On the relationship between water table depth and water vapor and carbon dioxide fluxes in a minerotrophic fen

The focus of this study is the relationship between water table depth (WTD) and water vapor [evapotranspiration (ET)] and carbon dioxide [CO₂; net ecosystem exchange (NEE)] fluxes in a fen in western Canada. We analyzed hydrological and eddy covariance measurements from four snow‐free periods (2003–2006) with contrasting meteorological conditions to establish the link between daily WTD and ET and gross ecosystem CO₂ exchange (GEE) and ecosystem respiration (R_eco; NEE=R_eco?GEE), respectively: 2003 was warm and dry, 2004 was cool and wet, and 2005 and 2006 were both wet. In 2003, the water table (WT) was below the ground surface. In 2004, the WT rose above the ground surface, and in 2005 and 2006, the WT stayed well above the ground surface. There were no significant differences in total ET (～316 mm period^?1), but total NEE was significantly different (2003: 8 g C m^?2 period^?1; 2004: ?139 g C m^?2 period^?1; 2005: ?163 g C m^?2 period^?1; 2006: ?195 g C m^?2 period^?1), mostly due to differences in total GEE (2003: 327 g C m^?2 period^?1; 2004: 513 g C m^?2 period^?1; 2005: 411 g C m^?2 period^?1; 2006: 556 g C m^?2 period^?1). Variation in ET is mostly explained by radiation (67%), and the contribution of WTD is only minor (33%). WTD controls the compensating contributions of different land surface components, resulting in similar total ET regardless of the hydrological conditions. WTD and temperature each contribute about half to the explained variation in GEE up to a threshold ponding depth, below which temperature alone is the key explanatory variable. WTD is only of minor importance for the variation in R_eco, which is mainly controlled by temperature. Our study implies that future peatland modeling efforts explicitly consider topographic and hydrogeological influences on WTD.     

The global carbon sink: a grassland perspective

The challenge to identify the biospheric sinks for about half the total carbon emissions from fossil fuels must include a consideration of below-ground ecosystem processes as well as those more easily measured above-ground. Recent studies suggest that tropical grasslands and savannas may contribute more to the ‘missing sink’ than was previously appreciated, perhaps as much as 0.5 Pg (= 0.5 Gt) carbon per annum. The rapid increase in availability of productivity data facilitated by the Internet will be important for future scaling-up of global change responses, to establish independent lines of evidence about the location and size of carbon sinks.     

CO2 stabilization,climate change and the terrestrial carbon sink

A nonequilibrium, dynamic, global vegetation model, Hybrid v4.1, with a subdaily timestep, was driven by increasing CO₂ and transient climate output from the UK Hadley Centre GCM (HadCM2) with simulated daily and interannual variability. Three IPCC emission scenarios were used: (i) IS92a, giving 790 ppm CO₂ by 2100, (ii) CO₂ stabilization at 750 ppm by 2225, and (iii) CO₂ stabilization at 550 ppm by 2150. Land use and future N deposition were not included. In the IS92a scenario, boreal and tropical lands warmed 4.5 °C by 2100 with rainfall decreased in parts of the tropics, where temperatures increased over 6 °C in some years and vapour pressure deficits (VPD) doubled. Stabilization at 750 ppm CO₂ delayed these changes by about 100 years while stabilization at 550 ppm limited the rise in global land surface temperature to 2.5 °C and lessened the appearance of relatively hot, dry areas in the tropics. Present‐day global predictions were 645 PgC in vegetation, 1190 PgC in soils, a mean carbon residence time of 40 years, NPP 47 PgC y^?1 and NEP (the terrestrial sink) about 1 PgC y^?1, distributed at both high and tropical latitudes. With IS92a emissions, the high latitude sink increased to the year 2100, as forest NPP accelerated and forest vegetation carbon stocks increased. The tropics became a source of CO₂ as forest dieback occurred in relatively hot, dry areas in 2060–2080. High VPDs and temperatures reduced NPP in tropical forests, primarily by reducing stomatal conductance and increasing maintenance respiration. Global NEP peaked at 3–4 PgC y^?1 in 2020–2050 and then decreased abruptly to near zero by 2100 as the tropical source offset the high‐latitude sink. The pattern of change in NEP was similar with CO₂ stabilization at 750 ppm, but was delayed by about 100 years and with a less abrupt collapse in global NEP. CO₂ stabilization at 550 ppm prevented sustained tropical forest dieback and enabled recovery to occur in favourable years, while maintaining a similar time course of global NEP as occurred with 750 ppm stabilization. By lessening dieback, stabilization increased the fraction of carbon emissions taken up by the land. Comparable studies and other evidence are discussed: climate‐induced tropical forest dieback is considered a plausible risk of following an unmitigated emissions scenario.     

A decade of boreal rich fen greenhouse gas fluxes in response to natural and experimental water table variability

Rich fens are common boreal ecosystems with distinct hydrology, biogeochemistry and ecology that influence their carbon (C) balance. We present growing season soil chamber methane emission (F_CH4), ecosystem respiration (ER), net ecosystem exchange (NEE) and gross primary production (GPP) fluxes from a 9‐years water table manipulation experiment in an Alaskan rich fen. The study included major flood and drought years, where wetting and drying treatments further modified the severity of droughts. Results support previous findings from peatlands that drought causes reduced magnitude of growing season F_CH4, GPP and NEE, thus reducing or reversing their C sink function. Experimentally exacerbated droughts further reduced the capacity for the fen to act as a C sink by causing shifts in vegetation and thus reducing magnitude of maximum growing season GPP in subsequent flood years by ~15% compared to control plots. Conversely, water table position had only a weak influence on ER, but dominant contribution to ER switched from autotrophic respiration in wet years to heterotrophic in dry years. Droughts did not cause inter‐annual lag effects on ER in this rich fen, as has been observed in several nutrient‐poor peatlands. While ER was dependent on soil temperatures at 2 cm depth, F_CH4 was linked to soil temperatures at 25 cm. Inter‐annual variability of deep soil temperatures was in turn dependent on wetness rather than air temperature, and higher F_CH4 in flooded years was thus equally due to increased methane production at depth and decreased methane oxidation near the surface. Short‐term fluctuations in wetness caused significant lag effects on F_CH4, but droughts caused no inter‐annual lag effects on F_CH4. Our results show that frequency and severity of droughts and floods can have characteristic effects on the exchange of greenhouse gases, and emphasize the need to project future hydrological regimes in rich fens.     

滨海蓝碳湿地碳汇速率测定方法及中国的研究现状和挑战

滨海盐沼、红树林和海草床蓝碳湿地生态系统具有高效的固碳-储碳能力,准确测定滨海蓝碳湿地生态系统碳汇速率,对于评估滨海湿地碳中和能力、生态恢复新增碳汇规模及碳贸易至关重要。深入思考滨海蓝碳湿地生态系统碳汇定义的内涵,提出狭义碳汇和广义碳汇的概念,介绍沉积物碳累积+植被净初级生产力法以及生态系统碳通量收支法2个目前国际上应用最多的滨海蓝碳湿地碳汇速率测定方法,特别是深入分析作为开放系统的滨海盐沼生态系统和海草床生态系统碳汇速率测定面临的诸多问题与挑战,梳理中国红树林、滨海盐沼和海草床生态系统碳汇速率的测定结果及国家尺度滨海蓝碳湿地生态系统碳汇规模,最后提出中国在滨海蓝碳湿地碳汇速率测定实践中急需加强的基础研究领域,以期为科学地计量中国滨海蓝碳湿地生态系统碳汇速率与碳汇规模提供方法参考和技术支撑。     

A method of determining rooting depth from a terrestrial biosphere model and its impacts on the global water and carbon cycle

We outline a method of inferring rooting depth from a Terrestrial Biosphere Model by maximizing the benefit of the vegetation within the model. This corresponds to the evolutionary principle that vegetation has adapted to make best use of its local environment. We demonstrate this method with a simple coupled biosphere/soil hydrology model and find that deep rooted vegetation is predicted in most parts of the tropics. Even with a simple model like the one we use, it is possible to reproduce biome averages of observations fairly well. By using the optimized rooting depths global Annual Net Primary Production (and transpiration) increases substantially compared to a standard rooting depth of one meter, especially in tropical regions that have a dry season. The decreased river discharge due to the enhanced evaporation complies better with observations. We also found that the optimization process is primarily driven by the water deficit/surplus during the dry/wet season for humid and arid regions, respectively. Climate variability further enhances rooting depth estimates. In a sensitivity analysis where we simulate changes in the water use efficiency of the vegetation we find that vegetation with an optimized rooting depth is less vulnerable to variations in the forcing. We see the main application of this method in the modelling communities of land surface schemes of General Circulation Models and of global Terrestrial Biosphere Models. We conclude that in these models, the increased soil water storage is likely to have a significant impact on the simulated climate and the carbon budget, respectively. Also, effects of land use change like tropical deforestation are likely to be larger than previously thought.     

Net primary and ecosystem production and carbon stocks of terrestrial ecosystems and their responses to climate change

Evaluating the role of terrestrial ecosystems in the global carbon cycle requires a detailed understanding of carbon exchange between vegetation, soil, and the atmosphere. Global climatic change may modify the net carbon balance of terrestrial ecosystems, causing feedbacks on atmospheric CO₂ and climate. We describe a model for investigating terrestrial carbon exchange and its response to climatic variation based on the processes of plant photosynthesis, carbon allocation, litter production, and soil organic carbon decomposition. The model is used to produce geographical patterns of net primary production (NPP), carbon stocks in vegetation and soils, and the seasonal variations in net ecosystem production (NEP) under both contemporary and future climates. For contemporary climate, the estimated global NPP is 57.0 Gt C y^–1, carbon stocks in vegetation and soils are 640 Gt C and 1358 Gt C, respectively, and NEP varies from –0.5 Gt C in October to 1.6 Gt C in July. For a doubled atmospheric CO₂ concentration and the corresponding climate, we predict that global NPP will rise to 69.6 Gt C y^–1, carbon stocks in vegetation and soils will increase by, respectively, 133 Gt C and 160 Gt C, and the seasonal amplitude of NEP will increase by 76%. A doubling of atmospheric CO₂ without climate change may enhance NPP by 25% and result in a substantial increase in carbon stocks in vegetation and soils. Climate change without CO₂ elevation will reduce the global NPP and soil carbon stocks, but leads to an increase in vegetation carbon because of a forest extension and NPP enhancement in the north. By combining the effects of CO₂ doubling, climate change, and the consequent redistribution of vegetation, we predict a strong enhancement in NPP and carbon stocks of terrestrial ecosystems. This study simulates the possible variation in the carbon exchange at equilibrium state. We anticipate to investigate the dynamic responses in the carbon exchange to atmospheric CO₂ elevation and climate change in the past and future.     

Evidence for large carbon sink and long residence time in semiarid forests based on 15 year flux and inventory records

The rate of change in atmospheric CO₂ is significantly affected by the terrestrial carbon sink, but the size and spatial distribution of this sink, and the extent to which it can be enhanced to mitigate climate change are highly uncertain. We combined carbon stock (CS) and eddy covariance (EC) flux measurements that were collected over a period of 15 years (2001–2016) in a 55 year old 30 km² pine forest growing at the semiarid timberline (with no irrigating or fertilization). The objective was to constrain estimates of the carbon (C) storage potential in forest plantations in such semiarid lands, which cover ~18% of the global land area. The forest accumulated 145–160 g C m^?2 year^?1 over the study period based on the EC and CS approaches, with a mean value of 152.5 ± 30.1 g C m^?2 year^?1 indicating 20% uncertainty in carbon uptake estimates. Current total stocks are estimated at 7,943 ± 323 g C/m² and 372 g N/m². Carbon accumulated mostly in the soil (~71% and 29% for soil and standing biomass carbon, respectively) with long soil carbon turnover time (59 years). Regardless of unexpected disturbances beyond those already observed at the study site, the results support a considerable carbon sink potential in semiarid soils and forest plantations, and imply that afforestation of even 10% of semiarid land area under conditions similar to that of the study site, could sequester ~0.4 Pg C/year over several decades.     

江西省森林碳蓄积过程及碳源/汇的时空格局

森林碳蓄积是研究森林与大气碳交换以及估算森林吸收或排放含碳气体的关键参数,不同年龄森林的碳源/汇功能差异则体现出森林生态系统碳蓄积过程的时间特征。以森林资源清查的样方数据作为数据源,通过刻画主要树种的林分蓄积生长曲线、林龄与净初级生产力(NPP)之间的关系,驱动区域碳收支模型(InTEC)模拟江西省1950—2008年的森林碳蓄积过程,了解山江湖工程实施以来的森林碳源/汇状况。结果表明,20世纪80年代以前,江西省森林年平均NPP波动于450—813 gCm-2a-1之间,年净增生物量碳26.55—36.23 TgC/a,年净增木质林产品碳0.01—0.3 TgC/a;80年代初,NPP和年净增生物量碳分别降至307.39 gC m-2a-1和17.31 TgC/a,而年净增木质林产品碳却高达0.6 TgC/a,说明森林被大量砍伐进入林产品碳库;1985年山江湖工程实施后,大面积造林使得年净增碳蓄积呈现急剧上升趋势,生物量和木质林产品碳蓄积分别上升至目前的42.37 TgC/a和0.79 TgC/a,而平均NPP值增加缓慢、碳汇功能降低,说明林分质量有待提高;90年代后碳汇功能开始稳步增强,说明造林面积的迅速增加是引起江西省森林碳增汇的主要驱动因素,但未来森林增汇潜力应源于森林生长和有效的经营管理。     

长白山亚高山针叶林沼泽湿地碳源/汇沿水分环境梯度变化规律

高纬度和高海拔区为气候变化敏感区,该区域湿地碳循环与气候反馈关系倍受关注。为探究在全球变暖背景下高海拔区沼泽湿地碳源/汇功能是否发生了转化,以长白山高海拔区沿水分环境梯度分布的5种沼泽类型(草丛沼泽-C、灌丛沼泽-G、落叶松泥炭藓沼泽-LN、落叶松藓类沼泽-LX、落叶松苔草沼泽-LT)为对象,采用静态箱-气相色谱法和相对生长方程法,同步测定各沼泽类型全年尺度上的土壤异养呼吸碳排放量(CO₂和CH₄)、植被年净固碳量及相关环境因子(温度、水位和土壤有机碳等),并依据生态系统净碳收支平衡,量化各沼泽类型的碳源/汇作用,揭示其沿水分环境梯度变化规律及形成机制。结果表明：(1)5种沼泽类型土壤CO₂年均通量((97.68±8.64)—(291.01±18.31)mg m^-2 h^-1)沿水分环境梯度呈阶梯式递增规律性(环境梯度上部生境地段的落叶松苔草沼泽和落叶松藓类沼泽最高,中部生境地段的落叶松泥炭藓沼泽和灌丛沼泽居中,草丛沼泽最低);(2)CH₄年均通量((-0....     

Microbial activity and enzymic decomposition processes following peatland water table drawdown

Microbial activity and enzymic decomposition processes were followed during a field-based experimental lowering of the water table in a Welsh peatland. Respiration was not significantly affected by the treatment. However, the enzymes sulphatase, -glucosidase and phosphatase were stimulated by between 31 and 67% upon water table drawdown. A further enzyme, phenol oxidase, was not significantly affected. The observation of elevated enzyme activities without an associated increase in microbial respiratory activity suggests that drought conditions influence peatland mineralisation rates through a direct stimulation of existing enzymes, rather than through a generalised stimulation of microbial metabolism (with associated de-novo enzyme synthesis). Hydrochemical data suggest that the stimulation may have been caused by a reduction in the inhibitory action of iron and phenolics in the peat pore waters. Overall, the findings support the recent hypothesis that drier conditions associated with climate change could stimulate mineralisation within wetlands. ei]R Merckx     

Landscape-variability of the carbon balance across managed boreal forests

Boreal forests are important global carbon (C) sinks and, therefore, considered as a key element in climate change mitigation policies. However, their actual C sink strength is uncertain and under debate, particularly for the actively managed forests in the boreal regions of Fennoscandia. In this study, we use an extensive set of biometric- and chamber-based C flux data collected in 50 forest stands (ranging from 5 to 211 years) over 3 years (2016–2018) with the aim to explore the variations of the annual net ecosystem production (NEP; i.e., the ecosystem C balance) across a 68 km² managed boreal forest landscape in northern Sweden. Our results demonstrate that net primary production rather than heterotrophic respiration regulated the spatio-temporal variations of NEP across the heterogeneous mosaic of the managed boreal forest landscape. We further find divergent successional patterns of NEP in our managed forests relative to naturally regenerating boreal forests, including (i) a fast recovery of the C sink function within the first decade after harvest due to the rapid establishment of a productive understory layer and (ii) a sustained C sink in old stands (131–211 years). We estimate that the rotation period for optimum C sequestration extends to 138 years, which over multiple rotations results in a long-term C sequestration rate of 86.5 t C ha⁻¹ per rotation. Our study highlights the potential of forest management to maximize C sequestration of boreal forest landscapes and associate climate change mitigation effects by developing strategies that optimize tree biomass production rather than heterotrophic soil C emissions.     

Terrestrial carbon balance in a drier world: the effects of water availability in southwestern North America

Global modeling efforts indicate semiarid regions dominate the increasing trend and interannual variation of net CO₂ exchange with the atmosphere, mainly driven by water availability. Many semiarid regions are expected to undergo climatic drying, but the impacts on net CO₂ exchange are poorly understood due to limited semiarid flux observations. Here we evaluated 121 site‐years of annual eddy covariance measurements of net and gross CO₂ exchange (photosynthesis and respiration), precipitation, and evapotranspiration (ET) in 21 semiarid North American ecosystems with an observed range of 100 – 1000 mm in annual precipitation and records of 4–9 years each. In addition to evaluating spatial relationships among CO₂ and water fluxes across sites, we separately quantified site‐level temporal relationships, representing sensitivity to interannual variation. Across the climatic and ecological gradient, photosynthesis showed a saturating spatial relationship to precipitation, whereas the photosynthesis–ET relationship was linear, suggesting ET was a better proxy for water available to drive CO₂ exchanges after hydrologic losses. Both photosynthesis and respiration showed similar site‐level sensitivity to interannual changes in ET among the 21 ecosystems. Furthermore, these temporal relationships were not different from the spatial relationships of long‐term mean CO₂ exchanges with climatic ET. Consequently, a hypothetical 100‐mm change in ET, whether short term or long term, was predicted to alter net ecosystem production (NEP) by 64 gCm^?2 yr^?1. Most of the unexplained NEP variability was related to persistent, site‐specific function, suggesting prioritization of research on slow‐changing controls. Common temporal and spatial sensitivity to water availability increases our confidence that site‐level responses to interannual weather can be extrapolated for prediction of CO₂ exchanges over decadal and longer timescales relevant to societal response to climate change.     

On carbon sequestration in desert ecosystems

Recent reports of net ecosysytem production >100 gCm⁻² yr⁻¹ in deserts are incompatible with existing measurements of net primary production and carbon pools in deserts. The comparisions suggest that gas exchange measurements should be used with caution and better validation if they are expected to indicate the magnitude of carbon sink in these ecosysytems.     

陆地生态系统碳汇评估方法研究进展

“碳中和”是我国作出的一项重大的国家战略决策,陆地生态系统碳汇作为碳增汇的重要组成部分,在碳中和目标实现的过程中发挥着重要的作用。但当前基于不同观测数据和方法的陆地碳汇计算仍有很大的不确定性,为了全面了解陆地生态系统碳汇分布特征,提高陆地生态系统碳汇评估的准确性,梳理了近年来关于陆地生态系统碳汇评估的国内外研究进展,从“自下而上”和“自上而下”两类途径阐述了陆地生态系统碳汇评估的主要方法(样地清查法、涡度相关法、模型模拟法和碳同化反演法)的主要原理和特征,优势和缺陷,及在不同尺度碳汇研究中的应用,并从土地利用/覆盖变化、气候因素(大气CO₂浓度、氮沉降)、环境因素(太阳辐射、温度、降水)等因素阐述了陆地系统碳汇主要驱动因子;分析了我国陆地生态系统碳汇的主要特征及时空变化趋势,并从人类活动(生态工程)和环境因素阐述了中国陆地生态系统碳汇的驱动因素;最后,展望了新的监测手段和评估方法在提升陆地生态系统碳汇评估精度中的作用,从而更好的服务于我国“碳中和”的长远目标。     

气候变化背景下农林复合系统碳汇功能研究进展

农林复合系统作为一种土地综合利用体系,可以有效吸收和固定CO₂、增加碳储量,在达到收获目的的同时,可有力减轻温室效应.农林复合系统对CO₂的调控作用,使人们认识到农林复合系统较单一作物系统有着明显优势,因此,深入了解不同农林复合系统的碳汇功能及其影响因素,对全球碳循环研究及碳收支准确评估具有重要意义.本文综述了农林复合系统的概念与分类,探讨了农林复合系统不同组分的碳固存潜力及其影响因子,得出不同区域、不同类型农林复合系统内植被的固碳速率相差很大(0.59～11.08 t C·hm^-2·a^-1),其主要受到气候因子和农林复合系统自身特性(物种组成、林木密度和林龄)的影响.农林复合系统内土壤的固碳潜力受到系统内树木和非树木成分输入的生物量多少和质量、土壤质地、土壤结构的影响.不同地区的任何一个农林复合系统的碳储量多少主要依赖于复合系统中各组分的结构和功能.针对目前的研究现状,指出应重点加强农林复合系统优化结构的碳汇功能研究,以及加强农林复合系统碳储量的时空分布格局及其固碳机制的长期研究.     

Modeled interactive effects of precipitation, temperature, and [CO2] on ecosystem carbon and water dynamics in different climatic zones

Interactive effects of multiple global change factors on ecosystem processes are complex. It is relatively expensive to explore those interactions in manipulative experiments. We conducted a modeling analysis to identify potentially important interactions and to stimulate hypothesis formulation for experimental research. Four models were used to quantify interactive effects of climate warming (T), altered precipitation amounts [doubled (DP) and halved (HP)] and seasonality (SP, moving precipitation in July and August to January and February to create summer drought), and elevated [CO₂] (C) on net primary production (NPP), heterotrophic respiration (R_h), net ecosystem production (NEP), transpiration, and runoff. We examined those responses in seven ecosystems, including forests, grasslands, and heathlands in different climate zones. The modeling analysis showed that none of the three‐way interactions among T, C, and altered precipitation was substantial for either carbon or water processes, nor consistent among the seven ecosystems. However, two‐way interactive effects on NPP, R_h, and NEP were generally positive (i.e. amplification of one factor's effect by the other factor) between T and C or between T and DP. A negative interaction (i.e. depression of one factor's effect by the other factor) occurred for simulated NPP between T and HP. The interactive effects on runoff were positive between T and HP. Four pairs of two‐way interactive effects on plant transpiration were positive and two pairs negative. In addition, wet sites generally had smaller relative changes in NPP, R_h, runoff, and transpiration but larger absolute changes in NEP than dry sites in response to the treatments. The modeling results suggest new hypotheses to be tested in multifactor global change experiments. Likewise, more experimental evidence is needed for the further improvement of ecosystem models in order to adequately simulate complex interactive processes.     

The physics and ecology of mining carbon dioxide from the atmosphere by ecosystems

Reforesting and managing ecosystems have been proposed as ways to mitigate global warming and offset anthropogenic carbon emissions. The intent of our opinion piece is to provide a perspective on how well plants and ecosystems sequester carbon. The ability of individual plants and ecosystems to mine carbon dioxide from the atmosphere, as defined by rates and cumulative amounts, is limited by laws of physics and ecological principles. Consequently, the rates and amount of net carbon uptake are slow and low compared to the rates and amounts of carbon dioxide we release by fossil fuels combustion. Managing ecosystems to sequester carbon can also cause unintended consequences to arise. In this paper, we articulate a series of key take‐home points. First, the potential amount of carbon an ecosystem can assimilate on an annual basis scales with absorbed sunlight, which varies with latitude, leaf area index and available water. Second, efforts to improve photosynthesis will come with the cost of more respiration. Third, the rates and amount of net carbon uptake are relatively slow and low, compared to the rates and amounts and rates of carbon dioxide we release by fossil fuels combustion. Fourth, huge amounts of land area for ecosystems will be needed to be an effective carbon sink to mitigate anthropogenic carbon emissions. Fifth, the effectiveness of using this land as a carbon sink will depend on its ability to remain as a permanent carbon sink. Sixth, converting land to forests or wetlands may have unintended costs that warm the local climate, such as changing albedo, increasing surface roughness or releasing other greenhouse gases. We based our analysis on 1,163 site‐years of direct eddy covariance measurements of gross and net carbon fluxes from 155 sites across the globe.     

Warming and depth interact to affect carbon dioxide concentration in aquatic mesocosms

1. Climate change may significantly influence lake carbon dynamics and consequently the exchange of CO₂ with the atmosphere. Warming will accelerate multiple processes that either absorb or release CO₂, making predicting the net effect of warming on CO₂ exchange with the atmosphere difficult. Here we experimentally test how the CO₂ flux of deep and shallow systems responds to warming. To do this, we conducted a greenhouse experiment using mesocosms of two depths, experiencing either ambient or warmed temperatures. 2. Deeper mesocosms were found to have a lower average CO₂ concentration than shallower mesocosms under ambient temperature conditions. In addition, warming interacts with mesocosm depth to affect the average CO₂ concentration; there was no effect of warming on the average CO₂ concentration of deep mesocosms, but shallow mesocosms had significantly lower average CO₂ concentrations when warmed. 3. The difference in CO₂ concentration resulting from the depth manipulation was due to varying loss rates of particulate carbon to the sediments. There was a strong negative correlation between CO₂ and sedimentation rates in the deep mesocosms suggesting that high particulate carbon loss to the sediments lowered the CO₂ concentration in the water column. There was no correlation between CO₂ and sedimentation rates observed for shallow mesocosms suggesting enhanced carbon regeneration from the sediments was maintaining higher CO₂ concentrations in the water column. 4. Relationships between CO₂ and algal concentrations indicate that the reduction in CO₂ concentrations resulting from warming is due to increased per capita algal turnover rates depleting CO₂ in the water column. Our results suggest that the carbon dynamics and CO₂ flux of shallow systems will be affected more by climate warming than deep systems and the net effect of warming is to increase CO₂ uptake.