Global significance of the carbon cycle in the karst dynamic system: evidence from geological and ecological processes

On the basis of proposing the existence of a karst carbon cycle and carbon sink at a watershed scale, this paper provides four pieces of evidence for the integration of geology and ecology during the carbon cycle processes in the karst dynamic system, and estimated the karst carbon sink effect using the methods of comparative monitoring of paired watersheds and the carbon stable isotope tracer technique. The results of the soil carbon cycle in Maocun, Guilin, showed that the soil carbon cycle in the karst area, the weathering and dissolution of carbonate rocks under the soil, resulted in a lower soil respiration of 25% in the karst area than in a non-karst area (sandstone and shale), and the carbon isotope results indicated that 13.46% of the heavy carbon of the limestone is involved in the soil carbon cycle. The comparative monitoring results in paired watersheds, suggesting that the HCO₃^- concentration in a karst spring is 10 times that of a rivulet in a non-karst area, while the concentration of inorganic carbon flux is 23.8 times. With both chemical stoichiometry and carbon stable isotopes, the proportion of carbon in karst springs derived from carbonate rocks was found to be 58.52% and 37.65% respectively. The comparison on carbon exchange and isotopes at the water-gas interface between the granite and carbonate rock basins in the Li River showed that the CO₂ emission of the karst water is 10.92 times that of the allogenic water from the non-karst area, while the carbon isotope of HCO₃^- in karst water is lighter by 8.62‰. However, this does not mean that the karst water body has a larger carbon source effect. On the contrary, it means the karst water body has a greater karst carbon sink effect. When the karst subterranean stream in Zhaidi, Guilin, is exposed at the surface, carbon-rich karst water stimulated the growth of aquatic plants. The values of carbon stable isotopes in the same species of submerged plants gradually becomes heavier and heavier, and the 512 m flow process has a maximum range of 15.46‰. The calculation results showed that 12.52% of inorganic carbon is converted into organic carbon. According to the data that has been published, the global karst carbon sink flux was estimated to be 0.53-0.58 PgC/a, equivalent to 31.18%-34.41% of the global forest carbon sink flux. In the meanwhile, the karst carbon sink flux in China was calculated to be 0.051 PgC/a, accounting for 68% of its forest carbon sink flux.     

我国南方岩溶区和北方黄土区的大气CO2效应

我国南方岩溶区与北方黄土区都是巨大的碳库。碳酸盐的溶蚀及再结晶是两个碳库与大气ＣＯ^２交换的重要过程。碳的区域平衡是评价化学风化消耗或逸散ＣＯ^２的基础。岩溶区与黄土区在地球化学风化的环境背景、溶蚀过程、产物运移和归宿等差异很大。黄土区化学风化消耗大气ＣＯ^２通量较岩溶区小。目前评价两类地区土壤与大气ＣＯ^２的源汇关系尚不成熟,需要定量认识土壤ＣＯ^２与下伏碳酸盐岩溶蚀或与下伏黄土次生碳酸盐化作用。岩溶区湖泊沉积物中有机质分解产生的ＨＣＯ^３－制约外源及内生碳酸盐溶解和自生碳酸盐形成。     

基于碳酸盐岩风化的碳源分析及土壤的影响作用机制

在CO2 - H2O- 岩石系统中由于碳酸盐岩的可溶性,使其回收土壤/大气CO2 的通量比硅酸盐岩更大。通过大陆河流湖泊HCO-3 来源和海洋碳来源两种计算方式得出,碳酸盐岩溶解回收大气CO2 的量是其它岩类的3倍以上。与同纬度地带性土壤相比,岩溶地区土壤通过促进土下碳酸盐岩的溶解与固碳作用对大气CO2 产生更为强烈的汇效应: 土壤的覆盖使土下碳酸盐岩的溶蚀速率平均提高4.35倍,从而加快了对土壤/大气CO2 的回收速率;富钙的土壤地球化学背景使石灰土富含胡敏酸钙,胡敏酸在土壤中的存留时间长达780～ 3 000年,是其它类型土壤有机质如富里酸的4～ 5倍,稳定的胡敏酸钙使土壤有机碳稳定性增加、周转周期延长而得以累积,固碳作用将减少土壤CO2 向大气的排放。石灰土的平均有机质含量比同纬度红壤、黄壤分别提高了44%、33% ,固碳作用十分明显。      

典型岩溶水系统碳汇通量估算

现代岩溶学研究成果表明,碳酸盐岩在全球碳循环中响应极其迅速,水循环过程中的碳汇效应显著。本研究选取广西桂林寨底地下河系统、广西环江大安地下河系统、重庆北碚青木关地下河系统三个典型岩溶地下水系统,利用各系统地下河的流量和HCO3-浓度的监测资料,采用简单化学平衡模式法估算碳汇通量(CO2)。结果显示,寨底地下河系统的单位面积年碳汇通量为68.82 t/(km2.a),大安地下河系统的单位面积年碳汇通量为81.18 t/(km2.a),青木关地下河系统的单位面积年碳汇通量为100.07 t/(km2.a)。分析认为同一个岩溶水系统的结构特征和环境条件基本上是稳定的;地下河的流量和HCO3-浓度是影响岩溶碳汇强度的关键因素,尤其是地下河流量的变化对碳汇强度的影响显著;不同岩溶水系统的碳汇通量不但受水化学条件和地下水动力条件的控制,同时受土地利用变化的影响。该研究对于改进碳循环模型和评价岩溶地质碳汇有重要意义。     

Contribution of carbonate rock weathering to the atmospheric CO2 sink

To accurately predict future CO₂ levels in the atmosphere, which is crucial in predicting global climate change, the sources and sinks of the atmospheric CO₂ and their change over time must be determined. In this paper, some typical cases are examined using published and unpublished data. Firstly, the sensitivity of carbonate rock weathering (including the effects by both dissolution and reprecipitation of carbonate) to the change of soil CO₂ and runoff will be discussed, and then the net amount of CO₂ removed from the atmosphere in the carbonate rock areas of mainland China and the world will be determined by the hydrochem-discharge and carbonate-rock-tablet methods, to obtain an estimate of the contribution of carbonate rock weathering to the atmospheric CO₂ sink. These contributions are about 0.018 billion metric tons of carbon/a and 0.11 billion metric tons of carbon/a for China and the world, respectively. Further, by the DBL (Diffusion Boundary Layer)-model calculation, the potential CO₂ sink by carbonate rock dissolution is estimated to be 0.41 billion metric tons of carbon/a for the world. Therefore, the potential CO₂ source by carbonate reprecipitation is 0.3 billion metric tons of carbon/a. Received: 12 May 1999 · Accepted: 16 August 1999     

硫酸型酸雨参与碳酸盐岩溶蚀的研究进展

碳酸盐岩的H2CO3溶蚀产生岩溶碳汇,占整个岩石风化碳汇的 94%。西南岩溶区硫酸型酸雨严重,硫酸型酸雨广泛参与碳酸盐岩的溶蚀。H2SO4参与的碳酸盐岩风化是一个大气CO2净释放过程,具有减汇作用巨大。另一方面,岩溶区石灰土壤和地下水具有较高的pH值及盐基饱和度,对H+有巨大的缓冲作用,大气酸沉降在碳酸盐岩地区可能并不会造成地下水的HCO3-和pH降低;相反,较高浓度的SO42-所产生的盐效应和SO2-4与各种阳离子形成的离子对会增大方解石、白云石溶解度,可增强H2CO3对碳酸盐的溶蚀,这可能会使岩溶作用产生更大的碳汇效应。因此,硫酸型酸雨参与碳酸盐岩风化的减汇效应不仅可能被高估,硫酸型酸雨还可能增强碳酸盐岩的H2CO3溶蚀,具有增加岩溶碳汇效应的作用。应结合石灰土壤对大气酸沉降的缓冲容量和阈值及大气酸沉降的H+与土壤中盐基离子的交换量,并综合考虑盐效应、离子对作用、同离子效应,客观评价硫酸型酸雨流经石灰土壤层后对碳酸盐岩溶蚀吸收大气/土壤CO2的影响      

Ecosystem-driven karst carbon cycle and carbon sink effects

It is recognized that karst processes are actively involved in the current global carbon cycle based on twenty years research, and the carbon sink occurred in karst processes is possibly an important part of “missing sink” in global carbon cycle. In this paper, an overview is given on karst carbon cycle research, and influence factors, formed carbon pools (background carbon sink) and sink increase potentials of current karst carbon cycle are analyzed. Carbonate weathering could contribute to the imbalance item (B_IM) and land use change item (E_LUC) in the global carbon cycle model, owing to its uptake of both atmospheric CO₂ (carbon sink effect) and CO₂ produced by soil respiration (carbon source reduction effect). Karst carbon sink includes inorganic carbon sink resulted from hydrogeochemical process and organic carbon sink generated by aquatic photosynthetic DIC conversion, forming relatively stable river (reservoir) water body or sediment carbon sink. The sizes of both sinks are controlled by terrestrial ecosystems and aquatic ecosystems, respectively. Desertification rehabilitation and carbon sequestration by aquatic plants are two effective ways to increase the carbon sink in karst area. It is estimated that the rate of carbon sink is at least 381 000 t CO₂/a with vegetation restoration and afforestation in southwest China karst area, while the annual organic carbon sink generated by aquatic photosynthesis is about 84 200 t C in the Pearl River Basin. The development of a soil CO₂ based model for assessment of regional dissolution intensity will help to improve the estimation accuracy of carbon sink increase and potential, thus provide a more clear and efficient karst sink increase scheme and pathway to achieve the goals of “double carbon”. With the deep investigation on karst carbon cycle, mechanism and carbon sink effect, and the improvement of watershed carbon sink measurement methods and regional sink increase evaluation approaches. Karst carbon sink is expected to be included in the list of atmospheric CO₂ sources/sinks of the global carbon budget in the near future.     

岩溶系统中土壤氮肥施用对岩溶碳汇的影响

有资料显示陆地碳酸盐岩风化消耗大气CO2的碳通量与世界森林碳汇通量量级相当。但农业地区过量施用氮肥形成的硝酸对碳酸盐岩的溶解会减弱岩溶碳汇效应,其量可达到7%~38%,而适量施用氮肥在增加农作物产量的同时,能降低土壤C/N比,增加土壤微生物活性,促进有机物料分解,从而提高土壤CO2浓度,提高土下碳酸盐岩的溶解速率。因此,要从两方面分析岩溶系统中土壤氮肥施用对岩溶碳汇效应的影响。同时,岩溶区碳酸盐岩风化形成的土壤具有较高的pH值及盐基饱和度,对H+有较强的缓冲作用,可能是导致自然条件下,河流中溶解无机碳(DIC)与水体中钙、镁等离子并不守恒的原因之一,因此,运用端元法可能过高估算了硝酸对碳酸盐岩的溶解量。岩溶区土壤环境中硝化作用产生的硝酸到底多少能对碳酸盐岩产生溶蚀,并影响到岩溶碳汇效应还有待研究。应结合土壤本身的特性及河流生物地球化学过程,综合研究不同施氮水平、土壤硝化产酸及其影响下的土下碳酸盐岩溶解及碳汇效应过程,客观评价岩溶区土壤氮肥施用对岩溶碳汇的影响,并寻求适宜氮肥施用量及促进岩溶碳循环,提高岩溶碳汇效应的技术方法。      

岩溶试片的岩性差异对估算岩溶速率和碳通量的影响

研究岩性差异对溶蚀速率的影响有助于提高溶蚀试片法估算岩溶碳汇强度的精确度。本文以贵州省普定县为研究区,将埋放地的主要基岩类型（石灰岩与白云岩）制成标准尺寸的试片,并将其埋设于不同土地利用类型和土壤深度下,经过4个水文年的监测后将估算的结果与前人在同一区域使用标准溶蚀试片的研究结果进行对比分析,结果表明:（1）在相同气候和土壤环境条件下,岩性对溶蚀试片的溶蚀速率有显著影响,且溶蚀速率与岩石中CaO含量呈正相关关系,与MgO含量呈负相关关系;（2）石灰岩与白云岩试片溶蚀速率的差异程度受土地利用及埋放深度的调控,整体上石灰岩溶蚀速率比白云岩溶蚀速率大14%;（3）不同岩性试片估算的岩溶碳汇强度相差较大,标准溶蚀试片估算的结果比埋放地基岩试片估算的结果高。故使用溶蚀试片法估算区域岩溶碳通量时应考虑埋放地基岩的岩性,或者对基于标准溶蚀试片的估算结果进行校正,才能准确反映区域尺度真实的岩溶碳通量大小。      

中国岩溶碳汇通量估算与人工干预增汇途径

中国是岩溶大国,岩溶作用吸收土壤或大气CO₂形成溶解无机碳,并随河流排向海洋,这是陆地碳循环的重要组成部分。中国地质调查局从2009年开始对岩溶碳汇进行探索性调查,基本查明了岩溶碳汇的作用机理、影响因素和计量方法。研究表明: 碳酸盐岩溶蚀试片、径流-水化学和回归模型等方法均揭示了中国岩溶碳汇潜力巨大; 植被恢复、土壤改良、外源水灌溉和水生植物培育等是人工干预增加岩溶碳汇的重要途径; 流域尺度岩溶碳循环及碳汇效应调查技术的应用,助力取得了岩溶碳循环地质调查和碳汇效应评价方面的理论、技术和平台建设等系列成果。值得注意的是,岩溶碳汇计量、核查和人工干预固碳增汇试验示范等方面还面临巨大挑战,这需要在“十四五”期间及以后的研究中不断攻关、完善,从而满足地质碳汇服务碳中和目标的需求。     

中国南方表层岩溶系统的碳循环及其生态效应

表层岩溶系统因碳酸盐岩－水－CO²(气)三相的化学动态不平衡过程而产生特殊的碳循环环节,参加循环的碳包括碳酸盐岩中的碳、大气和土壤空气CO²部分。中国南方表层岩溶系统的碳循环非常活跃,并敏感地响应岩溶动力因素的变化,从而促进了地球化学过程和生物化学过程的结合,成为大气CO²汇的重要项。中国南方表层岩溶系统的碳循环通过驱动环境的元素迁移,促进土壤有机质的积累,并影响植物所需要的矿物质营养元素的全量和有效态,进而影响岩溶区的植物物种、特有性和作物的发育。     

Impact of Carbonate Precipitation on Riverine Inorganic Carbon Mass Transport from a Mid-continent, Forested Watershed

Physiochemical controls on the carbonate geochemistry of large river systems are important regulators of carbon exchange between terrestrial and marine reservoirs on human time scales. Although many studies have focused on large-scale river carbon fluxes, there are few investigations of mechanistic aspects of carbonate mass balance and transport at the catchment scale. We determined elemental and carbonate geochemistry and mass balances for net carbonate dissolution fluxes from the forested, mid-latitude Huron River watershed, established on carbonate-rich unconfined glacial drift aquifers. Shallow groundwaters are near equilibrium with respect to calcite at pCO₂ values up to 25 times atmospheric values. Surface waters are largely groundwater fed and exhibit chemical evolution due to CO₂ degassing, carbonate precipitation in lakes and wetlands, and anthropogenic introduction of road salts (NaCl and CaCl₂). Because the source groundwater Mg²⁺/HCO₃ ^? ratio is fairly constant, this parameter permits mass balances to be made between carbonate dissolution and back precipitation after groundwater discharge. Typically, precipitation does not occur until IAP/K calcite values exceed 10 times supersaturation. Stream chemistry changes little thereafter even though streams remain highly supersaturated for calcite. Our data taken together with historical United States Geological Survey (USGS) data show that alkalinity losses to carbonate precipitation are most significant during periods of lowest discharge. Thus, on an annual basis, the large carbon flux from carbonate dissolution in soil zones is only decreased by a relatively small amount by the back precipitation of calcium carbonate.     

Carbonate weathering and connate seawater influencing karst groundwaters in the Gevas–Gurpinar–Güzelsu basins,Turkey

There are 59 springs at the Gevas–Gurp?nar–Güzelsu basins, 38 of these springs emerge from the fractured karst aquifers (recrystallized limestone and travertine) and 21 emerge from the Yuksekova ophiolites, K?rkgeçit formation and alluvium. The groundwater samples collected from 38 out of the total of 59 springs, two streams, one lake and 12 wells were analyzed physico-chemically in the year 2002. EC and TDS values of groundwater increased from the marble (high altitude) to the ophiolites and alluvium (toward Lake Van) as a result of carbonate dissolution and connate seawater. Five chemical types of groundwater are identified: Ca–Mg–HCO₃, Mg–Ca–HCO₃, Mg–Na–HCO₃, Na–Ca–HCO₃ and Mg–Ca–Na–HCO₃. The calculations and hydrochemical interpretations show that the high concentrations of Ca²⁺, Mg²⁺ and HCO₃ ^? as predominant ions in the waters are mainly attributed to carbonate rocks and high pCO₂ in soil. Most of the karst springs are oversaturated in calcite, aragonite and dolomite and undersaturated in gypsum, halite and anhydrite. The water–rock interaction processes that singly or in combination influence the chemical composition of each water type include dissolution of carbonate (calcite and dolomite), calcite precipitation, cation exchange and freshening of connate seawater. These processes contribute considerably to the concentration of major ions in the groundwater. Stable isotope contents of the groundwater suggest mainly direct integrative recharge.     

Quantifying the carbon source of pedogenic calcite veins in weathered limestone: implications for the terrestrial carbon cycle

The continent is the second largest carbon sink on Earth’s surface. With the diversification of vascular land plants in the late Paleozoic, terrestrial organic carbon burial is represented by massive coal formation, while the development of soil profiles would account for both organic and inorganic carbon burial. As compared with soil organic carbon, inorganic carbon burial, collectively known as the soil carbonate, would have a greater impact on the long-term carbon cycle. Soil carbonate would have multiple carbon sources, including dissolution of host calcareous rocks, dissolved inorganic carbon from freshwater, and oxidation of organic matter, but the host calcareous rock dissolution would not cause atmospheric CO₂ drawdown. Thus, to evaluate the potential effect of soil carbonate formation on the atmospheric pCO₂ level, different carbon sources of soil carbonate should be quantitatively differentiated. In this study, we analyzed the carbon and magnesium isotopes of pedogenic calcite veins developed in a heavily weathered outcrop, consisting of limestone of the early Paleogene Guanzhuang Group in North China. Based on the C and Mg isotope data, we developed a numerical model to quantify the carbon source of calcite veins. The modeling results indicate that 4–37 wt% of carbon in these calcite veins was derived from atmospheric CO₂. The low contribution from atmospheric CO₂ might be attributed to the host limestone that might have diluted the atmospheric CO₂ sink. Nevertheless, taking this value into consideration, it is estimated that soil carbonate formation would lower 1 ppm atmospheric CO₂ within 2000 years, i.e., soil carbonate alone would sequester all atmospheric CO₂ within 1 million years. Finally, our study suggests the C–Mg isotope system might be a better tool in quantifying the carbon source of soil carbonate.

     

Processes controlling precipitation of carbonate cement and dissolution of silica in reef and near-reef settings

Carbonate cementation in the surface layer of reefs and beachrock eliminates porosity and partially replaces detrital quartz grains. The uptake and release of CO₂ by photosynthesis and respiration in reef communities cause a shift in the carbonate buffer system of seawater. Field studies and experimentation simulating the natural settings show minimum values of CO₂ (1.9 mmoles 1^?1) and HCO₃^? (2.4 meq 1^?1) in association with maximum values of pH (9.8) and O₂ (> 100% oxygen saturation) in waters covering corals and algae prior to sunset. The converse is true for these variables prior to sunrise, when minimum values of pH (7.6) and O₂ (<66% oxygen saturation) occur with maximum values of CO₂ (2.7 mmoles 1^?1) and HCO₃^? (2.7 meq 1^?1). Experimental tanks containing plain seawater showed almost no diurnal variability in pH (a constant 7.5–7.6) or O₂ (80–90% oxygen saturation) measurements. Seawater adjacent to reef biomass, with elevated pH and supersaturated with calcium carbonate, is periodically pumped into the underlying reef and beach sediments due to pressure-buildups between the reef framework or algal zones and open waters. Carbonate precipitation and quartz dissolution follow. As the system equilibrates, pH values progressively decrease.     

模拟酸雨条件下石灰土—碳酸盐岩体系的碳汇效应

为揭示不同酸度降雨对石灰土—碳酸盐岩体系内岩溶碳汇效应的影响,以贵阳市花溪区历史降水量为参照,选取贵阳市青岩镇纯灰岩发育土壤与贵阳市花溪水库三叠纪大冶组纯灰岩为样本进行淋溶试验。测定了6个月时长内不同酸度降水﹑不同土壤深度下模拟石灰土—碳酸盐岩体系降水淋出液的HCO3-﹑DOC含量和土壤呼吸速率,研究了模拟酸雨对石灰土-石灰岩体系碳汇的影响。结果表明:（1）在土壤深度10～50 cm匀质状态的样本中,随着土壤厚度的增加,淋出液中HCO3-含量逐渐增大﹑DOC含量逐渐减小﹑土壤呼吸速率逐渐增大,显示出土壤厚度对石灰土—碳酸盐岩体系的碳汇效应有着明显的影响;(2)在pH=3.5~6.8的范围内,降水酸度的增强可以抑制岩溶作用与有机碳的溶解,并降低岩溶碳汇效应;（3）在日降水量90～230 mm范围内,随着降水量的增大碳汇效应也会随之增强。      

Carbonate equilibrium in the water of the Razdol’naya River

The paper suggests an accurate approach to studying carbonate equilibrium in the water of the Razdol’naya River. The approach involves measuring pH by Pitzer’s scale, using a cell without liquid junction; measuring the total alkalinity by Bruevich’s technique; and using apparent constants of carbonate equilibrium with regard for the organic alkalinity. The Pitzer technique was employed to calculate the apparent constants of carbonate equilibrium in solution that models the riverine water: Ca(HCO₃)₂–NaCl–H₂O within the range of alkalinity of 0–0.005 mol/kg and temperatures of 0–25°C. Carbonate equilibrium in the water of the Razdol’naya River was sampled for studying at eight sites during all four seasons. Although the contents of biogenic compounds in the water are high, they can merely insignificantly affect the acid–base equilibrium, which is controlled in the riverine water by carbonate equilibrium and the concentrations of humic substances, which play the greater role, the greater the discharge of the river. In addition to the production and destruction of organic matter, carbonate equilibrium in the river is also affected by the supply of humic substances with soil waters and total alkalinity with groundwaters. The fluxes of alkalinity and humic substances annually brought by the Razdol’naya River to Amur Bay are evaluated at 1.33 × 10⁹ mol and 9.9 × 10⁶ kgC, respectively. The carbon dioxide export with the Razdol’naya River is equal to the alkalinity flux and does not depend on the weathering mechanisms.     

Carbonate and carbon isotopic evolution of groundwater contaminated by produced water brine with hydrocarbons

The major ionic and dissolved inorganic carbon (DIC) concentrations and the stable carbon isotope composition of DIC (δ¹³C_DIC) were measured in a freshwater aquifer contaminated by produced water brine with petroleum hydrocarbons. Our aim was to determine the effects of produced water brine contamination on the carbonate evolution of groundwater. The groundwater was characterized by three distinct anion facies: HCO₃⁻-rich, SO₄²⁻-rich and Cl⁻-rich. The HCO₃⁻-rich groundwater is undergoing closed system carbonate evolution from soil CO_2(g) and weathering of aquifer carbonates. The SO₄²⁻-rich groundwater evolves from gypsum induced dedolomitization and pyrite oxidation. The Cl⁻-rich groundwater is contaminated by produced water brine and undergoes common ion induced carbonate precipitation. The δ¹³C_DIC of the HCO₃⁻-rich groundwater was controlled by nearly equal contribution of carbon from soil CO_2(g) and the aquifer carbonates, such that the δ¹³C of carbon added to the groundwater was −11.6‰. In the SO₄²⁻-rich groundwater, gypsum induced dedolomitization increased the ¹³C such that the δ¹³C of carbon added to the groundwater was −9.4‰. In the produced water brine contaminated Cl⁻-rich groundwater, common ion induced precipitation of calcite depleted the ¹³C such that the δ¹³C of carbon added to the groundwater was −12.7‰. The results of this study demonstrate that produced water brine contamination of fresh groundwater in carbonate aquifers alters the carbonate and carbon isotopic evolution.     

珠江流域岩石风化作用消耗大气/土壤CO2量的估算

以流域的岩性、径流量和水化学分析数据为主要资料,利用基于GIS空间分析的GEM-CO2模型,估算珠江流域陆地岩石风化作用消耗大气/土壤空气中的CO2,评价河流流域的碳汇能力。结果表明,珠江流域因岩石溶蚀和风化作用消耗大气/土壤中的CO2量为252×109 mol·a-1(571×103 mol·km-2·a-1),从岩性分析,碳酸盐岩区大气/土壤CO2消耗量为180×109 mol·a-1(1030×103 mol·km-2·a-1),占总量的71.4%。二级流域以西江流域CO2消耗量最大,占珠江流域总CO2消耗量79.4%,北江、东江分别占总量的13.0%、4.9%。珠江流域大气/土壤CO2消耗量大约为世界大河流域平均值的2.3倍。     

Spatial analysis of land use change effect on soil organic carbon stocks in the eastern regions of China between 1980 and 2000

Spatial distributions of 0-20 cm soil carbon sources/sinks caused by land use changes from the year 1980 to 2000 in an area of 2.97 × 10~6 km~2 in eastern China were investigated using a land use dataset from a recent soil geochemical survey.A map of soil carbon sources/sinks has been prepared based on a spatial analysis scheme with GIS.Spatial statistics showed that land use changes had caused 30.7 ± 13.64 Tg of surface soil organic carbon loss,which accounts for 0.33%of the total carbon storage of 9.22 Pg.The net effect of the carbon source was estimated to be ~ 71.49 Tg soil carbon decrease and ~40.80 Tg increase.Land use changes in Northeast China(NE) have the largest impact on soil organic carbon storage compared with other regions.Paddy fields,which were mainly transformed into dry farmland in NE,and constructed land in other regions,were the largest carbon sources among the land use types.Swamp land in NE was also another large soil carbon source when it was transformed into dry farmland or paddy fields.Dry farmland in the NE region formed the largest soil organic carbon sink,as some were transformed into paddy fields,forested land,and other land use types with high SOCD.