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.     

Carbon monoxide as an inhibitor of N2ase activity(C2H2) in control measurements of endogenous formation of ethylene by forest soils

Two problems with the recently suggested method to measure endogenous formation of C₂H₄ in an atmosphere enriched with C₂H₂ and CO in studies of N₂ase activity (C₂H₂) in forest soils were analysed, namely the effect of consumption of CO during incubation and the effect of water-saturated conditions.After an initial addition of 100 ml C₂H₂ and 20 ml CO 1^?1 to soil incubation vessels, CO was gradually consumed and followed by a recovery of N₂ase activity when the concentration of CO was lower than about 10 ml 1^?1. The shortest period within which this concentration was achieved was 1 day when incubating fresh soil cores at 15°C, and it was concluded that longer incubations should be avoided.The inhibition of N₂ase activity by CO was strongly suppressed when all soil pores were filled with water. Dissolved inorganic N (0.1% of dry mass soil) was much more efficient in inhibiting N₂ase activity under such conditions.     

Modelling energy and CO2 fluxes with an interactive vegetation land surface model-Evaluation at high and middle latitudes

We developed ISBA-CC, a new version of the ISBA land surface model, to represent the fluxes and reservoirs of the terrestrial carbon cycle. The simulated latent heat flux, sensible heat flux and net ecosystem exchange are validated against in-situ measurements at 26 sites of the FLUXNET network, located at temperate and high latitudes of the Northern Hemisphere. Overall, ISBA-CC captures well the temporal variations of the fluxes, from the diurnal to the seasonal and the inter-annual scales. The best results are obtained for deciduous broadleaf and needle leaf forests, while the worst are found for agricultural sites, suggesting the need to take into account agricultural practices in order to better simulate both the energy fluxes and the net carbon flux. The ranking of the sites in terms of scores is consistent for the different fluxes. We show that taking into account the response of heterotrophic respiration to soil moisture allows a better representation of the variations of the net ecosystem exchange along with soil water content. ISBA-CC also compares favourably with other terrestrial ecosystem models. All these results show that the model matches the observed variability driven by the climate and can be used to assess the response of the terrestrial biosphere to future climate changes.     

Long-term (42 years) effect of thinning on soil CO2 emission in a mixed broadleaved-Korean pine (Pinus koraiensis) forest in Northeast China

Thinning is an important forest management practice that has great potential to influence regional soil carbon storage and dynamics.The present study measured soil respiration(RS,the efflux of CO2 emitted)and its two components(heterotrophic(RH)and autotrophic(RA)respiration)from soil 42 years after thinning in comparison to un-thinning(control).Autotrophic respiration was significantly greater in the thinning plot,approximately 44%higher compared to the control,while both RSand RHwere slightly,but not significantly,higher in the thinning plot.Higher fine root biomass might have contributed to the higher RAin the thinning plot.Both RSand RHshowed clear soil temperature-dependent seasonal patterns,whereas RAwas less responsive to changes in temperature,especially within one specific season.The annual and season-specific temperature sensitivities of RSand RHwere lower in the thinning plot,specifically during the mid-growing season.Furthermore,variations in the season-specific temperature sensitivity of RSand RHwere less intense in the thinning plot.We conclude that forest thinning can reduce the temperature sensitivity of RSand RHduring the mid-growing season and increase soil CO2 emission in the long term.     

Wavelet analysis of wintertime and spring thaw CO2 and N2O fluxes from agricultural fields

Fluxes of N₂O and CO₂ are not limited to the growing season; winter and spring thaw can represent a significant emission period. The objective of this study was to apply wavelet analysis to winter and spring thaw CO₂ and N₂O fluxes and soil temperatures, to yield additional information about underlying processes, examining temporal patterns and relationships among them. Fluxes used in this analysis were measured over 4 years using micrometeorological methods, in a study comparing two agricultural management practices, best management (BM) and conventional (CONV) practices. Cross-wavelet transform (XWT) and wavelet coherence (WCO) were applied to daily mean time series of N₂O fluxes for BM and CONV replicates and treatments, CO₂ vs. N₂O fluxes, CO₂ flux vs. air and soil temperatures, and N₂O flux vs. air and soil temperatures. N₂O fluxes for replicate plots had small differences in temporal variation while N₂O fluxes from BM and CONV treatments showed a large difference in their time series. XWT and WCO analysis confirmed differences in N₂O fluxes between management practices due to differences in temporal trends in the time series. Field emissions of N₂O and CO₂ fluxes showed times of common high fluxes, such as thaw events. Nitrous oxide and CO₂ flux time series showed a strong coherence with surface (air) temperatures. The relationship between N₂O fluxes and temperature decreased with depth but the relationship between CO₂ flux and temperature was similar for surface and at depth. The strong coherence between emissions and surface conditions does not support the suggested mechanism of trapped gas release. A release of trapped gases from below the ice formation would have been indicated by a strong coherence from CO₂ and N₂O with temperatures at depth as the trapping ice barrier melted. This study demonstrates the effectiveness of wavelets as a tool to investigate temporal relationships in GHG emissions, which is a relatively new application for this type of analysis.     

农村能源建设对减排SO2和CO2贡献分析方法

农村能源建设从节约能源和开发利用可再生能源以替代常规能源两个方面对减排ＳＯ₂和ＣＯ₂作出贡献。该文以国际通用的减排量计算方法为依据,在具体分析农村能源特点的基础上,提出了农村能源建设对减排ＳＯ₂和ＣＯ₂贡献的定量分析方法、计算公式和参数,并对１９９６年农村能源建设的环境效益进行了计算。此方法可为农村能源政策分析提供有益参考。     

Impacts of elevated CO2 and O3 on aspen leaf litter chemistry and earthworm and springtail productivity

Human alteration of atmospheric composition affects foliar chemistry and has possible implications for the structure and functioning of detrital communities. In this study, we explored the impacts of elevated carbon dioxide and ozone on aspen (Populus tremuloides) leaf litter chemistry, earthworm (Lumbricus terrestris) individual consumption and growth, and springtail (Sinella curviseta) population growth. We found that elevated carbon dioxide reduced nitrogen and increased condensed-tannin concentrations in leaf litter. These changes were associated with decreases in earthworm individual growth, earthworm growth efficiency, and springtail population growth. Elevated ozone increased fiber and lignin concentrations of leaf litter. These changes were not associated with earthworm consumption or growth, but were associated with increased springtail population growth. Our results suggest that changes in litter chemistry caused by increased carbon dioxide concentrations will have negative impacts on the productivity of diverse detritivore taxa, whereas those caused by increased ozone concentrations will have variable, taxon-specific effects.     

A comparison of linear and exponential regression for estimating diffusive CH4 fluxes by closed-chambers in peatlands

The closed-chamber method is the most common approach to determine CH₄ fluxes in peatlands. The concentration change in the chamber is monitored over time, and the flux is usually calculated by the slope of a linear regression function. Theoretically, the gas exchange cannot be constant over time but has to decrease, when the concentration gradient between chamber headspace and soil air decreases. In this study, we test whether we can detect this non-linearity in the concentration change during the chamber closure with six air samples. We expect generally a low concentration gradient on dry sites (hummocks) and thus the occurrence of exponential concentration changes in the chamber due to a quick equilibrium of gas concentrations between peat and chamber headspace. On wet (flarks) and sedge-covered sites (lawns), we expect a high gradient and near-linear concentration changes in the chamber. To evaluate these model assumptions, we calculate both linear and exponential regressions for a test data set (n = 597) from a Finnish mire. We use the Akaike Information Criterion with small sample second order bias correction to select the best-fitted model. 13.6%, 19.2% and 9.8% of measurements on hummocks, lawns and flarks, respectively, were best fitted with an exponential regression model. A flux estimation derived from the slope of the exponential function at the beginning of the chamber closure can be significantly higher than using the slope of the linear regression function. Non-linear concentration-over-time curves occurred mostly during periods of changing water table. This could be due to either natural processes or chamber artefacts, e.g. initial pressure fluctuations during chamber deployment. To be able to exclude either natural processes or artefacts as cause of non-linearity, further information, e.g. CH₄ concentration profile measurements in the peat, would be needed. If this is not available, the range of uncertainty can be substantial. We suggest to use the range between the slopes of the exponential regression at the beginning and at the end of the closure time as an estimate of the overall uncertainty.     

Increasing net CO2 uptake by a Danish beech forest during the period from 1996 to 2009

The exchange of CO₂ between the atmosphere and a beech forest near Sorø, Denmark, was measured continuously over 14 years (1996-2009). The simultaneous measurement of many parameters that influence CO₂ uptake makes it possible to relate the CO₂ exchange to recent changes in e.g. temperature and atmospheric CO₂ concentration. The net CO₂ exchange (NEE) was measured by the eddy covariance method. Ecosystem respiration (RE) was estimated from nighttime values and gross ecosystem exchange (GEE) was calculated as the sum of RE and NEE. Over the years the beech forest acted as a sink of on average of 157 g C m⁻² yr⁻¹. In one of the years only, the forest acted as a small source. During 1996-2009 a significant increase in annual NEE was observed. A significant increase in GEE and a smaller and not significant increase in RE was also found. Thus the increased NEE was mainly attributed to an increase in GEE. The overall trend in NEE was significant with an average increase in uptake of 23 g C m⁻² yr⁻². The carbon uptake period (i.e. the period with daily net CO₂ gain) increased by 1.9 days per year, whereas there was a non significant tendency of increase of the leafed period. This means that the leaves stayed active longer. The analysis of CO₂ uptake by the forest by use of light response curves, revealed that the maximum rate of photosynthetic assimilation increased by 15% during the 14-year period. We conclude that the increase in the overall CO₂ uptake of the forest is due to a combination of increased growing season length and increased uptake capacity. We also conclude that long time series of flux measurements are necessary to reveal trends in the data because of the substantial inter-annual variation in the flux.     

Spatial structures of N2O, CO2, and CH4 fluxes from Acacia mangium plantation soils during a relatively dry season in Indonesia

We investigated spatial structures of N₂O, CO₂, and CH₄ fluxes during a relatively dry season in an Acacia mangium plantation stand in Sumatra, Indonesia. The fluxes and soil properties were measured at 1-m intervals in a 1 × 30-m plot (62 grid points) and at 10-m intervals in a 40 × 100-m plot (55 grid points) at different topographical positions of the upper plateau, slope, and valley bottom in the plantation. Spatial structures of each gas flux and soil property were identified using geostatistical analysis. The means (±SD) of N₂O, CO₂, and CH₄ fluxes in the 10-m grids were 0.54 (±0.33) mg N m⁻² d⁻¹, 2.81 (±0.71) g C m⁻² d⁻¹, and −0.84 (±0.33) mg C m⁻² d⁻¹, respectively. This suggests that A. mangium soils function as a larger source of N₂O than natural forest soils in the adjacent province on Sumatra during the relatively dry season, while CO₂ and CH₄ emissions from the A. mangium soils were less than or consistent with those in the natural forest soils. Multiple spatial dependence of N₂O fluxes within 3.2 m (1-m grids) and 35.0 m (10-m grids), and CO₂ fluxes within 1.8 m (1-m grids) and over 65 m (10-m grids) was detected. From the relationship among N₂O and CO₂ gas fluxes, soil properties, and topographic elements, we suggest that the multiple spatial structures of N₂O and CO₂ fluxes are mainly associated with soil resources such as readily mineralizable carbon and nitrogen in a relatively dry season. The soil resource distributions were probably controlled by the meso- and microtopography. Meanwhile, CH₄ fluxes were spatially independent in the A. mangium soils, and the water-filled pore space appeared to mainly control the spatial distribution of these fluxes.     

Emissions of N2O and CO2, denitrification measurements and soil properties in red clover and ryegrass stands

The relationships between the fluxes of nitrous oxide (N₂O) and carbon dioxide (CO₂), and their concentrations in the soil air, three different measures of potential denitrification, soil moisture, soil temperature and precipitation were investigated in soils from beneath ryegrass (Lolium multiflorum Lam.), red clover (Trifolium pratense L.) and mixture of ryegrass-red clover stands on a gleic cambisol. Investigations were carried out in order to test the hypothesis that the measure(s) of potential denitrification are good predictor(s) of N₂O fluxes and thus may be used in empirical models of N₂O emission. Potential denitrification characteristics used in this study involved (i) short-term denitrifying enzyme activity (DEA), (ii) long-term denitrification potential (DP), both determined in soils amended with nitrate and glucose, and (iii) denitrification rate (DR) measured using intact soil cores. Flux measurements were made using cylindrical chambers (internal diameter 31 cm, volume 0.015 m³). The fluxes of N₂O and CO₂ and many other characteristics showed large spatial and temporal variability. Emissions of N₂O from the grass plots were closely related to N₂O concentrations in the soil atmosphere at 22.5 cm depth. Most soil properties did not correlate with N₂O fluxes. It was concluded that DP was not a good predictor for N₂O flux. DEA did not show significant relationship with N₂O flux, but it is suggested that if determined in representative, large soil samples, DEA could be a predictor of N₂O fluxes; this assumption needs, however, verification. The only potential denitrification characteristic which was significantly related to N₂O emission both in grass and clover treatments was DR, which was determined in soil cores.     

CO2 and N2O emissions from gleyic soils in the Russian tundra and a German forest during freeze-thaw periods—a microcosm study

Knowledge is scarce on mineralization of soil organic carbon (SOC) in and N₂O emissions from tundra soils in periods of alternate freezing and thawing. Our objectives were to study the CO₂ and N₂O emissions from two silty gleyic soils formed in different climate zones (a gleyic Cryosol located in the Russian tundra, and a stagnic Gleysol located in an oak stand in central Germany) during freeze-thaw events. Soils were adjusted to a matric potential of −0.2 kPa and emissions were measured in 3-h intervals during an incubation period of 50 days including three freeze-thaw cycles. CO₂ emissions from the German oak forest soil were twofold higher than those of the tundra soil. The ratios of the mean CO₂ production rate before the freezing to the mean CO₂ production rate after thawing ranged from 0.63 to 0.73 for the forest soil and from 0.85 to 0.89 for the tundra soil. The specific CO₂-C production rate (CO₂-C/SOC) was 0.16 for the tundra soil and 0.57 for the forest soil. The results indicate that bioavailability of SOC was markedly smaller in the tundra soil than in the forest soil. Large N₂O emissions were found for the German forest soil, but no N₂O emissions were observed for the tundra soil. The main reason for the absence of N₂O emissions was most likely the negligible availability of nitrate for denitrification. There was some indication that the initial increase in mineralization of SOC induced by freezing and thawing differs between soils from various climatic regions, probably mainly due to a differing bioavailability of the SOC and differing releases of nutrients after thawing.     

Concentration of sugars, phenolic acids, and amino acids in forest soils exposed to elevated atmospheric CO2 and O3

Concentrations of soluble soil sugars, soluble phenolic acids, and free amino acids were measured in three forest communities at the FACTS-II Aspen FACE Site near Rhinelander, WI, in order to better understand how elevated atmospheric CO₂ and O₃ are influencing soil nutrient availability and cycling. Sugars, phenolic acids, and amino acids are mostly derived from plant and microbial processes, and have the potential to be influenced by changes in carbon inputs. We hypothesized that concentrations in the soil would parallel increases seen in biological activity, due to greater net primary productivity under elevated CO₂ and seasonal patterns of root growth. Chemical analysis of soils revealed marginally significant increases of total soluble sugars and total soluble phenolic acids in the elevated CO₂ treatment (+27 mg kg⁻¹, +0.02 μmol g⁻¹), but there were no significant differences in concentrations due to elevated O₃ or CO₂+O₃. Total free amino acid concentrations were not affected by any of the treatments, but significant shifts in individual amino acids were observed. Elevated CO₂ and the interaction treatment (elevated CO₂+O₃) increased aspartic acid concentrations, while elevated O₃ treatment decreased the concentration of valine. Concentrations of sugars increased throughout the growing season, while phenolic acids were constant and amino acids decreased. The birch-aspen community had the highest concentration of phenolic acids and sugars overall, while maple-aspen had the lowest. These findings suggest that concentrations of soluble sugars, soluble phenolic acids, and free amino acids in the soil are strongly influenced by soil properties, plant and microbial activity, plant community composition, and to a lesser degree, changes in atmospheric CO₂ and O₃.     

Soil CO2 flux from a norfolk loamy sand after 25 years of conventional and conservation tillage

Tillage affects the ability of coarse-textured soils of the southeastern USA to sequester C. Our objectives were to compare tillage methods for soil CO₂ flux, and determine if chemical or physical properties after 25 years of conventional or conservation tillage correlated with flux rates. Data were collected for several weeks during June and July in 2003, October and November in 2003, and April to July in 2004 from a tillage study established in 1978 on a Norfolk loamy sand (fine-loamy, kaolinitic, thermic Typic Kandiudults). Conventional tillage consisted of disking to a depth of approximately 15 cm followed by smoothing with an S-tined harrow equipped with rolling baskets. Conservation tillage consisted of direct seeding into surface residues. Flux rates in conservation tillage averaged 0.84 g CO₂ m⁻² h⁻¹ in Summer 2003, 0.36 g CO₂ m⁻² h⁻¹ in Fall 2003, 0.46 g CO₂ m⁻² h⁻¹ in Spring 2004, and 0.86 g CO₂ m⁻² h⁻¹ in Summer 2004. Flux rates from conventional tillage were greater for most measurement times. Conversely, water content of the surface soil layer (6.5 cm) was almost always higher with conservation tillage. Soil CO₂ flux was highly correlated with soil water content only in conventional tillage. In conservation tillage, no significant correlations occurred between soil CO₂ flux and soil N, C, C:N ratio, pH, bulk density, sand fraction, or clay fraction of the surface 7.5 cm. In conventional tillage, sand fraction was positively correlated, while bulk density and clay fraction were negatively correlated with soil CO₂ flux rate, but only when the soil was moist. Long-term conservation tillage management resulted in more uniform within- and across-season soil CO₂ flux rates that were less affected by precipitation events.     

A numerical study of the influence of a clearcut on eddy-covariance fluxes of CO2 measured above a forest

We have compared the contributions of unit source fluxes over a clearcut area in a forest and over the contiguous forested area to the eddy-covariance (EC) flux of CO₂ measured at a tall tower standing in the center of the clearcut under daytime convective conditions. Based on the comparison, the clearcut influence on EC flux measurements is evaluated from the footprint (or source area) perspective. The large-eddy simulation (LES) technique is used to simulate the bottom-up dispersion of two conservative and passive tracers that are released from the clearcut and from the forested area, respectively. Time series of LES-generated vertical velocity and mixing ratios of both tracers at all the levels are recorded every model time step at the tower location during the last hour of each LES run; this somewhat mimics real EC flux measurements at the tower. The contribution of the unit surface flux over the clearcut relative to that over the forested area decreases with increasing measurement height, decreasing convective boundary layer depth, increasing atmospheric stability, and decreasing size of the clearcut. These results are fitted to an empirical relation and used to evaluate flux measurements at the 447-m tall tower in Wisconsin, USA. The contribution of the unit flux over the clearcut to the EC flux measured at the 30-m level of the tower is larger than 50% of that over the forested area under most unstable conditions, while smaller than 2.5% at the 396-m level. Existing analytical footprint models underestimate the clearcut influence because effects of clearcut-induced heterogeneity of the turbulent flow are not taken into account.     

耕作措施对温带半干旱地区土壤温室气体(CO2、CH4、N2O)通量的影响

通过设置在甘肃省定西市李家堡镇的不同耕作措施试验, 利用CO₂分析仪、静态箱-气相色谱法对双序列轮作次序下春小麦地、豌豆地生育期内CO₂、CH₄和N₂O通量进行了测定。试验结果表明: 4种耕作措施下春小麦地和豌豆地在生育期内均表现为CO₂源、N₂O源和CH₄汇的功能。传统耕作不覆盖、免耕不覆盖、免耕秸秆覆盖和传统耕作结合秸秆还田下, 春小麦生育期内平均土壤CO₂通量(μmol·m^-2·s^-1)分别为0.203 6、0.221 2、0.241 8、0.224 9, CH₄通量(mg·m^-2·h^-1)分别为-0.041 6、-0.078 0、-0.081 8、-0.053 7, N₂O通量(mg·m^-2·h^-1)分别为0.089 1、0.069 2、0.046 1、0.065 6; 豌豆生育期内平均土壤CO₂通量(μmol·m^-2·s^-1)分别为0.273 6、0.261 6、0.218 1、0.236 0, CH₄通量(mg·m^-2·h^-1)分别为-0.055 0、-0.073 7、-0.066 2、-0.054 5, N₂O通量(mg·m^-2·h^-1)分别为0.123 4、0.084 7、0.080 6、0.035 0。少免耕及小麦秸秆覆盖有利于减少土壤CO₂排放通量, 免耕不覆盖、免耕秸秆覆盖及传统耕作结合秸秆还田均能不同程度地增加CH₄吸收通量、减少N₂O排放通量。综合来看, 免耕不覆盖、免耕秸秆覆盖和传统耕作结合秸秆还田3种保护性耕作措施有助于减少土壤温室气体的排放量。春小麦地CO₂通量随着土壤温度、土壤含水量的逐渐升高而增大; CH₄吸收通量随着土壤含水量的逐渐升高而增大, 而随着土壤温度的逐渐升高而减小。豌豆地CO₂通量的变化与土壤含水量存在极显著正相关关系; 而春小麦地N₂O通量则与平均土壤温度呈显著正相关, 豌豆地则为极显著正相关。