Reconciling estimates of the contemporary North American carbon balance among terrestrial biosphere models,atmospheric inversions,and a new approach for estimating net ecosystem exchange from inventory‐based data

We develop an approach for estimating net ecosystem exchange (NEE) using inventory‐based information over North America (NA) for a recent 7‐year period (ca. 2000–2006). The approach notably retains information on the spatial distribution of NEE, or the vertical exchange between land and atmosphere of all non‐fossil fuel sources and sinks of CO₂, while accounting for lateral transfers of forest and crop products as well as their eventual emissions. The total NEE estimate of a ?327 ± 252 TgC yr^?1 sink for NA was driven primarily by CO₂ uptake in the Forest Lands sector (?248 TgC yr^?1), largely in the Northwest and Southeast regions of the US, and in the Crop Lands sector (?297 TgC yr^?1), predominantly in the Midwest US states. These sinks are counteracted by the carbon source estimated for the Other Lands sector (+218 TgC yr^?1), where much of the forest and crop products are assumed to be returned to the atmosphere (through livestock and human consumption). The ecosystems of Mexico are estimated to be a small net source (+18 TgC yr^?1) due to land use change between 1993 and 2002. We compare these inventory‐based estimates with results from a suite of terrestrial biosphere and atmospheric inversion models, where the mean continental‐scale NEE estimate for each ensemble is ?511 TgC yr^?1 and ?931 TgC yr^?1, respectively. In the modeling approaches, all sectors, including Other Lands, were generally estimated to be a carbon sink, driven in part by assumed CO₂ fertilization and/or lack of consideration of carbon sources from disturbances and product emissions. Additional fluxes not measured by the inventories, although highly uncertain, could add an additional ?239 TgC yr^?1 to the inventory‐based NA sink estimate, thus suggesting some convergence with the modeling approaches.     

State of the science in reconciling top‐down and bottom‐up approaches for terrestrial CO2 budget

Robust estimates of CO₂ budget, CO₂ exchanged between the atmosphere and terrestrial biosphere, are necessary to better understand the role of the terrestrial biosphere in mitigating anthropogenic CO₂ emissions. Over the past decade, this field of research has advanced through understanding of the differences and similarities of two fundamentally different approaches: “top‐down” atmospheric inversions and “bottom‐up” biosphere models. Since the first studies were undertaken, these approaches have shown an increasing level of agreement, but disagreements in some regions still persist, in part because they do not estimate the same quantity of atmosphere–biosphere CO₂ exchange. Here, we conducted a thorough comparison of CO₂ budgets at multiple scales and from multiple methods to assess the current state of the science in estimating CO₂ budgets. Our set of atmospheric inversions and biosphere models, which were adjusted for a consistent flux definition, showed a high level of agreement for global and hemispheric CO₂ budgets in the 2000s. Regionally, improved agreement in CO₂ budgets was notable for North America and Southeast Asia. However, large gaps between the two methods remained in East Asia and South America. In other regions, Europe, boreal Asia, Africa, South Asia, and Oceania, it was difficult to determine whether those regions act as a net sink or source because of the large spread in estimates from atmospheric inversions. These results highlight two research directions to improve the robustness of CO₂ budgets: (a) to increase representation of processes in biosphere models that could contribute to fill the budget gaps, such as forest regrowth and forest degradation; and (b) to reduce sink–source compensation between regions (dipoles) in atmospheric inversion so that their estimates become more comparable. Advancements on both research areas will increase the level of agreement between the top‐down and bottom‐up approaches and yield more robust knowledge of regional CO₂ budgets.     

FLUXNET and modelling the global carbon cycle

Measurements of the net CO₂ flux between terrestrial ecosystems and the atmosphere using the eddy covariance technique have the potential to underpin our interpretation of regional CO₂ source–sink patterns, CO₂ flux responses to forcings, and predictions of the future terrestrial C balance. Information contained in FLUXNET eddy covariance data has multiple uses for the development and application of global carbon models, including evaluation/validation, calibration, process parameterization, and data assimilation. This paper reviews examples of these uses, compares global estimates of the dynamics of the global carbon cycle, and suggests ways of improving the utility of such data for global carbon modelling. Net ecosystem exchange of CO₂ (NEE) predicted by different terrestrial biosphere models compares favourably with FLUXNET observations at diurnal and seasonal timescales. However, complete model validation, particularly over the full annual cycle, requires information on the balance between assimilation and decomposition processes, information not readily available for most FLUXNET sites. Site history, when known, can greatly help constrain the model‐data comparison. Flux measurements made over four vegetation types were used to calibrate the land‐surface scheme of the Goddard Institute for Space Studies global climate model, significantly improving simulated climate and demonstrating the utility of diurnal FLUXNET data for climate modelling. Land‐surface temperatures in many regions cool due to higher canopy conductances and latent heat fluxes, and the spatial distribution of CO₂ uptake provides a significant additional constraint on the realism of simulated surface fluxes. FLUXNET data are used to calibrate a global production efficiency model (PEM). This model is forced by satellite‐measured absorbed radiation and suggests that global net primary production (NPP) increased 6.2% over 1982–1999. Good agreement is found between global trends in NPP estimated by the PEM and a dynamic global vegetation model (DGVM), and between the DGVM and estimates of global NEE derived from a global inversion of atmospheric CO₂ measurements. Combining the PEM, DGVM, and inversion results suggests that CO₂ fertilization is playing a major role in current increases in NPP, with lesser impacts from increasing N deposition and growing season length. Both the PEM and the inversion identify the Amazon basin as a key region for the current net terrestrial CO₂ uptake (i.e. 33% of global NEE), as well as its interannual variability. The inversion's global NEE estimate of −1.2 Pg [C] yr⁻¹ for 1982–1995 is compatible with the PEM‐ and DGVM‐predicted trends in NPP. There is, thus, a convergence in understanding derived from process‐based models, remote‐sensing‐based observations, and inversion of atmospheric data. Future advances in field measurement techniques, including eddy covariance (particularly concerning the problem of night‐time fluxes in dense canopies and of advection or flow distortion over complex terrain), will result in improved constraints on land‐atmosphere CO₂ fluxes and the rigorous attribution of mechanisms to the current terrestrial net CO₂ uptake and its spatial and temporal heterogeneity. Global ecosystem models play a fundamental role in linking information derived from FLUXNET measurements to atmospheric CO₂ variability. A number of recommendations concerning FLUXNET data are made, including a request for more comprehensive site data (particularly historical information), more measurements in undisturbed ecosystems, and the systematic provision of error estimates. The greatest value of current FLUXNET data for global carbon cycle modelling is in evaluating process representations, rather than in providing an unbiased estimate of net CO₂ exchange.     

A large proportion of North American net ecosystem production is offset by emissions from harvested products,river/stream evasion,and biomass burning

Diagnostic carbon cycle models produce estimates of net ecosystem production (NEP, the balance of net primary production and heterotrophic respiration) by integrating information from (i) satellite‐based observations of land surface vegetation characteristics; (ii) distributed meteorological data; and (iii) eddy covariance flux tower observations of net ecosystem exchange (NEE) (used in model parameterization). However, a full bottom‐up accounting of NEE (the vertical carbon flux) that is suitable for integration with atmosphere‐based inversion modeling also includes emissions from decomposition/respiration of harvested forest and agricultural products, CO₂ evasion from streams and rivers, and biomass burning. Here, we produce a daily time step NEE for North America for the year 2004 that includes NEP as well as the additional emissions. This NEE product was run in the forward mode through the CarbonTracker inversion setup to evaluate its consistency with CO₂ concentration observations. The year 2004 was climatologically favorable for NEP over North America and the continental total was estimated at 1730 ± 370 TgC yr^?1 (a carbon sink). Harvested product emissions (316 ± 80 TgC yr^?1), river/stream evasion (158 ± 50 TgC yr^?1), and fire emissions (142 ± 45 TgC yr^?1) counteracted a large proportion (35%) of the NEP sink. Geographic areas with strong carbon sinks included Midwest US croplands, and forested regions of the Northeast, Southeast, and Pacific Northwest. The forward mode run with CarbonTracker produced good agreement between observed and simulated wintertime CO₂ concentrations aggregated over eight measurement sites around North America, but overestimates of summertime concentrations that suggested an underestimation of summertime carbon uptake. As terrestrial NEP is the dominant offset to fossil fuel emission over North America, a good understanding of its spatial and temporal variation – as well as the fate of the carbon it sequesters ─ is needed for a comprehensive view of the carbon cycle.     

Comparison of Vegetation and CO2 Dynamics Between a Restored Cut‐Away Peatland and a Pristine Fen: Evaluation of the Restoration Success

We studied the restoration success of a cut‐away peatland 10 years after restoration by comparing the vegetation and CO₂ dynamics with those of a pristine peatland of similar nutrition level and climate. Vegetation and CO₂ dynamics were monitored during one growing season. We used DCA (detrended correspondence analysis) and diversity indices to study the vegetation composition within and between the sites, and non‐linear regression models to estimate the seasonal CO₂ fluxes and balances of the sites. Based on both DCA and diversity indices, the study plots in the restored site differed more in the vegetation composition than the study plots in the pristine site. The variation in the CO₂ fluxes and balance was greater in the restored than in the pristine site, resulting from the heterogeneous vegetation in the restored site. The seasonal net CO₂ balance was positive (sink) at both sites, the restored site binding on average 500 ± 410 g CO₂/m² and the pristine site 390 ± 265 g CO₂/m² (statistically not different, p = 0.575). The results indicate that the restoration of the vegetation composition is still incomplete but the vegetation coverage is sufficient for the restored site to function as a sink of atmospheric CO₂.     

Reconstruction and attribution of the carbon sink of European forests between 1950 and 2000

European forests are an important carbon sink; however, the relative contributions to this sink of climate, atmospheric CO₂ concentration ([CO₂]), nitrogen deposition and forest management are under debate. We attributed the European carbon sink in forests using ORCHIDEE‐FM, a process‐based vegetation model that differs from earlier versions of ORCHIDEE by its explicit representation of stand growth and idealized forest management. The model was applied on a grid across Europe to simulate changes in the net ecosystem productivity (NEP) of forests with and without changes in climate, [CO₂] and age structure, the three drivers represented in ORCHIDEE‐FM. The model simulates carbon stocks and volume increment that are comparable – root mean square error of 2 m³ ha^?1 yr^?1 and 1.7 kg C m^?2 respectively – with inventory‐derived estimates at country level for 20 European countries. Our simulations estimate a mean European forest NEP of 175 ± 52 g C m^?2 yr^?1 in the 1990s. The model simulation that is most consistent with inventory records provides an upwards trend of forest NEP of 1 ± 0.5 g C m^?2 yr^?2 between 1950 and 2000 across the EU 25. Furthermore, the method used for reconstructing past age structure was found to dominate its contribution to temporal trends in NEP. The potentially large fertilizing effect of nitrogen deposition cannot be told apart, as the model does not explicitly simulate the nitrogen cycle. Among the three drivers that were considered in this study, the fertilizing effect of increasing [CO₂] explains about 61% of the simulated trend, against 26% to changes in climate and 13% only to changes in forest age structure. The major role of [CO₂] at the continental scale is due to its homogeneous impact on net primary productivity (NPP). At the local scale, however, changes in climate and forest age structure often dominate trends in NEP by affecting NPP and heterotrophic respiration.     

Effect of eutrophication on air–sea CO2 fluxes in the coastal Southern North Sea: a model study of the past 50 years

The RIVERSTRAHLER model, an idealized biogeochemical model of the river system, has been coupled to MIRO‐CO₂, a complex biogeochemical model describing diatom and Phaeocystis blooms and carbon and nutrient cycles in the marine domain, to assess the dual role of changing nutrient loads and increasing atmospheric CO₂ as drivers of air–sea CO₂ exchanges in the Southern North Sea with a focus on the Belgian coastal zone (BCZ). The whole area, submitted to the influence of two main rivers (Seine and Scheldt), is characterized by variable diatom and Phaeocystis colonies blooms which impact on the trophic status and air–sea CO₂ fluxes of the coastal ecosystem. For this application, the MIRO‐CO₂ model is implemented in a 0D multibox frame covering the eutrophied Eastern English Channel and Southern North Sea and receiving loads from the rivers Seine and Scheldt. Model simulations are performed for the period between 1951 and 1998 using real forcing fields for sea surface temperature, wind speed and atmospheric CO₂ and RIVERSTRAHLER simulations for river carbon and nutrient loads. Model results suggest that the BCZ shifted from a source of CO₂ before 1970 (low eutrophication) towards a sink during the 1970–1990 period when anthropogenic DIN and P loads increased, stimulating C fixation by autotrophs. In agreement, a shift from net annual heterotrophy towards autotrophy in BCZ is simulated from 1980. The period after 1990 is characterized by a progressive decrease of P loads concomitant with a decrease of primary production and of the CO₂ sink in the BCZ. At the end of the simulation period, the BCZ ecosystem is again net heterotroph and acts as a source of CO₂ to the atmosphere. R‐MIRO‐CO₂ scenarios testing the relative impact of temperature, wind speed, atmospheric CO₂ and river loads variability on the simulated air–sea CO₂ fluxes suggest that the trend in air–sea CO₂ fluxes simulated between 1951 and 1998 in the BCZ was mainly controlled by the magnitude and the ratio of inorganic nutrient river loads. Quantitative nutrient changes control the level of primary production while qualitative changes modulate the relative contribution of diatoms and Phaeocystis to this flux and hence the sequestration of atmospheric CO₂.     

Net uptake of atmospheric CO2 by coastal submerged aquatic vegetation

‘Blue Carbon’, which is carbon captured by marine living organisms, has recently been highlighted as a new option for climate change mitigation initiatives. In particular, coastal ecosystems have been recognized as significant carbon stocks because of their high burial rates and long‐term sequestration of carbon. However, the direct contribution of Blue Carbon to the uptake of atmospheric CO₂ through air‐sea gas exchange remains unclear. We performed in situ measurements of carbon flows, including air‐sea CO₂ fluxes, dissolved inorganic carbon changes, net ecosystem production, and carbon burial rates in the boreal (Furen), temperate (Kurihama), and subtropical (Fukido) seagrass meadows of Japan from 2010 to 2013. In particular, the air‐sea CO₂ flux was measured using three methods: the bulk formula method, the floating chamber method, and the eddy covariance method. Our empirical results show that submerged autotrophic vegetation in shallow coastal waters can be functionally a sink for atmospheric CO_2. This finding is contrary to the conventional perception that most near‐shore ecosystems are sources of atmospheric CO₂. The key factor determining whether or not coastal ecosystems directly decrease the concentration of atmospheric CO₂ may be net ecosystem production. This study thus identifies a new ecosystem function of coastal vegetated systems; they are direct sinks of atmospheric CO₂.     

Tundra ecosystems observed to be CO2 sources due to differential amplification of the carbon cycle

Are tundra ecosystems currently a carbon source or sink? What is the future trajectory of tundra carbon fluxes in response to climate change? These questions are of global importance because of the vast quantities of organic carbon stored in permafrost soils. In this meta‐analysis, we compile 40 years of CO₂ flux observations from 54 studies spanning 32 sites across northern high latitudes. Using time‐series analysis, we investigated if seasonal or annual CO₂ fluxes have changed over time, and whether spatial differences in mean annual temperature could help explain temporal changes in CO₂ flux. Growing season net CO₂ uptake has definitely increased since the 1990s; the data also suggest (albeit less definitively) an increase in winter CO₂ emissions, especially in the last decade. In spite of the uncertainty in the winter trend, we estimate that tundra sites were annual CO₂ sources from the mid‐1980s until the 2000s, and data from the last 7 years show that tundra continue to emit CO₂ annually. CO₂ emissions exceed CO₂ uptake across the range of temperatures that occur in the tundra biome. Taken together, these data suggest that despite increases in growing season uptake, tundra ecosystems are currently CO₂ sources on an annual basis.     

Critical land change information enhances the understanding of carbon balance in the United States

Large‐scale terrestrial carbon (C) estimating studies using methods such as atmospheric inversion, biogeochemical modeling, and field inventories have produced different results. The goal of this study was to integrate fine‐scale processes including land use and land cover change into a large‐scale ecosystem framework. We analyzed the terrestrial C budget of the conterminous United States from 1971 to 2015 at 1‐km resolution using an enhanced dynamic global vegetation model and comprehensive land cover change data. Effects of atmospheric CO₂ fertilization, nitrogen deposition, climate, wildland fire, harvest, and land use/land cover change (LUCC) were considered. We estimate annual C losses from cropland harvest, forest clearcut and thinning, fire, and LUCC were 436.8, 117.9, 10.5, and 10.4 TgC/year, respectively. C stored in ecosystems increased from 119,494 to 127,157 TgC between 1971 and 2015, indicating a mean annual net C sink of 170.3 TgC/year. Although ecosystem net primary production increased by approximately 12.3 TgC/year, most of it was offset by increased C loss from harvest and natural disturbance and increased ecosystem respiration related to forest aging. As a result, the strength of the overall ecosystem C sink did not increase over time. Our modeled results indicate the conterminous US C sink was about 30% smaller than previous modeling studies, but converged more closely with inventory data.     

Land‐atmosphere exchange of methane from soil thawing to soil freezing in a high‐Arctic wet tundra ecosystem

The land‐atmosphere exchange of methane (CH₄) and carbon dioxide (CO₂) in a high‐Arctic wet tundra ecosystem (Rylekærene) in Zackenberg, north‐eastern Greenland, was studied over the full growing season and until early winter in 2008 and from before snow melt until early winter in 2009. The eddy covariance technique was used to estimate CO₂ fluxes and a combination of the gradient and eddy covariance methods was used to estimate CH₄ fluxes. Small CH₄ bursts were observed during spring thawing 2009, but these existed during short periods and would not have any significant effect on the annual budget. Growing season CH₄ fluxes were well correlated with soil temperature, gross primary production, and active layer thickness. The CH₄ fluxes remained low during the entire autumn, and until early winter. No increase in CH₄ fluxes were seen as the soil started to freeze. However, in autumn 2008 there were two CH₄ burst events that were highly correlated with atmospheric turbulence. They were likely associated with the release of stored CH₄ from soil and vegetation cavities. Over the measurement period, 7.6 and 6.5 g C m^?2 was emitted as CH₄ in 2008 and in 2009, respectively. Rylekærene acted as a C source during the warmer and wetter measurement period 2008, whereas it was a C sink for the colder and drier period of 2009. Wet tundra ecosystems, such as Rylekærene may thus play a more significant role for the climate in the future, as temperature and precipitation are predicted to increase in the high‐Arctic.     

Spatial variation in landscape‐level CO2 and CH4 fluxes from arctic coastal tundra: influence from vegetation,wetness, and the thaw lake cycle

Regional quantification of arctic CO₂ and CH₄ fluxes remains difficult due to high landscape heterogeneity coupled with a sparse measurement network. Most of the arctic coastal tundra near Barrow, Alaska is part of the thaw lake cycle, which includes current thaw lakes and a 5500‐year chronosequence of vegetated thaw lake basins. However, spatial variability in carbon fluxes from these features remains grossly understudied. Here, we present an analysis of whole‐ecosystem CO₂ and CH₄ fluxes from 20 thaw lake cycle features during the 2011 growing season. We found that the thaw lake cycle was largely responsible for spatial variation in CO₂ flux, mostly due to its control on gross primary productivity (GPP). Current lakes were significant CO₂ sources that varied little. Vegetated basins showed declining GPP and CO₂ sink with age (R² = 67% and 57%, respectively). CH₄ fluxes measured from a subset of 12 vegetated basins showed no relationship with age or CO₂ flux components. Instead, higher CH₄ fluxes were related to greater landscape wetness (R² = 57%) and thaw depth (additional R² = 28%). Spatial variation in CO₂ and CH₄ fluxes had good satellite remote sensing indicators, and we estimated the region to be a small CO₂ sink of ?4.9 ± 2.4 (SE) g C m^?2 between 11 June and 25 August, which was countered by a CH₄ source of 2.1 ± 0.2 (SE) g C m^?2. Results from our scaling exercise showed that developing or validating regional estimates based on single tower sites can result in significant bias, on average by a factor 4 for CO₂ flux and 30% for CH₄ flux. Although our results are specific to the Arctic Coastal Plain of Alaska, the degree of landscape‐scale variability, large‐scale controls on carbon exchange, and implications for regional estimation seen here likely have wide relevance to other arctic landscapes.     

Boundary layer budgets for regional estimates of scalar fluxes

The paper considers the theory and application of budget techniques for regional scalar flux estimation using the daytime convective boundary layer (CBL) and the nocturnal boundary layer (NBL). CBL techniques treat the well mixed layer of air between heights of, say, 100 m and 1000 m as an integrator of surface fluxes along the path of a column of air moving over the landscape. They calculate the average surface flux from the scalar concentration in and above the mixed layer, and the CBL height. The flux estimates are averaged over regions of 10–10⁴ km² extending 10 to 100 km upwind. An integral form of the CBL budget is used to estimate daily regional rates of CO₂ uptake and evaporation from three data sets. There was plausible agreement between the estimates and locally measured fluxes. CBL budgets have great potential for estimating regional scalar fluxes, but there is an urgent need for validation through direct measurements of fluxes and budget parameters. NBL budgets are useful when low-level, radiative inversions inhibit vertical mixing. Surface scalar fluxes can then be estimated from the rate of concentration change below the inversion. An example application for estimating the nocturnal CO₂ flux is given. While simple in concept, NBL budgets are more difficult to apply in practice because of the unpredictability of the depth of the layer and sometimes, its absence altogether. On the other hand, the depth of the atmospheric mixing chamber is better defined, few assumptions are required and the concentration changes usually will be larger and hence more easily detectable than in CBL budgetting.     

Climate change reduces the net sink of CH4 and N2O in a semiarid grassland

Atmospheric concentrations of methane (CH₄) and nitrous oxide (N₂O) have increased over the last 150 years because of human activity. Soils are important sources and sinks of both potent greenhouse gases where their production and consumption are largely regulated by biological processes. Climate change could alter these processes thereby affecting both rate and direction of their exchange with the atmosphere. We examined how a rise in atmospheric CO₂ and temperature affected CH₄ and N₂O fluxes in a well‐drained upland soil (volumetric water content ranging between 6% and 23%) in a semiarid grassland during five growing seasons. We hypothesized that responses of CH₄ and N₂O fluxes to elevated CO₂ and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO₂ increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO₂ and warming would have opposing effects on CH₄ and N₂O fluxes. Methane was taken up throughout the growing season in all 5 years. A bell‐shaped relationship was observed with soil moisture with highest CH₄ uptake at intermediate soil moisture. Both N₂O emission and uptake occurred at our site with some years showing cumulative N₂O emission and other years showing cumulative N₂O uptake. Nitrous oxide exchange switched from net uptake to net emission with increasing soil moisture. In contrast to our hypothesis, both elevated CO₂ and warming reduced the sink of CH₄ and N₂O expressed in CO₂ equivalents (across 5 years by 7% and 11% for elevated CO₂ and warming respectively) suggesting that soil moisture changes were not solely responsible for this reduction. We conclude that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH₄ and N₂O expressed in CO₂‐equivalents.     

Quantifying the erosion effect on current carbon budget of European agricultural soils at high spatial resolution

The idea of offsetting anthropogenic CO₂ emissions by increasing global soil organic carbon (SOC), as recently proposed by French authorities ahead of COP21 in the ‘four per mil’ initiative, is notable. However, a high uncertainty still exits on land C balance components. In particular, the role of erosion in the global C cycle is not totally disentangled, leading to disagreement whether this process induces lands to be a source or sink of CO₂. To investigate this issue, we coupled soil erosion into a biogeochemistry model, running at 1 km² resolution across the agricultural soils of the European Union (EU). Based on data‐driven assumptions, the simulation took into account also soil deposition within grid cells and the potential C export to riverine systems, in a way to be conservative in a mass balance. We estimated that 143 of 187 Mha have C erosion rates <0.05 Mg C ha^?1 yr^?1, although some hot‐spot areas showed eroded SOC >0.45 Mg C ha^?1 yr^?1. In comparison with a baseline without erosion, the model suggested an erosion‐induced sink of atmospheric C consistent with previous empirical‐based studies. Integrating all C fluxes for the EU agricultural soils, we estimated a net C loss or gain of ?2.28 and +0.79 Tg yr^?1 of CO₂eq, respectively, depending on the value for the short‐term enhancement of soil C mineralization due to soil disruption and displacement/transport with erosion. We concluded that erosion fluxes were in the same order of current carbon gains from improved management. Even if erosion could potentially induce a sink for atmospheric CO₂, strong agricultural policies are needed to prevent or reduce soil erosion, in order to maintain soil health and productivity.     

Simulated variations in atmospheric CO2 over a Wisconsin forest using a coupled ecosystem–atmosphere model

Ecosystem fluxes of energy, water, and CO₂ result in spatial and temporal variations in atmospheric properties. In principle, these variations can be used to quantify the fluxes through inverse modelling of atmospheric transport, and can improve the understanding of processes and falsifiability of models. We investigated the influence of ecosystem fluxes on atmospheric CO₂ in the vicinity of the WLEF‐TV tower in Wisconsin using an ecophysiological model (Simple Biosphere, SiB2) coupled to an atmospheric model (Regional Atmospheric Modelling System). Model parameters were specified from satellite imagery and soil texture data. In a companion paper, simulated fluxes in the immediate tower vicinity have been compared to eddy covariance fluxes measured at the tower, with meteorology specified from tower sensors. Results were encouraging with respect to the ability of the model to capture observed diurnal cycles of fluxes. Here, the effects of fluxes in the tower footprint were also investigated by coupling SiB2 to a high‐resolution atmospheric simulation, so that the model physiology could affect the meteorological environment. These experiments were successful in reproducing observed fluxes and concentration gradients during the day and at night, but revealed problems during transitions at sunrise and sunset that appear to be related to the canopy radiation parameterization in SiB2.     

The greenhouse gas balance of European grasslands

The greenhouse gas (GHG) balance of European grasslands (EU‐28 plus Norway and Switzerland), including CO₂, CH₄ and N₂O, is estimated using the new process‐based biogeochemical model ORCHIDEE‐GM over the period 1961–2010. The model includes the following: (1) a mechanistic representation of the spatial distribution of management practice; (2) management intensity, going from intensively to extensively managed; (3) gridded simulation of the carbon balance at ecosystem and farm scale; and (4) gridded simulation of N₂O and CH₄ emissions by fertilized grassland soils and livestock. The external drivers of the model are changing animal numbers, nitrogen fertilization and deposition, land‐use change, and variable CO₂ and climate. The carbon balance of European grassland (NBP) is estimated to be a net sink of 15 ± 7 g C m^?2 year^?1 during 1961–2010, equivalent to a 50‐year continental cumulative soil carbon sequestration of 1.0 ± 0.4 Pg C. At the farm scale, which includes both ecosystem CO₂ fluxes and CO₂ emissions from the digestion of harvested forage, the net C balance is roughly halved, down to a small sink, or nearly neutral flux of 8 g C m^?2 year^?1. Adding CH₄ and N₂O emissions to net ecosystem exchange to define the ecosystem‐scale GHG balance, we found that grasslands remain a net GHG sink of 19 ± 10 g C‐CO₂ equiv. m^?2 year^?1, because the CO₂ sink offsets N₂O and grazing animal CH₄ emissions. However, when considering the farm scale, the GHG balance (NGB) becomes a net GHG source of ?50 g C‐CO₂ equiv. m^?2 year^?1. ORCHIDEE‐GM simulated an increase in European grassland NBP during the last five decades. This enhanced NBP reflects the combination of a positive trend of net primary production due to CO₂, climate and nitrogen fertilization and the diminishing requirement for grass forage due to the Europe‐wide reduction in livestock numbers.     

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。     

Climatic role of terrestrial ecosystem under elevated CO2: a bottom‐up greenhouse gases budget

The net balance of greenhouse gas (GHG) exchanges between terrestrial ecosystems and the atmosphere under elevated atmospheric carbon dioxide (CO₂) remains poorly understood. Here, we synthesise 1655 measurements from 169 published studies to assess GHGs budget of terrestrial ecosystems under elevated CO₂. We show that elevated CO₂ significantly stimulates plant C pool (NPP) by 20%, soil CO₂ fluxes by 24%, and methane (CH₄) fluxes by 34% from rice paddies and by 12% from natural wetlands, while it slightly decreases CH₄ uptake of upland soils by 3.8%. Elevated CO₂ causes insignificant increases in soil nitrous oxide (N₂O) fluxes (4.6%), soil organic C (4.3%) and N (3.6%) pools. The elevated CO₂‐induced increase in GHG emissions may decline with CO₂ enrichment levels. An elevated CO₂‐induced rise in soil CH₄ and N₂O emissions (2.76 Pg CO₂‐equivalent year^?1) could negate soil C enrichment (2.42 Pg CO₂ year^?1) or reduce mitigation potential of terrestrial net ecosystem production by as much as 69% (NEP, 3.99 Pg CO₂ year^?1) under elevated CO₂. Our analysis highlights that the capacity of terrestrial ecosystems to act as a sink to slow climate warming under elevated CO₂ might have been largely offset by its induced increases in soil GHGs source strength.     

Elevated CO2 alters carbon fluxes in early successional Mediterranean ecosystems

Annual carbon budgets of ecosystems are central to our understanding of the biotic control of atmospheric composition, but they are not available under elevated CO₂ for most vegetation types. Using gas exchange techniques, we assessed carbon fluxes of four early successional Mediterranean model communities, consisting of grasses, legumes and composites. The assemblages were grown on the same monoliths for three consecutive years in greenhouses tracking field conditions except for CO₂ maintained at ambient (370 μmol mol^?1) or elevated (700 μmol mol^?1) concentration. During the third year of study, CO₂ enrichment consistently shifted the annual carbon balance towards lower efflux, with displacements between 4.3 and 26.2 mol m^?2 y^?1 (one assemblage became a net CO₂ sink, another just reached equilibrium, and the remaining two remained as a CO₂ source). At least 50% of the shift under elevated CO₂ originated from a decrease in belowground respiration. This indicates that, during this year, CO₂ enrichment did not predominantly enhance C‐cycling, but on the contrary inhibited root respiration or microbial C‐utilization. Although elevated‐CO₂‐grown systems acted as a net CO₂ sink during a longer period of the year (4–7 months) compared with ambient‐CO₂‐grown systems (3–3.5 months), gross canopy photosynthesis was modified only to a limited extent (between ?5.9 and + 14.8%). Interaction between the carbon and the water cycle was apparently responsible for this weak stimulation. In particular, reduced evapotranspiration under elevated CO₂ coincided with inhibited canopy photosynthesis in early spring, most likely resulting from water saturation of the soil. In addition, only the earliest‐planted assemblages had an increased gross canopy photosynthesis during late autumn and early winter. This suggests that a longer summer drought, by delaying the establishment of such an annual type of vegetation, would reduce the positive impact of elevated CO₂ on productivity. Water regime appears to strongly govern the influence of CO₂ on the carbon fluxes in Mediterranean ecosystems with annual herbaceous vegetation.