Carbon emissions and sinks in agro-ecosystems of China

Besides ruminant animals and their wastes, soil is an important regulating medium incarbon cycling. The soil can be both a contributor to climate change and a recipient of impacts. Inthe past, land cultivation has generally resulted in considerable depletion of soil organic matter andthe release of greenhouse gases (GHGs) into the atmosphere. The observation in the North-South Transect of Eastern China showed that climate change and land use strongly impact all soil proc-esses and GHG exchanges between the soil and the atmosphere. Soil management can restoreorganic carbon by enhancing soil structure and fertility and by doing so mitigating the negative im-pacts of atmospheric greenhouses on climate. A wide estimation carried out in China shows that carbon sequestration potential is about 77.2 MMt C/a (ranging from 26.1-128.3 MMt C/a) using proposed IPCC activities during the next fifty years.     

Long‐term changes in forest carbon under temperature and nitrogen amendments in a temperate northern hardwood forest

Currently, forests in the northeastern United States are net sinks of atmospheric carbon. Under future climate change scenarios, the combined effects of climate change and nitrogen deposition on soil decomposition, aboveground processes, and the forest carbon balance remain unclear. We applied carbon stock, flux, and isotope data from field studies at the Harvard forest, Massachusetts, to the ForCent model, which integrates above‐ and belowground processes. The model was able to represent decadal‐scale measurements in soil C stocks, mean residence times, fluxes, and responses to a warming and N addition experiment. The calibrated model then simulated the longer term impacts of warming and N deposition on the distribution of forest carbon stocks. For simulation to 2030, soil warming resulted in a loss of soil organic matter (SOM), decreased allocation to belowground biomass, and gain of aboveground carbon, primarily in large wood, with an overall small gain in total system carbon. Simulated nitrogen addition resulted in a small increase in belowground carbon pools, but a large increase in aboveground large wood pools, resulting in a substantial increase in total system carbon. Combined warming and nitrogen addition simulations showed a net gain in total system carbon, predominately in the aboveground carbon pools, but offset somewhat by losses in SOM. Hence, the impact of continuation of anthropogenic N deposition on the hardwood forests of the northeastern United States may exceed the impact of warming in terms of total ecosystem carbon stocks. However, it should be cautioned that these simulations do not include some climate‐related processes, different responses from changing tree species composition. Despite uncertainties, this effort is among the first to use decadal‐scale observations of soil carbon dynamics and results of multifactor manipulations to calibrate a model that can project integrated aboveground and belowground responses to nitrogen and climate changes for subsequent decades.     

Temperature and substrate controls on intra-annual variation in ecosystem respiration in two subarctic vegetation types

Arctic ecosystems are important in the context of climate change because they are expected to undergo the most rapid temperature increases, and could provide a globally significant release of CO₂ to the atmosphere from their extensive bulk soil organic carbon reserves. Understanding the relative contributions of bulk soil organic matter and plant‐associated carbon pools to ecosystem respiration is critical to predicting the response of arctic ecosystem net carbon balance to climate change. In this study, we determined the variation in ecosystem respiration rates from birch forest understory and heath tundra vegetation types in northern Sweden through a full annual cycle. We used a plant biomass removal treatment to differentiate bulk soil organic matter respiration from total ecosystem respiration in each vegetation type. Plant‐associated and bulk soil organic matter carbon pools each contributed significantly to ecosystem respiration during most phases of winter and summer in the two vegetation types. Ecosystem respiration rates through the year did not differ significantly between vegetation types despite substantial differences in biomass pools, soil depth and temperature regime. Most (76–92%) of the intra‐annual variation in ecosystem respiration rates from these two common mesic subarctic ecosystems was explained using a first‐order exponential equation relating respiration to substrate chemical quality and soil temperature. Removal of plants and their current year's litter significantly reduced the sensitivity of ecosystem respiration to intra‐annual variations in soil temperature for both vegetation types, indicating that respiration derived from recent plant carbon fixation was more temperature sensitive than respiration from bulk soil organic matter carbon stores. Accurate assessment of the potential for positive feedbacks from high‐latitude ecosystems to CO₂‐induced climate change will require the development of ecosystem‐level physiological models of net carbon exchange that differentiate the responses of major C pools, that account for effects of vegetation type, and that integrate over summer and winter seasons.     

Conventional intensive logging promotes loss of organic carbon from the mineral soil

There are few data, but diametrically opposed opinions, about the impacts of forest logging on soil organic carbon (SOC). Reviews and research articles conclude either that there is no effect, or show contradictory effects. Given that SOC is a substantial store of potential greenhouse gasses and forest logging and harvesting is routine, resolution is important. We review forest logging SOC studies and provide an overarching conceptual explanation for their findings. The literature can be separated into short‐term empirical studies, longer‐term empirical studies and long‐term modelling. All modelling that includes major aboveground and belowground biomass pools shows a long‐term (i.e. ≥300 years) decrease in SOC when a primary forest is logged and then subjected to harvesting cycles. The empirical longer‐term studies indicate likewise. With successive harvests the net emission accumulates but is only statistically perceptible after centuries. Short‐term SOC flux varies around zero. The long‐term drop in SOC in the mineral soil is driven by the biomass drop from the primary forest level but takes time to adjust to the new temporal average biomass. We show agreement between secondary forest SOC stocks derived purely from biomass information and stocks derived from complex forest harvest modelling. Thus, conclusions that conventional harvests do not deplete SOC in the mineral soil have been a function of their short time frames. Forest managers, climate change modellers and environmental policymakers need to assume a long‐term net transfer of SOC from the mineral soil to the atmosphere when primary forests are logged and then undergo harvest cycles. However, from a greenhouse accounting perspective, forest SOC is not the entire story. Forest wood products that ultimately reach landfill, and some portion of which produces some soil‐like material there rather than in the forest, could possibly help attenuate the forest SOC emission by adding to a carbon pool in landfill.     

土地利用变化对陆地生态系统碳贮量的影响

陆地生态系统是重要的碳库之一,在碳素生物地球化学循环中起着重要作用．本文就森林、农田生态系统,综述了土地利用变化对陆地生态系统碳贮量的影响及其可能的作用机制．土地利用变化显著地影响陆地生态系统的结构和功能,造成系统碳贮量的变化,这很大程度取决于生态系统类型和土地利用方式的改变．森林砍伐后变为农田和草地,使生态系统中植被和土壤碳贮量大大降低．土壤碳含量的降低主要是由于凋落物输入的减少,有机质分解速度的提高,以及耕种措施对有机质物理保护的破坏造成的．土壤碳损失主要发生在森林砍伐后较短的时期内,而其降低速率取决于诸多因素以及土壤理化和生物过程．农田和草地弃耕恢复为森林,以及农田保护性管理措施的利用．能够使大气中的碳在植被和土壤中得到汇集．森林恢复过程中植被可以大量汇集大气中的碳,而由于农田耕种历史不同以及土壤空间异质性。导致土壤碳汇集速率差异极大．保护性农田管理措施(诸如免耕、合理的种植制度、化肥的施用等)可以影响土壤理化特性、作物根系生长以及残茬数量和质量、土壤微生物数量和活性,维持和提高土壤碳含量水平．     

马尾松幼苗根系和光合碳供应对土壤性质变化和温室气体排放的驱动作用

森林植被受全球气候变化、森林经营活动及病虫害等多种干扰,导致林地光合碳供应水平及根系输入量发生变化。在此背景下,土壤性质及土壤温室气体排放的响应及其机理是预测森林碳汇功能变化及森林可持续经营的重要依据。以2年生马尾松盆栽苗为对象,通过单株/盆和3株/盆栽植密度控制根系输入量、通过环割和截干控制光合碳向地下的供应能力,模拟森林植被干扰导致的根系输入量及光合碳供应变化对土壤理化性质、微生物群落结构及温室气体排放的影响。结果表明,苗木根系非结构性碳水化合物(TNC)含量和氮含量比单株/盆低;3株/盆的土壤速效氮含量比单株/盆低,土壤革兰氏阳性菌、厌氧菌、放线菌及丛枝菌根真菌丰富度均比单株/盆显著增加,3株/盆的土壤二氧化碳(CO₂)排放速率较高,但土壤氧化亚氮(N₂O)排放速率差异不显著。无论是单株/盆还是3株/盆,环割和截干处理后,根系生物量、根系长度及表面积均比对照显著下降;根系TNC含量显著下降。土壤和根系氮含量都有增加趋势;土壤微生物生物量碳(SMBC)含量降低,而土壤微生物生物量氮(SMBN)则提高。环割和截干后,土壤中各种微生物组成丰富度均有下降趋势,土壤CO₂排放速率显著下降,土壤N₂O排放速率则显著提高。根系输入量及光合碳供应对土壤细菌和真菌含量均有显著影响,土壤细菌含量与根系生物量、SMBC和SMBN显著正相关;土壤真菌含量与土壤温度显著负相关,与根系生物量、SMBC和SMBN显著正相关。相关分析表明,土壤CO₂排放通量与土壤温度、土壤湿度及根系生物量显著正相关,与土壤硝态氮显著负相关;土壤N₂O排放通量与土壤温度和土壤湿度显著正相关。以上研究表明,根系输入量与地上光合碳供应共同作用,改变土壤理化性质及微生物环境,进而影响土壤温室气体排放。     

火烧对森林土壤有机碳的影响研究进展

对国内外火烧影响森林土壤有机碳动态的研究成果进行了综合述评。较多研究表明低强度火烧不会造成土壤有机碳贮量的明显变化,但火烧非常强烈而彻底,土壤有机碳明显减少。有限研究表明火烧对森林土壤呼吸的影响结果有增加、降低或无影响,因火烧强度、火后观测时间、森林类型、火烧迹地上植被恢复进程和气候条件等而异。同时,火烧对土壤有机碳组分(活性有机碳和黑碳)也具有不同程度的影响。随着全球变化研究的深入,火烧作为森林主要管理措施对大气CO2浓度影响亦愈来愈受重视,今后应着重开展以下几方面研究:(1)扩大气候和经营管理的变化对森林土壤有机碳贮量时空动态影响研究;(2)深入探讨火烧影响土壤CO2释放的过程及机理;(3)加强火烧历史和频率对黑碳影响的研究;(4)从广度和深度上加强火烧等经营措施对亚热带森林土壤碳动态影响的研究。     

林火干扰对广东木荷林生态系统碳库的影响

森林碳库在调节CO₂浓度及减缓温室效应中发挥重要作用。选择广东木荷林为研究对象,通过相邻样地法,进行植被生物量、凋落物生物量和土壤样品的采样与分析,研究不同林火干扰强度对生态系统各碳库(植被、凋落物和土壤有机碳)及生态系统碳库产生的变化规律和空间分布格局及其影响因素。结果表明:(1)植被碳密度随着林火干扰强度增强而减少,但不同组分的植被碳密度表现不同,乔木碳密度在不同林火干扰强度下变化与植被碳密度变化一致,而草本碳密度则呈现相反的变化趋势。相同林火干扰强度下,植被各组分碳密度均以乔木层降低幅度最大。林火干扰均显著降低了凋落物碳密度(P<0.05),并随林火干扰强度的增加其降低幅度增大,但不同林火干扰强度对凋落物碳密度的影响有所差异。林火干扰降低了土壤有机碳密度,且降低幅度随土层深度增加而逐渐变小。(2)林火干扰有效改变了生态系统碳库的空间分布格局。对照样地木荷林土壤有机碳库占比为61.59%,重度林火干扰后,土壤有机碳库占比为70.96%呈上升趋势,占生态系统碳库的优势地位,而植被和凋落物碳库占比呈下降趋势,处于生态系统碳库的次要地位。(3)双因素方差分析表明,林火干扰强度和土层深度及其交互作用均对土壤有机碳密度有显著影响。林火干扰强度解释了土壤有机碳密度变异的8.78%,土层深度解释了土壤有机碳密度变异的70.29%,林火干扰强度和土层深度之间的交互作用解释了土壤有机碳密度变异的8.16%。研究发现:林火干扰降低了生态系统碳库,且随林火干扰强度增加,生态系统碳库减少幅度增大。轻度林火干扰对森林生态系统碳库的影响差异不显著,而中度和重度林火干扰对森林生态系统碳库的影响差异显著。研究结果对深化亚热带森林固碳效应的影响机制提供理论支撑。     

土地利用变化和林业活动碳贮量变化测定与监测中的方法学问题

土地利用、土地利用变化和林业 (L U L U CF)活动是引起大气温室气体浓度上升的主要因素之一 ,评价、监测 L UL UCF活动的碳源 /汇功能还存在很大的不确定性。近年来我国在该方面开展了一些研究和测定工作 ,但研究力度还远远不够 ,研究方法还存在一些问题。针对 L U L UCF活动对碳贮量影响的测定和监测中的碳库选择、监测间隔期、样地数量以及土壤容重影响和校正等有关方法学问题进行了阐述 ,以期为我国该方面的研究和监测有所裨益。     

增温对南亚热带季风常绿阔叶林土壤有机碳及其组分的影响

全球变暖是当前全球气候变化的主要现象,影响着陆地生态系统的碳循环。森林土壤是陆地生态系统中最大的碳库,森林土壤有机碳及其不同组分的积累受到气候变暖的影响,许多研究普遍发现短期增温减少土壤有机碳及其活性碳组分,但尚不清楚这种负效应在长期增温下是否仍存在和有机碳组分是否变化。以鼎湖山季风常绿阔叶林为研究对象,采用红外辐射模拟增温,探究长期增温对南亚热带森林土壤有机碳及其组分的影响。2017—2021年的连续增温观测结果表明:与对照相比,在表层土壤中,增温处理下土壤有机碳含量显著增加4.5%,其中土壤重组有机碳库显著降低9.1%,轻组有机碳库显著增加9.8%,易氧化有机碳含量显著增加5.8%,但微生物生物量碳、可溶性有机碳、惰性有机碳和络合态碳含量不变。增温持续时间显著影响土壤有机碳、微生物生物量碳、易氧化有机碳、可溶性有机碳、轻组有机碳库、重组有机碳库、惰性有机碳和络合态碳。增温处理与增温持续时间的交互作用显著影响微生物生物量碳、易氧化有机碳和重组有机碳库,但对土壤有机碳、土壤可溶性有机碳、惰性有机碳、络合态碳和轻组有机碳库无显著影响。综上所述,长期增温背景下南亚热带季风林的土壤有机碳因土壤活性有机碳组分的增加而增加,使总有机碳增加的生物调控作用可能比矿物保护作用强,但减少的惰性碳组分和增加的活性碳组分可能会使土壤有机碳稳定性下降。本研究结果探讨了南亚热带森林表层土壤有机碳及其组分对长期增温的响应,与大多数研究所发现的短期增温使表层土壤有机碳含量减少形成对比,结果可为预测未来该地区土壤碳库的变化特征提供科学依据和理论支持。     

暖温带落叶阔叶林碳循环的初步估算

 森林生态系统碳循环过程与大气中二氧化碳含量有密切的关系,直接影响着大气成分的组成,进而对全球气候变化有重要影响。以我国暖温带落叶阔叶林生态系统近10年的定位研究为基础,初步建立了该类生态系统碳循环数值模式。结果表明：暖温带落叶阔叶林典型生态系统每年从外界主要是大气中吸收的碳是10.3 t·hm-2·a-1,植物呼吸释放到大气中的碳通量为5.5 t·hm-2·a-1。森林植物干物质积存的碳量为4.8 t·hm-2·a-1,通过凋落物分解释放到大气中的碳通量为2.46 t·hm-2·a-1。森林同化的碳绝大部分以活生物呼吸和凋落物分解的形式释放到大气中去了,存留在活生物体和凋落物中的很少。通过对碳现存量的研究发现,所研究的森林生态系统碳现存量为165.05 t·hm-2,其中活生物体碳现存量为61.2 t·hm-2,死生物体碳现存量为104.05 t·hm-2 (包括土壤中碳),土壤碳现存量为96 t·hm-2。土壤碳储量占总碳储量的58%,土壤是该地区森林生态系统主要的碳库,森林生态系统土壤中碳储量的变化必然引起整个区域碳储量整体动态的变化。     

Structure and dynamics of organic matter pools in clearings in the lichen pine forests of middle taiga subzone of Yenisei Siberia

This paper studies the structure and dynamics of the major pools of organic matter in clearings in the lichen pine forests of the middle taiga subzone of Yenisei Siberia, in the range of measurements of the international observatory Zotto, a high-tech research platform for long-term (>30 years) monitoring of the atmosphere and climate changes. The contribution of phytocenoses at different stages of post-clearing succession in the process of deposition of atmospheric carbon is analyzed. In conjunction with the data on atmospheric measurements of a wide range of greenhouse gases carried out at the base of the observatory, the results of this work will be used for modeling the stocks and flows of carbon in forest ecosystems of Central Siberia.     

Global and regional importance of the tropical peatland carbon pool

Accurate inventory of tropical peatland is important in order to (a) determine the magnitude of the carbon pool; (b) estimate the scale of transfers of peat‐derived greenhouse gases to the atmosphere resulting from land use change; and (c) support carbon emissions reduction policies. We review available information on tropical peatland area and thickness and calculate peat volume and carbon content in order to determine their best estimates and ranges of variation. Our best estimate of tropical peatland area is 441 025 km² (～11% of global peatland area) of which 247 778 km² (56%) is in Southeast Asia. We estimate the volume of tropical peat to be 1758 Gm³ (～18–25% of global peat volume) with 1359 Gm³ in Southeast Asia (77% of all tropical peat). This new assessment reveals a larger tropical peatland carbon pool than previous estimates, with a best estimate of 88.6 Gt (range 81.7–91.9 Gt) equal to 15–19% of the global peat carbon pool. Of this, 68.5 Gt (77%) is in Southeast Asia, equal to 11–14% of global peat carbon. A single country, Indonesia, has the largest share of tropical peat carbon (57.4 Gt, 65%), followed by Malaysia (9.1 Gt, 10%). These data are used to provide revised estimates for Indonesian and Malaysian forest soil carbon pools of 77 and 15 Gt, respectively, and total forest carbon pools (biomass plus soil) of 97 and 19 Gt. Peat carbon contributes 60% to the total forest soil carbon pool in Malaysia and 74% in Indonesia. These results emphasize the prominent global and regional roles played by the tropical peat carbon pool and the importance of including this pool in national and regional assessments of terrestrial carbon stocks and the prediction of peat‐derived greenhouse gas emissions.     

Transplantation of organic surface horizons of boreal soils into warmer regions alters microbiology but not the temperature sensitivity of decomposition

Changes in soil carbon, the largest terrestrial carbon pool, are critical for the global carbon cycle, atmospheric CO₂ levels and climate. Climate warming is predicted to be most pronounced in the northern regions and therefore the large soil carbon pool residing in boreal forests will be subject to larger global warming impact than soil carbon pools in the temperate or the tropical forest. A major uncertainty in current estimates of the terrestrial carbon balance is related to decomposition of soil organic matter (SOM). We hypothesized that when soils are exposed to warmer climate the structure of the ground vegetation will change much more rapidly than the dominant tree species. This change will alter the quality and amount of litter input to the soil and induce changes in microbial communities, thus possibly altering the temperature sensitivity of SOM decomposition. We transferred organic surface soil sections from the northern borders of the boreal forest zone to corresponding forest sites in the southern borders of the boreal forest zone and studied the effects of warmer climate after an adaptation period of 2 years. The results showed that initially ground vegetation and soil microbial community structure and community functions were different in northern and southern forest sites and that 2 years of exposure to warmer climate was long enough to cause changes in these ecological indicators. The rate of SOM decomposition was approximately equally sensitive to temperature irrespective of changes in vegetation or microbial communities in the studied forest sites. However, as temperature sensitivity of the decomposition increases with decreasing temperature regime, the proportional increase in the decomposition rate in northern latitudes could lead to significant carbon losses from the soils.     

Global climate change and carbon balance in forest ecosystems of boreal zones: Simulation modeling as a forecast tool

The individual-based system of models EFIMOD simulating carbon and nitrogen flows in forest ecosystems has been used for forecasting the response of forest ecosystems to various forest management regimes with climate change. As input data the forest inventory data for the Manturovskii forestry of the Kostroma region were used. It has been shown that increase of mid-annual temperatures and precipitation influence the redistribution of carbon and nitrogen supply in organic form: supply increase of these elements in phytomass simultaneously with depletion of them in soil occurred. The most carbon and nitrogen accumulation in forest ecosystems occurs in the scenario without felling. In addition, in this scenario only the ecosystems of the modeling territory function as a carbon sink; in the other two scenarios (with selective and clear cutting) they function as a source of carbon. Climate changes greatly influence the decomposition rate of organic matter in soil, which leads to increased emission of carbon dioxide. The second consequence of the increase in the destruction rate is nitrogen increase in the soil in a form available for plants that entails productivity increase of stands.     

土壤团聚体固碳的研究方法

增加土壤有机碳含量对维持土壤肥力和农业可持续发展,缓解温室气体增加和全球气候变化的影响具有重要意义.土壤团聚体是土壤的重要组成部分,影响着土壤的各种物理化学性质.土壤团聚体对土壤有机碳的物理保护是土壤碳固定的重要机制.本文综述了土壤团聚体对有机碳的固定作用,土壤团聚体固碳机制研究的经典方法和现代常规方法,并且针对研究中存在的问题探讨了今后的发展趋势.     

中国东部森林土壤有机碳组分的纬度格局及其影响因子

土壤有机碳是森林碳库的重要组成部分,其活性有机碳组分不仅是土壤碳周转过程的重要环节,还是气候变化最敏感的指标。以中国东部南北森林样带(NSTEC,North-South Transect of Eastern China)为对象,选择了9个典型森林生态系统(尖峰岭、鼎湖山、九连山、神农架、太岳山、东灵山、长白山、凉水和呼中),涵盖了我国热带森林、亚热带森林和温带森林的主要类型,测定其0—10 cm土壤有机碳(SOC)、易氧化有机碳(EOC)、微生物碳(MBC)和可溶性有机碳(DOC)含量,结合气候、土壤质地、土壤微生物和植被生物量等因素,探讨了森林土壤有机碳组分的纬度格局及其主要影响因素。实验结果表明:SOC、EOC、MBC和DOC含量分别为23.12—77.00 g/kg、4.62—17.24 g/kg、41.92—329.39 mg/kg和212.63—453.43 mg/kg。SOC、EOC和MBC随纬度增加呈指数增长(P0.05),而DOC则随纬度增加呈指数降低(P0.05)。在不同气候带上,SOC和EOC含量表现为热带森林亚热带森林温带森林(P0.05),DOC含量表现为热带森林亚热带森林温带森林(P0.001)。气候、植被生物量、土壤质地和土壤微生物可解释土壤有机碳组分纬度格局的大部分空间变异(SOC 74%;EOC 65%;MBC 51%和DOC 76%)。其中,气候是土壤有机碳组分呈现纬度格局的主要影响因素,土壤质地是SOC和EOC的次要影响因素,而土壤微生物和植被生物量是MBC和DOC的次要影响因素。     

土壤和沉积物中黑碳的环境行为及效应研究进展

土壤和沉积物是全球黑碳排放的主要归宿,土壤和沉积物中黑碳具有复杂的环境行为和环境效应。分析了黑碳的概念,指出应以环境意义为出发点去理解黑碳概念的丰富内涵;描述了黑碳形成过程及其对黑碳理化性质的影响,以及基于此的黑碳分类;总结了黑碳来源辨析的若干种常用方法;讨论了黑碳在土壤/沉积物与其他环境介质之间的迁移循环过程,以及在土壤和沉积物内部的迁移行为;探讨了土壤和沉积物中黑碳的降解行为与稳定性,及其与地-气碳氮温室气体通量、土壤稳定碳库的关系,以及在土壤碳循环模型中的作用;综述了土壤和沉积物中黑碳对有机物、重金属和营养盐的吸附行为及主要机制;提出了今后研究的主要方向,以供相关研究者参考。     

Carbon Stocks in Silvopastoral Systems: A Study from Four Communities in Southeastern Ecuador

Agriculture, particularly pasture, is the second largest source of greenhouse gas emissions from tropical regions. Silvopastoral systems may increase carbon pools in pastures while maintaining productivity. Adding trees to pasture provides carbon sinks in woody biomass, and may improve degraded soils and increase the stability of soil carbon pools. In this study we quantified the biomass carbon stored in spontaneous silvopastoral systems in southeastern Ecuador. We compared the stem density, basal area, aboveground biomass, and organic carbon in the top 20 cm of soil in 100 pastures, ranging from 3 to 250 hectares, in four different communities. Aboveground live woody biomass, calculated using allometric equations and two different wood densities, varied from 10.99 to 66.1 Mg per hectare. Soil organic carbon pools ranged from 85.0 to 97.6 Mg per hectare. Stem density, basal area, and aboveground live biomass all positively correlated with pasture age. We found no relationship between pasture age or stem density and soil organic carbon pools. We measured live woody biomass carbon pools of 34–1070 Mg of carbon per farm in these silvopastoral systems. We found no effects on productivity of the herbaceous layer, suggesting that having a low density of trees in pastures could substantially increase the number of trees and the associated carbon sequestration without affecting cattle production.     

Past, Present and Future Levels of Greenhouse Gases in the Atmosphere and Model Projections of Related Climatic Changes

Ice core analyses of polar ice reveal a high correlation betweenclimatic change and variations in the atmospheric concentrationsof greenhouse gases (carbon dioxide and methane) over the last160 000 years. Although the resolution of the data is not sufficientto determine the phase relationship between the respective variations,it is generally believed that climate change occurred firstas a result of the quasi-periodic variations of the Earth'sorbital parameters. However, data and model results are consistentwith the hypothesis that climate and atmospheric concentrationsof greenhouse gases interact via a positive feedback loop. The more recent increase in greenhouse gases since pre-industrialtimes can be related to human activities. Climate models predicta significant global warming of several degrees within the nextcentury if the industrial emissions increase unabated. On theother hand, accelerated policies on emission control will significantlyreduce the warming after a response time of a few decades.