Free Atmospheric CO2 Enrichment (FACE) Increased Labile and Total Carbon in the Mineral Soil of a Short Rotation Poplar Plantation

The global net terrestrial carbon sink was estimated to range between 0.5 and 0.7 Pg C y⁻¹ for the early 1990s. FACE (free atmospheric CO₂ enrichment) studies conducted at the whole-tree and community scale indicate that there is a marked increase of primary production, mainly allocated into below-ground biomass. The enhanced carbon transfer to the root system may result in enhanced rhizodeposition and subsequent transfer to soil C pools. During the first rotation of the POP/EuroFACE experiment in a short-rotation Poplar plantation, total soil C content increased more under ambient CO₂ treatment than under FACE, while under FACE more new C was incorporated than under ambient CO₂. These unexpected and opposite effects may have been caused by a priming effect, where priming effect is defined as the stimulation of SOM decomposition caused by the addition of labile substrates. In order to gain insight into these processes affecting SOM decomposition, we obtained the labile, refractory and stable pools of soil C and N by chemical fractionation (acid hydrolysis) and measured rates of N-mineralization. Results of the first 2 years of the second rotation show a larger increase of total soil C% under FACE than under ambient CO₂. In contrast to the first rotation, total C% is now increasing faster under FACE than under ambient CO₂. Based on these observations we infer that the priming effect ceased during the second rotation. FACE treatment increased the labile C fraction at 0–10 cm depth, which is in agreement with the larger input of plant litter and root exudates under FACE. N-mineralization rates were not affected by FACE. We infer that the system switched from a state where extra labile C and sufficient N-availability (due to the former agricultural use of the soil) caused a priming effect (first rotation), to a state where extra C input is accumulating due to limited N-availability (second rotation). Our results on N-mineralization (second rotation) are in agreement with observations made at three forest FACE sites (Duke Forest, Oak Ridge, and Rhinelander), but our finding of increasing mineral soil C content contrasted with results at the Duke Forest where no significant increase in C content of the mineral soil occurred. However, the FACE induced increase in total C content occurred within the fraction with the shortest turnover time, i.e. the labile fraction. The refractory and stable fractions were not affected. The question remains whether the currently observed larger increase of total soil C and the increase of labile C under FACE will eventually result in long-term C storage in refractory and stable organic matter fractions.     

C4作物FACE(free-air CO2 enrichment)研究进展

持续迅速上升的大气二氧化碳浓度([CO₂])是全球变暖最大的驱动因子,但其作为光合作用底物直接增加了作物的生产力。相比C₃作物,人们对未来高浓度CO₂情形下C₄作物的响应规律认识较少。与封闭或半封闭气室研究相比,FACE(free-air CO₂ enrichment)试验在空气自由流动的大田条件下对作物表现进行研究,它提供了对未来作物生长环境的真实模拟,因此提供了评估CO₂肥料效应以及揭示植物响应机制的最好机会。作为人类重要的粮食和饲料来源,高粱和玉米是最重要的C₄作物。在简介美国玉米和高粱FACE系统的基础上,综述了FACE情形下高浓度CO₂(模拟本世纪中叶大气CO₂浓度,即550 μmol/mol)对两大作物生理、生长和产量以及土壤特性等方面的影响,同时比较了与气室研究结果的异同点。(1)FACE使干旱条件下两作物光合作用显著增强,但湿润条件下没有影响;FACE条件下高粱出现光合适应现象,而玉米没有;(2)FACE使两作物气孔导度大幅下降,导致叶温升高、蒸腾速率下降、蒸发蒸腾总量减少或没有变化、叶片总水势和水分利用效率增加或没有变化;(3)FACE对两作物物候期和化学组分影响很少;(4)FACE使干旱条件下两作物生长和产量略有增加,但湿润条件下没有影响;(5)FACE使高粱田土壤丛枝状菌根真菌的长度和易提取胶状物质浓度显著增加,导致水稳性土壤团聚体增加;FACE对高粱田N₂O或含氮气体(N₂O+N₂)的排放没有影响;(6)高浓度CO₂对两作物气孔导度的影响FACE试验明显大于气室试验,而对生长和产量的影响呈相反趋势。阐明CO₂与基因型、土壤湿度和大气温度间的互作效应及其机制是下一轮C₄作物FACE研究优先考虑的方向,技术的不断进步已为利用大型FACE系统来研究这些互作效应提供了可能。     

Diurnal cycle of carbon isotope ratio in soil CO2 in various ecosystems

Our investigations of diurnal variations of the ¹³C/¹²C ratio and CO₂ content in soil air were carried out in three environments during periods of high biosphere activity. It has been observed that diurnal variation of CO₂ concentration is negatively correlated ¹³. Particularly great variations occurred at shallow soil depths (10–30 cm) when the plant cover activity was high while the soil temperature was rather low. Under such conditions the ¹³ variations had the magnitude of 4, while the CO₂ concentration varied more than doubly. The maximum of the ¹³C/¹²C ratlo and the minimum of the CO₂ concentration in a cultivated field with winter wheat took place in the afternoon, whereas in deciduous forest similar patterns were observed at dawn. In these cases soil temperatures at 10 cm depths varied less than 2°C. Hence, under wheat the variation in root respiration rate seem to be the main reason of the recorded varations. In an uncultivated grass-field during the hottest period in summer we did not measure any distinct variations of CO₂ properties in spite of the fact that soil temperature varied up to 5°C. This might be due to dominant microbial respiration at the high soil temperature, which exceeded 20°C.     

Carbon isotopes and carbon turnover in cotton and wheat FACE experiments

The Maricopa cotton and wheat FACE (free-air CO₂ enrichment) experiments offer propitious opportunity to quantify carbon turnover. The commercial CO₂ (% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiTdq2aaW% baaSqabeaacaaIXaGaaG4maaaakiaaboeacqGHijYUcqGHsislcaaI% ZaGaaG4naiaacwcaliaad+gaaaa!3FCB!\[\delta ^{13} {\text{C}} \approx - 37\% o\]) used to elevate CO₂ concentration in field plots provided a strongly ¹³C-depleted tracer. Soil CO₂ and ¹³C of soil organic carbon (SOC) in CO₂-enriched and Control plots were measured between the final cotton FACE project (October 1991) and the end of the second wheat experiment (June 1994). The initial ¹³C-depletion in SOC of cotton FACE plots (measured by the difference in ¹³C between FACE and Control plots) persisted at the same level (1.9) 1.5 years after the experiment ended. A similar depletion was observed in soil CO₂ evolved in the same plots, indicating ongoing decomposition of the new SOC. The SOC ¹³C of wheat plots before and after two growing seasons showed increasing ¹³C-depletion in FACE relative to Control. Isotopic mass balance was consistent with 5–6% new carbon input from the two wheat crops. This is lower than the 12–13% calculated for FACE cotton and perhaps a consequence of the larger root system of cotton or the 3-year duration of the cotton experiments versus 2 years for the wheat.     

植物对开放式CO2 浓度增高(FACE)的响应与适应研究进展

开放式CO2浓度增高(FACE)系统是近年研究植物对高CO2浓度响应和适应的新手段,它比以往密闭和半密闭系统对实验植物生长环境的干扰少.利用FACE系统进行研究更有助于正确地预测未来大气CO2浓度增高对植物的影响.该文结合作者的研究工作简要评介了FACE系统与以往密闭和半密闭式CO2浓度增高实验系统的不同之处以及近年来利用FACE系统所作的最新研究进展.     

Soil properties differently influence estimates of soil CO2 efflux from three chamber-based measurement systems

Soil CO₂ efflux is a major component of net ecosystem productivity (NEP) of forest systems. Combining data from multiple researchers for larger-scale modeling and assessment will only be valid if their methodologies provide directly comparable results. We conducted a series of laboratory and field tests to assess the presence and magnitude of soil CO₂ efflux measurement system × environment interactions. Laboratory comparisons were made with a dynamic, steady-state CO₂ flux generation apparatus, wherein gas diffusion drove flux without creating pressure differentials through three artificial soil media of varying air-filled porosity. Under these conditions, two closed systems (Li-6400-09 and SRC-1) exhibited errors that were dependent on physical properties of the artificial media. The open system (ACES) underestimated CO₂ flux. However, unlike the two other systems, the ACES results could be corrected with a single calibration equation that was unaffected by physical differences in artificial media. Both scale and rank changes occurred among the measurement systems across four sites. Our work clearly shows that soil CO₂ efflux measurement system × environment interactions do occur and can substantially impact estimates of soil CO₂ efflux. Until reliable calibration techniques are developed and applied, such interactions make direct comparison of published rates, and C budgets estimated using such rates, difficult.     

Kinetic Study of Carbon Dioxide (CO2) Respiration Rate in Bioremediation of Soil Contaminated with Spent Motor Oil

This study was carried out to investigate the kinetics of CO₂ generation from the bioremediation of soil contaminated with spent motor oil (SCSMO) in an aerobic fixed-bed bioreactor designed and fabricated using indigenous technology. Six contaminated soil samples with different compositions at the same contaminant strength were investigated under a controlled air flow rate of 10 L/h. Results obtained revealed that treatment option “6” (i.e., the biostimulation option) gave the best result, with 75% reduction in the initial oil and grease content (O&G) and CO₂ generation of 6,242 mg/kg contaminated soil (CS). Therefore, the biostimulation option was chosen as the treatment technology and kinetic investigation was based on the treatment technology. Results of kinetic investigation of the treatment technology showed that the growth rate can be represented by the popular Monod equation and the kinetic parameters are μ_max = 1.115 × 10^?16 h^?1; K _O&G = 37,490.9 mg/kg; yield of CO₂ = 42.59%; and yield of biomass = 57.41%. The rate equation and kinetic parameters can be used to design a bioreactor and monitor biodegradation reaction during bioremediation process.     

开放式空气CO2浓度增高对水稻产量形成的影响

在大田栽培条件下 ,研究开放式空气CO2 浓度增加 (FACE) 2 0 0 μmol·mol-1的处理对水稻产量及产量构成因素的影响 .结果表明 ,FACE处理对水稻株高和主茎叶片数没有明显影响 ,但使水稻生育进程加快 ,全生育期显著缩短 ,增加施N量可减缓FACE处理对水稻全生育期缩短的程度 ;FACE处理能显著增加分蘖数 ,极显著增加穗数 ,提高结实率 ,但使每穗颖花数显著减少 ;FACE处理能显著提高水稻产量 ,在高N条件下增产幅度更大 ;提高FACE处理的每穗颖花数和单位面积颖花数能极显著提高水稻产量 ,增加施N量是提高FACE处理每穗颖花数和单位面积颖花数的重要措施 .     

冬小麦光合作用对开放式空气CO2 浓度增高(FACE)的非气孔适应

在同样CO2浓度下测定时,开放式空气CO2浓度增高(FACE,580 μmol CO2 /mol)条件下生长的冬小麦叶片的净光合速率、气孔导度和羧化效率都显著低于普通空气(380 μmol CO2 /mol)中生长的对照叶片.与此相一致,FACE叶片的可溶性蛋白、二磷酸核酮糖羧化酶/加氧酶(Rubisco)和Rubisco活化酶含量也都显著低于对照叶片.这些结果表明,在根系生长不受限制的田间条件下,冬小麦叶片的光合作用对高浓度CO2产生了适应现象,其主要原因可能是碳同化的关键酶Rubisco等含量的降低.     

Effect of elevated atmospheric CO2 concentration on soil and root respiration in winter wheat by using a respiration partitioning chamber

Soil respiration in a cropland is the sum of heterotrophic (mainly microorganisms) and autotrophic (root) respiration. The contribution of both these types to soil respiration needs to be understood to evaluate the effects of environmental change on soil carbon cycling and sequestration. In this paper, the effects of free-air CO₂ enrichment (FACE) on hetero- and autotrophic respiration in a wheat field were differentiated and evaluated by a novel split-root growth and gas collection system. Elevated atmospheric pCO₂ of approximately 200 μmol mol⁻¹ above the ambient pCO₂ significantly increased soil respiration by 15.1 and 14.8% at high nitrogen (HN) and low nitrogen (LN) application rates, respectively. The effect of elevated atmospheric pCO₂ on root respiration was not consistent across the wheat growth stages. Elevated pCO₂ significantly increased and decreased root respiration at the booting-heading stage (middle stage) and the late-filling stage (late stage), respectively, in HN and LN treatments; however, no significant effect was found at the jointing stage (early stage). Thus, the effect of increased pCO₂ on cumulative root respiration for the entire wheat growing season was not significant. Cumulative root respiration accounted for approximately 25–30% of cumulative soil respiration in the entire wheat growing season. Consequently, cumulative microbial respiration (soil respiration minus root respiration) increased by 22.5 and 21.1% due to elevated pCO₂ in HN and LN, respectively. High nitrogen application significantly increased root respiration at the late stage under both elevated pCO₂ and ambient pCO₂; however, no significant effects were found on cumulative soil respiration, root respiration, and microbial respiration. These findings suggest that heterotrophic respiration, which is influenced by increased substrate supplies from the plant to the soil, is the key process to determine C emission from agro-ecosystems with regard to future scenarios of enriched pCO₂.     

Soil respiration of Alaskan tundra at elevated atmospheric carbon dioxide concentrations

Summary CO₂ efflux from tussock tundra in Alaska that had been exposed to elevated CO₂ for 2.5 growing seasons was measured to assess the effect of long- and short-term CO₂ enrichment on soil respiration. Long-term treatments were: 348, 514, and 683 μll⁻¹ CO₂ and 680 μll⁻¹ CO₂+4°C above ambient. Measurements were made at 5 CO₂ concentrations between 87 and 680 μll⁻¹ CO₂. Neither long- or short-term CO₂ enrichment significantly affected soil CO₂ efflux. Tundra developed at elevated temperature and 680 μll⁻¹ CO₂ had slightly higher, but not statistically different, mean respiration rates compared to untreated tundra and to tundra under CO₂ control alone.     

Production of CO2 in Soil Profiles of a California Annual Grassland

Soils play a key role in the global cycling of carbon (C), storing organic C, and releasing CO₂ to the atmosphere. Although a large number of studies have focused on the CO₂ flux at the soil–air interface, relatively few studies have examined the rates of CO₂ production in individual layers of a soil profile. Deeper soil horizons often have high concentrations of CO₂ in the soil air, but the sources of this CO₂ and the spatiotemporal dynamics of CO₂ production throughout the soil profile are poorly understood. We studied CO₂ dynamics in six soil profiles arrayed across a grassland hillslope in coastal southern California. Gas probes were installed in each profile and gas samples were collected weekly or biweekly over a three-year period. Using soil air CO₂ concentration data and a model based on Fick’s law of diffusion, we modeled the rates of CO₂ production with soil profile depth. The CO₂ diffusion constants were checked for accuracy using measured soil air ²²²Rn activities. The modeled net CO₂ production rates were compared with CO₂ fluxes measured at the soil surface. In general, the modeled and measured net CO₂ fluxes were very similar although the model consistently underestimated CO₂ production rates in the surficial soil horizons when the soils were moist. Profile CO₂ production rates were strongly affected by the inter- and intra-annual variability in rainfall; rates were generally 2–10 times higher in the wet season (December to May) than in the dry season (June to November). The El Niño event of 1997–1998, which brought above-average levels of rainfall to the study site, significantly increased CO₂ production in both the surface and subsurface soil horizons. Whole profile CO₂ production rates were approximately three times higher during the El Niño year than in the following years of near-average rainfall. During the dry season, when the net rates of CO₂ flux from the soil profiles are relatively low (4–11 mg C– CO₂ m⁻² h⁻¹), 20%–50% of the CO₂ diffusing out of the profiles appears to originate in the relatively moist soil subsurface (defined here as those horizons below 40 cm in depth). The natural abundance ¹⁴C signatures of the CO₂ and soil organic C suggest that the subsurface CO₂ is derived from the microbial mineralization of recent organic C, possibly dissolved organic C transported to the subsurface horizons during the wet season.     

Effects of CO2 enrichment on whole-plant carbon budget of seedlings of Fagus grandifolia and Acer saccharum in low irradiance

Carbon exchange rates (CER) and whole-plant carbon balances of beech (Fagus grandifolia) and sugar maple (Acer saccharum) were compared for seedlings grown under low irradiance to determine the effects of atmospheric CO₂ enrichment on shade-tolerant seedlings of co-dominant species. Under contemporary atmospheric CO₂, photosynthetic rate per unit mass of beech was lower than for sugar maple, and atmospheric CO₂ enrich ment enhanced photosynthesis for beech only. Aboveground respiration per unit mass decreased with CO₂ enrichment for both species while root respiration per unitmass decreased for sugar maple only. Under contemporary atmoapheric CO₂, beech had lower C uptake per plant than sugar maple, while C losses per plant to nocturnal aboveground and root respiration were similar for both species. Under elevated CO₂, C uptake per plant was similar for both species, indicating a significant relative increase in whole-seedling CER with CO₂ enrich ment for beech but not for sugar maple. Total C loss per plant to aboveground respiration was decreased for beech only because increase in sugar maple leaf mass counterbalanced a reduction in respiration rates. Carbon loss to root respiration per plant was not changed by CO₂ enrichment for either species. However, changes in maintenance respiration cost and nitrogen level suggest changes in tissue composition with elevated CO₂. Beech had a greater net daily C gain with CO₂ enrichment than did sugar maple in contrast to a lower one under contemporary CO₂. Elevated CO₂ preferentially enhances the net C balance of beech by increasing photosynthesis and reducing respiration cost. In all cases, the greatest C lost was by roots, indicating the importance of belowground biomass in net C gain. Relative growth rate estimated from biomass accumulation was not affected by CO₂ enrichment for either species possibly because of slow growth under low light. This study indicates the importance of direct effects of CO₂ enrichment when predicting potential change in species distribution with global climate change.     

Simulation of Stream Water Alkalinity Concentrations using Coupled Models of Soil air CO2 and Stream Water Chemistry

The ability to predict stream alkalinity values over timescales shorter than monthly or annually is needed to understand the response of stream chemistry to acidic inputs which occur across short time scales (days). We develop and apply a coupled series of physically-based models which are able to predict daily stream alkalinity values by first calculating soil air CO₂ concentrations. We apply the model to a 9 year record of discharge and stream chemistry from a small catchment in the Shenandoah National park of Virginia. We find that we are able to accurately predict the minimum daily stream alkalinity values for all years and we are able to accurately predict the entire annual cycle for 6 of the 9 years (Nash–Sutcliffe criterion equals 0.26). For the 3 years which we overpredict summer stream alkalinity, summer precipitation was greater than normal and much greater than the period for which the model was calibrated.     

Respiration of blue-green algae in the light

The CO₂ evolution in the light of Anabaena as well as several other blue-green algae is below 10% of the dark control. Addition of DCMU restores CO₂ evolution in the light almost to the dark level. Furthermore, by adding unlabeled NaHCO₃, a ¹⁴CO₂ release is observed with prelabeled algal cells attaining 15 to 100% of dark control. Analysis by double-reciprocal plots exhibits a competitive relationship between added and endogenously released carbon dioxide. We conclude that CO₂ evolved by respiration is immediately refixed in the light without being liberated.The degree of ¹⁴CO₂ release induced by unlabeled bicarbonate in the light allows to determine true photoinhibition of respiration. Anabaena variabilis Kütz. exhibits almost no inhibition while in eight other species respiration is light-inhibited between 50 and 85% of the dark control.Abbreviations CCCP carbonyl cyanide m-chlorophenylhydrazone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - TCA trichloroacetic acid     

Soil gas fluxes of N2O,CH4 and CO2 beneath Lolium perenne under elevated CO2: The Swiss free air carbon dioxide enrichment experiment

Fluxes of nitrous oxide, methane and carbon dioxide were measured from soils under ambient (350 µL L^-1) and enhanced (600 µL L^-1) carbon dioxide partial pressures (pCO₂) at the Free Air Carbon Dioxide Enrichment (FACE) experiment, Eidgenössische Technische Hochschule (ETH), Eschikon, Switzerland in July 1995, using a GC housed in a mobile laboratory. Measurements were made in plots of Lolium perenne maintained under high N input. During the data collection period N fertiliser was applied at a rate of 14 g m^-2 of N. Elevated pCO₂ appeared to result in an increased (27%) output of N₂O, thought to be the consequence of enhanced root-derived available soil C, acting as an energy source for denitrification. The climate, agricultural practices and soils at the FACE experiment combined to give rise to some of the largest N₂O emissions recorded for any terrestrial ecosystem. The amount of CO₂–C being lost from the control plot was higher (10%) than for the enhanced CO₂ plot, and is the reverse of that predicted. The control plot oxidised consistently more CH₄ than the enhanced plot, oxidising 25.5 ± 0.8 µg m^-2 hr^-1 of CH₄ for the control plot, with an average of 8.5 ± 0.4 µg m^-2 hr^-1 of CH₄ for the enhanced CO₂ plot. This suggests that elevated pCO₂ may lead to a feedback whereby less CH₄ is removed from the atmosphere. Despite the limited nature of the current study (in time and space), the observations made here on the interactions of elevated pCO₂ and soil trace gas release suggest that significant interactions are occurring. The feedbacks involved could have importance at the global scale.