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        植物根系分解及其對(duì)生物和非生物因素的響應(yīng)機(jī)理研究進(jìn)展

        2017-03-26 01:36:55羅永清趙學(xué)勇王濤李玉強(qiáng)左小安丁杰萍
        草業(yè)學(xué)報(bào) 2017年2期
        關(guān)鍵詞:細(xì)根菌根氮素

        羅永清, 趙學(xué)勇, 王濤, 李玉強(qiáng), 左小安, 丁杰萍

        (1.中國(guó)科學(xué)院西北生態(tài)環(huán)境資源研究院,甘肅 蘭州 730000;2.甘肅省環(huán)境科學(xué)設(shè)計(jì)研究院,甘肅 蘭州 730020;3.蘭州大學(xué)資源環(huán)境學(xué)院,甘肅 蘭州 730000)

        植物根系分解及其對(duì)生物和非生物因素的響應(yīng)機(jī)理研究進(jìn)展

        羅永清1, 趙學(xué)勇1, 王濤1, 李玉強(qiáng)1, 左小安1, 丁杰萍2,3

        (1.中國(guó)科學(xué)院西北生態(tài)環(huán)境資源研究院,甘肅 蘭州 730000;2.甘肅省環(huán)境科學(xué)設(shè)計(jì)研究院,甘肅 蘭州 730020;3.蘭州大學(xué)資源環(huán)境學(xué)院,甘肅 蘭州 730000)

        植物根系在生態(tài)系統(tǒng)物質(zhì)循環(huán)和能量流動(dòng)過(guò)程中具有重要意義,其分解是固定于植物體內(nèi)的物質(zhì)返回土壤和大氣環(huán)境的重要過(guò)程,受生物和非生物等多重因子影響。其中:生物因素方面,根系化學(xué)特征是影響根分解的主要因素,根系壽命、直徑、菌根及物種差異等生物因素主要是通過(guò)改變根系化學(xué)性質(zhì)產(chǎn)生作用,根系分泌物、土壤微生物等主要通過(guò)改變分解者的數(shù)量和活性影響根系分解。非生物因素方面,水熱因子是影響根分解的關(guān)鍵。另外,在全球變化背景下,大氣氮沉降、CO2濃度增加等因素也對(duì)植物根系分解產(chǎn)生影響。本文從植物根系分解過(guò)程和影響根分解的因素兩方面,綜述了植物根系分解過(guò)程中物質(zhì)釋放規(guī)律,總結(jié)和歸納了主要生物和非生物因素對(duì)植物根系分解的影響機(jī)理,并對(duì)根系分解的研究進(jìn)行展望。

        根系;分解過(guò)程;生物因素;環(huán)境因素;響應(yīng)機(jī)理

        凋落物分解是陸地生態(tài)系統(tǒng)植物體向土壤和大氣環(huán)境中返還養(yǎng)分和碳元素的主要途徑。凋落物包括地上凋落物和地下凋落物兩部分。受水熱資源限制,一些生態(tài)系統(tǒng)地上凋落物的產(chǎn)生存在明顯的季節(jié)差異性[1-4]。同時(shí),由于風(fēng)力活動(dòng)等因素,地上凋落物存在再分配的現(xiàn)象[5]。另外,放牧、刈割等人類活動(dòng)在不同程度上減少了地上生物量向土壤中的轉(zhuǎn)移,地上凋落物向土壤中的物質(zhì)返還存在不確定性。而對(duì)于地下凋落物,植物根系分解存在時(shí)間和空間上的連續(xù)性,其物質(zhì)循環(huán)與能量流動(dòng)是一個(gè)持續(xù)的過(guò)程;同時(shí),植物根系具有快速周轉(zhuǎn)的特性[6-7],尤其是細(xì)根,其周轉(zhuǎn)速率較快。研究表明,一些森林樹種細(xì)根年死亡量在40%~90%[8],通過(guò)細(xì)根更新可產(chǎn)生5倍于地上凋落物的量[9]。此外,植物根系產(chǎn)生并輸入到土壤中的穩(wěn)定性有機(jī)碳高于地上部分等量凋落物的輸入,植物地下部分腐殖化系數(shù)為地上部分的2.3倍[10],通過(guò)根系分解實(shí)現(xiàn)的物質(zhì)返還高于地上部分[11-12]。因此,植物根系分解是固定于植物體的物質(zhì)返回土壤和大氣環(huán)境的重要過(guò)程,在陸地生態(tài)系統(tǒng)物質(zhì)循環(huán)和能量流動(dòng)中具有重要意義。

        1 根分解的過(guò)程

        根系分解是指在物理、化學(xué)和生物學(xué)等一系列復(fù)雜過(guò)程的作用下,植物死亡根系與土壤環(huán)境進(jìn)行物質(zhì)交換的現(xiàn)象[9],主要包括物理淋溶、破碎與生化分解等[13]。其中破碎是指在土壤動(dòng)態(tài)過(guò)程(凍融交替、干濕循環(huán)等)或動(dòng)物行為的作用下,植物死亡根系分裂和破碎成小段(小塊)的現(xiàn)象。破碎擴(kuò)大了植物根系的比表面積,有利于微生物附著和一系列生化反應(yīng)的發(fā)生。淋溶是指根系中的物質(zhì)及元素在水溶及重力作用下,由根系轉(zhuǎn)移到土壤中的現(xiàn)象。淋溶作用是根分解過(guò)程中的一個(gè)重要環(huán)節(jié)。研究發(fā)現(xiàn),仙女木(Dryasoctopetala)、挪威虎耳草(Saxifragaoppositifolia)的細(xì)根在分解過(guò)程中,干物質(zhì)損失量與呼吸不存在顯著相關(guān)性,淋溶作用在細(xì)根分解中占有很大比重[14]。淋溶的主要對(duì)象為可溶性有機(jī)物質(zhì)和無(wú)機(jī)物質(zhì),該過(guò)程主要發(fā)生在植物根系分解的初期。研究表明,亞熱帶森林細(xì)根分解過(guò)程中,分解的前170 d內(nèi)95%的可溶性糖通過(guò)淋溶作用由根系轉(zhuǎn)移到土壤中[9];經(jīng)150 d分解,水曲柳(Fraxinusmandshurica)和落葉松(Larixgmelinii)根系可溶性糖的釋放超過(guò)90%,K的釋放超過(guò)95%[15]。采伐后的森林細(xì)根分解結(jié)果顯示,K、Mg、P、N等可溶性無(wú)機(jī)離子在根系分解的3個(gè)月內(nèi)發(fā)生了不同程度的淋溶,其中以K離子的淋溶速率最大,3個(gè)月內(nèi)淋溶量高達(dá)80%[16]。生化分解是指在由植物根系或根際微生物產(chǎn)生的酶的作用下,土壤微生物通過(guò)新陳代謝將儲(chǔ)存在植物體死根中的物質(zhì)緩慢分解的過(guò)程,生化分解的底物多為難溶、難分解型物質(zhì),主要包括木質(zhì)素、纖維素及單寧等。

        受環(huán)境、物種類型等諸多因素影響,根系分解的一些研究至今尚沒有統(tǒng)一的結(jié)論。如干物質(zhì)變化方面,有的研究發(fā)現(xiàn),根系分解過(guò)程中的干物質(zhì)變化呈3個(gè)階段,Arunachalam等[17]在濕潤(rùn)亞熱帶砍伐后再生林細(xì)根分解的研究結(jié)果表明,根系的分解表現(xiàn)出“慢-快-慢”的特征,指出初期(0~60 d)的緩慢分解可能是由于微生物種群建立與擴(kuò)散導(dǎo)致的滯后性所致;中期的高速率分解可能是由于土壤中的能量資源或水溶性物質(zhì)比例較高,微生物活性較強(qiáng)所致;而后期的緩慢分解可能是由于難溶性物質(zhì)(木質(zhì)素、纖維素等)積累造成。而更多的研究結(jié)論顯示,根分解過(guò)程中干物質(zhì)變化包含快速和緩慢分解兩個(gè)階段[18-22],其原因是由于分解開始后水溶性物質(zhì)及易分解物質(zhì)的比例較高,形成快速分解與釋放。而在分解后期,木質(zhì)素等難分解性物質(zhì)的相對(duì)含量升高,分解速率降低。導(dǎo)致這種差異的原因可能與分解起始時(shí)間有關(guān)。許多研究表明,凋落物分解初期主要受環(huán)境因子控制[23-26],分解后期主要受凋落物質(zhì)量和分解者狀況影響[27,28]。李榮華等[20]通過(guò)不同起始時(shí)間對(duì)亞熱帶森林凋落物分解的研究發(fā)現(xiàn),起始時(shí)間對(duì)凋落物的分解速率具有顯著影響,起始于雨季的分解速率高于起始于旱季,其大小與分解前期的環(huán)境因子相關(guān)性較高,與整個(gè)分解過(guò)程中的環(huán)境因子相關(guān)性較低。據(jù)此推斷,分解初期相對(duì)不適宜的環(huán)境可能是導(dǎo)致前期分解速率偏低、分解率峰值滯后的原因。

        根分解過(guò)程中的氮素變化也存在一定的爭(zhēng)議性。研究發(fā)現(xiàn),根系分解過(guò)程中氮濃度降低[18,29],而另一些研究則表明,根系分解過(guò)程中存在氮富集現(xiàn)象,如林成芳等[19]在杉木(Cunninghamialanceolata)、木荷(Schimasuperba)[30]等根分解的研究中發(fā)現(xiàn),各個(gè)莖級(jí)(0~1 mm、1~2 mm、2~4 mm)根系分解中氮素均表現(xiàn)出富集的特征。氮素的釋放與富集,可能與環(huán)境有關(guān),楊麗韞等[18]在長(zhǎng)白山原始林和次生林根分解的研究中發(fā)現(xiàn),原始紅松闊葉林在演替過(guò)程中,根系分解時(shí)會(huì)出現(xiàn)氮素的富集或釋放,且氮素的釋放或富集與環(huán)境溫度和水分有關(guān):降雨較少、溫度較低的春季,根分解過(guò)程中氮素表現(xiàn)出釋放現(xiàn)象,而在降雨較多、溫度較高的夏季,氮素表現(xiàn)出富集現(xiàn)象[18]。另外,有研究表明,根分解中氮素的釋放與富集與底物化學(xué)特征有關(guān):Chen等[31-32]通過(guò)分析底物N濃度與根分解過(guò)程中氮素動(dòng)態(tài)的關(guān)系發(fā)現(xiàn),氮素的變化有固定的閾值,當(dāng)初始N濃度高于0.4%時(shí),根分解過(guò)程中N素表現(xiàn)出釋放的特征,初始N濃度低于該值時(shí),N素表現(xiàn)為富集現(xiàn)象。

        2 影響根分解的因素

        2.1 生物因素

        2.1.1 根系化學(xué)特征 與地上部分凋落物不同,植物根系質(zhì)量被認(rèn)為是影響其分解速率的第一因素[7,33]。早期的研究發(fā)現(xiàn)[31],碳氮比對(duì)分解速率具有決定作用,最適于微生物分解的底物碳氮比為25,一部分真菌和細(xì)菌也能分解碳氮比較高的底物。碳素的存在形式也可對(duì)植物根系分解產(chǎn)生影響,如木質(zhì)素、纖維素、單寧等難分解型物質(zhì)含量的高低對(duì)根系分解具有顯著意義[23,32,34-35]。此外,Ca2+與P含量對(duì)根系的分解也有一定程度的影響。研究表明,P濃度與根分解速率呈正相關(guān)[31]。也有研究發(fā)現(xiàn),Ca2+濃度與分解速率呈正相關(guān)性[7,36],其原因?yàn)镃a2+不但是微生物生活的必需元素,而且真菌和部分異養(yǎng)菌能將儲(chǔ)存于根系中的Ca2+轉(zhuǎn)化為草酸鹽,在脅迫條件下可為微生物代謝提供養(yǎng)分[8]。

        2.1.2 根壽命與直徑 根系壽命與根直徑的大小對(duì)根系分解速率具有一定的影響[35,37]。Ruess等[34]以黑云杉(Piceamariana)為材料的研究發(fā)現(xiàn),較長(zhǎng)壽命根系的分解速率高于較短壽命的根系,壽命為2、8和18個(gè)月的細(xì)根對(duì)應(yīng)的分解消失所需時(shí)間分別為41、50和156 d,其原因是長(zhǎng)壽命根系中的木質(zhì)素和單寧等難分解物質(zhì)含量較高,這與宋森等[37]的研究結(jié)果一致。林成芳等[19]以杉木為材料的研究中發(fā)現(xiàn),隨根直徑增加,其分解速率有降低的趨勢(shì),0~1 mm、1~2 mm及2~4 mm根系的分解速率分別為54.8%、41.2%和38.2%。但也有研究發(fā)現(xiàn),在細(xì)根的分解過(guò)程中,根直徑越小,分解速率反而越慢,如Fahey等[16]對(duì)不同直徑細(xì)根研究結(jié)果表明,經(jīng)過(guò)兩年分解后直徑<0.6 mm的細(xì)根分解速率低于0.6~1.0 mm的木質(zhì)根;林成芳等[30]以木荷為材料的研究中也發(fā)現(xiàn),0~1 mm根系的分解速率(47.8%)低于1~2 mm根系(57.2%)。導(dǎo)致這種現(xiàn)象的原因可能是細(xì)根中表皮組織的分解速率慢于木質(zhì)部[31],較細(xì)根系中的表皮組織比例相對(duì)較高,導(dǎo)致其分解速率低于直徑較大的細(xì)根。宋森等[37]對(duì)興安落葉松(Larixgmelinii)根系分解的研究表明,根表皮比例的提高可能是導(dǎo)致小直徑細(xì)根分解慢的主要原因。另一方面,較細(xì)根系中N含量較高,容易形成N-木質(zhì)素絡(luò)合物,從而抑制根系的分解[38]。也有研究發(fā)現(xiàn),細(xì)根中較多形成外生菌根,大量外生菌根的存在改變了根系的物理結(jié)構(gòu)和化學(xué)性質(zhì),形成抗分解機(jī)制,從而降低根系的分解速率[39]。

        但也有研究認(rèn)為根分解與直徑無(wú)關(guān)。在以多年生草本植物柳枝稷(Panicumvirgatum)為材料的研究發(fā)現(xiàn),不同直徑根系的分解速率不存在顯著差異性[40]。也有研究表明,在直徑較大根系中,根直徑與分解速率無(wú)關(guān),如以新西蘭輻射松(Pinusradiata)為材料的研究[41]發(fā)現(xiàn),10~15 mm粗根分解中不存在直徑效應(yīng)(diameter effect)。2.1.3 物種 物種被認(rèn)為是影響凋落物分解的重要因素之一[14,18,42-44]。在不同物種根分解速率的研究中發(fā)現(xiàn),草本植物根系的分解速率顯著高于木本植物,其原因可能是草本植物根系中的木質(zhì)素含量較低而Ca2+含量較高所致[7,36]。榮麗等[45]通過(guò)微生物群落特征變化研究了光皮樺(Betulaluminifera)和扁穗牛鞭草(Hemarthriacompressa)根系的分解特征,結(jié)果發(fā)現(xiàn),分解120 d以后,扁穗牛鞭草處理的微生物總數(shù)及優(yōu)勢(shì)放線菌的數(shù)量均顯著高于光皮樺處理。在木本植物的對(duì)比研究中發(fā)現(xiàn),闊葉樹種根系分解速率高于針葉樹種,如林成芳等[19,30]的研究結(jié)果顯示,常綠闊葉樹種木荷1~2 mm和2~4 mm根分解速率分別為57.2%和39.5%,而常綠針葉樹種杉木根系相應(yīng)莖級(jí)的分解速率分別為41.2%和38.2%,均低于木荷。也有研究[46]發(fā)現(xiàn),墨西哥柏(Cupressuslusitanica)細(xì)根分解速率高于展業(yè)松(Pinuspatula)和巨桉(Eucalyptusgrandis)。Robinson 等[14]的研究表明,不同物種的質(zhì)量差異是造成其細(xì)根分解差異的重要原因。

        此外,混合物種根系分解的研究表明,柔毛橡(Quercuspubescens)、蒙彼利埃楓(Acermonspessulanum)、黃櫨(Cotinuscoggygria)和地中海白松(Pinushalepensis)4種植物根系組合后,在干旱脅迫和正常狀況下,分別有24.2%和63.6%表現(xiàn)出分解速率的協(xié)同交互作用(synergistic interactions),并且這種協(xié)同交互作用隨混合物種數(shù)量的增加而加劇,2種、3種和4種混合后的這種協(xié)同交互作用的出現(xiàn)率分別為53%、71%和100%[44]。這種混合協(xié)同交互作用的機(jī)理性研究目前多集中于葉凋落物,根系方面探索相對(duì)較少。凋落物分解的混合協(xié)同交互作用的機(jī)理可能為:1)不同分解底物化學(xué)性質(zhì)的差異導(dǎo)致混合樣品性質(zhì)的多樣性[47-48],2)分解底物豐富度的提高可能增加了土壤微生物的生態(tài)位,提高了分解者的豐富度和活性[49],3)混合分解過(guò)程中,一些物種可改變土壤水分等微環(huán)境[50-51],從而提高分解速率。

        2.1.4 根系分泌物 根系分泌物對(duì)植物殘?bào)w的分解有重要意義。Phillips等[52]的研究發(fā)現(xiàn),在CO2倍增條件下,森林植物主要通過(guò)根系分泌物增加植物殘?bào)w的分解速率來(lái)為植物提供更多的氮素,從而保證了生物量的增加,并指出,根系分泌是促進(jìn)分解、增加氮源的關(guān)鍵環(huán)節(jié),是生物量增加的保證。此外,根系分泌物對(duì)土壤氮素的賦存形態(tài)具有重要影響,研究表明,根系分泌物可促進(jìn)土壤氮素由有機(jī)態(tài)向無(wú)機(jī)態(tài)的轉(zhuǎn)化[53-54],而這種轉(zhuǎn)化可能是促進(jìn)根系分解的關(guān)鍵因素。另外,植物根系分泌物可促進(jìn)土壤中大分子有機(jī)物的降解[55-57],這可能對(duì)植物殘?bào)w分解過(guò)程產(chǎn)生影響。根系分泌物對(duì)凋落物分解的影響主要是通過(guò)土壤微生物實(shí)現(xiàn)。一方面,根系分泌物可直接對(duì)土壤微生物產(chǎn)生影響。研究發(fā)現(xiàn),根系分泌物可促進(jìn)土壤細(xì)菌數(shù)量和活性的增加[54,58-59]。根系分泌物的這種直接作用在微生物群落組成[60]和群落發(fā)育[61-64]兩個(gè)層面均可產(chǎn)生作用。另一方面,通過(guò)根系分泌物的形成,植物在根際形成并保持了一個(gè)特殊的群落結(jié)構(gòu),該群落可產(chǎn)生大量的次級(jí)代謝物[65]。已有的研究發(fā)現(xiàn),這類次級(jí)代謝物可顯著提高土壤氮素固定和氮素的可利用性[66]。根系分泌物通過(guò)對(duì)土壤酶活性的影響產(chǎn)生作用可能是根系分泌物影響植物殘?bào)w分解的另一重要途徑。趙小亮等[67]的研究發(fā)現(xiàn),棉花(Gossypiumspp.)根系分泌物對(duì)土壤轉(zhuǎn)化酶活性增強(qiáng)具有顯著作用,且這種促進(jìn)作用與根系分泌物濃度存在顯著的正相關(guān)性。根系分泌物對(duì)植物殘?bào)w分解的影響可能與其組成有關(guān),已有的研究表明[68],根系分泌物除直接產(chǎn)生多種酶類物質(zhì)(淀粉酶、轉(zhuǎn)化酶、磷酸酯酶、多聚半乳糖醛酸酶、硝酸還原酶、木聚糖酶、蛋白酶、尿酶、接觸酶等)以外,根系分泌物中的糖類(葡萄糖、果糖、半乳糖、核糖、蔗糖、木糖等)物質(zhì)和酸類(脂肪酸、有機(jī)酸、酚酸及多種氨基酸類)物質(zhì)可一方面向土壤微生物提供碳源,另一方面改變土壤酸性來(lái)影響土壤微生物的數(shù)量與活性[69]。

        2.1.5 菌根 菌根在植物根系生長(zhǎng)發(fā)育和“植物-土壤”生態(tài)系統(tǒng)碳氮循環(huán)方面具有重要作用[70]。90%以上的陸地植物可與菌根真菌形成共生關(guān)系,菌根真菌包括叢枝菌根(AM)、外生菌根(ECM)、內(nèi)外菌根(EEM)、歐石南菌根(ERM)和蘭科菌根(OM)等形式,將植物與土壤環(huán)境進(jìn)行聯(lián)結(jié),從而提高植物的養(yǎng)分吸收效率[71]。研究表明,菌根真菌對(duì)于植物養(yǎng)分獲取具有重要的作用,然而,其對(duì)凋落物分解的影響卻經(jīng)常被忽視[72]。目前,菌根對(duì)凋落物分解影響方面的研究尚存在較大的爭(zhēng)議。早期的研究表明[73],凋落物分解過(guò)程中,菌根通過(guò)抑制土壤微生物活性,減緩了其對(duì)凋落物的分解,這種抑制效應(yīng)也被稱之為“Gadgil effect”,該結(jié)論得到了許多研究的支持[39,74-75]。但也有許多研究[76-81]表明,菌根對(duì)凋落物的分解具有顯著的促進(jìn)作用。菌根對(duì)凋落物分解的影響過(guò)程中,氮素是一個(gè)關(guān)鍵因素,Colpaert等[82]研究認(rèn)為,外生菌根真菌并不能有效利用凋落物中的有機(jī)氮,而是通過(guò)腐生微生物從難分解的有機(jī)物中獲取氮。然而,Hodge 等[83]的研究表明,一些叢枝菌根真菌可提高凋落物的分解速率,同時(shí)獲取分解釋放的無(wú)機(jī)氮,通過(guò)對(duì)分解產(chǎn)物的有效利用促進(jìn)菌絲生長(zhǎng)。Cheng 等[84]研究表明,AM真菌通過(guò)對(duì)土壤中銨態(tài)氮和硝態(tài)氮含量的改變加速了凋落物的降解速率。

        2.1.6 土壤微生物 真菌和細(xì)菌被認(rèn)為是陸地生態(tài)系統(tǒng)中植物組織死亡后的初始分解者,占總分解者生物量和呼吸量的80%以上[13]。真菌和細(xì)菌分泌產(chǎn)生的酶,在根系大分子物質(zhì)的降解中起重要作用。不同種類真菌對(duì)植物組織殘?bào)w的分解具有各自的特點(diǎn):白腐菌專門降解木質(zhì)素且分解速率快;褐腐菌只能分解木質(zhì)素中苯酚的側(cè)鏈部分,余下含有酚單元的部分使根系變成褐色,分解速率慢[13]。也有研究表明,西黃松和黑松的根系由于大量白腐菌的寄生而使分解加快,而褐腐菌寄生的花旗松和西加云杉根系分解慢[32]。

        2.2 環(huán)境因素

        2.2.1 土壤水分 土壤水分是影響植物有機(jī)殘?bào)w分解的重要因素[85]。雖然在雨量充沛的熱帶亞熱帶等地區(qū),水分不是有機(jī)殘?bào)w分解的限制因子[20],甚至由于土壤水分過(guò)高,土壤通透性下降,氧氣供應(yīng)量降低,需氧型微生物受到抑制,導(dǎo)致根系分解速率與土壤濕度呈顯著負(fù)相關(guān)[86],但在干旱和半干旱地區(qū),土壤水分是影響有機(jī)殘?bào)w分解最重要的環(huán)境因子[87-89]。一方面,干旱可直接抑制植物根系的分解,另一方面,土壤水分可通過(guò)與溫度的耦合,共同作用于土壤微環(huán)境,從而對(duì)有機(jī)殘?bào)w的分解產(chǎn)生影響[90]。另外,干旱可通過(guò)顯著降低根系分解的協(xié)同作用從而導(dǎo)致混合根樣分解速率的降低[44]。

        2.2.2 土壤溫度 溫度對(duì)植物根分解的影響與水分類似,即在溫度梯度上,植物根系分解速率表現(xiàn)為單峰曲線的特征。Chen 等[31]通過(guò)測(cè)定根系分解過(guò)程中的呼吸特征發(fā)現(xiàn),在一定范圍內(nèi)(0~30 ℃或0~40 ℃),根分解過(guò)程中的呼吸強(qiáng)度隨溫度增加而上升,但當(dāng)溫度增加到一定程度時(shí)則表現(xiàn)為抑制效應(yīng)。

        通常情況下,土壤溫度對(duì)植物根系的分解具有促進(jìn)作用。秦艷等[91]以毛烏素沙地臭柏(Sabinavulgari)和油蒿(Artemisiaordosic)為材料的研究發(fā)現(xiàn),二者細(xì)根分解量與低溫表現(xiàn)出顯著的正相關(guān)性,表明低溫的升高促進(jìn)了臭柏和油蒿的細(xì)根分解。劉艷等[92]通過(guò)室內(nèi)培養(yǎng)的方式研究了不同溫度條件下杉木、榿木(Alnuscremastogyne)和火力楠(Micheliamacclurei)細(xì)根的分解動(dòng)態(tài),結(jié)果表明,隨溫度的增加,各物種細(xì)根的分解均表現(xiàn)為增加的特征。此外,土壤溫度也可通過(guò)凍融循環(huán)等間接作用于植物根系的分解。魏圓云等[93]通過(guò)網(wǎng)袋法研究了不同凍融時(shí)期高山/亞高山森林物種粗枝云杉(Piceaasperata)、紅樺(Betulaalbosinensis)和岷江冷杉(Abiesfaxoniana)等細(xì)根分解動(dòng)態(tài),結(jié)果表明,由溫度引起的凍融循環(huán)是影響細(xì)根分解變化的重要原因。

        2.2.3 土壤養(yǎng)分 一般認(rèn)為,土壤養(yǎng)分對(duì)微生物的數(shù)量和活性具有直接的影響,因此土壤養(yǎng)分被認(rèn)為是影響根系分解的重要因素,許多研究對(duì)此結(jié)論有所驗(yàn)證[13]。但也有研究表明,不同土壤養(yǎng)分對(duì)根分解的影響不存在差異性[94]。因此,土壤養(yǎng)分對(duì)根分解的影響可能受土壤碳元素影響[95],當(dāng)碳元素含量較低,對(duì)微生物碳供應(yīng)不足時(shí),碳元素可能是微生物活動(dòng)及根分解的限制因子,而當(dāng)碳供應(yīng)充足時(shí),土壤養(yǎng)分供應(yīng)對(duì)根分解產(chǎn)生促進(jìn)作用。

        2.2.4 CO2濃度 全球氣候變化背景下,CO2濃度增加及其效應(yīng)引起了廣泛的關(guān)注。研究發(fā)現(xiàn),CO2濃度增加導(dǎo)致了根系分解的減緩[40,96-97]。但也有一些研究表明,CO2濃度增加對(duì)根系分解不存在顯著作用,如以橡樹-棕櫚灌叢生態(tài)系統(tǒng)(oak-palmetto scrub ecosystem)的研究結(jié)果表明,培養(yǎng)1年后,CO2濃度增加(700 μL/L)與普通濃度處理下,根系的干物質(zhì)損失不存在顯著差異[98];以草本植物多年生黑麥草(Loliumperenne)、毛狀剪股穎(Agrostiscapillaris)和羊茅(Festucaovina)為材料的研究表明,與常規(guī)濃度(350 μL/L)相比較,CO2濃度增加(700 μL/L)對(duì)根和葉分解速率不存在顯著影響[99];以小麥(Triticumaestivum)為材料的研究同樣發(fā)現(xiàn),CO2濃度增加320 μL/L處理對(duì)根系中易分解成分的分解無(wú)顯著影響[100]。此外,也有研究發(fā)現(xiàn),在森林生態(tài)系統(tǒng)CO2濃度增加后凋落物分解速率的增加及其養(yǎng)分釋放的加快是植物初級(jí)生產(chǎn)增加的保證[52,70]。

        2.2.5 N沉降/添加 研究表明,氮添加/氮沉降對(duì)植物根系分解存在促進(jìn)和抑制兩方面的影響。以濕地松(Pinuselliottii)為材料的研究表明,高氮(12 g N/m2·yrs)處理顯著抑制了細(xì)根和針葉的分解,與針葉胞外酶活性受抑制導(dǎo)致分解減緩不同,酸不溶解性殘余物與無(wú)機(jī)氮的結(jié)合導(dǎo)致細(xì)根分解速率降低[22]。也有研究表明,長(zhǎng)期氮沉降顯著降低了華西雨屏區(qū)苦竹(Pleioblastusamarus)細(xì)根的分解速率,增加了根系氮素的殘留率,其主要機(jī)理為:1)分解后期外源無(wú)機(jī)氮與細(xì)根中木質(zhì)素分解的中間產(chǎn)物發(fā)生了一系列的非生物反應(yīng),并形成抗分解物質(zhì),從而導(dǎo)致分解減緩[22,101];2)長(zhǎng)期氮沉降可能降低土壤pH值,改變生產(chǎn)木質(zhì)素降解酶的真菌環(huán)境,導(dǎo)致相關(guān)酶活性降低,從而抑制后期根系分解[101];3)氮沉降可改變根系的化學(xué)特征,長(zhǎng)期或高濃度氮沉降可導(dǎo)致氮素在根系中的積累,這一類氮素在根系中一方面形成有機(jī)化合物,對(duì)細(xì)根分解產(chǎn)生影響[101],另一方面,當(dāng)根系中的氮素累積到一定程度,高出根分解所需要的水平,達(dá)到了飽和后氮素會(huì)減少微生物多樣性而改變微生物群落的結(jié)構(gòu),從真菌轉(zhuǎn)向以細(xì)菌為主,降低了凋落物的分解速率[102]。

        也有研究表明,氮添加可促進(jìn)植物根系的分解。Huxham 等[43]研究發(fā)現(xiàn),氮添加促進(jìn)了木欖(Bruguieagymnorrhiza)根系分解速率。在紅三葉草(Trifoliumincarnatum)和毛野豌豆(Viciavillosa)的研究發(fā)現(xiàn),由于氮添加導(dǎo)致土壤碳氮比降低,以及由氮素引起的激發(fā)效應(yīng),促進(jìn)了土壤微生物的生長(zhǎng),從而促進(jìn)了細(xì)根分解的加速[103]。

        氮添加與根分解的關(guān)系可能與氮素濃度有關(guān),如以天山云杉的研究表明,低濃度氮素促進(jìn)分解,高濃度氮素抑制分解。

        2.2.6 土層深度 研究表明[37,104-108],根分解速率隨土層深度增加而顯著下降,其原因由不同土層的水熱因子及土壤生物活性差異所導(dǎo)致。但也有研究發(fā)現(xiàn)[109-112],不同深度根樣的分解速率不存在顯著差異性,如Cai等[110]以玉米(Zeamays)根系為材料的研究發(fā)現(xiàn),15 cm深度和45 cm深度的根樣分解速率差異性不顯著;Solley等[112]以歐洲山毛櫸(Fagussylvatica)為材料的研究也表明,5,20和35 cm深度的根分解速率不存在顯著差異性。不同土層根分解速率研究結(jié)論的爭(zhēng)議性可能是由于不同深度土層的水熱差異所致。Cai等[110]通過(guò)實(shí)驗(yàn)觀測(cè)表明,45 cm深度土壤較高的含水率和土壤溫度可能會(huì)產(chǎn)生深層土壤生物活性的補(bǔ)償,從而減弱甚至抵消根分解的差異性;Solley等[112]認(rèn)為,冬季淺層土壤的低溫對(duì)夏季深層土壤溫度的平衡作用可能導(dǎo)致了不同土層根分解差異不顯著。綜上所述,土層深度對(duì)根分解的影響主要是通過(guò)水熱因子產(chǎn)生作用。

        3 展望

        植物根分解是陸地生態(tài)系統(tǒng)物質(zhì)循環(huán)和能量流動(dòng)的關(guān)鍵環(huán)節(jié),根系分解過(guò)程中的氮素釋放是植被初級(jí)生產(chǎn)的重要保證。根系在植物生長(zhǎng)和發(fā)育過(guò)程中通過(guò)多種生物和非生物因素影響著植物死根的分解。這些過(guò)程通過(guò)單一或耦合的形式影響著植被-土壤系統(tǒng)的物質(zhì)周轉(zhuǎn),該方面研究對(duì)準(zhǔn)確評(píng)價(jià)植物根系生態(tài)學(xué)作用、進(jìn)一步探討地下凋落物分解規(guī)律及植物-土壤系統(tǒng)氮素轉(zhuǎn)化機(jī)制具有重要意義。根分解的研究已有較長(zhǎng)的歷史和大量的研究結(jié)論,但針對(duì)其分解機(jī)理,目前仍存在以下方面的問(wèn)題。

        已有的研究多以單一物種為研究對(duì)象,而在自然環(huán)境中,植被多以群落形式存在,多種根系混合后,根系的分解特征、根分解是否存在激發(fā)效應(yīng)?混合分解對(duì)生態(tài)系統(tǒng)碳排放的估算有多大影響?自然環(huán)境中,植物根系是一個(gè)連續(xù)周轉(zhuǎn)的過(guò)程,在該過(guò)程中,活根的活動(dòng)和根系的死亡分解并存,而活根生長(zhǎng)和發(fā)育過(guò)程中所形成的水分差異(水分再分配、水力提升等)、根系分泌物、菌根及根際微生物等對(duì)根分解的影響機(jī)理尚不清楚。近期的研究表明,根系分泌物和菌根可在一定程度上刺激根系分解,如在CO2濃度增加的狀況下,植物通過(guò)根系分泌物和菌根對(duì)根分解的刺激加速N循環(huán)速率,從而維持較高的初級(jí)生產(chǎn)。因此,根系活動(dòng)對(duì)根分解的影響可能對(duì)生態(tài)系統(tǒng)碳平衡和植被-土壤系統(tǒng)的物質(zhì)循環(huán)產(chǎn)生重要影響,目前該方面的研究較少。

        根分解過(guò)程中的物質(zhì)分配,尤其是碳分配方面,目前的研究主要集中在根系碳氮濃度變化,而分解過(guò)程中的物質(zhì)分配比例,即由根系分解釋放的物質(zhì)在土壤、大氣和淋溶進(jìn)入水體中的量或比例,直接關(guān)系到大氣組成、土壤肥力、水質(zhì)等生態(tài)和環(huán)境問(wèn)題。受研究方法的限制,該方面的研究較少,更準(zhǔn)確的研究方法(如同位素示蹤技術(shù)等)亟需進(jìn)一步探索。

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        Plant root decomposition and its responses to biotic and abiotic factors

        LUO Yong-Qing1, ZHAO Xue-Yong1, WANG Tao1, LI Yu-Qiang1, ZUO Xiao-An1, DING Jie-Ping2,3

        1.NorthwestInstituteofEco-EnvironmentandResources,ChineseAcademyofSciences,Lanzhou730000,China; 2.GansuAcademyofEnvironmentalSciences,Lanzhou730020,China; 3.CollegeofEarthandEnvironmentalSciences,LanzhouUniversity,Lanzhou730000,China

        The plant root system plays an important role in ecosystem matter exchange and energy flux, and root decomposition is one of the main contributors to these processes. Matter accumulates in roots and is then released back into the roots’ surroundings, including the atmosphere and soil, via the decomposition process. Root decomposition is affected by multiple biotic and abiotic factors. In terms of biotic factors, the chemical characteristics of roots are among the main factors affecting their decomposition. Other factors such as root lifespan, size/diameter, and the abundance and types of fungi and other microbes in soil also affect plant root decomposition, and their effects can be explained by their ability to alter the chemical characteristics of plant roots. Root exudates and changes in the abundance and activities of soil microorganisms that degrade plant roots affect the root decomposition rate. In terms of abiotic factors, water and temperature are the main factors affecting root decomposition. Other factors related to climate change, for example, nitrogen deposition and carbon dioxide concentrations, will affect the root decomposition rate to different degrees, the extent of which is currently unknown. In this study, we reviewed the process of matter release during plant root decomposition, and summarized the current knowledge on how different biotic and abiotic factors affect plant root decomposition. Finally, we proposed some related perspectives based on the current status of plant root decomposition research.

        plant root system; decomposition; biotic factors; abiotic factors; response mechanism

        10.11686/cyxb2016136

        http://cyxb.lzu.edu.cn

        2016-03-29;改回日期:2016-04-28

        國(guó)家自然基金項(xiàng)目(31500369)和中國(guó)科學(xué)院“百人計(jì)劃”項(xiàng)目(Y551821001, Y451H31001)資助。

        羅永清(1984-),男,陜西寶雞人,助理研究員,博士。E-mail: luoyongqing8401@sina.com

        羅永清, 趙學(xué)勇, 王濤, 李玉強(qiáng), 左小安, 丁杰萍. 植物根系分解及其對(duì)生物和非生物因素的響應(yīng)機(jī)理研究進(jìn)展. 草業(yè)學(xué)報(bào), 2017, 26(2): 197-207.

        LUO Yong-Qing, ZHAO Xue-Yong, WANG Tao, LI Yu-Qiang, ZUO Xiao-An, DING Jie-Ping. Plant root decomposition and its responses to biotic and abiotic factors. Acta Prataculturae Sinica, 2017, 26(2): 197-207.

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