bluuebunny/crossref_metadata_2025 huggingface dataset:
Interpreting tracer breakthrough tailing from different forced‐gradient tracer experiment configurations in fractured bedrock
Authors: Becker Matthew W. , Shapiro Allen M.
Published: May 2003

Conceptual and mathematical models are presented that explain tracer breakthrough tailing in the absence of significant matrix diffusion. Model predictions are compared to field results from radially convergent, weak‐dipole, and push‐pull tracer experiments conducted in a saturated crystalline bedrock. The models are based upon the assumption that flow is highly channelized, that the mass of tracer in a channel is proportional to the cube of the mean channel aperture, and the mean transport time in the channel is related to the square of the mean channel aperture. These models predict the consistent −2 straight line power law slope observed in breakthrough from radially convergent and weak‐dipole tracer experiments and the variable straight line power law slope observed in push‐pull tracer experiments with varying injection volumes. The power law breakthrough slope is predicted in the absence of matrix diffusion. A comparison of tracer experiments in which the flow field was reversed to those in which it was not indicates that the apparent dispersion in the breakthrough curve is partially reversible. We hypothesize that the observed breakthrough tailing is due to a combination of local hydrodynamic dispersion, which always increases in the direction of fluid velocity, and heterogeneous advection, which is partially reversed when the flow field is reversed. In spite of our attempt to account for heterogeneous advection using a multipath approach, a much smaller estimate of hydrodynamic dispersivity was obtained from push‐pull experiments than from radially convergent or weak dipole experiments. These results suggest that although we can explain breakthrough tailing as an advective phenomenon, we cannot ignore the relationship between hydrodynamic dispersion and flow field geometry at this site. The design of the tracer experiment can severely impact the estimation of hydrodynamic dispersion and matrix diffusion in highly heterogeneous geologic media.
Simulation of the development of karst aquifers using a coupled continuum pipe flow model
Authors: Liedl Rudolf , Sauter Martin , Hückinghaus Dirk , Clemens Torsten , Teutsch Georg
Published: May 2003

This paper is intended to provide insight into the controlling mechanisms of karst genesis based on an advanced modeling approach covering the characteristic hydraulics in karst systems, the dissolution kinetics, and the associated temporal decrease in flow resistance. Karst water hydraulics is strongly governed by the interaction between a highly conductive low storage conduit network and a low‐conductive high‐storage rock matrix under variable boundary conditions. Only if this coupling of flow mechanisms is considered can an appropriate representation of other relevant processes be achieved, e.g., carbonate dissolution, transport of dissolved solids, and limited groundwater recharge. Here a parameter study performed with the numerical model Carbonate Aquifer Void Evolution (CAVE) is presented, which allows the simulation of the genesis of karst aquifers during geologic time periods. CAVE integrates several important features relevant for different scenarios of karst evolution: (1) the complex hydraulic interplay between flow in the karst conduits and in the small fissures of the rock matrix, (2) laminar as well as turbulent flow conditions, (3) time‐dependent and nonuniform recharge to both flow systems, (4) the widening of the conduits accounting for appropriate physicochemical relationships governing calcite dissolution kinetics. This is achieved by predefining an initial network of karst conduits (“protoconduits”) which are allowed to grow according to the amount of aggressive water available due to hydraulic boundary conditions. The increase in conduit transmissivity is associated with an increase in conduit diameters while the conductivity of the fissured system is assumed to be constant in time. The importance of various parameters controlling karst genesis is demonstrated in a parameter study covering the recharge distribution, the upgradient boundary conditions for the conduit system, and the hydraulic coupling between the conduit network and the rock matrix. In particular, it is shown that conduit diameters increase in downgradient or upgradient direction depending on the spatial distribution (local versus uniform) of the recharge component which directly enters the conduit system.
Interaction between deterministic trend and autoregressive process
Authors: Yue Sheng , Pilon Paul
Published: May 2003

When both an autoregressive (AR) process (stochastic trend) and a deterministic trend (systematic changes over mean) exist within a time series, there may be some interactions between them. This study investigates whether these two components interact with each other. Only time series consisting of a linear trend and a lag 1 autoregressive AR(1) process are explored, which are commonly used in hydrology. Results indicate that (1) the presence of a deterministic trend will overestimate positive serial correlation and underestimate negative serial correlation, while (2) the existence of an AR(1) process does not affect the estimate of the magnitude of the deterministic trend. However, a positive AR(1) will inflate the variance of the trend and a negative AR(1) will shrink the variance of the trend. For a time series with a deterministic trend the trend has to be removed in order to correctly model the stochastic properties of the time series. In the literature, both differencing and detrending have been proposed to fulfill such a task. Their applicability is based on the assumption that they cannot distort the residual series. However, no evidence has been provided to certify whether this assumption is appropriate. This study examines this issue and it indicates that differencing can remove a deterministic trend from a time series but it can seriously damage the existing AR(1) process. In contrast to differencing, detrending can remove a deterministic trend without distorting the existing AR process.
A physical basis for calculating instrument spatial weighting functions in homogeneous systems
Authors: Molz Fred J. , Dinwiddie Cynthia L. , Wilson John L.
Published: May 2003

Starting with a streamline‐based argument, the theoretical basis for a spatial weighting function in a steady, homogenous, and isotropic flow system is developed for both compressible and incompressible flow. A physical interpretation of spatial weighting functions in terms of the ratio of steady state energy dissipation rate per unit volume of porous medium to total energy dissipation rate over the entire flow domain is formulated. Finally, applications of the spatial weighting functions are presented based on calculated gas minipermeameter mass flux fields for the conventional surface probe and a new small drill hole probe configuration. Spatial weighting function distributions indicate that with diverging flow field instruments, such as the gas minipermeameter, porous medium volumes in the inlet vicinity are heavily weighted, with volumes near the seal boundaries shown to be extremely important. The technique described allows one to quantify the size and shape of the subvolume of a domain contributing to a hydraulic conductivity or intrinsic permeability measurement. For heterogeneous systems it is suggested that the spatial weighting function for a given instrument will be heterogeneity dependent and therefore nonunique. However, extending the weighting function concept to heterogeneous and/or transient systems is an important research priority.
A particle‐based model of size or anion exclusion with application to microbial transport in porous media
Authors: Scheibe Timothy D. , Wood Brian D.
Published: May 2003

This paper presents a novel approach to the representation of pore‐scale exclusion processes, based on the truncation of the distribution of local dispersive displacements in a random‐walk particle model. This approach increases the mean velocity of colloidal‐sized particles relative to inert solute tracers, and decreases the apparent dispersion. An equivalent continuum model, with modified velocity and dispersion parameters, is also derived. Both the particle and the equivalent continuum models are applied to the results of laboratory experiments on bacterial transport in intact cores from a research site near Oyster, Virginia. The particle‐based model requires only modest truncation (8% maximum) of the particle velocity distribution function to closely reproduce the significant observed decrease in bacterial arrival times relative to a bromide tracer. The approach provides a conceptually appealing and consistent means of incorporating the exclusion process into groundwater transport models.
The response of river water quality and quantity to the development of irrigated agriculture in the last 4 decades in the Yellow River Basin, China
Authors: Chen Jingsheng , He Dawei , Cui Shubin
Published: May 2003

Irrigation agriculture is a major economic activity throughout the Yellow River Basin and has doubled in area in the last 5 decades. Monitoring data for major ions at 63 monitoring stations in the Yellow River system, ranging from 1960 to 2000, were studied. The concentration of major ions in much of the basin, and especially in downstream of the major irrigation areas of the upper basin, increases significantly between 1960 and 2000. We conclude that this increasing trend is mainly a result of saline irrigation return waters, especially from irrigated lands in arid zones of northern China where sodium increases by 100% and total dissolved solids increases by 29% in the main stem. Since 1972 a critical water quantity issue, “duanliu,” has appeared, which refers to the drying up of the lower reach of the Yellow River. This phenomenon is due mainly to increasing irrigation water withdrawal, which is officially reported to be 91% of all surface water abstraction from the Yellow River. High rates of evapotranspiration and loss to groundwater, especially in the arid irrigated areas in two large irrigation areas in Ningxia and Inner Mongolia, where abstracted water is some 25–30% of annual main stem flow, creates major water losses. There is a need to develop and enforce more rational water quotas as part of a reform of water management in the basin so that basin‐wide management is given at least as much importance as provincial‐level benefits.
Uniform steady free surface flow with recharge in heterogeneous porous formations
Authors: Amir Orna
Published: May 2003

Free surface flow in a randomly heterogeneous medium with recharge is studied in a stochastic setting. A three‐dimensional gravitational flow model with sloped mean uniform flow is used to describe flow in a phreatic aquifer experiencing natural gradient flow, far from wells. The model seeks to investigate the effects of medium heterogeneity and recharge on flow on and below the free surface. Statistical moments of free surface position, head, and specific discharge are derived using a first‐order approximation and compared with their parallel in an unbounded medium. Analytic solutions for the variance of free surface fluctuations and of the specific discharge on the free surface are developed as well as semianalytic solutions for the statistical moments of head and specific discharge beneath the free surface. The effect of recharge, angle of mean free surface and anisotropy on the statistical moments is analyzed.
An experimental study of the effects of density and viscosity contrasts on macrodispersion in porous media
Authors: Kretz V. , Berest P. , Hulin J. P. , Salin D.
Published: May 2003

We study the vertical miscible displacements of two fluids of different densities and/or viscosities in two model porous media, constructed using different arrangements of blocks of packed glass beads with different sizes. The two configurations have the same permeability distributions but different spatial arrangements and structural features. Time variations of the mean fluid concentration in different sections along the samples are monitored by an acoustic technique. For stable viscosity or density contrasts, the spreading of the displacement front is predominantly macrodispersive. For fluids of the same viscosity but different densities, the macrodispersivities approach at large velocities, where the displacement is stable, the passive tracer limit, ld∞, which is controlled only by the heterogeneity of the medium. This is true, regardless of the density contrast. At lower velocities, where gravity instabilities can exist, the normalized dispersivities ld/ld∞ vary exponentially with the normalized flow rate, with opposite exponents in the stable and unstable configurations. These results are compared to existing theoretical works based on stochastic approaches and linear stability analyses.
Methods for using groundwater model predictions to guide hydrogeologic data collection, with application to the Death Valley regional groundwater flow system
Authors: Tiedeman Claire R. , Hill Mary C. , D'Agnese Frank A. , Faunt Claudia C.
Published: May 2003

Calibrated models of groundwater systems can provide substantial information for guiding data collection. This work considers using such models to guide hydrogeologic data collection for improving model predictions by identifying model parameters that are most important to the predictions. Identification of these important parameters can help guide collection of field data about parameter values and associated flow system features and can lead to improved predictions. Methods for identifying parameters important to predictions include prediction scaled sensitivities (PSS), which account for uncertainty on individual parameters as well as prediction sensitivity to parameters, and a new “value of improved information” (VOII) method presented here, which includes the effects of parameter correlation in addition to individual parameter uncertainty and prediction sensitivity. In this work, the PSS and VOII methods are demonstrated and evaluated using a model of the Death Valley regional groundwater flow system. The predictions of interest are advective transport paths originating at sites of past underground nuclear testing. Results show that for two paths evaluated the most important parameters include a subset of five or six of the 23 defined model parameters. Some of the parameters identified as most important are associated with flow system attributes that do not lie in the immediate vicinity of the paths. Results also indicate that the PSS and VOII methods can identify different important parameters. Because the methods emphasize somewhat different criteria for parameter importance, it is suggested that parameters identified by both methods be carefully considered in subsequent data collection efforts aimed at improving model predictions.
Influences of the stream groundwater hydrology on nitrate concentration in unsaturated riparian area bounded by an intermittent Mediterranean stream
Authors: Butturini A. , Bernal S. , Hellin C. , Nin E. , Rivero L. , Sabater S. , Sabater F.
Published: May 2003

Stream aquifer hydrology and nitrate removal were studied, over a period of 2 years, in an unsaturated riparian zone, bounded by an intermittent Mediterranean stream, (Fuirosos, northeastern Spain). The riparian groundwater system is characterized by drastic hydrological changes and by mixing of stream water with hillslope groundwater. The hillslope groundwater flowed through a medium with low hydraulic conductivity (9.6 10−3 < ks < 0.1 m d−1) and low specific discharges (1.7 10−3 < qhll < 15 10−3 m d−1). In contrast, stream water infiltrated through the near stream porous medium with relatively high hydraulic conductivity (4.8 < ks < 19 m d−1) and variable specific discharges (i.e., 0.03 < qst < 1.5 m d−1). An intense and short stream discharge period occurred in autumn, when stream water infiltrated a maximum of 10 m into the riparian zone. Nitrate concentration and nitrate removal spatial rates (ηNO3) showed wide spatial heterogeneity. Higher nitrate concentrations (3.4 NO3‐N mg L−1) and effective nitrate removal (ηNO3 = 0.098 ± 0.04 m−1) were found in the deep groundwater of hillslope zone associated to low water fluxes. In contrast, in the stream edge zone (with higher water fluxes), nitrate release predominated over depletion (ηNO3 = −0.13 ± 0.04 m−1) during the stream discharge period. This opposite pattern of nitrate removal observed in the study area suggests that the depletion of diffuse nitrate inputs in riparian zones bounded by intermittent streams requires careful consideration.
Annual CO2 exchange and CH4 fluxes on a subarctic palsa mire during climatically different years
Authors: Nykänen Hannu , Heikkinen Juha E. P. , Pirinen Leena , Tiilikainen Karoliina , Martikainen Pertti J.
Published: May 2003

Carbon dioxide and methane fluxes in a palsa mire characterized by dry (palsa) and wet surfaces in a subarctic zone in Finland were measured using a static chamber technique during two climatically different years, 1998 and 1999. Each of the 24 collars was individually studied for water table level, peat temperature, pH, vegetation, frost depth, CO2 exchange and CH4 fluxes. The annual gaseous carbon balance varied from −36.9 g C m−2 to −138.6 g C m−2 (uptake) on wet surfaces. During both years, palsa surfaces with shrub vegetation were sinks for carbon, whereas palsa surfaces with sparse vegetation lost carbon to the atmosphere. In 1999, a wet year, a lower NEE resulted mainly from a decrease in photosynthesis. The annual emissions of CH4 ranged from 1.0 g CH4‐C m−2 in the palsa to 24.7 g CH4‐C m−2 at the palsa margin. On wet surfaces, photosynthesis, respiration and CH4 fluxes were tightly linked. Annual NEE and CH4 fluxes were close to the values reported for boreal peatlands (fens).
Transports and budgets of total inorganic carbon in the subpolar and temperate North Atlantic
Authors: Álvarez Marta , Ríos Aida F. , Pérez Fiz F. , Bryden Harry L. , Rosón Gabriel
Published: May 2003

Transports of Total Inorganic Carbon (TIC), Total Alkalinity (TA) and Anthropogenic Carbon (CANT) are calculated across a densely sampled World Ocean Circulation Experiment (WOCE) section at the southern boundary of the subpolar North Atlantic (WOCE A25, 4x cruise). The circulation pattern was approximated using an inverse model constrained with measured mass transports at specific sites, while conserving the mass and salt transports, and forcing the silicate flux to equal the river input north of the section. The mass and chemical fluxes are decomposed into their barotropic, baroclinic and horizontal components. The TA transport is negligible (transport ± maximum estimate of uncertainty, −135 ± 507 kmol s−1), while TIC is transported southwards (−1015 ± 490 kmol s−1) and CANT northwards (116 ± 125 kmol s−1). Combining our results with those from Rosón et al. [2002] across 24.5°N (WOCE A5) we examine the contemporary and preindustrial TIC budgets in the subpolar and temperate North Atlantic based on two different approximations for the budget definitions. Initially, river input, biological production of TIC, along with sedimentation of calcium carbonate are ignored. Then, extended contemporary and preindustrial TIC budgets are discussed including rough estimates of the former processes, mainly based on values from the literature. Our findings point to the North Atlantic Ocean north of 24.5°N as a strong sink for atmospheric CO2 both today (2932 ± 2057 kmol s−1) and preindustrially (2439 ± 1721 kmol s−1). Only 17% of the contemporary CO2 air‐sea uptake corresponds to CANT, which is mainly taken up in the temperate North Atlantic (between the 4x and 24.5°N sections). North of 24.5°N the Atlantic Ocean stores CANT at a rate of 1123 ± 200 kmol s−1. This CANT is mainly advected into the area in the upper limb of the overturning circulation, while 44% is directly introduced by air‐sea uptake.
Seasonal contribution of CO2fluxes in the annual C budget of a northern bog
Authors: Roehm Charlotte L. , Roulet Nigel T.
Published: May 2003

Peatlands are sinks for carbon dioxide (CO2) because net primary production exceeds decomposition. The contribution of non‐growing‐season fluxes to the annual C budget of a peatland is, to date, little studied. We therefore measured the changes in the pattern of carbon exchange with seasons in a bog located in the cool temperate climate region. The growing season CO2‐C uptake was of −113 g m−2. During the non‐growing season, 36 g C m−2was lost to the atmosphere, resulting in an estimated net ecosystem production of −76 g C m−2. Despite the non‐growing‐season loss equaling 33 to 40% of the summer uptake, the net annual accumulation of was 3 times the long‐term average net accumulation rate usually cited in the literature. The high rate of non‐growing‐season efflux could be supported directly by temporal concurrent respiration and the release of stored CO2from prior production. These results indicate the need to revise current models to address peat thermal properties inducing CO2production at lower temperature ranges.
Worldwide distribution of continental rock lithology: Implications for the atmospheric/soil CO2 uptake by continental weathering and alkalinity river transport to the oceans
Authors: Amiotte Suchet Philippe , Probst Jean‐Luc , Ludwig Wolfgang
Published: May 2003

The silicate rock weathering followed by the formation of carbonate rocks in the ocean, transfers CO2 from the atmosphere to the lithosphere. This CO2 uptake plays a major role in the regulation of atmospheric CO2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks. This leads us to develop the first world lithological map with a grid resolution of 1° × 1°. This paper analyzes the spatial distribution of the six main rock types by latitude, continents, and ocean drainage basins and for 49 large river basins. Coupling our digital map with the GEM‐CO2 model, we have also calculated the amount of atmospheric/soil CO2 consumed by rock weathering and alkalinity river transport to the ocean. Among all silicate rocks, shales and basalts appear to have a significant influence on the amount of CO2 uptake by chemical weathering.
Space and time variation of δ18O and δD in Antarctic precipitation revisited
Authors: Kavanaugh J. L. , Cuffey Kurt M.
Published: May 2003

To better understand the variations over time of precipitation water isotopes measured in polar ice cores, we have developed an intermediate complexity model (ICM) of atmospheric water vapor transport and associated isotopic distillation. The model builds directly from earlier work by D. Fisher and M. Hendricks and is calibrated against the measured modern spatial distribution of δD and deuterium excess (d). Model improvements include a correction to the equation governing advective transport, which solves a major puzzle arising from the earlier work. The new model shows isotopic data to be consistent with dominance of eddy diffusive transport at high latitudes. Model experiments are used to show that a wide variety of climate changes in subtropical source regions can affect Antarctic d and that such d changes are consistently anticorrelated with changes of δD. Magnitudes of ΔδD/Δd are ∼−1 to −5 and are much higher at the ice sheet margin than in the interior of the continent. Changes in the relative dominance of diffusive and advective transport are also shown to cause anticorrelated changes of similar magnitude. Isotopic sensitivities to temperature change (ΔδD/ΔT) are shown to also vary spatially, with low values in marginal zones and higher values inland. Effects of ocean surface relative humidity on Antarctic d are shown to be nearly uniform across the continent (in contrast to earlier results given by Petit et al. [1991]) but quantitatively small. The method used by Vimeux et al. [2001] for folding humidity effects into source temperature effects is supported. We draw attention to a pervasive misapplication of marine composition corrections to Antarctic δD records. Arguments are made in support of simple interpretations of calibrated ice core isotopic time series in terms of temperature changes despite the potential complexities in the system.
Isotopic trends and background fluxes of atmospheric lead in northern Europe: Analyses of three ombrotrophic bogs from south Sweden
Authors: Klaminder Jonatan , Renberg Ingemar , Bindler Richard , Emteryd Ove
Published: May 2003

Knowledge about the natural atmospheric background deposition rate of lead (Pb) prior to anthropogenic pollution is critical in the understanding of present‐day pollution and for establishing realistic goals for the reduction of atmospheric Pb. We utilize stable Pb isotopes (206Pb and207Pb) in radiocarbon‐dated peat cores from three ombrotrophic bogs from south Sweden, to calculate fluxes and to survey atmospheric Pb trends prior 3500 BP (the so far known onset of large‐scale anthropogenic pollution). The estimated atmospheric Pb deposition rate was between 1 and 10 μg Pb m2yr−1between 5900 and 3700 calendar years BP, which is 100 to 1000 times lower than present‐day deposition rates. The majority of the samples older than 3500 calendar years BP had206Pb/207Pb ratios ≤1.20, which is significantly lower than unpolluted Swedish mineral soils (206Pb/207Pb > 1.30), suggesting that even the natural atmospheric deposition of Pb was dominated by long‐range transport, rather than local inputs from soil dust. Low206Pb/207Pb ratios (1.16–1.18) of several samples indicate that this distant transport originated at least partly from early pollution sources. A possible climatic connection with the observed Pb deposition trends is suggested.
Sustainability of terrestrial carbon sequestration: A case study in Duke Forest with inversion approach
Authors: Luo Yiqi , White Luther W. , Canadell Josep G. , DeLucia Evan H. , Ellsworth David S. , Finzi Adrien , Lichter John , Schlesinger William H.
Published: May 2003

A sound understanding of the sustainability of terrestrial carbon (C) sequestration is critical for the success of any policies geared toward stabilizing atmospheric greenhouse concentrations. This includes the Kyoto Protocol and/or other greenhouse strategies implemented by individual countries. However, the sustainability of C sinks and pools has not been carefully studied with either empirical or theoretical approaches. This study was intended to develop a conceptual framework to define the sustainability based on C influx and residence time (τ). The latter τ quantifies the capacity for C storage in various plant and soil pools. We estimated τ via inverse analysis of multiple data sets from a Free‐Air CO2 Enrichment (FACE) experiment in Duke Forest, North Carolina, United States. This study suggested that estimated residence times at elevated CO2 decreased for plant C pools and increased for litter and soil pools in comparison to those at ambient CO2. The ensemble of the residence times from all the pools at elevated CO2, however, was well correlated with that at ambient CO2. We then used the estimated residence times, combined with C influx, to simulate C sequestration rates in response to a gradual increase in atmospheric CO2 concentration (Ca). The simulated C sequestration rate gradually increased from 69 g m−2 yr−1 in 2000 when Ca was 378 ppm to 201 g m−2 yr−1 in 2100 when Ca was at 710 ppm. Thus, the current evidence from both experimental observations and inverse analysis suggested that C sequestration in the forest ecosystem was likely to increase gradually as Ca gradually increases. The model projection of the C sequestration will improve as more data on long‐term processes become available in coming years. In addition, such a modeled increase in terrestrial C sequestration is too small to balance the anthropogenic C emission.
Dissolved inorganic phosphorus, dissolved iron, and Trichodesmium in the oligotrophic South China Sea
Authors: Wu Jingfeng , Chung Shi‐Wei , Wen Liang‐Saw , Liu Kon‐Kee , Chen Yuh‐ling Lee , Chen Houng‐Yung , Karl David M.
Published: May 2003

Dissolved inorganic phosphorus (DIP) concentrations in the oligotrophic surface waters of the South China Sea decrease from ∼20 nM in March 2000 to ∼5 nM in July 2000, in response to seasonal water column stratification. These minimum DIP concentrations are one order of magnitude higher than those in the P‐limited, iron‐replete stratified surface waters of the western North Atlantic, suggesting that the ecosystem in the South China Sea may be limited by bioavailable nitrogen or some trace nutrient rather than DIP. Nutrient enrichment experiments using either nitrate, phosphate or both indicate that nitrogen limits the net growth of phytoplankton in the South China Sea, at least during March and July 2000. The fixed nitrogen limitation may result from the excess phosphate (N:P<16) transported into the South China Sea from the North Pacific relative to microbial population needs, or from iron control of nitrogen fixation. The iron‐limited nitrogen fixation hypothesis is supported by the observation of low population densities of Trichodesmium spp. (<48 × 103 trichomes/m3), the putative N2 fixing cyanobacterium, and with low concentrations of dissolved iron (∼0.2–0.3 nM) in the South China Sea surface water. Our results suggest that nitrogen fixation can be limited by available iron even in regions with a high rate of atmospheric dust deposition such as in the South China Sea.
Effects of CO2 and nutrient availability on mineral weathering in controlled tree growth experiments
Authors: Williams Erika L. , Walter Lynn M. , Ku Timothy C. W. , Kling George W. , Zak Donald R.
Published: May 2003

We sought to determine the effect of elevated atmospheric CO2 on mineral weathering reactions in midlatitude carbonate‐bearing forest soils of differing nutrient availability. Increased plant growth and soil respiration under elevated atmospheric CO2 suggest increased rates of carbon cycling, which may affect mineral weathering. A randomized complete block experiment was conducted, where aspen and maple saplings were grown in open top chambers under two levels of atmospheric CO2 and soil N. Soil solution chemistry and soil gas PCO2 profiles beneath aspen were collected from planting (1997) to harvest (1999). Carbonate mineral weathering products (Ca2+, Mg2+, HCO3−) dominated solutions, which were saturated with respect to calcite. Soil PCO2 values at 25 cm depth were 41% higher in high N soils, but CO2 treatment was not significant. An ANOVA model tested treatment effects on spring 1998 solution chemistry. CO2 treatment had a significant effect on DIC, which was 12% higher in elevated than ambient CO2 chambers. Little effect of CO2 treatment was observed in low N soils. In high N soils, solutions had higher concentrations of carbonate weathering products (DIC, 15%; HCO3−, 27%; Ca2+, 3%, not significant; Mg2+, 5%, not significant). Soil N availability had a significant, positive, effect on mean concentrations of Ca2+, Mg2+, K+, Na+, NO3−, SO42−, and DOC. The soil N treatment difference in solutes may result from differences in PCO2 and, additionally, NO3− from organic matter decomposition. Our results suggest that increased carbonate weathering may occur under increased atmospheric CO2 and in fertile soils.
Effects of patchy ocean fertilization on atmospheric carbon dioxide and biological production
Authors: Gnanadesikan Anand , Sarmiento Jorge L. , Slater Richard D.
Published: May 2003

Increasing oceanic productivity by fertilizing nutrient‐rich regions with iron has been proposed as a mechanism to offset anthropogenic emissions of carbon dioxide. Earlier studies examined the impact of large‐scale fertilization of vast reaches of the ocean for long periods of time. We use an ocean general circulation model to consider more realistic scenarios involving fertilizing small regions (a few hundred kilometers on a side) for limited periods of time (of order 1 month). A century after such a fertilization event, the reduction of atmospheric carbon dioxide is between 2% and 44% of the initial pulse of organic carbon export to the abyssal ocean. The fraction depends on how rapidly the surface nutrient and carbon fields recover from the fertilization event. The modeled recovery is very sensitive to the representation of biological productivity and remineralization. Direct verification of the uptake would be nearly impossible since changes in the air‐sea flux due to fertilization would be much smaller than those resulting from natural spatial variability. Because of the sensitivity of the uptake to the long‐term fate of the iron and organic matter, indirect verification by measurement of the organic matter flux would require high vertical resolution and long‐term monitoring. Finally, the downward displacement of the nutrient profile resulting from an iron‐induced productivity spurt may paradoxically lead to a long‐term reduction in biological productivity. In the worst‐case scenario, removing 1 ton of carbon from the atmosphere for a century is associated with a 30‐ton reduction in biological export of carbon.
Effect of elevated CO2 on the community metabolism of an experimental coral reef
Authors: Langdon Chris , Broecker Wallace S. , Hammond Douglas E. , Glenn Edward , Fitzsimmons Kevin , Nelson Steven G. , Peng Tsung‐Hung , Hajdas Irka , Bonani Georges
Published: May 2003

The effect of elevated pCO2 on the metabolism of a coral reef community dominated by macroalgae has been investigated utilizing the large 2650 m3 coral reef mesocosm at the Biosphere‐2 facility near Tucson, Arizona. The carbonate chemistry of the water was manipulated to simulate present‐day and a doubled CO2 future condition. Each experiment consisted of a 1–2 month preconditioning period followed by a 7–9 day observational period. The pCO2 was 404 ± 63 μatm during the present‐day pCO2 experiment and 658 ± 59 μatm during the elevated pCO2 experiment. Nutrient levels were low and typical of natural reefs waters (NO3− 0.5–0.9 μM, NH4+ 0.4 μM, PO43− 0.07–0.09 μM). The temperature and salinity of the water were held constant at 26.5 ± 0.2°C and 34.4 ± 0.2 ppt. Photosynthetically available irradiance was 10 ± 2 during the present‐day experiment and 7.4 ± 0.5 mol photons m−2 d−1 during the elevated pCO2 experiment. The primary producer biomass in the mesocosm was dominated by four species of macroalgae; Haptilon cubense, Amphiroa fragillisima, Gelidiopsis intricata and Chondria dasyphylla. Algal biomass was 10.4 mol C m−2 during the present‐day and 8.7 mol C m−2 and during the elevated pCO2 experiments. As previously observed, the increase in pCO2 resulted in a decrease in calcification from 0.041 ± 0.007 to 0.006 ± 0.003 mol CaCO3 m−2 d−1. Net community production (NCP) and dark respiration did not change in response to elevated pCO2. Light respiration measured by a new radiocarbon isotope dilution method exceeded dark respiration by a factor of 1.2 ± 0.3 to 2.1 ± 0.4 on a daily basis and by 2.2 ± 0.6 to 3.9 ± 0.8 on an hourly basis. The 1.8‐fold increase with increasing pCO2 indicates that the enhanced respiration in the light was not due to photorespiration. Gross production (GPP) computed as the sum of NCP plus daily respiration (light + dark) increased significantly (0.24 ± 0.03 vs. 0.32 ± 0.04 mol C m−2 d−1). However, the conventional calculation of GPP based on the assumption that respiration in the light proceeds at the same rate as the dark underestimated the true rate of GPP by 41–100% and completely missed the increased rate of carbon cycling due to elevated pCO2. We conclude that under natural, undisturbed, nutrient‐limited conditions elevated CO2 depresses calcification, stimulates the rate of turnover of organic carbon, particularly in the light, but has no effect on net organic production. The hypothesis that an increase pCO2 would produce an increase in net production that would counterbalance the effect of decreasing saturation state on calcification is not supported by these data.
Influence of soil texture on carbon dynamics and storage potential in tropical forest soils of Amazonia
Authors: Telles Everaldo de Carvalho Conceição , de Camargo Plínio Barbosa , Martinelli Luiz A. , Trumbore Susan E. , da Costa Enir Salazar , Santos Joaquim , Higuchi Niro , Oliveira Raimundo Cosme
Published: May 2003

Stable and radiocarbon isotopes were used to investigate the role of soil clay content in the storage and dynamics of soil carbon in tropical forest soils. Organic matter in clay‐rich Oxisols and Ultisols contains at least two distinct components: (1) material with light δ13C signatures and turnover times of decades or less; and (2) clay‐associated, 13C‐enriched, carbon with turnover times of decades at the surface to millennia at depths >20 cm. Soil texture, in this case clay content, exerts a major control on the amount of slowly cycling carbon and therefore influences the storage and dynamics of carbon in tropical forest soils. Soils in primary tropical forest have been proposed as a potentially large sink for anthropogenic carbon. Comparison of carbon stocks in Oxisols sampled near Manaus, Brazil, shows no measurable change in organic carbon stocks over the past 20 years. Simple models estimating the response of soil carbon pools to a sustained 0.5% yr−1 increase in productivity result in C storage rates of 0.09 to 0.13 MgC ha−1 yr−1 in soil organic matter, with additional potential storage of 0.18 to 0.27 MgC ha−1 yr−1 in surface litter and roots. Most storage occurs in organic matter pools with turnover times less than a decade. Export of carbon in dissolved form from upland terra firme Oxisols likely accounts for <0.2 MgC ha−1 yr−1, but more work is required to assess the export potential for periodically inundated Spodosols.
Connecting methane fluxes to vegetation cover and water table fluctuations at microsite level: A modeling study
Authors: Kettunen Anu
Published: May 2003

The paper presents a process‐based model describing methane flux dynamics in different microsites of boreal peatlands. The simulated fluxes matched well to measurements in all microsites without any parameter adjustment. The model emphasizes the importance of microsite characteristics, water table and vegetation cover for methane fluxes. Water level determines the moisture and oxygen profile in peat matrix and therefore affects methane production and oxidation rates in peat profile. Vascular plants provide methanogenesis with substrates, form a pathway for methane to liberate from peat to the atmosphere and enhance methane oxidation by transporting oxygen to water saturated peat. The model connects methane fluxes to the seasonal photosynthetic cycle of plants at the microsite level and hence dynamically combines the microbial processes in peat to changing environmental factors in the level of peatland ecosystem. Sensitivity analysis of the model reveals the importance of substrate supply to methane fluxes. In addition, the capability of the vascular plants to transport oxygen downwards has a large effect on model outcome. Lack of oxygen and methane keep methane oxidation at a low level in the model simulations, and changes that compensate for these lacks have a remarkable decreasing effect on simulated flux. Dry periods decrease the simulated methane flux considerably, especially if the drought prevails long, threshold for a dramatic decrease lying between 4 and 6 weeks of drought. Increase in air temperature enhances methane flux, especially if the effect of increased temperature on gross primary production is taken into account.
Substrate regulation of soil respiration in a tallgrass prairie: Results of a clipping and shading experiment
Authors: Wan Shiqiang , Luo Yiqi
Published: May 2003

Changes in soil respiration, one of the major fluxes of global carbon cycling, could significantly slow down or accelerate the increase in atmospheric CO2, with consequent feedbacks to climate change. It is critical to understand how substrate availability regulates soil respiration in projecting the response of carbon cycling to changed climate. We conducted a clipping and shading experiment for 1 year in a tallgrass prairie of the Great Plains, United States, to manipulate substrate supply to soil respiration. Our results showed that reduced substrate supply under clipping and/or shading significantly decreased soil respiration at all the timescales (diurnal, transient, and annual) irrespective of the minor concurrent changes in soil temperature and moisture. Annual mean soil respiration decreased significantly by 33, 23, and 43% for the clipping, shading, and clipping plus shading treatments, respectively. Temperature sensitivity of soil respiration decreased from 1.93 in the control plots to 1.88, 1.75, and 1.83 in the clipped, shaded, and clipped plus shaded plots, respectively. Rhizosphere respiration, respiration from decomposition of aboveground litter, and respiration from oxidation of soil organic matter and dead roots accounted for 30, 14, and 56% of annual mean soil respiration, respectively. Rhizosphere respiration was more sensitive to temperature than the other two components. Our results suggest a critical role of substrate supply in regulating soil respiration and its temperature sensitivity.
Isotopic evidence for the role of plant development on transpiration in deciduous forests of southern United States
Authors: Mora Germán , Jahren A. Hope
Published: May 2003

We evaluated D/H ratios of soil‐ and plant‐extracted water during the 1997 growing season to assess the influence of temperature, humidity, and rainfall on water distribution in deciduous forests. Three state parks (Chicot in Arkansas; Natchez in Mississippi, and St. Bernard in Louisiana) were identified along a 13.5‐cm precipitation gradient established during the studied growing season within the Mississippi River basin. Samples were collected for isotopic determinations from five to six species at each site early (March) and late (June) in the growing season. To capture the isotopic variability in water sources, samples of rainwater, groundwater, and soil water were collected. Isotopic results for rainwater showed an average increase of 4‰ from March to June. This increase did not transfer to soil water: soil water δD values throughout the growing season showed values close to those measured for March rainwater. In contrast, leaf water showed δD values that were 15‰ to 20‰ higher in March compared to June δD values. Elevated March δD values in leaf water were observed in virtually all species at the three sites. Change in leaf water δD value during the growing season was not correlated with precipitation rate, temperature, humidity, or changes in atmospheric water vapor isotopic composition. We propose that this widespread March isotopic enrichment resulted from enhanced evaporative demand induced by accelerated plant growth early in the growing season. This suggestion implies a decoupling of environmental factors and plant response, pointing to the important role of plant developmental timing in ecosystem functioning.
Interannual variability in the peatland‐atmosphere carbon dioxide exchange at an ombrotrophic bog
Authors: Lafleur Peter M. , Roulet Nigel T. , Bubier Jill L. , Frolking Steve , Moore Tim R.
Published: May 2003

Eddy covariance measurements of net ecosystem carbon dioxide (CO2) exchange (NEE) were taken at an ombrotrophic bog near Ottawa, Canada from 1 June 1998 to 31 May 2002. Temperatures during this period were above normal except for 2000 and precipitation was near normal in 1998 and 1999, above normal in 2000, and well below normal in 2001. Growing period maximum daytime uptake (−0.45 mg CO2 m−2 s−1) was similar in all years and nighttime maximum respiration was typically near 0.20 mg CO2 m−2 s−1, however, larger values were recorded during very dry conditions in the fourth year of study. Winter CO2 flux was considerably smaller than in summer, but persistent, resulting in significant accumulated losses (119–132 g CO2 m−2 period−1). This loss was equivalent to between 30 and 70% of the net CO2 uptake during the growing season. During the first 3 years of study, the bog was an annual sink for CO2 (∼−260 g CO2 m−2 yr−1). In the fourth year, with the dry summer, however, annual NEE was only −34 g CO2 m−2 yr−1, which is not significantly different from zero. We examined the performance of a peatland carbon simulator (PCARS) model against the tower measurements of NEE and derived ecosystem respiration (ER) and photosynthesis (PSN). PCARS ER and PSN were highly correlated with tower‐derived fluxes, but the model consistently overestimated both ER and PSN, with slightly poorer comparisons in the dry year. As a result of both component fluxes being overestimated, PCARS simulated the tower NEE reasonably well. Simulated decomposition and autotrophic respiration contributed about equal proportions to ER. Shrubs accounted for the greatest proportion of PSN (85%); moss PSN declined to near zero during the summer period due to surface drying.
Eddy‐driven sources and sinks of nutrients in the upper ocean: Results from a 0.1° resolution model of the North Atlantic
Authors: McGillicuddy D. J. , Anderson L. A. , Doney S. C. , Maltrud M. E.
Published: May 2003

A nitrate‐based model of new production is incorporated into eddy‐resolving (0.1°) simulations of the North Atlantic. The biological model consists of light and nutrient limited production within the euphotic zone and relaxation of the nitrate field to climatology below. Sensitivity of the solutions to the parameters of the biological model is assessed in a series of simulations. Model skill is quantitatively evaluated with observations using an objective error metric; simulated new production falls within the range of observed values at several sites throughout the basin. Results from the “best fit” model are diagnosed in detail. Mean and eddying components of the nutrient fluxes are separated via Reynolds decomposition. In the subtropical gyre, eddy‐driven vertical advection of nutrients is sufficient to overcome the mean wind‐driven downwelling in the region and fuels a significant fraction of the annual new production in that area. In contrast, eddies constitute a net sink of nutrients in the subpolar gyre. Geostrophic adjustment to deep winter convection through mesoscale processes causes a net flux of nutrients out of the euphotic zone; the magnitude of this sink is sufficient to counterbalance the mean wind‐driven upwelling of nutrients over much of the region. On the basis of these simulations it appears that the oceanic mesoscale has major impacts on nutrient supply to, and removal from, the euphotic zone.
Dynamics of plant‐mediated organic matter and nutrient cycling following water‐level drawdown in boreal peatlands
Authors: Laiho Raija , Vasander Harri , Penttilä Timo , Laine Jukka
Published: May 2003

If boreal peatlands face drought more often due to climatic warming, the responses of vegetation may drastically change the functions of the ecosystem. We assessed the effects of water‐level drawdown on plant‐mediated organic matter (OM) and nutrient fluxes in a chronosequence of undrained and drained, originally sparsely treed fens. The chronosequence mimicked the reduced growing season moisture predicted by current climate change scenarios. In a pristine state, OM and nutrient fluxes were characterized by annual cycling through graminoids and mosses. Water‐level drawdown initiated a “forest succession,” in which the OM and nutrient cycles shifted from being dominated by graminoids and mosses to dominance by arboreal vegetation in two decades. Simultaneously, the quantity and tissue type composition of annual litterfall, as well as the quantity and allocation of annual nutrient uptake, changed radically. The changes may have contrasting but as yet unexplored implications for the carbon and nutrient balances of these sites.