劉捷豹,陳光水,2,郭劍芬,2,楊智杰,2,*,李一清,林成芳,2，楊玉盛,2

1 福建師范大學(xué)地理科學(xué)學(xué)院, 福州 350007 2 福建師范大學(xué)濕潤亞熱帶山地生態(tài)國家重點(diǎn)實(shí)驗(yàn)室培育基地, 福州 350007

森林土壤酶對(duì)環(huán)境變化的響應(yīng)研究進(jìn)展

劉捷豹1,陳光水1,2,郭劍芬1,2,楊智杰1,2,*,李一清1,林成芳1,2，楊玉盛1,2

1 福建師范大學(xué)地理科學(xué)學(xué)院, 福州 350007 2 福建師范大學(xué)濕潤亞熱帶山地生態(tài)國家重點(diǎn)實(shí)驗(yàn)室培育基地, 福州 350007

全球氣候變化已是不爭的事實(shí),對(duì)陸地生態(tài)系統(tǒng)特別是森林生態(tài)系統(tǒng)物質(zhì)循環(huán)將產(chǎn)生顯著的影響。土壤酶是森林土壤物質(zhì)循環(huán)的主要限制因素之一,對(duì)氣候變化的響應(yīng)近年來受到廣泛關(guān)注。由于森林土壤酶對(duì)全球氣候變化的響應(yīng)研究是預(yù)測未來環(huán)境變化對(duì)森林生態(tài)系統(tǒng)過程影響的關(guān)鍵，因此,著重綜述了森林土壤酶對(duì)環(huán)境變化尤其是全球變暖和氮沉降響應(yīng)方面的研究,并分析了未來研究的主要方向。環(huán)境變化會(huì)引起土壤pH、水分及其營養(yǎng)成分的變化,而這些變化會(huì)反作用于土壤酶的活性和穩(wěn)定性。森林土壤酶對(duì)增溫的響應(yīng),不僅與酶的種類以及增溫的溫度范圍和持續(xù)時(shí)間有關(guān),還與土壤類型有關(guān),是多種因子綜合作用的結(jié)果。森林土壤酶對(duì)氮添加的響應(yīng)與林分類型和土層類型有關(guān),受復(fù)合氮的影響更大。建議未來的研究應(yīng)加強(qiáng)酶的基本性質(zhì)對(duì)環(huán)境變化的響應(yīng)研究,注重林分類型、土層類型導(dǎo)致的差異,強(qiáng)化多因素的交互作用,并進(jìn)行長期、綜合的觀測。

氣候變化；增溫；氮沉降；土壤酶；活性

隨著人類社會(huì)的快速發(fā)展及隨之而來的大量施肥和化石燃料的燃燒,氮沉降已經(jīng)成普遍現(xiàn)象,影響到土壤養(yǎng)分循環(huán)、尤其是有機(jī)營養(yǎng)物的礦化[1-4]。由于溫室效應(yīng),21世紀(jì)末氣溫預(yù)計(jì)還將增加0.3—4.8 ℃(IPCC,2014)；氣候變暖和氮沉降已經(jīng)成為影響人類活動(dòng)的兩個(gè)最主要的全球環(huán)境變化因子。

環(huán)境改變會(huì)對(duì)微生物活性和群落組成產(chǎn)生直接的影響,也能通過影響植物和土壤動(dòng)物而對(duì)微生物群落產(chǎn)生間接的作用,最終影響土壤酶活性[6-7],從而影響到陸地土壤碳、氮、磷等的物質(zhì)循環(huán)。酶是有機(jī)質(zhì)分解、周轉(zhuǎn)和礦化的主要生物催化劑,也是土壤碳、氮、磷循環(huán)的主要限制因素之一[8-9],因此,土壤酶活性監(jiān)測已經(jīng)成為土壤學(xué)研究的重要內(nèi)容之一。

森林是陸地最主要的碳庫之一[10],在全球碳循環(huán)中扮演了重要角色[11-12]。因此,研究森林土壤酶對(duì)環(huán)境因子變化的響應(yīng),不僅有利于了解土壤碳、氮、磷等養(yǎng)分循環(huán)過程對(duì)環(huán)境變化的響應(yīng),也可為深入研究環(huán)境變化對(duì)森林土壤的作用機(jī)制以及未來氣候變化下土壤碳排放的預(yù)測提供依據(jù),有助于促進(jìn)生態(tài)系統(tǒng)與全球變化研究的理論拓展。1985年至2016年10月,科學(xué)引文索引數(shù)據(jù)庫(web of knowledge)核心合集的檢索結(jié)果表明,土壤酶方面研究論文已超1400篇,而森林土壤酶的研究論文僅有兩千多篇,這顯然與當(dāng)前森林碳匯研究熱點(diǎn)不成比例。

近幾年來,國內(nèi)外關(guān)于土壤酶的綜述文章主要以農(nóng)田生態(tài)系統(tǒng)為主[7,13-14],森林土壤酶的專題綜述文章很鮮見,這未能滿足當(dāng)前森林生態(tài)學(xué)和森林土壤酶學(xué)發(fā)展的要求。因此,本文綜述了森林土壤酶對(duì)環(huán)境變化響應(yīng)的研究動(dòng)態(tài),為森林土壤酶的進(jìn)一步研究提供有益的參考。

1 森林土壤酶及其主要影響因素

環(huán)境變化(如氣候變暖和氮沉降等)會(huì)引起土壤pH[15,62]、水分[17]及其營養(yǎng)成分的變化[2],而土壤水分、pH值、空氣、溫度、團(tuán)聚體、礦質(zhì)元素和有機(jī)質(zhì)均不同程度地影響著土壤酶的活性及穩(wěn)定性[18]。因此,本節(jié)主要簡述與環(huán)境變化有關(guān)的pH、溫度、水分以及營養(yǎng)成分同森林土壤酶的關(guān)系。

土壤水分狀態(tài)對(duì)酶活性有一定的影響[19],在某些情況下甚至是關(guān)鍵性的驅(qū)動(dòng)因子[20]。一般情況下,在土壤含水量較低時(shí),土壤酶活性較低,但當(dāng)土壤過濕時(shí),酶活性也會(huì)受到抑制[17-18,21,26]。A′Bear等[27]對(duì)腐生擔(dān)子菌介導(dǎo)的山毛櫸(Fagussylvatica)分解實(shí)驗(yàn)結(jié)果表明,提高土壤濕度后,除亮氨酸氨肽酶外,β-葡萄糖苷酶、纖維二糖水解酶、β-木糖苷酶、N-乙酰-氨基葡糖苷酶、酸性磷酸酶、過氧化物酶以及酚氧化酶活性顯著提高。

土壤pH不僅決定有機(jī)分子功能基團(tuán)的離子化、影響酶和底物的構(gòu)象、酶對(duì)土壤顆粒的吸附性以及酶必需輔助因子(非蛋白組分)的溶解性,使大多數(shù)土壤酶在特定的pH范圍內(nèi)表現(xiàn)出最大的活性和穩(wěn)定性[28-30]；土壤pH還可通過影響微生物的活性而影響酶的活性,甚至使得微生物分泌的胞外酶適應(yīng)了土壤的酸堿環(huán)境、產(chǎn)生更穩(wěn)定的微生物酶[33]。因此,不同的酶在不同的土壤中,酶活性最大時(shí)的最適pH會(huì)有所不同[34]。從區(qū)域的角度來看,Sinsabaugh等[35]對(duì)溫帶為主的40種生態(tài)系統(tǒng)的酶進(jìn)行Meta分析發(fā)現(xiàn),β-1,4-葡萄糖苷酶、纖維二糖水解酶、β-1,4-N-乙酰氨基葡萄糖苷酶、磷酸酶、亮氨酸氨肽酶、酚氧化酶和過氧化酶的活性均與土壤pH顯著相關(guān),其中亮氨酸氨肽酶、酚氧化酶和過氧化酶活性總體上隨pH的增高而上升,而β-1,4-N-乙酰氨基葡萄糖苷酶的活性與土壤pH呈負(fù)相關(guān)。

在不同地區(qū)及不同的植被下,土壤pH和酶活性的關(guān)系有所不同。例如：Rodríguez-Loinaz等[36]對(duì)伊比利亞半島的混合橡木林土壤研究后發(fā)現(xiàn),其pH與酰胺酶和芳基硫酸酯酶活性呈正相關(guān),而與酸性磷酸酶的活性呈負(fù)相關(guān)關(guān)系。吳際友等[37]研究了長沙市城郊地區(qū)馬尾松混交林、杉木混交林、香樟林、楓香林和濕地松林5種典型林分土壤的脲酶、過氧化氫酶、酸性磷酸酶、纖維素酶后發(fā)現(xiàn),4種酶與pH值均呈不同程度的負(fù)相關(guān)。易海燕[38]對(duì)岷江上游山地森林/干旱河谷交錯(cuò)帶的幼林、闊葉混交林、灌木林、針闊混交林、次生林等5個(gè)植被類型的土壤酶進(jìn)行研究表明,脲酶、過氧化氫酶、蔗糖酶和蛋白酶活性均與土壤pH呈負(fù)相關(guān)。Guan等[39]對(duì)新疆三公河流域附近的4種植物群落研究發(fā)現(xiàn),除過氧化物酶外,土壤pH值與多酚氧化酶、α-1,4-葡萄糖苷酶、β-1,4-葡萄糖苷酶、N-乙酰-β-氨基葡糖苷酶、β-D-纖維二糖水解酶以及β-木糖苷酶活性呈正相關(guān)。

此外,不同森林土壤酶具有不同的最適pH值。Wittmann等[33]對(duì)芬蘭中部的樟子松林和云杉冷杉林的土壤進(jìn)行分析后,發(fā)現(xiàn)半纖維素酶和纖維素酶的最適pH是3—4,磷酸單酯酶、硫酸酯酶、氨肽酶、N-乙酰-氨基葡糖苷酶的最適pH為4—5。Niemi等[40]研究發(fā)現(xiàn),土壤中亮氨酸氨肽酶最適pH一般為7.5或更高,芳香基硫酸酯酶、β-葡萄糖苷酶、β-木糖苷酶、酯酶、磷酸二酯酶的最適pH為4-5.5,纖維二糖水解酶的最適pH為4-5,α-葡萄糖苷酶的最適pH低于5.5,但在pH7.0時(shí)也能表現(xiàn)出高活性。幾丁質(zhì)酶、酯酶、磷酸二酯酶、磷酸單酯酶的最適pH隨著土壤pH而改變。用0.5mol/L的乙酸緩沖液在pH5.5的條件下測定土壤中的芳香基硫酸酯酶、磷酸二酯酶和磷酸單酯酶的活性最大。Turner[34]研究了巴拿馬熱帶森林7種土壤中與碳、氮、硫、磷循環(huán)相關(guān)的8種水解酶后發(fā)現(xiàn),土壤酶活性最適pH的不同與酶的來源、和(或)土壤表面的穩(wěn)定程度有關(guān)；土壤中的部分酶為同工酶(Isoenzyme),即來源不同但可以作用于同一底物、催化相同的化學(xué)反應(yīng),而酶的結(jié)構(gòu)和性質(zhì)不盡相同,其最適pH也會(huì)隨之發(fā)生變化,比如磷酸酶,細(xì)菌來源磷酸酶的最適pH一般比真菌來源的高。在全球氣候變化的背景下,微生物群落結(jié)構(gòu)的變化,會(huì)導(dǎo)致酶的來源以及同工酶的比例發(fā)生改變,從而最終改變了土壤酶的性質(zhì)。

溫度和pH值會(huì)共同影響森林土壤酶的活性,Min等[41]研究發(fā)現(xiàn),在5—25 ℃的溫度范圍內(nèi),β-葡萄糖苷酶在pH 5.5—8.5時(shí)的活性均比較高,而N-乙酰-氨基葡糖苷酶表現(xiàn)出相對(duì)比較狹窄的最適pH(5.5—6.5)。β-葡萄糖苷酶在不同的pH下對(duì)溫度的響應(yīng)有明顯差異,在pH為4.5時(shí)該酶對(duì)溫度響應(yīng)最敏感。不同的是,N-乙酰-氨基葡糖苷酶隨pH變化沒有表現(xiàn)出明顯的溫度響應(yīng)。

森林土壤酶活性還受到土壤中營養(yǎng)物質(zhì)的影響[16-17,22,42-43],這必然與微生物營養(yǎng)需求的化學(xué)計(jì)量有關(guān)[35],尤其在氮沉降的背景下,土壤中的C∶N∶P的比例發(fā)生了改變,促使微生物隨之發(fā)生適應(yīng)性變化,如Kivlin等[17]發(fā)現(xiàn)土壤中碳氮的濃度和pH均與微生物水解酶有明顯的關(guān)系。

2 森林土壤酶對(duì)全球氣候變化的響應(yīng)

2.1 森林土壤酶對(duì)土壤增溫的響應(yīng)

大部分酶為蛋白質(zhì),對(duì)熱敏感。升高溫度會(huì)直接影響土壤酶活性[2],也會(huì)通過影響土壤的水熱條件[44]、土壤微生物的群落結(jié)構(gòu)[45]、微生物生物量[6]、微生物代謝作用[46]、土壤的呼吸作用[47]、土壤有機(jī)物質(zhì)分解[49]等因素,間接影響土壤酶的活性。常用溫室或密閉腔室、紅外加熱、開頂式生長室(OTC)和電纜加熱的增溫方式,通過對(duì)大氣加熱或者土壤直接加熱來模擬未來氣候變暖的情況[48]。

森林土壤酶活性對(duì)增溫的響應(yīng),與酶的種類以及增溫的溫度范圍有關(guān)。Criquet等[21]對(duì)法國常綠櫧的研究發(fā)現(xiàn),酸性磷酸酶活性與溫度呈負(fù)相關(guān),而堿性磷酸酶活性與溫度呈正相關(guān)。Waldrop等[45]發(fā)現(xiàn),升高溫度降低了土壤中水解酶的活性,提高了酚氧化酶的活性,而對(duì)過氧化物酶活性沒有顯著影響。McDaniel[50]對(duì)砍伐后的森林土壤進(jìn)行模擬增溫(紅外加熱,白天增溫1.5 ℃, 夜間增溫3 ℃)實(shí)驗(yàn),研究表明,增溫后β-1,4-葡萄糖苷酶、N-乙酰氨基葡萄糖苷酶和多酚氧化酶的活性下降。楊玉蓮[51]以地處青藏高原東緣和長江上游地區(qū)的川西高山森林土壤為研究對(duì)象,通過海拔梯度的自然溫度差異進(jìn)行模擬增溫實(shí)驗(yàn)發(fā)現(xiàn),土壤轉(zhuǎn)化酶、脲酶、酸性磷酸酶、中性磷酸酶和堿性磷酸酶的活性受到顯著影響。增溫2 ℃明顯提高了土壤有機(jī)層和礦質(zhì)土壤層的轉(zhuǎn)化酶活性(生長季節(jié)末期除外),而模擬增溫4 ℃卻顯著降低了土壤轉(zhuǎn)化酶活性。增溫2 ℃和4 ℃時(shí)土壤脲酶活性都明顯上升,而土壤酸性磷酸酶和中性磷酸酶的活性卻總體呈現(xiàn)出下降的趨勢。

有機(jī)土壤層和礦質(zhì)土壤層中有機(jī)質(zhì)含量的不同,會(huì)導(dǎo)致相應(yīng)土層中酶對(duì)溫度變化的響應(yīng)不同。馮瑞芳[52]采用控制環(huán)境生長室的方法研究了川西亞高山森林生態(tài)系統(tǒng)中土壤的性質(zhì),發(fā)現(xiàn)升高溫度顯著增加了土壤有機(jī)層和礦質(zhì)層的轉(zhuǎn)化酶、脲酶、硝酸還原酶、酸性磷酸酶活性,并且土壤有機(jī)層的轉(zhuǎn)化酶、硝酸還原酶和脲酶活性增加更顯著。

土壤酶活性對(duì)溫度升高的響應(yīng)還與增溫的持續(xù)時(shí)間有關(guān)。潘新麗等[53]對(duì)川西米亞羅60a人工針葉林土壤進(jìn)行原位OTC增溫的研究結(jié)果表明,增溫1a后,蔗糖酶、過氧化氫酶、蛋白酶、脲酶和多酚氧化酶活性均增強(qiáng),其中0—10 cm土層的蔗糖酶和多酚氧化酶活性顯著提高；但增溫2a后,蔗糖酶、蛋白酶和脲酶活性仍處于增強(qiáng)的趨勢,而過氧化氫酶和多酚氧化酶活性與對(duì)照相比則呈下降趨勢。此外,實(shí)驗(yàn)結(jié)果還發(fā)現(xiàn),增溫2a比增溫1a導(dǎo)致有機(jī)碳降低的幅度要小。因此,土壤酶對(duì)長時(shí)間增溫的響應(yīng),應(yīng)該是與土壤微生物對(duì)環(huán)境的適應(yīng)性、碳的有效性有關(guān)。隨著碳有效性的降低,土壤酶活性也隨之發(fā)生變化。

由于地下生物過程的復(fù)雜性,不同環(huán)境因子的共同作用會(huì)導(dǎo)致森林土壤酶活性受增溫的影響很小甚至不明顯[54]。徐振鋒等[55]采用OTC模擬增溫(增溫約0.6 ℃)的方式,研究增溫1a后對(duì)川西亞高山兩類針葉林土壤酶活性的影響,結(jié)果表明,雖然模擬增溫易于增加土壤酶的活性,但增溫效應(yīng)和林型、酶種類和季節(jié)變化還有一定關(guān)系。A′Bear等[27]對(duì)腐生擔(dān)子菌介導(dǎo)的山毛櫸分解實(shí)驗(yàn)結(jié)果表明,增溫3 ℃能夠彌補(bǔ)干燥對(duì)酶活性帶來的負(fù)效應(yīng),但單獨(dú)增溫以及同時(shí)增溫+增濕并沒有增加胞外酶的活性。

由此可見,不同種類土壤酶活性對(duì)增溫的響應(yīng)不同,不同增溫幅度下土壤酶活性的變化也有差異,土壤酶活性對(duì)增溫的響應(yīng)還受到其他土壤及環(huán)境因子的制約,是多種因子綜合作用的結(jié)果[44]。

2.2 森林土壤酶對(duì)氮添加的響應(yīng)

因?yàn)橥寥烂笇?duì)生態(tài)系統(tǒng)的變化敏感,所以其可以作為植物-土壤系統(tǒng)對(duì)氮沉降響應(yīng)的指示劑[56]。自Carreir等[1]首次發(fā)現(xiàn),氮添加會(huì)引起生態(tài)系統(tǒng)中木質(zhì)素降解酶活性的下降后,氮添加與土壤酶活性關(guān)系的研究逐年上升。

不同種類的外源氮添加對(duì)森林土壤酶的影響不同,復(fù)合氮添加對(duì)土壤酶的影響更大[57]。Guo等[57]在南京紫金山松木林進(jìn)行1a的施氮肥實(shí)驗(yàn),發(fā)現(xiàn)胞外酶對(duì)混合氮肥表現(xiàn)出不同的響應(yīng)。轉(zhuǎn)化酶、纖維素酶、纖維二糖水解酶、堿性磷酸酶、過氧化氫酶在不同比例(無機(jī)氮∶有機(jī)氮=10∶0,7∶3,3∶7以及1∶9)的混合氮肥(硝酸銨為無機(jī)氮,尿素和甘胺酸等量混合物為有機(jī)氮)下的活性比單一的無機(jī)氮(硝酸銨)的活性要高。施無機(jī)氮抑制了多酚氧化酶活性,而施混合氮?jiǎng)t相反；上述各種施氮條件均促進(jìn)了酸性磷酸酶活性,而施不同混合氮肥對(duì)其影響不顯著。因此,單一的無機(jī)氮肥可能會(huì)抑制土壤酶活性以及微生物生物量。Guo等認(rèn)為,氮肥中無機(jī)氮和有機(jī)氮的比例為3∶7時(shí),酶活性和微生物量最大。Li等[58]進(jìn)一步研究發(fā)現(xiàn),添加無機(jī)N(硝酸銨)抑制了轉(zhuǎn)化酶、β-葡萄糖苷酶、纖維素酶、酚氧化酶和磷酸酶的活性,而添加有機(jī)N(尿素和甘胺酸)則提高這些酶的活性；土壤中的真菌對(duì)氮肥更加敏感。2005年到2009年,Zhang等[59]收集了我國32個(gè)地點(diǎn)的雨水,分析表明,可溶性有機(jī)氮(DON)占大氣氮沉降量的28%。而有機(jī)氮可能會(huì)使森林從碳匯變成碳源[60],因此研究或者模擬大氣氮沉降對(duì)森林生態(tài)系統(tǒng)影響時(shí),也應(yīng)重視有機(jī)氮沉降的影響。

近幾年來的研究表明,氮添加對(duì)土壤酶活性的影響還與林分類型有關(guān)。Zheng等[61]在廣東鼎湖山自然保護(hù)區(qū)內(nèi),對(duì)馬尾松幼林和季風(fēng)常綠闊葉原始林進(jìn)行氮(150 kg N hm-2a-1)添加實(shí)驗(yàn),結(jié)果發(fā)現(xiàn)氮添加能夠顯著提高常綠闊葉林土壤酸性磷酸酶的活性,而對(duì)馬尾松幼林土壤的酸性磷酸酶活性影響不顯著。Wang 等[62]對(duì)廣東鼎湖山自然保護(hù)區(qū)的松木林、闊葉林和混交林的研究發(fā)現(xiàn),在松木林土壤中添加100 kg hm-2a-1的氮,能夠顯著提高土壤過氧化物酶的活性、抑制土壤β-1,4-葡萄糖苷酶的活性；而在闊葉林中,氮添加提高了多酚氧化酶的活性；但是混交林中土壤酶對(duì)氮添加的響應(yīng)不顯著。Weand等[63]在紐約的卡斯基山北方硬木林添加NH4NO3(50 kg hm-2a-1)進(jìn)行12a的實(shí)驗(yàn)表明,氮添加后,微生物群落組成和土壤酶活性發(fā)生變化的強(qiáng)度跟樹木種類有關(guān),變化趨勢不可預(yù)知。總之,在氮增加的條件下,不同林分類型下土壤酶的反應(yīng)有所不同,這不僅與森林土壤中原本存在的碳氮濃度高低有關(guān),還與氮的添加量及不同施氮類型引起的土壤酸化[64-65]、從而影響到微生物和土壤酶的活性有關(guān)[62,66],這還需要更加系統(tǒng)的定量研究。

氮添加不僅影響到碳循環(huán),甚至與磷循環(huán)也存在交互作用[72]。Zheng等[61]研究發(fā)現(xiàn),氮添加顯著激發(fā)常綠闊葉天然林中酸性磷酸酶的活性和馬尾松幼林地中β-葡萄糖苷酶的活性,而添加磷能夠減弱這種激發(fā)效應(yīng)。這些現(xiàn)象表明,氮添加改變了土壤中氮磷化學(xué)計(jì)量學(xué)關(guān)系,導(dǎo)致土壤微生物生長受到磷濃度的影響,因此未來的氮添加研究中,增加磷對(duì)照實(shí)驗(yàn)是很有必要的。

3 研究展望

森林土壤酶系統(tǒng)與森林土壤肥力(如土壤營養(yǎng)成分、土壤微生物等)和林分類型等密切相關(guān),也是土壤質(zhì)量的生物活性指標(biāo),可以為森林的經(jīng)營和管理提供指導(dǎo)依據(jù)。因此,在全球氣候變化的條件下,森林土壤酶在以下幾個(gè)方面還需要進(jìn)一步的研究。

第一,在實(shí)驗(yàn)環(huán)境的選擇上,大部分研究還是基于單因素的水平,比如單一的氮添加實(shí)驗(yàn)、或者是單一的增溫影響,多因素交互作用的研究還比較少。生態(tài)系統(tǒng)對(duì)一種環(huán)境因子改變的響應(yīng),可能會(huì)被其他環(huán)境因子的改變所干擾[27,73],因此,未來的研究應(yīng)該著力進(jìn)行多因素的交互實(shí)驗(yàn),既要充分利用現(xiàn)有的大型野外觀測平臺(tái)、發(fā)揮平臺(tái)優(yōu)勢,又要避免重復(fù)投資浪費(fèi)。

在全球氣候變化中,降水變化對(duì)土壤酶影響的研究,研究對(duì)象主要集中在草地、灌木林[7]上。根據(jù)政府間氣候變化專業(yè)委員會(huì)(IPCC)第五次評(píng)估報(bào)告,氣候變化將導(dǎo)致未來干旱發(fā)生頻率增加。因此降水變化對(duì)土壤酶影響,特別是森林土壤酶的影響研究有待加強(qiáng)。

第二,需進(jìn)一步拓寬以下研究內(nèi)容：

(1)區(qū)別于普通的催化劑,大部分酶是蛋白質(zhì),其活性與土壤中的pH、溫度、水分、離子濃度、底物濃度,甚至與土壤質(zhì)地均有關(guān)系。在氣候變化條件下,單純地測定酶活性高低的變化,未必能精確表征土壤酶性質(zhì)的改變。因此,在未來的研究中,要監(jiān)測酶的最適pH和最適溫度、甚至酶的特征常數(shù)(Km)是否發(fā)生漂移,以此來驗(yàn)證酶的來源或者存在狀態(tài)等在全球環(huán)境改變下的相應(yīng)變化。

(2)傳統(tǒng)的土壤酶學(xué)研究主要側(cè)重于土壤酶的時(shí)空分布研究,比如酶在不同生態(tài)系統(tǒng)下、不同植被類型、不同土層等的分布情況以及隨季節(jié)的變化情況等。但由于酶在土壤中主要與有機(jī)質(zhì)、礦物質(zhì)等結(jié)合在一起而加強(qiáng)了穩(wěn)定性[13],因此,森林酶學(xué)研究應(yīng)結(jié)合區(qū)域特點(diǎn),加強(qiáng)土壤酶與有機(jī)質(zhì)、土壤質(zhì)地等關(guān)系的研究,考察酶復(fù)合體對(duì)環(huán)境因子變化的響應(yīng)。

(3)內(nèi)生菌根(AM)樹種和外生菌根(ECM)樹種對(duì)土壤中N的利用方式不同[74],這可能是導(dǎo)致不同林分土壤酶對(duì)全球氣候變化響應(yīng)不同的原因之一。因此要重新審視傳統(tǒng)的植物-土壤界面的過程,加強(qiáng)植物根系-微生物-土壤界面的生理生化過程研究。

(4)土壤中的氧化還原酶,既參與高分子物質(zhì)的分解作用,也參與高分子物質(zhì)的合成作用[18],因此,酚氧化酶和過氧化物酶對(duì)土壤和環(huán)境因子的動(dòng)態(tài)響應(yīng),是影響土壤有機(jī)質(zhì)積累的重要機(jī)制[53,75]。

第三,在實(shí)驗(yàn)的周期上,不管是全球變暖,還是氮沉降對(duì)森林土壤酶的影響研究,大部分研究都是數(shù)年內(nèi)的結(jié)論,10a以上的長期定位觀測研究較少,這未必能代表森林對(duì)全球氣候變化響應(yīng)的真實(shí)情況,所得實(shí)驗(yàn)結(jié)論可能會(huì)導(dǎo)致未來的評(píng)估預(yù)測發(fā)生偏差。比如Peterjohn等[76]發(fā)現(xiàn),哈佛森林土壤在進(jìn)行增溫5 ℃實(shí)驗(yàn)的前幾年,土壤的CO2通量急劇提高,而Melillo等[77]發(fā)現(xiàn),增溫10a后,該森林土壤中的CO2通量與對(duì)照組沒有顯著的差異。

總之,雖然人類對(duì)環(huán)境變化下土壤酶的作用及其機(jī)制有一定的認(rèn)識(shí),但是,如何通過調(diào)控土壤中微生物以及土壤酶的功能特性來增加土壤碳儲(chǔ)量、減弱氣候變化所帶來負(fù)面影響的研究還比較有限。

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Advances in research on the responses of forest soil enzymes to environmental change

LIU Jiebao1, CHEN Guangshui1,2, GUO Jianfen1,2, YANG Zhijie1,2,*, LI Yiqing1,2, LIN Chengfang1,2,*,YANG Yusheng1,2

1SchoolofGeographicalScience,FujianNormalUniversity,Fuzhou350007,China2StateKeyLaboratoryofSubtropicalMountainEcology(FoundedbyMinistryofScienceandTechnologyandFujianProvince),FujianNormalUniversity,Fuzhou350007,China

Global climate change is an indisputable fact and this change has a significant impact on terrestrial ecosystems, especially the material cycles of forest ecosystems. Because soil enzymes play a predominant role in soil material cycles, the responses of forest soil enzymes to environmental change have been receiving increasing attention recently. These responses are key for the prediction of the impacts of future climate change on forest ecosystem processes. Therefore, this review summarizes the latest research on the responses of forest soil enzymes to environmental change, especially global warming and nitrogen deposition, and provides some suggestions for future areas of research. Environmental change will lead to changes in soil pH, moisture, and nutrients, but these changes will feed back into the activity and stability of soil enzymes. The responses of forest soil enzymes to warming are comprehensive, not only related to the type of enzyme and the range and duration of the increased temperature but also to soil types. The responses of soil enzymes to nitrogen addition are related to the types of forest and soil, and the effects of compound nitrogen deposition are greater. Future research should strengthen the dynamic response of enzymatic characterization and pay more attention to the different enzymatic responses induced by different forests and soils and to interactions between multiple factors. Additionally, long-term comprehensive observations are necessary.

climate change; global warming; nitrogen deposition; soil enzymes; potential activity

國家自然科學(xué)基金重點(diǎn)資助項(xiàng)目(31130013) ;國家自然科學(xué)基金面上資助項(xiàng)目(31270584);國家“973”計(jì)劃資助項(xiàng)目(2014CB954003)

2016- 08- 01;

2016- 11- 17

10.5846/stxb201608011581

*通訊作者Corresponding author.E-mail: zhijieyang@fjnu.edu.cn

劉捷豹,陳光水,郭劍芬,楊智杰,李一清,林成芳,楊玉盛.森林土壤酶對(duì)環(huán)境變化的響應(yīng)研究進(jìn)展.生態(tài)學(xué)報(bào),2017,37(1):110- 117.

Liu J B, Chen G S, Guo J F, Yang Z J, Li Y Q, Lin C F,Yang Y S.Advances in research on the responses of forest soil enzymes to environmental change.Acta Ecologica Sinica,2017,37(1):110- 117.