排水对若尔盖高寒沼泽CO2和CH4排放通量的影响

周文昌,崔丽娟,王义飞,李 伟,康晓明

排水对若尔盖高寒沼泽CO2和CH4排放通量的影响

周文昌1,2,崔丽娟1,3*,王义飞1,3,李 伟1,3,康晓明1,3

(1.中国林业科学研究院湿地研究所,湿地生态过程与功能北京市重点实验,北京 100091；2.湖北省林业科学研究院,湖北 武汉 430075；3.四川若尔盖高寒湿地生态系统国家定位观测研究站,四川 若尔盖 624500)

建立3块标准样地(天然沼泽、1990s和1970s排水沼泽),于2014年生长季期间,采用静态箱-快速温室气体分析仪野外原位观测CO2和CH4排放通量.结果表明:沼泽排水增加了土壤温度(5,20,45cm),但降低沼泽水位;1990s[(680±329) mg CO2/(m2×h)]和1970s排水沼泽[(973±234) mg CO2/(m2×h)]生态系统CO2排放通量分别较天然沼泽增加了200%和330%,但CH4排放通量[(0.78±0.52) mg CH4/(m2×h)]和[(-0.01±0.02) mg CH4/(m2×h)]较天然沼泽分别降低了90%和100%;综合考虑两者排放通量,1990s[(186±89) mg C/(m2×h)]和1970s排水沼泽[(265±64) mg C/(m2×h)]生态系统碳(C)排放通量较天然沼泽分别增加了180%和300%.天然沼泽、1990s和1970s排水沼泽生态系统CO2排放通量与5cm土壤温度存在显著正相关,而仅1990s排水沼泽生态系统CO2排放通量与水位存在显著负相关.天然沼泽生态系统CH4排放通量与土壤温度(5,20,45cm)存在显著正相关,但1970s排水沼泽生态系统CH4排放通量与土壤温度(20,45cm)存在显著负相关,1990s排水沼泽生态系统CH4排放通量与水位存在显著正相关.沼泽排水显著增加了若尔盖高寒沼泽生态系统C排放通量,降低了沼泽C汇功能,可能增强区域气候变暖.

排水；CO2排放通量；CH4排放通量；温室气体；若尔盖沼泽

温室气体CO2、CH4和N2O是导致全球气候变暖的主要影响因素[1].天然沼泽地作为大气CO2的汇,同时作为大气CH4排放源[2-5],植被每年通过光合作用同化的CO2有4%~20%转化为CH4排放到大气中[6-8],约占全球CH4排放总量的20%~40%[1,9].天然沼泽因其长期具备的微弱碳汇[12.8~23g/(m2×a)],导致土壤储存了大约600×1015g 碳[10-11],相当于全球陆地土壤碳库的30%[10]和大气中碳库(828×1015g碳)的70%[1].因此,沼泽温室气体CO2和CH4排放通量变化,将影响着未来气候的变化趋势.

由于人类活动的加剧,大量沼泽地被人为疏干,排除沼泽地多余的水分和控制地下水位,是沼泽地开发利用的关键,沼泽地为林业、农业排水或为能源开采等利用,已导致全球5000万hm2的沼泽地损失[12].人类活动严重改变了沼泽地土壤地下水位,影响沼泽地温室气体CO2和CH4排放通量[13-15],从而影响沼泽地碳汇功能和大气中CO2和CH4含量.据Martikainen等[16]研究,北方泥炭地排水导致矿养型泥炭地(fen)和雨养型泥炭地(bog)的CO2和CH4排放通量存在差异,排水均是增加CO2排放通量和降低CH4排放通量;但是Strack等[17]研究的北方泥炭地排水,导致了微地貌草丘(hummock)区泥炭地CH4排放通量显著降低97%,而微地貌洼地(hollow)区泥炭地与对照地无显著影响,并且仍观测到较高CH4排放通量.不同泥炭地类型,排水导致沼泽地CO2和CH4排放通量具有显著差异,进而影响沼泽地CO2和CH4的排放通量.

若尔盖高寒沼泽地约46万hm2,分别占全国泥炭地面积和碳储量的12%和40%,是我国泥炭最大分布区,亦是世界最大高原沼泽之一[18].自1955年便开始对本区沼泽地开沟排水,到20世纪70年代,全区普遍采取大规模开沟排水,改造沼泽,经过多次开沟排水,累积达20万hm2沼泽采取了不同程度的改造,以增加区域草场用地,导致沼泽地大量锐减[18-21],目前有关该区域的沼泽地温室气体CO2和CH4排放通量的研究集中天然沼泽地[22-24],很少报道人类活动(如沼泽地排水)对若尔盖高寒沼泽地温室气体CO2和CH4排放通量的影响研究.作者选择若尔盖湿地自然保护区不同时期排水沼泽(1970s和1990s排水沼泽)和天然沼泽(未排水沼泽),采用静态箱和快速温室气体分析仪原位观测沼泽温室气体CO2和CH4排放通量,旨在揭示排水对若尔盖高寒沼泽CO2和CH4排放通量的影响规律及控制因子.

1 研究区域与研究方法

1.1 研究区概况

若尔盖湿地自然保护区(102°29′E~102°59′E, 33°25′N~34°80′N),面积16670.6hm2.若尔盖高原位于青藏高原的东北边隅,平均海拔3400~3700m,沼泽是本区域的特色,土壤主要类型为亚高山草甸土、草甸土和沼泽土[18].该区域属于高原寒温带湿润气候,常年无夏,11月至次年4月受西伯利亚和蒙古冷空气控制,5~10月受西南季风控制,无绝对无霜期,每年9月下旬土壤开始冻结,5月中旬完全解冻,冻土最深达72cm,年均气温0.7~1.0℃,最高气温和最低气温出现于7月和1月,各自气温均值10.7,-10.3℃,年均降水量650mm,集中在6~9月,相对湿度78%[24-26].

1.2 样地选择

2013年夏季期间在若尔盖湿地自然保护区选取天然沼泽(33°55.39′N,102°52.16′E,海拔3439m)、1990s(33°55.2′N,102°48.9′E,海拔3438m)和1970s排水沼泽(33°55.2′N,102°44.1′E,海拔3432m)的3个样地,3个样地位于花湖湖泊周边,沼泽地形成时间大概一致.天然沼泽水位接近地表,植物类型以木里薹草()和西藏嵩草()为主,伴生条叶垂头菊(),并发育许多微地貌草丘(hummocks)或洼地(hollows);为了扩大草场牧地,自1955年以来对本区域采取大规模开沟排水,疏干沼泽,经调研1990s排水沼泽地,地表无积水,湿润,植被类型主要以西藏嵩草为主,伴生少些花葶驴蹄草();1970s排水沼泽,地表无积水,极度干旱,主要植被类型以矮生嵩草()、鹅绒委陵菜()为主,伴生海乳草(),因样地排水和放牧多年,网状草丘已消失,出现危害草场动物黑唇鼠()和中华鼢鼠(sp.),随之出现较多鼠兔洞穴,样地趋向于退化阶段,该样地于2008年通过构筑坝堤的堵沟方法,沿着排水沟间隔100m建立坝堤,以抬升水位,实施湿地恢复工程,但是本研究期间,自2010年后,该坝堤已遭受雨季期的雨水冲刷而损坏,从而水位逐年降低.

1.3 CO2和CH4排放通量测量

2014年5~10月,开展野外实验研究,CO2和CH4排放通量测量采用静态箱和快速温室气体分析仪(FGGA,DLT-100,Los Gatos Research Inc.,San Jose, USA)原位观测[24].每个样地重复设置3个以上静态箱(天然沼泽3个,1990s排水沼泽3个,1970s排水沼泽设置9个静态箱,是由于样地排水沟相隔约为100、200和300m).静态箱由铝皮制作,规格为50cm× 50cm×50cm,为防止箱内温度波动,箱外用塑料泡沫包裹,箱内顶部有2个小型风扇,静态箱顶部中央附有2个橡皮塞小圆孔,连接快速温室气体分析仪的2根附有橡皮塞的透明导气管,长度约4m(内径为4mm),仪器通过12V的蓄电池供电,设置数据观测频率为1Hz[27-28].仪器启动后,在测量CO2和CH4排放通量前,将底座水槽加满水,防止顶箱扣在底座后,箱内气体泄漏.然后密闭静态箱,测量箱内顶部气体CO2和CH4含量,密闭测量5min(盖箱的读数时间),然后揭开静态箱,置于开放状态约2min,目的是等待箱顶部气体含量稳定,紧接着连续操作以上过程,测量下一个静态箱位置区沼泽地两种排放通量,直到完成所有测量样点.排放通量测量时间:对照地(天然沼泽)测量时间为5~9月,1990s排水沼泽测量为5~10月,1970s排水沼泽测量为5~9月,均是北京时间早上9:30~11:00.CO2和CH4排放通量是以封闭箱内顶部CO2和CH4含量随时间变化的直线斜率计算[27-28], CO2和CH4排放通量计算公式见文献[29].

同时,采用数字温度计(JM624,天津今明仪器有限公司),测量采样点5,20,45cm深度的地温;在静态箱附近挖了一个小井,测量地下水位,测量井的深度为地表之下100cm,如超出100cm深度,以-105cm记录水位值,水位直接用直尺测量,水位为正值表示水位位于土壤地表之上,水位为负值表示水位位于土壤地表之下.

采用独立样本检验方法,利用SPSS 18.0软件,CO2和CH4排放通量的差异性采用配对-检验.CO2和CH4排放通量与温度和水位环境因子采用Pearson相关分析.显著差异水平=0.05.利用Origin 9.0软件制图.

2 结果与分析

2.1 排水对沼泽土壤温度和水位的影响

CK: 天然沼泽; D90s: 1990s排水沼泽; D70s: 1970s排水沼泽

由图1可知,2014年5~9月观测期间,沼泽排水后,沼泽土壤温度升高.天然沼泽、1990s和1970s排水沼泽土壤5cm温度平均值分别为10.1,10.4, 12.9℃;土壤20cm温度平均值分别为10.3,10.6, 10.1℃;土壤45cm温度平均值分别为8.3,10.6, 8.9 ℃.沼泽排水后,水位显著降低(图1d).天然沼泽、1990s排水沼泽水位平均值分别为0.7, -56.8cm,而1970s排水沼泽水位观测期间,测量井100cm深度始终不见有水,从而以-105cm作为记录1970s排水沼泽水位.

2.2 排水对沼泽生态系统CO2和CH4排放通量的影响

2014年生长季期间,沼泽排水后,沼泽生态系统CO2排放通量增加(图2a).天然沼泽、1990s和1970s排水沼泽生态系统CO2排放通量(平均值±标准差)分别为96~402mg CO2/(m2×h)[(223±94) mg CO2/ (m2×h)]、95~1284mg CO2/(m2×h)[(680±329) mg CO2/ (m2×h)]和579~1329mg/(m2×h)[(973±234) mg/(m2×h)]. 1990s和1970s排水沼泽生态系统CO2排放通量平均值较天然沼泽分别增加了约200%和330%.配对-检验发现,天然沼泽与2个排水沼泽生态系统CO2排放通量平均值之间存在极显著差异(<0.01),2个排水沼泽生态系统CO2排放通量平均值之间存在显著差异(<0.05).

2014年生长季期间,沼泽排水后,沼泽生态系统CH4排放通量降低(图2b).天然沼泽、1990s和1970s排水沼泽生态系统CH4排放通量(平均值±标准差)分别为1.02~17.43mg CH4/(m2×h)[(7.04±4.46) mg CH4/(m2×h)]、0.11~2.06mg CH4/(m2×h)[(0.78±0.52) mg CH4/(m2×h)]和-0.04~0.04mg CH4/(m2×h)[-0.01± 0.02] mg CH4/(m2×h)](负值为大气CH4汇).1990s和1970s排水沼泽生态系统CH4排放通量平均值较天然沼泽降低了约90%和100%.配对-检验发现,天然沼泽、1990s和1970s排水沼泽生态系统CH4排放通量平均值之间存在极显著差异(<0.01).

综合沼泽生态系统CO2和CH4排放通量,2014年生长季期间,沼泽排水后,沼泽生态系统碳(C)排放通量显著增加(图2c).天然沼泽、1990s和1970s排水沼泽生态系统C排放通量(平均值±标准差)分别为29~123mg C/(m2×h)[(66±28) mg C/(m2×h)]、25~ 351mg C/(m2×h)[(186±89) mg C/(m2×h)]和158~ 362mg C/(m2×h)[(265±64) mg C/(m2×h)].1990s和1970s排水沼泽生态系统C排放通量较天然沼泽增加了约180%和300%.配对-检验发现,天然沼泽与2个排水沼泽生态系统C排放通量平均值之间存在极显著差异(<0.01),2个排水沼泽生态系统C排放通量之间存在显著差异(<0.05).

CK:天然沼泽; D90s: 1990s排水沼泽; D70s: 1970s排水沼泽

2.3 沼泽生态系统CO2和CH4排放通量与土壤温度和水位的关系

2014年生长季期间,天然沼泽、1990s和1970s排水沼泽生态系统CO2排放通量与5cm土壤温度均存在显著线性正相关(<0.05),仅1990s排水沼泽生态系统CO2排放通量与水位存在显著线性负相关(<0.05,表1).天然沼泽生态系统CH4排放通量与土壤温度(5,20,45cm)存在显著线性正相关(<0.05),然而1970s排水沼泽生态系统CH4排放通量与20,45cm土壤温度存在显著负相关(<0.05),仅1990s排水沼泽生态系统CH4排放通量与水位存在显著线性正相关(<0.05,表1).

表1 沼泽生态系统CO2和CH4排放通量与温度和水位的相互关系

CK: 天然沼泽; D90s: 1990s排水沼泽; D70s: 1970s排水沼泽.

3 讨论

3.1 与其他研究的沼泽地CO2和CH4排放通量的比较

本研究沼泽CO2[96~1329mg CO2/(m2×h)]和CH4排放通量[-0.04~17.43mg CH4/(m2×h)]与其他研究范围吻合[CO2排放通量:14~1050mg CO2/(m2×h), CH4排放通量:-0.02~86.78mg CO2/(m2×h),表2],不过有研究表明,北方排水泥炭地生态系统CO2排放通量峰值高达3310mg CO2/(m2×h)[33].然而,泥炭地排水多集中在温带和寒温带沼泽区域,这些实验研究表明,沼泽地排水后,水位的降低,CO2排放通量增加达30%~380%[16,33,36-37],这与本研究增加的比例吻合(200%和330%).而CH4排放通量降低达60%~ 98%[16,36,38]或转为大气CH4吸收汇[13,38-39],这也与本研究降低的比例吻合(90%和100%)(表2).

表2 不同区域沼泽湿地CO2和CH4排放通量

3.2 水位和温度对沼泽地CO2和CH4排放通量的影响

据研究报道,沼泽地CO2和CH4排放通量主要受水位和温度的影响[4,16,36-37].沼泽地排水,降低了沼泽地水位深度,2014年生长季5~9月上旬期间,1990s排水沼泽水位低于地表之下60cm深度,1970s排水沼泽水位低于地表之下100cm,沼泽水位的降低,将增加了空气进入下层土壤,加速土壤有机质分解,增加土壤呼吸通量(CO2排放通量).另外,沼泽水位下降,土壤暴露于太阳辐射下,土壤温度增加,土壤微生物活性增强,进而提高土壤呼吸,增大了土壤呼吸通量[40].这可从本研究沼泽地CO2排放通量与5cm土壤温度均存在显著线性相关得到证实,与其他研究的北方沼泽的结论吻合[16,36-37,41].沼泽地排水降低了CH4排放通量[16,38,42],这是由于沼泽地释放到大气中的CH4是由产甲烷菌在严格厌氧条件下产生的[24,43],沼泽地在CH4产生量很低的情况下,还需经过甲烷氧化菌氧化CH4,从而减少CH4排放通量[14].因此,水位在调控沼泽CH4排放通量的过程中发挥关键性作用[43-44].在水位较高条件下,随着气温增加,沼泽土壤温度增加,植被生产力也逐渐增加,从而增强产甲烷菌活性和产CH4量,通常加速CH4生成和大量CH4释放[22,44],这就是沼泽地CH4排放通量常与土壤温度存在显著线性正相关和在夏季7月或8月观测到峰值的原因[22,44].反之,沼泽水位降低后,CH4排放通量与土壤温度存在显著负相关或不相关[42,44].这是由于当沼泽受到干扰后,沼泽CH4产生和释放过程的环境因子(水位、温度和植被类型)发生了改变[45],导致沼泽CH4排放通量与环境因子之间的关系趋向复杂[46],很难再使用沼泽CH4排放通量与环境因子(水位或地温)之间的关系定量化[45].因此,人为干扰(如森林沼泽采伐、沼泽排水)改变了原先沼泽CH4排放通量与土壤温度或与水位的相互关系[46-47],这可能是1990s排水沼泽CH4排放通量与水位存在正相关,1970s排水沼泽CH4排放通量与土壤温度存在负相关的原因.

3.3 沼泽地排水对C汇功能和温室效应的影响

综述以上结果,沼泽排水增加CO2排放通量,但降低CH4排放通量.本研究表明,排水沼泽地C排放通量较天然沼泽显著增加了180%和300%,这可能降低沼泽作为大气的C汇强度,从而降低沼泽土壤碳储量.这种推测验证了本团队曾研究的若尔盖高寒沼泽排水导致土壤有机碳储量降低了349.7t C/hm2的结论[48],这说明沼泽排水通过显著增加C排放通量,降低了土壤碳储量,使得沼泽地从大气C吸收汇转为大气C排放源,随后降低碳汇功能.但据Renou-Wilson等[49]研究表明排水的雨养型泥炭地还湿后,并加强管理,已使得泥炭地转变为碳汇.因此,在今后的沼泽地管理中,要避免沼泽地排水的管理方式,从而通过项目工程加紧堵塞沼泽排水沟,不断抬升沼泽地下水位,减少温室气体排放,从而增强沼泽固碳功能.

在全球气候变暖的背景下,可能降低沼泽水位,并增加土壤温度.据若尔盖高寒区域近50年的气候变化(1957~2011年)表明,该区域降水量以每年-0.978mm的速率减少,气温和潜在蒸发量却以每年0.029℃和0.755mm的速率增加[50],该区域将可能趋于气候变暖变干,可能降低该地区沼泽水位和增加土壤温度,从而将影响温室气体排放通量[51].因此,根据本研究结论,未来将增加天然沼泽、排水沼泽CO2排放通量,但对于天然沼泽来说,可能增加CH4排放通量,也可能降低CH4排放通量,从而存在不确定性;反而对于排水沼泽,一定会降低CH4排放通量.根据IPCC第五次评估报告,CH4单分子(摩尔)的温室效应跨过100年时间是CO2的34倍[1];再根据Ding等[26]预算若尔盖高寒沼泽生长季CH4排放量以165d计算,基于这两个数值和本研究的生长季沼泽生态系统CO2和CH4排放通量平均值,估算1990s和1970s排水沼泽温室气体CO2当量(分别为61和88mol CO2/m2)较天然沼泽(20mol CO2/m2)增加了200%和335%,这种大量的温室气体CO2排放到大气中,将加速该区域气候变暖.如果不加以人类干扰控制沼泽地的破坏活动和恢复退化的沼泽地,最终会使得这种气候变化进入到一种恶性循环中.

4 结论

4.1 若尔盖高寒沼泽地排水,显著增加了CO2排放通量,但降低了CH4排放通量,甚至1970s排水沼泽地转为大气CH4吸收汇.综合考虑生态系统C排放通量,沼泽地排水显著增加了C排放通量达180%~ 300%.

4.2 3种沼泽地(天然沼泽地、1990s和1970s排水沼泽地)CO2排放通量与5cm土壤温度均存在正相关,仅1990s排水沼泽地CO2排放通量与水位存在显著负相关.天然沼泽地CH4排放通量与土壤温度(5,20,45cm)存在显著正相关,而1970s排水沼泽地CH4排放通量与土壤温度(20,45cm)存在显著负相关,仅1990s排水沼泽地CH4排放通量与水位存在显著正相关.

4.3 若尔盖高寒区域的气候变化可能导致沼泽地温室气体CO2和CH4的排放存在不确定.2种排水沼泽地温室气体CO2当量(含CH4折算)比未排水沼泽地增加了200%和335%,这种人类活动(如沼泽地排水)可能加剧该区域气候变暖.

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Effects of drainage on the CO2and CH4emission fluxes in the Zoigê Plateau Marsh.

ZHOU Wen-chang1,2, CUI Li-juan1,3*, WANG Yi-fei1,3, LI Wei1,3, KANG Xiao-ming1,3

(1.Beijing Key Laboratory of Wetland Services and Restoration, Institute of Wetland Research, Chinese Academy of Forestry, Beijing 100091, China；2.Hubei Academy of Forestry, Wuhan 430075, China；3.Sichuan Zoigê Wetland Ecosystem Research Station, Ruoergai 624500, China)., 2019,39(7)：3040~3047

The CO2and CH4emission fluxes in the Zoigê Plateau Marsh were monitored by using the dark static chamber and Fast Greenhouse Gas Analyzer at three typical sampling sites (natural marsh, drainage marshes in the 1990s and 1970s) from May to October in 2014. The results showed that the drainage increased the soil temperature (5, 20 and 45cm depth) and reduced the water level. The ecosystem CO2emission fluxes of the drained marshes in 1990s [(680±329) mg CO2/(m2×h)] and 1970s [(973±234) mg CO2/(m2×h)] were 200% and 330% higher than that in the natural marsh. While, the ecosystem CH4emission fluxes of the drained marshes in 1990s [(0.78±0.52) mg CH4/(m2×h)] and 1970s [(-0.01±0.02) mg CH4/(m2×h)] were 90% and 100% lower than that in the natural marsh, respectively. Taking into consideration of both CO2and CH4emission fluxes, the ecosystem carbon (C) emission fluxes of the drained marshes in 1990s [(186±89) mg C/(m2×h)] and 1970s [(265±64) mg C/(m2×h)] were 180% and 300% higher than that in the natural marsh. Besides, there was a significant positive correlation between the ecosystem CO2emission fluxes and the soil temperature of 5cm depth in the three typical sampling sites. Whereas, there was an obvious negative correlation between the ecosystem CO2emission flux and the water level in the 1990s drained marsh. In addition, the ecosystem CH4emission flux in the natural marsh was notably positively correlated with the soil temperatures at 5, 20 and 45cm depth. However, the ecosystem CH4emission flux in the 1970s drained marsh was remarkably negatively correlated with the soil temperatures at 20 and 45cm depth. Meanwhile, we also found that there was a significant positive relation between the ecosystem CH4emission flux in the 1990s drained marsh and the water level. This research suggested that the marsh drainage could significantly increase the ecosystem C emission flux from the soil into the atmosphere in the Zoigê Plateau Marsh, subsequently decrease the soil C sink function, and further enhance the regional climate warming.

drainage；CO2emission flux；CH4emission flux；greenhouse gas；Zoigê Plateau Marsh

X53

A

1000-6923(2019)07-3040-08

周文昌(1983-),贵州镇远人,助理研究员,博士,主要研究方向为湿地生态学.发表论文20余篇.

2018-12-01

国家重点研发计划项目(2016YFC0501804);中央级公益性科研院所基本科研业务费专项(CAFYBB2017QB009);国家林业局公益行业科研专项项目(201204201)

* 责任作者, 研究员, lkyclj@126.com