Net Ecosystem CO2 Exchange Research Articles

Seasonal and diurnal variations of carbon dioxide and energy fluxes over three land cover types of Nepal

This study examines the seasonal and diurnal variations of carbon dioxide and energy fluxes over three land cover types of Nepal by using the eddy covariance method from March to November 2016. The surface energy balance closures were moderate with the values of about 56%, 61%, and 64% closure at Kirtipur, Simara, and Tarahara sites respectively. The monthly average values of net radiation flux and latent heat flux peaked in August at Kirtipur and Tarahara sites whereas in June at the Simara site respectively. The maximum monthly average measured sensible heat flux was 37 W m−2, 43.6 W m−2, and 36.3 W m−2 in April for all the sites whereas soil heat flux was 5.1 W m−2 and 2.9 W m−2 in April for Kirtipur and Simara sites and 6.2 W m−2 in June for the Tarahara site. The magnitude of diurnal peak of net ecosystem CO2 exchange (NEE) reached up to 11.04 μmol m−2 s−1 at Kirtipur, 15.04 μmol m−2 s−1 at Simara, and 10.44 μmol m−2 s−1 at Tarahara sites respectively. Among the three study sites, the ecosystem at the Kirtipur site was a good carbon source; the ecosystems at Simara and Tarahara sites were low and good carbon sink in the growing season. In addition, all three different land cover ecosystem were carbon source when accounted for the measurement period.

Direct and indirect effects of environmental factors on daily CO2 exchange in a rainfed maize cropland—A SEM analysis with 10 year observations

The carbon budget of agricultural ecosystems is of great importance to the global carbon balance. In this study, the structural equation modeling (SEM) method was employed to quantify the direct and indirect effects of environmental factors on daily net ecosystem CO2 exchange (NEE) in a rainfed maize cropland, northeast China. The results showed that net radiation (Rn) was the most important factor controlling daily NEE, followed by the leaf area index (LAI), air temperature (Ta), vapor pressure deficit (VPD), and clearness index (Kt). The strong effect of Rn was mainly attributed to its direct effect, while Ta and VPD showed comparable or even higher indirect effects than direct effects, indicating that Ta and VPD influenced NEE mainly through regulating canopy development on a daily scale. Moreover, the responses of NEE to individual environmental factors differed greatly among multiyear climatic conditions. Ta and VPD were shown to be more important in warm (WM) years and warm and dry (WD) years than in normal (NM) years and cold and wet (CW) years. LAI was the primary controlling factor of daily NEE in WD years, resulting in large indirect effects of Ta and VPD on NEE. Kt had a large effect on daily NEE only in CW years. SWC had significant effects on daily NEE in WM and WD years but in the opposite direction, i.e., daily NEE increased with SWC in WD years but decreased with SWC in CW years. The partitioning of direct and indirect effects of environmental factors with SEM can greatly enhance the understanding of the controlling mechanism of NEE and remind researchers to consider the distinct effects of environmental factors on NEE among multiyear climatic conditions in future models.

Environmental factors affecting greenhouse gas fluxes of green roofs in temperate zone

This paper investigates the full seasonal greenhouse gas (GHG) dynamics of fluxes from three green roof systems (lightweight clay aggregate-based green roof – LR; grass roof – GR; sod roof – SR) and natural control site on shallow Leptosol (NC), using closed static chambers in the period April 2014 to December 2015. CO2, CH4 and N2O fluxes are measured and their relationships to meteorological parameters and substrate physicochemical characteristics are quantified. Median CO2 flux values were 21 (LR), 38 (GR), 62 (SR), and 82 (NC) mg CO2-C m−2 h−1. The results show ecosystem respiration (Reco) clearly increased until July and then decreased until November. Net ecosystem CO2 exchange (NEE) was more variable than Reco, depending on plant growth phase and weather conditions. Median NEE values for study period (from April to November 2015) were −7 (LR), −17 (GR), −136 (SR), and −82 (NC) mg CO2-C m−2 h−1. The percentage of autotrophic respiration (Ra) in Reco showed clear rise from LR (35%) to NC (62%). CH4 consumption dominated resulting in median fluxes as follows: −2 (LR), −1 (GR), −15 (SR), and −23 (NC) μg CH4-C m−2 h−1. N2O flux was low and highly variable in time, with median values varying from −0.07 (GR) to 2.18 (NC) μg N2O-N m−2 h−1. During the maximum vegetation growth, NEE exceeded Reco value. Green roofs are effective CH4 sinks, but they do not significantly affect N2O flux. The main environmental factors determining GHG fluxes in linear models were parameters describing moisture regime, meteorological parameters and soil physical characteristics. These models can be used to predict GHG fluxes from similar green roof systems in analogous climatic conditions. We conclude that green roof technology may be used to mitigate excessive ambient GHG levels in urban areas.

El Niño–Southern Oscillation (ENSO) event reduces CO<sub>2</sub> uptake of an Indonesian oil palm plantation

Abstract. The El Niño–Southern Oscillation (ENSO) in 2015 was one of the strongest observed in almost 20 years and set the stage for a severe drought and the emergence of widespread fires and related smoke emission over large parts of Southeast Asia. In the tropical lowlands of Sumatra, which were heavily affected by the drought and haze, large areas of tropical rainforest have been converted into oil palm (Elaeis guineensis Jacq.) plantations during the past decades. In this study, we investigate the impact of drought and smoke haze on the net ecosystem CO2 exchange, evapotranspiration, yield and surface energy budget in a commercial oil palm plantation in Jambi province (Sumatra, Indonesia) by using micrometeorological measurements, the eddy covariance method, yield data and a multiple linear regression model (MLRM). With the MLRM we identify the contribution of meteorological and environmental parameters to the net ecosystem CO2 exchange. During the initial part of the drought, when incoming shortwave radiation was elevated, net CO2 uptake increased by 50 % despite a decrease in upper-layer soil moisture by 35 %, an increase in air temperature by 10 % and a tripling of atmospheric vapour pressure deficit. Emerging smoke haze decreased incoming solar radiation by 35 % compared to non-drought conditions and diffuse radiation almost became the sole shortwave radiation flux for 2 months, resulting in a strong decrease in net CO2 uptake by 86 %. Haze conditions resulted in a complete pause of oil palm net carbon accumulation for about 1.5 months and contributed to a decline in oil palm yield by 35 %. With respect to a projected pronounced drying trend over the western Pacific during a future El Niño, our model showed that an increase in drought may stimulate net CO2 uptake, while more severe smoke haze, in combination with drought, can lead to pronounced losses in productivity and net CO2 uptake, highlighting the importance of fire prevention.

Subtropical mangrove wetland is a stronger carbon dioxide sink in the dry than wet seasons

Coastal mangrove wetlands have an excellent potential in sequestrating atmospheric carbon dioxide (CO2) owing to their high primary productivity as well as slow anaerobic decomposition of organic matter. Yet, there is hitherto a paucity of researches examining the temporal variations and environmental controls of ecosystem-scale CO2 fluxes in subtropical mangroves using quasi-continuous, high temporal resolution measurements. In this study, we measured the net ecosystem CO2 exchange (NEE) between the atmosphere and a subtropical estuarine mangrove dominated by Kandelia obovata using an eddy covariance system for two full years to investigate the seasonal variability and key biophysical drivers of NEE. During the wet seasons, the magnitude of increase in ecosystem respiration (Re) was greater than that in gross primary productivity (GPP) owing to a combination of higher temperature and lower percentage of inundation, tidal height and water salinity, which subsequently resulted in an overall decrease in net CO2 uptake as compared to the dry seasons. Our results of path analysis showed that temperature was a dominant control of the temporal variations in CO2 flux during the wet seasons, while its influence became weaker during the dry seasons. On the other hand, an increase in water salinity during the dry seasons had a positive influence on GPP, which was likely related to a greater availability of ions (mainly Cl− and Na+) in supporting photosynthesis by mangrove trees in this coastal wetland with relatively low salinity (∼5–15 ppt). Our subtropical mangrove wetland was shown to be a significant carbon (C) sink, with annual C uptake rates of 890 and 758 g CO2-C m-2 yr−1 in the two years of study. We found a strong control of mangrove CO2 fluxes by biophysical factors such as temperature and salinity, implying that global warming and a reduction in water salinity in response to a greater river discharge could potentially reduce the C sink strength of estuarine mangroves in the subtropical regions.

Maximum carbon uptake rate dominates the interannual variability of global net ecosystem exchange

Terrestrial ecosystems contribute most of the interannual variability (IAV) in atmospheric carbon dioxide (CO2 ) concentrations, but processes driving the IAV of net ecosystem CO2 exchange (NEE) remain elusive. For a predictive understanding of the global C cycle, it is imperative to identify indicators associated with ecological processes that determine the IAV of NEE. Here, we decompose the annual NEE of global terrestrial ecosystems into their phenological and physiological components, namely maximum carbon uptake (MCU) and release (MCR), the carbon uptake period (CUP), and two parameters, α and β, that describe the ratio between actual versus hypothetical maximum C sink and source, respectively. Using long-term observed NEE from 66 eddy covariance sites and global products derived from FLUXNET observations, we found that the IAV of NEE is determined predominately by MCU at the global scale, which explains 48% of the IAV of NEE on average while α, CUP, β, and MCR explain 14%, 25%, 2%, and 8%, respectively. These patterns differ in water-limited ecosystems versus temperature- and radiation-limited ecosystems; 31% of the IAV of NEE is determined by the IAV of CUP in water-limited ecosystems, and 60% of the IAV of NEE is determined by the IAV of MCU in temperature- and radiation-limited ecosystems. The Lund-Potsdam-Jena (LPJ) model and the Multi-scale Synthesis and Terrestrial Model Inter-comparison Project (MsTMIP) models underestimate the contribution of MCU to the IAV of NEE by about 18% on average, and overestimate the contribution of CUP by about 25%. This study provides a new perspective on the proximate causes of the IAV of NEE, which suggest that capturing the variability of MCU is critical for modeling the IAV of NEE across most of the global land surface.

Net Ecosystem Exchange, Gross Primary Production And Ecosystem Respiration In Ridge-Hollow Complex At Mukhrino Bog

The continuous field measurements of net ecosystem exchange (NEE) of CO 2 were provided at ridge-hollow oligotrophic bog in the Middle Taiga zone of West Siberia, Russia in 2017-2018. The model of net ecosystem exchange of CO 2 was suggested to describe the influence of different environmental factors on NEE and to estimate the total carbon budget of the bog over the growing season. The model uses air and soil temperature, incoming photosynthetically active radiation (PAR) and water table depth, as the key factors influencing gross primary production (GPP) and ecosystem respiration (ER). The model coefficients were calibrated using the data collected by automated soil CO 2 flux system with two transparent long-term chambers placed at large hollow and small ridge sites. Experimental and modeling results showed that the Mukhrino bog acted over the study period as a carbon sink, with an average NEE of –87.7 gC m -2 at the hollow site and –50.2 gC m -2 at the ridge site. GPP was – 344.8 and –228.5 gC m -2 whereas ER was 287.6 and 140.9 gC m -2 at ridge and hollow sites, respectively. Despite of a large difference in NEE estimates between 2017 and 2018 the growing season variability of NEE were quite similar.

Evaluating a Lagrangian inverse model for inferring isotope CO2 exchange in plant canopies

Multi-layer Lagrangian models could be useful techniques for studying stable isotope exchange within and just above plant canopies. The main objective of this study was to evaluate the use of an analytical Lagrangian analysis (localized near-field theory, LNF), to study 13CO2 and C18OO isotope exchange in different plant canopies by comparing the LNF estimates with those provided by the eddy covariance (EC) technique and the isotope flux ratio method (IFR). Mixing ratios of stable isotopes of CO2 were measured within and above a temperate deciduous forest, tallgrass prairie and corn field using a multi-port sampling system and the tunable diode laser spectroscopy technique. Wind velocity data and the net CO2 ecosystem exchange (NEE) were measured above the plant canopies using an EC system. The wind velocity data and CO2 stable isotope mixing ratios were combined with the LNF theory to infer NEE and source/sinks of isotopes inside canopies. The LNF NEE estimates were likely affected by the flux decoupling in the forest canopy, resulting in a low correlation (R2 ranging from 0.03 to 0.35) between LNF and EC NEE estimates. On the other hand, LNF NEE estimates for corn and grassland canopies showed better correlation with EC NEE estimates (R2 ranging from 0.58 to 0.85), suggesting better coupling between in and above canopy air flows. Although, both LNF and IFR estimates showed large variability, our results show that the LNF approach reduced the uncertainties of the isotope compositions of NEE when compared to the IFR approach. These results suggest that LNF is a useful tool to study CO2 isotope exchange within short canopies where flux measurements are more challenging than inside tall canopies.

Contrasting Temperature Sensitivity of CO2 Exchange in Peatlands of the Hudson Bay Lowlands, Canada

AbstractIt remains uncertain how the net ecosystem CO2 exchange (NEE) of diverse peatlands will respond to warming. Here we compare five years of eddy covariance measurements of NEE and estimates of gross primary productivity and ecosystem respiration between a fen dominated by deciduous vegetation and an adjacent bog with evergreen vegetation in the Canadian Hudson Bay Lowlands. At the bog, daily net CO2 uptake lasted from snowmelt to snow cover onset, while at the fen, net CO2 uptake was delayed in spring and ended earlier in fall. Greater midsummer net CO2 uptake at the fen compensated for shoulder season net CO2 losses resulting in similar annual NEE at the two sites (fen: −52 ± 16 g C m−2, bog: −80 ± 14 g C m−2). Observations of a satellite‐based productivity index also suggest lower shoulder season and higher peak vegetation productivity at these deciduous versus evergreen plant‐dominated peatlands. The response of NEE to warmer weather differed between sites. Warming during the shoulder seasons increased net CO2 uptake at the evergreen plant‐dominated bog, while it increased net CO2 losses at the fen where deciduous leaves had not yet emerged or had senesced. In contrast, warmer weather during the peak growing season appeared to reduce net CO2 uptake more at the bog than the fen resulting from both increasing ecosystem respiration and decreasing gross primary productivity. In the short term, warming will likely decrease annual net CO2 uptake of these and similar peatlands, although the magnitude will depend on factors including vegetation dynamics and seasonality of warming.

Experimental warming amplified opposite impacts of drought vs. wet extremes on ecosystem carbon cycle in a tallgrass prairie

Climate warming is leading to greater precipitation variability, resulting in increased frequency and intensity of both drought and wet extremes. However, how these extreme events interact with climate warming and hay-harvest in grasslands to impact ecosystem functions has not yet been well explored. In this study, we took advantage of a long-term experiment to examine how climate warming and clipping (i.e., mimicking hay harvest) regulated impacts of naturally occurring drought and wet extremes on ecosystem CO2 fluxes of a tallgrass prairie in the Great Plains, USA. Warming resulted in net ecosystem carbon release (i.e., positive net ecosystem CO2 exchange, NEE) in the extreme drought year of 2011, but significantly enhanced net carbon uptake in the extremely wet year of 2015 in comparison with NEE in normal years. Warming-induced carbon release in the drought year was due to significantly enhanced ecosystem respiration (ER) from mid-summer to early-autumn, whereas warming-enhanced NEE in the wet year was due to an increase in aboveground net primary production (ANPP) compared to those in normal years. Drought diminished warming-induced increases in ANPP to about one sixth of that in the wet year in the unclipped plots. Interestingly, clipping offset the drought-mediated ecosystem carbon loss by increasing GPP and weakened the wet-enhanced ANPP. Overall, our results suggest that a future, warmer climate may exacerbate carbon losses in terrestrial ecosystems during drought extremes but stimulate the ecosystem carbon sink under wet extremes.

Isotopically-tracked hydrological changes in carbon cycling and its sources in a Chinese subtropical forested watershed

Forests are important components of the terrestrial ecosystem that play major roles in carbon cycling and associated environmental changes, but more knowledge of the processes involved is required. Therefore, carbon wet deposition and rainfall-runoff processes were examined, as well as carbon fluxes and sources in water in the Xiangxi River watershed, a typical subtropical forested watershed in China. Wet deposition of carbon amounted to 1.31 × 104 kg km−2 between June 2015 and May 2016 (inclusive). The concentration of measured forms of carbon in rainwater of single rainfall event peaked in summer, when the concentration of dissolved total carbon (DTC) was 16.5 mg L−1. In 2016, rainfall-runoff amounted to 7440 kg km−2, accounting for about 57% of the carbon wet deposition in the watershed. The forest ecosystem in the watershed was found to be a carbon sink every month of the year, with net ecosystem CO2 exchange (NEE) in 2016 of −5.11 × 105 kg C km−2, and monthly NEE was lowest in July (−8.93 × 104 kg C km−2). The 13C/12C ratio in dissolved inorganic carbon in the river water, relative to Pee Dee Belemnite (δ13CDIC), was higher in the dry season (range, −12.00 to −9.70‰; mean, −11.0‰) than in the rainy season (range, −14.00 to −12.20‰; mean, −12.90‰), and higher in downstream reaches (mean values in winter and spring: −10.45 and −12.35‰, respectively) than in upstream and middle reaches (−10.63 and −13.30‰, −11.63 and −12.87‰). In addition, atmospheric precipitation made the highest contributions to the C concentration in the river water in spring and winter (48.6 and 43.2%, respectively).

Water and carbon dioxide exchange of an alpine meadow ecosystem in the northeastern Tibetan Plateau is energy-limited

The alpine meadow in the Tibetan Plateau (TP) is a fragile ecosystem and very sensitive to climate change and human activities. However, the flux (energy, water, and carbon dioxide) exchanges (FEs) and their controls in this ecosystem are not well understood. Using two years (2015–2016) of observations from three eddy covariance towers and different grazing management types, we quantified the FEs of an alpine meadow ecosystem in the northeastern TP. A boosted regression trees (BRT)-based method was used to determine the environmental controls of FEs and clarify the factors causing the differences in FEs among the sites. The mean annual latent heat flux (LE), sensible heat flux (H), and evapotranspiration (ET) of the sites were 36.3 W m−2, 23.2 W m−2, and 458.2 mm, respectively. The mean annual gross primary productivity (GPP) and ecosystem respiration (Re) of the sites were 504.8 and 317.2 g C m−2, respectively. The mean annual net ecosystem CO2 exchange (NEE) of the sites was –187.6 g C m–2, indicating that the ecosystem was a CO2 sink, not accounting for grazing losses. The BRT-based analysis showed that, (i) The LE (ET) and H were mostly determined by soil temperature (ST) and downward shortwave radiation (DR), while the NEE, GPP, and Re were primarily correlated with ST. As a result, the FEs of the ecosystem were energy-limited; (ii) The high spatial heterogeneity in multiple environmental variables jointly determined the differences in FEs of the ecosystem. The differences in LE (ET) and H among the sites resulted from multiple environmental variables, including DR, ST, precipitation, and SM; and differences in NEE, GPP, and Re were mainly caused by precipitation, SM, and RH. The study highlights that future warming will enhance GPP and Re of the ecosystem, while reducing the CO2 sink because of the larger increases in Re than in GPP.

The initial three years of carbon dioxide exchange between the atmosphere and a reclaimed oil sand wetland

Northern peatlands contain up to 20% of the ∼3000 Pg of global soil organic carbon. Carbon-rich peatlands cover upwards of 65% of the landscape in northern Canada where resource extraction activities disturb both the carbon pools and the future carbon sequestration capacity of the landscape. Previous estimates of the carbon losses from this disturbance predict a complete loss of the region’s peatland carbon pool. Mining industries operating in these sensitive environments have recently begun constructing closure landscapes which are intended to develop carbon cycle processes similar to undisturbed northern peatlands. This study investigates eddy covariance fluxes of carbon dioxide (CO2) at one of Canada’s first fully constructed boreal plains watersheds, the Sandhill Fen Watershed. During the first three years since inception, only the lowland region had an annual net ecosystem exchange of CO2 (NEE) indicative of increasing CO2 sink potential. The lowland region was characterized by saturated salvaged peat soils, standing water, thriving communities of Typha and Carex spp. and was a net CO2 sink of 77 g C m−2 in the third year. At the same time the upland and the midland regions characterized by moist salvaged peat soils and a mix of herbaceous, shrub and planted Picea glauca and Pinus banksiana remained net sources of CO2. Despite similar rates of gross primary production, ecosystem and plot-level respiration rates in the lowland were significantly lower than in the midland region, likely due to very low reduction potentials within the lowland’s saturated soils. With no other significant outflows of carbon, the lowland of the Sandhill Fen Watershed may be in the early stages of organic matter accumulation. Due to limited oxidation of the salvaged peat substrate in the lowland region, wetland reclamation employing these techniques may reduce the disturbance loss of the carbon pool in the boreal plains.

Interannual and seasonal variability in carbon dioxide and methane fluxes of a pine peat bog in the Eastern Alps, Austria

Intact peat bogs are carbon dioxide (CO2) sinks and methane (CH4) sources. Facing drought and drainage, they may turn into CO2 sources and decreased CH4 sources. Information on the CO2 and CH4 exchange of alpine peat bogs in Central Europe has been missing so far. Here, we present data from two years of CO2 and CH4 exchange between an alpine low-shrub pine bog in the Eastern Alps and the atmosphere using the eddy covariance method. The annual net CO2 ecosystem exchange of the peatland differs substantially between the two measurement years, with -24 ± 13 g C m−2 yr−1 for the drought affected first year and -84 ± 13 g C m−2 yr−1 for the more humid second year. We found ecosystem respiration (Reco) to depend on variations in soil temperature and soil moisture, and gross primary production (GPP) to be strongly linked to net radiation and daylength. The summer drought in 2015 shifted the peatland from a C sink to a C source, as increases in Reco clearly exceeded enhanced GPP. Annual CH4 emission was 4.40 ± 2.40 g C m−2 yr−1 during the drought-affected year and 5.24 ± 2.57 g C m−2 yr−1 during the wetter year. Summer CH4 fluxes contribute 44% to the annual balance, followed by autumn (27%), spring (20%) and winter season fluxes (9%). CH4 fluxes most strongly depend on soil temperatures, soil moisture effects increase at smaller time-scales. Annual CH4 emissions are low compared to other temperate bogs, which most likely is the result of the ongoing degradation, indicated by a shift in vegetation composition. Net flux of both greenhouse gases was positive in the first year (+75 g CO2-eq m−2) and negative in the second year (−110 g CO2-eq m−2). Our results indicate that drought events and seasonal and interannual variations in temperature and precipitation strongly affect the C cycle of alpine peat bogs.

Net Ecosystem Exchange of CO2 in Deciduous Pine Forest of Lower Western Himalaya, India

Carbon cycle studies over the climate-sensitive Himalayan regions are relatively understudied and to address this gap, systematic measurements on carbon balance components were performed over a deciduous pine forest with an understory layer. We determined annual net carbon balance, seasonality in components of carbon balance, and their environmental controls. Results indicated a strong seasonality in the behavior of carbon exchange components. Net primary productivity (NPP) of pine forest exceeded soil respiration during the growing phase. Consequently, net ecosystem exchange exhibited a net carbon uptake. In the initial phase of the growing season, daily mean uptake was −3.93 (±0.50) g C m−2 day−1, which maximizes (−8.47 ± 2.3) later during post-monsoon. However, a brief phase of carbon release was observed during peak monsoon (August) owing to an overcast condition. Nevertheless, annually the forest remained as a carbon sink. The understory is extensively distributed and it turned out to be a key component of carbon balance because of sustained NPP during the pine leafless period. Temperature and evaporative fraction exhibited a prime control over the seasonal carbon dynamics. Our observations could lend certain useful insights into the application of coupled climate-carbon cycle models for the Himalaya and ecological functions in the region.

Hydro-meteorological influences on the growing season CO2 exchange of an alpine meadow in the northeastern Tibetan Plateau permafrost region: observations using eddy covariance method

Hydro-meteorological variations are fundamental environmental factors affecting site-level ecosystem carbon balance variations; however, how hydro-meteorological variations influence continuous CO2 exchange over alpine meadows during the growing season in permafrost regions on the Tibetan Plateau is poorly understood. Net ecosystem CO2 exchange (NEE) measured using the eddy covariance method was partitioned into its components of ecosystem respiration (ER) and gross ecosystem CO2 exchange (GPP). ER and GPP had a significant relationship with air temperature, vapor pressure deficit, and net radiation, during the growing season of an alpine meadow. There was a positive correlation between GPP and active layer soil temperatures at 5–40 cm except soil temperature at 80 cm, located in the vicinity of the bottom of the active layer. The microbial activity of underground rhizospheric microorganism, mainly distributed in in shallow soil layers at 0–20 cm in alpine permafrost regions on the Tibetan Plateau, can significantly promote plant photosynthesis. Thus, the correlation between GPP and soil temperatures at 5–20 cm was stronger than that between GPP and air temperature. The correlation between ER (GPP) and active layer soil moisture at 5–80 cm was complicated, especially for soils at 5–20 cm. With the increase of active layer soil moisture, ER (GPP) increased first and then decreased. This was related to the thawing process of the active layer, increase of rainfalls, the frozen layer acting as a weak permeable layer, and the replenishment of super-permafrost water. Our results suggested that long-term observations on how hydro-meteorological variations affect the continuous CO2 exchange of alpine meadows with different active layer thicknesses should be carried out in alpine permafrost regions.

Stability of Ecosystem CO2 Flux in Response to Changes in Precipitation in a Semiarid Grassland

Carbon dioxide (CO2) flux provides feedback between C cycling and the climatic system. There is considerable uncertainty regarding the direction and magnitude of the responses of this process to precipitation changes, hindering accurate prediction of C cycling in a changing world. We examined the responses of ecosystem CO2 flux to ambient precipitation and experimentally decreased (−35%) and increased precipitation (+20%) in a semiarid grassland in China between July 2013 and September 2015. The measured CO2 flux components included the gross ecosystem productivity (GEP), net ecosystem CO2 exchange (NEE), ecosystem respiration (Re), and soil respiration (Rs). The results showed that the seasonal and diurnal patterns of most components of ecosystem CO2 flux were minimally affected by precipitation treatments, with less than 4% changes averaged across the three growing seasons. GEP and NEE had a quadratic relationship, while Re and Rs increased exponentially with soil temperature. GEP, RE, and Rs, however, decreased with soil moisture. Decreased precipitation reduced the dependence of CO2 flux on soil temperature but partly increased the dependence on soil moisture; in contrast, increased precipitation had the opposite influence. Our results suggested a relatively stable CO2 flux in this semiarid grassland across the tested precipitation regimes.

Flux Puppy – An open-source software application and portable system design for low-cost manual measurements of CO2 and H2O fluxes

Manual chamber-based measurements of CO2 (and H2O) fluxes are important for understanding ecosystem carbon metabolism. Small opaque chambers can be used to measure leaf, stem and soil respiration. Larger transparent chambers can be used to measure net ecosystem exchange of CO2, and small jars often serve this purpose for laboratory incubations of soil and plant material. We developed an Android application (app), called Flux Puppy, to facilitate chamber-based flux measurements in the field and laboratory. The app is designed to run on an inexpensive handheld Android device, such as a tablet or phone, and it has a graphical user interface that communicates with a LI-COR LI-820 and LI-830 (CO2) or LI-840 and LI-850 (CO2/H2O) infrared gas analyzer. The app logs concentrations of CO2 and H2O, cell temperature and pressure at 1 Hz, displays the output graphically, and calculates the linear regression slope, R-squared, and standard error of the CO2 time series. A metadata screen allows users to enter operator, site, and plot information, as well as take a photograph using the Android device’s built-in camera, and log measurement location using the device GPS. Additionally, there is a notes field, which can be revised after the measurements are taken. Data files (the 1 s raw data, photograph, and metadata including statistics calculated from the raw data) are then transmitted off the device through file sharing options (Gmail, Outlook, Google Drive, Dropbox etc.). Because Flux Puppy code is open-source (available on GitHub) and the flux measurement system we describe is relatively inexpensive and straightforward to assemble, it should be of broad interest to the carbon cycling community.

Soil N2O, CH4, and CO2 Fluxes in Forest, Grassland, and Tillage/No-Tillage Croplands in French Guiana (Amazonia)

The agricultural landscape of French Guiana (Amazonia) is expected to undergo substantial change as a result of rapid population growth in the region. Such changes in the landscape will lead to the conversion of tropical forests into land destined for agricultural use. Little information is available on the effect of different agricultural systems on greenhouse gas (GHG) emissions in French Guiana. For our experiment, two hectares of forest were cleared, without the use of fire, at the Combi experimental site (sandy-clayey Ferralsol) at the end of 2008. After one year with legume and grass cover, the site was modified to include the following three fertilized agricultural systems: (1) Grassland (Brachiaria ruziziensis, mowed), (2) cropland (maize/soybean rotation) with disc tillage, and (3) cropland (maize/soybean rotation) with no-tillage in direct seeding. Soil N2O, CH4, and CO2 fluxes were measured with dark chambers from May 2011 to November 2014. Our results show that grassland was a significantly lower emitter of N2O but a significantly higher emitter of CH4 compared to the two cropland systems studied. We did not observe significant differences between the two cropland systems for N2O and CH4 fluxes. Measurements of the net ecosystem CO2 exchange would be useful to better compare the role of different agricultural systems as a source of GHGs.

Quantifying the UK's carbon dioxide flux: an atmospheric inverse modelling approach using a regional measurement network

Abstract. We present a method to derive atmospheric-observation-based estimates of carbon dioxide (CO2) fluxes at the national scale, demonstrated using data from a network of surface tall-tower sites across the UK and Ireland over the period 2013–2014. The inversion is carried out using simulations from a Lagrangian chemical transport model and an innovative hierarchical Bayesian Markov chain Monte Carlo (MCMC) framework, which addresses some of the traditional problems faced by inverse modelling studies, such as subjectivity in the specification of model and prior uncertainties. Biospheric fluxes related to gross primary productivity and terrestrial ecosystem respiration are solved separately in the inversion and then combined a posteriori to determine net ecosystem exchange of CO2. Two different models, Data Assimilation Linked Ecosystem Carbon (DALEC) and Joint UK Land Environment Simulator (JULES), provide prior estimates for these fluxes. We carry out separate inversions to assess the impact of these different priors on the posterior flux estimates and evaluate the differences between the prior and posterior estimates in terms of missing model components. The Numerical Atmospheric dispersion Modelling Environment (NAME) is used to relate fluxes to the measurements taken across the regional network. Posterior CO2 estimates from the two inversions agree within estimated uncertainties, despite large differences in the prior fluxes from the different models. With our method, averaging results from 2013 and 2014, we find a total annual net biospheric flux for the UK of 8±79 Tg CO2 yr−1 (DALEC prior) and 64±85 Tg CO2 yr−1 (JULES prior), where negative values represent an uptake of CO2. These biospheric CO2 estimates show that annual UK biospheric sources and sinks are roughly in balance. These annual mean estimates consistently indicate a greater net release of CO2 than the prior estimates, which show much more pronounced uptake in summer months.