Knowledge Resource Center for Ecological Environment in Arid Area
| 半干旱草原生产力和碳循环对降水变化的非对称响应 | |
| 其他题名 | Asymmetric Responses of Ecosystem Productivity and Carbon Cycle to Precipitation Change in a Semiarid Steppe |
| 张兵伟 | |
| 出版年 | 2016 |
| 学位类型 | 博士 |
| 导师 | 韩兴国 |
| 学位授予单位 | 中国科学院大学 |
| 中文摘要 | 半干旱区生态系统是陆地上分布最广泛的生态系统类型，也是对水分变化最敏感的系统之一。当前，全球增温导致的降水格局也将发生剧烈变化：在中国北方草原降水将呈现出持续增加的趋势，同时伴随着剧烈的年际波动。降水的多少决定了该类型生态系统的碳源-汇关系及强度，进而影响了全球碳平衡。因而，研究半干旱区生态系统碳循环对降水变化的响应对于准确预测和评估全球碳汇功能的变化具有重要意义。尽管对这一问题已经进行了大量的研究，人们对降水变化下半干旱生态系统碳循环过程的响应机制依然不够清楚。基于2012-2015年在内蒙古西乌珠穆沁旗半干旱草原建立的降水变化控制实验（处理包括5个降水梯度：生长季减少降水60%和30%，对照，增加降水30%和60%；以及两种干旱处理模式：持续减少降水以观测长期干旱的影响，减少降水2年后接受自然降水以观测生态系统在干旱后的恢复过程），通过连续4年对环境因子、生态系统生产力和碳循环过程的监测，本研究着重探讨了生态系统对降水变化的响应规律和机制过程。取得了以下主要结果：1）4年当中，不同处理下生长季降水量的变化范围为122-610 mm，超过了当地过去56年（1956-2011年）最大的降水量变化幅度（118-509 mm）。降水变化显著改变了土壤环境和地上植被的生长状况。随着降水的增加，土壤含水量和植被的冠层盖度呈非线性增加，而土壤含水量的季节变异和温度呈非线性降低趋势；微生物生物量和无机氮含量逐渐增加，但土壤可溶性有机碳氮逐渐降低。2）降水变化显著改变了生态系统生产力。随着降水量的增加，生态系统地上和地下净初级生产力均呈现非线性增加的趋势，即对降水减少的敏感性更大。降水变化没有影响地下净初级生产力对总生产力的贡献比例（42%），但改变了根系生长在地下不同土层间的分配。随降水增加，植物将更多根系分配到表层土壤（0-10 cm），这主要是由于降水增加时表层土壤水分和养分含量的改善，以及土壤水分季节波动的降低引起的；而干旱促使更多根系进入到下层土壤（10-50 cm）。说明通过调整自身根系的垂直分布，植物可以充分利用湿润环境或者应对干旱胁迫的影响。3）降水变化没有显著影响群落的物种数、物种多样性和个体数量，但改变了群落中不同物种的构成比例。在群落中，根系分布越浅表化和地上叶茎比越大的物种对降水变化的响应越强。如浅根且叶茎比最大的知母（Anemarrhena asphodeloides）和黄囊苔草（Carex korshinskyi）对地上生物量的贡献随降水增加而增加；而稀有种的贡献则逐渐降低。其中，知母主要通过个体大小的变化来响应降水变化，黄囊苔草的变化则由个体数量的变化所主导。这说明在半干旱草原中，不同物种可以根据其自身特征来调整对降水变化的响应强度和方式，进而通过物种间的互相补偿作用维持生态系统的相对稳定。4）生态系统的碳水循环过程（生态系统总光合、呼吸、净碳交换、蒸散，以及碳水利用效率）均随着降水的增加而呈非线性增加趋势，即对减少降水响应的敏感性高于其对增加降水的敏感性。其中土壤含水量对降水变化的非线性响应是引起生态系统总光合、呼吸和蒸散非对称响应的最重要原因。但是生态系统的碳平衡，即生态系统净碳交换和碳水利用效率则更加受制于土壤温度的变化。这是因为减少降水处理下升高的温度对呼吸和蒸散的促进作用更大，并部分抵消掉干旱的抑制作用，使得生态系统呼吸和蒸散对降水变化的响应变弱，进而决定了生态系统净碳交换和碳水利用效率的大小。5）土壤呼吸包括54%的异养呼吸和46%的自养呼吸。随降水增加，土壤呼吸呈非线性增加趋势，且异养呼吸对总呼吸的贡献也随之增加。其中，异养呼吸和自养呼吸主要受到土壤含水量和植被冠层盖度的影响。土壤含水量对降水变化响应的敏感性显著大于盖度，可以部分解释异养呼吸对降水变化更大的敏感性，进而增大其对总呼吸的贡献。此外，自养呼吸的温度敏感性显著高于异养呼吸。减少降水处理下，升高的土壤温度会对自养呼吸产生更大的促进作用，部分抵消掉干旱的抑制作用，使得异养呼吸主导了土壤呼吸对降水变化的响应。未来降水增加可能会通过异养呼吸的增加而降低土壤碳库的大小和稳定性，但是这一过程也受到了土壤可溶性有机碳降低所产生的负反馈影响。因而降水增加后土壤呼吸的变化会如何影响土壤碳库还需要更长期的实验来研究。6）生态系统碳循环的不同过程对干旱响应的敏感性、抵抗力和恢复力是不同的。其中敏感性最大的生态系统净碳交换和异养呼吸，抵抗力最小，恢复力最大；而敏感性最小的自养呼吸、生态系统呼吸和水分利用效率，抵抗力最大，恢复力最小。本研究首次发现了生态系统碳循环的不同过程之间：对干旱响应中抵抗力越大的过程恢复力越小，反之亦然。综上，未来降水的增加会有利于半干旱生态系统生产过程，但其影响程度小于等量降水减少所产生的影响，因而在预测具体变化时需要考虑到降水变化影响的非对称性。不同生态系统过程在对降水变化的响应上存在着显著的差异，说明在研究未来气候变化对生态系统的影响上，多考虑组分间的相同或不同响应有利于精确地评价生态系统的变化。而对于生态系统中变化缓慢但将会产生深远影响的过程，如群落结构和土壤碳库，在未来降水变化情况下会发生怎样的改变还需要更长时期的实验研究。 |
| 英文摘要 | Semiarid ecosystems are the most widespread terrestrial ecosystem type, and extremely sensitive to precipitation change. With rising global temperature, precipitation is predicted to change greatly in the future. In the northern China, precipitation has been predicted to increase within the next hundred-year, and with greater inner- and inter-annual variations. Recent studies have revealed that semiarid ecosystems play a dominant role in regulating the dynamics of the global terrestrial CO2 sink, which are mainly driven by increasing inter-annual precipitation variability. Therefore, understanding how ecosystem carbon (C) processes respond to precipitation change in semiarid ecosystems can greatly improve our ability in predicting changes in the global C cycle. However, how ecosystem carbon cycle processes will respond to precipitation change is not well recognized yet, for the lack of substantially experimental evidences with multi-level precipitation.Here, a 4-year (2012-2015) field manipulation experiment with 5-level precipitation amount (reduced 30% (P-30) and 60% (P-60) growing season precipitation, control (P) and increased 30% (P+30) and 60% (P+60) growing season precipitation) and two drought treatments (one receives 30% and 60% reduced growing season precipitation in the four seasons to detect the influence of long-term drought on ecosystem; the other receives the ambient precipitation after two-year’s drought to detect the recovery of ecosystem (P-30R and P-60R)) were conducted in a semiarid steppe in West Ujinmqin Banner, Inner Mongolia, China. Soil environmental factors, ecosystem productivity and ecosystem C processes were determined, to study the patterns and underlying mechanisms of them in response to precipitation. The key findings are as follows.1) During the four seasons, precipitation received in all plots ranged from 122 mm to 610 mm, which was greater than historical fluctuation (118-509 mm during 1956-2011). As precipitation increased, soil water content (SWC) and canopy greenness cover (Cover) increased nonlinearly, while the seasonal variable coefficient of SWC (CVSWC) and soil temperature (Ts) decreased nonlinearly; soil microbial biomass carbon (MBC) and nitrogen (MBN) and inorganic nitrogen increased, but soil dissolved organic carbon (DOC) and nitrogen (DON) decreased.2) Altered precipitation caused significant effects on ecosystem productivity. Both above- (ANPP) and below-ground (BNPP) net primary productivity showed nonlinear responses to precipitation change, with a greater response sensitivity to the decreased than to increased precipitation. Despite that precipitation change did not affect the allocation ratio of NPP into belowground (fBNPP with 42%), it did alter the vertical distribution of BNPP with more roots in the surface soil (0-10 cm) under wet conditions, which was related to enhanced soil resource (water and nitrogen) availability. While drought treatment induced more roots in the deeper soil (10-50 cm). Therefore, through altering roots vertical distribution, plants could satisfy their water requirements whether in drought or wet conditions.3) Precipitation change did not affect species richness, biodiversity and stem density of community, but altered the relative contribution of plant species. Species with shallower rooting depth and greater leaf to stem ratio showed greater response ratio to precipitation change. As precipitation increased, the proportion of two species with shallow roots and great leaf to stem ratio: Anemarrhena asphodeloides (A.a.) and Carex korshinskyi (C.k.), increased significantly, while that of rare species decreased. While, the changings were different between A.a. and C.k., that is, the increased biomass of A.a. was determined by its tiller size, while that of C.k. was determined by enhanced tiller density. Therefore, with different rooting characteristic and leaf to stem ratio, plants would respond differently to precipitation change, and then stabilize ecosystem structure with the compensatory effect among species.4) Ecosystem CO2 (GEP: gross ecosystem photosynthesis, ER: ecosystem respiration, NEE: net ecosystem CO2 exchange), water (ET: ecosystem evapotranspiration) exchange and resource use efficiency (CUE: carbon use efficiency, WUE: water use efficiency) were all more sensitive in response to the reduced than to increased precipitation; therefore, showed a positive nonlinear response to precipitation change. Soil water availability was the most important driver determining the changes in GEP, ER and ET in response to precipitation change. While the responses of NEE, CUE and WUE were predominately regulated by soil temperature. Because higher soil temperature under decreased precipitation treatments could promote plant respiration and soil evaporation greatly, offsetting the suppression effect of drought on ER and ET. Then the greater response of GEP to precipitation change than ER and ET determine the relative magnitude of NEE, CUE and WUE.5) Total soil respiration (SRtot) consists of 54% heterotrophic part and 46% autotrophic part. Increased precipitation promoted SRtot nonlinearly. But the response sensitivity of SRh to precipitation change was much greater than that of SRa, which led to an increasing SRh/SRtot ratio as precipitation increase. Changes in SRh and SRa were mainly determined by SWC and plant growth (Cover) respectively. Sensitivity of SWC to precipitation change was greater than Cover, which could partially explain the distinguishing response patterns of SRh and SRa. Moreover, SRa showed a greater sensitivity to soil temperature (Q10) than SRh; higher soil temperature in drought treatments would promote SRa more than SRh, which offset the influence from water stress, and induced a weaker apparent response of SRa to precipitation change than SRh. As precipitation increase, greater SRh meant faster decomposition of soil C, and would affect soil C sequestration in further. Meanwhile, a negative feedback to SRh from declining DOC concentration was also dected. Therefore, to figure out how the magnitude and stability of soil C would change under future precipitation change, long-term precipitation manipulation experiments are vital necessary.6) Sensitivity, resistance and resilience of different C cycle processes in response to drought event were various. Among these processes, NEE and SRh were most sensitive to dought, but with smallest resistance and greatest resilience. While SRa, ER and WUE exhibited smallest sensitivity to drought, greatest resistance and smallest resilience. This work firstly revealed a negative relationship between resistance and resilience of different ecosystem C cycle processes in response to drought events.In summary, increased precipitation in the future would be favorable to ecosystem productivity in semiarid steppes, but its influence was much smaller than reduced precipitation. The asymmetric response of ecosystem functions to precipitation change indicates that influence of increased precipitation and reduced precipitation should not be treated equally. This study also revealed distinguishing sensitivity in response to precipitation change among different ecosystem processes. Research including multi ecosystem processes would be helpful to better understand how future climate change affects ecosystem function. Moreover, to figure out how precipitation change will affect community structure and soil C pool, the relative steady processes which are closely related to ecosystem stability, long-term manipulation experiment is also vital necessary. |
| 中文关键词 | 降水变化 ; 碳循环 ; 生产力 ; 草原 ; 分配 |
| 英文关键词 | precipitation change carbon cycle ecosystem productivity grassland allocation |
| 语种 | 中文 |
| 国家 | 中国 |
| 来源学科分类 | 生态学 |
| 来源机构 | 中国科学院植物研究所 |
| 资源类型 | 学位论文 |
| 条目标识符 | http://119.78.100.177/qdio/handle/2XILL650/287824 |
| 推荐引用方式 GB/T 7714 | 张兵伟. 半干旱草原生产力和碳循环对降水变化的非对称响应[D]. 中国科学院大学,2016. |
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