程学刚,张彩虹,杨焕祥,刘浩栋,付战勇,张光灿,李传荣

(泰山森林生态系统国家定位观测研究站山东农业大学农业生态与环境重点实验室,271018,山东泰安)

连作对杨树人工林土壤呼吸及各组分的影响

程学刚,张彩虹,杨焕祥,刘浩栋,付战勇,张光灿,李传荣†

(泰山森林生态系统国家定位观测研究站山东农业大学农业生态与环境重点实验室,271018,山东泰安)

土壤呼吸是整个陆地生态系统碳循环的关键过程之一。以山东大汶河沿岸沙地不同连作代数杨树人工林(1代林、2代林和3代林)为研究对象,利用ACE自动土壤呼吸监测系统(UK),对3种林分一个生长季(4—10月)的土壤呼吸速率及温湿度进行测定,同时采用壕沟法对3种林分的土壤呼吸进行组分分离,并对土壤呼吸及各组分与土壤温湿度的关系进行模型模拟。结果表明:3种林分的土壤呼吸速率(RS)、自养呼吸速率(RA)和异养呼吸速率(RH)的月变化均为明显的单峰格局;生长季内,3种林分RA贡献率月差异明显,平均贡献率为40.04%;RS及其组分与5 cm处土壤温度存在显著指数关系,与土壤体积含水量没有相关性,土壤温度与土壤体积含水量的复合模型对土壤呼吸速率变化解释能力为80%～94%;3种林分生长季平均土壤呼吸速率分别为3.12、3.08和2.66 μmol/(m2·s),3代林RS和RH均显著低于1代林和2代林。连作导致杨树人工林地土壤呼吸速率减弱,土壤理化性质和微生物量的差异是导致林分间土壤呼吸速率差异的主要原因。揭示连作对杨树人工林土壤呼吸及各组分的影响,以及作用机制,为全面探究杨树人工林连作效应及土壤碳循环,提供数据支撑。

自养呼吸;连作;异养呼吸;壕沟法

自工业革命以来,大气中温室气体浓度急剧增加,其中CO2浓度在250年间升高约32%[1],由此引起的全球变暖是人类目前所面临的主要环境问题之一。地球上的CO2库主要是土壤和海洋,前者约为后者的2倍[2]。土壤碳排放的主要途径是土壤呼吸,每年向大气中释放大约80～110 Pg[3],是化石燃料燃烧年排放量的13倍[4];因此,任何土壤呼吸速率的微小变化,都会对大气CO2浓度乃至整个生态系统碳循环产生较大影响[5]。准确掌握土壤呼吸对土壤碳排放的影响,在应对全球气候变化中具有重要的意义。

杨树(PopulusL.)是我国最重要的速生丰产树种,到2009年,全国杨树人工林总面积已经达到757.23万hm2[6],在缓解我国木材市场的供需矛盾中发挥了重要的作用;此外,杨树人工林可降低风速,增加地表粗糙度,亦能够改良土壤,增加黏粒质量分数,以此减弱河岸沙地的风蚀,发挥一定的水土保持效益。在栽植面积快速增长的同时,由于多采用短轮伐期集约栽培体制,杨树连作的现象也越来越普遍,由此产生的杨树人工林连作效应逐渐显露出来,限制了其生态系统服务功能的实现[7]。目前研究多集中于其根系分泌物的化感作用、土壤养分有效性以及微生物区系等方面。研究认为,连作会使林分生长放缓,树高降低,同时令土壤理化性质下降,显著影响林地生产力和水土保持效益的实施[8-10];然而,杨树连作是否会影响土壤呼吸,以及进一步的土壤碳循环过程,目前尚不清楚:因此,笔者以大汶河沿岸沙地不同连作代数杨树人工林为研究对象,定位观测土壤呼吸及相关环境因子,旨在揭示连作对杨树人工林土壤呼吸及各组分的影响,以及作用机制,为全面探究杨树人工林连作效应及土壤碳循环,提供数据支撑。

1 研究区概况

研究区位于山东省泰安市宁阳县国有高桥林场(E 116°52＇,N 35°54＇)。该地区属温带大陆性半湿润季风气候区,四季分明,夏季高温多雨,冬季寒冷干燥。年均降水量689.6 mm,年际变化大,且年内分配不均,主要集中在6—9月,蒸发量1 428.8 mm。多年平均气温13.7℃,≥10℃的活动积温4 493.3℃,日照总时间2 679.3 h,无霜期206 d。土壤类型为沙壤质潮土,土壤黏粒较少,保水保肥性差,春季常有大风沙尘天气,微度风蚀。林下草本植物主要有狗尾草(Setaria viridis(L.)Beauv.)、荩草(Arthraxon hispidus(Thunb.)Makino)、小飞蓬(Conyza canadensis(L.)Cronq.)、反枝苋(Amaranthus retroflexusL.)、葎草(Humulusscandens(Lour.) Merr.)、蒺藜(Tribulus terresterL.)、鬼针草(Bidens pilosaL.)等,盖度为60%～90%。不同连作代数的杨树人工林基本概况见表1。

表1 不同连作代数的杨树人工林基本概况Tab.1 Basic profile of poplar plantations in different continuous cropping systems

2 材料与方法

2.1 样地选择

选择立地条件和林龄相似的I-107杨树(Populus×euramericanacv.‘Neva')人工林,其中1代林(首次造林)、2代林(皆伐一轮后造林)和3代林(皆伐两轮后造林)各1块样地,轮伐期为15年。各林地经营管理措施一致,造林株行距均为3 m×5 m。3种林分在首次造林前均为农田,各林地土壤特性见表2。

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

2.2 测定方法

2014年12月,分别在3个林分内,各布设2个20 m×20 m的样地。每个样地内分别设置保留根系(对照)和切断根系(视为异养呼吸)2种处理,每个处理6个重复。把对照呼吸速率与断根处理呼吸速率的差值作为自养呼吸速率。断根处理采用壕沟法[11],选择1 m×1 m的区域,四周垂直挖深1 m以切断根系,在其周围布设100目的尼龙网,然后回填土壤。土壤呼吸速率采用ACE自动土壤呼吸监测系统(UK)测定。2015年4—10月,选择每月中旬,且雨后至少5 d的晴朗天气,使用3台仪器,对3个样地进行同步观测,每次测定时间间隔为2 h,连续测定24 h。为消除土壤扰动带来的影响,每次测定前24 h,埋设钢圈并剪除杂草。采用ACE自带的温湿度传感器,同步测定地下5 cm处的土壤温度和土壤体积含水量。

2015年6月,进行3种林分的土壤样品采集和土壤特性测定。土壤密度采用环刀法测定,土壤全氮采用半微量凯氏定氮法测定,用电位法测定土壤pH值,土壤有机碳测定采用重铬酸钾外加热法,土壤微生物量碳采用氯仿熏蒸-K2SO4浸提法测定。每个样地3个重复,每个林分的样本数n=6。

2.3 数据处理

采用常用的3种经验模型,对土壤温度、土壤含水量和土壤呼吸的关系,进行拟合分析。[10]

式中:RS为土壤呼吸速率,μmol·m-2·s-1;t为5 cm深处的土壤温度,℃;W为5 cm深土壤体积含水量,%;a、b、c为待定参数。

方差分析和回归分析利用IBM SPSS 20.0软件进行,显著性水平设定为α=0.05。图表采用Qrigin 7.5和Microsoft Excel 2003绘制。

3 结果与分析

3.1 连作对土壤呼吸及各组分的影响

整个生长季,3个杨树人工林地的土壤呼吸(RS)、自养呼吸(RA)和异养呼吸(RH)的月变化趋势一致,4月最小,7月最大,均为明显的单峰曲线(图1)。1代林、2代林和3代林生长季土壤呼吸速率的变化范围分别为1.42～6.01、1.07～5.91和1.21～5.13 μmol/m2·s,平均值分别为3.12、3.08和2.66 μmol/m2·s。由表3可知,RS随连作代数的增加有下降趋势。单因素方差分析显示,1代林和2代林显著高于3代林(P＜0.05),1代林同2代林之间无显著差异(P＞0.05)。其中,3种林分的自养呼吸速率差异不显著;而在异养呼吸速率方面,1代林和2代林显著高于3代林。

图1 土壤呼吸速率及各组分的月变化(平均值±标准偏差)Fig.1 Monthly change of soil respiration rate and its components(mean±SD)

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

3.2 连作对土壤呼吸各组分贡献率的影响

图2显示,3种林分自养呼吸贡献率平均值为40.04%,1代林、2代林和3代林分别为39.39%、38.47%和42.25%,三者之间无显著差异(P＞0.05)。不同月份相比,夏季的6—8月,3个月自养呼吸占土壤呼吸的比值相对较高,其余4、5、9和10月,均低于总平均值。

图2 3种林分自养呼吸贡献率及其总平均值Fig.2 Contribution rates of autotrophic respiration and overall means of three forest types

3.3 土壤呼吸及各组分与土壤温湿度的关系

图3显示,3个林分5 cm处土壤温度呈现明显的单峰曲线,无显著差异(P＞0.05),4月份为最低值,7月份达到最高值。土壤含水量呈现较强的变异性,3个林分同样无显著差异。

分别采用指数模型、线性模型和幂-指数模型,对土壤呼吸及各组分与土壤温湿度进行拟合。结果显示,土壤呼吸速率与5 cm处土壤温度之间呈极显著的指数关系(P＜0.01),土壤温度能够解释3种林分土壤呼吸75%以上的变化(表4)。土壤呼吸速率与5 cm深处土壤含水量之间无相关关系(P＞0.05)。采用双因素模型,对土壤呼吸与土壤温度和土壤含水量进行模拟,发现R2值较单纯的指数模型均有不同程度的提高,土壤温湿度能够共同解释3种林分土壤呼吸90%～94%的变化;因此,幂-指数模型能够更好地模拟三者的关系。

图3 3种林分土壤温度和含水量月变化(平均值±标准偏差)Fig.3 Monthly variation of soil temperature and soil water content in three polar plantations(mean±SD)

表4 土壤呼吸速率与土壤温度(t)和土壤湿度(w)不同模型的参数Tab.4 Parameters for different models showing the relationships of soil respiration with soil temperature(t)and soil water content(W)

4 结论和讨论

4.1 连作对杨树人工林土壤呼吸及各组分的影响

研究结果表明,土壤呼吸的月变化呈单峰型,生长旺季RS显著高于非生长旺季(图1),该趋势与土壤温度的变化一致(图3)。生长旺季植物活根系及微生物活动最剧烈,从而导致其RS最高。杨树连作3代之后,RS和RH较1代林和2代林有明显下降(表3)。试验期间,3种林分的土壤温度和湿度并无显著差异(P＞0.05),可以断定土壤温湿度并非是引起3种林分RS差异的主要原因;同时,3种林分RA无显著差异(表2):因此,其RS的差异主要体现在RH的差异上。除土壤温湿度外,土壤环境的生物物理学性质(微生物数量和活性、土壤物理性质)以及底物可利用性(土壤有机质质量分数)等,被认为是土壤异养呼吸的主要影响因素[12-13]。秦越等[14]研究发现,RS与土壤有机质质量分数呈正相关关系;臧逸飞等[15]也认为土壤有机质、土壤微生物生物量碳与土壤呼吸有极显著的相关性;笔者在研究中发现,随连作代数的增加,土壤有机碳、全氮和微生物量碳等指标均有明显下降(表2),这与多数对连作障碍的研究一致[16-17]:因此,2代林和3代林相对较低的呼吸底物浓度和微生物量,可能是导致其RS和RH较小的主要原因。

4.2 环境因子对土壤呼吸及各组分的影响

土壤呼吸是一个极为复杂的生物过程,受诸多环境因子的影响[18]。其中,土壤温度和土壤含水量被认为是最主要的限制因子[19]。土壤温度能够影响植物物候特征、根系活动、土壤和根际微生物活性以及呼吸底物的供应[20-21],进而对RS产生影响。试验发现,3种林分的RS、RA和RH均与土壤温度呈极显著指数关系(表3),这与多数温带地区森林的研究结果一致[22-23];同时,基于指数模型拟合发现,土壤温度对土壤呼吸速率变化的解释能力为75%～92%:因此,土壤温度是本区域杨树人工林土壤呼吸速率的关键限制因子。通过线性模型拟合发现,土壤呼吸速率与土壤含水量相关关系不显著,原因可能是试验样地内,土壤水分并未限制到植物根系和微生物的活动。Wang等[24]认为,包含温湿度的复合模型能够更好地解释土壤呼吸的变化,笔者采用复合函数模型对三者进行拟合,R2值均有提高,说明双因素模型能够更好地模拟土壤呼吸与土壤温湿度的关系,这也印证了上文的论断。

4.3 土壤呼吸各组分的贡献率

采用壕沟法区分RA和RH,无根呼吸(异养呼吸)观测点样方内的根系,只是切断而并未排除,由于杨树具有明显的无性系繁殖特性,使得处理样方内的根系能够存活很长一段时间,并进行呼吸作用;另外,死亡的根系会被微生物分解,其相当于人为添加异养呼吸的底物,可增强土壤异养呼吸速率[25-27]。应用壕沟法进行土壤呼吸的组分分离,显然会导致自养呼吸的贡献率小于实际值。唐罗忠等[28]研究发现,杨树＜10 mm的根系在切断4个月后,活性基本消失(死亡);因此,笔者在挖壕沟处理的4个月后,再进行异养呼吸的测定,此时壕沟样方内的根系已经基本死亡,使得RH的观测值能够更加接近实际值。

吴君君等[11]收集不同气候带森林土壤呼吸数据发现,自养呼吸的贡献率在18.4%～83.1%之间,笔者研究3种林分自养呼吸的平均贡献率分别为39.39%、38.47%和42.25%,都在上述范围之内。各林分RA的平均贡献率为40.04%,这一数值略高于D.D.Saurette等[29]对加拿大阿尔伯塔杨树人工林37%的研究结果。除此之外,RA/RS的比值存在明显的季节变化,即夏季自养呼吸的贡献率显著高于春、秋2个季节(图2),这也印证了RA的温度敏感性高于RH,自养呼吸速率相比异养呼吸速率更容易受到温度的影响,夏季温度较高时,自养呼吸速率升高较快,从而导致RA/RS的值大于春秋2季。

综上所述,笔者对不同连作代数杨树人工林生长季土壤呼吸速率及其差异原因进行测定和分析发现,在连作条件下,2代林和3代林的土壤理化性质和微生物生物量较1代林有所下降,导致其土壤呼吸速率包括异养呼吸速率降低。杨树人工林长期连作,会抑制土壤呼吸作用;而连作过后,整个杨树人工林生态系统是碳“源”亦或是“汇”,仍需要进一步研究确定。

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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

项目名称:国家林业公益性科研专项“森林生态服务功能分布式定位观测与模型模拟”(201204101-7);国家自然科学基金“黄泛平原农田林网的生态因子场形成机制的研究”(31170662);教育部博士点基金“基于生态因子场的拟法正农田林网可持续更新机制研究”(20133702110007)

程学刚(1989—),男,硕士研究生。主要研究方向:恢复生态学。E-mail:shuoyueliuxing@163.com

†通信作者简介:李传荣(1968—),男,教授。主要研究方向:恢复生态和林业生态工程。E-mail:chrli@sdau.edu.cn