)根生長(zhǎng)，提高根際周轉(zhuǎn)速率并增加根際分泌[32-33]。這些可利用資源的增加將促進(jìn)土壤微生物的生長(zhǎng)與繁殖[34-35]，并進(jìn)一步通過上行效應(yīng)影響土壤線蟲的種群密度[36]。另一方面， CO2濃度升高將降低植物氣孔導(dǎo)度，減少蒸騰作用，從而增加土壤濕度[37-39]，對(duì)線蟲產(chǎn)生有利影響。

1.2 二氧化碳濃度升在高營(yíng)養(yǎng)類群及種屬水平對(duì)土壤線蟲的影響

線蟲群落中，不同的營(yíng)養(yǎng)類群對(duì)CO2濃度升高的響應(yīng)不同。Eisenhauer在美國(guó)明尼蘇達(dá)Cedar Creek草地的研究發(fā)現(xiàn)，13a的FACE實(shí)驗(yàn)顯著增加了食真菌線蟲的數(shù)量，但降低了雜捕類線蟲的數(shù)量[40]；而Hoeksema[27]及Neher[41]在森林FACE實(shí)驗(yàn)中卻得出了完全相反的結(jié)論。土壤有機(jī)質(zhì)含量的差異可能是造成線蟲群落結(jié)構(gòu)在草地及森林生態(tài)系統(tǒng)中對(duì)二氧化碳濃度升高出現(xiàn)不一致響應(yīng)的重要原因[35,42]。

另外，由于線蟲種類繁多，不同種屬線蟲在繁殖策略、生活史周期及體型大小方面存在諸多差異，傳統(tǒng)的按營(yíng)養(yǎng)類群進(jìn)行功能分類的方法往往會(huì)掩蓋線蟲個(gè)體生物學(xué)特征對(duì)環(huán)境變化的響應(yīng)。Yeates對(duì)草地CO2富集實(shí)驗(yàn)的研究發(fā)現(xiàn)，CO2濃度升高顯著增加了Longidoruselongatus線蟲的種群密度[30]，而Neher在北方森林的研究結(jié)果表明，在CO2升高處理下，Aphelenchoides線蟲種群密度明顯下降，而Filenchus線蟲種群密度明顯增加。線蟲種屬水平上對(duì)CO2濃度升高響應(yīng)的差異將改變?nèi)郝涓?jìng)爭(zhēng)關(guān)系，并引起線蟲群落結(jié)構(gòu)的變化，進(jìn)而影響其生態(tài)功能[18]。

1.3 不同時(shí)空尺度下，線蟲群落對(duì)二氧化碳濃度升高響應(yīng)的差異

土壤線蟲對(duì)CO2濃度升高的響應(yīng)與生態(tài)系統(tǒng)類型無關(guān)，且CO2濃度升高的正效應(yīng)隨處理時(shí)間的延長(zhǎng)而減弱。Sonnemann[28]通過研究溫帶草原地下食物網(wǎng)對(duì)二氧化碳濃度升高的響應(yīng)發(fā)現(xiàn)，土壤線蟲種群密度在實(shí)驗(yàn)處理第1年顯著增加，然而，實(shí)驗(yàn)處理第3年時(shí)線蟲種群密度重新回歸到處理前水平。這一結(jié)果與Blankinship利用整合分析方法得出的結(jié)論相一致[43]。究其原因，一方面可能是由于長(zhǎng)期[CO2]升高導(dǎo)致諸如氮元素更多流向植物體內(nèi)和土壤有機(jī)質(zhì)中，而通過凋落物分解返還到土壤中的氮素減少，以及固氮作用的下降[44]等，結(jié)果最終導(dǎo)致地下生態(tài)系統(tǒng)由最初的碳限制轉(zhuǎn)變?yōu)榈拗芠45]，這種“漸進(jìn)性氮限制”通過改變土壤微生物群落組成及結(jié)構(gòu)進(jìn)而影響土壤線蟲群落構(gòu)成。另一方面，長(zhǎng)期[CO2]升高還會(huì)導(dǎo)致土壤離子流失及土壤酸化[46]并進(jìn)一步影響土壤線蟲的數(shù)量及取食活性[47]。

2 全球變暖對(duì)土壤線蟲的影響

1880—2012 年全球平均溫度已升溫0.85℃。溫度是調(diào)節(jié)陸地生態(tài)系統(tǒng)生物地球化學(xué)過程的重要環(huán)境因子。碳循環(huán)的關(guān)鍵過程如植物光合作用、凋落物分解、土壤呼吸、微生物及土壤動(dòng)物活性等都受環(huán)境溫度的調(diào)控[47-50],是線蟲群落結(jié)構(gòu)變化的重要驅(qū)動(dòng)因子[15,51]。

2.1 全球變暖在群落水平上對(duì)土壤線蟲的影響

諸多研究發(fā)現(xiàn)，溫度升高處理下，土壤線蟲的種群密度顯著降低而種群多樣性及勻均度顯著增加，Simmons 等[52]研究發(fā)現(xiàn)，在南極州干谷，增溫通過直接或間接改變土壤微環(huán)境顯著降低了線蟲的種群密度。而Matute[47]對(duì)位于美國(guó)阿肯色州一處農(nóng)場(chǎng)研究也發(fā)現(xiàn)，土壤線蟲表現(xiàn)出明顯的畏熱性(thermophobic)。雖然有研究認(rèn)為，增溫促進(jìn)了植物特別是細(xì)根的生長(zhǎng)，增加了異養(yǎng)呼吸的底物供應(yīng)[53]， 有利于線蟲種群密度的擴(kuò)增，然而，另一方面，溫度升高可導(dǎo)致蒸騰作用增強(qiáng)，加重土壤水分流失[54-55]及熱應(yīng)激[17,56]，從而影響土壤線蟲群落。Dong對(duì)農(nóng)田生態(tài)系統(tǒng)土壤線蟲對(duì)溫度升高響應(yīng)的研究發(fā)現(xiàn)，線蟲群落物種多樣性與溫度呈正相關(guān)關(guān)系[57]， Bakonyi對(duì)半干旱草原的研究也得出了相似結(jié)論[15]。

2.2 全球變暖在營(yíng)養(yǎng)類群及種屬水平上對(duì)土壤線蟲的影響

土壤線蟲群落中不同營(yíng)養(yǎng)類群及種屬對(duì)增溫的響應(yīng)不盡相同。Ruess等[51]認(rèn)為，食細(xì)菌類線蟲及食真菌類線蟲的種群密度與土壤溫度呈負(fù)相關(guān)而植食性線蟲的種群密度則不受溫度變化的影響。而Stevnbak[58]的研究結(jié)果表明，溫度升高1℃后，線蟲種群密度顯著下降，特別是植食性線蟲，降低幅度達(dá)50%，并且群落構(gòu)成上以世代周期較長(zhǎng)的種屬占優(yōu)勢(shì)。Bakonyi 及Papatheodorou等[15,21]發(fā)現(xiàn)在屬的水平上，小桿屬線蟲(Rhabditida)及滑刃屬線蟲(Tylenchida)對(duì)低溫敏感；麗突(Acrobeles)、擬麗突(Acrobeloides)，繞線(plectus)屬線蟲與溫度呈正相關(guān),而頭葉屬線蟲(Cephalobus)與溫度呈負(fù)相關(guān)。Bakonyi等[15]的研究結(jié)果則顯示出擬麗突屬線蟲(Acrobeloide)與溫度呈負(fù)相關(guān)。Anderson[59]發(fā)現(xiàn)擬麗突Acrobeloides具有很寬的生態(tài)幅(12—35℃)，這似乎可以解釋以上實(shí)驗(yàn)結(jié)果的差異。Sohlenius等[20]同樣也發(fā)現(xiàn)了不同屬線蟲對(duì)溫度變化的響應(yīng)不同的現(xiàn)象，但他認(rèn)為這一結(jié)論的前提是所有種屬共同出現(xiàn)。然而，由于復(fù)雜的種間關(guān)系的存在，人們無法判斷某一特定屬線蟲對(duì)溫度變化的響應(yīng)，同時(shí)，也很難將溫度變化的直接影響與其間接影響區(qū)分開來。

2.3 不同時(shí)空尺度下，線蟲群落對(duì)溫度升高響應(yīng)的差異

土壤線蟲對(duì)氣溫升高的響應(yīng)與當(dāng)?shù)鼐唧w的氣候類型密切相關(guān)。Ruess通過極地土壤增溫實(shí)驗(yàn)發(fā)現(xiàn)，土壤線蟲與溫度之間存在顯著的正相關(guān)關(guān)系[51]，而Simmons對(duì)南極州干谷的研究卻得出了相反的結(jié)論[52]。同時(shí)，一些對(duì)北方森林、農(nóng)田及沙漠土壤的研究則顯示，增溫對(duì)土壤線蟲的群落組成及結(jié)構(gòu)無顯著影響[60-61]。這些矛盾的結(jié)果說明，線蟲對(duì)增溫的響應(yīng)受不同的生態(tài)系統(tǒng)類型及地區(qū)氣候條件的控制。Blankinship等[43]采用整合分析的方法對(duì)不同生態(tài)系統(tǒng)類型下土壤動(dòng)物對(duì)氣溫升高響應(yīng)研究發(fā)現(xiàn)，土壤動(dòng)物對(duì)增溫處理的響應(yīng)與類群、體型及生態(tài)系統(tǒng)類型無關(guān)，主要受年均溫度與年均降水量的影響，在氣溫較低及降水較少區(qū)域，增溫顯著降低了土壤線蟲的數(shù)量，在年降水量超過626mm時(shí)，增溫對(duì)土壤動(dòng)物的數(shù)量表現(xiàn)為正效應(yīng)。同時(shí)，土壤動(dòng)物對(duì)增溫的響應(yīng)與增溫幅度無關(guān)，而與增溫的持續(xù)時(shí)間成負(fù)相關(guān)，其原因主要是由于增溫減少了土壤可利用水分含量，限制了土壤生物的生長(zhǎng)和繁殖[62]。

3 大氣氮沉降對(duì)土壤線蟲的影響

工業(yè)革命以來，由于化石燃料的燃燒及土地利用方式的改變，大氣氮(N)沉降現(xiàn)象日趨嚴(yán)重，成為目前人類面臨的重要全球變化因子[63-64]。大氣氮沉降通過影響植物生長(zhǎng)[42,65]，改變種間競(jìng)爭(zhēng)關(guān)系[66-67]；影響土壤理化性質(zhì)[68-69]，減少根系分泌及抑制微生物胞外酶活性[70-71]等方式影響土壤線蟲的資源有效性及棲息地微環(huán)境。然而，目前只有少數(shù)通過氮素添加模擬大氣氮沉降的野外控制實(shí)驗(yàn)用以研究大氣氮沉降對(duì)土壤線蟲群落組成及結(jié)構(gòu)的影響[19,72-74]。而這些研究的結(jié)果還存在諸多不確定性。

3.1 群落水平線蟲對(duì)大氣氮沉降的響應(yīng)

大氣氮沉降對(duì)土壤線蟲群落表現(xiàn)出明顯的負(fù)效應(yīng)，降低了種群密度及物種多樣性，使群落結(jié)構(gòu)趨于簡(jiǎn)單化。雖然Sjursen[75]等認(rèn)為，低水平的氮素添加因減緩了生態(tài)系統(tǒng)的氮限制，促進(jìn)了植物生長(zhǎng)，增加了地下生物量，從而提高了土壤線蟲的種群密度。然而，諸多森林、草地、農(nóng)田實(shí)驗(yàn)研究中，氮素的添加顯著降低了線蟲的種群數(shù)量及物種多樣性[76]。Liang研究中國(guó)北方地區(qū)長(zhǎng)期氮素添加對(duì)土壤線蟲的影響發(fā)現(xiàn)，氮素添加顯著降低了線蟲種群密度及物種豐富度[77]，Eisenhauer[40]在明尼蘇達(dá)州草原生態(tài)系統(tǒng)的研究也得到了相似的結(jié)論，究其原因，一方面，長(zhǎng)期較高濃度的N 沉降能造成土壤酸化[14 ], 而pH 值是土壤線蟲分布的重要限制性因素, 大多數(shù)土壤線蟲適宜在微酸和中性條件下生活, 因而土壤pH值的變化將影響土壤線蟲的種群密度[78-79]。另一方面，氮添加導(dǎo)致土壤中銨鹽濃度增高，產(chǎn)生的銨毒性對(duì)線蟲種群特別是根食性線蟲具有較強(qiáng)的抑制作用。直接接觸這些銨毒性物質(zhì)可以引起土壤線蟲機(jī)體衰弱, 成長(zhǎng)速度減緩、繁殖能力減退,數(shù)量減少，甚至導(dǎo)致其死亡[19]。氮添加還可通過降低根際分泌，減少植物光合產(chǎn)物向地下的分配[74]，改變植物群落構(gòu)成進(jìn)而間接影響土壤線蟲群落。土壤線蟲通過植物根系及凋落物獲得碳源[80-81]，因此，植物物種構(gòu)成及多樣性的改變將導(dǎo)致其碳輸入數(shù)量及質(zhì)量的變化，并間接影響土壤線蟲的群落結(jié)構(gòu)和功能[82]。

3.2 營(yíng)養(yǎng)類群及種屬水平土壤線蟲對(duì)氮素添加的響應(yīng)。

不同線蟲營(yíng)養(yǎng)類群及種屬對(duì)氮素添加的響應(yīng)方向與程度不盡相同。Sun[83]對(duì)森林生態(tài)系統(tǒng)研究發(fā)現(xiàn)，氮素添加顯著降低了群落中食真菌性線蟲、根食性線蟲及雜食-捕食類線蟲的種群密度。這種變化主要是由于氮添加對(duì)土壤真菌的抑制作用導(dǎo)致了食真菌性線蟲種群密度的降低。雜-捕類線蟲多為r策略線蟲，更易受氮添加引起的環(huán)境攪動(dòng)影響[19,84]。Gruber[85]及 Papanikolaou[86]等研究表明，根食性線蟲，特別是Geocenamusarcticus屬線蟲及食真菌線蟲豐度與土壤中銨態(tài)氮及硝態(tài)濃度呈顯著負(fù)相關(guān)，表明了施氮對(duì)土壤線蟲的直接影響。

3.3 不同時(shí)空尺度下，線蟲群落對(duì)氮沉降響應(yīng)的差異

氮素添加的效應(yīng)與生態(tài)系統(tǒng)類型無關(guān)。Sun[83]對(duì)森林生態(tài)系統(tǒng)研究發(fā)現(xiàn)，土壤線蟲群落對(duì)氮素添加的響應(yīng)模式受降水的影響。氮循環(huán)的諸多過程如凋落物分解，有機(jī)氮礦化及有效氮淋溶等都與降水條件密切相關(guān)[87]，另外Liang[77]研究發(fā)現(xiàn)，線蟲群落對(duì)氮素添加的響應(yīng)隨處理持續(xù)時(shí)間的不同而不同，表現(xiàn)為處理初期，根食性線蟲種群密度顯著降低，而隨著處理時(shí)間的延長(zhǎng)，其種群密度逐漸恢復(fù)。因此，在研究氮素添加對(duì)土壤線蟲生態(tài)效應(yīng)時(shí)應(yīng)充分考慮不同地區(qū)的降水格局及實(shí)驗(yàn)處理持續(xù)時(shí)間的影響。

4 降水格局變化對(duì)土壤線蟲的響應(yīng)

據(jù)聯(lián)合國(guó)政府間氣候變化專門委員會(huì)第一工作組第五次評(píng)估報(bào)告(IPCC 2013)預(yù)測(cè)，到21世紀(jì)末，高緯度地區(qū)和熱帶太平洋區(qū)域的年降水量將會(huì)增加；許多中緯度的潮濕地區(qū)，平均降水也將增加。但在中緯度干燥地區(qū)、副熱帶的干燥地區(qū)平均降水將減少。在全球持續(xù)變暖的趨勢(shì)下，到21世紀(jì)末，中緯度大部分陸地區(qū)域與熱帶區(qū)域的濕區(qū)，極端降水事件將很可能更劇烈并更頻繁[1]。線蟲生活在土壤孔隙水中，降水格局的變化將對(duì)土壤線蟲產(chǎn)生重要影響。

4.1 群落水平土壤線蟲對(duì)降水格局變化的響應(yīng)

大多數(shù)實(shí)驗(yàn)研究表明，在一定濕度范圍內(nèi)，土壤線蟲種群密度與土壤濕度呈顯著的正相關(guān)關(guān)系。Ruan對(duì)中國(guó)北方半干旱草原土壤線蟲研究發(fā)現(xiàn)，增加降水顯著增了線蟲的種群密度[88]。Landesman對(duì)美國(guó)新澤西州森林土壤線蟲的研究也得出了相似的結(jié)論[87]。水分對(duì)土壤線蟲種群密度的正效應(yīng)一方面是增加降水有利于線蟲的攝食、運(yùn)動(dòng)、生長(zhǎng)及繁殖[55]。另一方面，降水的增加促進(jìn)植物生長(zhǎng)，增加植物凈初級(jí)生長(zhǎng)力[89]，促進(jìn)氮礦化，提高土壤有效氮含量[90]，增加微生物量進(jìn)而提高土壤線蟲的種群密度。

4.2 營(yíng)養(yǎng)類群及種屬水平土壤線蟲對(duì)降水格局變化的響應(yīng)

不同營(yíng)養(yǎng)類群及種屬線蟲對(duì)增加降水及干旱的響應(yīng)不同。濕潤(rùn)環(huán)境利于食細(xì)菌性線蟲的生長(zhǎng)繁殖而干燥環(huán)境更利于食真菌線蟲種群擴(kuò)增。Ruan對(duì)內(nèi)蒙草原的研究發(fā)現(xiàn)，增加降水處理，增加了植食性線蟲數(shù)量而降低了細(xì)菌性及雜捕類線蟲的數(shù)量[88]。Landesman的研究發(fā)現(xiàn)，所有營(yíng)養(yǎng)類群線蟲均對(duì)干旱敏感，食細(xì)菌性線種表現(xiàn)最為明顯，特別是plectidae的數(shù)量在干旱處理下顯著降低，然而，對(duì)于同樣屬于食細(xì)菌性線蟲的Cephalobidae和qudsianematidae則對(duì)干旱處理無響應(yīng)[87]。Sohlenius[91]通過實(shí)驗(yàn)室培養(yǎng)實(shí)驗(yàn)發(fā)現(xiàn)擬麗突線蟲Acrobeloides與水分呈正相關(guān)，滑刃屬線蟲Tylenchida與水分呈負(fù)相關(guān)。他認(rèn)為這是由于兩種線蟲間的競(jìng)爭(zhēng)關(guān)系導(dǎo)致的。然而，由于這兩種線蟲的食性不同，因此這種差異也可能是由于資源有效性所導(dǎo)致的。Bakonyi[15]認(rèn)為麗突(Acrobeles)與擬麗突(Acrobeloides)屬線蟲種群密度與水分條件無關(guān)，而Griffiths等發(fā)現(xiàn)擬麗突屬線蟲(Acrobeloides)更適合于干燥環(huán)境。另一項(xiàng)田間實(shí)驗(yàn)與證明了繞線屬線蟲(Plectus)在干燥環(huán)境中密度增加[92]。植食性線蟲中不同種屬對(duì)土壤水分條件變化的響應(yīng)也不盡相同[15,93-96]，Griffin研究發(fā)現(xiàn)矮化屬線蟲(Tylenchorhynchusacutoides)及劍屬線蟲(Xiphinemaamericanum)對(duì)濕度變化比短針屬線蟲(Pratylenchusneglectus)更敏感。對(duì)于雜捕類線蟲，Porazinska等通過農(nóng)田灌溉實(shí)驗(yàn)發(fā)現(xiàn)除孔咽屬線蟲(Aporcelaimellus)和真矛屬線蟲(Eudorylaimus)與灌溉呈正相關(guān)外，其它種屬線蟲的種群密度不受土壤水分條件的影響[97]。線蟲種屬水平上對(duì)降水格局變化的響應(yīng)不同導(dǎo)致其群落組成及結(jié)構(gòu)的變化， 并影響其生態(tài)功能。

4.3 不同時(shí)空尺度下，線蟲群落對(duì)降水格局變化響應(yīng)的差異

森林生態(tài)系統(tǒng)土壤線蟲更易受降水格局變化影響，增加降水的正效應(yīng)隨處理時(shí)間的延長(zhǎng)而增強(qiáng)，土壤線蟲群落對(duì)干旱的響應(yīng)強(qiáng)于增加降水的響應(yīng)。Eisenhauer 對(duì)美國(guó)明尼蘇達(dá)州草原土壤研究發(fā)現(xiàn)，線蟲群落對(duì)夏季干旱無響應(yīng)。Landesman[82]等在森林生態(tài)系統(tǒng)得出的結(jié)論與這一結(jié)果相反。由于土壤生物對(duì)環(huán)境具有一定的適應(yīng)能力，因此，相對(duì)于干旱，碳源是土壤線蟲更重要的限制因子。在美國(guó)新澤西州森林進(jìn)行的為期1a的降水控制實(shí)驗(yàn)結(jié)果表明，干旱對(duì)線蟲群落的影響強(qiáng)于增加降水[98]，這可能是與干旱對(duì)凋落物層及礦質(zhì)土層影響程度不同有關(guān)[99]，另外，水分不足情況下，由于植被與土壤動(dòng)物之間對(duì)于水分的競(jìng)爭(zhēng)能力亦不相同[100]，因此導(dǎo)致了土壤線蟲對(duì)干旱的響應(yīng)強(qiáng)于增加降水。

根據(jù)不同全球變化驅(qū)動(dòng)因子對(duì)線蟲群落影響機(jī)制與模式繪制模式圖如下。

圖1 土壤線蟲對(duì)不同氣候變化驅(qū)動(dòng)因子(二氧化碳濃度升高；氣候變暖；大氣氮沉降；降水格局變化)的響應(yīng)Fig.1 The responses of soil nematode to global change (elevated CO2; warming; nitrogen deposition; changing precipitation)虛線表示長(zhǎng)期效應(yīng); “↑”表示增高，“↓”表示降低

5 多因子交互作用對(duì)土壤線蟲群落的影響

全球變化是以大氣CO2濃度上升，氣候變暖，大氣氮沉降及降水格局變化為主要特征的多個(gè)驅(qū)動(dòng)因子共同作用的結(jié)果，各個(gè)驅(qū)動(dòng)因子之間存在復(fù)雜的交互作用。如氣候變暖，可通過改變土壤呼吸速率及大氣環(huán)流而影響大氣CO2濃度及降水格局，并進(jìn)一步影響植物群落及土壤微環(huán)境，并從資源與環(huán)境兩方面影響土壤線蟲群落。因此，前期的單因子控制實(shí)驗(yàn)結(jié)果無法準(zhǔn)確模擬未來氣候變化情境下土壤線蟲群落的真實(shí)響應(yīng)[101-102]，近年來，關(guān)于不同全球變化因子間的交互作用對(duì)地下生態(tài)系統(tǒng)結(jié)構(gòu)與功能影響的野外控制實(shí)驗(yàn)陸續(xù)開展[88,103-104]，Kardol[105]發(fā)現(xiàn)自然CO2濃度下增溫和干旱表現(xiàn)出對(duì)線蟲群落的負(fù)效應(yīng)，而在增高CO2的處理中，增溫及干旱對(duì)線蟲群落無影響。Chung[106]認(rèn)為土壤動(dòng)物群落的組成及結(jié)構(gòu)受植物多樣性、CO2濃度及氮素水平的共同影響；Hoeksema[27]及Li[107] 的實(shí)驗(yàn)結(jié)果表明，土壤線蟲的種群密度及多樣性受CO2和氮素添加間交互作用的影響。Fu2功能團(tuán)線蟲對(duì)CO2及氮肥的交互作用響應(yīng)最顯著[108]?？傊?，各種全球變化因子通過復(fù)雜的交互作用共同調(diào)節(jié)地上植物群落及土壤微環(huán)境，進(jìn)而改變土壤線蟲食物資源有效性及生境穩(wěn)定性，觸發(fā)養(yǎng)分介導(dǎo)的“上行效應(yīng)”及種間關(guān)系介導(dǎo)的“下行效應(yīng)”，共同改變土壤線蟲群落的組成及結(jié)構(gòu)。作為地下食物網(wǎng)的重要組分，線蟲群落的變化將引起整個(gè)地下食物網(wǎng)結(jié)構(gòu)與功能的變化，進(jìn)而改變土壤養(yǎng)分循環(huán)過程，對(duì)陸地生態(tài)系統(tǒng)結(jié)構(gòu)與功能造成廣泛而深遠(yuǎn)的影響[109]。

6 問題與展望

土壤線蟲是地下食物網(wǎng)的重要組成部分，在維持生態(tài)系統(tǒng)生物多樣性及營(yíng)養(yǎng)元素循環(huán)過程中發(fā)揮重要作用，是研究未來氣候變化情景下地下生態(tài)系統(tǒng)結(jié)構(gòu)、功能變化的重要指示性生物。然而，目前，在研究線蟲群落組成及結(jié)構(gòu)對(duì)全球變化響應(yīng)方面還存在諸多不足，主要表現(xiàn)在：

(1)對(duì)于線蟲的生態(tài)學(xué)功能認(rèn)識(shí)不足。以往研究，特別是在傳統(tǒng)農(nóng)牧業(yè)管理措施中，線蟲往往被作為害蟲加以防控，直到近年來，線蟲在土壤質(zhì)量評(píng)估、生境穩(wěn)定性檢測(cè)，環(huán)境污染、人為及自然干擾強(qiáng)度及物質(zhì)循環(huán)等過程中的作用才逐漸引起人們的重視。

(2)研究方法落后，研究尺度較窄。目前對(duì)土壤線蟲的研究主要沿習(xí)傳統(tǒng)的形態(tài)學(xué)鑒定方法，這一方法雖然具有簡(jiǎn)便、準(zhǔn)確等優(yōu)點(diǎn)，但鑒定速度較慢，不適用于大尺度、大樣本量研究。同時(shí)，形態(tài)學(xué)鑒定只能反映線蟲群落的組成及結(jié)構(gòu)變化，無法反應(yīng)不同營(yíng)養(yǎng)級(jí)間的物質(zhì)及能量流通情況。分子生物學(xué)及同位素示蹤技術(shù)可有效彌補(bǔ)形態(tài)學(xué)鑒定方法的不足，在未來，現(xiàn)代生物學(xué)技術(shù)與傳統(tǒng)方法相結(jié)合成為必然的趨勢(shì)。

(3)關(guān)于土壤線蟲在全球變化領(lǐng)域中的作用研究有待細(xì)化。目前，在全球變化研究領(lǐng)域，土壤線蟲的作用越來越受到生態(tài)學(xué)家的重視，然而，其研究?jī)?nèi)容往往主要集中于土壤線蟲種群密度、營(yíng)養(yǎng)類群組成的變化等方面。土壤線蟲類群極其豐富，簡(jiǎn)單地按食性進(jìn)行功能群劃分往往會(huì)掩蓋不同屬線蟲的個(gè)體生物學(xué)差異，因此，有必要對(duì)不同屬線蟲的繁殖策略，生境選擇，對(duì)干擾的抵抗力與恢復(fù)力，種間競(jìng)爭(zhēng)關(guān)系等問題進(jìn)行深入細(xì)致的研究。

(4)全球變化是由諸多驅(qū)動(dòng)因子共同作用的結(jié)果，各個(gè)驅(qū)動(dòng)因子之間存在復(fù)雜的交互作用。因此，前期的單因子控制實(shí)驗(yàn)結(jié)果無法準(zhǔn)確模擬未來氣候變化情景下地下食物網(wǎng)的真實(shí)響應(yīng)，然而，受經(jīng)費(fèi)及人力等因素制約，目前包括以上四種變化因子的大型、長(zhǎng)期野外控制實(shí)驗(yàn)平臺(tái)還十分有限。

(5)目前關(guān)于全球變化對(duì)土壤線蟲群落影響的研究多限于2—3a的短期實(shí)驗(yàn)，而氣候變化所導(dǎo)致的如植被構(gòu)成、土壤理化性質(zhì)的變化是一個(gè)長(zhǎng)期的漸變過程，因此迫切需要建立長(zhǎng)期野外控制實(shí)驗(yàn)以揭示土壤線蟲對(duì)氣候變化的真實(shí)響應(yīng)。

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Response of soil nematodes to climate change:a review

SONG Min*, LIU Yinzhan, JING Shuishui

CollegeofLifeSciences,HenanUniversity,Kaifeng475004,China

Global climate change constitutes an important influence on terrestrial ecosystem functions. Soil nematodes exist widely in diverse ecological systems. As an important component of the soil food web, the soil nematode is crucial in maintaining biodiversity and regulating soil nutrient cycling. The various factors driving global change affect the soil nematode community via diverse mechanisms. Changes in temperature and precipitation directly affect soil nematode populations by changing the habitat. In addition, the positive effects would increase with time. CO2enrichment and atmospheric nitrogen deposition can indirectly affect soil nematode communities via changes in vegetation, litter quality, and soil physiochemical properties (e.g. soil moisture, pH, ammonium concentration). At the same time, a better understanding of the complex interactions among the drivers of global change, and of how these interactions influence soil nematode communities, is required to predict how terrestrial ecosystems will respond to future climate changes.

warming; changing precipitation regime; elevated CO2concentration; nitrogen deposition; soil nematode community

博士后基金資助項(xiàng)目(2012M520066, 2013T 60699)

2014-04-22; < class="emphasis_bold">網(wǎng)絡(luò)出版日期:

日期:2014-12-18

10.5846/stxb201404220796

*通訊作者Corresponding author.E-mail: smsm2000@163.com

宋敏, 劉銀占, 井水水.土壤線蟲對(duì)氣候變化的響應(yīng)研究進(jìn)展.生態(tài)學(xué)報(bào),2015,35(20):6857-6867.

Song M, Liu Y Z, Jing S S.Response of soil nematodes to climate change:a review.Acta Ecologica Sinica,2015,35(20):6857-6867.