放牧对新疆草地生态系统碳源/汇的影响模拟研究

<正>确评估新疆草地生态系统碳源/汇效应,对区域尺度碳循环研究具有重要意义。放牧是新疆草地生态系统中主要的人类活动,但放牧对草地碳平衡与碳动态的影响还具有很大的不确定性。利用生态系统放牧模型Biome-BGC grazing,通过情景模拟综合评价新疆草地生态系统碳源/汇的动态。结果表明:1)1979—2007年新疆草地生态系统的碳源总量为0.38Pg C,其中由放牧导致的碳释放为0.37Pg C;2)当平均放牧率小于0.24头标准羊/hm~2时,放牧能够促进草地碳固定。研究实现了BiomeBGC grazing模型在区域尺度的应用,研究结果将有助于理解气候变化及放牧对干旱区草地生态系统碳动态变化的驱动机理,对探明干旱区草原生态系统的源/汇特征具有重要意义。     

青藏高原高寒草甸生态系统碳增汇潜力

为了揭示青藏高原高寒草甸生态系统植被变化对碳储量的影响,以原生矮嵩草草甸、退化草甸、人工草地以及农田为研究对象,对比分析了该4种不同土地格局下生态系统的有机碳现状.以原生矮嵩草草甸土壤碳储量为基准对不同类型高寒生态系统的碳增汇潜力进行了估算.结果表明:不同类型生态系统的碳储量和碳增汇潜力有很大差异,在0-40cm土层中,(1)原生草甸碳储量最高,达到17098 g C/m2,退化草甸、人工草地和农田的有机碳汇增加潜力分别为:5637、3823、1567 g C/m2.(2)对于退化草甸和人工草地,土壤有机碳含量和密度明显低于原生草甸和农田.(3)地下生物量碳储量主要集中在0-20cm,且原生草甸地下生物量的碳储量比其他3个植被类型高3.6-5倍.总体上,青藏高原草地生态系统存在巨大的碳增汇潜力.     

滨海盐沼净碳汇能力研究方法综述

滨海盐沼较高的初级生产力以及缓慢的有机质降解速率使其具有显著的碳汇功能，对其净固碳能力开展定量化研究对于制定我国增汇减排政策、实现碳中和目标具有重要的技术支撑作用。目前，国内外有关湿地净碳汇量的观测手段、计算思路及预测方法众多，导致各种研究成果的可比性不高。从碳通量原位观测、碳收支过程测定、土壤碳储量变化、碳循环过程模拟等角度出发，对滨海盐沼净碳汇能力研究方法进行系统梳理，在总结各类方法优缺点、剖析已有研究存在问题的基础上，提出未来滨海盐沼净固碳能力确定的主要研究方向：基于湿地生态系统碳的生物地球化学循环机理，结合卫星遥感技术，构建碳源/汇过程模型，将碳通量观测及碳收支测量数据用于模型参数率定和校验，实现不同时空尺度盐沼生态系统碳收支模拟，是定量评估滨海盐沼净碳汇能力的发展趋势和有效途径。     

中国陆地生态系统碳源/汇整合分析

国家尺度陆地生态系统碳收支及其循环过程的研究对于提升地球系统科学与全球变化科学的科技创新能力、提高我国参与应对全球气候变化国际行动和维护国家利益的话语权、保障国家生态安全和改进生态系统管理都具有重要意义。近年来,我国已经在气候变化与陆地生态系统碳循环领域开展了大量的研究工作,主要包括国家清查、生态系统模型模拟、大气反演等手段。然而,由于大尺度陆地生态系统碳源/汇的估算存在很大的不确定性,目前尚未形成国家尺度的陆地生态系统碳源/汇的整合分析。通过搜集已发表的关于中国陆地生态系统及其组分碳源/汇的59篇文献,整合国家清查、生态系统模型模拟、大气反演3种研究手段,分析中国陆地生态系统碳源/汇大小以及时间尺度上的动态变化。结果表明,在1960s-2010s期间中国陆地生态系统碳汇整体呈上升趋势,平均为（0.213±0.030）Pg C/a,其中森林、草地、农田和灌木生态系统碳汇分别为（0.101±0.023）Pg C/a、（0.032±0.007）Pg C/a、（0.043±0.010）Pg C/a和（0.028±0.010）Pg C/a。森林生态系统中的植被碳汇远大于土壤碳汇,然而这种格局在草地和农田生态系统却相反,而且1960s-2010s期间中国主要植被类型的生态系统碳汇总体上随时间呈增加趋势。融合多源数据（地面观测、激光雷达、卫星遥感等）、多尺度数据（样地尺度、站点尺度、区域尺度）以及多手段数据（联网观测、森林清查、模型模拟）,有助于全面准确地评估中国陆地生态系统碳源/汇及其对气候变化的响应。     

服务双碳目标的中国人工林生态系统碳增汇途径

中国在相对较低的经济发展水平条件下提出了"碳达峰、碳中和"目标，在全球气候治理中起着关键作用。中国是全球人工林面积最多的国家，中国森林生态系统碳储量增加的主要贡献者是人工林，是中国陆地碳汇的主要来源，具有较高的碳汇增长潜力，加强人工林碳增汇方案研究对中国实现"碳达峰、碳中和"目标具有非常重要的作用。研究梳理了中国人工林生态系统碳汇能力提升的主要因子和环节，分别从增加碳汇强度型增汇、保护修复型增汇、减少碳排放型增汇、技术提高型增汇和市场引领型增汇5个方面提出了12条人工林碳增汇途径，以期为中国实现"碳达峰、碳中和"目标作出更大贡献。     

陆地生态系统碳源与碳汇及其影响机制研究进展

全球碳循环研究中发现,目前已知碳源与碳汇不能达到平衡。存在一个很大的碳失汇。大气、海洋和陆地生态系统是人工源CO2的3个可能的容纳汇,其中陆地生态系统最复杂、最具不确定性,因此陆地生态系统碳源与碳汇研究是全球碳循环研究的核心问题之一。大气成分监测、CO2通量测定、森林资源清查以及模型模拟等方面的研究都表明,CO2施肥效应、氮沉降增加、污染、全球气候变化以及土地利用变化,是影响陆地生态系统碳储量的主要生态机制,但不确定在过去的10～100年以及未来哪一种机制起最主要的作用。     

低碳视角下太原市旅游业碳均衡分析

低碳旅游是解决旅游发展与环境保护矛盾的重要途径。碳均衡分析作为新时期旅游业低碳转型的探索性工作,突破了以往研究中无法明确判断碳减排压力的困境,逐步成为新的学术热点。本文基于旅游业碳排放与生态系统碳吸收模型的构建,结合区域旅游经济贡献,对太原市旅游碳赤字进行初步估算。结果表明:2010—2016年,太原市旅游业碳排放从48.85×10~7kg增长到66.84×10~7kg,其中,旅游交通、住宿、购物及餐饮部门是主要碳源;旅游交通和住宿部门在选定时段内的比重不断减小,其余要素部门占比逐年增加,这在一定程度上反映了旅游消费结构的变化;太原市陆地生态系统年均碳吸收总量为122.98×10~7kg,其中,森林和农田系统是主要碳汇,草地和水域系统的增汇能力相对较弱;旅游业年均碳赤字的估算值为34.30×10~7kg,呈现严重的碳失衡状态;太原市旅游业成为一个显著的碳源,节能减排和生态增汇将是未来发展的有效途径。     

葡萄园生态系统碳源/汇及碳减排策略研究进展

葡萄园生态系统是农业生态系统的重要组成部分, 集中连片栽培的葡萄园具有重要的生态价值。开展葡萄园生态系统碳源/汇的研究, 是完整探讨葡萄园生态系统碳循环必不可少的内容。随着葡萄生态学研究的进一步深入, 如何直观地揭示葡萄园生态系统碳循环规律和碳汇功能已经成为葡萄生态学领域关注的热点问题。研究发现, 葡萄园生态系统固定大量碳, 将碳封存在葡萄果实等一年生器官、主干等多年生器官以及土壤碳库中。葡萄园生态系统碳输入量大于碳输出量, 是碳汇; 土壤是葡萄园生态系统最大的碳库, 占总碳储量的70%, 尤其是土藤界面; 覆盖和免耕作为葡萄园的碳减排策略, 可以减少碳排放, 提高葡萄园土壤肥力。基于此, 为了阐明葡萄园生态系统的碳汇价值, 该文围绕葡萄生态学最新研究进展, 系统回顾了葡萄园生态系统中碳循环规律、碳汇研究进展及碳减排策略, 为葡萄生态学的研究提供理论基础, 并对本领域未来的研究方向和应用前景进行展望。     

草地农业生态学研究进展与趋势

阐述了草地农业生态学的内涵,发展过程,前沿领域和发展趋势,草地农业生态系统的界面理论,结构与功能,系统耦合与系统相悖以及系统健康评价是该学科的4个前沿领域,草地农业生态系统存在3个基本界面,即草丛-地境,草地-动物以及草畜生产-经营管理界面,研究发生于各个界面的一系列生态学过程,是揭示系统行为特征的关键,草地农业生态系统有4个生产层,即前植物,植物,动物和外生物生产层,系统耦合与系统相悖是生产层之间互作的2个重要概念,不同生产层之间的系统耦合,可产生系统进化,多方面释放系统潜势,生态系统健康水平是其结构和功能的反映,健康评价是对其有序度和服务价值的度量。     

葡萄园生态系统碳源/汇及碳减排策略研究进展

葡萄园生态系统是农业生态系统的重要组成部分,集中连片栽培的葡萄园具有重要的生态价值。开展葡萄园生态系统碳源/汇的研究,是完整探讨葡萄园生态系统碳循环必不可少的内容。随着葡萄生态学研究的进一步深入,如何直观地揭示葡萄园生态系统碳循环规律和碳汇功能已经成为葡萄生态学领域关注的热点问题。研究发现,葡萄园生态系统固定大量碳,将碳封存在葡萄果实等一年生器官、主干等多年生器官以及土壤碳库中。葡萄园生态系统碳输入量大于碳输出量,是碳汇;土壤是葡萄园生态系统最大的碳库,占总碳储量的70%,尤其是土藤界面;覆盖和免耕作为葡萄园的碳减排策略,可以减少碳排放,提高葡萄园土壤肥力。基于此,为了阐明葡萄园生态系统的碳汇价值,该文围绕葡萄生态学最新研究进展,系统回顾了葡萄园生态系统中碳循环规律、碳汇研究进展及碳减排策略,为葡萄生态学的研究提供理论基础,并对本领域未来的研究方向和应用前景进行展望。     

Gross Direct and Embodied Carbon Sinks for Urban Inventories

Cities and urban regions are undertaking efforts to quantify greenhouse (GHG) emissions from their jurisdictional boundaries. Although inventorying methodologies are beginning to standardize for GHG sources, carbon sequestration is generally not quantified. This article describes the methodology and quantification of gross urban carbon sinks. Sinks are categorized into direct and embodied sinks. Direct sinks generally incorporate natural process, such as humification in soils and photosynthetic biomass growth (in urban trees, perennial crops, and regional forests). Embodied sinks include activities associated with consumptive behavior that result in the import and/or storage of carbon, such as landfilling of waste, concrete construction, and utilization of durable wood products. Using methodologies based on the Intergovernmental Panel on Climate Change 2006 guidelines (for direct sinks) and peer‐reviewed literature (for embodied sinks), carbon sequestration for 2005 is calculated for the Greater Toronto Area. Direct sinks are found to be 317 kilotons of carbon (kt C), and are dominated by regional forest biomass. Embodied sinks are calculated to be 234 kt C based on one year's consumption, though a complete life cycle accounting of emissions would likely transform this sum from a carbon sink to a source. There is considerable uncertainty associated with the methodologies used, which could be addressed with city‐specific stock‐change measurements. Further options for enhancing carbon sink capacity within urban environments are explored, such as urban biomass growth and carbon capture and storage.     

Mitigating the greenhouse gas balance of ruminant production systems through carbon sequestration in grasslands

Soil carbon sequestration (enhanced sinks) is the mechanism responsible for most of the greenhouse gas (GHG) mitigation potential in the agriculture sector. Carbon sequestration in grasslands can be determined directly by measuring changes in soil organic carbon (SOC) stocks and indirectly by measuring the net balance of C fluxes. A literature search shows that grassland C sequestration reaches on average 5 ± 30 g C/m2 per year according to inventories of SOC stocks and -231 and 77 g C/m2 per year for drained organic and mineral soils, respectively, according to C flux balance. Off-site C sequestration occurs whenever more manure C is produced by than returned to a grassland plot. The sum of on- and off-site C sequestration reaches 129, 98 and 71 g C/m2 per year for grazed, cut and mixed European grasslands on mineral soils, respectively, however with high uncertainty. A range of management practices reduce C losses and increase C sequestration: (i) avoiding soil tillage and the conversion of grasslands to arable use, (ii) moderately intensifying nutrient-poor permanent grasslands, (iii) using light grazing instead of heavy grazing, (iv) increasing the duration of grass leys; (v) converting grass leys to grass-legume mixtures or to permanent grasslands. With nine European sites, direct emissions of N2O from soil and of CH4 from enteric fermentation at grazing, expressed in CO2 equivalents, compensated 10% and 34% of the on-site grassland C sequestration, respectively. Digestion inside the barn of the harvested herbage leads to further emissions of CH4 and N2O by the production systems, which were estimated at 130 g CO2 equivalents/m2 per year. The net balance of on- and off-site C sequestration, CH4 and N2O emissions reached 38 g CO2 equivalents/m2 per year, indicating a non-significant net sink activity. This net balance was, however, negative for intensively managed cut sites indicating a source to the atmosphere. In conclusion, this review confirms that grassland C sequestration has a strong potential to partly mitigate the GHG balance of ruminant production systems. However, as soil C sequestration is both reversible and vulnerable to disturbance, biodiversity loss and climate change, CH4 and N2O emissions from the livestock sector need to be reduced and current SOC stocks preserved.     

Do grasslands act as a perpetual sink for carbon?

It is increasingly commonly suggested that grasslands are a perpetual sink for carbon, and that just maintaining grasslands will yield a net carbon sink. I examine the evidence for this from repeated soil surveys, long term grassland experiments and simple mass balance calculations. I conclude that it is untenable that grasslands act as a perpetual carbon sink, and the most likely explanation for observed grassland carbon sinks over short periods is legacy effects of land use and land management prior to the beginning of flux measurement periods. Simply having grassland does not result is a carbon sink, but judicious management or previously poorly managed grasslands can increase the sink capacity. Given that grasslands are a large store of carbon, and that it is easier and faster for soils to lose carbon that it is for them to gain carbon, it is an important management target to maintain these stocks.     

Terrestrial carbon sinks in China and around the world and their contribution to carbon neutrality

Enhancing the terrestrial ecosystem carbon sink (referred to as terrestrial C sink) is an important way to slow down the continuous increase in atmospheric carbon dioxide (CO₂) concentration and to achieve carbon neutrality target. To better understand the characteristics of terrestrial C sinks and their contribution to carbon neutrality, this review summarizes major progress in terrestrial C budget researches during the past decades, clarifies spatial patterns and drivers of terrestrial C sources and sinks in China and around the world, and examines the role of terrestrial C sinks in achieving carbon neutrality target. According to recent studies, the global terrestrial C sink has been increasing from a source of (?0.2±0.9) Pg C yr^?1 (1 Pg=10¹⁵ g) in the 1960s to a sink of (1.9±1.1) Pg C yr^?1 in the 2010s. By synthesizing the published data, we estimate terrestrial C sink of 0.20–0.25 Pg C yr^?1 in China during the past decades, and predict it to be 0.15–0.52 Pg C yr^?1 by 2060. The terrestrial C sinks are mainly located in the mid- and high latitudes of the Northern Hemisphere, while tropical regions act as a weak C sink or source. The C balance differs much among ecosystem types: forest is the major C sink; shrubland, wetland and farmland soil act as C sinks; and whether the grassland functions as C sink or source remains unclear. Desert might be a C sink, but the magnitude and the associated mechanisms are still controversial. Elevated atmospheric CO₂ concentration, nitrogen deposition, climate change, and land cover change are the main drivers of terrestrial C sinks, while other factors such as fires and aerosols would also affect ecosystem C balance. The driving factors of terrestrial C sink differ among regions. Elevated CO₂ concentration and climate change are major drivers of the C sinks in North America and Europe, while afforestation and ecological restoration are additionally important forcing factors of terrestrial C sinks in China. For future studies, we recommend the necessity for intensive and long term ecosystem C monitoring over broad geographic scale to improve terrestrial biosphere models for accurately evaluating terrestrial C budget and its dynamics under various climate change and policy scenarios.

     

国内农业碳源/汇效应研究:视角、进展与改进

结构复杂、自然生态与社会经济过程交织、地域特征显著的农业碳源/汇效应存在基于内部结构、双重属性、区域尺度等多种研究视角。国内基于内部结构视角的农业碳源/汇效应研究多局限于种植业,且多以单一的碳源或碳汇研究为主;农业组分细化与组分关联的系统综合研究明显不足。在基于双重属性视角的研究中,自然生态视角侧重于碳吸收和直接碳排放,社会经济视角则聚焦于间接碳排放;由自然与经济相互割裂到相互融合的二元农业碳源/汇综合方法尚需深化。基于区域尺度视角的研究多集中在大中尺度和样地尺度,小尺度县域研究比较薄弱;农产品供应链关联的区域间碳流及减排值得关注。针对中国小规模家庭农场和种养关联的多种经营特征,农业碳源/汇效应研究应将农业产业组分、农产品碳源/汇构成解析与种养关联碳源/汇综合并重,发展和完善评估模型、提高参数本土化程度,追踪农产品生产、流通与消费等区域间碳流全过程。研究综述国内基于内部结构、双重属性和区域尺度等视角的农业碳源/汇效应研究进展,并提出基于我国农业经营特征的碳源/汇效应改进研究。     

The greenhouse gas balance of European grasslands

The greenhouse gas (GHG) balance of European grasslands (EU‐28 plus Norway and Switzerland), including CO₂, CH₄ and N₂O, is estimated using the new process‐based biogeochemical model ORCHIDEE‐GM over the period 1961–2010. The model includes the following: (1) a mechanistic representation of the spatial distribution of management practice; (2) management intensity, going from intensively to extensively managed; (3) gridded simulation of the carbon balance at ecosystem and farm scale; and (4) gridded simulation of N₂O and CH₄ emissions by fertilized grassland soils and livestock. The external drivers of the model are changing animal numbers, nitrogen fertilization and deposition, land‐use change, and variable CO₂ and climate. The carbon balance of European grassland (NBP) is estimated to be a net sink of 15 ± 7 g C m^?2 year^?1 during 1961–2010, equivalent to a 50‐year continental cumulative soil carbon sequestration of 1.0 ± 0.4 Pg C. At the farm scale, which includes both ecosystem CO₂ fluxes and CO₂ emissions from the digestion of harvested forage, the net C balance is roughly halved, down to a small sink, or nearly neutral flux of 8 g C m^?2 year^?1. Adding CH₄ and N₂O emissions to net ecosystem exchange to define the ecosystem‐scale GHG balance, we found that grasslands remain a net GHG sink of 19 ± 10 g C‐CO₂ equiv. m^?2 year^?1, because the CO₂ sink offsets N₂O and grazing animal CH₄ emissions. However, when considering the farm scale, the GHG balance (NGB) becomes a net GHG source of ?50 g C‐CO₂ equiv. m^?2 year^?1. ORCHIDEE‐GM simulated an increase in European grassland NBP during the last five decades. This enhanced NBP reflects the combination of a positive trend of net primary production due to CO₂, climate and nitrogen fertilization and the diminishing requirement for grass forage due to the Europe‐wide reduction in livestock numbers.     

Changes in European ecosystem productivity and carbon balance driven by regional climate model output

Climate change resulting from the enhanced greenhouse effect together with the direct effect of increased atmospheric CO₂ concentrations on vegetation growth are expected to produce changes in the cycling of carbon in terrestrial ecosystems. Impacts will vary across Europe, and regional-scale studies are needed to resolve this variability. In this study, we used the LPJ-GUESS ecosystem model driven by a suite of regional climate model (RCM) scenarios from the European Union (EU) project PRUDENCE to estimate climate impacts on carbon cycling across Europe. We identified similarities and discrepancies in simulated climate impacts across scenarios, particularly analyzing the uncertainties arising from the range of climate models and emissions scenarios considered. Our results suggest that net primary production (NPP) and heterotrophic respiration (Rh) will generally increase throughout Europe, but with considerable variation between European subregions. The smallest NPP increases, and in some cases decreases, occurred in the Mediterranean, where many ecosystems switched from sinks to sources of carbon by 2100, mainly as a result of deteriorating water balance. Over the period 1991–2100, modeled climate change impacts on the European carbon balance ranged from a sink of 11.6 Gt C to a source of 3.3 Gt C, the average annual sink corresponding with 1.85% of the current EU anthropogenic emissions. Projected changes in carbon balance were more dependent on the choice of the general circulation model (GCM) providing boundary conditions to the RCM than the choice of RCM or the level of anthropogenic greenhouse gases emissions.     

Effect of climate change,CO2 trends,nitrogen addition,and land‐cover and management intensity changes on the carbon balance of European grasslands

Several lines of evidence point to European managed grassland ecosystems being a sink of carbon. In this study, we apply ORCHIDEE‐GM a process‐based carbon cycle model that describes specific management practices of pastures and the dynamics of carbon cycling in response to changes in climatic and biogeochemical drivers. The model is used to simulate changes in the carbon balance [i.e., net biome production (NBP)] of European grasslands over 1991–2010 on a 25 km × 25 km grid. The modeled average trend in NBP is 1.8–2.0 g C m^?2 yr^?2 during the past two decades. Attribution of this trend suggests management intensity as the dominant driver explaining NBP trends in the model (36–43% of the trend due to all drivers). A major change in grassland management intensity has occurred across Europe resulting from reduced livestock numbers. This change has ‘inadvertently’ enhanced soil C sequestration and reduced N₂O and CH₄ emissions by 1.2–1.5 Gt CO₂‐equivalent, offsetting more than 7% of greenhouse gas emissions in the whole European agricultural sector during the period 1991–2010. Land‐cover change, climate change and rising CO₂ also make positive and moderate contributions to the NBP trend (between 24% and 31% of the trend due to all drivers). Changes in nitrogen addition (including fertilization and atmospheric deposition) are found to have only marginal net effect on NBP trends. However, this may not reflect reality because our model has only a very simple parameterization of nitrogen effects on photosynthesis. The sum of NBP trends from each driver is larger than the trend obtained when all drivers are varied together, leaving a residual – nonattributed – term (22–26% of the trend due to all drivers) indicating negative interactions between drivers.     

低碳导向下土地覆被演变模拟——以深圳市为例

全球碳排放水平的不断增加引起的全球变暖越发严重,导致了严重的自然灾害和经济损失,这种失衡发展的态势促使着各个国家开始探索低碳环保的发展模式。为了探究何种土地利用组成可以更好的为低碳城市服务,以深圳市为研究区,结合2020年土地利用现状结构和2020年土地利用规划结构分别估算出碳汇最大化情景和碳排放量最小化情景下2020年各土地利用类型的数量结构,并运用FLUS模型模拟出深圳市土地利用类型在这两种情景下的空间分布特征。最后,从碳密度和碳排放视角对比这两种情景的低碳效益。研究结果如下：①碳汇最大化和碳排放最小化情景下土地利用总碳盈余均比2020年少,且碳汇最大化情景下土地利用总碳盈余最小。碳汇最大化情景下耕地、园地和林地面积增加而水域和建设用地减少,碳排放最小化情景下园地和林地面积增加来源于草地、水域和建设用地的减少,这两种低碳情景的碳汇能力增强而碳排放量减少;②碳汇最大化和碳排放最小化情景下林地明显增加故而土地利用总碳盈余均比实际情景小,而园地和草地的缩减和扩张是引起两种低碳情景碳密度和碳排放量有差异的主要原因。碳汇最大化和碳排放最小化情景下,西部和东南部主要是碳密度增加和碳排放减少的区域,而中部是碳密度减少和碳排放增加的区域。因此对中部区域进行重点调控,有利于深圳市碳中和和碳达峰的实现。研究可以为深圳的低碳发展提供规划建议,同时给其他区域的低碳规划提供参考意见。     

Land use effects on terrestrial carbon sources and sinks

Current and past land use practices are critical in determining the distribution and sizeof global terrestrial carbon (C) sources and sinks. Although fossil fuel emissions dominate the an-thropogenic perturbation of the global C cycle, land use still drives the largest portion of anthropo-genic emissions in a number of tropical regions of Asia. The size of the emission flux owing to landuse change is still the biggest uncertainty in the global C budget. The Intergovernmental Panel on Climate Change (IPCC) reported a flux term of 1.7 PgC·a~(-1) for 1990-1995 but more recent es-timates suggest the magnitude of this source may be only of 0.96 PgC·a~(-1) for the 1990s. In add-ition, current and past land use practices are now thought to contribute to a large degree to the northern hemisphere terrestrial sink, and are the dominant driver for some regional sinks. However,mechanisms other than land use change need to be invoked in order to explain the inferred C sink in the tropics. Potential candidates are the carbon dioxide (CO_2) fertilization and climate change;fertilization due to nitrogen (N) deposition is believed to be small or nil. Although the potential formanaging C sinks is limited, improved land use management and new land uses such as refores-tation and biomass fuel cropping, can further enhance current terrestrial C sinks. Best manage-ment practices in agriculture alone could sequester 0.4-0.8 PgC per year in soils if implemented globally. New methodologies to ensure verification and permanency of C sequestration need to be developed.