bluuebunny/crossref_metadata_2025_split huggingface dataset:
An experimental comparison of three direct methods of marine source signature estimation
Authors: Laws Robert , Landrø Martin , Amundsen Lasse 
Published: March 2003

Three methods for estimation of the pressure wavefield generated by a marine airgun array are tested experimentally and compared. In the trial a variety of radiation angles and array configurations were used and some large synchronization errors were deliberately introduced. The source was equipped with near‐field hydrophones and a subsource ministreamer. A tethered far‐field hydrophone was used so that the three estimated far‐field signatures could be compared with an independent measurement.The knowledge of the source signature is important for on‐board source array QC, deconvolution, multiple attenuation, stratigraphic trap prediction, modelling and inversion, AVO analysis and reservoir monitoring.The methods perform very well and give estimates whose frequency‐domain spectra match the measured spectra to within a few dB and within a few tens of degrees of phase over the tested bandwidth of 3.5–110 Hz. The time‐domain error‐energy is typically only a few per cent of the signal energy for radiation angles within about 30° of the vertical. The third method proved to be sensitive to an experimental shortcoming leading to overloading of the ministreamer and meaningful comparison was not possible for some test configurations.
2D finite‐difference viscoelastic wave modelling including surface topography
Authors: O. Hestholm Stig , Ruud Bent 
Published: March 2003

We have pursued two‐dimensional (2D) finite‐difference (FD) modelling of seismic scattering from free‐surface topography. Exact free‐surface boundary conditions for the particle velocities have been derived for arbitrary 2D topographies. The boundary conditions are combined with a velocity–stress formulation of the full viscoelastic wave equations. A curved grid represents the physical medium and its upper boundary represents the free‐surface topography. The wave equations are numerically discretized by an eighth‐order FD method on a staggered grid in space, and a leap‐frog technique and the Crank–Nicholson method in time.In order to demonstrate the capabilities of the surface topography modelling technique, we simulate incident point sources with a sinusoidal topography in seismic media of increasing complexities. We present results using parameters typical of exploration surveys with topography and heterogeneous media. Topography on homogeneous media is shown to generate significant scattering. We show additional effects of layering in the medium, with and without randomization, using a von Kármán realization of apparent anisotropy. Synthetic snapshots and seismograms indicate that prominent surface topography can cause back‐scattering, wave conversions and complex wave patterns which are usually discussed in terms of inter‐crust heterogeneities.
Effects of climate change on nitrogen dynamics in upland soils. 2. A soil warming study
Authors: Ineson Philip , Benham David G. , Poskitt Janet , Harrison Anthony F. , Taylor Kenneth , Woods Clive 
Published: March 2003

AbstractA new warming technique has been developed in a field experimental study of the potential effects of climatic change on N leaching from hill land plant/soil systems. Thermocouple compensating cable has been utilized to provide a small cross‐section, flexible, low voltage heating cable, mounted on a framework of stainless steel mesh, to provide uniform heating at the vegetation/soil interface of zero‐tension lysimeters and surrounding turf. We describe a specially designed heat controller capable of maintaining a temperature differential of 3 °C above ambient at a soil depth of 0.8 cm. The equipment raises temperatures down the soil profile and within the grass sward, whilst tracking normal diurnal temperature variation. Results presented here illustrate the efficacy of the warming technique, together with the consequences for the release of nitrate from lysimeters. The responses of soil solution concentrations of nitrate varied markedly between soil types, but showed a significant decrease in the brown earth during the first 5 months of additional heating. This suggests that increased nutrient release is masked by plant uptake in this soil, but the responses in the other two soils were less marked.
The effect of elevated CO2 concentration and soil pH on the relationship between plant growth and rhizosphere denitrification potential
Authors: Hall J. M. , Paterson E. , Killham K. 
Published: March 2003

AbstractThe effect of CO2 concentration on plant growth and the size of the rhizosphere denitrifier population was investigated for ryegrass grown at 3 different soil pH values (pH 4.3, 5.9 and 7.0). Soil microcosms were planted with ryegrass and maintained under constant growth conditions at either ambient (450ppm) or elevated (720ppm) CO2 concentration. At harvest, the rhizosphere soil was collected and subjected to a potential denitrification assay to provide an estimate of the size of the denitrifier population present. Ryegrass dry matter production varied across the pH range studied and contrary to other studies, elevated CO2 concentration did not consistently increase growth. Plant growth was reduced by ≈ 35% and 23% at pH 4.3 and pH 5.9, respectively, under elevated CO2 concentration. At pH 7.0, however, plant growth was increased by ≈ 45% under elevated CO2. Potential denitrification rates within the rhizosphere followed a similar pattern to plant growth in the different treatments, suggesting that plant growth and the size of denitrifier population within the rhizosphere are coupled. This study investigates the relationship between plant growth and rhizosphere denitrification potential, thereby providing an estimate of the size of the denitrifier population under increased CO2 concentration and soil pH.
Effects of short‐term drainage and aeration on the production of methane in submerged rice soil
Authors: Ratering Stefan , Conrad RalF. 
Published: March 2003

AbstractEmission rates of CH4 were measured in microcosms of submerged soil which were planted with rice. Drainage of the rice microcosms for 48 h resulted in drastically decreased CH4 emission rates which only slowly recovered to the rates of the undrained controls. Drainage also resulted in drastically increased sulphate concentrations which only slowly decreased to nearly zero background values after the microcosms were submerged again. The mechanisms responsible for the decrease of CH4 production by aeration were investigated in slurries of a loamy and a sandy Italian rice soil. Incubation of the soil slurries under anoxic conditions resulted first in the reduction of nitrate, sulphate and ferric iron before CH4 production started. Incubation of the soil slurries for 48 h under air resulted in immediate and complete inhibition of CH4 production. Although the soil slurries were then again incubated under anoxic conditions (N2 atmosphere), the inhibition of CH4 production persisted for more than 30 days. The redox potential of the soil increased after the aeration but returned within 15 days to the low values typical for CH4 production. However, the concentrations of sulphate and of ferric iron increased dramatically after the aeration and stayed at elevated levels for the period during which CH4 production was inhibited. These observations show that even brief exposure of the soil to O2 allowed the production of sulphate and ferric iron from their reduced precursors. Elevated sulphate and ferric iron concentrations allowed sulphate‐reducing and ferric iron‐reducing bacteria to outcompete methanogenic bacteria on H2 as common substrate. Indeed, concentrations of H2 were decreased as long as sulphate and ferric iron were high so that the Gibbs free energy of CH4 production from H2/CO2 was also increased (less exergonic). On the other hand, concentrations of acetate, the more important precursor for CH4, were not much affected by the short aeration of the soil slurries, and the Gibbs free energy of CH4 production from acetate was highly exergonic suggesting that acetotrophic methanogens were not outcompeted but were otherwise inhibited. Aeration also resulted in increased rates of CO2 production and in a short‐term increase of N2O production. However, these increases were < 10% of the decreased production of CH4 and did not represent a trade‐off in terms of CO2 equivalents. Hence, short‐term drainage and aeration of submerged paddy fields may be a useful mitigation option for decreasing the emission of greenhouse gases.
Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings
Authors: JANSSENS IVAN. A. , CROOKSHANKS MEG. , TAYLOR GAIL. , CEULEMANS REINHART 
Published: March 2003

AbstractIn this study, we investigated the impact of elevated atmospheric CO2 (ambient + 350 μmol mol–1) on fine root production and respiration in Scots pine (Pinus sylvestris L.) seedlings. After six months exposure to elevated CO2, root production measured by root in‐growth bags, showed significant increases in mean total root length and biomass, which were more than 100% greater compared to the ambient treatment. This increased root length may have lead to a more intensive soil exploration. Chemical analysis of the roots showed that the roots in the elevated treatment accumulated more starch and had a lower C/N‐ratio. Specific root respiration rates were significantly higher in the elevated treatment and this was probably attributed to increased nitrogen concentrations in the roots. Rhizospheric respiration and soil CO2 efflux were also enhanced in the elevated treatment. These results clearly indicate that under elevated atmospheric CO2 root production and development in Scots pine seedlings is altered and respiratory carbon losses through the root system are increased.
Crop water availability in early agriculture: evidence from carbon isotope discrimination of seeds from a tenth millennium BP site on the Euphrates
Authors: Araus J. L. , Febrero A. , Catala M. , Molist M. , Voltas J. , Romagosa I. 
Published: March 2003

AbstractThe analysis of carbon isotope discrimination (Δ) in crop plant remains from archaeological sites may help to assess water availability for early agriculture. This study presents the analysis of Δ in seeds of naked wheat (Triticum aestivum/durum), lentil (Lens orientalis/culinaris), and flax (Linum sp.) found at the archaeological site of Tell Halula in the valley of the Middle Euphrates (Syria). This Neolithic site is the oldest in this region of the Fertile Crescent where the cultivation of domesticated plants has been reported, with seed remains ranging from 9550 to 8465 BP. Most of the seeds analysed showed Δ values greater than 16 ‰, reaching 20 ‰ for some samples of flax. For wheat, Δ values were much higher than those reported in present‐day (1996) durum wheat crops cultivated under rainfed conditions in north‐west Syria under environments with somewhat higher rainfall than Tell Halula. Similarly, grains of present‐day (1997) barley cultivated in the archaeological site also showed lower values than those found in archaeological kernels. An empirical relationship between Δ of mature kernels and total precipitation (plus irrigation where applicable) from heading to maturity (r2 = 0.82, n = 11) was established for durum wheat, currently cultivated in different environments of the Mediterranean basin. The resulting relationship was applied to the data on Δ of wheat fossil kernels from Tell Halula to estimate the accumulated water inputs during the time (about 6 weeks) the kernels were produced. Calculated water inputs for wheat during early agriculture were (over 110 mm) at least 5 times higher than current‐day rainfall accumulated in Tell Halula during the same phenological period. These results strongly suggest that early agriculture wheat was cultivated at Tell Halula under much wetter conditions than are currently to be found in the area. The presence of flax and its very high Δ values also support this conclusion. Whether such humid conditions during cultivation were due to moister conditions prevailing at this time, by planting in alluvial areas or by irrigation works is discussed.
Elevated CO2 influences the responses of two birch species to soil moisture: implications for forest community structure
Authors: Catovsky S. , Bazzaz F. A. 
Published: March 2003

AbstractIncreased levels of atmospheric CO2 may alter the structure and composition of plant communities by affecting how species respond to their physical and biological environment. We investigated how elevated CO2 influenced the response of paper birch ( Betula papyrifera Marsh.) and yellow birch (Betula alleghaniensis Britt.) seedlings to variation in soil moisture. Seedlings were grown for four months on a soil moisture gradient, individually and in mixed species stands, in controlled environment facilities at ambient (375 μL L–1) and elevated (700 μL L–1) atmospheric CO2. For both individually and competitively grown paper birch seedlings, there was a greater CO2 growth enhancement for seedlings watered less frequently than for well‐watered seedlings. This differential change in CO2 responsiveness across the moisture gradient reduced the difference in seedling growth between high and low water levels and effectively broadened the regeneration niche of paper birch. In contrast, for yellow birch seedlings, elevated CO2 only produced a significant growth enhancement at the wet end of the soil moisture gradient, and increased the size difference between seedlings at the two ends of the gradient. Gas exchange measurements showed that paper birch seedlings were more sensitive than yellow birch seedlings to declines in soil moisture, and that elevated CO2 reduced this sensitivity. Additionally, elevated CO2 improved survival of yellow birch seedlings growing in competition with paper birch in dry stands. Thus, elevated CO2 may influence regeneration patterns of paper birch and yellow birch on sites of differing soil moisture. In the future, as atmospheric CO2 levels rise, growth of paper birch seedlings and survival of yellow birch seedlings may be enhanced on xeric sites, while yellow birch may show improved growth on mesic sites.
Predicted disappearance of coral‐reef ramparts: a direct result of major ecological disturbances
Authors: Williams Ernest H. , Bartels PauL. J. , Bunkley‐Williams LucY. 
Published: March 2003

SummaryTwo coral cays near La Parguera, Puerto Rico, have large, exposed coral ramparts composed almost entirely of loose pieces of elkhorn coral Acropora palmata (88% of horizontal transects, 98% of vertical transects). The total volume of elkhorn coral in the ramparts of the two cays was estimated at 3600 and 12 800 m3. The present volume of living elkhorn coral on these two reefs was estimated at 7 and 14 m3 and previous volumes at 11 000 and 34 900 m3. White‐band disease was found on 8.5% of living elkhorn colonies. Lang’s boring sponge Cliona langae covered 10.8% of the total transect area, overgrowing both dead and living corals. White‐band disease and coral‐reef bleaching have drastically reduced the populations of elkhorn coral, thus, skeletal coral materials to replenish the plate ramparts are severely reduced, disrupting the process of forming and maintaining these coral reef ramparts. We predict that the next series of major storms striking these prominent cay ramparts will remove them. These disappearances will represent a quick, obvious and permanent consequence of global disturbances. Loss of cay ramparts will modify the environments on and around Atlantic coral reefs. Ramparts may be similarly lost from Indo‐Pacific reefs. The lack of any other indisputable definitive indicators of long‐term, major disturbances on coral reefs makes the distinct loss of coral‐reef ramparts an important physical sign.
Modelling changes in VOC emission in response to climate change in the continental United States
Authors: . Constable JohN. V. H , Guenther AleX. B. , Schimel David S. , Monson Russell K. 
Published: March 2003

SummaryThe alteration of climate is driven not only by anthropogenic activities, but also by biosphere processes that change in conjunction with climate. Emission of volatile organic compounds (VOCs) from vegetation may be particularly sensitive to changes in climate and may play an important role in climate forcing through their influence on the atmospheric oxidative balance, greenhouse gas concentration, and the formation of aerosols. Using the VEMAP vegetation database and associated vegetation responses to climate change, this study examined the independent and combined effects of simulated changes in temperature, CO2 concentration, and vegetation distribution on annual emissions of isoprene, monoterpenes, and other reactive VOCs (ORVOCs) from potential vegetation of the continental United States. Temperature effects were modelled according to the direct influence of temperature on enzymatic isoprene production and the vapour pressure of monoterpenes and ORVOCs. The effect of elevated CO2 concentration was modelled according to increases in foliar biomass per unit of emitting surface area. The effects of vegetation distribution reflects simulated changes in species spatial distribution and areal coverage by 21 different vegetation classes. Simulated climate warming associated with a doubled atmospheric CO2 concentration enhanced total modelled VOC emission by 81.8% (isoprene + 82.1%, monoterpenes + 81.6%, ORVOC + 81.1%), whereas a simulated doubled CO2 alone enhanced total modelled VOC emission by only + 11.8% (isoprene + 13.7%, monoterpenes + 4.1%, ORVOC + 11.7%). A simulated redistribution of vegetation in response to altered temperatures and precipitation patterns caused total modelled VOC emission to decline by 10.4% (isoprene – 11.7%, monoterpenes – 18.6%, ORVOC 0.0%) driven by a decline in area covered by vegetation classes emitting VOCs at high rates. Thus, the positive effect of leaf‐level adjustments to elevated CO2 (i.e. increases in foliar biomass) is balanced by the negative effect of ecosystem‐level adjustments to climate (i.e. decreases in areal coverage of species emitting VOC at high rates).
Ocean province classification using ocean colour data: observing biological signatures of variations in physical dynamics
Authors: Esaias Wayne E. , Iverson Richard L. , Turpie Kevin 
Published: March 2003

SummaryWe have used satellite colour data to classify ocean environments for monitoring interannual changes in the ocean. The unsupervised classification method is based on our observation that the frequency distributions of Coastal Zone Color Scanner (CZCS) annual pigment means and standard deviations are nonuniform and contain distinct clusters. The frequency distributions are used to objectively determine ocean areas with similar pigment statistical characteristics. A major separation between high variance, high pigment and lower variance, lower pigment waters is observed in terms of global ocean area. The ocean areas determined with our method reflect different bio‐logical responses to variations in ocean physical dynamics. Pigment means and variances around the Joint Global Ocean Flux Study (JGOFS) Time Series stations are used as fiducial characteristics. Hawaii Ocean Time‐series (HOT) station is associated with the low‐variance portion of the global annual pigment distribution characteristic of the central gyres, but shows slightly higher mean and variance than the minima in the central Pacific gyre. The Bermuda Atlantic Time Series (BATS) pigment associations comprise a transitional region between the gyres and high‐variance pigment areas, and circumscribe the HOT pigment associations. Together, these associations encompass 23% (HOT‐like) and 48% (BATS‐like) of the Northern Hemisphere open ocean. The Pacific regions delineated by the JGOFS station pigment‐based patterns are similar to distributions described historically for Pacific zooplankton communities. Interannual variation for the northern hemisphere gyre area is on the order of by 10% for the 11/78–10/81 period.
Canopy transpiration in a chronosequence of Central Siberian pine forests
Authors: Zimmermann Reiner , Schulze Ernst‐ Detlef , Wirth Christian , Schulze Ernst‐Eckart , Mcdonald Kyle C. , Vygodskaya Natascha N. , Ziegler Waldemar 
Published: March 2003

SummaryTree transpiration was measured in 28, 67, 204 and 383‐y‐old uniform stands and in a multicohort stand (140–430 y) of Pinus sylvestris ssp. sibirica Lebed. in Central Siberia during August 1995. In addition transpiration of three codominant trees was monitored for two years in a 130‐y‐old stand. All stands established after fire. Leaf area index (LAI) ranged between 0.6 (28‐y‐old stand) and 1.6 for stands older than 67‐y. Stand xylem area at 1.3 m height increased from 4 cm2 m−2 (28‐y) to 11.5 cm2 m−2 (67‐y) and decreased again to 7 cm2 m−2 in old stands. Above‐ground living biomass increased from 1.5 kg dry weight m−2 (28‐y) to 14 kg dry weight m−2 (383‐y). Day‐to‐day variation of tree transpiration in summer was dependent on net radiation, vapour pressure deficit, and soil water stress. Tree‐to‐tree variation of xylem flux was small and increased with heterogeneity in canopy structure. Maximum rates of xylem flux density followed the course of net radiation from mid April when a constant level of maximum rates was reached until mid September when low temperatures and light strongly reduced flux density. Maximum sap flux density (60 g m−2 s−1) and canopy transpiration (1.5 mm d−1) were reached in the 67‐y stand. Average canopy transpiration of all age classes was 0.72 ± 0.3 mm d−1. Canopy transpiration (E) was not correlated with LAI but related to stand sapwood area SA (E = − 0.02 + 1.15SA R2) which was determined by stand density and tree sapwood area.
Methane dynamics of a restored cut‐away peatland
Authors: Tuittila Eeva‐Stiina , Komulainen Veli‐Matti , Vasander Harri , Nykänen Hannu , Martikainen Pertti J. , Laine Jukka 
Published: March 2003

SummaryWe measured a cut‐away peatland's CH4 dynamics using the static chamber technique one year before and two years after restoration (rewetting). The CH4 emissions were related to variation in vegetation and abiotic factors using multiple linear regression. A statistical model for CH4 flux with cottongrass cover (Eriophorum vaginatum L.), soil temperature, water level, and effective temperature sum index as driving variables explained most (r2 = 0.81) of the temporal and spatial variability in the fluxes. In addition to the direct increasing effect of raised water level on CH4 emissions, rewetting also promoted an increase of cottongrass cover which consequently increased carbon flux (substrate availability) into the system. The seasonal CH4 dynamics in tussocks followed seasonal CO2 dynamics till mid August but in late autumn CH4 emissions increased while CO2 influxes decreased. The reconstructed seasonal CH4 exchange was clearly higher following the rewetting, although it was still lower than emissions from pristine mires in the same area. However, our simulation for closed cottongrass vegetation showed that CH4 emissions from restored peatlands may remain at a lower level for a longer period of time even after sites have become fully vegetated and colonized by mire plants.