Carbon dioxide fluxes in coastal Douglas-fir stands at different stages of development after clearcut harvesting

Forests play a significant role in the global carbon (C) cycle. Variability in weather, species, stand age, and current and past disturbances are some of the factors that control stand-level C dynamics. This study examines the relative roles of stand age and associated structural characteristics and weather variability on the exchange of carbon dioxide between the atmosphere and three different coastal Douglas-fir stands at different stages of development after clearcut harvesting. The eddy covariance technique was used to measure carbon dioxide fluxes and a portable soil chamber system was used to measure soil respiration in the three stands located within 50 km of each other on the east coast of Vancouver Island, British Columbia, Canada. In 2002, the recently clearcut harvested stand (HDF00) was a large C source, the pole/sapling aged stand (HDF88) was a moderate C source, and the rotation-aged stand (DF49) was a moderate C sink (net ecosystem production of −606, −133, and 254 g C m⁻² year⁻¹, respectively). Annual gross ecosystem production and ecosystem respiration also increased with increasing stand age. Differences in stand structural characteristics such as species composition and phenology were important in determining the timing and magnitude of maximum gross ecosystem production and net ecosystem production through the year. Both soil and ecosystem respiration were exponentially related to soil temperature in each stand with total ecosystem respiration differing more among stands than soil respiration. Between 1998 and 2003, annual net ecosystem production ranged from 254 to 424 g C m⁻² year⁻¹ over 6 years for DF49, from −623 to −564 g C m⁻² year⁻¹ over 3 years for HDF00, and from −154 to −133 g C m⁻² year⁻¹ over 2 years for HDF88. Interannual variations in C exchange of the oldest, most structurally stable stand (DF49) were related to variations in spring weather while the rapid growth of understory and pioneer species influenced variations in HDF00. The differences in net ecosystem production among stands (maximum of 1000 g C m⁻² year⁻¹ between the oldest and youngest stands) were an order of magnitude greater than the differences among years within a stand and emphasized the importance of age-related differences in stand structure on C exchange processes.     

Eight years of carbon dioxide exchange above a mixed forest at Borden, Ontario

This paper summarizes results from 8 years (1996–2003) of eddy covariance-based ecosystem CO₂ exchange measurements at the Borden Forest Research Station (44°19′N, 79°56′W). The site represents a mid-latitude, 100-year-old, mixed deciduous and coniferous forest dominated by red maple, aspen and white pine. The years 1996 and 1997 were relatively cold, had a late spring and received below average photosynthetic photon flux density (PPFD). This contrasts with an early spring, warmer soil and air temperatures during 1998–1999, and with distinctly wet year of 2000 and dry years of 2001–2003. The combination of early spring, warmer air and soil temperature and relatively high level of PPFD was associated with higher net ecosystem productivity (NEP) that peaked during 1999. Photosynthetic capacity was reduced and NEP showed a mid-growing season depression during the dry years of 2001–2003. Annual average ecosystem respiration (R) determined from a light response model was 30% less than R derived from a logistic respiration equation, relating night time CO₂ flux and soil temperature. However these independently determined R values were well correlated indicating that the site is unaffected by fetch and spatial heterogeneity problems. Based on the combined 8 years of growing season daytime data, an air temperature of 20–25 °C and a vapor pressure deficit (VPD) of 1.3 kPa were found to be the optimal conditions for CO₂ uptake by the canopy. Over the 1996–2003 period, the forest sequestered carbon at an average rate of 140 ± 111 gC m⁻² y⁻¹. The corresponding gross ecosystem photosynthesis (GEP) and R over this period were 1116 ± 93 gC m⁻² y⁻¹ and 976 ± 68 gC m⁻² y⁻¹, respectively. The annual carbon sequestration ranged from 19 gC m⁻² in 1996 to 281 gC m⁻² in 1999. However, these estimates were sensitive to frictional velocity threshold () used for screening data associated with poor turbulent mixing at night. Increasing from 0.2 m s⁻¹ (based on the inflection point in the nighttime CO₂ flux vs. u_* relationship) to 0.35 m s⁻¹ (determined using a selection algorithm based on change-point detection) modified the 8-year mean NEP estimate from 140 ± 111 gC m⁻² y⁻¹ to 65 ± 120 gC m⁻² y⁻¹. Both approaches show that the Borden forest was a low to moderate sink of carbon over the 8-year period.     

Comparison of net ecosystem CO2 exchange in two peatlands in western Canada with contrasting dominant vegetation, Sphagnum and Carex

Net ecosystem carbon dioxide exchange was measured in two contrasting peatlands in northern Alberta, Canada using the eddy covariance technique during the growing season (May–October). Sphagnum spp. made up approximately 66% of the total LAI (1.52 m² m⁻²) at the poor fen and the total N content of Sphagnum capitula was 7.8 mg g⁻¹ at the peak of the growing season. In contrast, the dominant plant species at the extreme-rich fen site, the perennial sedge, Carex lasiocarpa, accounted for approximately 60% of the total LAI (1.09 m² m⁻²), and had leaf total N content of 19.3 mg g⁻¹ at peak biomass. In addition, the peak aboveground biomass was higher at the poor fen (230.9 g m⁻²) than at the extreme-rich fen (157.1 g m⁻²). Both sites had maximum daily rates of net CO₂ uptake of approximately 5 μmol m⁻² s⁻¹, and typical nighttime rates of CO₂ loss of approximately 2 μmol m⁻² s⁻¹ during the peak of the growing season. Calculations of maximum photosynthetic and respiratory capacity were consistently higher at the extreme-rich fen. The poor fen was a net sink for CO₂ during 4 of the 6 months (peaking at 44 g C m⁻² in July), while only slight net losses of CO₂ (3 g C m⁻²) occurred in May and September. In contrast, the extreme-rich fen was calculated to be a significant net sink for CO₂ only during 2 months of the growing season (peaking at 30 g C m⁻² in August), while significant net losses of CO₂ occurred in May (8 g C m⁻²) and in October (13 g C m⁻²). The plant species at the poor fen site were active earlier and later in the growing season, while it took longer for C. lasiocarpa to develop leaf tissue, and leaf senescence and reduction in photosynthetic activity occurred earlier in the fall at the extreme-rich fen. When integrated over the 6-month growing season, the poor fen was a net sink (90 g C m⁻²) that was three times larger than the extreme-rich fen (31 g C m⁻²). The ratio of cumulative total ecosystem respiration to gross primary production was 0.7 at the poor fen and 0.9 at the extreme-rich fen.     

Interacting effects of temperature, soil moisture and plant biomass production on ecosystem respiration in a northern temperate grassland

Chamber measurements of total ecosystem respiration (TER) in a native Canadian grassland ecosystem were made during two study years with different precipitation. The growing season (April–September) precipitation during 2001 was less than one-half of the 30-year mean (1971–2000), while 2002 received almost double the normal growing season precipitation. As a consequence soil moisture remained higher in 2002 than 2001 during most of the growing season and peak aboveground biomass production (253.9 g m⁻²) in 2002 was 60% higher than in 2001. Maximum respiration rates were approximately 9 μmol m⁻² s⁻¹ in 2002 while only approximately 5 μmol m⁻² s⁻¹ in 2001. Large diurnal variation in TER, which occurred during times of peak biomass and adequate soil moisture, was primarily controlled by changes in temperature. The temperature sensitivity coefficient (Q₁₀) for ecosystem respiration was on average 1.83 ± 0.08, and it declined in association with reductions in soil moisture. Approximately 94% of the seasonal and interannual variation in R₁₀ (standardized rate of respiration at 10 °C) data was explained by the interaction of changes in soil moisture and aboveground biomass, which suggested that plant aboveground biomass was good proxy for accounting for variations in both autotrophic and heterotrophic capacity for respiration. Soil moisture was the dominant environmental factor that controlled seasonal and interannual variation in TER in this grassland, when variation in temperature was held constant. We compared respiration rates measured with chambers and that determined from nighttime eddy covariance (EC) measurements. Respiration rates measured by both techniques showed very similar seasonal patterns of variation in both years. When TER was integrated over the entire growing season period, the chamber method produced slightly higher values than the EC method by approximately 4.5% and 13.6% during 2001 and 2002, respectively, much less than the estimated uncertainty for both measurement techniques. The two methods for calculating respiration had only minor effects on the seasonal-integrated estimates of net ecosystem CO₂ exchange and ecosystem gross photosynthesis.     

Environmental control of net ecosystem CO2 exchange in a treed, moderately rich fen in northern Alberta

Peatlands cover about 21% of the landscape and contain about 80% of the soil carbon stock in western Canada. However, the current rates of carbon accumulation and the environmental controls on ecosystem photosynthesis and respiration in peatland ecosystems are poorly understood. As part of Fluxnet-Canada, we continuously measured net ecosystem carbon dioxide exchange (NEE) using the eddy covariance technique in a treed fen dominated by stunted Picea mariana and Larix laricina trees during August 2003–December 2004. The total carbon stock in the ecosystem was approximately 51,000 g C m⁻², with only 540 g C m⁻² contributed by live above ground vegetation. The NEE measurements were used to parameterize simple physiological models to assess temporal variation in maximum ecosystem photosynthesis (A_max) and ecosystem respiration rate at 10 °C (R₁₀). During mid-summer the ecosystem had a relatively high A_max (approx. 30 μmol m⁻² s⁻¹) with relatively low R₁₀ (approx. 4 μmol m⁻² s⁻¹). The peak mid-day NEE uptake rate during July and August was 10 μmol m⁻² s⁻¹. The ecosystem showed large seasonal variation in photosynthetic and respiratory activity that was correlated with shifts in temperature, with both spring increases and fall decreases in A_max well predicted by the mean daily air temperature averaged over the preceding 21 days. Leaf-level gas exchange and spectral reflectance measurements also suggested that seasonal changes in photosynthetic activity were primarily controlled by shifts in temperature. Ecosystem respiration was strongly correlated with changes in ecosystem photosynthesis during the growing season, suggesting important links between plant activity and mycorrhizae and microbial activity in the shallow layers of the peat. Only very low rates of respiration were observed during the winter months. During 2004, the peatland recorded a net annual gain of 144 g C m⁻² year⁻¹, the result of a difference between gross photosynthesis of 713 and total ecosystem respiration of 569 g C m⁻² year⁻¹.     

Carbon dioxide and energy fluxes from a boreal mixedwood forest ecosystem in Ontario, Canada

A long-term flux measurement station has been established in a 74-year-old mixedwood forest ecosystem, located approximately 80 km west of Timmins in northern Ontario, as part of the Fluxnet-Canada Research Network (FCRN). Measurements of energy, water vapour, and carbon dioxide fluxes have been made continuously since August 2003 using the eddy covariance technique, along with ancillary meteorological variables. The spatial structure of the site was evaluated using a variety of sources and techniques, including remote sensing, showing that this forest is mixed but relatively homogeneous. The canopy top height is remarkably constant at between 30 and 32 m. The basal area varies from 18 to 27 m² ha⁻¹, and the aboveground biomass ranges from 82 to 122 Mg ha⁻¹. In this paper, we summarize the diurnal and seasonal patters of carbon dioxide exchange and water loss from September 1, 2003 to August 31, 2004. Net ecosystem productivity (NEP) is strongly related to temperature. Atmospheric vapour pressure deficit (VPD) in this ecosystem exerted strong biophysical control on the daily gross ecosystem productivity (GEP) and evapotranspiration. Seasonal change in shortwave albedo, as a result of the presence of mixed deciduous and coniferous species, was clearly evident. Albedo changes were comparable to the seasonal pattern of NEP. The dormant season lasts more than 6 months of the year at this station. This forest was a moderate sink of carbon over the measurement period. Annual values of GEP, ecosystem respiration (R), and NEP were 1075, 919, and 156 ± 35 g C m⁻², respectively.     

Carbon balance of a three crop succession over two cropland sites in South West France

Long term flux measurements of different crop species are necessary to improve our understanding of management and climate effects on carbon flux variability as well as cropland potential in terrestrial carbon sequestration. The main objectives of this study were to analyse the seasonal dynamics of CO₂ fluxes and to establish the effects of climate and cropland management on the annual carbon balance.CO₂ fluxes were measured by means of the eddy correlation (EC) method over two cropland sites, Auradé and Lamasquère, in South West France for a succession of three crops: rapeseed, winter wheat and sunflower at Auradé, and triticale, maize and winter wheat at Lamasquère. The net ecosystem exchange (NEE) was partitioned into gross ecosystem production (GEP) and ecosystem respiration (R_E) and was integrated over the year to compute net ecosystem production (NEP). Different methodologies tested for NEP computation are discussed and a methodology for estimating NEP uncertainty is presented.NEP values ranged between −369 ± 33 g C m⁻² y⁻¹ for winter wheat at Lamasquère in 2007 and 28 ± 18 g C m⁻² y⁻¹ for sunflower at Auradé in 2007. These values were in good agreement with NEP values reported in the literature, except for maize which exhibited a low development compared to the literature. NEP was strongly influenced by the length of the net carbon assimilation period and by interannual climate variability. The warm 2007 winter stimulated early growth of winter wheat, causing large differences in GEP, R_E and NEE dynamics for winter wheat when compared to 2006. Management had a strong impact on CO₂ flux dynamics and on NEP. Ploughing interrupted net assimilation during voluntary re-growth periods, but it had a negligible short term effect when it occurred on bare soil. Re-growth events after harvest appeared to limit carbon loss: at Lamasquère in 2005 re-growth contributed to store up to 50 g C m⁻². Differences in NEE response to climatic variables (VPD, light quality) and vegetation index were addressed and discussed.Net biome production (NBP) was calculated yearly based on NEP and considering carbon input through organic fertilizer and carbon output through harvest. For the three crops, the mean NBP at Auradé indicated a nearly carbon balanced ecosystem, whereas Lamasquère lost about 100 g C m⁻² y⁻¹; therefore, the ecosystem behaved as a carbon source despite the fact that carbon was imported through organic fertilizer. Carbon exportation through harvest was the main cause of this difference between the two sites, and it was explained by the farm production type. Lamasquère is a cattle breeding farm, exporting most of the aboveground biomass for cattle bedding and feeding, whereas Auradé is a cereal production farm, exporting only seeds.     

Resolving systematic errors in estimates of net ecosystem exchange of CO2 and ecosystem respiration in a tropical forest biome

The controls on uptake and release of CO₂ by tropical rainforests, and the responses to a changing climate, are major uncertainties in global climate change models. Eddy-covariance measurements potentially provide detailed data on CO₂ exchange and responses to the environment in these forests, but accurate estimates of the net ecosystem exchange of CO₂ (NEE) and ecosystem respiration (R_eco) require careful analysis of data representativity, treatment of data gaps, and correction for systematic errors. This study uses the comprehensive data from our study site in an old-growth tropical rainforest near Santarem, Brazil, to examine the biases in NEE and R_eco potentially associated with the two most important sources of systematic error in Eddy-covariance data: lost nighttime flux and missing canopy storage measurements. We present multiple estimates for the net carbon balance and R_eco at our site, including the conventional “u* filter”, a detailed bottom-up budget for respiration, estimates by similarity with ²²²Rn, and an independent estimate of respiration by extrapolation of daytime Eddy flux data to zero light. Eddy-covariance measurements between 2002 and 2006 showed a mean net ecosystem carbon loss of 0.25 ± 0.04 μmol m⁻² s⁻¹, with a mean respiration rate of 8.60 ± 0.11 μmol m⁻² s⁻¹ at our site. We found that lost nocturnal flux can potentially introduce significant bias into these results. We develop robust approaches to correct for these biases, showing that, where appropriate, a site-specific u* threshold can be used to avoid systematic bias in estimates of carbon exchange. Because of the presence of gaps in the data and the day–night asymmetry between storage and turbulence, inclusion of canopy storage is essential to accurate assessments of NEE. We found that short-term measurements of storage may be adequate to accurately model storage for use in obtaining ecosystem carbon balance, at sites where storage is not routinely measured. The analytical framework utilized in this study can be applied to other Eddy-covariance sites to help correct and validate measurements of the carbon cycle and its components.     

Carbon dioxide and energy flux partitioning between the understorey and the overstorey of a maritime pine forest during a year with reduced soil water availability

Carbon dioxide, water vapour and energy fluxes were measured above and within a maritime pine forest during an atypical year with long-lasting reduced soil water availability. Energy balance closure was adequately good at both levels. As compared with what is usually observed at this site the ecosystem dissipated less energy via latent heat flux and more via sensible heat flux. The understorey canopy was responsible for a variable, significant component of the whole canopy fluxes of water vapour and carbon dioxide. The annual contribution of the understorey was 38% (154 mm) of the overall evaporation (399 mm) and 32% (89 mm) of the overall sensible heat flux (274 mm). The participation of the understorey reached 45% of the overall evaporation and 30% of the daytime overall assimilation during significant soil water deficit periods in summertime. Even during winter, understorey photosynthesis was consistent as it compensated soil and understorey respiration. The ecosystem behaved as a sink of carbon, with a negative annual carbon budget (−57 g C m⁻²). However, due to high soil water deficit, the annual ecosystem GPP was 40% less than usually observed at this site. This budget resulted from a sink of −131 g C m⁻² for the overstorey and a source of +74 g C m⁻² for the understorey. Moreover, on an annual basis the overstorey layer contributed to almost two-thirds of the ecosystem respiration. Finally, the effect of long-lasting soil water deficit on the maritime pine forest was found more important than the effect of the heat wave and drought of summer 2003.     

Net ecosystem productivity of boreal aspen forests under drought and climate change: Mathematical modelling with Ecosys

The net ecosystem productivity (NEP) of boreal aspen is strongly affected by comparative rates of annual potential evapotranspiration (E_a) and precipitation (P_a). Changes in E_a versus P_a during future climate change will likely determine changes in aspen NEP and consequently the magnitude of the carbon sink/source of a significant part of the boreal forest. We hypothesize that the effects of E_a versus P_a on aspen NEP can be modelled with a soil–root–canopy hydraulic resistance scheme coupled to a canopy energy balance closure scheme that determines canopy water status and thereby CO₂ uptake. As part of the ecosystem model ecosys, these schemes were used to model diurnal declines in CO₂ and latent heat (LE) exchange during a 3-year drought (2001–2003) at the Fluxnet-Canada Research Network (FCRN) southern old aspen site (SOA). These declines were consistent with those measured by eddy covariance (EC) at SOA, except that ecosystem CO₂ effluxes modelled during most nights were larger that those measured by EC or gap-filled from other EC measurements. Soil CO₂ effluxes in the model were close to, but sometimes smaller than, those measured by automated surface chambers at SOA. Diurnal declines in CO₂ exchange during the drought caused declines in annual NEP in the model, and in gap-filled EC measurements (model versus EC in g C m⁻²: 275 versus 367 ± 110 in 2001, 82 versus 144 ± 43 in 2002 and 23 versus 104 ± 31 in 2003). Lower modelled NEP was attributed to the larger modelled CO₂ effluxes. Ecosys was then used to predict changes in aspen net biome productivity (NBP = NEP − C lost from disturbance) caused by 6-year versus 3-year recurring droughts during 100-year fire cycles under current climate versus climate change projected under the IPCC SRES A1B scenario. Although NBP was adversely affected during recurring 6-year droughts under current climate, it recovered quickly during non-drought years so that long-term NBP was maintained at 4 g C m⁻² year⁻¹. NBP rose by 10, 108 and 126 g C m⁻² year⁻¹ during the first, second and third centuries under climate change with recurring 3-year droughts, indicating a gradual rise in sink activity by boreal aspen. However recurring 6-year droughts during climate change caused recurring negative NBP (C losses), gradually depleting aspen C reserves and eventually causing dieback of the aspen overstory during the third century of climate change. This dieback was followed by a large decline in NBP.We conclude that NBP of boreal aspen will rise gradually under current projections of climate change, except under prolonged (e.g. 6 years) recurring droughts, which would eventually cause aspen to die back and substantial amounts of C to be lost.     

Impacts of nitrogen addition on the carbon balance in a temperate semiarid grassland ecosystem

Understanding carbon (C) cycling and sequestration in vegetation and soils, and their responses to nitrogen (N) deposition, is important for quantifying ecosystem responses to global climate change. Here, we describe a 2-year study of the C balance in a temperate grassland in northern China. We measured net ecosystem CO₂ exchange (NEE), net ecosystem production (NEP), and C sequestration rates in treatments with N addition ranging from 0 to 25 g N m^?2 year^?1. High N addition significantly increased ecosystem C sequestration, whose rates ranged from 122.06 g C m^?2 year^?1 (control) to 259.67 g C m^?2 year^?1 (25 g N). Cumulative NEE during the growing season decreased significantly at high and medium N addition, with values ranging from ?95.86 g C m^?2 (25 g N) to 0.15 g C m^?2 (5 g N). Only the highest N rate increased significantly cumulative soil microbial respiration compared with the control in the dry 2014 growing season. High N addition significantly increased net primary production (NPP) and NEP in both years, and NEP ranged from ?5.83 to 128.32 g C m^?2. The C input from litter decomposition was significant and must be quantified to accurately estimate NPP. Measuring C sequestration and NEP together may allow tracking of the effects of N addition on grassland C budgets. Overall, adding 25 or 10 g N m^?2 year^?1 improved the CO₂ sink of the grassland ecosystem, and increased grassland C sequestration.     

Measuring forest floor CO2 fluxes in a Douglas-fir forest

CO₂ exchange was measured on the forest floor of a coastal temperate Douglas-fir forest located near Campbell River, British Columbia, Canada. Continuous measurements were obtained at six locations using an automated chamber system between April and December, 2000. Fluxes were measured every half hour by circulating chamber headspace air through a sampling manifold assembly and a closed-path infrared gas analyzer. Maximum CO₂ fluxes measured varied by a factor of almost 3 between the chamber locations, while the highest daily average fluxes observed at two chamber locations occasionally reached values near 15 μmol C m⁻² s⁻¹. Generally, fluxes ranged between 2 and 10 μmol C m⁻² s⁻¹ during the measurement period. CO₂ flux from the forest floor was strongly related to soil temperature with the highest correlation found with 5 cm depth temperature. A simple temperature dependent exponential model fit to the nighttime fluxes revealed Q₁₀ values in the normal range of 2–3 during the warmer parts of the year, but values of 4–5 during cooler periods. Moss photosynthesis was negligible in four of the six chambers, while at the other locations, it reduced daytime half-hourly net CO₂ flux by about 25%. Soil moisture had very little effect on forest floor CO₂ flux. Hysteresis in the annual relationship between chamber fluxes and soil temperatures was observed. Net exchange from the six chambers was estimated to be 1920±530 g C m⁻² per year, the higher estimates exceeding measurement of ecosystem respiration using year-round eddy correlation above the canopy at this site. This discrepancy is attributed to the inadequate number of chambers to obtain a reliable estimate of the spatial average soil CO₂ flux at the site and uncertainty in the eddy covariance respiration measurements.     

Soil–plant nitrogen cycling modulated carbon exchanges in a western temperate conifer forest in Canada

Nitrogen controls, on the seasonal and inter-annual variability of net ecosystem productivity (NEP) in a western temperate conifer forest in British Columbia, Canada, were simulated by a coupled carbon and nitrogen (C&N) model. The model was developed by incorporating plant–soil nitrogen algorithms in the Carbon-Canadian Land Surface Scheme (C-CLASS). In the coupled C&N-CLASS, the maximum carboxylation rate of Rubisco (V_cmax) is determined non-linearly from the modelled leaf Rubisco-nitrogen, rather than being prescribed. Hence, variations in canopy assimilation and stomatal conductance are sensitive to leaf nitrogen status through the Rubisco enzyme. The plant–soil nitrogen cycle includes nitrogen pools from photosynthetic enzymes, leaves and roots, as well as organic and mineral reservoirs from soil, which are generated, exchanged, and lost by biological fixation, atmospheric deposition, fertilization, mineralization, nitrification, root uptake, denitrification, and leaching. Model output was compared with eddy covariance flux measurements made over a 5-year period (1998–2002). The model performed very well in simulating half-hourly and monthly mean NEP values for a range of environmental conditions observed during the 5 years. C&N-CLASS simulated NEP values were 274, 437, 354, 352 and 253 g C m⁻² for 1998–2002, compared to observed NEP values of 269, 360, 381, 418 and 264 g C m⁻², for the respective years. Compared to the default C-CLASS, the coupled C&N model showed improvements in simulating the seasonal and annual dynamics of carbon fluxes in this forest. The nitrogen transformation to soil organic forms, mineralization, plant nitrogen uptake and leaf Rubisco-nitrogen concentration patterns were strongly influenced by seasonal and annual temperature variations. In contrast, the impact of precipitation was insignificant on the overall forest nitrogen budget. The coupled C&N modelling framework will help to evaluate the impact of nitrogen cycle on terrestrial ecosystems and its feedbacks on Earth's climate system.     

华北平原农田生态系统碳过程与环境效应研究

本文总结了25年来针对华北平原小麦-玉米两熟系统,农田的碳循环对气候变化(温度升高)和管理措施(氮肥施入、秸秆还田和耕作方式等)响应机制的研究成果。自2001年起我们在中国科学院栾城农业生态系统试验站建立了3个长期定位碳循环试验:耕作试验、有机循环试验和增温试验,并完善了4种农田碳过程监测方法体系:隔离罐-碱液吸收CO_2法、静态箱-气相色谱法、涡度相关技术和浓度梯度法。量化了华北平原小麦-玉米两熟系统碳输入-输出的平衡,并对华北平原施氮农田土壤碳截留进行了再评价,指出秸秆还田下高水高肥的精细管理农田正在以77 g(C)·m~(-2)·a~(-1)的速度丢失碳;此外长期氮施入虽然显著增加0~100 cm土体的土壤有机碳含量,但同时会造成0~60 cm土体土壤无机碳含量显著降低。我们在对碳过程环境效应的研究中进一步指出:增温和施氮均会降低CH4汇强度,但对土壤呼吸无显著影响,这可能主要是由于试验增温诱发的土壤干旱抵消了土壤温度的部分影响和土壤呼吸对土壤温度升高的适应性造成的。我们对剖面土壤气体的研究表明施氮对剖面CH4和CO_2均无显著影响。进一步将静态箱法和浓度梯度法相结合的研究结果表明0~40cm土层是北方旱地无氮农田土壤CO_2产生和CH4吸收的主要发生层。     

Modelling multi-year coupled carbon and water fluxes in a boreal aspen forest

In order to test two hypotheses: (i) that carbon (C) and energy exchanges between terrestrial ecosystems and the atmosphere are closely constrained by soil water availability, and (ii) that vegetation is able to optimize soil water uptake from different soil layers; two model simulations were conducted. The Boreal Ecosystem Productivity Simulator (BEPS) model was run to simulate an aspen forest in Saskatchewan, Canada during the period 1997–2004. In Simulation 1, the effect of soil water availability in different soil layers on stomatal conductance was weighted only by root fraction. In Simulation 2, the influence of soil water availability in different soil layers on stomatal conductance was weighted according to both the root fraction and soil water availability, in order to allow easier access of roots to soil layers containing more water.Comparison against measured fluxes showed that Simulation 2 was an improvement over Simulation 1 in predicting C, water and energy fluxes at different time scales in dry years. In Simulation 1, the daytime C and water fluxes were underestimated during the transition from adequate to insufficient soil water content in the upper layers. In this run, the model captured 92, 79 and 91% of the daily variances in gross primary productivity (GPP), net ecosystem productivity (NEP), and ecosystem respiration (R_e) during 1997–2004. In Simulation 2, the daily variances of GPP, NEP, and R_e explained by the model increased to 93, 82 and 92%, respectively. In Simulation 1, the annual NEP was considerably underestimated in the dry years and years with dry periods, with a root mean square error (RMSE) of 45 g C m⁻² year⁻¹ (n = 8) from 1997 to 2004. In Simulation 2, the RMSE value of simulated annual NEP was reduced to 14 g C m⁻² year⁻¹, a relatively small value compared with the average NEP of 157 g C m⁻² year⁻¹ during 1997–2004. This suggested that the ability of plant roots to extract water from deep soil layers is critical for the forest to maintain growth when surface layers dried out. Our model results showed that NEP was very sensitive to water conditions at this site. In wet years, heterotrophic respiration was enhanced and NEP was reduced.     

不同生长时期极端降水事件对人工针叶林净生产力的影响——以江西省吉安市千烟洲生态试验站为例

[目的]研究不同生长时期极端降水事件对生态系统碳通量的影响,为准确评估江西省吉安市千烟洲人工针叶林生态系统应对极端天气的响应机制提供理论基础。[方法]对比2008年与2010年植物生长初期(4月)与生长旺盛期(6月)碳通量对环境因子(净辐射RN、温度TA、土壤含水量SWC与增强型植被指数EVI)变化的响应差异。[结果] 4月份净生态系统生产力(NEP)降低22.87%,主要是初级生产总量和生态系统呼吸分别降低17.36%[GEP, 9.56 g/(m~2·d)]和2.84%[RE,2.86 g/(m~2·d)]导致,而6月份GEP降低3.58%[7.17 g/(m~2·d)]以及RE增长12.8%[20.37 g/(m~2·d)]导致NEP降低65.77%[27.55 g/(m~2·d)]。[结论]生长季初期TA降低对RE的抑制大于土壤含水量增加的影响,而生长旺盛期土壤含水量增加则会抵消降温对呼吸的抑制,造成更大的碳损失,因此后续研究森林生态系统碳通量对极端气候响应时还需要考虑植物生长时期的影响。     

The REFLEX project: Comparing different algorithms and implementations for the inversion of a terrestrial ecosystem model against eddy covariance data

We describe a model-data fusion (MDF) inter-comparison project (REFLEX), which compared various algorithms for estimating carbon (C) model parameters consistent with both measured carbon fluxes and states and a simple C model. Participants were provided with the model and with both synthetic net ecosystem exchange (NEE) of CO₂ and leaf area index (LAI) data, generated from the model with added noise, and observed NEE and LAI data from two eddy covariance sites. Participants endeavoured to estimate model parameters and states consistent with the model for all cases over the two years for which data were provided, and generate predictions for one additional year without observations. Nine participants contributed results using Metropolis algorithms, Kalman filters and a genetic algorithm. For the synthetic data case, parameter estimates compared well with the true values. The results of the analyses indicated that parameters linked directly to gross primary production (GPP) and ecosystem respiration, such as those related to foliage allocation and turnover, or temperature sensitivity of heterotrophic respiration, were best constrained and characterised. Poorly estimated parameters were those related to the allocation to and turnover of fine root/wood pools. Estimates of confidence intervals varied among algorithms, but several algorithms successfully located the true values of annual fluxes from synthetic experiments within relatively narrow 90% confidence intervals, achieving >80% success rate and mean NEE confidence intervals <110 gC m⁻² year⁻¹ for the synthetic case. Annual C flux estimates generated by participants generally agreed with gap-filling approaches using half-hourly data. The estimation of ecosystem respiration and GPP through MDF agreed well with outputs from partitioning studies using half-hourly data. Confidence limits on annual NEE increased by an average of 88% in the prediction year compared to the previous year, when data were available. Confidence intervals on annual NEE increased by 30% when observed data were used instead of synthetic data, reflecting and quantifying the addition of model error. Finally, our analyses indicated that incorporating additional constraints, using data on C pools (wood, soil and fine roots) would help to reduce uncertainties for model parameters poorly served by eddy covariance data.     

Winter wheat carbon exchange in Thuringia,Germany

Eddy covariance measurements and estimates of biomass net primary production (NPP) in combination with soil carbon turnover modelled by the Roth-C model were used to assess the ecosystem carbon balance of an agricultural ecosystem in Thuringia, Germany, growing winter wheat in 2001. The eddy CO₂ flux measurements indicate an annual net ecosystem exchange (NEE) uptake in the range from −185 to −245 g C m⁻² per year. Main data analysis uncertainty in the annual NEE arises from night-time u¹ screening, other effects (e.g. coordinate rotation scheme) have only a small influence on the annual NEE estimate. In agricultural ecosystems the fate of the carbon removed during harvest plays a role in the net biome production (NBP) of the ecosystem, where NBP is given by net ecosystem production (NEP=−NEE) minus non-respiratory losses of the ecosystem (e.g. harvest). Taking account of the carbon removed by the wheat harvest (290 g C m⁻²), the agricultural field becomes a source of carbon with a NBP in the order of −45 to −105 g C m⁻² per year. Annual carbon balance modelled with the Roth-C model also indicated that the ecosystem was a source for carbon (NBP −25 to −55 g C m⁻² per year). Based on the modelling most of carbon respired resulted from changes in the litter and fast soil organic matter pool. Also, the crop and management history, particularly the C input to soil in the previous year, significantly affect next year’s CO₂ exchange.     

CO2/heat fluxes in rice fields: Comparative assessment of flooded and non-flooded fields in the Philippines

The seasonal fluxes of heat, moisture and CO₂ were investigated under two different rice environments: flooded and aerobic soil conditions, using the eddy covariance technique during 2008 dry season. The fluxes were correlated with the microclimate prevalent in each location. This study was intended to monitor the environmental impact, in terms of C budget and heat exchange, of shifting from lowland rice production to aerobic rice cultivation as an alternative to maintain crop productivity under water scarcity.The aerobic rice fields had higher sensible heat flux (H) and lower latent heat flux (LE) compared to flooded fields. On seasonal average, aerobic rice fields had 48% more sensible heat flux while flooded rice fields had 20% more latent heat flux. Consequently, the aerobic rice fields had significantly higher Bowen ratio (0.25) than flooded fields (0.14), indicating that a larger proportion of the available net radiation was used for sensible heat transfer or for warming the surrounding air.The total C budget integrated over the cropping period showed that the net ecosystem exchange (NEE) in flooded rice fields was about three times higher than in aerobic fields while gross primary production (GPP) and ecosystem respiration (R_e) were 1.5 and 1.2 times higher, respectively. The high GPP of flooded rice ecosystem was evident because the photosynthetic capacity of lowland rice is naturally large. The R_e of flooded rice fields was also relatively high because it was enhanced by the high photosynthetic activities of lowland rice as manifested by larger above-ground plant biomass. The NEE, GPP, and R_e values for flooded rice fields were −258, 778, and 521 g C m⁻², respectively. For aerobic rice fields, values were −85, 515, and 430 g C m⁻² for NEE, GPP, and R_e, respectively. The ratio of R_e/GPP in flooded fields was 0.67 while it was 0.83 for aerobic rice fields.This short-term data showed significant differences in C budget and heat exchange between flooded and aerobic rice ecosystems. Further investigation is needed to clarify seasonal and inter-annual variations in microclimate, carbon and water budget of different rice production systems.