Regional Carbon Sink Research Articles

Analyzing land-cover change and corresponding impacts on carbon budget in a fast developing sub-tropical region by integrating MODIS and Landsat TM/ETM+ images

Land-cover change has significant impacts on regional carbon dynamics. Understanding the carbon consequences of land-cover change is necessary for decision makers to address the issues of carbon reduction and climate change mitigation. Optical remote sensing images have been widely used for detecting regional land-cover change. However, it is difficult to acquire desirable images for regions that are frequently affected by cloudy and rainy weather. In this study, we proposed an approach to deal with this problem by integrating moderate-resolution imaging spectroradiometer (MODIS) and Landsat images based on the mixed-label analysis (MLA) model. We tested this model in Guangdong Province, a fast developing sub-tropical region in China, to derive the provincial land-cover data for the analysis of land-cover change between 2000 and 2009 and its impacts on regional carbon dynamics. Results show that forest land decreased by 3.03%, while built-up area rapidly expanded by 73.01% from 2000 to 2009. The regional vegetation carbon sink declined by 2.6%, whereas the regional carbon emissions increased by more than 100% due to the fast urbanization and economic development. The regional vegetation carbon sink can only offset 4.1% of total carbon emissions in 2009, far below the national level (about 7.0–7.7%) at the same period. Future efforts to improve the regional carbon budget should focus more on the control of land development and the advance of energy efficiency.

IBIS Model-Based Simulation Study on Variation of Ecological Carbon Sink Potential in Shenzhen City

IBIS (Integrated Biosphere Simulator) model is an ecosystem process model, which represents the research direction of a global carbon cycle simulation. Based on the existing research achievements, the regional ecological carbon sink potential is simulated from point to surface with a long time serie of meteorological data (1954-2010), data on different ecosystem vegetation types, soil texture data, terrain data and remote sensing data. The results are listed as follows: there is no significant variation of potential carbon sink of Shenzhen vegetation over the past 30 years, carbon sequestration caapcity ranges within 43.52-55.82×104Mg C/a, and the potential carbon sink intensity value within 0-624 C/m2/a. Precipitation is a main factor which influnces the ecosystem carbon sink intensity in Shenzhen City. The carbon sink intensity of ecosystem is increased with more annual precipitation.

Carbon dynamics of temperate grassland ecosystems in China from 1951 to 2007: an analysis with a process-based biogeochemistry model

Grasslands account for 40 % of the Chinese land area. About 80 % of the total grasslands are in the northern temperate zone. These grassland ecosystems provide goods and services to the local people and play an important role in the global carbon cycle. Remote sensing and ecosystem modeling approaches have been used to quantify the carbon budget of these grasslands. However, the intensive site measurements and meteorological data acquired in these ecosystems in the last few decades have not been adequately used to improve ecosystem model capabilities, in turn, better quantify their carbon budget. In this study an effort was made to examine the carbon budget and its spatial–temporal variation of the temperate grasslands in China from 1951 to 2007 using a process-based biogeochemistry model. It was found that the regional grasslands acted as a small carbon sink at 11.25 g C m−2 year−1 in the study area of 64.96 million hectares with a high inter-annual variability ranging from −124 to 122.7 g C m−2 year−1 during the study period. As a result, the temperate grasslands sequestered about 410 Tg C in their vegetation and soils during the study period. The carbon sink occurred in typical steppe in central Inner Mongolia within the 300–400 mm rainfall zone and forest steppe in central and western China. By contrast, forest steppe in northeastern China mainly acted as a carbon source. Three major ecosystem types of forest steppe, typical steppe and desert steppe account for 54, 34, and 12 % of the total sink (7.3 Tg C year−1) during 1951–2007, respectively. Soil moisture and evapotranspiration had a dominant effect on carbon budget in the typical steppe and the forest steppe while both water conditions and nitrogen mineralization rate were the major factors in the desert steppe. At a decadal scale, the air temperature significantly increased by 0.4 °C and annual precipitation insignificantly decreased by 0.2 mm; the regional carbon sink increased by 2.2 Tg C per decade during the period 1951–2007. However, further sensitivity analysis suggests that the sink of temperate grasslands will be reduced if the climate gets warmer and drier during this century since the increasing net primary production does not keep up with the increase of heterotrophic respiration.

Carbon dynamics of terrestrial ecosystems on the Tibetan Plateau during the 20th century: an analysis with a process‐based biogeochemical model

ABSTRACTAim The Tibetan Plateau accounts for about a quarter of the total land area of China and has a variety of ecosystems ranging from alpine tundra to evergreen tropics. Its soils are dominated by permafrost and are rich in organic carbon. Its climate is unique due to the influence of the Asian monsoon and its complex topography. To date, the carbon dynamics of the Tibetan Plateau have not been well quantified under changes of climate and permafrost conditions. Here we use a process‐based biogeochemistry model, the Terrestrial Ecosystem Model (TEM), which was incorporated with a soil thermal model, to examine the permafrost dynamics and their effects on carbon dynamics on the plateau during the past century.Location The Tibetan Plateau.Methods We parameterize and verify the TEM using the existing data for soil temperature, permafrost distribution and carbon and nitrogen from the region. We then extrapolate the model and parameters to the whole plateau.Results During the 20th century, the Tibetan Plateau changed from a small carbon source or neutral in the early part of the century to a sink later, with a large inter‐annual and spatial variability due to changes of climate and permafrost conditions. Net primary production and soil respiration increased by 0.52 and 0.22 Tg C year−1, respectively, resulting in a regional carbon sink increase of 0.3 Tg C year−1. By the end of the century, the regional carbon sink reached 36 Tg C year−1 and carbon storage in vegetation and soils is 32 and 16 Pg C, respectively. On the plateau, from west to east, the net primary production, soil respiration and net ecosystem production increased, due primarily to the increase of air temperature and precipitation and lowering elevation. In contrast, the decrease of carbon fluxes from south to north was primarily controlled by precipitation gradient. Dynamics of air temperature and associated soil temperature and active layer depth resulted in a higher plant carbon uptake than soil carbon release, strengthening the regional carbon sink during the century.Main conclusions We found that increasing soil temperature and deepening active layer depth enhanced soil respiration, increasing the net nitrogen mineralization rate. Together with the effects of warming air temperature and rising CO2 concentrations on photosynthesis, the stronger plant nitrogen uptake due to the enhanced available nitrogen stimulates plant carbon uptake, thereby strengthening the regional carbon sink as the rate of increase net primary production was faster than that of soil respiration. Further, the warming and associated soil thermal dynamics shifted the regional carbon sink from the middle of July in the early 20th century to early July by the end of the century. Our study suggests that soil thermal dynamics should be considered for future quantification of carbon dynamics in this climate‐sensitive region.

Effects of forest management intensity on carbon and nitrogen content in different soil size fractions of a North Florida Spodosol

Pine plantations of the southeastern USA are regional carbon (C) sinks. In spite of large increases in woody biomass due to advanced growing systems, studies have shown little or even negative eVects on the C content of the extremely sandy soils of this region. Hence, it is important to understand the mechanisms that determine the impact of intensive forest manage- ment on soil organic carbon (SOC) sequestration. This study was conducted to examine the C proWle in a 4-year-old loblolly pine (Pinus taeda L.) plantation managed under two levels of management intensity (chemical understory control and fertilizer inputs). Soil organic C and nitrogen (N) pools were evaluated using two size fractionation methods, dry and wet sieving (2000-250 m, 250-150 m, 150-53 m and <53 m). Dry sieving was preferred over wet sieving for soil size fractionation, as it preserved more structure and water- soluble SOC components such as esters and amides and did not aVect the N distribution. DiVuse ReXectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra were used to examine the chemical composition of the size fractions, which showed the presence of recently added organic matter in the largest sand frac- tion, as well as more decomposed organic matter in the <53 m fraction. Intensive forest management reduced SOC in all three 2000-53 m fractions, most likely due to reduced root input of understory plants that were con- trolled using herbicides. The 2000-250 m fractions contained nearly half of the total SOC and showed a 23% decrease in C content due to the intensive manage- ment regime. Results from this study indicated the sig- niWcance and responsiveness of sand size SOC fractions in Florida Spodosols. Results also showed that reduc- tions in SOC due to intensive management occurred after four years and highlighted the need to understand the long-term impacts and the mechanisms responsible.

Terrestrial carbon pools in southeast and south-central United States

Analyses of regional carbon sources and sinks are essential to assess the economical feasibility of various carbon sequestration technologies for mitigating atmospheric CO2 accumulation and for preventing global warming. Such an inventory is a prerequisite for regional trading of CO2 emissions. As a U.S. Department of Energy Southeast Regional Carbon Sequestration Partner, we have estimated the state-level terrestrial carbon pools in the southeast and south-central US. This region includes: Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, Texas, and Virginia. We have also projected the potential for terrestrial carbon sequestration in the region. Texas is the largest contributor (34%) to greenhouse gas emission in the region. The total terrestrial carbon storage (forest biomass and soils) in the southeast and south-central US is estimated to be 130 Tg C/year. An annual forest carbon sink (estimated as 76 Tg C/year) could compensate for 13% of the regional total annual greenhouse gas emission (505 Tg C, 1990 estimate). Through proper policies and the best land management practices, 54 Tg C/year could be sequestered in soils. Thus, terrestrial sinks can capture 23% of the regional total greenhouse emission and hence are one of the most cost-effective options for mitigating greenhouse emission in the region.

Recent trends in hydrologic balance have enhanced the terrestrial carbon sink in the United States

Climate data show significant increases in precipitation and humidity over the U.S. since 1900, yet the role of these hydro‐climatic changes on the reported U.S. carbon sink is incompletely understood. Using a prognostic terrestrial ecosystem model, we simulated 1900–1993 continental U.S. carbon fluxes and found that increased growth by natural vegetation was associated with increased precipitation and humidity, especially during the 1950–1993 period. CO2 trends and warmer temperatures had a lesser effect. Two thirds of the increase in observed forest growth rates could be accounted for by observed climatic changes, including the confluence of earlier springs and wetter autumns leading to a lengthening of the vegetation carbon uptake period. However, regional differences in precipitation trends produced differing regional carbon sink responses. The strong coupling between carbon and hydrologic cycles implies that global carbon budget studies, currently dominated by temperature analyses, should consider changes in the hydrologic cycle.

Towards robust regional estimates of CO2 sources and sinks using atmospheric transport models

Information about regional carbon sources and sinks can be derived from variations in observed atmospheric CO2 concentrations via inverse modelling with atmospheric tracer transport models. A consensus has not yet been reached regarding the size and distribution of regional carbon fluxes obtained using this approach, partly owing to the use of several different atmospheric transport models. Here we report estimates of surface-atmosphere CO2 fluxes from an intercomparison of atmospheric CO2 inversion models (the TransCom 3 project), which includes 16 transport models and model variants. We find an uptake of CO2 in the southern extratropical ocean less than that estimated from ocean measurements, a result that is not sensitive to transport models or methodological approaches. We also find a northern land carbon sink that is distributed relatively evenly among the continents of the Northern Hemisphere, but these results show some sensitivity to transport differences among models, especially in how they respond to seasonal terrestrial exchange of CO2. Overall, carbon fluxes integrated over latitudinal zones are strongly constrained by observations in the middle to high latitudes. Further significant constraints to our understanding of regional carbon fluxes will therefore require improvements in transport models and expansion of the CO2 observation network within the tropics.

Variable Carbon Sinks

Over half of the anthropogenic carbon dioxide emitted into the atmosphere is partially absorbed by carbon sinks. As Fung explains in this Perspective, determining the location and magnitude of these sinks remains a major challenge. Bousquet et al. find that regional and global terrestrial carbon sinks are much more variable than their oceanic counterparts and that the tropical land masses may play a major role. Given that the land sink is so variable, it is uncertain how it will repond to climate change.

New production in the Northeast Water Polynya: 1993

The Northeast Water Polynya has been suggested as acting as a sink for carbon, especially during the spring and summer when phytoplankton growth is active. During 1993 the polynya was sampled for the entire growing period (late May through mid-August) in order to more accurately assess the magnitude, controls and patterns of new and total (ammonium, nitrate and urea) nitrogen production. This represents the first assessment of new production throughout an entire season in the Arctic. We found that, in 1993, new production, based on 15N-tracer techniques and integrated over the euphotic zone, was 0.141 mmol N m−2 h−1 (0.361 g C m−2 d−1 when converted using observed C/N ratios). Measured f ratios averaged 0.65 and demonstrate that the system, to a great extent, was using nitrate as a nitrogen source. In general f ratios were greatest early in the season and minimal in mid-summer. Urea uptake was highly variable and contributed slightly less than ammonium to phytoplankton nitrogen demand. Nitrate uptake at stations with low (< 0.5μM) nitrate concentrations was significantly reduced, implying that nitrate concentrations limited phytoplankton growth late in the growing season. Long-term new production rates calculated from nutrient depletion patterns from the polynya as a whole as well as a time-series constructed from a single location were ca. 0.144-0.281 g C m−2 d−1. The relationship between new production as measured by incubations and nutrient depletion budgets suggests that phytoplankton growth is the dominant factor influencing the nitrogen budget of the polynya. The amount of material available for removal from the euphotic zone is limited and constrains the degree to which the polynya can act as a regional carbon sink.