bluuebunny/crossref_metadata_2025_split huggingface dataset:
Imaging magnetic sources using Euler's equation
Authors: Hsu Shu‐Kun 
Published: March 2003

ABSTRACTThe conventional Euler deconvolution method has the advantage of being independent of magnetization parameters in locating magnetic sources and estimating their corresponding depths. However, this method has the disadvantage that a suitable structural index must be chosen, which may cause spatial diffusion of the Euler solutions and bias in the estimation of depths to the magnetic sources. This problem becomes more serious when interfering anomalies exist. The interpretation of the Euler depth solutions is effectively related to the model adopted, and different models may have different structural indices. Therefore, I suggest a combined inversion for the structural index and the source location from the Euler deconvolution, by using only the derivatives of the magnetic anomalies. This approach considerably reduces the diffusion problem of the location and depth solutions. Consequently, by averaging the clustered solutions satisfying a given criterion for the solutions, we can image the depths and attributes (or types) of the causative magnetic sources. Magnetic anomalies acquired offshore northern Taiwan are used to test the applicability of the proposed method.
Effects of pore aspect ratios on velocity prediction from well‐log data
Authors: Yan Jun , Li Xiang‐Yang , Enru Liu 
Published: March 2003

ABSTRACTWe develop a semi‐empirical model which combines the theoretical model of Xu and White and the empirical formula of Han, Nur and Morgan in sand–clay environments. This new model may be used for petrophysical interpretation of P‐ and S‐wave velocities. In particular, we are able to obtain an independent estimation of aspect ratios based on log data and seismic velocity, and also the relationship between velocities and other reservoir parameters (e.g. porosity and clay content), thus providing a prediction of shear‐wave velocity. To achieve this, we first use Kuster and Toksöz's theory to derive bulk and shear moduli in a sand–clay mixture. Secondly, Xu and White's model is combined with an artificial neural network to invert the depth‐dependent variation of pore aspect ratios. Finally these aspect ratio results are linked to the empirical formula of Han, Nur and Morgan, using a multiple regression algorithm for petrophysical interpretation. Tests on field data from a North Sea reservoir show that this semi‐empirical model provides simple but satisfactory results for the prediction of shear‐wave velocities and the estimation of reservoir parameters.
Carbon cycling in cultivated land and its global significance
Authors: Buyanovsky Gregory A. , Wagner George H. 
Published: March 2003

AbstractLong‐term data from Sanborn Field, one of the oldest experimental fields in the USA, were used to determine the direction of soil organic carbon (SOC) dynamics in cultivated land. Changes in agriculture in the last 50 years including introduction of more productive varieties, wide scale use of mineral fertilizers and reduced tillage caused increases in total net annual production (TNAP), yields and SOC content. TNAP of winter wheat more than doubled during the last century, rising from 2.0–2.5 to 5–6 Mg ha–1 of carbon, TNAP of corn rose from 3–4 to 9.5–11.0 Mg ha–1 of carbon. Amounts of carbon returned annually with crop residues increased even more drastically, from less than 1 Mg ha–1 in the beginning of the century to 3–3.5 Mg ha–1 for wheat and 5–6 Mg ha–1 for corn in the 90s. These amounts increased in a higher proportion because in the early 50s removal of postharvest residues from the field was discontinued. SOC during the first half of the century, when carbon input was low, was mineralized at a high rate: 89 and 114 g m–2 y–1 under untreated wheat and corn, respectively. Application of manure decreased losses by half, but still the SOC balance remained negative. Since 1950, the direction of the carbon dynamics has reversed: soil under wheat monocrop (with mineral fertilizer) accumulated carbon at a rate about 50 g m–2 y–1, three year rotation (corn/wheat/clover) with manure and nitrogen applications sequestered 150 g m2 y–1 of carbon. Applying conservative estimates of carbon sequestration documented on Sanborn Field to the wheat and corn production area in the USA, suggests that carbon losses to the atmosphere from these soils were decreased by at least 32 Tg annually during the last 40–50 years. Our computations prove that cultivated soils under proper management exercise a positive influence in the current imbalance in the global carbon budget.
A free‐air enrichment system for exposing tall forest vegetation to elevated atmospheric CO2
Authors: Hendrey George R. , Ellsworth David S. , Lewin Keith F. , Nagy JohN. 
Published: March 2003

AbstractA free‐air CO2 enrichment (FACE) system was designed to permit the experimental exposure of tall vegetation such as stands of forest trees to elevated atmospheric CO2 concentrations ([CO2]a) without enclosures that alter tree microenvironment. We describe a prototype FACE system currently in operation in forest plots in a maturing loblolly pine (Pinus taeda L.) stand in North Carolina, USA. The system uses feedback control technology to control [CO2] in a 26 m diameter forest plot that is over 10 m tall, while monitoring the 3D plot volume to characterize the whole‐stand CO2 regime achieved during enrichment. In the second summer season of operation of the FACE system, atmospheric CO2 enrichment was conducted in the forest during all daylight hours for 96.7% of the scheduled running time from 23 May to 14 October with a preset target [CO2] of 550 μmol mol–1, ≈ 200 μmol mol–1 above ambient [CO2]. The system provided spatial and temporal control of [CO2] similar to that reported for open‐top chambers over trees, but without enclosing the vegetation. The daily average daytime [CO2] within the upper forest canopy at the centre of the FACE plot was 552 ± 9 μmol mol–1 (mean ± SD). The FACE system maintained 1‐minute average [CO2] to within ± 110 μmol mol–1 of the target [CO2] for 92% of the operating time. Deviations of [CO2] outside of this range were short‐lived (most lasting < 60 s) and rare, with fewer than 4 excursion events of a minute or longer per day. Acceptable spatial control of [CO2] by the system was achieved, with over 90% of the entire canopy volume within ± 10% of the target [CO2] over the exposure season. CO2 consumption by the FACE system was much higher than for open‐top chambers on an absolute basis, but similar to that of open‐top chambers and branch bag chambers on a per unit volume basis. CO2 consumption by the FACE system was strongly related to windspeed, averaging 50 g CO2 m–3 h–1 for the stand for an average windspeed of 1.5 m s–1 during summer. The [CO2] control results show that the free‐air approach is a tractable way to study long‐term and short‐term alterations in trace gases, even within entire tall forest ecosystems. The FACE approach permits the study of a wide range of forest stand and ecosystem processes under manipulated [CO2]a that were previously impossible or intractable to study in true forest ecosystems.
Do open‐top chambers overestimate the effects of rising CO2 on plants? An analysis using spring wheat
Authors: Van Oijen M. , Schapendonk A. H. C. M. , Jansen M. J. H. , Pot C. S. , Maciorowski R. 
Published: March 2003

AbstractThe microclimate in facilities for studying effects of elevated CO2 on crops differs from ambient conditions. Open‐top chambers (OTCs) increase temperature by 1–3 °C. If temperature and CO2 interact in their effect on crops, this would limit the value of OTC experiments. Furthermore, interaction of CO2 and temperature deserves study because increases in atmospheric CO2 concentration are expected to cause global warming.This paper describes two experiments in which a recently developed cooling system for OTCs was used to analyse the effects of temperature on photosynthesis, growth and yield of spring wheat (Triticum aestivum L., cv. Minaret). Two levels of CO2 were used (350 and 700 ppm), and two levels of temperature, with cooled OTCs being 1.6–2.4 °C colder than noncooled OTCs.Photosynthetic rates were increased by elevated CO2, but no effect of temperature was found. Cross‐switching CO2 concentrations as well as determination of A–Ci curves showed that plant photosynthetic capacity after anthesis acclimated to elevated CO2. The acclimation may be related to the effects of CO2 on tissue composition: elevated CO2 decreased leaf nitrogen concentrations and increased sugar content. Calculations of the seasonal mean crop light‐use efficiency (LUE) were consistent with the photosynthesis data in that CO2 increased LUE by 20% on average whereas temperature had no effect. Both elevating CO2 and cooling increased grain yield, by an average of 11% and 23%, respectively. CO2 and temperature stimulated yield via different mechanisms: CO2 increased photosynthetic rate, but decreased crop light interception capacity (LAI), whereas cooling increased grain yield by increasing LAI and extending the growing season with 10 days. The effects of CO2 and temperature were not additive: the CO2 effect was about doubled in the noncooled open‐top chambers. In most cases, effects on yield were mediated through increased grain density rather than increased individual grain weights.The higher growth response to elevated CO2 in noncooled vs. cooled OTCs shows that a cooling system may remove a bias towards overestimating crop growth response to CO2 in open‐top chambers.
Carbon balance of a boreal patterned peatland
Authors: Waddington J. M. , Roulet N. T. 
Published: March 2003

SummaryMeasurements of the spatial variability of methane (CH4) emissions, net CO2 ecosystem exchange (NEE), and dissolved carbon (CH4, CO2, and DOC) were made in a boreal patterned peatland in northern Sweden in the summers (May to September) of 1992 and 1993. Carbon balance terms were measured and the carbon balance inferred at different peatland surface topography features (e.g. ridges, lawns, and pools) and at different positions within the peatland (e.g. plateau, margin). Combining these data permits a comparison of the carbon balance at the peatland scale for the two field seasons.Trends in the spatial variability of the net carbon storage, as determined by the difference between inputs and outputs, suggest that carbon storage decreased in lawns from the margin of the peatland to the central plateau, while the reverse trend occurred in ridges. This indicates a difference in carbon exchange processes between sites with different surface topography due to differences in soil moisture and temperature.Total carbon storage for the peatland, weighted for topographic variability, indicates that the peatland gained carbon in 1992 (2.0 g C m− 2), but lost carbon in 1993 ( −  7.6 g C m− 2). There was little variation in mean seasonal air temperature and total precipitation between the two years suggesting that the timing and magnitude of temperature and precipitation variation within the growing season are important for the season carbon balance. Because the carbon storage differences were small relative to the potential errors we conclude that the peatland was neither a net sink nor source of atmospheric carbon. This research demonstrates the importance of position in a peatland for the inference of long‐term carbon accumulation and the assessment of contemporary exchange rates.
A new global 1‐km dataset of percentage tree cover derived from remote sensing
Authors: Defries R. S. , Hansen M. C. , Townshend J. R. G. , Janetos A. C. , Loveland T. R. 
Published: March 2003

SummaryAccurate assessment of the spatial extent of forest cover is a crucial requirement for quantifying the sources and sinks of carbon from the terrestrial biosphere. In the more immediate context of the United Nations Framework Convention on Climate Change, implementation of the Kyoto Protocol calls for estimates of carbon stocks for a baseline year as well as for subsequent years. Data sources from country level statistics and other ground‐based information are based on varying definitions of ‘forest’ and are consequently problematic for obtaining spatially and temporally consistent carbon stock estimates. By combining two datasets previously derived from the Advanced Very High Resolution Radiometer (AVHRR) at 1 km spatial resolution, we have generated a prototype global map depicting percentage tree cover and associated proportions of trees with different leaf longevity (evergreen and deciduous) and leaf type (broadleaf and needleleaf). The product is intended for use in terrestrial carbon cycle models, in conjunction with other spatial datasets such as climate and soil type, to obtain more consistent and reliable estimates of carbon stocks. The percentage tree cover dataset is available through the Global Land Cover Facility at the University of Maryland at http://glcf.umiacs.umd.edu.
Biosynthesis of plant phenolic compounds in elevated atmospheric CO2
Authors: Hartley Susan E. , Jones Clive G. , Couper Gordon C. , Jones T. Hefin 
Published: March 2003

SummaryExperiments were carried out to determine the effects of elevated atmospheric carbon dioxide (CO2) on phenolic biosynthesis in four plant species growing over three generations for nine months in a model plant community. Results were compared to those obtained when the same species were grown individually in pots in the same soils and controlled environment. In the model herbaceous plant community, only two of the four species showed any increase in biomass under elevated CO2, but this occurred only in the first generation for Spergula arvensis and in the second generation for Poa annua. Thus, the effects of CO2 on plant biomass and carbon and nitrogen content were species‐ and generation‐specific. The activity of the principle phenolic biosynthetic enzyme, phenylalanine ammonia lyase (PAL), increased under elevated CO2 in Senecio vulgaris only in Generation 1, but increased in three of the four plant species in Generation 2. There were no changes in the total phenolic content of the plants, except for P. annua in Generation 1. Lignin content decreased under elevated CO2 in Cardamine hirsuta in Generation 1, but increased in Generation 2, whilst the lignin content of P. annua showed no change, decreased, then increased in response to elevated CO2 over the three generations. When the species were grown alone in pots, elevated CO2 increased PAL activity in plants grown in soil taken from the Ecotron community after nine months of plant growth, but not in plants grown in the soil used at the start of the experiment (‘initial' soil). In P. annua, phenolic biosynthesis decreased under elevated CO2 in initial soil, and in both P. annua and S. vulgaris there was a significant interaction between effects of soil type and CO2 level on PAL activity. In this study, plant chemical composition altered more in response to environmental factors such as soil type than in response to carbon supply. Results were species‐specific and changed markedly between generations.
Long‐term dynamics of pine and hardwood litter in contrasting environments: toward a global model of decomposition
Authors: Gholz Henry L. , Wedin David A. , Smitherman Stephen M. , Harmon Mark E. , Parton William J. 
Published: March 2003

SummaryWe analysed data on mass loss after five years of decomposition in the field from both fine root and leaf litters from two highly contrasting trees, Drypetes glauca, a tropical hardwood tree from Puerto Rico, and pine species from North America as part of the Long‐Term Intersite Decomposition Experiment (LIDET). LIDET is a reciprocal litterbag study involving the transplanting of litter from 27 species across 28 sites in North and Central America reflecting a wide variety of natural and managed ecosystems and climates, from Arctic tundra to tropical rainforest. After 5 years, estimated k‐values ranged from 0.032 to 3.734, lengths of Phase I (to 20% mass remaining) from 0.49 to 47.92 years, and fractional mass remaining from 0 to 0.81. Pine litter decomposed more slowly than Drypetes litter, supporting the notion of strong control of substrate quality over decomposition rates. Climate exerted strong and consistent effects on decomposition. Neither mean annual temperature or precipitation alone explained the global pattern of decomposition; variables including both moisture availability and temperature (i.e. actual evapotranspiration and DEFAC from the CENTURY model) were generally more robust than single variables. Across the LIDET range, decomposition of fine roots exhibited a Q10 of 2 and was more predictable than that of leaves, which had a higher Q10 and greater variability. Roots generally decomposed more slowly than leaves, regardless of genus, but the ratio of above‐ to belowground decomposition rates differed sharply across ecosystem types. Finally, Drypetes litter decomposed much more rapidly than pine litter in ‘broadleaved habitats’ than in ‘conifer habitats’, evidence for a ‘home‐field advantage’ for this litter. These results collectively suggest that relatively simple models can predict decomposition based on litter quality and regional climate, but that ecosystem‐specific problems may add complications.
Microbial processes influencing methane emission from rice fields
Authors: Krüger Martin , Frenzel Peter , Conrad Ralf 
Published: March 2003

SummaryIrrigated rice fields are an important source of atmospheric methane. In order to improve our understanding of the controlling processes, we measured in situ CH4 emission and CH4 oxidation in an Italian rice field in 1998 and 1999, and studied CH4 production in soil and root samples. The CH4 emission rates were correlated with diurnal temperature variations and showed pronounced seasonal and interannual variations. The contribution of CH4 oxidation to total CH4 flux, determined by specific inhibition with difluoromethane, decreased from 40% at the beginning to zero at the end of the season. The stable carbon isotopic composition of the emitted CH4 also decreased. The CH4‐oxidizing bacteria probably became limited by nitrogen as indicated by the seasonal decrease of NH4+. Thus, CH4 oxidation had little effect on CH4 emission. Methane production on rice roots was relatively constant over the season. Methane production in soil slowly increased after flooding and was highest in the middle of the season. Pore water concentrations of CH4 showed a similar seasonal pattern. In 1999, CH4 production increased later in the season and reached lower rates than in 1998. An additional drainage in 1999 resulted in higher ferric iron concentrations, higher soil redox potentials and lower acetate concentrations. As a result, acetate‐utilizing methanogens were probably out‐competed by iron‐reducers so that a larger percentage of [2–14C]acetate was converted to 14CO2 instead of 14CH4. The residual CH4 production was relatively low and was mainly due to H2/CO2‐dependent methanogenesis. Experiments with radioactive bicarbonate and with methyl fluoride as specific inhibitor showed that the theoretical ratio of 7:3 of methanogenesis from acetate vs. H2/CO2 was only reached later in the season when total CH4 production was at the maximum. In conclusion, our results give a mechanistic explanation for the intraseasonal and interannual differences in CH4 emission.
Paired‐tower measurements of carbon and energy fluxes following disturbance in the boreal forest
Authors: Amiro Brian D. 
Published: March 2003

SummaryDisturbances by fire and harvesting are thought to regulate the carbon balance of the Canadian boreal forest over scales of several decades. However, there are few direct measurements of carbon fluxes following disturbances to provide data needed to refine mathematical models. The eddy covariance technique was used with paired towers to measure fluxes simultaneously at disturbed and undisturbed sites over periods of about one week during the growing season in 1998 and 1999. Comparisons were conducted at three sites: a 1‐y‐old burned jackpine stand subjected to an intense crown fire at the International Crown Fire Modelling Experiment site near Fort Providence, North‐west Territories; a 1‐y‐old clearcut aspen area at the EMEND project near Peace River, Alberta; and a 10‐y‐old burned, mixed forest near Prince Albert National Park, Saskatchewan. Nearby mature forest stands of the same types were also measured as controls. The harvested site had lower net radiation (Rn), sensible (H) and latent (LE) heat fluxes, and greater ground heat fluxes (G) than the mature forest. Daytime CO2 fluxes were much reduced, but night‐time CO2 fluxes were identical to that of the mature aspen forest. It is hypothesized that the aspen roots remained alive following harvesting, and dominated soil respiration. The overall effect was that the harvested site was a carbon source of about 1.6 gC m−2 day−1, while the mature site was a sink of about −3.8 gC m−2 day−1. The one‐year‐old burn had lower Rn, H and LE than the mature jackpine forest, and had a continuous CO2 efflux of about 0.8 gC m–2 day−1 compared to the mature forest sink of − 0.5 g C m−2 day−1. The carbon source was likely caused by decomposition of fire‐killed vegetation. The 10‐y‐old burned site had similar H, LE, and G to the mature mixed forest site. Although the diurnal amplitude of the CO2 fluxes were slightly lower at the 10‐y‐old site, there was no significant difference between the daily integrals (− 1.3 gC m−2 day−1 at both sites). It appears that most of the change in carbon flux occurs within the first 10 years following disturbance, but more data are needed on other forest and disturbance types for the first 20 years following the disturbance event.
Detecting potential regional effects of increased atmospheric CO2 on growth rates of western juniper
Authors: Knapp Paul A. , Soulé Peter T. , Grissino‐Mayer Henri D. 
Published: March 2003

AbstractEvidence of an atmospheric CO2 fertilization effect on radial growth rates was uncovered by examining climate–growth relationships for seven western juniper tree‐ring chronologies in central Oregon using multiple regression models. Consistent upward trends of the residuals from dendroclimatic models indicated a decreased ability for climate parameters to predict growth with time. Additionally, an assessment was made of whether enhanced growth was detectable under drought conditions, because a major benefit of elevated atmospheric CO2 is the reduction of water stress. Mean ring indices were compared between ecologically comparable drought years, when atmospheric CO2 was lower (1896–1949), and more recent drought years that occurred under higher atmospheric CO2 concentrations (1950–96/98). The results presented herein show that: (i) residuals from climate/growth models had a significant positive trend at six of seven sites, suggesting the presence of a nonclimatic factor causing increased growth during recent decades; (ii) overall growth was 23% greater in the latter half of the 20th century; (iii) growth indices during matched drought and matched wet years were 63% and 30% greater, respectively, in the later 20th century than the earlier 20th century; and (iv) harsher sites had greater responses during drought periods between early and late periods. While it is not possible to rule out other factors, these results are consistent with expectations for CO2 fertilization effects.