程學剛,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業(yè)大學農業(yè)生態(tài)與環(huán)境重點實驗室,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業(yè)大學農業(yè)生態(tài)與環(huán)境重點實驗室,271018,山東泰安)

土壤呼吸是整個陸地生態(tài)系統(tǒng)碳循環(huán)的關鍵過程之一。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自動土壤呼吸監(jiān)測系統(tǒng)(UK),對3種林分一個生長季(4—10月)的土壤呼吸速率及溫濕度進行測定,同時采用壕溝法對3種林分的土壤呼吸進行組分分離,并對土壤呼吸及各組分與土壤溫濕度的關系進行模型模擬。結果表明: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,3種林分RA貢獻率月差異明顯,平均貢獻率為40.04%;RS及其組分與5 cm處土壤溫度存在顯著指數(shù)關系,與土壤體積含水量沒有相關性,土壤溫度與土壤體積含水量的復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。連作導致楊樹人工林地土壤呼吸速率減弱,土壤理化性質和微生物量的差異是導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,以及作用機制,為全面探究楊樹人工林連作效應及土壤碳循環(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,都會對大氣CO2濃度乃至整個生態(tài)系統(tǒng)碳循環(huán)產生較大影響[5]。準確掌握土壤呼吸對土壤碳排放的影響,在應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樹種,到2009年,全國楊樹人工林總面積已經(jīng)達到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速,增加地表粗糙度,亦能夠改良土壤,增加黏粒質量分數(shù),以此減弱河岸沙地的風蝕,發(fā)揮一定的水土保持效益。在栽植面積快速增長的同時,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,由此產生的楊樹人工林連作效應逐漸顯露出來,限制了其生態(tài)系統(tǒng)服務功能的實現(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。研究認為,連作會使林分生長放緩,樹高降低,同時令土壤理化性質下降,顯著影響林地生產力和水土保持效益的實施[8-10];然而,楊樹連作是否會影響土壤呼吸,以及進一步的土壤碳循環(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,以及作用機制,為全面探究楊樹人工林連作效應及土壤碳循環(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℃的活動積溫4 493.3℃,日照總時間2 679.3 h,無霜期206 d。土壤類型為沙壤質潮土,土壤黏粒較少,保水保肥性差,春季常有大風沙塵天氣,微度風蝕。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個林分內,各布設2個20 m×20 m的樣地。每個樣地內分別設置保留根系(對照)和切斷根系(視為異養(yǎng)呼吸)2種處理,每個處理6個重復。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100目的尼龍網(wǎng),然后回填土壤。土壤呼吸速率采用ACE自動土壤呼吸監(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臺儀器,對3個樣地進行同步觀測,每次測定時間間隔為2 h,連續(xù)測定24 h。為消除土壤擾動帶來的影響,每次測定前24 h,埋設鋼圈并剪除雜草。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。每個樣地3個重復,每個林分的樣本數(shù)n=6。

2.3 數(shù)據(jù)處理

采用常用的3種經(jīng)驗模型,對土壤溫度、土壤含水量和土壤呼吸的關系,進行擬合分析。[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軟件進行,顯著性水平設定為α=0.05。圖表采用Qrigin 7.5和Microsoft Excel 2003繪制。

3 結果與分析

3.1 連作對土壤呼吸及各組分的影響

整個生長季,3個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±標準偏差)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率的影響

圖2顯示,3種林分自養(yǎng)呼吸貢獻率平均值為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系

圖3顯示,3個林分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月份達到最高值。土壤含水量呈現(xiàn)較強的變異性,3個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行擬合。結果顯示,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關系(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系。

圖3 3種林分土壤溫度和含水量月變化(平均值±標準偏差)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最劇烈,從而導致其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,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)境的生物物理學性質(微生物數(shù)量和活性、土壤物理性質)以及底物可利用性(土壤有機質質量分數(shù))等,被認為是土壤異養(yǎng)呼吸的主要影響因素[12-13]。秦越等[14]研究發(fā)現(xiàn),RS與土壤有機質質量分數(shù)呈正相關關系;臧逸飛等[15]也認為土壤有機質、土壤微生物生物量碳與土壤呼吸有極顯著的相關性;筆者在研究中發(fā)現(xiàn),隨連作代數(shù)的增加,土壤有機碳、全氮和微生物量碳等指標均有明顯下降(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

土壤呼吸是一個極為復雜的生物過程,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、土壤和根際微生物活性以及呼吸底物的供應[20-21],進而對RS產生影響。試驗發(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)關系(表3),這與多數(shù)溫帶地區(qū)森林的研究結果一致[22-23];同時,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關系不顯著,原因可能是試驗樣地內,土壤水分并未限制到植物根系和微生物的活動。Wang等[24]認為,包含溫濕度的復合模型能夠更好地解釋土壤呼吸的變化,筆者采用復合函數(shù)模型對三者進行擬合,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系,這也印證了上文的論斷。

4.3 土壤呼吸各組分的貢獻率

采用壕溝法區(qū)分RA和RH,無根呼吸(異養(yǎng)呼吸)觀測點樣方內的根系,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,使得處理樣方內的根系能夠存活很長一段時間,并進行呼吸作用;另外,死亡的根系會被微生物分解,其相當于人為添加異養(yǎng)呼吸的底物,可增強土壤異養(yǎng)呼吸速率[25-27]。應用壕溝法進行土壤呼吸的組分分離,顯然會導致自養(yǎng)呼吸的貢獻率小于實際值。唐羅忠等[28]研究發(fā)現(xiàn),楊樹＜10 mm的根系在切斷4個月后,活性基本消失(死亡);因此,筆者在挖壕溝處理的4個月后,再進行異養(yǎng)呼吸的測定,此時壕溝樣方內的根系已經(jīng)基本死亡,使得RH的觀測值能夠更加接近實際值。

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)呼吸的平均貢獻率分別為39.39%、38.47%和42.25%,都在上述范圍之內。各林分RA的平均貢獻率為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)呼吸的貢獻率顯著高于春、秋2個季節(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,自養(yǎng)呼吸速率升高較快,從而導致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和微生物生物量較1代林有所下降,導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,會抑制土壤呼吸作用;而連作過后,整個楊樹人工林生態(tài)系統(tǒng)是碳“源”亦或是“匯”,仍需要進一步研究確定。

[1] 陳泮勤.地球系統(tǒng)碳循環(huán)[M].北京:科學出版社, 2004:21. CHEN Panqin.Earth system carbon cycle[M].Beijing: Science Press,2004:21.

[2] HUGELIUS G,STRAUSS J,ZUBRZYCKI S,et al.Estimated stocks of circumpolar permafrost carbon with quantified uncertainty ranges and identified data gaps[J]. Biogeosciences,2014,11(23):6573.

[3] SCHLESINGER W H,ANDREWS J A.Soil respiration and the global carbon cycle[J].Biogeochemistry,2000, 48(1):7.

[4] CANADELL J G,RAUPACH M R.Managing forests for climate change mitigation[J].Science,2008,320 (5882):1456.

[5] DAVIDSON E A,JANSSENS I A.Temperature sensitivity of soil carbon decomposition and feedbacks to climate change[J].Nature,2006,440(7081):165.

[6] 吳迪,張蕊,高升華,等.模擬氮沉降對長江中下游灘地楊樹林土壤呼吸各組分的影響[J].生態(tài)學報, 2015,35(3):717. WU Di,ZHANG Ri,GAO Shenghua,et al.Effects of simulated nitrogen deposition on the each component of soil respiration in the Populous L.plantations in a riparian zone of the mid-lower Yangtze River[J].Acta Ecologica Sinica,2015,35(3):717.

[7] 森林生態(tài)系統(tǒng)服務功能評估規(guī)范:LY/T 1721—2008 [S].北京:標準出版社,2016:1. Specification for assessment of forest ecosystem services in China:LY/T 1721-2008.[S].Beijing:Standard Press of China,2016,1.

[8] 王華田,楊陽,王延平,等.外源酚酸對歐美楊‘I-107'水培幼苗硝態(tài)氮吸收利用的影響[J].植物生態(tài)學報, 2011,35(2):214. WANG Huatian,YANG Yang,WANG Yanping,et al. Effects of exogenous phenolic acids on nitrate absorption and utilization of hydroponic cuttings of Populus×euramericana‘Neva'[J].Chinese Journal of Plant Ecology,2011,35(2):214.

[9] 王延平,王華田,許壇,等.酚酸對楊樹人工林土壤養(yǎng)分有效性及酶活性的影響[J].應用生態(tài)學報,2013, 24(3):667. WANG Yanping,WANG Huatian,XU Tan,et al. Effects of exogenous phenolic acid on soil nutrient availability and enzyme activities in a poplar plantation[J]. Chinese Journal of Applied Ecology,2013,24(3):667.

[10] 許壇,王華田,王延平,等.楊樹人工林土壤養(yǎng)分有效性變化及其與土壤細菌群落演變的相關性[J].應用與環(huán)境生物學報,2014,20(3):491. XU Tan,WANG Huatian,WANG Yanping,et al.Correlation between soil nutrient availability and bacteria community succession in poplar plantations[J].Chinese Journal of Applied and Environmental Biology, 2014,20(3):491.

[11] 吳君君,楊智杰,劉小飛,等.米櫧和杉木人工林土壤呼吸及其組分分析[J].植物生態(tài)學報,2014,38 (1):45. WU Junjun,YANG Zhijie,LIU Xiaofei,et al.Analysis of soil respiration and components in Castanopsis carlesii and Cunninghamia lanceolata plantations[J].Chinese Journal of Plant Ecology,2014,38(1):45.

[12] RYAN M G,LAW B E.Interpreting,measuring,and modeling soil respiration[J].Biogeochemistry,2005, 73(1):3.

[13] TANG J,BALDOCCHI D D,Xu L.Tree photosynthesis modulates soil respiration on a diurnal time scale[J]. Global Change Biology,2005,11(8):1298.

[14] 秦越,李彬彬,武蘭芳.不同耕作措施下秸稈還田土壤CO2排放與溶解性有機碳的動態(tài)變化及其關系[J].農業(yè)環(huán)境科學學報,2014,33(7):1442. QIN Yue,LI Binbin,WU Lanfang.Dynamics and interrelationship of CO2emissions and dissolved organic carbon in soils with crop residue retention under different tillage practices[J].Journal of Agro-Envirnment Science,2014,33(7):1442.

[15] 臧逸飛,郝明德,張麗瓊,等.26年長期施肥對土壤微生物量碳、氮及土壤呼吸的影響[J].生態(tài)學報, 2015,35(5):1445. ZANG Yifei,HAO Mingde,ZHANG Liqiong,et al. Effects of wheat cultivation and fertilization on soil microbial biomass carbon,soil microbial biomass nitrogen and soil basal respiration in 26 years[J].Acta Ecologica Sinica,2015,35(5):1445.

[16] 許婷婷,李紅麗,王峰,等.I-107楊樹連作對土壤有機碳和全氮的影響[J].水土保持學報,2014,28 (5):182. XU Tingting,LI Hongli,WANG Feng,et al.Effect of continuous cropping of I-107 poplar plantation on soil organic carbon and total nitrogen[J].Journal of Soil and Water Conservation,2014,28(5):182.

[17] 陳莉莎,張金池,陸茜,等.楊樹多代連作對土壤養(yǎng)分特征和生物活性的影響[J].南京林業(yè)大學學報(自然科學版),2014,38(5):85. CHEN Shasha,ZHANG Jinchi,LU Xi,et al.Effects of continuous planting of poplars on soil biological activity and nutrients[J].Journal of Nanjing Forestry University(Natural Sciences Edition),2014,38(5):85.

[18] HUBBARD R M,RYAN M G,ELDER K,et al.Seasonal patterns in soil surface CO2flux under snow cover in 50 and 300 year old subalpine forests[J].Biogeochemistry,2005,73(1):93.

[19] INURE T,KOIZUMI H.Effects of environmental factors upon variation in soil respiration of a Zoysia japonica grassland,central Japan[J].Ecological research, 2012,27(2):445.

[20] DUNNE J,HARTE J,TAYLOR K.Response of subalpine meadow plant reproductive phenology to manipulated climate change and natural climate variability[J]. Ecological Monograph,2003,73(5):69.

[21] CHEN B,LIU S,GE J,et al.Annual and seasonal variations of Q10 soil respiration in the sub-alpine forests of the Eastern Qinghai-Tibet Plateau,China[J].Soil Biology and Biochemistry,2010,42(2):1735.

[22] MIKAN C J,SCHIMEL J P,DOYLE A P.Temperature controls of microbial respiration in arctic tundra soils above and below freezing[J].Soil Biology&Biochemistry,2002,34(11):1785.

[23] WAN S Q,LUO Y Q.Substrate regulation of soil respiration in a tallgrass prairie:results of a clipping and shading experiment[J].Global Biogeochemical Cycles, 2003,17(2):23

[24] WANG C,YANG J,ZHANG Q.Soil respiration in six temperate forests in China[J].Global Change Biology, 2006,12(11):2103.

[25] BUCHMANN N.Biotic and abiotic factors controlling soil respiration rates in Picea abies stands[J].Soil Biology&Biochemistry,2000,32(11):1625.

[26] LEE M S,NAKANE K,NAKATSUBO T,et al.Seasonal changes in the contribution of root respiration to total soil respiration in a cool-temperature deciduous forest [J].Plant and Soil,2003,255(5):311.

[27] BOND-LAMBERTY B,WANG C K,GOWER S T.A global relationship between the heterotrophic and autotrophic components of soil respiration[J].Global Change Biology,2004,10(10):1756.

[28] 唐羅忠,葛曉敏,吳麟,等.南方型楊樹人工林土壤呼吸及其組分分析[J].生態(tài)學報,2012,32(22):7000. TANG Luozhong,GE Xiaomin,WU Lin,et al.Partitioning of autotrophic and heterotrophic soil respiration in southern type poplar plantations[J].Acta Ecologica Sinica,2012,32(22):7000.

[29] SAURETTE D D,CHANG S X,THOMAS B R.Autotrophic and heterotrophic respiration rates across a chronosequence of hybrid poplar plantations in northern Alberta[J].Canadian Journal of Soil Science,2008,88 (3):261.

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

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

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

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