Habitat type determines herbivory controls over CO2 fluxes in a warmer Arctic

High-latitude ecosystems store large amounts of carbon (C); however, the C storage of these ecosystems is under threat from both climate warming and increased levels of herbivory. In this study we examined the combined role of herbivores and climate warming as drivers of CO2 fluxes in two typical high-latitude habitats (mesic heath and wet meadow). We hypothesized that both herbivory and climate warming would reduce the C sink strength of Arctic tundra through their combined effects on plant biomass and gross ecosystem photosynthesis and on decomposition rates and the abiotic environment. To test this hypothesis we employed experimental warming (via International Tundra Experiment [ITEX] chambers) and grazing (via captive Barnacle Geese) in a three-year factorial field experiment. Ecosystem CO2 fluxes (net ecosystem exchange of CO2, ecosystem respiration, and gross ecosystem photosynthesis) were measured in all treatments at varying intensity over the three growing seasons to capture the impact of the treatments on a range of temporal scales (diurnal, seasonal, and interannual). Grazing and warming treatments had markedly different effects on CO2 fluxes in the two tundra habitats. Grazing caused a strong reduction in CO2 assimilation in the wet meadow, while warming reduced CO2 efflux from the mesic heath. Treatment effects on net ecosystem exchange largely derived from the modification of gross ecosystem photosynthesis rather than ecosystem respiration. In this study we have demonstrated that on the habitat scale, grazing by geese is a strong driver of net ecosystem exchange of CO2, with the potential to reduce the CO2 sink strength of Arctic ecosystems. Our results highlight that the large reduction in plant biomass due to goose grazing in the Arctic noted in several studies can alter the C balance of wet tundra ecosystems. We conclude that herbivory will modulate direct climate warming responses of Arctic tundra with implications for the ecosystem C balance; however, the magnitude and direction of the response will be habitat-specific.     

Net emissions of CH4 and CO2 in Alaska: implications for the region's greenhouse gas budget.

We used a biogeochemistry model, the Terrestrial Ecosystem Model (TEM), to study the net methane (CH4) fluxes between Alaskan ecosystems and the atmosphere. We estimated that the current net emissions of CH4 (emissions minus consumption) from Alaskan soils are approximately 3 Tg CH4/yr. Wet tundra ecosystems are responsible for 75% of the region's net emissions, while dry tundra and upland boreal forests are responsible for 50% and 45% of total consumption over the region, respectively. In response to climate change over the 21st century, our simulations indicated that CH4 emissions from wet soils would be enhanced more than consumption by dry soils of tundra and boreal forests. As a consequence, we projected that net CH4 emissions will almost double by the end of the century in response to high-latitude warming and associated climate changes. When we placed these CH4 emissions in the context of the projected carbon budget (carbon dioxide [CO2] and CH4) for Alaska at the end of the 21st century, we estimated that Alaska will be a net source of greenhouse gases to the atmosphere of 69 Tg CO2 equivalents/yr, that is, a balance between net methane emissions of 131 Tg CO2 equivalents/yr and carbon sequestration of 17 Tg C/yr (62 Tg CO2 equivalents/yr).     

基于DNDC模型评估水位变化对滨海湿地净生态系统CO2交换的影响

水位是影响滨海湿地生态系统蓝碳功能的重要因素。气候变化引起的海平面上升以及极端气候事件的频发,可能加快水位的变化,从而改变生态系统碳交换的过程。然而,滨海湿地碳汇功能响应水位变化的机制尚不清楚。为了评估水位对滨海湿地净生态系统CO₂交换(NEE)特征的影响,以及验证DNDC(denitrification-decomposition)模型对模拟预测滨海湿地生态系统碳交换的适用性,该研究设计了野外水位控制试验(自然水位,地下20 cm水位、地表10 cm水位),并利用DNDC模型模拟和预测水位变化对滨海湿地NEE的影响。结果表明:(1)不同水位处理之间NEE差异显著,地表10 cm水位处理促进CO₂吸收,地下20 cm水位则抑制CO₂吸收;(2)经过校准和验证的DNDC模型可以准确模拟水位变化对黄河三角洲湿地NEE的影响,NEE模拟值的日动态与田间观测结果显著相关(R²>0.6);(3)通过改变气候、土壤和田间管理等输入参数对DNDC模型进行灵敏度检验,生态系统碳交换过程对日均温、降雨和水位改变的响应最为显著,其中,水位对NEE的影响主要作用于土壤呼吸(Rs)。未来气候情境下,不同水位变化下的生态系统碳交换过程随年份增长呈现不同的规律,因此未来的模拟研究应关注DNDC中水文模块和植被演替过程的完善。该研究可为预测水文变化情境下滨海湿地碳汇功能的未来发展以及政策制定提供参考。     

气候变暖背景下森林土壤碳循环研究进展

由人类活动引起的温室效应以及由此造成的气候变暖对森林牛态系统的影响已引起人们的普遍关注.森林土壤碳循环作为全球碳循环的重要组成部分,是决定未来陆地牛物嘲表现为碳源/碳汇的关键环节,揭示这一作用对于准确理解全球变化背景下陆地生态系统碳循环过程具有重要的指导意义.本文主要通过论述影响土壤碳循环过程的5个方面(土壤呼吸、土壤微生物、土壤酶活性、凋落物输入与分解、土壤碳库),综述了近10 a来全球气候变暖对土壤碳循环过程的影响.近年来,尽管已开展了大量有关土壤碳循环对气候变暖的响应及反馈机制的研究,并取得了一定的成果,但研究结果仍然存在很大的不确定性.整合各种密切关联的全球变化现象,完善研究方法和实验手段,加强根际微生态系统碳循环过程与机理研究将是下一步研究的方向和重点.参70     

大气CO2升高对土壤碳循环影响的研究进展

土壤有机碳是陆地碳库的重要组成部分,其积累和分解的变化直接影响全球的碳平衡。据估计,全球土壤（表层1m）有机碳积累总量相当于大气中碳总量的2～3倍。土壤是温室气体的源或汇,土壤碳库的变化将影响大气C02的浓度,因此,土壤碳库对人类活动的响应也是全球碳循环和全球变化研究的热点。在全球变化的大背景下,大气CO2升高导致植被生态系统碳平衡的改变进而对土壤碳循环产生影响。总结了陆地生态系统碳循环对大气C02浓度升高响应的主要生物学机制及过程,简述了大气C02浓度升高对影响土壤碳输入和输出的各因素的研究进展,并指出未来研究的主要方向。在大气C02浓度升高条件下,陆地生态系统碳循环的变化主要反映在以下几个方面：1）不同类型植物群落的净初级生产力（NPP）显著增加,但湿地植物的净初级生产力也有可能降低;2）光合产物向根系分配的数量增加,地上／地下生物量降低,根系形态发生变化,根系周转速率和根系分泌等过程的碳流量提高;3）植物含氮量降低,C／N提高,次生代谢产物增加,微生物生长受到抑制,植物残体分解速率降低;4）土壤呼吸速率显著增加,提高幅度受植物类型与土壤状况的影响;5）进入土壤的植物残体及分泌物的数量和性质影响土壤酶的活性,脱氢酶和转化酶活性增加,酚氧化酶和纤维素酶受植物类型与环境条件的影响;6）土壤中真菌的数量的增加幅度要高于细菌;7）CH4释放量增加,在植物的生长期表现更为明显。由于陆地生态系统碳循环的复杂性,研究结果仍有很大的不确定性。大气C02浓度升高与全球变化的其它表现间的交互作用将是今后研究的重点,同时由于土壤碳循环是一个由微生物介导的生物地球化学循环过程,因此,加强陆地生态系统碳循环的微生物机制研究也将为全面理解碳循环的过程提供更加准确的研究理论基础。     

Microbial community composition and function across an arctic tundra landscape

Arctic landscapes are characterized by a diversity of ecosystems, which differ in plant species composition, litter biochemistry, and biogeochemical cycling rates. Tundra ecosystems differing in plant composition should contain compositionally and functionally distinct microbial communities that differentially transform dissolved organic matter as it moves downslope from dry, upland to wet, lowland tundra. To test this idea, we studied soil microbial communities in upland tussock, stream-side birch-willow, and lakeside wet sedge tundra in arctic Alaska, USA. These are a series of ecosystems that differ in topographic position, plant composition, and soil drainage. Phospholipid fatty acid (PLFA) analyses, coupled with compound-specific 13C isotope tracing, were used to quantify microbial community composition and function; we also assayed the activity of extracellular enzymes involved in cellulose, chitin, and lignin degradation. Surface soil from each tundra ecosystem was labeled with 13C-cellobiose,13C-N-acetylglucosamine, or 13C-vanillin. After a five-day incubation, we followed the movement of 13C into bacterial and fungal PLFAs, microbial respiration, dissolved organic carbon, and soil organic matter. Microbial community composition and function were distinct among tundra ecosystems, with tussock tundra containing a significantly greater abundance and activity of soil fungi. Although the majority of 13C-labeled substrates rapidly moved into soil organic matter in all tundra soils (i.e., 50-90% of applied 13C), microbial respiration of labeled substrates in wet sedge tundra soil was lower than in tussock and birch-willow tundra; approximately 8% of 13C-cellobiose and approximately 5% of 13C-vanillin was respired in wet sedge soil vs. 26-38% of 13C-cellobiose and 18-21% of 13C-vanillin in the other tundra ecosystems. Despite these differences, wet sedge tundra exhibited the greatest extracellular enzyme activity. Topographic variation in plant litter biochemistry and soil drainage shape the metabolic capability of soil microbial communities, which, in turn, influence the chemical composition of dissolved organic matter across the arctic tundra landscape.     

森林土壤氧化（吸收）甲烷研究进展

甲烷是一种重要的温室气体,对全球气候变暖的贡献仅次于CO2,约为25%。大气甲烷可以被土壤中甲烷氧化细菌在有氧条件下吸收利用,陆地生态系统森林土壤氧化吸收甲烷的研究已有大量报道。甲烷氧化菌是以甲烷作为唯一的碳源和能源的一类细菌的总称。但森林土壤在全球甲烷核算中具有一定的不确定性,取决于产甲烷菌和甲烷氧化菌的相对活性。甲烷氧化菌的研究集中在环境对氧化能力的影响和自身氧化能力上。大气甲烷氧化过程为高氧化能力低亲力氧化,受到一些因子,如土壤温度、土壤通气状况、pH、氮肥等的影响,具体机理的研究还有待进一步深入。土壤通气状况受土壤质地与土壤水分影响,土地利用类型可能改变土壤容重、土壤结构和土壤水分,进而影响土壤甲烷氧化。植物可以通过自身对生境的作用或化感作用影响土壤甲烷氧化。土壤动物的研究则相对较少,目前排放清单中仅有白蚁是全球甲烷核算所包括的。从甲烷氧化菌的分类出发,对甲烷氧化菌氧化甲烷的机理、菌的生态分布及甲烷氧化的影响因素、时空异质性、观测方法等作出了综述,为正确认识和准确预测森林土壤在一定气候和土地利用类型条件下的甲烷氧化强度提供理论依据。     

Light-stress avoidance mechanisms in a Sphagnum-dominated wet coastal Arctic tundra ecosystem in Alaska

The Arctic experiences a high-radiation environment in the summer with 24-hour daylight for more than two months. Damage to plants and ecosystem metabolism can be muted by overcast conditions common in much of the Arctic. However, with climate change, extreme dry years and clearer skies could lead to the risk of increased photoxidation and photoinhibition in Arctic primary producers. Mosses, which often exceed the NPP of vascular plants in Arctic areas, are often understudied. As a result, the effect of specific environmental factors, including light, on these growth forms is poorly understood. Here, we investigated net ecosystem exchange (NEE) at the ecosystem scale, net Sphagnum CO2 exchange (NSE), and photoinhibition to better understand the impact of light on carbon exchange from a moss-dominated coastal tundra ecosystem during the summer season 2006. Sphagnum photosynthesis showed photoinhibition early in the season coupled with low ecosystem NEE. However, later in the season, Sphagnum maintained a significant CO2 uptake, probably for the development of subsurface moss layers protected from strong radiation. We suggest that the compact canopy structure of Sphagnum reduces light penetration to the subsurface layers of the moss mat and thereby protects the active photosynthetic tissues from damage. This stress avoidance mechanism allowed Sphagnum to constitute a significant percentage (up to 60%) of the ecosystem net daytime CO2 uptake at the end of the growing season despite the high levels of radiation experienced.     

大气主要温室气体源汇及其研究进展

全球气候变暖是全球生态学的重点研究领域,近10余年来在世界范围内对引起气候变暖的温室气体源和汇进行了广泛的研究。文章综述了大气中几种主要温室气体(CO2、CH4和N2O)源和汇的种类及大小,认为在3种温室气体的源汇通量方面仍缺乏准确的定量认识,并存在许多不确定因素。通过分析近10年的国内外文献,总结出大气主要温室气体源汇的国际研究趋势主要是研究方法日益先进、研究地域不断扩大、注重温室气体通量对全球变化的响应以及多学科综合研究等方面。国内对温室气体源汇研究起步较晚,且存在观测点少、观测频度低、研究不够系统等不足,近几年在全国范围内实施的有关碳循环的重大科研项目在很大程度上促进了我国在温室气体源汇研究的发展。     

Life-cycle model of terrestrial carbon exchange

We present here a terrestrial carbon cycle model based on a scheme of the phytomass change, which is continuous in time. The experimental information about net primary production, living and dead phytomass, and soil organic matter for various ecosystems is used for calibration of the model. The suggested model enables to characterize terrestrial ecosystems as carbon sources or carbon sinks and to evaluate intensity of these sources and sinks. The model is applied for the European territory of Russia as a case study. Intensity of the total exchange carbon flux for this territory is evaluated. The obtained results allow to conclude that the given territory is the sink of carbon.     

Effect of nitrogen deposition on China's terrestrial carbon uptake in the context of multifactor environmental changes

The amount of atmospheric nitrogen (N) deposited on the land surface has increased globally and by nearly five times in China from 1901 to 2005. Little is known about how elevated reactive N input has affected the carbon (C) sequestration capability of China's terrestrial ecosystems, largely due to the lack of reliable data on N deposition. Here we have used a newly developed data set of historical N deposition at a spatial resolution of 10 km x 10 km in combination with other gridded historical information on climate, atmospheric composition, land use, and land management practices to drive a process-based ecosystem model, the dynamic land ecosystem model (DLEM) for examining how increasing N deposition and its interactions with other environmental changes have affected C fluxes and storage in China's terrestrial ecosystems during 1901-2005. Our model simulations indicate that increased N deposition has resulted in a net C sink of 62 Tg C/yr (1 Tg = 1012 g) in China's terrestrial ecosystems, totaling up to 6.51 Pg C (1 Pg = 10(15) g) in the past 105 years. During the study period, the N-induced C sequestration can compensate for more than 25% of fossil-fuel CO2 emission from China. The largest C sink was found in southeast China, a region that experienced the most significant increase of N deposition in the period 1901-2005. However, the net primary productivity induced by per-unit N deposition (referred to as ecosystem N use efficiency, ENUE, in this paper) has leveled off or declined since the 1980s. This indicates that part of the deposited N may not be invested to stimulate plant growth, but instead leave the ecosystem by various pathways. Except shrubland and northwest/southwest China, signs of N saturation are apparent in the rest major biome types and regions, with ENUE peaking in the 1980s and leveling off or declining thereafter. Therefore, to minimize the excessive N pollution while keeping the N-stimulated C uptake in China's terrestrial ecosystems, optimized management practices should be taken to increase N use efficiency rather than to keep raising N input level in the near future.     

Application of a coupled ecosystem-chemical equilibrium model, DayCent-Chem, to stream and soil chemistry in a Rocky Mountain watershed

Atmospheric deposition of sulfur and nitrogen species have the potential to acidify terrestrial and aquatic ecosystems, but nitrate and ammonium are also critical nutrients for plant and microbial productivity. Both the ecological response and the hydrochemical response to atmospheric deposition are of interest to regulatory and land management agencies. We developed a non-spatial biogeochemical model to simulate soil and surface water chemistry by linking the daily version of the CENTURY ecosystem model (DayCent) with a low temperature aqueous geochemical model, PHREEQC. The coupled model, DayCent-Chem, simulates the daily dynamics of plant production, soil organic matter, cation exchange, mineral weathering, elution, stream discharge, and solute concentrations in soil water and stream flow. By aerially weighting the contributions of separate bedrock/talus and tundra simulations, the model was able to replicate the measured seasonal and annual stream chemistry for most solutes for Andrews Creek in Loch Vale watershed, Rocky Mountain National Park. Simulated soil chemistry, net primary production, live biomass, and soil organic matter for forest and tundra matched well with measurements. This model is appropriate for accurately describing ecosystem and surface water chemical response to atmospheric deposition and climate change.     

区域能源碳足迹计算模型比较研究——以湖北省为例

能源消费排放是最主要的碳排放来源,研究能源碳足迹重要理论和现实意义。如何准确计算和表征能源碳足迹的大小成为该研究领域的重要问题。在文献回顾的基础上,文章总结了3种目前应用较广泛的计算区域能源消费碳足迹的方法和模型,它们分别是碳汇法、净初级生产力改进模型和净生态系统生产力改进模型。分别介绍了其计算公式、输入参数和计算过程。以湖北省1998-2009年能源消费碳足迹的计算为例,分别用这几种方法计算了能源消费碳足迹的总量和人均碳足迹。得到的主要结论有：研究区域能源消费碳排放量增长较快,3种方法和模型计算得到的碳足迹总量和人均量从时间序列上看,整体变化趋势一致,碳足迹亦呈现快速增长。但3种模型计算出的碳足迹大小差异较大,碳汇法模型足迹最大,净生态系统生产力改进模型次之,净初级生产力改进模型结果最小,并且碳汇法的计算结果明显大于其他两种模型。计算结果差异的原因在于,传统碳汇法模型只考虑林地的碳吸收能力,忽略了区域其他用地类型的碳吸收能力。净初级生产力改进模型考虑了各种用地类型的吸收能力,但从生态系统来说忽略了异氧呼吸的碳释放,高估了区域的碳吸收能力。几种模型都运用了固定值或者平均统计量,未能考虑地域差异,同时未考虑各统计量随时间和气候等变化而变化的可能,存在一定的不合理性,这也是今后研究值得深人研究的方向。     

Modeling seasonal course of carbon fluxes and evapotranspiration in response to low temperature and moisture in a boreal Scots pine ecosystem

Environmental conditions act above and below ground, and regulate carbon fluxes and evapotranspiration. The productivity of boreal forest ecosystems is strongly governed by low temperature and moisture conditions, but the understanding of various feedbacks between vegetation and environmental conditions is still unclear. In order to quantify the seasonal responses of vegetation to environmental factors, the seasonality of carbon and heat fluxes and the corresponding responses for temperature and moisture in air and soil were simulated by merging a process-based model (CoupModel) with detailed measurements representing various components of a forest ecosystem in Hyytiälä, southern Finland. The uncertainties in parameters, model assumptions, and measurements were identified by generalized likelihood uncertainty estimation (GLUE). Seasonal and diurnal courses of sensible and latent heat fluxes and net ecosystem exchange (NEE) of CO₂ were successfully simulated for two contrasting years. Moreover, systematic increases in efficiency of photosynthesis, water uptake, and decomposition occurred from spring to summer, demonstrating the strong coupling between processes. Evapotranspiration and NEE flux both showed a strong response to soil temperature conditions via different direct and indirect ecosystem mechanisms. The rate of photosynthesis was strongly correlated with the corresponding water uptake response and the light use efficiency. With the present data and model assumptions, it was not possible to precisely distinguish the various regulating ecosystem mechanisms. Our approach proved robust for modeling the seasonal course of carbon fluxes and evapotranspiration by combining different independent measurements. It will be highly interesting to continue using long-term series data and to make additional tests of optional stomatal conductance models in order to improve our understanding of the boreal forest ecosystem in response to climate variability and environmental conditions.     

Effects of interpolation and data resolution on methane emission estimates from rice paddies

Rice paddies are an important source of the greenhouse gas methane (CH₄). Global methane emission estimates are highly uncertain and do not account for effects of interpolation or data resolution errors. This paper determines such scaling effects for the influence of soil properties on calculated CH₄ emissions for the island of Java, Indonesia. The effects of different interpolation techniques, variograms and neighbor optimization were tested for soil properties by cross-validation. Interpolated organic carbon values were not significantly different from the original soil samples, in contrast to interpolated soil iron contents. Interpolation of soil properties coupled to a process-based model on CH₄emissions led to a significant change in distribution of calculated CH₄ emissions, i.e., the variance decreased. Effects of data resolution were examined by interpolating soil properties to derive data at different data resolutions and then calculating CH₄ emissions by applying the process-based model at these resolutions. The soil properties did not differ significantly for different data resolutions, in contrast to calculated CH₄ emissions. These scaling effects were caused by the combination of interpolation and a non-linear model. Real scaling effects may even be larger because small-scale variability was not accounted for. Scaling effects, including those caused by small-scale variability, have to be considered to achieve unbiased and less uncertain global CH₄emissions estimates from rice paddies.     

Response of tree growth to a changing climate in boreal central Canada: A comparison of empirical,process-based,and hybrid modelling approaches

The impact of 2 × CO₂ driven climate change on radial growth of boreal tree species Pinus banksiana Lamb., Populus tremuloides Michx. and Picea mariana (Mill.) BSP growing in the Duck Mountain Provincial Forest of Manitoba (DMPF), Canada, is simulated using empirical and process-based model approaches. First, empirical relationships between growth and climate are developed. Stepwise multiple-regression models are conducted between tree-ring growth increments (TRGI) and monthly drought, precipitation and temperature series. Predictive skills are tested using a calibration–verification scheme. The established relationships are then transferred to climates driven by 1× and 2 × CO₂ scenarios using outputs from the Canadian second-generation coupled global climate model. Second, empirical results are contrasted with process-based projections of net primary productivity allocated to stem development (NPP_s). At the finest scale, a leaf-level model of photosynthesis is used to simulate canopy properties per species and their interaction with the variability in radiation, temperature and vapour pressure deficit. Then, a top-down plot-level model of forest productivity is used to simulate landscape-level productivity by capturing the between-stand variability in forest cover. Results show that the predicted TRGI from the empirical models account for up to 56.3% of the variance in the observed TRGI over the period 1912–1999. Under a 2 × CO₂ scenario, the predicted impact of climate change is a radial growth decline for all three species under study. However, projections obtained from the process-based model suggest that an increasing growing season length in a changing climate could counteract and potentially overwhelm the negative influence of increased drought stress. The divergence between TRGI and NPP_s simulations likely resulted, among others, from assumptions about soil water holding capacity and from calibration of variables affecting gross primary productivity. An attempt was therefore made to bridge the gap between the two modelling approaches by using physiological variables as TRGI predictors. Results obtained in this manner are similar to those obtained using climate variables, and suggest that the positive effect of increasing growing season length would be counteracted by increasing summer temperatures. Notwithstanding uncertainties in these simulations (CO₂ fertilization effect, feedback from disturbance regimes, phenology of species, and uncertainties in future CO₂ emissions), a decrease in forest productivity with climate change should be considered as a plausible scenario in sustainable forest management planning of the DMPF.     

中国农田主要温室气体排放特征与控制技术

全球气候变化已成为不争的事实,已经引起了各国科学家和政府的高度重视。人类活动向大气中排放的二氧化碳（CO2）、甲烷（CH4）和氧化亚氮（N2O）等温室气体浓度的增加是导致气候变化的重要原因之一。农田土壤是这三种温室气体的重要来源。文章概述了农田土壤CO2、CH4和N2O三种主要温室气体的排放机制,系统总结了国内对这三种温室气体排放通量的观测研究,提出了相应的减排技术对策,并对目前的研究问题和未来的发展方向作了深入的探讨,以期为控制农田土壤温室气体排放、发展低碳农业提供参考依据。     

湿地生态系统碳汇与碳源过程的控制因子和临界条件

湿地生态系统由于其自身的结构组分特征,已成为地球表层系统中最为重要的碳汇。但是近年来对于湿地系统的不合理开发利用、降水减少等原因使其碳＂汇＂功能减弱,湿地的碳蓄积能力下降且有转变为碳＂源＂的趋势。文章从湿地生态系统的水份、植物类型、土壤厚度、微生物（底物、pH、温度、氧化还原条件）等方面总结了影响湿地碳汇/源过程的控制因子和临界交替条件。湿地水位的高低决定湿地的氧气环境,与甲烷产生量成正相关,但却与二氧化碳产生量有一定的负相关关系。湿地植物通过通气组织与根系分泌物等影响湿地碳的吸收与排放通量,湿地植株的高度、覆盖率等也是影响湿地作为碳汇与碳源的重要因素。不同深度土层由于其产甲烷菌、甲烷氧化菌等微生物活性不同导致各个土层碳吸收、排放通量的差异,通常浅层土壤中的CO2、CH4的产生率高于深层土壤。微生物的活跃程度直接影响到湿地碳的吸收与排放,影响活跃程度的因素包括湿地底物、pH、温度与氧化还原条件等。湿地底物浓度的增加会在一定程度上提高甲烷的产生率,中性或者是弱碱性环境是产甲烷菌的最适宜条件,在一定范围内温度越高,甲烷产生量越大,而温度对于二氧化碳的影响则是通过改变光合作用来实现。氧化还原电位与甲烷产生量成负相关关系,-150 mV是产甲烷菌产生甲烷的最高电位。总体上,由于湿地生态系统的复杂性和碳吸收与排放过程的复杂,以上这些因子相互作用,且在一定条件下会相互转化。最后针对如何充分发挥湿地生态系统的碳＂汇＂功能,控制湿地向碳＂源＂转化的条件措施方面进行了讨论,包括间歇灌溉、种植多年生草本植物或木本植物等来增强湿地的固碳能力。     

Life-cycle assessment of net greenhouse-gas flux for bioenergy cropping systems.

Bioenergy cropping systems could help offset greenhouse gas emissions, but quantifying that offset is complex. Bioenergy crops offset carbon dioxide emissions by converting atmospheric CO2 to organic C in crop biomass and soil, but they also emit nitrous oxide and vary in their effects on soil oxidation of methane. Growing the crops requires energy (e.g., to operate farm machinery, produce inputs such as fertilizer) and so does converting the harvested product to usable fuels (feedstock conversion efficiency). The objective of this study was to quantify all these factors to determine the net effect of several bioenergy cropping systems on greenhouse-gas (GHG) emissions. We used the DAYCENT biogeochemistry model to assess soil GHG fluxes and biomass yields for corn, soybean, alfalfa, hybrid poplar, reed canarygrass, and switchgrass as bioenergy crops in Pennsylvania, USA. DAYCENT results were combined with estimates of fossil fuels used to provide farm inputs and operate agricultural machinery and fossil-fuel offsets from biomass yields to calculate net GHG fluxes for each cropping system considered. Displaced fossil fuel was the largest GHG sink, followed by soil carbon sequestration. N20 emissions were the largest GHG source. All cropping systems considered provided net GHG sinks, even when soil C was assumed to reach a new steady state and C sequestration in soil was not counted. Hybrid poplar and switchgrass provided the largest net GHG sinks, >200 g CO2e-C x m(-2) x yr(-1) for biomass conversion to ethanol, and >400 g CO2e-C x m(-2) x yr(-1) for biomass gasification for electricity generation. Compared with the life cycle of gasoline and diesel, ethanol and biodiesel from corn rotations reduced GHG emissions by approximately 40%, reed canarygrass by approximately 85%, and switchgrass and hybrid poplar by approximately 115%.     

Modelling global-scale potential grassland changes in spatio-temporal patterns to global climate change

Grassland is one of the most widespread vegetation types worldwide and plays a significant role in global carbon cycling. Understanding the sensitivity of grassland to climate change and the effect of climate changes on the grassland ecosystems is a key issue in global carbon cycling. One of the goals of this study was to evaluate the three net primary productivity (NPP)–climate models, i.e. the Miami model, the Schuur model and the classification indices-based model. Results indicated that the classification indices-based model was the most effective model at estimating large-scale grassland NPP. In this research, changes in the spatial pattern of global potential grassland from recent past (1950–2000) to future (2001–2050) A2a scenario were analysed with the integrated orderly classification system of grassland (IOCSG) approach in a Geographic Information System (GIS) environment. NPP was evaluated with the classification indices-based model. Results indicate that under recent past climatic conditions, the main parts of global grassland are the savanna and tundra and alpine grassland and will be converted into the savanna, steppe and semi-desert grassland in A2a scenario. As a whole, areas of grassland will increase by 31.76 million hectares. The classification indices-based model estimated a 12.40% increase of total NPP in grassland from recent past to A2a scenario. It will impose a new issue for future grassland researches to support sustainable development and to provide action relevant knowledge to meet the challenge of climate change.