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中国沙漠, 2025, 45(3): 60-71 doi: 10.7522/j.issn.1000-694X.2025.00046

干旱沙区植被恢复与重建对碳循环关键过程的影响研究进展

虎瑞,, 高艳红,, 张鹏, 李小军

中国科学院西北生态环境资源研究院 沙坡头沙漠研究试验站/干旱区生态安全与可持续发展全国重点实验室,甘肃 兰州 730000

Review on the effects of vegetation revegetation on key carbon cycle processes in arid sandy regions

Hu Rui,, Gao Yanhong,, Zhang Peng, Li Xiaojun

Shapotou Desert Research and Experiment Station / National Key Laboratory of Ecological Safety and Sustainable Development in Arid Lands,Northwest Institute of Eco-Environment and Resources,Chinese Academy of Sciences,Lanzhou 730000,China

通讯作者: 高艳红(Email: gao_yanhong@lzb.ac.cn

收稿日期: 2025-03-28   修回日期: 2025-05-08  

基金资助: 宁夏回族自治区重点研发计划项目.  2023BEG02040
国家自然科学基金项目.  32471729
甘肃省自然科学基金项目.  23JRRA621

Received: 2025-03-28   Revised: 2025-05-08  

作者简介 About authors

虎瑞(1984—),女,甘肃兰州人,副研究员,研究方向为土壤生态学E-mail:hurui@lzb.ac.cn , E-mail:hurui@lzb.ac.cn

摘要

植被恢复与重建是干旱区沙化土地修复的关键措施,该过程通过改变地表覆盖、生物多样性和土壤有机质含量等影响区域碳循环过程。本文综述了近70年来中国北方沙区植被恢复与重建对碳循环关键过程的影响及其机制。结果表明:植被恢复通过植被-结皮-土壤复合体提升光合固碳能力,其中人工林地、灌丛和草本群落的净生态系统碳交换(NEE)分别为-386~-245、-280~-156、-210~-125 g·m-2·a-1,生物土壤结皮(BSCs)的年固碳量可达11.36~26.75 g·m-2·a-1。土壤呼吸和有机碳矿化速率随植被恢复时间延长呈增加趋势,土壤CO2释放受植被组成、恢复年限、BSCs发育程度及季节性水热波动的综合调控,表现出显著的空间异质性和时间动态特征。植被恢复通过增加生物量碳输入、BSCs发育及改善土壤团聚体结构,显著提升土壤有机碳(SOC)储量,0~100 cm土层SOC储量0.19~7.71 kg·m-2,其固存速率受生态恢复措施配置、土壤基质属性及水热耦合关系等多因子协同控制,土壤碳氮耦合机制对系统碳汇功能具有关键调控作用。植被恢复与重建显著改变地表温室气体通量格局,CO₂通量动态呈现复杂的环境响应特征。未来需加强多尺度长期监测,深化BSCs功能、气候变化响应及碳氮耦合机制研究,为优化沙区植被恢复模式及实现“碳中和”目标提供科学支撑。

关键词: 碳循环 ; 土壤呼吸 ; 土壤有机碳储量 ; 碳通量 ; 植被恢复 ; 北方沙区

Abstract

Vegetation restoration and reconstruction are key measures for the remediation of desertified lands in arid regions. This process profoundly influences regional carbon cycling by altering surface cover, biodiversity, and soil organic matter. This paper reviews the impact and mechanisms of vegetation restoration and reconstruction on key carbon cycling processes over the past 70 years in the sandy areas of northern China. The results indicate that vegetation restoration enhances photosynthetic carbon sequestration through the "vegetation-biocrust-soil" complex, with net ecosystem carbon exchange (NEE) measurements of -386 to -245, -280 to -156, and -210 to -125 g·m-2·a-1 for artificial forests, shrublands, and herbaceous communities, respectively. The annual carbon sequestration by biological soil crusts (BSCs) can reach 11.36 to 26.75 g·m-2·a-1. Soil respiration and organic carbon mineralization rates tend to increase with longer vegetation restoration periods. Soil CO2-C release is regulated by a combination of factors including vegetation composition, restoration duration, BSCs development level, and seasonal hydrological and thermal fluctuations, demonstrating significant spatial heterogeneity and temporal dynamics. Vegetation restoration significantly enhances soil organic carbon (SOC) storage by increasing biomass carbon input, BSC development, and improving soil aggregate structure, with SOC storage in the 0-100 cm soil layer reaching 0.19 to 7.71 kg·m-2. The sequestration rate is co-controlled by multiple factors such as ecological restoration measures, soil substrate properties, and hydrothermal coupling. The carbon-nitrogen coupling mechanism plays a key regulatory role in the carbon sink function of the system. Vegetation restoration and reconstruction significantly alter the surface greenhouse gas flux pattern, with CO2 flux dynamics showing complex environmental response characteristics. Future research should strengthen multi-scale long-term monitoring and deepen studies on BSC function, climate change responses, and carbon-nitrogen coupling mechanisms to provide scientific support for optimizing vegetation restoration models in sandy areas and achieving the "dual carbon" goals.

Keywords: carbon cycle ; soil respiration ; SOC storage ; carbon flux ; revegetation ; sandy areas in North China

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本文引用格式

虎瑞, 高艳红, 张鹏, 李小军. 干旱沙区植被恢复与重建对碳循环关键过程的影响研究进展. 中国沙漠[J], 2025, 45(3): 60-71 doi:10.7522/j.issn.1000-694X.2025.00046

Hu Rui, Gao Yanhong, Zhang Peng, Li Xiaojun. Review on the effects of vegetation revegetation on key carbon cycle processes in arid sandy regions. Journal of Desert Research[J], 2025, 45(3): 60-71 doi:10.7522/j.issn.1000-694X.2025.00046

0 引言

干旱半干旱区占陆地面积的45%,拥有全球陆地生态系统碳储量的25%,是地球上最大的生物群系1-3。然而,气候变化和人类活动导致该地区超过2/3的土地退化,主要表现为荒漠化,造成生态系统中20~30 Pg(10亿t)C流失,影响碳循环过程,显著降低土壤质量、生态系统生产力和生物多样性,同时增加了大气中温室气体浓度,对人类健康和生产活动构成严重威胁2-6。研究显示,生态恢复措施能在25~50年内恢复60%~70%的旱地历史碳损失37。最新研究表明,通过在6%适宜恢复的旱地上造林,每年可固碳0.4 Gt,相当于同期预计商业排放量的1%8。因此,修复退化旱地并促进生态系统碳氮固存已成为逆转荒漠化、提升土壤质量以及缓解气候变化研究的核心议题2-39-10

植被恢复与重建是修复旱地生态系统的核心措施和有效方法211。自20世纪50年代以来,中国在北方沙区实施了一系列生态建设工程,累计种植788.2万hm2防风固沙林12,显著改善了生态环境。沙区植被恢复与重建不仅提高了地表覆盖和生物多样性,增加了净初级生产力和土壤有机质13-16,还通过改善土壤物理、化学及微生物特性,从而影响碳积累与转化过程91417。因此,研究植被恢复过程中碳循环变化及其机制,是揭示恢复生态系统结构、功能和可持续性的核心问题9-101518-23

本文基于1950—2020年中国北方沙区生态恢复的植物-土壤系统长期观测研究,综述了植被恢复对碳循环关键过程的影响及机制,以期为植被恢复与重建模式优化、可持续土地利用与管理决策提供科学依据,也为准确预测旱地生态系统对全球碳循环中的贡献以及实现“碳中和”目标提供支持。

1 沙区植被恢复与重建对光合固碳的影响

植被恢复与重建主要通过植被和BSCs光合作用影响土壤有机碳累积过程32024-26,形成植被-结皮-土壤复合体,通过多个生态过程协同作用,提升生态系统的生物碳固定能力20。研究表明,中国北方沙区的人工林地净生态系统碳交换(NEE,负值表示碳汇,以C计)为-386~-245 g·m-2·a-1,灌丛系统为-280~-156 g·m-2·a-1,而草本群落为-210~-125 g·m-2·a-1[27。这种差异主要源于物种特征和环境条件。例如,樟子松人工林年净初级生产力(NPP,以C计)为586~725 g·m-2,显著高于同区域柠条灌丛(425~560 g·m-228。长期定位观测表明,植被碳固定能力随时间动态变化:在恢复初期(1~5年),碳固定能力较弱,而随着群落发育和演替,碳固定速率逐渐增加,通常在15~20年达到峰值29。此外,BSCs在植被恢复中碳固定作用也至关重要。在腾格里沙漠,随着BSCs发育和演替,其光合固碳能力显著提高:早期藻结皮日固碳量为0.12~0.25 g·m-2,后期藓藻结皮固碳量0.45~0.86 g·m-2,表明BSCs的恢复显著增加了荒漠生态系统碳输入20

2 沙区植被恢复与重建对土壤呼吸的影响

土壤呼吸是陆地生态系统碳循环的重要环节,也是土壤碳释放到大气的主要途径30-32。在植被重建和恢复过程中,植物群落通过根系活动和凋落物输入改变土壤的物理结构、微环境、有机质以及生物化学过程,进而影响土壤呼吸速率33-34。研究表明,凋落物积累增加土壤呼吸,而根系呼吸在土壤总呼吸中也占有较高比例。根据Hanson等35的研究,根系呼吸在生长季占土壤呼吸一半以上36。此外,植被恢复导致土壤微生物群落组成和活性发生变化,促进了土壤呼吸过程,该过程与植被变化密切相关。

土壤呼吸速率通常与植被类型和物种丰富度密切相关。物种丰富度越高,土壤呼吸速率通常越大,凋落物累积和根系呼吸增强了该过程37-38。干旱半干旱区,不同灌木林土壤呼吸差异显著。例如研究显示新疆准噶尔盆地外围柽柳(Tamarix chinensis)+芦苇(Phragmites australis)群落和梭梭(Haloxylon ammodendron)群落土壤呼吸日变化呈单峰型39,而艾比湖流域盐肤木(Rhus chinensis)土壤呼吸呈双峰型40。甘肃民勤石羊河下游不同发育阶段白刺(Nitraria sibirica)灌丛土壤呼吸变化日最大值均出现在12:00,平均呼吸速率依次为稳定阶段>初期发育阶段>衰退阶段>严重衰退阶段41。随着植被恢复时间延长土壤呼吸速率呈增加趋势42-43,这与土壤有机质和微生物数量增加有关44。但在腾格里沙漠的研究显示,只有在高土壤含水量时,植被区各演替阶段的土壤呼吸存在显著差异42。此外,有研究显示土壤呼吸通量随着植被演替降低45-46,随着正向演替进程,植被盖度增加会降低土壤温度,从而抑制土壤呼吸47。植被结构和组成的变化,通过影响土壤温度和水分显著影响土壤呼吸。

除了地表维管束植物影响土壤呼吸外,BSCs对土壤呼吸的影响也不容忽视。BSCs对土壤呼吸速率的调节主要涉及以下方面:首先,BSCs中隐花植物通过光合作用固碳,从而促进微生物活性,增加土壤呼吸速率。内蒙古沙地覆盖有BSCs的土壤呼吸速率明显高于裸露土壤48。其次,不同类型BSCs对土壤水分的调控能力影响土壤呼吸。自然降水条件下,藓类结皮的土壤呼吸速率平均为1.09 μmol·m-2·s-1,较藻-地衣结皮的0.94 μmol·m-2·s-1高14%49。藓类结皮具有更高的土壤微生物量和有机质,且长时间保持水分,有利于土壤呼吸50。同时,BSCs改变了土壤表层结构和稳定性,降低侵蚀、减缓气体扩散并调节土壤温度,抑制土壤呼吸。研究证实在科尔沁沙地沙漠化逆转过程中BSCs对土壤呼吸也有同样的影响51。BSCs对土壤呼吸的影响受BSCs类型及土壤环境的影响。

温度和水分也是影响土壤呼吸的关键非生物因子。干旱沙区土壤呼吸受两者协同影响,并呈现出明显的季节特征52。沙区土壤含水量较高时,土壤呼吸对温度的敏感性(Q10)增加;土壤含水量偏低时,Q10下降53。干旱沙区地表水分含量低,蒸发量大,低温条件下(如非生长季)土壤呼吸主要受温度控制;高温条件下土壤水分成为限制土壤呼吸的主要因素54。尤其是夏季,土壤呼吸通常受水分限制,导致微生物和根系活性下降,从而抑制土壤呼吸55。对腾格里沙漠BSCs呼吸的研究表明不同降水事件下BSCs呼吸具有明显的时空异质性,且BSCs呼吸速率与降水量显著正相关49。年际尺度上,土壤含水量是不同植被群落土壤剖面CO2浓度变化的关键因子,日尺度上土壤温度是主要限制因子56。温度和水分对土壤呼吸的影响存在明显的协同作用,但两者之间的相对重要性和作用机理仍需进一步研究57-58

3 沙区植被恢复与重建对土壤碳矿化的影响

植被恢复与重建对土壤碳矿化有重要影响,涉及土壤生态系统碳循环以及土壤有机碳(SOC)周转。土壤碳矿化过程反映土壤中碳保留或释放动态,是评估土壤功能变化和养分供给能力的重要指标。研究表明随着植被演替,土壤碳矿化可能增加59、减少60或先减少后增加61,该过程由多种因素综合调控,决定碳矿化速率,影响土壤碳库稳定性和生态系统碳汇功能。

不同植被类型凋落物数量、种类及根系分泌物化学成分等影响土壤理化性质、微生物群落结构和多样性,进而改变SOC的矿化特征62。通常凋落物数量及分解速度可表征土壤碳矿化状况63。干旱沙区,不同固沙灌木因凋落物化学组成差异导致微生物关键类群和分解速率显著不同,从而影响SOC矿化64。李云飞等65研究证实各物种间凋落物的差异能够显著影响土壤碳矿化速率。此外,灌木根系通过增加根系碳输入(包括根系死亡和分泌物)比地上凋落物输入更有效地促进SOC矿化66。豆科灌木通过固氮作用缓解氮素缺乏,加速有机质分解,有助于碳转化。腾格里沙漠经过64年植被重建,显著提高了土壤碳矿化67。随着植被恢复、土壤SOC质量和微生物数量显著提高,为土壤微生物提供了基础物质,激发微生物活性,提高SOC矿化速率68。研究显示,腾格里沙漠经过62年的植被恢复,土壤碳矿化速率显著增加69-70。这是由于恢复期间植被盖度和多样性提高,凋落物和根系分泌物中的有机质含量增加,为土壤提供了更多碳输入。

BSCs作为主要干旱沙区地表生物体,对土壤碳转化的作用日益受到重视。随着BSCs的演替土壤微环境改善,代谢活性增强71,从而有效调节土壤碳矿化过程。谢婷等72发现随着植被恢复,BSCs层的SOC矿化速率逐渐增大,且显著高于其下0~5 cm层土壤。随植被恢复,BSCs群落从藻类优势逐渐演变为藻类和藓类共存,期间SOC矿化速率从0.03 g·kg-1·d-1增加到0.04 g·kg-1·d-1[73

土壤温度和湿度是影响土壤SOC矿化的主要环境因子74。季节性温湿度变化通过控制可矿化底物的可用性引起土壤微生物数量和活性波动影响土壤碳矿化过程75-76。干旱地区降水和气温的季节性波动改变土壤微环境和生物地球化学过程,使微生物对基质温湿度变化更加敏感,导致碳矿化呈现出显著的季节动态77-78。Stoyan等79发现夏季土壤微生物生物量及活性最高,促进SOC矿化。而非生长季由于干旱加剧和低温,微生物活性显著降低,导致碳矿化速率减小67;相反,在降水相对较多的生长季,强降雨事件能够迅速增加土壤湿度,激活微生物活性,从而显著提高SOC矿化速率80-81

4 沙区植被恢复与重建对生态系统碳储量的影响

4.1 固沙植被区碳固存现状、速率及潜力

生态系统碳输入增加或减少时表现为碳积累或损失982。植被恢复与重建促使维管束植物和隐花植物盖度持续增加915,不断将大气中CO2转化为植物生物量碳83。枯死根系、菌根和根系分泌物释放则进一步促进碳固存84。在固沙地区,植物生物量中的碳储量通常为0.07~3.62 kg·m-2,积累速率为2.80~144.80 g·m-2·a-1。植物生物量碳积累速率与气候条件、物种选择等生物和非生物因素密切相关,主要影响植被地上和地下生物量以及植被覆盖度16-178385-86。许多研究表明随着固沙年限延长,生物量碳储量呈现持续增加趋势1787-88,但在植被演替过程中,木本植物退化可能导致生物量碳积累速率显著下降1517-1883。例如腾格里沙漠的研究表明植被重建20年后生物量碳储量达到峰值,但由于土壤水分的浅层化,旱生灌木逐渐被草本植物取代,导致生物量显著下降1883

尽管植被恢复与重建对土壤碳变化的影响受到广泛关注,但SOC变化方向和幅度仍存在不确定性。目前研究结果显示植被恢复后SOC可能增加916-17、减少89-90或无显著变化91。该差异可能与气候变化、物种选择、土壤质地以及土地利用历史差异相关,或者由于缺乏长期观测991。特别是以流沙为起始条件的植被恢复能够有效促进SOC的长期恢复,在0~100 cm土壤剖面SOC储量为0.19~7.71 kg·m-2,固存速率为7.49~308.4 g·m-2·a-1[916-17838692-96。然而,由于地上凋落物和根系,以及其他碳输入(如BSCs和大气降尘)主要在表土层发生,植被恢复引发的SOC储量变化主要在0~20 cm土层917。此外,由于SOC主要依赖于植物来源的土壤有机质输入97,SOC的变化往往滞后于生物量碳变化16-178398,表明尽管植物生物量迅速增长和增加碳输入,SOC的显著变化可能需要更长期的积累和转化过程。

4.2 植被恢复与重建过程中影响碳固存的因素

植被恢复与重建措施和所选植被物种很大程度上决定了碳固存过程的变化方向和幅度99。多项研究表明流沙地区采用旱生灌木+沙障进行植被重建,可以显著增加植物和土壤碳储量916-17。石羊河流域研究表明黏土沙障通过提供大量的细颗粒物质,比其他类型沙障更有利于土壤、植被和BSCs发育,更有效地促进碳固存86100-101。而对于沙化草地来说,人工植被(如种植灌木和草本植物)和禁牧是增加碳汇的主要途径。大量研究表明禁牧能有效提高干旱区草地生态系统的碳固存能力102-104。然而,全球尺度的整合分析显示,禁牧初期可能导致地下生物量碳的显著下降,随着时间推移该生物量碳呈上升趋势98105。毛乌素沙化草地种植草本植物后SOC呈单峰曲线变化趋势106,而种植旱生灌木则能显著增加沙化草地的SOC107-108。这说明人工植被建设比单纯的自然修复更有效地促进植被恢复和碳固存,与采取的恢复措施变化密切相关106-108。合理的恢复措施与适宜的人为干预可以促进植物群落恢复,也有助于形成可持续的碳固存能力。

种植乡土物种是沙区生态恢复的关键措施109-110。通常情况下,乔木林地碳变化速率大于灌木林地,针叶林地变化速率大于阔叶林地。此外,木本植物凋落物分解速率通常大于草本植物91786111,该变化取决于不同植物地上和地下凋落物的数量和质量及其分解速率8699112。中国北方沙区多采用柠条锦鸡儿(Caragana korshinskii)、小叶锦鸡儿(Caragana microphylla)和花棒(Hedysarum scoparium)等豆科植物为建群种,其凋落物分解并融入土壤速率显著低于非豆科植物,从而影响碳累积112-115。通常认为,植物多样性高的固沙植被物种组成能够产生更丰富的根系分泌物和凋落物,提高凋落物的数量和质量,为微生物提供更多的碳源和能量,促进微生物生长,进而改变碳累积进程215。塔克拉玛干沙漠研究中疏叶骆驼刺(Alhagi sparsifolia)纯林的生物量显著高于其与花花柴(Karelinia caspia)的混交林,而花花柴纯林表层土壤碳含量也显著高于混交林109。适宜的物种选择能够显著提高沙区碳固存能力,促进沙区生态系统健康发展。

沙区碳固存速率与土壤黏粉粒以及磷(P)和钾(K)等元素的含量正相关285。SOM的分解释放营养元素3116,营养元素增加可以促进植物生长和微生物活性,从而影响植物残体和凋落物的分解速率,改变碳固存过程29117。SOC固存往往受N和P等营养元素限制3116。有研究表明旱地每固存10 kg的SOC需要0.83 kg N、0.2 kg P和0.14 kg S116。增加土壤养分对提升碳固存能力至关重要。植被和地表凋落物覆盖减少太阳辐射,降低土壤温度和水分蒸发,而细颗粒物质增加提高土壤斥水性和导热性21117。微环境改善有利于凋落物分解,也能通过促进养分循环增加凋落物输入921117-118。沙土中SOM输入减小pH值,有助于减少SOM分解和SOC累积2119。生态恢复引起的植被组成、土壤理化性质以及微环境变化均会显著增加微生物多样性和活性267120,从而促进凋落物的分解并融入土壤21121

温度和降水是影响碳固存及其分布格局的关键气候因子122。温度和降水不仅影响植被类型及其生产力,输入生态系统的SOM数量和质量122-123,还通过调节水热过程影响微生物群落组成、活性及相互作用和生化反应,从而改变SOM的分解和转化速率123-124。不同气候区域温度和降水对碳固存的影响不同。干旱生态系统中水分是限制净初级生产力的关键因素,从而影响碳固存25。较湿润的区域由于较快的植被演替和根系周转,大量碳被释放到土壤中,因而该地区SOC随着时间的推移呈显著增长趋势17123

植被恢复年限是影响碳固存速率的重要因素。恢复初期,地上和地下生物量变化导致SOM输入发生变化9125。同时,恢复过程中土壤理化性质持续改变对碳循环过程也有重要影响985-86114。土壤微生物群落组成和功能演变使SOM分解和维持过程存在差异21121126

碳-氮相互作用是陆地生态系统碳汇持久性的关键影响因素127-128。氮的有效性通过限制初级生产力,影响旱地碳固存能力和其可持续性127-129。碳输入能激发氮固定和大气氮沉降增加,氮供应增加能够提高植被生产力,增加SOM的输入和土壤碳汇潜能130-131。反之,若土壤氮储量不随时间增加,SOC固存受限127。随着土壤C∶N增大,土壤氮矿化速率降低。植物对氮需求超过土壤氮供应,植物生长受限,导致生物量氮积累速率下降127。尽管近年来沙区植被恢复与重建引发的SOC和N动态变化受到广泛关注,但现有数据主要来源于短期CO2富集实验132-133,尚不足以对生态系统N储量变异进行全面量化9133。基于年代序列的腾格里沙漠的研究调查显示,SOC和N均随固沙年限的延长显著增加,且二者呈正相关关系,表明SOC和N之间存在耦合关系9。这说明随着植被恢复年限增加,N累积不足可能导致其对SOC固存限制增强127-128134,成为植被恢复后期SOC固存速率下降的主要原因。这种现象可以部分归因于植物生长对N的消耗以及高C∶N凋落物输入的增加135

5 植被恢复与重建对生态系统CO2 通量的影响

沙区植被恢复与重建对碳通量的影响是复杂多维的生态过程。生物圈和大气圈之间的CO2交换关系复杂且动态变化,其源-汇特性和交换强度受到植被类型、土壤条件、气候因子(如光照、温度、水分)以及植被演替阶段等多重因素的综合影响。

干旱沙区不同地点碳源汇关系表现出显著的空间异质性。例如研究表明科尔沁沙地的沙质草地在生长季通常表现为碳汇136,但在年际尺度上,尤其是在干旱年份,可转变为碳源137。而新疆古尔班通古特沙漠138-139和宁夏盐池140的研究也印证了生态系统在干旱年份为碳源,湿润年份为碳汇。相比之下,腾格里沙漠东南缘沙坡头地区通过几十年的植被恢复,将流动沙丘转变为稳定的碳汇95141。该人工生态系统即使在干旱年份也保持碳汇状态,其年净固碳量最高可达91.61±36.17 g·m-2·a-1[142。比较不同恢复阶段发现25龄人工植被区的碳汇能力最强,其次为58龄人工植被区,两者均显著高于邻近的天然荒漠植被区143。以上研究结果表明固沙植被重建重塑了区域生态系统的CO₂交换格局,且其碳汇功能强度随恢复时间呈现非线性变化。

影响沙区碳循环的气候因素主要包括温度、降水、土壤含水量以及与之相关的植被演替动态。而干旱沙区的极端气候条件和脆弱生态系统更为突出,温度变化直接影响植物生长,还显著调控土壤呼吸和净CO2交换过程141-142144。沙区植物进化的独特生理适应机制,在适宜范围内,温度升高促进光合速率;然而存在一个最优温度范围,当温度超过该阈值时,光合效率会下降,尤其在水分胁迫条件下145,高温更易导致光合抑制,从而削弱碳固定能力146。因此,生态系统碳固定能力随温度变化呈现先增后降的非线性响应146。沙坡头地区生态系统呼吸对温度变化敏感,通常随温度升高呈指数增加,但也受到土壤含水量的调节141147。同时,春季增温增强了沙坡头沙漠生态系统碳汇功能以及年际变化趋势142。然而,温度对生态系统碳通量的影响,很大程度上受限于土壤水分条件145。由于沙土保水能力差,增温加剧人工灌丛生态系统的干旱胁迫,从而抑制总初级生产力。当气温增加3 ℃时,人工灌丛生产力彻底崩溃,引发生态系统结构和功能的全面退化148

降水和土壤水分是决定沙区生态系统为碳源或碳汇的关键因素149,但年降水量并不能完全解释年CO2交换量140,这可能是由于降水的遗留效应150-151以及光合固碳和碳释放对降水的不对称响应95152。干旱沙区土壤水分是生态系统的关键资源,显著影响植被生长和碳动态过程。土壤水分对植被光合作用153-154、碳固定能力155以及土壤碳分解与释放具有重要作用。尤其在气候变化背景下,水分时空异质性对沙区碳循环过程产生深远影响。气候变暖和干旱耦合通过降低土壤水分有效性,抑制土壤温室气体排放,减少碳损失156,也减少了以苔藓为主的BSCs对碳的吸收157,从而改变了沙区的碳循环过程。

植被演替对碳通量有重要影响。不同植被类型(如灌木、草本和乔木)在光合速率、根系分布和土壤有机碳积累方面存在显著差异158。草本植物虽然生长周期较短,但其通过快速生长和再生能力调控光合作用与土壤呼吸的关系,对碳循环产生重要影响159。然而,灌木光合能力则随着演替进程发生变化,通常在植被恢复初期和中期具有较高的光合速率,后期可能下降160。基于涡度相关技术的碳通量观测研究同样证实,人工固沙植被区碳汇强度随恢复年限增加呈现下降趋势,但整体上仍显著高于邻近的天然植被区143。值得注意的是,随着沙区人工植被演替,尽管优势灌木盖度逐渐降低,但草本植物以及整个植被群落盖度和物种多样性通常呈现增加趋势161。多样化的植物群落能够更有效地利用资源,增强生态系统的整体稳定性和生产力162。因此,选择合理的植被类型,积极维护生物多样性,对于优化沙区碳循环过程和提升碳汇功能起着关键作用,并能够为应对气候变化提供重要的生态系统服务支持。

6 问题与展望

尽管沙区具有巨大的碳固存潜力,但植被恢复与重建所引发的碳固存是一个极其缓慢的过程。该过程不仅依赖于植被恢复与重建本身,还受到多种环境因素和生态系统管理方式的制约。因此,为有效提升沙区的碳固存潜力,对生态系统的精细管理至关重要。此外,固沙区碳动态变化是复杂且强烈受非生物因素影响的过程。因此,有必要进行长期观测研究,以揭示植被恢复过程中碳循环机理,为实现干旱沙区的碳中和目标提供科学支持。

沙区植被恢复与重建对碳循环影响非常复杂,涉及植被类型与恢复阶段、土壤理化性质、微生物群落、气候因素与人为干扰等多个生态过程和因素之间的相互作用。因此,开展多尺度的综合监测十分必要,这不仅有助于深入揭示植被恢复与土壤碳循环之间的相互作用机制和调控途径,还能为提升碳固存能力和制定优化植被恢复措施提供科学依据。

未来的研究应重点量化不同类型BSCs在各种气候条件下的长期效应,并结合遥感技术,监测大尺度植被覆盖率变化及其对土壤呼吸的影响。此外,研究在气候变化情景下,极端天气事件如干旱和暴雨对BSCs稳定性和功能的影响,将有助于预测和管理生态系统碳平衡与气候适应性。同时,应聚焦沙区植被恢复与重建对温室气体通量的多因素交互作用,特别是在气候变化背景下的响应机制。应加强对土壤微生物群落、功能基因丰度及其在碳循环中的作用研究,深入理解其对CO₂、CH₄和N₂O排放的影响。此外,推动多尺度、长期监测实验研究,为沙区生态修复和应对气候变化提供科学依据。

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