摘要:

We compare the simulations of three biogeography models (BIOME2, Dynamic Global Phytogeography Model (DOLY), and Mapped Atmosphere‐Plant Soil System (MAPSS)) and three biogeochemistry models (BIOME‐BGC (BioGeochemistryCycles), CENTURY, and Terrestrial Ecosystem Model (TEM)) for the conterminous United States under contemporary conditions of atmospheric CO2and climate. We also compare the simulations of these models under doubled CO2and a range of climate scenarios. For contemporary conditions, the biogeography models successfully simulate the geographic distribution of major vegetation types and have similar estimates of area for forests (42 to 46% of the conterminous United States), grasslands (17 to 27%), savannas (15 to 25%), and shrublands (14 to 18%). The biogeochemistry models estimate similar continental‐scale net primary production (NPP; 3125 to 3772 × 1012gC yr−1) and total carbon storage (108 to 118 × 1015gC) for contemporary conditions. Among the scenarios of doubled CO2and associated equilibrium climates produced by the three general circulation models (Oregon State University (OSU), Geophysical Fluid Dynamics Laboratory (GFDL), and United Kingdom Meteorological Office (UKMO)), all three biogeography models show both gains and losses of total forest area depending on the scenario (between 38 and 53% of conterminous United States area). The only consistent gains in forest area with all three models (BIOME2, DOLY, and MAPSS) were under the GFDL scenario due to large increases in precipitation. MAPSS lost forest area under UKMO, DOLY under OSU, and BIOME2 under both UKMO and OSU. The variability in forest area estimates occurs because the hydrologic cycles of the biogeography models have different sensitivities to increases in temperature and CO2. However, in general, the biogeography models produced broadly similar results when incorporating both climate change and elevated CO2concentrations. For these scenarios, the NPP estimated by the biogeochemistry models increases between 2% (BIOME‐BGC with UKMO climate) and 35% (TEM with UKMO climate). Changes in total carbon storage range from losses of 33% (BIOME‐BGC with UKMO climate) to gains of 16% (TEM with OSU climate). The CENTURY responses of NPP and carbon storage are positive and intermediate to the responses of BIOME‐BGC and TEM. The variability in carbon cycle responses occurs because the hydrologic and nitrogen cycles of the biogeochemistry models have different sensitivities to increases in temperature and CO2. When the biogeochemistry models are run with the vegetation distributions of the biogeography models, NPP ranges from no response (BIOME‐BGC with all three biogeography model vegetations for UKMO climate) to increases of 40% (TEM with MAPSS vegetation for OSU climate). The total carbon storage response ranges from a decrease of 39% (BIOME‐BGC with MAPSS vegetation for UKMO climate) to an increase of 32% (TEM with MAPSS vegetation for OSU and GFDL climates). The UKMO responses of BIOME‐BGC with MAPSS vegetation are primarily caused by decreases in forested area and temperature‐induced water stress. The OSU and GFDL responses of TEM with MAPSS vegetations are primarily caused by forest expansion and temperature‐

ISSN:0886-6236    

DOI:10.1029/95GB02746

年代:1995

数据来源: WILEY

摘要:

Variations in the isotopic composition (δ13C, δD) of methane produced within a landfill site near Mainz, Germany, were studied using a newly developed tunable diode laser absorption spectrometer (TDLAS) method. Additional data on the mixing ratios of CO2, O2, N2, CH4itself and δ13C of the CO2in the landfill gas were also acquired. Samples taken from several branches of the landfill biogas collection system had methane isotopic compositions in the range δ13C = −62.3 to −55.3‰ VPDB (n= 23) and δD = −327 to −287‰ VSMOW (n= 23). Although the variability of the stable isotope ratios is small, several significant correlations were found between these and the other measured parameters, which provide insight into the microbiological processes occurring within the landfill. Several samples showed evidence of admixture of atmospheric air which occurs when the pumping rate in the collection branch exceeds the local methane production rate. A fraction of the atmospheric oxygen is consumed during the passage through the landfill and CO2is produced in addition to the CO2associated with methanogenesis. The consumption of oxygen is correlated with the δ13C and δD of CH4and the δ13C of CO2. The correlation is consistent with partial bacterial oxidation of CH4resulting in the progressive enrichment of the remaining CH4(α(δ13C) = 1.008 ± 0.003 and α(δD) = 1.044 ± 0.020) and in the formation of very depleted CO2. For samples showing no evidence of oxidation, there was a negative correlation between δD and δ13C(CH4)(r= −0.86,n= 14) and between δ13C(CO2) and δ13C(CH4) (r= −0.95,n= 14), which we interpret as originating from slightly varying contributions from the two methanogenic pathways CO2r

ISSN:0886-6236    

DOI:10.1029/95GB02582

年代:1995

数据来源: WILEY

摘要:

Soil radiocarbon measurements show that mineral soil carbon under a recovering temperate forest in South Carolina turns over twice as fast as carbon in undisturbed soil. The observed 12‐year turnover time influences the design and interpretation of CO2fertilization experiments. Experiments conducted on formerly disturbed sites will show a soil carbon fertilization response considerably faster than experiments conducted on native sites. Calculating the soil carbon CO2fertilization factor from observed increases in soil carbon requires values for the turnover time and inventory of active soil carbon. We also use the observed turnover time to estimate the rate of atmospheric carbon dioxide sequestration in soil following agricultural abandonment. Although using the observed turnover rate increases estimates of soil carbon uptake on abandoned land, the amount of carbon sequestered globally is minimal because the net area of land being abandoned is smal

ISSN:0886-6236    

DOI:10.1029/95GB02380

年代:1995

数据来源: WILEY

摘要:

Methane emissions were measured by a static chamber technique in a diverse peatland complex in the Northern Study Area (NSA) of the Boreal Ecosystem Atmosphere Study (BOREAS). Sampling areas represented a wide range of plant community and hydrochemical gradients (pH 3.9–7.0). Emissions were generally larger than those reported from other boreal wetland environments at similar latitude. Seasonal average fluxes from treed peatlands (including palsas) ranged from 0 to 20 mg CH4m−2d−1compared with 92 to 380 mg CH4m−2d−1in open graminoid bogs and fens (with maximum single fluxes up to 1355 mg CH4m−2d−1). Permafrost‐related collapse scars had similarly high CH4emissions, particularly in the lag areas where continuous measurements of water table, peat surface elevation, and peat temperature showed that the peat surface adjusted to a falling water table in the abnormally dry 1994 season, maintaining warm, saturated conditions and high CH4flux later into the season than nonfloating sites. A predictive model for CH4flux and environmental variables was developed using multiple stepwise regression. A combined variable of mean seasonal peat temperature at the average position of the water table explained most of the spatial variability in log CH4flux (r2= 0.64), with height above mean water table (HMWT), water chemistry (Kcorr, pH, Ca), tree cover, and herbaceous plant cover explaining additional variance (r2= 0.81). Canonical correspondence analysis (CCA) of combined vascular and bryophyte data with environmental variables showed that CH4flux was negatively correlated with HMWT, the second axis of vegetation variability, and was only weakly correlated with chemistry, the first axis. Sedge and tree cover were correlated with high and low CH4fluxes, respectively, while shrub cover was of less predictive value. Microtopographic groupings of hummocks and hollows were separated in terms of CH4flux at the intermediate ranges of the moisture gradient. These data show that multivariate vegetation analyses may provide a useful framework for integrating the complex environmental controls on CH4flux and extrapolating single point chamber measurements to the landscape scale using remote sensing. (Key words: CH4flux, peatland, vegetation, and

ISSN:0886-6236    

DOI:10.1029/95GB02379

年代:1995

数据来源: WILEY

摘要:

A global primary productivity and phytogeography model is described. The model represents the biochemical processes of photosynthesis and the dependence of gas exchange on stomatal conductance, which in turn depends on temperature and soil moisture. Canopy conductance controls soil water loss by evapotranspiration. The assignment of nitrogen uptake to leaf layers is proportional to irradiance, and respiration and maximum assimilation rates depend on nitrogen uptake and temperature. Total nitrogen uptake is derived from soil carbon and nitrogen and depends on temperature. The long‐term average annual carbon and hydrological budgets dictate canopy leaf area. Although observations constrain soil carbon and nitrogen, the distribution of vegetation types is not specified by an underlying map. Variables simulated by the model are compared to experimental results. These comparisons extend from biochemical processes to the whole canopy, and the comparisons are favorable for both current and elevated CO2atmospheres. The model is used to simulate the global distributions of leaf area index and annual net primary productivity. These distributions are sufficiently realistic to demonstrate that the model is useful for analyzing vegetation responses to global environmental chang

ISSN:0886-6236    

DOI:10.1029/95GB02432

年代:1995

数据来源: WILEY

摘要:

Vegetation fire residues of various kinds of plant materials were analyzed for black carbon (BC) and the corresponding hydrogen. These data clearly show that BC can be defined as the fire produced carbon fraction with a molar H/C ratio of ≤ 0.2 which is resistant to heating to 340°C in pure oxygen. Relationships of black carbon production to fire characteristics such as fuel type, gaseous emissions, etc., were studied demonstrating that black carbon is mainly a product of the hot flaming combustion and that more than 80% of the black carbon produced remains on ground after the fire with the rest being emitted with the smoke. This black carbon mainly in the fine residue may significantly contribute to the background concentrations of black carbon in the atmosphere. From the ratios of black carbon in the residue to carbon exposed to the fire, and to carbon emitted as CO2we estimate the global black carbon formation to be 50–270 Tg yr−l(teragram = 1012g). As this carbon is very resistant to microbial decay, it may well represent a sink for biospheric carbon. This sink may be significantly greater, for example, up to a factor of 2 when reducing the pretreatment temperature to 300°C, if some of the less resistant carbon with higher H/C ratios which is removed by our method would also show significant resistance to microbial breakdown. The fire‐induced sequestration of carbon from the short‐term biospheric to the long‐term geological cycle due to the formation of black carbon may represent a significant sink of atmospheric CO2and source of O2and possibly has influenced today's atmospheric o

ISSN:0886-6236    

DOI:10.1029/95GB02742

年代:1995

数据来源: WILEY

摘要:

Among all global ecosystems, tropical savannas are the most severely and extensively affected by anthropogenic burning. Frequency of fire in cerrado, a type of tropical savanna covering 25% of Brazil, is 2 to 4 years. In 1992 we measured soil fluxes of NO, N2O, CH4, and CO2from cerrado sites that had been burned within the previous 2 days, 30 days, 1 year, and from a control site last burned in 1976. NO and N2O fluxes responded dramatically to fire with the highest fluxes observed from newly burned soils after addition of water. Emissions of N‐trace gases after burning were of similar magnitude to estimated emissions during combustion. NO fluxes immediately after burning are among the highest observed for any ecosystem studied to date. These rates declined with time after burning and had returned to control levels 1 year after the burn. An assessment of our data suggested that tropical savanna, burned or unburned, is a major source of NO to the troposphere. Cerrado appeared to be a minor source of N2O and a sink for atmospheric CH4. Burning also elevated CO2fluxes, which remained detectably elevated 1 year late

ISSN:0886-6236    

DOI:10.1029/95GB02086

年代:1995

数据来源: WILEY

摘要:

Forests in seasonally dry areas of eastern Amazonia near Paragominas, Pará, Brazil, maintain an evergreen forest canopy through an extended dry season by taking up soil water through deep (>1 m) roots. Belowground allocation of C in these deep‐rooting forests is very large (1900 g C m−2yr−1) relative to litterfall (460 g C m−2yr−1). The presence of live roots drives an active carbon cycle deeper than l m in the soil. Although bulk C concentrations and14C contents of soil organic matter at>l‐m depths are low, estimates of turnover from fine‐root inputs, CO2production, and the14C content of CO2produced at depth show that up to 15% of the carbon inventory in the deep soil has turnover times of decades or less. Thus the amount of fast‐cycling soil carbon between 1 and 8‐m depths (2–3 kg C m−2, out of 17–18 kg C m−2) is significant compared to the amount present in the upper meter of soil (3–4 kg C m−2out of 10–11 kg C m−2). A model of belowground carbon cycling derived from measurements of carbon stocks and fluxes, and constrained using carbon isotopes, is used to predict C fluxes associated with conversion of deep‐rooting forests to pasture and subsequent pasture management. The relative proportions and turnover times of active (including detrital plant material; 1–3 year turnover), slow (decadal and shorter turnover), and passive (centennial to millennial turnover) soil organic matter pools are determined by depth for the forest soil, using constraints from measurements of C stocks, fluxes, and isotopic content. Reduced carbon inputs to the soil in degraded pastures, which are less productive than the forests they replace, lead to a reduction in soil carbon inventory and Δ14C, in accord with observations. Managed pastures, which have been fertilized with phosphorous and planted with more productive grasses, show increases in C and14C over forest values. Carbon inventory increases in the upper meter of managed pasture soils are partially offset by predicted carbon losses due to death and decomposition of fine forest roots at depths>1 m in the soil. The major adjustments in soil carbon inventory in response to land management changes occur within the first decade after conversion. Carbon isotopes are shown to be more sensitive indicators of recent accumulation or loss of soil organic matter than d

ISSN:0886-6236    

DOI:10.1029/95GB02148

年代:1995

数据来源: WILEY

摘要:

The emission fluxes and the distribution of dissolved methane (CH4) and carbon dioxide (CO2) were determined for 11 sampling stations in two hydroelectric reservoirs (flooded since 1978 and 1993) located in the James Bay territory of northern Québec. The measured benthic fluxes for the two greenhouse gases were found to be either higher or similar to those determined at the water‐air interface during the ice‐free sampling periods. For the 2 year duration of the study, emission fluxes of CH4to the atmosphere generally varied between 5 and 10 mg m−2d−1, while those for CO2ranged from 500 to 1100 mg m−2d−1. Furthermore, through the use of static chambers at the water‐air interface, we determined that the emission fluxes for the gases are controlled by molecular diffusion. Our calculated fluxes have been separated into two groups: (1) regular emission fluxes and (2) above‐average emission fluxes. The first type comprises the majority of fluxes measured during the sampling periods (i.e., 88% for CH4and 87% for CO2). The second group reflects unusual sampling conditions (e.g., strong winds, water column depths of less than 1 m, or flooded peatland mats floating at the surface). Although data for this group are limited, our preliminary results suggest that they may be an important component in an atmospheric emissions budget for large reservoirs. Concentration profiles for CH4and CO2dissolved in the water column clearly show that oxidation and/or horizontal advection of these gases are controlling factors in their subsequent release to the atmosphere. Most of the CH4is oxidized within the first 25 cm above the flooded soil‐water interface. Consequently, neither benthic emissions of CH4and CO2nor the type of flooded soil appear to control atmospheric emissions of these gases from hydroe

ISSN:0886-6236    

DOI:10.1029/95GB02202

年代:1995

数据来源: WILEY

摘要:

A gridded biospheric carbon model is used to investigate the impact of the atmospheric CO2increase on terrestrial carbon storage. The analysis shows that the calculated CO2fertilization sink is dependent not just on the mathematical formulation of the “β factor” but also on the relative controls of net primary productivity (NPP), carbon residence times, and resource availability. The modeled evolution of the biosphere for the period 1850–1990 shows an increasing lag between NPP and the heterotrophic respiration. The time evolution of the modeled biospheric sink (i.e., difference between enhanced NPP and enhanced respiration) does not match that obtained by deconvolution of the ice core CO2time series. Agreement between the two is reasonable for the first half of the period, but during the recent decades the deconvoluted CO2increase is much too fast to be explained by the CO2fertilization effect only. Therefore other mechanisms than CO2fertilization should also contribute to the missing sink. Our results suggest that about two thirds to three fourths of the 1850–1990 integrated missing sink is due to the CO2greening of the biosphere. The remainder may be due to the increased level of nitrogen deposition starting aro

ISSN:0886-6236    

DOI:10.1029/95GB02381

年代:1995

数据来源: WILEY