中國東部南北樣帶典型針葉林土壤酶活性分布格局

王永生, 于貴瑞, 程淑蘭, 方華軍, 高文龍

1 中國科學(xué)院地理科學(xué)與資源研究所生態(tài)網(wǎng)絡(luò)觀測與模擬重點實驗室CERN綜合研究中心, 北京 100101 2 中國科學(xué)院大學(xué), 北京 100049

中國東部南北樣帶典型針葉林土壤酶活性分布格局

王永生1,2, 于貴瑞1, 程淑蘭2, 方華軍1,*, 高文龍1,2

1 中國科學(xué)院地理科學(xué)與資源研究所生態(tài)網(wǎng)絡(luò)觀測與模擬重點實驗室CERN綜合研究中心, 北京 100101 2 中國科學(xué)院大學(xué), 北京 100049

選取中國東部南北樣帶不同氣候區(qū)(寒溫帶、溫帶和亞熱帶)的針葉林作為研究對象，分析了多酚氧化酶、過氧化物酶、幾丁質(zhì)酶以及β-葡萄糖苷酶活性的變化。結(jié)果表明，土壤溶解性有機(jī)碳(DOC)、微生物生物量碳(MBC)、微生物生物量氮(MBN)含量從北到南依次減小，寒溫帶顯著高于亞熱帶；土壤pH值在3個氣候區(qū)差異顯著，溫帶最高，亞熱帶最低。分解木質(zhì)素的多酚氧化酶和過氧化物酶活性不存在顯著性差異；而與氮循環(huán)密切相關(guān)的幾丁質(zhì)酶活性表現(xiàn)為溫帶顯著高于寒溫帶和亞熱帶；與碳循環(huán)密切的β-葡萄糖苷酶活性在溫帶最高，并顯著高于亞熱帶地區(qū)。逐步回歸分析表明，土壤MBN與pH值影響土壤幾丁質(zhì)酶活性，而土壤β-葡萄糖苷酶活性主要受pH的控制。研究認(rèn)為，在中國東部南北樣帶針葉林中，土壤幾丁質(zhì)酶和β-葡萄糖苷酶活性存在顯著性差異，而pH與MBN可能是影響土壤酶活性的主要因素。

氣候區(qū); 土壤酶活性; 纖維素; 木質(zhì)素

土壤有機(jī)質(zhì)碳庫約為植物與動物碳庫的3倍，是陸地上最大的碳庫[1]，土壤有機(jī)質(zhì)的轉(zhuǎn)化是全球碳循環(huán)中的關(guān)鍵過程。植物凋落物與根系是森林土壤有機(jī)質(zhì)的主要來源[2- 3]，碳貯存與礦化的平衡影響土壤有機(jī)質(zhì)碳庫的大小與周轉(zhuǎn)，從而改變大氣CO2濃度[4]。纖維素、木質(zhì)素和幾丁質(zhì)是陸地生物區(qū)系中最主要的3類生物聚合物，是碳的主要存在與轉(zhuǎn)化形式，也是碳循環(huán)過程中的主要基質(zhì)和產(chǎn)物[5]。纖維素和木質(zhì)素是凋落物的兩類主要生物化學(xué)組分，能被不同的微生物胞外酶分解。木質(zhì)素能被多酚氧化酶和過氧化物酶分解氧化，纖維素能被外切葡聚糖酶、內(nèi)切葡聚糖酶以及β-葡萄糖苷酶水解[6]。幾丁質(zhì)并非來源于植物，而是主要來源于動物殘體。幾丁質(zhì)的水解受內(nèi)切幾丁質(zhì)酶、殼二糖酶和N-乙酰葡萄糖胺糖苷酶作用[5]。對活性和惰性有機(jī)質(zhì)轉(zhuǎn)化過程相關(guān)酶活性格局的探討，有助于深入研究森林土壤有機(jī)質(zhì)轉(zhuǎn)化和固定的生物化學(xué)機(jī)制。

有機(jī)質(zhì)的分解作用受凋落物數(shù)量和質(zhì)量的影響[7- 8]，同時也受水分和溫度的影響[9- 10]。土壤酶活性除了受到底物有機(jī)質(zhì)質(zhì)量的影響外，還受到水熱條件的影響[11- 13]。中國東部南北森林樣帶(NSTEC)是 (IGBP)的第15條樣帶，以熱量為主要驅(qū)動因子，其次為降水。年均溫、年均降水量與氮沉降通量呈現(xiàn)從南到北遞減趨勢，從北到南分布有寒溫帶針葉林、溫帶針闊混交林、暖溫帶落葉闊葉林、亞熱帶常綠闊葉林和熱帶季雨林，為探討森林土壤活性分布格局和主控因子提供了很好的研究平臺。有關(guān)NSTEC樣帶典型森林土壤碳儲量動態(tài)、碳氮?dú)怏w排放等關(guān)鍵過程的分布格局和主控因子已有大量的研究[14- 16]，對不同森林群區(qū)典型森林土壤生物化學(xué)過程尤其是酶活性的分布格局和主控因子研究較少。本文主要研究目的是：(1)探討不同氣候區(qū)針葉林土壤活性碳氮和主要胞外酶活性的分布格局；(2)分析土壤活性碳氮含量與酶活性之間的內(nèi)在聯(lián)系。研究NSTEC針葉林土壤酶活性差異及其主控因子，對于加強(qiáng)該區(qū)域針葉林生態(tài)系統(tǒng)碳循環(huán)，尤其是研究從有機(jī)質(zhì)到溫室氣體排放的“轉(zhuǎn)換”過程提供科學(xué)依據(jù)。

1 材料與方法

1.1 研究區(qū)概況

沿著NSTEC由北向南選擇寒溫帶大興安嶺站、溫帶東靈山站和亞熱帶鼎湖山站作為研究區(qū)域，分別代表寒溫帶、溫帶和亞熱帶氣候特征。其中，大興安嶺站選擇興安落葉松成熟林作為研究對象；東靈山站選擇油松林為研究對象；而鼎湖山站內(nèi)選擇馬尾松作為研究對象。從北到南，3個站點的年均溫，從寒溫帶的 -5.4 ℃到亞熱帶的21.4 ℃；多年平均降水量從500 mm到1600 mm[14]。0—20 cm的土壤理化性狀以及森林基本情況見表1。

表1 NSTEC中3個森林站主要特征及0—20 cm土壤理化性狀

1.2 樣品采集與理化性狀測定

在每個森林隨機(jī)設(shè)置3個15 m×15 m的樣方，在每個樣方去除凋落物層后，采集0—10 cm的表層礦質(zhì)土壤樣品。大興安嶺土壤樣品采自2011年8月15日，鼎湖山樣品采集于2011年9月19日，東靈山樣品采集于2011年9月21日。每個森林選擇3個重復(fù)小區(qū)進(jìn)行取樣，每個小區(qū)內(nèi)選擇5點取樣混合為一個樣品，所有樣品過2 mm篩，除根和砂石后，保存于4 ℃冰箱內(nèi)。土壤可溶性有機(jī)碳(DOC)采用去離子水浸提，過0.45 μm濾膜后，利用TOC儀器測定分析；微生物碳(MBC)和微生物氮(MBN)采用氯仿熏蒸法測定[19]；土壤pH 按2.5∶1的水土比，采用玻璃電極測定。

1.3 酶活性測定方法

多酚氧化酶和過氧化物酶的底物為5 mmol/L的 L- 3,4-dihydroxy phenyalanine (DOPA)，采用改進(jìn)的紫外分光光度計法測定[20- 21]。用50 mmol/L的pH值為5.0的醋酸鈉緩沖液配制基質(zhì)溶液，相當(dāng)于1 g干土的樣品中，加入2.5 mL 緩沖液和2.5 mL的DOPA底物溶液，在20 ℃下培養(yǎng)1 h，培養(yǎng)結(jié)束后，取濾液在分光光度計460 nm處測定吸光度值。其中，過氧化物酶的所有樣品和對照均加0.3%的H2O2溶液。

β-葡萄糖苷酶和幾丁質(zhì)酶的底物為5 mmol/L的pNP-β-D-glucopyranoside與pNP N-acetyl-β-D-glucosaminide，采用改進(jìn)的紫外分光光度計法測定[20,22]。用50 mmol/L的pH為5.0的醋酸鈉緩沖液配制基質(zhì)溶液，相當(dāng)于1g干土的樣品中，加入2.5 mL 緩沖液和2.5 mL的底物溶液，在25 ℃下培養(yǎng)4 h，培養(yǎng)結(jié)束后，加入0.2 mL的1.0 mol/L的NaOH溶液終止反應(yīng)并顯色。取濾液在紫外分光光度計410 nm處測定吸光度值。

以上實驗，均設(shè)置不加基質(zhì)的土樣為空白。酶活性用每小時每克樣品的基質(zhì)(μmol)轉(zhuǎn)化率表示(μmol h-1g-1土)。

1.4 數(shù)據(jù)分析

利用單因素方差分析分別比較3個氣候區(qū)之間的土壤DOC、MBC、MBN和 pH值以及土壤酶活性的差異；利用逐步線性回歸分析土壤理化性質(zhì)和土壤酶活性之間的相關(guān)性。統(tǒng)計分析采用SPSS (16.0)進(jìn)行，采用Sigma Plot 10.0 繪圖。

2 結(jié)果與分析

2.1 土壤可溶性碳氮與pH分析

從圖1中可以看出，土壤可溶性有機(jī)碳(DOC)與微生物碳(MBC)含量均呈現(xiàn)為由北到南不斷減小的趨勢，而且寒溫帶地區(qū)的興安落葉松土壤DOC與MBC含量顯著高于南亞熱帶的馬尾松林土壤含量(P<0.05)。溫帶和寒溫帶土壤微生物氮(MBN)含量不存在顯著性差異，但均顯著高于亞熱帶地區(qū)(P<0.05)。而土壤pH則表現(xiàn)為，溫帶>寒溫帶>亞熱帶，而且均存在顯著性差異(P<0.05)。

2.2 土壤酶活性研究

從圖2中可以看出，3個不同氣候區(qū)的針葉林土壤中，分解木質(zhì)素的多酚氧化酶和過氧化物酶活性不存在顯著性差異。溫帶東靈山地區(qū)的油松林土壤幾丁質(zhì)酶和β-葡萄糖苷酶活性最高；其中幾丁質(zhì)酶活性顯著高于寒溫帶興安落葉松和亞熱帶馬尾松森林土壤酶活性，而β-葡萄糖苷酶活性僅顯著高于亞熱帶馬尾松森林土壤酶活性(P<0.05)。

圖1 不同針葉林表層土壤可溶性碳氮與pH分析

圖2 不同針葉林表層土壤不同類型酶活性研究

2.3 土壤酶活性與理化性狀相關(guān)性分析

利用線性回歸分析了4種酶活性和土壤DOC、MBC、MBN以及pH值之間的相關(guān)性。結(jié)果見表2。多酚氧化酶和過氧化物酶與土壤碳氮以及pH值之間均不存在顯著的相關(guān)性。幾丁質(zhì)酶與pH值存在顯著相關(guān)性(R2=0.81，P<0.01)，與MBN存在弱的相關(guān)性(R2=0.35，P=0.09)；β-葡萄糖苷酶也與土壤MBN以及pH值存在顯著的相關(guān)性，相關(guān)系數(shù)分別為0.46(P<0.05)和0.57(P<0.05)。

逐步線性回歸的結(jié)果表明，幾丁質(zhì)酶主要受MBN與pH值的影響，兩者可以解釋幾丁質(zhì)酶變異的97.6%；而β-葡萄糖苷酶主要受土壤pH值的影響(R2=0.55，P=0.018)。

表2 土壤酶活性與理化性狀的線性回歸分析

3 討論

土壤酶活性受土壤有機(jī)質(zhì)質(zhì)量和數(shù)量、養(yǎng)分有效性、底物(易氧化或難氧化)、濕度、溫度以及pH的影響[23]。多酚氧化酶能利用O2作為最終的電子受體，催化腐殖質(zhì)中難以利用的芳香族化合物轉(zhuǎn)化分解為容易被利用的物質(zhì)，該過程中釋放的自由基和醌類是合成胡敏酸的重要組分[24]。過氧化物酶可以利用由于微生物活動和某些氧化酶的作用而在土壤中形成的H2O2和其他有機(jī)過氧化物中的氧，氧化土壤有機(jī)質(zhì)，對腐殖質(zhì)的形成具有重要作用。雖然本研究中3個針葉林所處的氣候條件不同，而且土壤有機(jī)質(zhì)、全氮以及碳氮比均存在一定差異(表1)，但分解木質(zhì)素的多酚氧化酶與過氧化物酶活性在亞熱帶、溫帶以及寒溫帶之間不存在顯著性差異。因此我們推斷，從土壤酶活性的角度來看，我國東部南北樣帶針葉林土壤碳氮的差異，可能主要是由分解幾丁質(zhì)和β-葡萄糖苷酶的酶活性差異造成的。相關(guān)性分析發(fā)現(xiàn)，多酚氧化酶與過氧化物酶活性與土壤DOC、MBC、MBN以及pH均不存在相關(guān)性。Sinsabaugh and Moorhead[9]也發(fā)現(xiàn)，土壤有機(jī)質(zhì)含量與多酚氧化酶以及過氧化物酶活性不存在相關(guān)性，其他的研究也發(fā)現(xiàn)，多酚氧化酶和過氧化物酶活性與土壤肥力的相關(guān)性較差[25- 26]。

幾丁質(zhì)酶與土壤有機(jī)碳和氮的轉(zhuǎn)化關(guān)系密切，能將幾丁質(zhì)轉(zhuǎn)化為氨基糖，是土壤礦質(zhì)氮的主要來源[27]，本研究中，3個針葉林土壤幾丁質(zhì)酶活性在溫帶顯著高于寒溫帶與亞熱帶地區(qū)。β-葡萄糖苷酶具有生物催化劑功能，能裂解二聚糖和多聚糖及β-葡萄糖苷中的β-葡萄糖苷糖苷鍵，在降解有機(jī)碳復(fù)合物的過程中發(fā)揮重要作用，其水解產(chǎn)物(糖類)是土壤微生物的主要能量來源[28]，本研究中，溫帶針葉林土壤β-葡萄糖苷酶活性高于亞熱帶針葉林。溫帶地區(qū)幾丁質(zhì)酶與β-葡萄糖苷酶活性較高的原因在于：(1)溫帶東靈山地區(qū)的油松林凋落物量較大，但由于林齡較小(約40a)，而且海拔較高，因此土壤有機(jī)質(zhì)含量較低(表1)；理論上處于碳氮解狀態(tài)。因此會引起分解有機(jī)質(zhì)合成酶的負(fù)反饋效應(yīng)，因此幾丁質(zhì)酶和β-葡萄糖苷酶活性顯著高于其他氣候區(qū)。而大興安嶺地區(qū)的興安落葉松土壤有機(jī)質(zhì)豐富，DOC、MBC以及MBN含量較高(圖1)，分解有機(jī)質(zhì)的酶產(chǎn)生負(fù)反饋效應(yīng)，減少用于胞外酶合成的能量，使養(yǎng)分更高效的用于微生物生長，從而限制養(yǎng)分釋放[29]，因此酶活性反而不高。(2)干旱指數(shù)(降水量/蒸發(fā)量)也是影響酶活性的主要因素。本研究選取的3個氣候區(qū)的降水量從北到南分別為500、508.9和1564 mm，然而蒸發(fā)量為800，1077.3與1115 mm，干旱指數(shù)分別為0.63，0.47與1.40，與幾丁質(zhì)酶以及β-葡萄糖苷酶活性的格局呈相反趨勢。其他的研究也發(fā)現(xiàn)，水分(%)與幾丁質(zhì)酶以及β-葡萄糖苷酶活性呈現(xiàn)負(fù)相關(guān)關(guān)系[12]，年均降水量、潛在蒸發(fā)蒸騰與實際蒸發(fā)蒸騰這些氣候變量與凋落物分解具有高度相關(guān)[30]。

關(guān)于土壤DOC、MBC和MBN含量以及pH值與土壤酶活性的研究結(jié)論不一致，存在相關(guān)[31]與不相關(guān)[32]兩種結(jié)果，這與土壤酶活性受復(fù)雜的生物以及非生物環(huán)境因素影響有關(guān)[33]。逐步線性回歸分析結(jié)果顯示，幾丁質(zhì)酶活性主要受MBN與pH控制，而β-葡萄糖苷酶活性主要受pH控制(表2)?？赡艿脑蛟谟冢?1)土壤pH不僅控制土壤微生物群落組成的多樣性，而且直接影響土壤酶參與生化反應(yīng)的速度。而本研究中3個氣候區(qū)之間的土壤pH值也存在顯著差異(圖1)，亞熱帶森林土壤pH值低于4，部分酶活性受到限制。因此，土壤pH可能是主要的控制因素之一；(2)北方溫帶和寒溫帶森林土壤多受N限制，其次本研究選取的亞熱帶馬尾松林處于演替的初級階段，土壤可以利用氮有限[34]，而且MBN含量顯著低于溫帶和寒溫帶針葉林土壤。因此3個針葉林土壤可利用氮的不足，成為限制土壤酶活性的主要因素之一。而MBN雖然占土壤氮的百分比較小，但生物有效性較高，也是影響土壤酶活性的一個重要的因素。由此可以看出，在中國東部南北樣帶針葉林中，pH與MBN可能是影響微生物活動，從而影響土壤酶活性的主要因素。

4 結(jié)論與展望

通過研究發(fā)現(xiàn)，分解木質(zhì)素類物質(zhì)的多酚氧化酶和過氧化物酶活性在3個氣候區(qū)無顯著性差異；而溫帶東靈山地區(qū)的油松林土壤幾丁質(zhì)酶和β-葡萄糖苷酶活性最高，亞熱帶馬尾松土壤酶活性最低。因此推斷，從土壤酶活性的角度來看，我國東部南北樣帶針葉林土壤碳氮的差異，可能主要是由分解幾丁質(zhì)和β-葡萄糖苷酶的酶活性差異造成的。森林土壤MBN含量與pH值控制土壤幾丁質(zhì)酶活性，而土壤β-葡萄糖苷酶活性主要受pH的控制。

下一步的研究應(yīng)從土壤微生物的角度剖析，通過Biolog技術(shù)評價我國東部南北森林樣帶土壤微生物對不同碳源利用的能力，利用PLFA技術(shù)分析該條樣帶上土壤微生物群落結(jié)構(gòu)的差異及其與土壤理化性狀的分析，對于加強(qiáng)我國東部南北森林樣帶碳循環(huán)研究具有重要意義。

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Pattern of enzyme activities of typical coniferous forest soils along the North-South Transect of Eastern China

WANG Yongsheng1,2, YU Guirui1, CHENG Shulan2, FANG Huajun1,*, GAO Wenlong1,2

1SynthesisResearchCenterofCERN,KeyLaboratoryofEcosystemNetworkObservationandModeling,InstituteofGeographicSciencesandNaturalResourcesResearch,ChineseAcademyofSciences,Beijing100101,China2UniversityofChineseAcademyofSciences,Beijing100049,China

Soil organic carbon (SOC) is the largest terrestrial pool of carbon, and the amount of carbon in soils represents about two thirds of total ecosystem. Therefore, any change in the size and turnover rate of SOC pools will potentially alter the atmospheric CO2concentration and the subsequent global climate. Soil enzymes are involved in a series of catalyzing reactions and play pivotal roles in litter and soil organic matter decomposition as well as nutrient cycling in terrestrial ecosystems. Soil enzyme activity can be used as a proxy for plant and microbial substrate availability, the interference between soil ecosystem and external environment, and microbial community structure and metabolic capabilities. Therefore, the determination of soil enzyme activities is a powerful tool for understanding soil carbon and nitrogen biogeochemical processes and their responses to climate change. Soil enzyme activity has been a frontier of forest ecology research and has been widespread concern. Unfortunately, most of the studies to date have been limited to a single forest biome or a single forest site, and few studies concern the pattern and controlling factors of soil enzyme activity at a terrestrial transect scale. North-South Transect of Eastern China (NSTEC) is the fifteenth standard transect established by International Geosphere-Biosphere Program (IGBP) in 2005 and is mainly driven by heat, followed by precipitation. It covers almost all forest types from boreal forest to tropical rain forest, which provides an ideal tool to investigate the pattern and environmental control of soil enzyme activities in the boreal, temperate and tropical forest biomes. In the past decade, many studies were conducted to measure the main factors affecting the pattern of SOC storage and soil-atmosphere exchange of greenhouse gases in the typical forests along the NSTEC. However, little information is available on the pattern of soil enzyme activities involved in soil carbon and nitrogen cycles and they relate to soil microbial biomass at a large spatial scale. In this study, the typical coniferous forests distributed in the cold-temperate, temperate and subtropical climatic zones were selected along the NSTEC. The activities of polyphenol oxidase, peroxidase, chitinase and β-glucosaccharase, soil dissolved organic carbon (DOC), microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) as well as soil pH in the three coniferous forest soils were measured. The results showed that soil DOC, MBC, and MBN contents tended to decrease from north to south, and they were higher in the cold-temperate forest soils than in the subtropical forest soils. There was a significant difference in the soil pH values among the three coniferous forests, following the order cold-temperate > temperate > subtropical forests. Also, no significant differences were found in the soil polyphenol oxidase and peroxidase activities among the three typical coniferous forests. In contrast, the soil chitinase activity involved in soil nitrogen cycle was significantly higher in the temperate forest than in the cold-temperate and subtropical forests, and the soil β-glucosaccharase activity in the temperate forest was also significantly higher than that of the subtropical forest. Stepwise regression analysis showed that the soil chitinase activity was closely associated with soil MBN content and soil pH value, and the soil β-glucosaccharase activity was significantly and positively related to soil pH value. Our results suggest that soil cellulose- and chitin-degrading enzymes (i.e., chintinase and β-glucosaccharase) sensitively respond to climate zone. Soil MBN content and pH value are the main factors controlling soil enzyme activities of coniferous forests along the NSTEC.

climatic region; soil enzyme activities; cellulose; lignin

國家自然科學(xué)基金(31070435, 31290222, 31290221); 國家重點基礎(chǔ)研究發(fā)展計劃項目(2012CB417103, 2010CB833502); 中國科學(xué)院地理科學(xué)與資源研究所“秉維”優(yōu)秀青年人才基金(2011RC202); 中國科學(xué)院戰(zhàn)略性先導(dǎo)科技專項(XDA05050600)

2013- 09- 27;

2014- 08- 22

10.5846/stxb201309272377

*通訊作者Corresponding author.E-mail: fanghj@igsnrr.ac.cn

王永生, 于貴瑞, 程淑蘭, 方華軍, 高文龍.中國東部南北樣帶典型針葉林土壤酶活性分布格局.生態(tài)學(xué)報,2015,35(11):3636- 3642.

Wang Y S, Yu G R, Cheng S L, Fang H J, Gao W L.Pattern of enzyme activities of typical coniferous forest soils along the North-South Transect of Eastern China.Acta Ecologica Sinica,2015,35(11):3636- 3642.