Net Ecosystem CO2 Flux Research Articles

Temperature dependence of spring carbon uptake in northern high latitudes during the past four decades.

In the northern high latitudes, warmer spring temperatures generally lead to earlier leaf onsets, higher vegetation production, and enhanced spring carbon uptake. Yet, whether this positive linkage has diminished under climate change remains debated. Here, we used atmospheric CO2 measurements at Barrow (Alaska) during 1979-2020 to investigate the strength of temperature dependence of spring carbon uptake reflected by two indicators, spring zero-crossing date (SZC) and CO2 drawdown (SCC). We found a fall and rise in the interannual correlation of temperature with SZC and SCC (RSZC-T and RSCC-T ), showing a recent reversal of the previously reported weakening trend of RSZC-T and RSCC-T . We used a terrestrial biosphere model coupled with an atmospheric transport model to reproduce this fall and rise phenomenon and conducted factorial simulations to explore its potential causes. We found that a strong-weak-strong spatial synchrony of spring temperature anomalies per se has contributed to the fall and rise trend in RSZC-T and RSCC-T , despite an overall unbroken temperature control on net ecosystem CO2 fluxes at local scale. Our results provide an alternative explanation for the apparent drop of RSZC-T and RSCC-T during the late 1990s and 2000s, and suggest a continued positive linkage between spring carbon uptake and temperature during the past four decades. We thus caution the interpretation of apparent climate sensitivities of carbon cycle retrieved from spatially aggregated signals.

Estimation of daily CO2 fluxes and of the components of the carbon budget for winter wheat by the assimilation of Sentinel 2-like remote sensing data into a crop model

Croplands contribute to greenhouse gas emissions but also have the potential to mitigate climate change through soil carbon storage. However, there is a lack of tools based on objective observations for assessing cropland C budgets at the plot scale over large areas. Such tools would allow us to more precisely establish the contribution of an agricultural plot to net CO2 emissions according to the plot management and identify levers for improving the C budget. In this study, we present a diagnostic regional modelling approach, called SAFY-CO2, that assimilates high spatial and temporal resolution (HSTR) optical remote sensing data in a simple crop model and evaluate the performance of this approach in quantifying crop production and the main components of the annual carbon budget for winter wheat.The SAFY-CO2 model simulates daily crop development (biomass, partition to leaves, etc.), the components of net ecosystem CO2 fluxes, and the annual yield and net ecosystem carbon budget (NECB).Multi-temporal green area index (GAI) maps derived from HSTR data from the Formosat-2 and SPOT satellites were used to calibrate the light-use efficiency and phenological parameters of the model. Data from the literature were used to set a priori values for a set of model parameters, and a large dataset of in situ data was used for model validation. This dataset includes 8 years of eddy-covariance net CO2 flux measurements and GAI, biomass and yield data acquired at 2 instrumented sites in southwest France. Biomass and yield data from 16 fields in the study area between 2005 and 2014 were also used for validation.The SAFY-CO2 model is able to reproduce both GAI dynamics (RRMSE = 14%, R2 = 0.97) and biomass production and yield (RRMSE of 27% and 21%, respectively) with high precisions under contrasting climatic, environmental and management conditions. Additionally, the net CO2 flux components estimated by the model generally agreed well with in situ data and presented very good and significant correlations (RMSE of 1.74, 1.13 and 1.29 gC.m−2.d-1 for GPP, Reco and NEE, respectively; R2 of 0.90, 0.75 and 0.85 for GPP, Reco and NEE, respectively) over the 8 studied years. This study also highlights the importance of accounting for post-harvest vegetative events (spontaneous re-growth, weed development and cover crops) for an accurate calculation of the annual net CO2 flux. This approach requires a limited number of input parameters for estimating yield and net CO2 flux components, which is promising for regional/global-scale applications based on Sentinel 2-like data; however, the approach requires plot-scale data concerning organic amendments and straw management (exportation) in animal farming systems to calculate field C budgets.

Memory effects of climate and vegetation affecting net ecosystem CO2 fluxes in global forests.

Forests play a crucial role in the global carbon (C) cycle by storing and sequestering a substantial amount of C in the terrestrial biosphere. Due to temporal dynamics in climate and vegetation activity, there are significant regional variations in carbon dioxide (CO2) fluxes between the biosphere and atmosphere in forests that are affecting the global C cycle. Current forest CO2 flux dynamics are controlled by instantaneous climate, soil, and vegetation conditions, which carry legacy effects from disturbances and extreme climate events. Our level of understanding from the legacies of these processes on net CO2 fluxes is still limited due to their complexities and their long-term effects. Here, we combined remote sensing, climate, and eddy-covariance flux data to study net ecosystem CO2 exchange (NEE) at 185 forest sites globally. Instead of commonly used non-dynamic statistical methods, we employed a type of recurrent neural network (RNN), called Long Short-Term Memory network (LSTM) that captures information from the vegetation and climate’s temporal dynamics. The resulting data-driven model integrates interannual and seasonal variations of climate and vegetation by using Landsat and climate data at each site. The presented LSTM algorithm was able to effectively describe the overall seasonal variability (Nash-Sutcliffe efficiency, NSE = 0.66) and across-site (NSE = 0.42) variations in NEE, while it had less success in predicting specific seasonal and interannual anomalies (NSE = 0.07). This analysis demonstrated that an LSTM approach with embedded climate and vegetation memory effects outperformed a non-dynamic statistical model (i.e. Random Forest) for estimating NEE. Additionally, it is shown that the vegetation mean seasonal cycle embeds most of the information content to realistically explain the spatial and seasonal variations in NEE. These findings show the relevance of capturing memory effects from both climate and vegetation in quantifying spatio-temporal variations in forest NEE.

Net ecosystem production, calcification and CO2 fluxes on a reef flat in Northeastern Brazil

The carbon cycle in coral reefs is usually dominated by the organic carbon metabolism and precipitation-dissolution of CaCO3, processes that control the CO2 partial pressure (pCO2) in seawater and the CO2 fluxes through the air–sea interface. In order to characterize these processes and the carbonate system, four sampling surveys were conducted at the reef flat of Coroa Vermelha during low tide (exposed flat). Net ecosystem production (NEP), net precipitation-dissolution of CaCO3 (G) and CO2 fluxes across the air–water interface were calculated. The reef presented net autotrophy and calcification at daytime low tide. The NEP ranged from −8.7 to 31.6 mmol C m−2 h−1 and calcification from −13.1 to 26.0 mmol C m−2 h−1. The highest calcification rates occurred in August 2007, coinciding with the greater NEP rates. The daytime CO2 fluxes varied from −9.7 to 22.6 μmol CO2 m−2 h−1, but reached up to 13,900 μmol CO2 m−2 h−1 during nighttime. Carbon dioxide influx to seawater was predominant in the reef flat during low tide. The regions adjacent to the reef showed a supersaturation of CO2, acting as a source of CO2 to the atmosphere (from −22.8 to −2.6 mol CO2 m−2 h−1) in the reef flat during ebbing tide. Nighttime gas release to the atmosphere indicates a net CO2 release from the Coroa Vermelha reef flat within 24 h, and that these fluxes can be important to carbon budget in coral reefs.

Modelling effects of seasonal variation in water table depth on net ecosystem CO<sub>2</sub> exchange of a tropical peatland

Abstract. Seasonal variation in water table depth (WTD) determines the balance between aggradation and degradation of tropical peatlands. Longer dry seasons together with human interventions (e.g. drainage) can cause WTD drawdowns making tropical peatland C storage highly vulnerable. Better predictive capacity for effects of WTD on net CO2 exchange is thus essential to guide conservation of tropical peat deposits. Mathematical modelling of basic eco-hydrological processes under site-specific conditions can provide such predictive capacity. We hereby deploy a process-based mathematical model ecosys to study effects of seasonal variation in WTD on net ecosystem productivity (NEP) of a drainage affected tropical peat swamp forest at Palangkaraya, Indonesia. Simulated NEP suggested that the peatland was a C source (NEP ~ −2 g C m−2 d−1, where a negative sign represents a C source and a positive sign a C sink) during rainy seasons with shallow WTD, C neutral or a small sink (NEP ~ +1 g C m−2 d−1) during early dry seasons with intermediate WTD and a substantial C source (NEP ~ −4 g C m−2 d−1) during late dry seasons with deep WTD from 2002 to 2005. These values were corroborated by regressions (P < 0.0001) of hourly modelled vs. eddy covariance (EC) net ecosystem CO2 fluxes which yielded R2 > 0.8, intercepts approaching 0 and slopes approaching 1. We also simulated a gradual increase in annual NEP from 2002 (−609 g C m−2) to 2005 (−373 g C m−2) with decreasing WTD which was attributed to declines in duration and intensity of dry seasons following the El Niño event of 2002. This increase in modelled NEP was corroborated by EC-gap filled annual NEP estimates. Our modelling hypotheses suggested that (1) poor aeration in wet soils during shallow WTD caused slow nutrient (predominantly phosphorus) mineralization and consequent slow plant nutrient uptake that suppressed gross primary productivity (GPP) and hence NEP (2) better soil aeration during intermediate WTD enhanced nutrient mineralization and hence plant nutrient uptake, GPP and NEP and (3) deep WTD suppressed NEP through a combination of reduced GPP due to plant water stress and increased ecosystem respiration (Re) from enhanced deeper peat aeration. These WTD effects on NEP were modelled from basic eco-hydrological processes including microbial and root oxidation-reduction reactions driven by soil and root O2 transport and uptake which in turn drove soil and plant carbon, nitrogen and phosphorus transformations within a soil-plant-atmosphere water transfer scheme driven by water potential gradients. Including these processes in ecosystem models should therefore provide an improved predictive capacity for WTD management programs intended to reduce tropical peat degradation.

Altitudinal variation of ecosystem CO2 fluxes in an alpine grassland from 3600 to 4200 m

Recent studies have recognized the alpine grasslands on the Qinghai–Tibetan plateau as a significant sink for atmospheric CO2. The carbon-sink strength may differ among grassland ecosystems at various altitudes because of contrasting biotic and physical environments. This study aims (i) to clarify the altitudinal pattern of ecosystem CO2 fluxes, including gross primary production (GPP), daytime ecosystem respiration (Redaytime) and net ecosystem production (NEP), during the period with peak above-ground biomass; and (ii) to elucidate the effects of biotic and abiotic factors on the altitudinal variation of ecosystem CO2 fluxes. Ecosystem CO2 fluxes and abiotic and biotic environmental factors were measured in an alpine grassland at four altitudes from 3600 to 4200 m along a slope of the Qilian Mountains on the northwestern Qinghai–Tibetan Plateau during the growing season of 2007. We used a closed-chamber method combined with shade screens and an opaque cloth to measure several carbon fluxes, GPP, Redaytime and NEP, and factors, light-response curve for GPP and temperature sensitivity of Redaytime. Above- and below-ground biomasses and soil C and N contents at each measurement point were also measured. (i) Altitudinal pattern of ecosystem CO2 fluxes: The maximum net ecosystem CO2 flux (NEPmax), i.e. the potential ecosystem CO2 sink strength, was markedly different among the four altitudes. NEPmax was higher at the highest and lowest sites, approximately −7.4 ± 0.9 and −6.7 ± 0.6 μmol CO2 m−2 s−1 (mean ± standard error), respectively, but smaller at the intermediate altitude sites (3800 and 4000 m). The altitudinal pattern of maximum gross primary production was similar to that of NEPmax. The Redaytime, however, was significantly higher at the lowest altitude (3.4 ± 0.3 μmol CO2 m−2 s−1) than at the other three altitudes. (ii) Altitudinal variation of vegetation biomass: The above-ground biomass was higher at the highest altitude (154 ± 27 g DW m−2) than at the other altitudes, which we attribute mainly to the large biomass in cushion plants at the highest altitude. The small above-ground biomass at the lower altitudes was probably due to heavy grazing during the growing season. (iii) Features of ecosystem CO2 fluxes: Redaytime and GPP were positively correlated with above-ground biomass. The low ratio of Redaytime to GPP at either the measurement point or the site level suggests that CO2 uptake efficiency tends to be higher at higher altitudes, which indicates a high potential sink strength for atmospheric CO2 despite the low temperature at high altitudes. The results suggest that the effect of grazing intensity on ecosystem carbon dynamics, partly by decreasing vegetation biomass, should be clarified further.

Ecosystem respiration depends strongly on photosynthesis in a temperate heath

We measured net ecosystem CO2 flux (F n) and ecosystem respiration (R E), and estimated gross ecosystem photosynthesis (P g) by difference, for two years in a temperate heath ecosystem using a chamber method. The exchange rates of carbon were high and of similar magnitude as for productive forest ecosystems with a net ecosystem carbon gain during the second year of 293 ± 11 g C m−2 year−1 showing that the carbon sink strength of heather-dominated ecosystems may be considerable when C. vulgaris is in the building phase of its life cycle. The estimated gross ecosystem photosynthesis and ecosystem respiration from October to March was 22% and 30% of annual flux, respectively, suggesting that both cold-season carbon gain and loss were important in the annual carbon cycle of the ecosystem. Model fit of R E of a classic, first-order exponential equation related to temperature (second year; R 2 = 0.65) was improved when the P g rate was incorporated into the model (second year; R 2 = 0.79), suggesting that daytime R E increased with increasing photosynthesis. Furthermore, the temperature sensitivity of R E decreased from apparent Q 10 values of 3.3 to 3.9 by the classic equation to a more realistic Q 10 of 2.5 by the modified model. The model introduces R photo, which describes the part of respiration being tightly coupled to the photosynthetic rate. It makes up 5% of the assimilated carbon dioxide flux at 0°C and 35% at 20°C implying a high sensitivity of respiration to photosynthesis during summer. The simple model provides an easily applied, non-intrusive tool for investigating seasonal trends in the relationship between ecosystem carbon sequestration and respiration.

Partitioning the net CO2 flux of a deciduous forest into respiration and assimilation using stable carbon isotopes

Partitioning net ecosystem CO2 fluxes measured by the eddy covariance technique into their components assimilation and respiration is crucial for predicting future responses and feedbacks of ecosystems to a changing climate. On the basis of an isotopic approach with 13C, we partitioned the daytime net CO2 fluxes of a deciduous forest in central Germany into assimilation and respiration fluxes over a period of 3 weeks. This is the first attempt so far to quantify component fluxes with stable isotopes over the period of 3 weeks, enabling us to investigate the impact of environmental factors on the partitioned fluxes. Large variability in environmental conditions during the 3‐week measurement campaign led to strong changes in isotopic disequilibrium between assimilation and respiration, ranging from 1 to 5‰. Although this approach is still associated with large uncertainty, we found reasonable patterns in ecosystem respiration and assimilation, and a significant correlation of daytime respiration with soil temperature (R2 = 0.48). The ratio of respiration to assimilation was highly variable on a day‐to‐day basis, ranging from 10% to more than 25%. This ratio was mainly controlled by soil temperature (R2 = 0.61), indicating a strong sensitivity of ecosystem carbon dynamics to temperature changes and higher carbon uptake efficiency during cooler days.

Year‐round measurements of net ecosystem CO2 flux over a montane larch forest in Mongolia

Mongolian boreal forest merits special attention since it is located in the transitional area between the southern Siberian boreal forest and the Asian steppe zone, a vulnerable region being potentially affected by global warming and anthropogenic activities. This paper presents the first full‐year‐long continuous measurements of net ecosystem CO2 flux (NEE) made over a montane larch (Larix sibirica Ledeb.) forest in Mongolia from 25 March 2003 to 24 March 2004 (366 days) using the eddy covariance technique. The hourly maximum uptake was −10.1 μmol m−2 s−1. The maximum daily uptake of −4.0 g C m−2 d−1 (negative NEE values denote net carbon uptake by the canopy from the atmosphere) occurred in July. The annual cumulative NEE was −85 g C m−2, indicating that the forest acted as a net sink of CO2. We examined the responses of NEE to environmental conditions in the growing season from May to September. Both daytime 30‐min mean and daily integrated NEE responded to incident photosynthetically active radiation (PAR) in a rectangular hyperbolic fashion. Model results show that the apparent quantum yield (α) was −0.0133 ± 0.0011 μmol CO2 per μmol of photons, and the bulk light use efficiency (LUE) on the daily basis was −6.7 mmol CO2 per mole of PAR photons over the entire growing season for this forest. Additionally, daily integrated NEE was also a linear function of the normalized difference vegetation index (NDVI), a linear function of mean daily air temperature (Ta), and a quadratic polynomial function of daily means of the atmospheric water vapor pressure deficit (VPD). Among these factors, LAI (as measured by NDVI) was dominant in affecting the dynamics of NEE, followed by Ta. Lower Ta was limiting the growth rate of this montane larch forest. As daily means of VPD exceeded 1.2 kPa, net CO2 uptake by the canopy declined. Nevertheless, water stress was not observed as a problem for the forest growth.

Synergy of Remote Sensing and Biophysical Process-based Modeling for Estimating Dynamics of CO<sub>2</sub> Flux in Agro-ecosystems

The objective of this study was to investigate the potential of the synergy between the biophysical/ecophysiological models and remote sensing signatures for dynamic estimation of key biophysical variables at the ecosystems-atmosphere interface. We obtained a long-term and comprehensive data set of micrometeorological, plant, and remote sensing (optical and thermal domains) measurements over well-managed uniform agricultural fields. The net ecosystem CO2 flux (NEECO2) was measured by the eddy covariance method (ECM). A soil-vegetation-atmosphere transfer (SVAT) model was used to describe the energy balance, water budget and physiological processes that allowed simulation of the seasonal change of CO2 and water fluxes as well as biomass, photosynthesis, soil water, and surface temperatures. Remotely sensed spectral reflectance was useful to effectively tune the process-based model, so that biomass, evapotranspiration, and CO2 flux were accurately simulated. Simulated NEECO2 agreed well with those measured by ECM, while simulated biomass agreed well with independent measurements. The synergy of remote sensing and process-based modeling was quite effective in utilizing infrequent and multi-source remote sensing data. This approach has great potential for quantitative and dynamic assessment of multiple variables in terrestrial ecosystems.

18O composition of CO2 and H2O ecosystem pools and fluxes in a tallgrass prairie: Simulations and comparisons to measurements

AbstractIn this paper we describe measurements and modeling of 18O in CO2 and H2O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the δ18O value of leaf water, depth‐resolved soil water, atmospheric water vapor, and Keeling plot δ18O intercepts for net soil‐surface CO2 and ecosystem CO2 and H2O fluxes during three periods of the 2000 growing season. Daytime discrimination against C18OO, as calculated from measured above‐canopy CO2 and δ18O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land‐surface and isotope model (ISOLSM) that simulates ecosystem H218O and C18OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the δ18O value of net ecosystem CO2 and H2O fluxes. Simulations indicate that incomplete equilibration between CO2 and H2O within C4 plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the δ18O value of above‐canopy vapor had a small impact on the predicted δ18O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the δ18O value of above‐canopy CO2 substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem 18O‐isoflux in CO2 during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C18OO fluxes requires accurate predictions of both soil and leaf 18O‐isofluxes.

Modelling of biospheric CO2 gross fluxes via oxygen isotopes in a spruce forest canopy: a 222Rn calibrated box model approach

One-dimensional box model estimates of biospheric CO2 gross fluxes are presented. The simulations are based on a set of measurements performed during the EUROSIBERIAN CARBONFLUX intensive campaign between 27 July and 1 August 1999 in a natural Picea abies forest in Russia. CO2 mixing ratios and stable isotope ratios of CO2 were measured on flask samples taken in two heights within the canopy. Simultaneously, soil and leaf samples were collected and analysed to derive the 18O/16O ratio of the respective water reservoirs and the 13 C/ 12 C ratio of the leaf tissue. The main objective of this project was to investigate biospheric gas exchange with soil and vegetation, and thereby take advantage of the potential of the 18 O/ 16 O ratio in atmospheric CO2. Via exchange of oxygen isotopes with associated liquid water reservoirs, leaf CO2 assimilation fluxes generally enrich while soil CO2 respiration fluxes generally deplete the 18 O/ 16 O ratio of atmospheric CO2. In the model, we parameterised intracanopy transport by exploiting soil-borne 222 Rn as a tracer for turbulent transport. Our model approach showed that, using oxygen isotopes, the net ecosystem CO2 flux can be separated into assimilation and respiration yielding fluxes comparable with those derived by other methods. However, partitioning is highly sensitive to the respective discrimination factors, and therefore also on the parameterisation of internal leaf CO2 concentrations and gradients.

BOWEN RATIO/ENERGY BALANCE AND SCALED LEAF MEASUREMENTS OF CO2FLUX OVER BURNEDPROSOPISSAVANNA

Prescribed fire is used as a management practice to maintain grassland dominance and reduce woody plant encroachment on grasslands and rangelands. Little is known regarding effects of these fires on CO2 fluxes and their potential contribution to atmospheric CO2. The objectives of this study were (1) to quantify the effect of fire on net ecosystem CO2 flux above Prosopis glandulosa Torr. (honey mesquite) mixed-grass savannas using the Bowen ratio/energy balance (BREB) method, and (2) to compare these fluxes to fluxes determined by an empirical model that scaled measurements of leaf photosynthesis and leaf area of dominant species and soil CO2 fluxes to the ecosystem level. Measurements were made during a wet year (1995) and a dry year (1996) with different savanna areas burned in late winter (February–March) each year. In the wet year, BREB-estimated daily (24-h) CO2 flux averaged −0.1, 14.6, and 0.8 g/m2 on the unburned plot, and 15.7, 19.7, and 15.2 g/m2 on the burned plot during spring, summer, and fall periods, respectively, during the first growing season following the fire (positive values mean net CO2 uptake). In the drought year, mean daily CO2 flux during spring, summer, and fall was −9.2, 4.1, and 4.8 g/m2 on the unburned plot, and 8.1, −0.9, and 4.6 g/m2 on the burned plot, respectively. The increased carbon uptake following a fire when compared to the unburned plot was estimated to offset the initial carbon loss from combustion within 28 growing-season days in the wet year, while it took 82 d in the dry year. Empirically determined daylight (12-h) CO2 fluxes, measured on two midsummer days each year, averaged 17.9 g/m2 when determined by BREB, and 12.0 g/m2 when calculated from scaled-up leaf-level measurements. By the end of the first growing season following the burn, it is likely that the amount of carbon emitted during the burn had been taken up by the regrowing vegetation, in both a wet and a drought year.

Modelling of biospheric CO2 gross fluxes via oxygen isotopes in a spruce forest canopy: a 222Rn calibrated box model approach

One-dimensional box model estimates of biospheric CO2 gross fluxes are presented. The simulations are based on a set of measurements performed during the EUROSIBERIAN CARBONFLUX intensive campaign between 27 July and 1 August 1999 in a natural Picea abies forest in Russia. CO2 mixing ratios and stable isotope ratios of CO2 were measured on flask samples taken in two heights within the canopy. Simultaneously, soil and leaf samples were collected and analysed to derive the 18O/16O ratio of the respective water reservoirs and the 13C/12C ratio of the leaf tissue. The main objective of this project was to investigate biospheric gas exchange with soil and vegetation, and thereby take advantage of the potential of the 18O/16O ratio in atmospheric CO2. Via exchange of oxygen isotopes with associated liquid water reservoirs, leaf CO2 assimilation fluxes generally enrich while soil CO2 respiration fluxes generally deplete the 18O/16O ratio of atmospheric CO2. In the model, we parameterised intra-canopy transport by exploiting soil-borne 222Rn as a tracer for turbulent transport. Our model approach showed that, using oxygen isotopes, the net ecosystem CO2 flux can be separated into assimilation and respiration yielding fluxes comparable with those derived by other methods. However, partitioning is highly sensitive to the respective discrimination factors, and therefore also on the parameterisation of internal leaf CO2 concentrations and gradients.

Measurements of CO2 exchange over a woodland savanna (Cerrado Sensu stricto) in southeast Brasil

The technique of eddy correlation was used to measure the net ecosystem exchange over a woodland savanna (Cerrado Sensu stricto) site (Gleba Pé de Gigante) in southeast Brazil. The data set included measurements of climatological variables and soil respiration using static soil chambers. Data were collected during the period from 10 October 2000 to 30 March 2002. Measured soil respiration showed average values of 4.8 molCO2 m-2s-1 year round. Its seasonal differences varied from 2 to 8 molCO2 m-2s-1 (Q10 = 4.9) during the dry (April to August) and wet season, respectively, and was concurrent with soil temperature and moisture variability. The net ecosystem CO2 flux (NEE) variability is controlled by solar radiation, temperature and air humidity on diel course. Seasonally, soil moisture plays a strong role by inducing litterfall, reducing canopy photosynthetic activity and soil respiration. The net sign of NEE is negative (sink) in the wet season and early dry season, with rates around -25 kgC ha-1day-1, and values as low as 40 kgC ha-1day-1. NEE was positive (source) during most of the dry season, and changed into negative at the onset of rainy season. At critical times of soil moisture stress during the late dry season, the ecosystem experienced photosynthesis during daytime, although the net sign is positive (emission). Concurrent with dry season, the values appeared progressively positive from 5 to as much as 50 kgC ha-1day-1. The annual NEE sum appeared to be nearly in balance, or more exactly a small sink, equal to 0.1 0.3 tC ha-1yr-1, which we regard possibly as a realistic one, giving the constraining conditions imposed to the turbulent flux calculation, and favourable hypothesis of succession stages, climatic variability and CO2 fertilization.

Net grassland carbon flux over a subambient to superambient CO2 gradient

AbstractIncreasing atmospheric CO2 concentrations may have a profound effect on the structure and function of plant communities. A previously grazed, central Texas grassland was exposed to a 200‐µmol mol−1 to 550 µmol mol−1 CO2 gradient from March to mid‐December in 1998 and 1999 using two, 60‐m long, polyethylene‐ covered chambers built directly onto the site. One chamber was operated at subambient CO2 concentrations (200–360 µmol mol−1 daytime) and the other was regulated at superambient concentrations (360–550 µmol mol−1). Continuous CO2 gradients were maintained in each chamber by photosynthesis during the day and respiration at night. Net ecosystem CO2 flux and end‐of‐year biomass were measured in each of 10, 5‐m long sections in each chamber. Net CO2 fluxes were maximal in late May (c. day 150) in 1998 and in late August in 1999 (c. day 240). In both years, fluxes were near zero and similar in both chambers at the beginning and end of the growing season. Average daily CO2 flux in 1998 was 13 g CO2 m−2 day−1 in the subambient chamber and 20 g CO2 m−2 day−1 in the superambient chamber; comparable averages were 15 and 26 g CO2 m−2 day−1 in 1999. Flux was positively and linearly correlated with end‐of‐year above‐ground biomass but flux was not linearly correlated with CO2 concentration; a finding likely to be explained by inherent differences in vegetation. Because C3 plants were the dominant functional group, we adjusted average daily flux in each section by dividing the flux by the average percentage C3 cover. Adjusted fluxes were better correlated with CO2 concentration, although scatter remained. Our results indicate that after accounting for vegetation differences, CO2 flux increased linearly with CO2 concentration. This trend was more evident at subambient than superambient CO2 concentrations.

Downward adjustment of carbon fluxes at the biochemical, leaf, and ecosystem scale in beech‐spruce model communities exposed to long‐term atmospheric CO2 enrichment

Young beech (Fagus sylvatica) and spruce (Picea abies) trees from different provenances or genotypes were grown in competition in large model ecosystems and were exposed to two concentrations of atmospheric CO2 (370 vs 570 μmol mol−1), two levels of wet nitrogen deposition (7 vs 70 kg N ha−1 yr−1), and two native forest soils (acidic vs calcareous) for four years in open‐top chambers. The 2×2×2 factorial experimental design was fully replicated (n=4) with each CO2×N combination applied to each soil type. Exposure to atmospheric CO2 enrichment stimulated daytime net ecosystem CO2 flux (NEC) as measured during sunny days in the middle of the third growing season. Nevertheless, we observed substantial down‐regulation of NEC, with larger adjustments on acidic than on calcareous soil. NEC adjustment was associated with slightly reduced leaf area index (LAI) on the acidic soil (no response on calcareous soil), enhanced soil CO2 efflux from both substrate types, and, most importantly, with down‐regulation of CO2 uptake at the leaf scale. Downward adjustment of light‐saturated single‐leaf photosynthesis (A) and of Rubisco was more pronounced in beech than in spruce and these species‐specific differences increased over time. By year four, A adjustment (except in one specific treatment combination in each species) had become complete in beech but had disappeared in spruce. At no time did we observe a genotype or provenance effect on the downward adjustment of carbon fluxes, and nitrogen deposition rate generally had little effect as well. Overall, our results suggest that tree species and soil quality will have profound effects on ecosystem CO2 fluxes under continued atmospheric CO2 enrichment.

Very high productivity of the C4 aquatic grass Echinochloa polystachya in the Amazon floodplain confirmed by net ecosystem CO2 flux measurements.

Fluxes of CO2 and H2O vapour from dense stands of the C4 emergent macrophyte grass Echinochloa polystachya were measured by eddy covariance in both the low water (LW) and high water (HW, flooded) phases of the annual Amazon river cycle at Manaus, Brazil. Typical clear-sky midday CO2 uptake rates by the vegetation stand (including detritus, sediment or water surface) were 30 and 35µmolCO2(ground)m-2s-1 in the LW and HW periods, respectively. A rectangular hyperbola model fitted the responses of "instantaneous" (20- or 30-min average) net CO2 exchange rates to incident photosynthetic photon flux densities (PFD) well. Stand evaporation rates were linearly related to PFD. The major difference in CO2 uptake rates between the two periods was the larger respiration flux during LW due to the CO2 efflux from sediment, roots and litter. Integrated 20- or 30-min fluxes were used to derive relationships between daily CO2 and H2O vapour fluxes and incident radiation. The daily CO2 fluxes were almost linearly related to incident radiation, but there was evidence of saturation at the highest daily radiation totals. Annual productivity estimated from the daily model in 1996-1997 agreed closely with that previously estimated for 1985-1986 from a leaf-scale photosynthetic model, but were some 15% less than those derived at that time from biomass harvests. Both CO2 uptake and water use efficiency were comparable with those found in fertilised maize fields in warm temperate conditions.

Trace gas exchange in a high‐Arctic valley: 1. Variationsin CO2 and CH4 Flux between tundra vegetation types

Ecosystem exchanges of CO2 and CH4 were studied by chamber techniques in five different vegetation types in a high arctic valley at Zackenberg, NE Greenland. The vegetation types were categorized as Cassiope heath, hummocky fen, continuous fen, grassland and Salix arctica snowbed. Integrated daytime fluxes for the different vegetation types of the valley showed that the fen areas and the grassland, were significant sources of CH4 with a mean efflux of 6.3 mg CH4 m−2 hr−1 and sinks for CO2, with almost ‐170 mg CO2 m−2 hr−1. The heath and snowbed areas had much lower carbon sequestration rates of about −25 mg CO2 m−2 hr−1 and were also sinks for CH4. Methane emissions from the valley dominated in the hummocky fens. Computation of area integrated mean daytime flux values across all vegetation types of the entire valley bottom revealed that it was a sink of CO2 in the order of −96 ± 33 mg CO2 m−2 hr−1 and a source of 1.9 ± 0.7 mg CH4 m−2 hr−1. These values were in accordance with eddy correlation measurements reported elsewhere in this issue and reflect a high‐carbon exchange despite the high arctic location. In the fens, where the water table was at or above the soil surface, methane emissions increased with net ecosystem CO2 flux. In places with the water table below the soil surface, such as particularly in the hummocky parts of the fen, oxidation tended to become the dominant controlling factor on methane efflux.

Effects of extended growing season and soil warming on carbon dioxide and methane exchange of tussock tundra in Alaska

The active season of tussock tundra was extended during two growing seasons (1995 and 1996) by snow removal in early season and prevention of snow accumulation in late season to test the effects of a longer growing season on tundra carbon exchange. Three treatments were established: extended season, extended season + soil warming, and controls. Soil warming was accomplished using cold‐frame, resistance heating wire installed the year prior to the initiation of treatments. Diurnal courses of CO2 exchange were measured weekly using infrared gas analysis with enclosed chamber techniques. Methane fluxes were measured two to three times a season also using enclosure methods. In 1995, snowmelt occurred unusually early, and snow removal treatments increased the season only 9–10 days. In 1996 the early season was increased approximately 24 days. As expected, thaw depth, soil temperature, and plant growth were greater earlier in the extended season and extended season + soil heating plots. Methane fluxes in both seasons were low but tended to be higher in the extended season and soil heated plots. Net ecosystem CO2 fluxes were similar among treatments early in the season, with a tendency toward more positive fluxes (system loss) for the snow removal and wanned plots, possibly due to higher belowground respiration. During midseason, fluxes were similar among the treatments. Later in the season, fluxes of extended season and warmed plots tended to be lower (less carbon loss) than controls, especially in 1995. Totaled over the season, however, the fluxes of the three treatments did not statistically differ and represented losses to the atmosphere. Measurements of dark respiration in 1996 indicate that both respiration and uptake were increased on the extended season plots, resulting in similar net fluxes to controls.