Impacts of urban forests on offsetting carbon emissions from industrial energy use in Hangzhou,China

This study quantified carbon storage and sequestration by urban forests and carbon emissions from energy consumption by several industrial sources in Hangzhou, China. Carbon (C) storage and sequestration were quantified using urban forest inventory data and by applying volume-derived biomass equations and other models relating net primary productivity (NPP) and mean annual biomass increments. Industrial energy use C emissions were estimated by accounting for fossil fuel use and assigning C emission factors. Total C storage by Hangzhou's urban forests was estimated at 11.74 Tg C, and C storage per hectare was 30.25 t C. Carbon sequestration by urban forests was 1,328, 166.55 t C/year, and C sequestration per ha was 1.66 t C/ha/year. Carbon emissions from industrial energy use in Hangzhou were 7 Tg C/year. Urban forests, through sequestration, annually offset 18.57% of the amount of carbon emitted by industrial enterprises, and store an amount of C equivalent to 1.75 times the amount of annual C emitted by industrial energy uses within the city. Management practices for improving Hangzhou's urban forests function of offsetting C emissions from energy consumption are explored. These results can be used to evaluate the urban forests' role in reducing atmospheric carbon dioxide.     

Potential contribution of the forestry sector in Bangladesh to carbon sequestration

The Kyoto Protocol provides for the involvement of developing countries in an atmospheric greenhouse gas reduction regime under its Clean Development Mechanism (CDM). Carbon credits are gained from reforestation and afforestation activities in developing countries. Bangladesh, a densely populated tropical country in South Asia, has a huge degraded forestland which can be reforested by CDM projects. To realize the potential of the forestry sector in developing countries for full-scale emission mitigation, the carbon sequestration potential of different species in different types of plantations should be integrated with the carbon trading system under the CDM of the Kyoto Protocol. This paper discusses the prospects and problems of carbon trading in Bangladesh, in relation to the CDM, in the context of global warming and the potential associated consequences. The paper analyzes the effects of reforestation projects on carbon sequestration in Bangladesh, in general, and in the hilly Chittagong region, in particular, and concludes by demonstrating the carbon trading opportunities. Results showed that tree tissue in the forests of Bangladesh stored 92tons of carbon per hectare (tC/ha), on average. The results also revealed a gross stock of 190tC/ha in the plantations of 13 tree species, ranging in age from 6 to 23 years. The paper confirms the huge atmospheric CO(2) offset by the forests if the degraded forestlands are reforested by CDM projects, indicating the potential of Bangladesh to participate in carbon trading for both its economic and environmental benefit. Within the forestry sector itself, some constraints are identified; nevertheless, the results of the study can expedite policy decisions regarding Bangladesh's participation in carbon trading through the CDM.     

Assessing the potential of native tree species for carbon sequestration forestry in Northeast China

Although the native forests of China are exceptionally diverse, only a small number of tree species have been widely utilized in forest plantations and reforestation efforts. We used dendrochronological sampling methods to assess the potential growth and carbon sequestration of native tree species in Jilin Province, Northeast China. Trees were sampled in and near the Changbaishan Biosphere Reserve, with samples encompassing old-growth, disturbed forest, and plantations. To approximate conditions for planted trees, sampling focused on trees with exposed crowns (dominant and co-dominant individuals). A log-linear relationship was found between diameter increment and tree diameter, with a linear decrease in increment with increasing local basal area; no significant differences in these patterns between plantations and natural stands were detected for two commonly planted species (Pinus koraiensis and Larix olgensis). A growth model that incorporates observed feedbacks with individual tree size and local basal area (in conjunction with allometric models for tree biomass), was used to project stand-level biomass increment. Predicted growth trajectories were then linked to the carbon process model InTEC to provide estimates of carbon sequestration potential. Results indicate substantial differences among species, and suggest that certain native hardwoods (in particular Fraxinus mandshurica and Phellodendron amurense), have high potential for use in carbon forestry applications. Increased use of native hardwoods in carbon forestry in China is likely to have additional benefits in terms of economic diversification and enhanced provision of "ecosystem services", including biodiversity protection.     

Quantifying Terrestrial Ecosystem Carbon Dynamics in the Jinsha Watershed, Upper Yangtze, China from 1975 to 2000

Quantifying the spatial and temporal dynamics of carbon stocks in terrestrial ecosystems and carbon fluxes between the terrestrial biosphere and the atmosphere is critical to our understanding of regional patterns of carbon budgets. Here we use the General Ensemble biogeochemical Modeling System to simulate the terrestrial ecosystem carbon dynamics in the Jinsha watershed of China’s upper Yangtze basin from 1975 to 2000, based on unique combinations of spatial and temporal dynamics of major driving forces, such as climate, soil properties, nitrogen deposition, and land use and land cover changes. Our analysis demonstrates that the Jinsha watershed ecosystems acted as a carbon sink during the period of 1975–2000, with an average rate of 0.36 Mg/ha/yr, primarily resulting from regional climate variation and local land use and land cover change. Vegetation biomass accumulation accounted for 90.6% of the sink, while soil organic carbon loss before 1992 led to a lower net gain of carbon in the watershed, and after that soils became a small sink. Ecosystem carbon sink/source patterns showed a high degree of spatial heterogeneity. Carbon sinks were associated with forest areas without disturbances, whereas carbon sources were primarily caused by stand-replacing disturbances. It is critical to adequately represent the detailed fast-changing dynamics of land use activities in regional biogeochemical models to determine the spatial and temporal evolution of regional carbon sink/source patterns.     

Net global warming potential and greenhouse gas intensity in irrigated cropping systems in northeastern Colorado

The impact of management on global warming potential (GWP), crop production, and greenhouse gas intensity (GHGI) in irrigated agriculture is not well documented. A no-till (NT) cropping systems study initiated in 1999 to evaluate soil organic carbon (SOC) sequestration potential in irrigated agriculture was used in this study to make trace gas flux measurements for 3 yr to facilitate a complete greenhouse gas accounting of GWP and GHGI. Fluxes of CO2, CH4, and N2O were measured using static, vented chambers, one to three times per week, year round, from April 2002 through October 2004 within conventional-till continuous corn (CT-CC) and NT continuous corn (NT-CC) plots and in NT corn-soybean rotation (NT-CB) plots. Nitrogen fertilizer rates ranged from 0 to 224 kg N ha(-1). Methane fluxes were small and did not differ between tillage systems. Nitrous oxide fluxes increased linearly with increasing N fertilizer rate each year, but emission rates varied with years. Carbon dioxide efflux was higher in CT compared to NT in 2002 but was not different by tillage in 2003 or 2004. Based on soil respiration and residue C inputs, NT soils were net sinks of GWP when adequate fertilizer was added to maintain crop production. The CT soils were smaller net sinks for GWP than NT soils. The determinant for the net GWP relationship was a balance between soil respiration and N2O emissions. Based on soil C sequestration, only NT soils were net sinks for GWP. Both estimates of GWP and GHGI indicate that when appropriate crop production levels are achieved, net CO2 emissions are reduced. The results suggest that economic viability and environmental conservation can be achieved by minimizing tillage and utilizing appropriate levels of fertilizer.     

“双碳”目标下我国海洋碳汇交易的发展机制研究

海洋具备高效的固碳能力和巨大的碳汇潜力，完善的海洋碳汇交易机制是我国应对全球变化、实现碳中和的重要途径。本文在总结国内外海洋碳汇相关政策与实践情况的基础上，探讨了我国海洋碳汇交易的动力机制、实现机制和保障机制，旨在为未来海洋碳汇交易的政策制定提供理论支撑。首先，海洋碳汇交易的实现需要以政策环境作为推力，利益动机作为引力，社会责任作为压力。其次，我国海洋碳汇交易的发展总体上可以分为以自愿市场为主的探索阶段、自愿市场到履约市场的过渡阶段和以履约市场为主的成熟阶段，不同阶段各利益相关方的作用和实践中的关键点需要加以分析。最后，在法律保障方面要注意将“自上而下”的顶层设计和“自下而上”的基层探索相结合；在资金方面可以形成以公共财政为主、多渠道融资为辅的资金保障；在技术方面需要组建专业机构开展科学与政策的相关研究工作。     

Long-term effect of a single application of organic refuse on carbon sequestration and soil physical properties

Restoration of degraded lands could be a way to reverse soil degradation and desertification in semiarid areas and mitigate greenhouse gases (GHG). Our objective was to evaluate the long-term effects of a single addition of organic refuse on soil physical properties and measure its carbon sequestration potential. In 1988, a set of five plots (87 m(2) each) was established in an open desert-like scrubland (2-4% cover) in Murcia, Spain, to which urban solid refuse (USR) was added in a single treatment at different rates. Soil properties were monitored over a 5-yr period. Sixteen years after the addition, three of the plots were monitored again (P0: control, P1: 13 kg m(-2), P2: 26 kg m(-2) of USR added) to assess the lasting effect of the organic addition on the soil organic carbon (SOC) pools and on the physical characteristics of the soil. The SOC content was higher in P2 (16.4 g kg(-1)) and in P1 (11.8 g kg(-1)) than in P0 (7.9 g kg(-1)). Likewise, aerial biomass increased from 0.18 kg m(-2) in P0 up to 0.27 kg m(-2) in P1 and 0.46 kg m(-2) in P2. This represents a total C sequestration of 9.5 Mg ha(-1) in P2 and 3.4 Mg ha(-1) in P1, most of the sequestered C remaining in the recalcitrant soil pool. Additionally, higher saturated hydraulic conductivity, aggregate stability, and available water content values and lower bulk density values were measured in the restored plots. Clearly, a single addition of organic refuse to the degraded soils to increase the potential for C sequestration was effective.     

Impact of Grazing on Plant Species Richness, Plant Biomass, Plant Attribute, and Soil Physical and Hydrological Properties of Vertisol in East African Highlands

Understanding the problems of grazing land in vertisol areas and seeking long-lasting solutions is the central point where mixed crop livestock is the second stay for the majority of the population. In order to understand this, the current study was conducted at two sites, one with 0–4% slope and the other with 4–8% slope at Ginchi watershed, 80 km west of Addis Ababa, Ethiopia. The specific objectives of the study were to quantify changes in plant species richness, biomass, plant cover, and soil physical and hydrological properties. The grazing regimes were: moderate grazing (regulated), heavy grazing (free grazing), and no grazing (closed to any grazing), which was considered the control treatment. The results showed that the biomass yield in nongrazed plots was higher than in the grazed plots. However, the biomass yield in grazed plots improved over the years. Species richness and percentage of dominant species attributes were better in medium grazed plots than the other treatments. Soil compaction was higher in very heavily grazed plots than in nongrazed and medium-grazed plots. In contrast to that, the soil water content and infiltration rate were better in nongrazed plots than in grazed plots. Soil loss in grazed plots decreased with the increase of biomass yields and as the soil was more compacted by livestock trampling during the wet season. Finally since the medium stocking rate is better in species richness and plant attributes, and lies between nongrazed and heavily grazed plots in the rest of the measured parameters, it could be the appropriate stocking rate to practice by the smallholder farmer.     

Thinning Pine Plantations to Reestablish Oak Openings Species in Northwestern Ohio

Globally the area in forest plantations is rising by 2% annually, increasing the importance of plantations for production of human goods and services and for ecological functions such as carbon storage and biodiversity conservation. Specifically in the Great Lakes states and provinces of Midwestern North America, thousands of hectares of pine plantations were established in the early and mid-1900s to revegetate abandoned agricultural fields that had replaced mixed-species forests and oak–prairie ecosystems. Plantation establishment also was intended to bolster the timber base. Management priorities have shifted, with many resource managers currently seeking to manage existing plantations for promoting mixed-species ecosystems. The purpose of this study was to assess plant succession and the reestablishment of oak savanna and prairie species after thinning 14 plantations of Pinus resinosa and strobus in northwestern Ohio, USA. Thinning reduced tree basal area by an average of 75%. Plant communities were sampled on 0.05-ha plots one and 3 years after thinning and compared to 10 unthinned control plantations. By 3 years after thinning, thinned plots contained 2–3 times more species and 14 times more plant cover than control plots. The species composition of colonizing plants was most strongly correlated with residual pine basal area and soil variables related to drainage (e.g., sand concentration, available water capacity). Although plant composition was dominated by widespread colonizers such as Erechtites hieraciifolia, the coefficient of conservatism (indicative of species of more intact, undisturbed communities) significantly increased on thinned plots from year 1 to 3. This finding, coupled with the presence of four rare, state-listed Ohio species whose eight plot occurrences all were on thinned plots, suggests that plant composition is moving towards species typifying more high-quality savanna and prairie habitats.     

Investigation on working fluids selection for organic rankine cycles with low-temperature heat sources

The Influence of mixed and pure working fluids on the performance of organic Rankine cycles (ORCs) is discussed. Specifically, the performance of mixed and pure working fluids is analyzed based on certain characteristics of low-temperature heat source and heat sink. A method of selecting binary zeotropic mixed working fluids that match with different heat sinks is introduced. Thermodynamic processes of ORCs for various heat sources are simulated in Matlab. The performance characteristics of pure and mixed working fluids are compared under different inlet temperatures and temperature gradients of sensible heat sources. The results demonstrate that when the initial temperature of a heat source is lower and its temperature gradient is higher, and the temperature gradient of the heat sink is higher, mixed working fluids have better performance than pure working fluids. However, for the opposite heat source and heat sink situations, pure working fluids perform better. Mixtures with low critical temperature components exhibit the best performance among all working fluids when the temperature gradient of the heat source is large. The analysis also shows that introduction of a recuperator may reduce the cycle efficiency when the heat source temperature is low and the temperature gradient of the heat source is large.     

Future carbon balance of China's forests under climate change and increasing CO2

The possible response of the carbon (C) balance of China's forests to an increase in atmospheric CO(2) concentration and climate change was investigated through a series of simulations using the Integrated Terrestrial Ecosystem Carbon (InTEC) model, which explicitly represents the effects of climate, CO(2) concentration, and nitrogen deposition on future C sequestration by forests. Two climate change scenarios (CGCM2-A2 and -B2) were used to drive the model. Simulations showed that China's forests were a C sink in the 1990 s, averaging 189 Tg C yr(-1) (about 13% of the global total). This sink peaks around 2020 and then gradually declines to 33.5 Tg C yr(-1) during 2091-2100 without climate and CO(2) changes. Effects of pure climate change of CGCM2-A2 and -B2 without allowing CO(2) effects on C assimilation in plants might reduce the average net primary productivity (NPP) of China's forests by 29% and 18% during 2091-2100, respectively. Total soil C stocks might decrease by 16% and 11% during this period. China's forests might broadly act as C sources during 2091-2100, with values of about 50 g Cm(-2)yr(-1) under the moderate warming of CGCM2-B2 and 50-200 g Cm(-2)yr(-1) under the warmer scenario of CGCM2-A2. An increase in CO(2) might broadly increase future C sequestration of China's forests. However, this CO(2) fertilization effect might decline with time. The CO(2) fertilization effects on NPP by the end of this century are 349.6 and 241.7 Tg C yr(-1) under CGCM2-A2 and -B2 increase scenarios, respectively. These effects increase by 199.1 and 126.6 Tg C yr(-1) in the first 50 years, and thereafter, by 150.5 and 115.1 Tg C yr(-1) in the second 50 years under CGCM2-A2 and -B2 increase scenarios, respectively. Under a CO(2) increase without climate change, the majority of China's forests would be C sinks during 2091-2100, ranging from 0 to 100 g Cm(-2)yr(-1). The positive effect of CO(2) fertilization on NPP and net ecosystem productivity would be exceeded by the negative effect of climate change after 2050. Under the CGCM2-A2 climate scenario and with direct CO(2) effects, China's forests may be a small C source of 7.6 Tg C yr(-1) during 2091-2100. Most forests act as C sources of 0-40 g Cm(-2)yr(-1). Under the CGCM2-B2 climate scenario and with direct CO(2) effects, China's forests might be a small C sink of 10.5 Tg C yr(-1) during 2091-2100, with C sequestration of most forests ranging from 0 to 40 g Cm(-2)yr(-1). Stand age structure plays a more dominant role in determining future C sequestration than CO(2) and climate change. The prediction of future C sequestration of China's forests is very sensitive to the Q(10) value used to estimate maintenance respiration and to soil water availability and less sensitive to N deposition scenario. The results are not yet comprehensive, as no forest disturbance data were available or predicted after 2001. However, the results indicate a range of possible responses of the C balance of China's forests to various scenarios of increase in CO(2) and climate change. These results could be useful for assessing measures to mitigate climate change through reforestation.     

Net Carbon Sequestration Potential and Emissions in Home Lawn Turfgrasses of the United States

Soil analyses were conducted on home lawns across diverse ecoregions of the U.S. to determine the soil organic carbon (SOC) sink capacity of turfgrass soils. Establishment of lawns sequestered SOC over time. Due to variations in ecoregions, sequestration rates varied among sites from 0.9 Mg carbon (C) ha^?1 year^?1 to 5.4 Mg C ha^?1 year^?1. Potential SOC sink capacity also varied among sites ranging from 20.8 ± 1.0–96.3 ± 6.0 Mg C ha^?1. Average sequestration rate and sink capacity for all sites sampled were 2.8 ± 0.3 Mg C ha^?1 year^?1 and 45.8 ± 3.5 Mg C ha^?1, respectively. Additionally, the hidden carbon costs (HCC) due to lawn mowing (189.7 kg Ce (carbon equivalent) ha^?1 year^?1) and fertilizer use (63.6 kg Ce ha^?1 year^?1) for all sites totaled 254.3 kg Ce ha^?1 year^?1. Considering home lawn SOC sink capacity and HCC, mean home lawn sequestration was completely negated 184 years post establishment. The potential SOC sink capacity of home lawns in the U.S. was estimated at 496.3 Tg C, with HCC of between 2,504.1 Gg Ce year^?1 under low management regimes and 7551.4 Gg Ce year^?1 under high management. This leads to a carbon-positive system for between 66 and 199 years in U.S. home lawns. More efficient and reduction of C-intensive maintenance practices could increase the overall sequestration longevity of home lawns and improve their climate change mitigation potential.     

Carbon sequestration potential in reclaimed mine sites in seven east-central states

Terrestrial systems represent a significant potential carbon (C) sink to help mitigate or offset greenhouse gas emissions. Nearly 3.2 Mha are permitted for mining activities in the United States, which are required to be reclaimed with vegetative cover. While site-specific studies have assessed C accumulation on reclaimed mine sites, regional analyses to estimate potential C increases have not been conducted. For this analysis, potential C sequestration is analyzed on 567,000 ha of mine land in a seven-state region reclaimed to cropland, pasture, or forest. Carbon accumulation is estimated for cropland, pasture, and forest soils, forest litter layer, and aboveground biomass by estimating average annual rates of C accumulation from site-specific and general C sequestration studies. The average annual rate of C storage is highest when mine land is reclaimed to forest, where the potential sequestration is 0.7 to 2.2 Tg yr(-1). The C from soils, litter layer, and biomass from mine lands reclaimed to forest represents 0.3 to 1.0% of the 1990 CO2 emissions from the study region (919 Tg CO2). To achieve the greenhouse gas (GHG) emission reduction goal of 7% below the 1990 level as proposed by the Kyoto Treaty requires CO2 emissions in the study area to be reduced by just over 64 Tg CO2. The potential carbon storage in mine sites reclaimed to forest could account for 4 to 12.5% of these required reductions.     

Natural vs. plantation forests: A case study of land reclamation strategies for the humid tropics

Biomass and productivity were compared in two plantations and in one stand of natural regeneration on similar sites in a premontane moist forest region of Puerto Rico. While initial growth rates of plantation species were higher, after four decades productivity of the natural regeneration plots was equal to or greater than productivity of the plantations. For the first 44 years, aboveground biomass of natural regeneration increased at an average annual rate of 3.8t·ha^–1·yr^–1, but the last year of the study it was 14.7t·ha^–1. Biomass increment of a pine plantation averaged between 8 and 10.5t·ha^–1·yr^–1 except for one year when the rate was much lower, possibly because of hurricane damage. A tropical hardwood plantation averaged close to 4t·ha^–1·yr^–1 for 41 years. It is suggested that in countries where funds for land reclamation are limited, intensive plantations may not always be the best strategy. Natural regeneration or shelterbelt plantations may be suitable alternatives.     

Soil carbon storage in silvopasture and related land-use systems in the brazilian cerrado

Silvopastoral management of fast-growing tree plantations is becoming popular in the Brazilian Cerrado (savanna). To understand the influence of such systems on soil carbon (C) storage, we studied C content in three aggregate size classes in six land-use systems (LUS) on Oxisols in Minas Gerais, Brazil. The systems were a native forest, a treeless pasture, 24- and 4-yr-old eucalyptus ( sp.) plantations, and 15- and 4-yr-old silvopastures of fodder grass plus animals under eucalyptus. From each system, replicated soil samples were collected from four depths (0-10, 10-20, 20-50, and 50-100 cm), fractionated into 2000- to 250-, 250- to 53-, and <53-μm size classes representing macroaggregates, microaggregates, and silt + clay, respectively, and their C contents determined. Macroaggregate was the predominant size fraction under all LUS, especially in the surface soil layers of tree-based systems. In general, C concentrations (g kg soil) in the different aggregate size fractions did not vary within the same depth. The soil organic carbon (SOC) stock (Mg C ha) to 1-m depth was highest under pasture compared with other LUS owing to its higher soil bulk density. The soils under all LUS had higher C stock compared with other reported values for managed tropical ecosystems: down to 1 m, total SOC stock values ranged from 461 Mg ha under pasture to 393 Mg ha under old eucalyptus. Considering the possibility for formation and retention of microaggregates within macroggregates in low management-intensive systems such as silvopasture, the macroaggregate dynamics in the soil seem to be a good indicator of its C storage potential.     

Changes in microbial biomass parameters of a heavy metal-contaminated calcareous soil during a field remediation experiment

Soil microbial biomass parameters give useful information about the restoration degree and quality of contaminated soils. These parameters were studied in a field experiment where the effect of two organic amendments on the bioavailability of heavy metals in an agricultural soil and on their accumulation in Beta vulgaris and Beta maritima was assessed. The soil was a calcareous Xeric Torriorthent and the total metal levels were (mg kg(-1)): 2706 Zn, 3235 Pb, and 39 Cu. The treatments were: fresh cow manure, olive husk, and inorganic fertilizer as a control. Two successive crops (B. vulgaris and B. maritima) were grown on the treated and untreated plots. The soil was sampled before each planting and after each harvest over a 15-mo period. Biomass C and N increased in all plots, especially in the organically amended ones. The ratio CO(2)-C/biomass C decreased in olive husk and manure-treated plots, in comparison with the control, and also during the experiment, suggesting a beneficial effect of the organic amendments. In olive husk-treated plots a significant increase in the ratio of biomass C/total organic carbon (TOC) with time was observed. This indicated a reduction of heavy metal stress on the microbial population. The amendments showed, in general, a beneficial effect on soil quality and fertility, while microbial biomass parameters were found to be useful indicators of the evolution of the remediation processes.     

Soil organic carbon and nitrogen accumulation in plots of rhizoma perennial peanut and bahiagrass grown in elevated carbon dioxide and temperature

Carbon sequestration in soils might mitigate the increase of carbon dioxide (CO2) in the atmosphere. Two contrasting subtropical perennial forage species, bahiagrass (BG; Paspalum notatum Flügge; C4), and rhizoma perennial peanut (PP; Arachis glabrata Benth.; C3 legume), were grown at Gainesville, Florida, in field soil plots in four temperature zones of four temperature-gradient greenhouses, two each at CO2 concentrations of 360 and 700 micromol mol(-1). The site had been cultivated with annual crops for more than 20 yr. Herbage was harvested three to four times each year. Soil samples from the top 20 cm were collected in February 1995, before plant establishment, and in December 2000 at the end of the project. Overall mean soil organic carbon (SOC) gains across 6 yr were 1.396 and 0.746 g kg(-1) in BG and PP, respectively, indicating that BG plots accumulated more SOC than PP. Mean SOC gains in BG plots at 700 and 360 micromol mol(-1) CO2 were 1.450 and 1.343 g kg(-1), respectively (not statistically different). Mean SOC gains in PP plots at 700 and 360 micromol mol(-1) CO2 were 0.949 and 0.544 g kg(-1), respectively, an increase caused by elevated CO2. Relative SON accumulations were similar to SOC increases. Overall mean annual SOC accumulation, pooled for forages and CO2 treatments, was 540 kg ha(-1) yr(-1). Eliminating elevated CO2 effects, overall mean SOC accumulation was 475 kg ha(-1) yr(-1). Conversion from cropland to forages was a greater factor in SOC accumulation than the CO2 fertilization effect.     

Carbon sinks and sources in China's forests during 1901-2001

This paper reports the annual carbon (C) balance of China's forests during 1901-2001 estimated using the Integrated Terrestrial Ecosystem C-budget model (InTEC). Annual carbon source and sink distributions are simulated for the same period using various spatial datasets including land cover and leaf area index (LAI) obtained from remote sensing, soil texture, climate, forest age, and nitrogen deposition. During 1901-1949, China's forests were a source of 21.0+/-7.8 Tg C yr(-1) due to disturbances (human activities). Its size increased to 122.3+/-25.3 Tg C yr(-1) during 1950-1987 due to intensified human activities in the late 1950s, early 1960s, 1970s and early 1980s. The forests became large sinks of 176.7+/-44.8 Tg C yr(-1) during 1988-2001, owing to large-scale plantation and forest regrowth in previously disturbed areas as well as growth stimulation by nondisturbance factors such as climatic warming, atmospheric CO(2) fertilization, and N deposition. From 1901 to 2001, China's forests were a small carbon source of 3.32 Pg C, about 32.9+/-22.3 Tg C yr(-1). The overall C balance in biomass from InTEC generally agrees with previous results derived from forest inventories of China's forests. InTEC results also include C stock variation in soils and are therefore more comprehensive than previous results. The uncertainty in InTEC results is still large, but it can be reduced if a detailed forest age map becomes available.