耿 靜,程淑蘭,方華軍,于貴瑞,徐敏杰,王 磊,李曉玉,司高月,何 舜

1 中國科學(xué)院地理科學(xué)與資源研究所,生態(tài)系統(tǒng)觀測與模擬重點實驗室, 北京 100101 2 中國科學(xué)院大學(xué)資源與環(huán)境學(xué)院, 北京 100049

氮素類型和劑量對寒溫帶針葉林土壤N2O排放的影響

耿 靜1,2,程淑蘭2,方華軍1,*,于貴瑞1,徐敏杰2,王 磊1,李曉玉1,司高月2,何 舜2

1 中國科學(xué)院地理科學(xué)與資源研究所,生態(tài)系統(tǒng)觀測與模擬重點實驗室, 北京 100101 2 中國科學(xué)院大學(xué)資源與環(huán)境學(xué)院, 北京 100049

大氣氮沉降；土壤N2O通量；氮素有效性；主控因子；北方森林

氧化亞氮(N2O)是地球大氣中三大溫室氣體之一,百年尺度上單分子N2O的增溫潛勢(GWP)是CH4和CO2的21倍和298倍,對全球變暖的貢獻約占6%[1]。同時,對流層中的N2O通過擴散進入平流層,與O3反應(yīng)生成NO破壞臭氧層,增加地面紫外輻射量[2]。IPCC第五次報告表明,2011年大氣中N2O濃度為324 ppb,較工業(yè)化前的數(shù)值高出20%,近幾十年平均增幅為0.25%/a。大氣N2O的源估計為17.7 Tg N/a,自然源(農(nóng)業(yè)、水體、生物燃燒等)和人為源(自然植被土壤、海洋)分別占37.8%和62.2%；大氣N2O的匯估計為12.6 Tg N/a,主要在平流層被光解為NOx,最終轉(zhuǎn)化成硝酸和硝酸鹽等反應(yīng)性氮[3]。其中,60%—70%的大氣反應(yīng)性氮以干濕沉降形式到達地表,導(dǎo)致當(dāng)前全球大氣氮沉降量高達105 Tg N/a[4-5],顯著改變陸地生態(tài)系統(tǒng)碳、氮循環(huán),降低生物多樣性,進而影響陸地生態(tài)系統(tǒng)的結(jié)構(gòu)和功能[6]。

北方森林(Boreal forests)是僅次于熱帶森林的第二大森林群區(qū),占全球陸地面積的14.5%,其土壤碳密度平均為296 t C/hm2[17]；此外,由于該區(qū)溫度較低,土壤氮素礦化緩慢,土壤有效氮極其匱乏,對外源性氮素響應(yīng)十分敏感[18]。研究表明,外源性氮素輸入會顯著改變北方森林植物和微生物群落組成[19]、土壤碳氮轉(zhuǎn)化與溫室氣體排放[20]、生態(tài)系統(tǒng)生產(chǎn)力和固碳潛力[21]。過去普遍認為,水熱條件較好、土壤風(fēng)化強烈的熱帶/亞熱帶森林土壤N2O排放較高,而溫度較低、有效氮貧乏的高緯度地區(qū)森林土壤N2O排放量可以忽略不計[22-23]。然而,近年來一些研究發(fā)現(xiàn),由于氣溫升高和氮沉降增加提高了高緯度地區(qū)森林、苔原等自然生態(tài)系統(tǒng)氮素的可利用性,導(dǎo)致該區(qū)土壤也大量排放N2O[24-25]。長期以來對高緯度地區(qū)自然植被土壤N2O排放的忽視,可能是導(dǎo)致全球N2O收支研究中諸多不確定性的原因之一[26]。大興安嶺寒溫帶針葉林是北方森林的南緣,面積占全國森林面積的29%,有關(guān)土壤N2O對外源性氮素輸入尚未有實驗報道。

1 材料與方法

1.1 研究區(qū)概況

研究區(qū)位于內(nèi)蒙古大興安嶺森林生態(tài)系統(tǒng)國家野外科學(xué)觀測研究站以東的開拉氣林場(50°20′—50°30′N, 121°45′—122°00′E),屬大興安嶺西北坡,海拔826 m。該地區(qū)是寒溫帶半濕潤氣候,冬季寒冷漫長,夏季涼爽多雨。年均氣溫-5.4℃,最高溫出現(xiàn)在7月,最低溫在1月。年降水量450—550 mm,其中60%集中于5—9月。年均日照2594 h,全年地表蒸發(fā)量800—1200 mm,無霜期80 d。該區(qū)主要物種為興安落葉松(Larixgmelini)、白樺(Betulaplatyphylla)、杜鵑(Rhododendronsimsii)、杜香(Ledumpalustre)、紅豆越橘(VacciniumVitisidaea)等。研究區(qū)的植被類型為杜香-落葉松林,林齡約150a。土壤類型為發(fā)育于花崗巖殘積物上的棕色針葉林土,土壤腐殖質(zhì)含量10%—30%,pH值為4.5—6.5。

1.2 試驗設(shè)計

1.3 土壤N2O排放通量監(jiān)測

土壤N2O排放量采用靜態(tài)箱-氣相色譜法測定,測定時段為2013年生長季(5—10月)。在每個樣地中分別設(shè)置帶槽的底座(50 cm×50 cm×10 cm)和蓋箱(50 cm×50 cm×20 cm),在測定時,將槽內(nèi)灌滿水,打開風(fēng)扇的電源,然后小心地把帶有溫度計和小風(fēng)扇的蓋箱沿槽放入。在40 min 時間段內(nèi),每隔10 min用100mL注射器抽取1次氣樣,同時記錄大氣溫度、箱內(nèi)溫度和地下5 cm的溫度值。測定N2O時氣相色譜的柱箱溫度為55 ℃,檢測器ECD的溫度為250℃；載氣(干空氣及高純H2)流量分別為300 mL/min和50 mL/min,尾吹氣(N2)流量為10 mL/min。利用土壤水分儀(TDR200,Spectrum Technologies, USA)測定10 cm土壤體積含水量。用氣相色譜儀(7890A,Agilent,USA)分析N2O氣體濃度,利用下述公式計算土壤N2O氣體通量：

(1)

式中,Qt為t時刻N2O的排放通量(μg N m-2h-1)；V為箱體的體積(m3)；A為取樣時箱體所覆蓋的面積(m2)；Ta為取樣時的大氣溫度(K)；P為取樣時的大氣壓值(kPa)；ΔC為Δt時間內(nèi)箱體內(nèi)N2O濃度增量(ppb)；Δt為時間變化量(s)。

1.4 土壤采集與分析

1.5 統(tǒng)計分析

利用重復(fù)測定方差分析(RANOVA)比較不同施氮水平和施氮類型對土壤溫度、含水量、無機氮含量和土壤N2O通量的影響,利用Tukey′s HSD進行均值間的多重比較。采用一元和多元逐步回歸分析方法探討土壤N2O通量與土壤環(huán)境因子之間的關(guān)系。所有數(shù)據(jù)利用SPSS 16.0軟件進行分析,利用SigmaPlot 12.5軟件進行繪圖。

2 結(jié)果與分析

2.1 土壤溫度和水分

整個生長季,土壤5cm溫度季節(jié)變化顯著,整體上呈現(xiàn)單峰季節(jié)變化(表1,P< 0.001)。對照處理土壤溫度最高值和最低值分別出現(xiàn)在7月初和5月初,平均變化范圍為0.70—15.03℃(圖1)。不同施氮處理下,土壤溫度變化格局相似,增氮對土壤溫度無明顯影響(表1)。

表1 月份、施氮水平、施氮類型對土壤N2O通量、土壤溫度、水分和無機氮含量影響的重復(fù)測量方差分析

Table 1 Repeated measures ANOVA of effects of month, N level and N form on soil N2O fluxes, soil temperature, soil moisture and inorganic N contents

變異來源Sourceofvariance土壤溫度Soiltemperature土壤水分Soilmoisture土壤N2O通量SoilN2Oflux土壤NO-3-N含量SoilNO-3-Ncontent土壤NH+4-N含量SoilNH+4-NcontentO層OlayerM層MlayerO層OlayerM層Mlayer組內(nèi)差異Withinsubjects(Multivariate)月份Month<0.001<0.001<0.0010.120.002<0.001<0.001月份×施氮水平Month×Nlevel0.980.020.180.430.160.710.06月份×施氮類型Month×Nform0.840.110.330.320.090.480.33組間差異Betweensubjects施氮水平Nlevel0.790.030.0030.050.14<0.001<0.001施氮類型Nform<0.0010.040.010.160.12<0.0010.003

圖1 土壤溫度和水分的季節(jié)變化及其對增氮的響應(yīng)Fig.1 The seasonal variations and responses of soil temperature and soil moisture to N additionCK：對照 control；LAC：低氮氯化銨 low-NH4Cl；LPN：低氮硝酸鉀 low-KNO3；LAN：低氮硝酸銨 low-NH4NO3；MAC：中氮氯化銨 medium-NH4Cl；MPN：中氮硝酸鉀 medium-KNO3；MAN：中氮硝酸銨 medium-NH4NO3；HAC：高氮氯化銨 high-NH4Cl；HPN：高氮硝酸鉀 high-KNO3；HAN：高氮硝酸銨 high-NH4NO3

整個生長季0—10 cm層土壤含水量季節(jié)波動明顯,呈逐漸遞減的趨勢(圖1)。由于5、6月份土壤處于凍融期,土壤含水量較高,5月初對照處理土壤含水量值最高(27.48%)。秋季降水明顯減少,8月中旬和9月末土壤含水量較低,最低值為5.86%。就某個月份而言,施氮水平對土壤含水量有顯著影響(表1,P=0.02)。施氮水平和施氮類型均顯著改變了土壤體積含水量(表1,P=0.03,P=0.04)。

2.2 土壤N2O通量

整個生長季土壤N2O排放通量季節(jié)變化顯著(表1,P< 0.001)。除7月份外,其他月份土壤N2O排放均很低,對照處理土壤N2O通量變化范圍為-1.19—5.13 μg N m-2h-1。施氮后土壤N2O排放急劇增加,7月份出現(xiàn)明顯的排放峰(47.77μg N m-2h-1)(圖2)。施氮水平和施氮類型均對土壤N2O有極顯著的影響(表1,P=0.003,P=0.01)。隨著增氮水平增加,土壤N2O排放量逐漸增加。就施氮類型而言,KNO3和NH4NO3的促進效應(yīng)顯著高于NH4Cl,說明硝態(tài)氮比銨態(tài)氮肥對土壤N2O排放量的影響更為顯著。與對照相比,施加NH4NO3對土壤N2O排放的促進效應(yīng)最強,不同施氮劑量處理土壤N2O通量的增幅度為442%—677%。

圖2 土壤N2O通量的季節(jié)變化及其對增氮的響應(yīng)Fig.2 The seasonal variations and responses of soil N2O fluxes to N addition

2.3 土壤無機氮含量

2.4 土壤N2O通量與土壤變量之間的關(guān)系

土壤變量Soilvariables回歸方程Equation決定系數(shù)R2P土壤溫度Soiltemperature(Ts)FN2O=0.68+0.37Ts0.040.0012有機層NH+4-N含量NH+4-NcontentinOlayer(NH+4-NO)FN2O=1.24+0.01NH+4-NO0.040.0053多元回歸MultipleregressionFN2O=-7.51+0.47Ts+0.011NH+4-NO+0.85LN+2.37MN+2.86HN0.27<0.001

3 討論

3.1 施氮類型和劑量對土壤無機氮累積的影響

3.2 施氮類型和劑量對土壤N2O通量的影響

4 結(jié)論

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The effects of types and doses of nitrogen addition on soil N2O flux in a cold-temperate coniferous forest, northern China

GENG Jing1, 2, CHENG Shulan2, FANG Huajun1,*, YU Guirui1, XU Minjie2, WANG Lei1, LI Xiaoyu1, SI Gaoyue2, HE Shun2

1KeyLaboratoryofEcosystemNetworkObservationandModeling,InstituteofGeographicalSciencesandNaturalResourcesResearch,ChineseAcademyofSciences,Beijing100101,China2CollegeofResourcesandEnvironment,UniversityofChineseAcademyofSciences,Beijing100049,China

atmospheric N deposition; soil N2O flux; N availability; controlling factors; boreal forest

國家自然科學(xué)基金項目(41471212, 31470558, 31290221, 31130009, 31290222)；國家重點基礎(chǔ)研究發(fā)展計劃項目(2012CB417103)；中國科學(xué)院地理科學(xué)與資源研究所“秉維”優(yōu)秀青年人才基金項目(2011RC202)；中國科學(xué)院戰(zhàn)略性先導(dǎo)科技專項(XDA05050600)

2015-08-04;

日期:2016-06-13

10.5846/stxb201508041639

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

耿靜,程淑蘭,方華軍,于貴瑞,徐敏杰,王磊,李曉玉,司高月,何舜.氮素類型和劑量對寒溫帶針葉林土壤N2O排放的影響.生態(tài)學(xué)報,2017,37(2):395-404.

Geng J, Cheng S L, Fang H J, Yu G R, Xu M J, Wang L, Li X Y, Si G Y, He S.The effects of types and doses of nitrogen addition on soil N2O flux in a cold-temperate coniferous forest, northern China.Acta Ecologica Sinica,2017,37(2):395-404.