程學(xué)剛,張彩虹,楊煥祥,劉浩棟,付戰(zhàn)勇,張光燦,李傳榮

(泰山森林生態(tài)系統(tǒng)國家定位觀測研究站山東農(nóng)業(yè)大學(xué)農(nóng)業(yè)生態(tài)與環(huán)境重點(diǎn)實(shí)驗(yàn)室,271018,山東泰安)

連作對楊樹人工林土壤呼吸及各組分的影響

程學(xué)剛,張彩虹,楊煥祥,劉浩棟,付戰(zhàn)勇,張光燦,李傳榮?

(泰山森林生態(tài)系統(tǒng)國家定位觀測研究站山東農(nóng)業(yè)大學(xué)農(nóng)業(yè)生態(tài)與環(huán)境重點(diǎn)實(shí)驗(yàn)室,271018,山東泰安)

土壤呼吸是整個(gè)陸地生態(tài)系統(tǒng)碳循環(huán)的關(guān)鍵過程之一。以山東大汶河沿岸沙地不同連作代數(shù)楊樹人工林(1代林、2代林和3代林)為研究對象,利用ACE自動(dòng)土壤呼吸監(jiān)測系統(tǒng)(UK),對3種林分一個(gè)生長季(4—10月)的土壤呼吸速率及溫濕度進(jìn)行測定,同時(shí)采用壕溝法對3種林分的土壤呼吸進(jìn)行組分分離,并對土壤呼吸及各組分與土壤溫濕度的關(guān)系進(jìn)行模型模擬。結(jié)果表明:3種林分的土壤呼吸速率(RS)、自養(yǎng)呼吸速率(RA)和異養(yǎng)呼吸速率(RH)的月變化均為明顯的單峰格局;生長季內(nèi),3種林分RA貢獻(xiàn)率月差異明顯,平均貢獻(xiàn)率為40.04%;RS及其組分與5 cm處土壤溫度存在顯著指數(shù)關(guān)系,與土壤體積含水量沒有相關(guān)性,土壤溫度與土壤體積含水量的復(fù)合模型對土壤呼吸速率變化解釋能力為80%～94%;3種林分生長季平均土壤呼吸速率分別為3.12、3.08和2.66 μmol/(m2·s),3代林RS和RH均顯著低于1代林和2代林。連作導(dǎo)致楊樹人工林地土壤呼吸速率減弱,土壤理化性質(zhì)和微生物量的差異是導(dǎo)致林分間土壤呼吸速率差異的主要原因。揭示連作對楊樹人工林土壤呼吸及各組分的影響,以及作用機(jī)制,為全面探究楊樹人工林連作效應(yīng)及土壤碳循環(huán),提供數(shù)據(jù)支撐。

自養(yǎng)呼吸;連作;異養(yǎng)呼吸;壕溝法

自工業(yè)革命以來,大氣中溫室氣體濃度急劇增加,其中CO2濃度在250年間升高約32%[1],由此引起的全球變暖是人類目前所面臨的主要環(huán)境問題之一。地球上的CO2庫主要是土壤和海洋,前者約為后者的2倍[2]。土壤碳排放的主要途徑是土壤呼吸,每年向大氣中釋放大約80～110 Pg[3],是化石燃料燃燒年排放量的13倍[4];因此,任何土壤呼吸速率的微小變化,都會(huì)對大氣CO2濃度乃至整個(gè)生態(tài)系統(tǒng)碳循環(huán)產(chǎn)生較大影響[5]。準(zhǔn)確掌握土壤呼吸對土壤碳排放的影響,在應(yīng)對全球氣候變化中具有重要的意義。

楊樹(PopulusL.)是我國最重要的速生豐產(chǎn)樹種,到2009年,全國楊樹人工林總面積已經(jīng)達(dá)到757.23萬hm2[6],在緩解我國木材市場的供需矛盾中發(fā)揮了重要的作用;此外,楊樹人工林可降低風(fēng)速,增加地表粗糙度,亦能夠改良土壤,增加黏粒質(zhì)量分?jǐn)?shù),以此減弱河岸沙地的風(fēng)蝕,發(fā)揮一定的水土保持效益。在栽植面積快速增長的同時(shí),由于多采用短輪伐期集約栽培體制,楊樹連作的現(xiàn)象也越來越普遍,由此產(chǎn)生的楊樹人工林連作效應(yīng)逐漸顯露出來,限制了其生態(tài)系統(tǒng)服務(wù)功能的實(shí)現(xiàn)[7]。目前研究多集中于其根系分泌物的化感作用、土壤養(yǎng)分有效性以及微生物區(qū)系等方面。研究認(rèn)為,連作會(huì)使林分生長放緩,樹高降低,同時(shí)令土壤理化性質(zhì)下降,顯著影響林地生產(chǎn)力和水土保持效益的實(shí)施[8-10];然而,楊樹連作是否會(huì)影響土壤呼吸,以及進(jìn)一步的土壤碳循環(huán)過程,目前尚不清楚:因此,筆者以大汶河沿岸沙地不同連作代數(shù)楊樹人工林為研究對象,定位觀測土壤呼吸及相關(guān)環(huán)境因子,旨在揭示連作對楊樹人工林土壤呼吸及各組分的影響,以及作用機(jī)制,為全面探究楊樹人工林連作效應(yīng)及土壤碳循環(huán),提供數(shù)據(jù)支撐。

1 研究區(qū)概況

研究區(qū)位于山東省泰安市寧陽縣國有高橋林場(E 116°52＇,N 35°54＇)。該地區(qū)屬溫帶大陸性半濕潤季風(fēng)氣候區(qū),四季分明,夏季高溫多雨,冬季寒冷干燥。年均降水量689.6 mm,年際變化大,且年內(nèi)分配不均,主要集中在6—9月,蒸發(fā)量1 428.8 mm。多年平均氣溫13.7℃,≥10℃的活動(dòng)積溫4 493.3℃,日照總時(shí)間2 679.3 h,無霜期206 d。土壤類型為沙壤質(zhì)潮土,土壤黏粒較少,保水保肥性差,春季常有大風(fēng)沙塵天氣,微度風(fēng)蝕。林下草本植物主要有狗尾草(Setaria viridis(L.)Beauv.)、藎草(Arthraxon hispidus(Thunb.)Makino)、小飛蓬(Conyza canadensis(L.)Cronq.)、反枝莧(Amaranthus retroflexusL.)、葎草(Humulusscandens(Lour.) Merr.)、蒺藜(Tribulus terresterL.)、鬼針草(Bidens pilosaL.)等,蓋度為60%～90%。不同連作代數(shù)的楊樹人工林基本概況見表1。

表1 不同連作代數(shù)的楊樹人工林基本概況Tab.1 Basic profile of poplar plantations in different continuous cropping systems

2 材料與方法

2.1 樣地選擇

選擇立地條件和林齡相似的I-107楊樹(Populus×euramericanacv.‘Neva')人工林,其中1代林(首次造林)、2代林(皆伐一輪后造林)和3代林(皆伐兩輪后造林)各1塊樣地,輪伐期為15年。各林地經(jīng)營管理措施一致,造林株行距均為3 m×5 m。3種林分在首次造林前均為農(nóng)田,各林地土壤特性見表2。

表2 3種林地土壤特性Tab.2 Soil properties of three forest types

2.2 測定方法

2014年12月,分別在3個(gè)林分內(nèi),各布設(shè)2個(gè)20 m×20 m的樣地。每個(gè)樣地內(nèi)分別設(shè)置保留根系(對照)和切斷根系(視為異養(yǎng)呼吸)2種處理,每個(gè)處理6個(gè)重復(fù)。把對照呼吸速率與斷根處理呼吸速率的差值作為自養(yǎng)呼吸速率。斷根處理采用壕溝法[11],選擇1 m×1 m的區(qū)域,四周垂直挖深1 m以切斷根系,在其周圍布設(shè)100目的尼龍網(wǎng),然后回填土壤。土壤呼吸速率采用ACE自動(dòng)土壤呼吸監(jiān)測系統(tǒng)(UK)測定。2015年4—10月,選擇每月中旬,且雨后至少5 d的晴朗天氣,使用3臺(tái)儀器,對3個(gè)樣地進(jìn)行同步觀測,每次測定時(shí)間間隔為2 h,連續(xù)測定24 h。為消除土壤擾動(dòng)帶來的影響,每次測定前24 h,埋設(shè)鋼圈并剪除雜草。采用ACE自帶的溫濕度傳感器,同步測定地下5 cm處的土壤溫度和土壤體積含水量。

2015年6月,進(jìn)行3種林分的土壤樣品采集和土壤特性測定。土壤密度采用環(huán)刀法測定,土壤全氮采用半微量凱氏定氮法測定,用電位法測定土壤pH值,土壤有機(jī)碳測定采用重鉻酸鉀外加熱法,土壤微生物量碳采用氯仿熏蒸-K2SO4浸提法測定。每個(gè)樣地3個(gè)重復(fù),每個(gè)林分的樣本數(shù)n=6。

2.3 數(shù)據(jù)處理

采用常用的3種經(jīng)驗(yàn)?zāi)Ｐ?對土壤溫度、土壤含水量和土壤呼吸的關(guān)系,進(jìn)行擬合分析。[10]

式中:RS為土壤呼吸速率,μmol·m-2·s-1;t為5 cm深處的土壤溫度,℃;W為5 cm深土壤體積含水量,%;a、b、c為待定參數(shù)。

方差分析和回歸分析利用IBM SPSS 20.0軟件進(jìn)行,顯著性水平設(shè)定為α=0.05。圖表采用Qrigin 7.5和Microsoft Excel 2003繪制。

3 結(jié)果與分析

3.1 連作對土壤呼吸及各組分的影響

整個(gè)生長季,3個(gè)楊樹人工林地的土壤呼吸(RS)、自養(yǎng)呼吸(RA)和異養(yǎng)呼吸(RH)的月變化趨勢一致,4月最小,7月最大,均為明顯的單峰曲線(圖1)。1代林、2代林和3代林生長季土壤呼吸速率的變化范圍分別為1.42～6.01、1.07～5.91和1.21～5.13 μmol/m2·s,平均值分別為3.12、3.08和2.66 μmol/m2·s。由表3可知,RS隨連作代數(shù)的增加有下降趨勢。單因素方差分析顯示,1代林和2代林顯著高于3代林(P＜0.05),1代林同2代林之間無顯著差異(P＞0.05)。其中,3種林分的自養(yǎng)呼吸速率差異不顯著;而在異養(yǎng)呼吸速率方面,1代林和2代林顯著高于3代林。

圖1 土壤呼吸速率及各組分的月變化(平均值±標(biāo)準(zhǔn)偏差)Fig.1 Monthly change of soil respiration rate and its components(mean±SD)

表3 3種林分土壤呼吸及其組分平均值的多重比較Tab.3 One way ANOVA for the mean of soil respiration and its compositions under different forest types

3.2 連作對土壤呼吸各組分貢獻(xiàn)率的影響

圖2顯示,3種林分自養(yǎng)呼吸貢獻(xiàn)率平均值為40.04%,1代林、2代林和3代林分別為39.39%、38.47%和42.25%,三者之間無顯著差異(P＞0.05)。不同月份相比,夏季的6—8月,3個(gè)月自養(yǎng)呼吸占土壤呼吸的比值相對較高,其余4、5、9和10月,均低于總平均值。

圖2 3種林分自養(yǎng)呼吸貢獻(xiàn)率及其總平均值Fig.2 Contribution rates of autotrophic respiration and overall means of three forest types

3.3 土壤呼吸及各組分與土壤溫濕度的關(guān)系

圖3顯示,3個(gè)林分5 cm處土壤溫度呈現(xiàn)明顯的單峰曲線,無顯著差異(P＞0.05),4月份為最低值,7月份達(dá)到最高值。土壤含水量呈現(xiàn)較強(qiáng)的變異性,3個(gè)林分同樣無顯著差異。

分別采用指數(shù)模型、線性模型和冪-指數(shù)模型,對土壤呼吸及各組分與土壤溫濕度進(jìn)行擬合。結(jié)果顯示,土壤呼吸速率與5 cm處土壤溫度之間呈極顯著的指數(shù)關(guān)系(P＜0.01),土壤溫度能夠解釋3種林分土壤呼吸75%以上的變化(表4)。土壤呼吸速率與5 cm深處土壤含水量之間無相關(guān)關(guān)系(P＞0.05)。采用雙因素模型,對土壤呼吸與土壤溫度和土壤含水量進(jìn)行模擬,發(fā)現(xiàn)R2值較單純的指數(shù)模型均有不同程度的提高,土壤溫濕度能夠共同解釋3種林分土壤呼吸90%～94%的變化;因此,冪-指數(shù)模型能夠更好地模擬三者的關(guān)系。

圖3 3種林分土壤溫度和含水量月變化(平均值±標(biāo)準(zhǔn)偏差)Fig.3 Monthly variation of soil temperature and soil water content in three polar plantations(mean±SD)

表4 土壤呼吸速率與土壤溫度(t)和土壤濕度(w)不同模型的參數(shù)Tab.4 Parameters for different models showing the relationships of soil respiration with soil temperature(t)and soil water content(W)

4 結(jié)論和討論

4.1 連作對楊樹人工林土壤呼吸及各組分的影響

研究結(jié)果表明,土壤呼吸的月變化呈單峰型,生長旺季RS顯著高于非生長旺季(圖1),該趨勢與土壤溫度的變化一致(圖3)。生長旺季植物活根系及微生物活動(dòng)最劇烈,從而導(dǎo)致其RS最高。楊樹連作3代之后,RS和RH較1代林和2代林有明顯下降(表3)。試驗(yàn)期間,3種林分的土壤溫度和濕度并無顯著差異(P＞0.05),可以斷定土壤溫濕度并非是引起3種林分RS差異的主要原因;同時(shí),3種林分RA無顯著差異(表2):因此,其RS的差異主要體現(xiàn)在RH的差異上。除土壤溫濕度外,土壤環(huán)境的生物物理學(xué)性質(zhì)(微生物數(shù)量和活性、土壤物理性質(zhì))以及底物可利用性(土壤有機(jī)質(zhì)質(zhì)量分?jǐn)?shù))等,被認(rèn)為是土壤異養(yǎng)呼吸的主要影響因素[12-13]。秦越等[14]研究發(fā)現(xiàn),RS與土壤有機(jī)質(zhì)質(zhì)量分?jǐn)?shù)呈正相關(guān)關(guān)系;臧逸飛等[15]也認(rèn)為土壤有機(jī)質(zhì)、土壤微生物生物量碳與土壤呼吸有極顯著的相關(guān)性;筆者在研究中發(fā)現(xiàn),隨連作代數(shù)的增加,土壤有機(jī)碳、全氮和微生物量碳等指標(biāo)均有明顯下降(表2),這與多數(shù)對連作障礙的研究一致[16-17]:因此,2代林和3代林相對較低的呼吸底物濃度和微生物量,可能是導(dǎo)致其RS和RH較小的主要原因。

4.2 環(huán)境因子對土壤呼吸及各組分的影響

土壤呼吸是一個(gè)極為復(fù)雜的生物過程,受諸多環(huán)境因子的影響[18]。其中,土壤溫度和土壤含水量被認(rèn)為是最主要的限制因子[19]。土壤溫度能夠影響植物物候特征、根系活動(dòng)、土壤和根際微生物活性以及呼吸底物的供應(yīng)[20-21],進(jìn)而對RS產(chǎn)生影響。試驗(yàn)發(fā)現(xiàn),3種林分的RS、RA和RH均與土壤溫度呈極顯著指數(shù)關(guān)系(表3),這與多數(shù)溫帶地區(qū)森林的研究結(jié)果一致[22-23];同時(shí),基于指數(shù)模型擬合發(fā)現(xiàn),土壤溫度對土壤呼吸速率變化的解釋能力為75%～92%:因此,土壤溫度是本區(qū)域楊樹人工林土壤呼吸速率的關(guān)鍵限制因子。通過線性模型擬合發(fā)現(xiàn),土壤呼吸速率與土壤含水量相關(guān)關(guān)系不顯著,原因可能是試驗(yàn)樣地內(nèi),土壤水分并未限制到植物根系和微生物的活動(dòng)。Wang等[24]認(rèn)為,包含溫濕度的復(fù)合模型能夠更好地解釋土壤呼吸的變化,筆者采用復(fù)合函數(shù)模型對三者進(jìn)行擬合,R2值均有提高,說明雙因素模型能夠更好地模擬土壤呼吸與土壤溫濕度的關(guān)系,這也印證了上文的論斷。

4.3 土壤呼吸各組分的貢獻(xiàn)率

采用壕溝法區(qū)分RA和RH,無根呼吸(異養(yǎng)呼吸)觀測點(diǎn)樣方內(nèi)的根系,只是切斷而并未排除,由于楊樹具有明顯的無性系繁殖特性,使得處理樣方內(nèi)的根系能夠存活很長一段時(shí)間,并進(jìn)行呼吸作用;另外,死亡的根系會(huì)被微生物分解,其相當(dāng)于人為添加異養(yǎng)呼吸的底物,可增強(qiáng)土壤異養(yǎng)呼吸速率[25-27]。應(yīng)用壕溝法進(jìn)行土壤呼吸的組分分離,顯然會(huì)導(dǎo)致自養(yǎng)呼吸的貢獻(xiàn)率小于實(shí)際值。唐羅忠等[28]研究發(fā)現(xiàn),楊樹＜10 mm的根系在切斷4個(gè)月后,活性基本消失(死亡);因此,筆者在挖壕溝處理的4個(gè)月后,再進(jìn)行異養(yǎng)呼吸的測定,此時(shí)壕溝樣方內(nèi)的根系已經(jīng)基本死亡,使得RH的觀測值能夠更加接近實(shí)際值。

吳君君等[11]收集不同氣候帶森林土壤呼吸數(shù)據(jù)發(fā)現(xiàn),自養(yǎng)呼吸的貢獻(xiàn)率在18.4%～83.1%之間,筆者研究3種林分自養(yǎng)呼吸的平均貢獻(xiàn)率分別為39.39%、38.47%和42.25%,都在上述范圍之內(nèi)。各林分RA的平均貢獻(xiàn)率為40.04%,這一數(shù)值略高于D.D.Saurette等[29]對加拿大阿爾伯塔楊樹人工林37%的研究結(jié)果。除此之外,RA/RS的比值存在明顯的季節(jié)變化,即夏季自養(yǎng)呼吸的貢獻(xiàn)率顯著高于春、秋2個(gè)季節(jié)(圖2),這也印證了RA的溫度敏感性高于RH,自養(yǎng)呼吸速率相比異養(yǎng)呼吸速率更容易受到溫度的影響,夏季溫度較高時(shí),自養(yǎng)呼吸速率升高較快,從而導(dǎo)致RA/RS的值大于春秋2季。

綜上所述,筆者對不同連作代數(shù)楊樹人工林生長季土壤呼吸速率及其差異原因進(jìn)行測定和分析發(fā)現(xiàn),在連作條件下,2代林和3代林的土壤理化性質(zhì)和微生物生物量較1代林有所下降,導(dǎo)致其土壤呼吸速率包括異養(yǎng)呼吸速率降低。楊樹人工林長期連作,會(huì)抑制土壤呼吸作用;而連作過后,整個(gè)楊樹人工林生態(tài)系統(tǒng)是碳“源”亦或是“匯”,仍需要進(jìn)一步研究確定。

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Effects of continuous cropping on soil respiration and its components of poplar plantations

CHENG Xuegang,ZHANG Caihong,YANG Huanxiang,LIU Haodong,FU Zhanyong, ZHANG Guangcan,LI Chuanrong
(Taishan Forest Ecosystem Research Station;Key Laboratory of Agricultural Ecology and Environment of Shandong Agricultural University,271018,Tai＇an,Shandong,China)

[Background]Soil respiration(RS)is a key step in the carbon cycle of forest ecosystem, which mainly consists of two parts:autotrophic respiration(RA)and heterotrophic respiration(RH).The decisive factors of autotrophic and heterotrophic respiration are different.In addition,the heterotrophic and autotrophic components of soil respiration may respond differently to climate change.Our goal is to assess the relationship between soil respiration and soil temperature and humidity,to determine the relative contribution of autotrophic and heterotrophic respiration to soil respiration,and to investigate the effect of continuous cropping on soil respiration and its components.The study site is located in the sandlot along the Dawen River,Shandong Province of eastern China.[Methods]We took different continuous cropping generations of poplar plantation(i.e.first generation,second generation and third generation)as theresearch objects.We used a field setup through trenching method to distinguish between heterotrophic and autotrophic respiration,and ACE automatic soil respiration monitoring system to measure the dynamics of soil respiration during the growing season in 2015.Meantime,soil temperature and soil water content at 5 cm depth were also measured by the self-contained temperature and moisture sensor of the instrument mentioned above.We used three empirical models to fit and analyze the relationship between soil respiration,soil temperature and volumetric water content.In addition,the soil bulk density,pH value, soil organic carbon(SOC),total nitrogen(TN)and microbial biomass carbon(MBC)content in 0-20 cm soil depth of three forest types were observed.[Results]1)Soil respiration and its components presented significant monthly variation as unimodal pattern,and were consistent with the change of soil temperature.2)The average soil respiration rate of three forest types on their growth seasons were 3.12 μmol/(m2·s),3.08 μmol/(m2·s)and 2.66 μmol/(m2·s)respectively.RSandRHof the third generation forest were significantly lower than that of the first and second generation forest,whileRAshowed no significant difference among the three forest types.3)Contribution rate ofRAof the first generation,second generation,third generation and overall mean value was 39.39%,38.47%,42.25% and 40.04%respectively,and showed seasonal differences,but the difference alone the three stands was not significant.4)Soil temperature and volumetric water content were not significant among the three types of forest during the observation period.Soil respiration and its components showed significant exponential relationship with soil temperature in 5 cm depth,and no significant relationship with volumetric water content.The goodness of the binary mixed model indicated that the combined effects of soil temperature and volumetric water content on soil respiration and its components were 80%-94%.The simulation results showed that the binary mixed model was the best.[Conclusions]In summary,continuous cropping of poplar plantation reduced soil respiration rate and heterotrophic respiration rate,and the difference on soil physical and chemical properties and microbial biomass is the main reason leading to the difference in soil respiration rate alone different stands.This study revealed the effects of continuous cropping on soil respiration and its components,and provided data support for the comprehensive study of continuous cropping effect and soil carbon cycle in poplar plantations.

autotrophic respiration;continuous cropping;heterotrophic respiration;trenching method

S718.55

:A

:2096-2673(2017)01-0105-08

10.16843/j.sswc.2017.01.013

2016- 04- 18

2016- 12- 08

項(xiàng)目名稱:國家林業(yè)公益性科研專項(xiàng)“森林生態(tài)服務(wù)功能分布式定位觀測與模型模擬”(201204101-7);國家自然科學(xué)基金“黃泛平原農(nóng)田林網(wǎng)的生態(tài)因子場形成機(jī)制的研究”(31170662);教育部博士點(diǎn)基金“基于生態(tài)因子場的擬法正農(nóng)田林網(wǎng)可持續(xù)更新機(jī)制研究”(20133702110007)

程學(xué)剛(1989—),男,碩士研究生。主要研究方向:恢復(fù)生態(tài)學(xué)。E-mail:shuoyueliuxing@163.com

?通信作者簡介:李傳榮(1968—),男,教授。主要研究方向:恢復(fù)生態(tài)和林業(yè)生態(tài)工程。E-mail:chrli@sdau.edu.cn