Trend, seasonal and diurnal variations of atmospheric CO2 in Beijing

The concentration of atmospheric CO₂ in Beijing increased rapidly at a mean growth rate of 3.7% · a⁻¹ from 1993 to 1995. After displaying a peak of (409.7±25.9) μmol · mol⁻¹ in 1995, it decreased slowly. Both the almost stable anthropogenic CO₂ source and increasing biotic CO₂ sink contribute to the drop of CO₂ concentration from 1995 to 2000. The seasonal variation of CO₂ concentration exhibits a clear cycle with a maximum in winter, averaging (426.8±20.6) μmol · mol⁻¹, and a minimum in summer, averaging (369.1±6.1) μmol·mol⁻¹. The seasonal variation of CO₂ concentration is mainly controlled by phenology. The mean diurnal variation of atmospheric CO₂ concentration for a year in Beijing is highly clear: daily maximum CO₂ concentration usually occurs at night, but daily minimum CO₂ concentration does in the daytime, with a mean diurnal difference more than 34.7 μmol·mol⁻¹. It has been revealed that the interannual variations of atmospheric CO₂ concentration in winter and autumn regulated the interannual trend of atmospheric CO₂, whereas the interannual variation of CO₂ concentration in summer affected the general tendency of atmospheric CO₂ in a less degree.     

CO2浓度升高对冬小麦叶片特性及蒸散影响研究初报

利用开顶式气室(OTC)设置不同CO2浓度处理,观测分析了CO2浓度升高对冬小麦叶片特性与蒸散的影响。结果表明,CO2浓度增加100%、50%和25%后冬小麦叶面积指数(LAI)、叶重、厚度及蒸散量均有不同程度的增加。其中,在叶片发育最旺盛的抽穗期LAI和叶重分别增加23%、11.4%、5.6%和21.3%、9.4%、4.3%;抽穗后,冬小麦平均旗叶面积和叶厚分别增加25.1%、8.4%、4.1%和12.9%、8.8%、7.5%;而旗叶叶绿素无明显变化;蒸散量在拔节-孕穗期略有增加,在孕穗-抽穗期分别增加33.8%、26.8%、11.9%,在开花-乳熟期分别增加了44%、23.9%、8.3%。可见,CO2浓度增加有利于冬小麦生物量的积累,但同时增加了农田生态系统的耗水量。     

Changes of photosynthetic capacity of some plant species under very high CO2 concentrations in Biosphere 2

Conclusions The initial slope of A/ light curve became steep under high CO₂ (700 μmol · mol^-1) compared with that under low CO₂ (350 μmol · mol^-1) for the C₃ species growing in very high CO₂(>2 200 μmol · mol^-1) for a long period. The light compensation points remained unchanged, but the light saturation points were found increased. The ϕ_m,_app and A_max of the C₃ species increased respectively by 79 % and 80 %, while those of C₄ species decreased by 10% and 14%, respectively. The shape of A/light curve of C₄ species did not change. Such results indicated that C₃ species increased the capacity of photosynthesis, while C₄ did not change, otherwise it decreased a little. This work only compared the changes of capacity in photosynthesis of some species under different CO₂ levels in Biosphere 2. We need further investigation on the effects of high CO₂ on the same species outside Biosphere 2, in order to fully undertand the effects and mechanism of response of plants to the elevated CO₂.     

Effects of elevated [CO2] on stem and root lodging among rice cultivars

Studies showed that elevated [CO2] would improve photosynthetic rates and enhance yields of rice;however,few studies have focused on the response of rice lodging,which is a major cause of cereal yield loss and quality reduction,under elevated [CO2].In this study,we examined the effects of elevated [CO2] on stem and root lodging using 4 rice cultivars(86Y8,japonica hybrid;LYP9,2-line indica hybrid;variety 9311,type of indica inbred rice,and SY63,3-line indica hybrid) grown under two [CO2] levels:400 and 680 μmol mol-1.Our results indicated that under elevated [CO2],the stem-lodging risk(SLR) of 9311 decreased,while in SY63 the SLR increased,86Y8 and LYP9 were not significantly affected;the risk of root lodging was reduced for all cultivars,because root biomass(instead of root number) and bending strength were increased significantly,and then the increase of anti-lodging ability is far higher than that of self-weight mass moment for all cultivars.These findings suggested that higher [CO2] can enhance the risk of stem-lodging for cultivars with strong-[CO2]-responses,but may not aggravate the root lodging for all rice cultivars.     

滇重楼光合作用与环境因子的关系

 利用LI-6400光合测定仪和人工气候箱研究了滇重楼叶片的光合补偿点、光合饱和点、羧化效率和CO₂补偿点,并进行温度、相对湿度对光合速率的单因子影响研究.滇重楼叶片的光合补偿点为5.7μmol/(m²·s),光合有效辐射增至2000μmol/(m·s)²仍未测到光合饱和点,最适光合有效辐射为750～2000μmol/(m·s)².叶片的羧化效率为0.0295,CO₂补偿点为65μmol/mol,光合速率随CO₂浓度升高而明显增加,高于大气正常值的CO₂浓度对增加滇重楼光合作用的光能利用率是有利的.光合速率在温度11～20℃范围内,随温度升高上升;20～35℃随温度升高下降,最适温度为16～28℃.相对湿度20%～85%的试验范围内,叶片光合速率随湿度增加而增大,最适相对湿度条件在75%以上.     

Effect of CO<Subscript>2</Subscript> and H<Subscript>2</Subscript>O on gasification dissolution and deep reaction of coke

To more comprehensively analyze the effect of CO₂ and H₂O on the gasification dissolution reaction and deep reaction of coke, the reactions of coke with CO₂ and H₂O using high temperature gas-solid reaction apparatus over the range of 950-1250℃ were studied, and the thermodynamic and kinetic analyses were also performed. The results show that the average reaction rate of coke with H₂O is about 1.3-6.5 times that with CO₂ in the experimental temperature range. At the same temperature, the endothermic effect of coke with H₂O is less than that with CO₂. As the pressure increases, the gasification dissolution reaction of coke shifts to the high-temperature zone. The use of hydrogen-rich fuels is conducive to decreasing the energy consumed inside the blast furnace, and a corresponding high-pressure operation will help to suppress the gasification dissolution reaction of coke and reduce its deterioration. The interfacial chemical reaction is the main rate-limiting step over the experimental temperature range. The activation energies of the reaction of coke with CO₂ and H₂O are 169.23 kJ·mol-1 and 87.13 kJ·mol-1, respectively. Additionally, water vapor is more likely to diffuse into the coke interior at a lower temperature and thus aggravates the deterioration of coke in the middle upper part of blast furnace.     

Biogeochemical sequestration of carbon within phytoliths of wetland plants: A case study of Xixi wetland, China

As an important long-term terrestrial carbon sequestration mechanism, biogeochemical sequestration of carbon within phytoliths may play a significant role in the global carbon cycle and climate change. The aim of this study is to explore the potential of carbon bio-sequestration within phytoliths produced by wetland plants. The results show that the occluded carbon content of phytoliths in wetland plants ranges from 0.49% to 3.97%, with a CV (coefficient of variation) value of 810%. The data also indicate that the phytolith-occluded carbon (PhytOC) content of biomass for wetland plants depends not only on the phytolith content of biomass, but also the efficiency of carbon occlusion within phytoliths during plant growth in herb-dominated fens. The fluxes of carbon bio-sequestration within phytoliths of herb-dominated fen plants range from 0.003 to 0.077 t CO2 equivalents t-e-CO2ha-1a-1 . In China, 0.04×106 to 1.05×106t CO2 equivalents per year may be sequestrated in phytoliths of herbaceous-dominated fen plants. Globally, taking a fen area of 1.48×108 ha and the largest phytolith carbon biosequestration flux (0.077 t-e-CO 2 ha-1a-1 ) for herb-dominated fen plants, about 1.14×10 7 t CO 2 equivalents per year would have been sequestrated in phytoliths of fen plants. If other wetland plants have similar PhytOC production flux with herb-dominated fen plants (0.077 t-e-CO2ha-1a-1 ), about 4.39×10 7 t-e-CO2a-1 may be sequestrated in the phytoliths of world wetland plants. The data indicate that the management of wetland ecosystems (e.g. selection of plant species) to maximize the production of PhytOC have the potential to bio-sequestrate considerable quantities of atmospheric CO2 .     

Coral reef ecosystems in the South China Sea as a source of atmospheric CO<Subscript>2</Subscript> in summer

Field measurements of air-sea CO2 exchange in three coral reef areas of the South China Sea (i.e. the Yongshu Reef atoll of the Nansha Islands, southern South China Sea (SCS); Yongxing Island of Xisha Islands, north-central SCS; and Luhuitou Fringing Reef in Sanya of Hainan Island, northern SCS) during the summers of 2008 and 2009 revealed that both air and surface seawater partial pressures of CO2 (pCO2) showed regular diurnal cycles. Minimum values occurred in the evening and maximum values in the morning. Air pCO2 in each of the three study areas showed small diurnal variations, while large diurnal variations were ob-served in seawater pCO2. The diurnal variation amplitude of seawater pCO2 was ~70 μmol mol–1 at the Yongshu Reef lagoon, 420–619 μmol mol–1 on the Yongxing Island reef flat, and 264–579 μmol mol–1 on the reef flat of the Luhuitou Fringing Reef, and 324–492 μmol mol–1 in an adjacent area just outside of this fringing reef. With respect to spatial relations, there were large differences in air-sea CO2 flux across the South China Sea (e.g. ~0.4 mmol CO2 m–2 d–1 at Yongshu Reef, ~4.7 mmol CO2 m–2 d–1 at Yongxing Island, and ~9.8 mmol CO2 m–2 d–1 at Luhuitou Fringing Reef). However, these positive values suggest that coral reef ecosystems of the SCS may be a net source of CO2 to the atmosphere. Additional analyses indicated that diurnal variations of surface seawater pCO2 in the shallow water reef flat are controlled mainly by biological metabolic processes, while those of deeper water lagoons and outer reef areas are regulated by both biological metabolism and hydrodynamic factors. Unlike the open ocean, inorganic metabolism plays a significant role in influencing seawater pCO2 variations in coral reef ecosystems.     

转SOD基因烟草的光合特性初探

 以转基因Fe-SOD,Mn-SOD高表达、转基因Mn-SOD反义低表达烟草及非转基因品系为供试材料,比较CO₂光强对烟草光合作用和蒸腾作用的影响及不同叶位叶片的光合差异.在相同条件下,Mn-SOD高表达烟草CO₂补偿点最高(105μmol·mol^-1),Mn-SOD低表达烟草最低(78μmol·mol^-1),CO₂浓度对非转基因烟草蒸腾作用的影响比对其他三个转基因品系的影响要强.Mn-SOD低表达烟草的光补偿点最高(45μmol·m^-2·s^-1),而Mn-SOD高表达烟草最低(25μmol·m^-2·s^-1),并且强光对非转基因烟草的光合作用抑制大,转基因品系对强光具有较强的耐受力.同一叶位叶片,转基因烟草的光合速率、蒸腾速率更强.但转基因SOD高表达烟草在整体上并没有表现出明显的光合作用优势.     

Responses of rice growth to copper stress under free-air CO2 enrichment （FACE）

The responses of rice to the second degree contamination of copper were studied by pot experiments under free-air CO2 enrichment （FACE） with 570 μmol ·mol^-1 of CO2. The results showed that the content of copper in rice leaves was reduced with the CO2 concentration reaching 570 μmol· mol^-1 and this happened more significantly under the second degree contamination of copper. Under FACE, activities of superoxide dismutase （SOD） enzyme in rice leaves treated by copper contamination were induced, whereas the contents of glutathione （GSH） and glutathione disulfide （GSSG） had no significant difference from the control. In the presence of ambient CO2, activities of SOD enzyme treated by copper pollution were suppressed during the whole rice growth, however, the contents of GSH and GSSG were induced at tillering and jointing stages, and then restored to the control levels in later growth under the second degree contamination of copper. With the rice growing, the content of malondialdehyde （MDA） rises continuously, but there had been no significant difference between the treatments at the same growth stage. Further studies are needed on the response mechanism of rice to Cu stress under elevated CO2.     

Effect of CO<Subscript>2</Subscript> concentrations on the activity of photosynthetic CO<Subscript>2</Subscript> fixation and extracelluar carbonic anhydrase in the marine diatom <Emphasis Type="Italic">Skeletonema costatum</Emphasis>

The growth and activity of photosynthetic CO2 uptake and extracellular carbonic anhydrase (CAext) of the marine diatom Skeletonema costatum were investigated while cultured at different levels of CO2 in order to see its physio-logical response to different CO2 concentrations under either a low (30 靘ol·m-2·s-1) or high (210 靘ol·m-2·s-1) irradiance. The changes in CO2 concentrations (4—31 靘ol/L) affected the growth and net photosynthesis to a greater extent under the low than under the high light re-gime. CAext was detected in the cells grown at 4 mol/L CO2 but not at 31 and 12 靘ol/L CO2, with its activity being about 2.5-fold higher at the high than at the low irradiance. Photo- synthetic CO2 affinity (1/ K1/2(CO2)) of the cells de-creased with increased CO2 concentrations in culture. The cells cultured under the high-light show significantly higher photosynthetic CO2 affinity than those grown at the low-light level. It is concluded that the regulations of CAext activity and photosynthetic CO2 affinity are dependent not only on CO2 concentration but also on light availability, and that the de-velopment of higher CAext activity and CO2 affinity under higher light level could sufficiently support the photosyn-thetic demand for CO2 even at low level of CO2.     

胡杨异胚发育叶片碳平衡研究

胡杨成熟个体植株上的叶片可大概分为3种叶形,即阔卵圆形(BO)、过渡形(O)及披针形(L).叶片尺度的碳平衡取决于光合作用和呼吸作用,但机理尚不明.作者研究了胡杨异形叶的气体交换、形态学和生化特性.结果显示:① 胡杨3种叶形比叶面积(SLA)存在显著差异,阔卵圆形叶比叶面积SLA=(6.69±0.24)m2·kg-1显著低于其余两者(过渡形叶SLA=(7.36±0.09)m2·kg-1,披针形叶SLA=(8.09±0.16)m2·kg-1),叶片单位质量氮含量(Nmass)存在差异但不具显著差异;② 阔卵圆形叶单位面积碳同化速率(Aarea=(11.92±1.22)μmol·m-2·s-1)显著高于过渡形叶(Aarea=(10.21±1.43)μmol·m-2·s-1)和披针形叶(Aarea=(9.98±1.04)μmol·m-2·s-1),然而,单位质量(Amass)和单位氮碳同化速率(AN)在3种叶形中并不具显著差异,披针形叶的Amass=(80.37±7.07)μmol·kg-1·s-1(DW)和AN=(80.37±7.07)μmol·kg-1·s-1(DW)略高于其它2种叶形;③ 阔卵圆形叶单位面积及单位质量暗呼吸速率(Rn-area=(1.90±0.17)μmol·m-2·s-1和Rn-mass=(12.58±0.77)μmol·kg-1·s-1(DW))显著高于过渡形叶(Rn-area=(1.60±0.19)μmol·m-2·s-1和Rn-mass=(11.85±0.15)μmol·kg-1·s-1(DW))及披针形叶(Rn-area=(1.57±0.29)μmol·m-2·s-1和Rn-mass=(11.25±0.92)μmol·kg-1·s-1(DW)),另外,3种叶形叶片光下线粒体呼吸速率(RL)均大于暗呼吸速率,说明光对暗呼吸有抑制作用,且不同叶形抑制程度也不同,从披针叶形50.37%到阔卵圆形叶的80.67%;④ 不同叶形叶片水平碳平衡(Ag/R)差异不显著,但披针形叶的Ag/R=7.90±1.39高于其余2种叶形(阔卵圆形叶Ag/R=6.19±0.87,过渡形叶Ag/R=7.65±1.71).由此推测,阔卵圆形叶片可能具更强结构支持能力和更高呼吸速率,利于个体的生理维持,而披针形叶具更高的碳同化能力则用于支持生长.     

屋顶绿地碳固定潜力的研究

屋顶绿化因为能减轻城市内涝、热岛效应和空气污染等一系列城市化带来的负面影响,在很多国家和地区已经得到推广和应用。通过野外调查对10个用草坪草建植的简单式屋顶绿地的碳固定潜力进行评价,并探讨影响其碳固定能力的各种因子。同化箱试验的结果表明,屋顶绿地在白天可以通过吸收和固定CO2,降低其周围空气中的CO2含量。在多云清冷的上午,由朝鲜结缕草或狗牙根为主建植而成的屋顶植被在1 h内吸收的CO2可达0.46 g·m-2,在5 min内同化箱里CO2体积分数的降低幅度可达42%(162×10-6);地上部分碳储量试验的结果表明,以草坪草为主建成的屋顶绿地植被的地上部平均固定的有机碳为92.55 g/m2。故屋顶绿地是城市内小型碳库,具有一定的固碳作用。同时,基质厚度与草坪草地上部分的有机碳含量呈显著的正相关,也说明基质厚度可能是影响屋顶绿地碳固定潜力的重要因素。     

亚高山红桦根及根际土壤生化特性对升高温度和CO2的响应

通过川西亚高山野外大型控制实验,研究了红桦不同密度下的根系生物量、根际土壤微生物数量和根际土壤酶活性对短期升高温度（ET,相对室外平均升温2.4±0.4 ℃）、升高大气CO2浓度（EC,平均增加15.5±1.0 μmol·L^-1）及交互作用（ETC,生长室相对室外平均升温2.2±0.5　℃并CO₂浓度平均增加15.8±1.2　μmol·L^-1）的响应.初步结果表明：升高大气温度或CO₂浓度均能够显著促进红桦低密度和高密度下单株根系生物量;升高温度和CO₂浓度及二者共同升高对微生物类群和数量影响不同,升高温度细菌、真菌数量以及低密度下放线菌数量显著增加,而高密度下放线菌数量显著下降;升高CO₂浓度下高密度时细菌和真菌数量增加而低密度下均显著下降;升高温度（ET）显著抑制高、低密度下红桦根际土壤多酚氧化酶活性,升高CO₂（EC）根际土壤过氧化氢酶活性在2种密度条件下均不同程度升高,土壤脲酶和多酚氧化酶活性则降低;ETC条件下,根际土壤多酚氧化酶和过氧化氢酶在2种密度下均表现出不同程度的降低,但脲酶活性在高、低密度条件下对ETC表现出不同的响应结果.     

A new estimate of global soil respiration from 1970 to 2008

Soil respiration(Rs)is one of the key processes that underline our understanding of carbon cycle in terrestrial ecosystems.Great uncertainty remains in the previous global Rs estimates with a difference of 70 Pg C a 1between the highest and lowest estimates.Thus,the present study aimed to estimate the global annual Rs and investigate the interannual and spatial variability in global annual Rs using a semi-mechanistic,empirically-based model which included climatic factors(temperature and precipitation)and topsoil(0–20 cm)organic carbon storage.About 657 published studies of annual Rs from 147 measurement sites were included in this meta-analysis.The global data sets from 1970 to 2008 on climate,surface air temperature,and soil properties were collected.The Monte Carlo method was used to propagate the simulation errors to global Rs.The results indicated that the mean annual global Rs was 94.4 Pg C a 1,increasing at roughly 0.04 Pg C a 1(~0.04%a 1)from 1970 to 2008.The Rs rate increased from colder,drier and less soil carbon-rich regions to warmer,moister and more carbon-rich regions.Highest Rs rates appeared in the tropical forest,while the lowest ones were in polar and desert regions.The annual Rs correlated directly with global temperature anomalies,suggesting that the interannual variability in temperature was responsible for the interannual variations in predicted global Rs.The global Rs increased from high-latitude zones to low-latitude zones.Further studies are recommended to explore the relationship between soil respiration and vegetation characters.     

Static opaque chamber-based technique for determination of net exchange of CO<Subscript>2</Subscript> between terrestrial ecosystem and atmosphere

Terrestrial carbon cycling is one of the hotspots in global change issues. In this paper, we presented the rationale for determination of net exchange of CO₂ between terrestrial and the atmosphere (NEE) and the methods for measuring several relevant components. Three key processes for determination of NEE were addressed, including the separation of shoot autotrophic respiration from total CO₂ emissions of the ecosystem, the partition of root respiration from soil CO₂ efflux, and the quantification of rhizodeposition C from NPP. With an understanding of the processes involved in the CO₂ exchange between terrestrial and the atmosphere, we estimated NEE of rice ecosystem in Nanjing based on field measurements of CO₂ emissions and several relevant biotic components as well as abiotic factors. The field measurements of CO₂ emissions were made over the rice-growing seasons in 2001 and 2002 with the static opaque chamber method. Calculations indicated that the seasonal pattern of NEE is comparable for two seasons. Either net carbon emission or fractional carbon fixation occurred during 3 weeks after rice transplanting and thereafter net carbon fixation appeared with an increasing trend as rice growing. Higher net carbon fixation occurred in the rice developmental period from elongating to heading. A decline trend in the fixation was documented after rice heading. The mean daily NEE was -6.06 gC·m⁻² in 2001 season and -7.95 gC·m⁻² in 2002 season, respectively. These values were comparable to the results obtained by Campbell et al. who made field measurements with the Bowen ratio-energy balance technique in irrigated rice, Texas USA. Moreover, the mean daily NEE in this study was also comparable to the values obtained from a Japanese rice paddy with the eddy covariance method under the similar water regime, either drainage course or waterlogged. It is concluded that NEE determined by the static opaque chamber method is comparable and in agreement with those measured by Bowen ratioenergy balance and eddy covariance methods.     

A meta-analysis of responses of wheat yield formation to elevated ozone concentration

The meta-analysis method was applied to quantitatively investigate effects of the elevated ozone concentration ([O₃]) on chlorophyll concentration, gas exchange and yield components of wheat. There were 39 effective references through Web of Science (ISI, USA) and Chinese journal full-text database (CNKI, China). The results of meta-analysis indicated that elevated [O₃] decreased grain yield, grain weight, grain number per ear, ear number per plant and harvest index by 26%, 18%, 11%, 5% and 11%, respectively, relative to ambient air. The decrease in leaf physiological characters was much greater than that in yield when wheat was expose to elevated [O₃], while light-saturated photosynthetic rate (Asat), stomatal conductance (Gs) and chlorophyll content (Chl) decreased by 40%, 31%, and 46%, respectively. The responses to elevated [O₃] between spring wheat and winter wheat were similar. Most of the variables showed a linear decrease trend with an increase of [O₃]. The most significant decrease for Asat, Gs and Chl was found in grain filling stage. Elevated [CO₂] could significantly ameliorated or offset the detrimental effects caused by elevated [O₃]. Supported by National Natural Science Foundation of China (Grant No. 30670387), Eco-Frontier Fellowship of the Ministry of Environment, Japan (Grant No. 07-C062-03) and Ministry of Science and Technology of People’s Republic China with 973 Project (Grant No. 2002CB410803)     

高浓度CO2对红松幼苗及土壤碳氮特征的影响

为探知红松苗对未来大气CO₂浓度的碳、氮响应策略,系统了解不同CO₂浓度下红松幼苗及其土壤碳、氮特征,采用生长箱培养法,分别研究了350、700 μmol/mol CO₂浓度下红松幼苗主要器官碳、氮浓度与积累(吸收)量变化,并分析其培养土壤的碳、氮含量。结果表明:与低浓度CO₂处理相比,高浓度CO₂处理并未对红松幼苗根、茎及叶的碳浓度产生显著影响,但导致叶碳积累量显著增加37.63%; 高浓度CO₂培养导致红松幼苗根、茎、叶氮浓度显著降低,茎氮吸收量显著下降27.45%,根、茎、叶的碳氮比升高,土壤溶解性有机碳含量显著增加28.82%,总有机碳、微生物量碳、全氮、微生物量氮及水解性氮含量均未发生显著变化,碳氮比增加。总体上,3年生的红松幼苗氮浓度、碳氮比、叶碳积累量及土壤溶解性有机碳对CO₂升高响应迅速。     

Features of major greenhouse gases in loess, Shanxi Province, China

According to the investigations of five loess sections in Shanxi Province, China, it was found that the concentrations of the major greenhouse gases CO₂, CH₄ and N₂O in loess-paleosol sequences are generally high, even sometimes may be several times or scores of times higher than their atmospheric concentrations respectively. Although the CO₂ concentration in the same loess section shows poor regularity among different layers, it increases slowly from north to south in space. The CH₄ concentration in the layers under Malan Loess is much higher than that in the atmosphere, although it is not high in Malan Loess. Most of the δ¹³C values of CO₂ in loess are -11.14‰—15.48‰ (relative to PDB standard). Analysis of carbon isotopic compositions of CO₂ indicates that the main source of CO₂ in loess section is decomposition of ‘stable’ organic matters by microbes. The δ¹³C_g of CO₂ is a little heavier than organic source for exchanging carbon isotope with carbonate in loess. The abundant carbonate in loess not only makes the loess a huge carbon reservior but also adjusts     

Soil aggregate fraction-based 14C analysis and its application in the study of soil organic carbon turnover under forests of different ages

There still exist uncertainties in the trend, magnitude and efficiency of carbon sequestration with regard to the changes in soil organic carbon (SOC) pools after afforestation. In this study, SOC turnover times of the meadow steppe and planted forests at Saihanba Forest Station of Hebei Province, China are estimated by means of the radiocarbon ( 14C) method. Our results show that the SOC turnover times can be as long as from 70 to 250 years. After planting the Pinus sylvestri var. mongolica in the Leymus chinensis meadow steppe, the turnover times of organic carbon in both bulk samples and soil aggregate fractions of the topsoils are decreased with an increase of the stand age. Such a lowering of the turnover time would cause an increase in soil CO2 flux, implying that afforestation of grassland may reduce the capacity of topsoil to sequestrate organic carbon. Combined stable isotope and 14C analyses on soil aggregate fractions suggest that there are different responses to afforestation of grassland between young and old carbon pools in topsoils. In the young and middle-age planted forests, the proportion of CO 2 emission from the older soil carbon pool shows an increasing trend. But in the mature planted forest, its proportion tends to decline, indicating that the stand age may influence the soil carbon sequestration mechanism. The CO2 emission from the topsoils estimated using the 14 C method is relatively low compared to those by other methods and may be caused by the partial isolation of the young carbon component from the soil aggregates. For more accurate estimation of CO2 flux, future studies should therefore employ improved methodology for more effective separation of different soil carbon components before isotope analyses.