Short-term changes in C and N distribution in soil particle size fractions induced by agricultural practices in a cultivated volcanic soil from Mexico   总被引：6，自引：0，他引：6  

Sara Covaleda  Silvia Pajares  Juan F. Gallardo  Jorge D. Etchevers   《Organic Geochemistry》2006,37(12):1943

The purpose of this study was to assess the effect of agricultural practices on the characteristics of soil organic carbon (SOC). The study area is located in the Central Volcanic Belt (Michoacán) in Mexico. The soil is an Acrisol, acidic and rich in clays and sesquioxides. Experimental plots were treated with four different agronomic management systems between 2002 and 2003: traditional, improved traditional, organic and fallow. Each treatment was replicated twice. Samples were taken at depths of 0–10 and 10–20 cm and were subjected to a physical fractionation process by way of particle size. SOC was fractionated into fulvic acids, humic acids and humin. After two years of cultivation, the SOC content increased significantly with the organic management (2.2 mg g⁻¹ at 0–10 cm and 5.8 mg g⁻¹ at 10–20 cm). The C content of the soil fine particle fraction increased with the traditional and organic managements. The organic C and N contents of the silt + clay particle-size fraction were mainly concentrated as humin, indicating that this SOM should be stable and have a low risk of mineralization. The humin C content only decreased significantly under the traditional and fallow treatments. The N content of the humin fraction decreased significantly under the traditional management system (from 69% to 54%), indicating the low sustainability of this soil management. The C/N ratio of the soil increased significantly under all treatments, but mainly under the traditional and organic treatments (from 12.1 and 12.8 to 13.7 and 14.0, respectively). This indicates a decrease in humus quality. In addition, the C/N of the humin increased significantly (from 13.3 and 12.7 to 19.2 and 16.0, respectively).  相似文献   

Alkyl nitrate formation from the reaction of a series of branched RO2 radicals with NO as a function of temperature and pressure     

Roger Atkinson  Sara M. Aschmann  Arthur M. Winer 《Journal of Atmospheric Chemistry》1987,5(1):91-102

Alkyl nitrate yields from the NO _x photooxidations of neopentane, 2-methylbutane and 3-methylpentane have been determined over the temperature and pressure ranges 281–323 K and 54–740 torr, respectively. The formation of the alkyl nitrates is attributed to the reaction pathway (1b) $${\text{RO}}_{\text{2}} + {\text{NO}}^{{\text{ }}\underrightarrow {\text{M}}} {\text{ RONO}}_{\text{2}}$$ and rate constant ratios k _1b/(k _1a+k _1b) are estimated, where (1a) is the reaction pathway (1a) $${\text{RO}}_{\text{2}} + {\text{NO}} \to {\text{RONO}}_{\text{2}} .$$ A method for estimating this rate constant ratio for primary, secondary and tertiary alkyl peroxy radicals is presented.  相似文献   

Far from Being "Home Alone": The Dynamics of Accompanied Fieldwork     

Julie Cupples  Sara Kindon 《Singapore journal of tropical geography》2003,24(2):211-228

Despite persistent images to the contrary, most fieldworkers are accompanied. Yet, there has been limited discussion on the nature of accompanied fieldwork, particularly by geographers. Drawing on our experiences in three countries in the tropics, we discuss the dynamics of being accompanied in “the field” by our children and female co‐researchers. Specifically, we focus on issues of access and rapport; the impacts of their presence on our positionality; and the implications these have for power relations and research outcomes. We demonstrate how being accompanied entangles our personal and professional selves and can result in more egalitarian power relations as we become “observers observed”. We argue that by paying attention to the dynamics of accompanied fieldwork, there is the potential to enhance the conceptual focus of our methodological concerns and to provide a more theoretically sophisticated mode of exploring the ways in which our multiple identities intersect while in “the field”.  相似文献   

Predicting fine sediment dynamics along a pool-riffle mountain channel   总被引：1，自引：0，他引：1  

Sara Rathburn  Ellen Wohl   《Geomorphology》2003,55(1-4):111

Fine sediment dynamics in mountain rivers are of concern because of implications for aquatic habitat, channel stability, and downstream sediment yields. Many mountain river systems have episodic fine sediment transport because of infrequent, point-source sediment inputs from landslides; basin instability triggered by land uses such as logging; or infrequent mobilization of the coarse surface layer in channels. Dam removal, which is now more likely along mountain rivers, may also provide a substantial fine sediment input to downstream channel reaches.Fine sediment storage in the interstices of spawning gravels and within pools along mountain rivers is of particular interest because of impacts to aquatic organisms. In this study we focus on sediment dynamics within pools of the North Fork Poudre River in Colorado as an example of the processes controlling fine sediment deposition, storage, and transport within laterally constricted pools. The 1996 release of 7000 m³ of silt-to gravel-sized sediment from a reservoir on the North Fork provided an opportunity to develop a field data set of fine sediment dynamics and to test the predictions of three different one- or two-dimensional sediment transport and hydraulic models against the field observations.The models were calibrated against quantitative measurements of pool scour and fill. One-dimensional HEC-6 results indicate that robust simulations yield the greatest agreement between predicted and measured pool bed elevation change. Model calibration on two pools and validation on one pool indicate that at least 58% of observed bed changes after the sediment release were predicted by HEC-6. Modeling accuracy using quasi-two-dimensional GSTARS 2.0 was considerably more variable, and no pool-wide trends were obtained. The two-dimensional model RMA2 substantially improved the representation of eddy pool hydraulics within a compound pool of the North Fork. Results from the hydraulic modeling, coupled with bed load and total load computations, delineate areas of scour and deposition which are consistent with observations in the field.A conceptual model of sediment delivery and storage for laterally confined pools suggests that persistent deposition of fine sediment within eddies distal from the sediment source may result from sediment releases. The original loss of channel capacity facilitated additional deposition within eddies as sediment within upstream proximal pools became mobilized. At high discharges, the development of a strong shear zone prevents degradation of sediment deposits within the eddy. Central portions of these proximal pools may clear according to existing models, whereas deposition within recirculating zones may be long-term. Water managers could use these models to estimate minimum pool volume for overwinter habitat and residence time of pool sediment.  相似文献   

From platform to basin to swell: orbital control on sedimentary sequences in the Oxfordian, Spain     

André Strasser  Marc Aurell  Beatriz Bádenas  Guillermo Meléndez  Sara Tomás 《地学学报》2005,17(5):407-413

Climatic, oceanographic and ecological changes that control the formation and deposition of sediment in shallow and deep depositional environments commonly occur with periodicities of a few 10 000 years. Consequently, in order to interpret sedimentary sequences in the geological past, high time resolution is required. This is best obtained by cyclostratigraphy. Three sections have been studied in the Oxfordian of north-eastern Spain: one represents a shallow, siliciclastic-carbonate platform with repetitive subaerial exposures, one an intraplatform basin with sponge bioherms, and one a swell where iron ooids and glauconite formed. The platform section displays a well-defined stacking pattern of depositional sequences; the deeper-water sections are well dated by ammonites. The correlation between the three sections is a best-fit solution integrating biostratigraphy, sequence stratigraphy and cyclostratigraphy. It is concluded that the small-scale depositional sequences formed in tune with the 100-ka orbital eccentricity cycle. An additional factor was differential subsidence that ruled basin morphology.  相似文献   

Imposex levels and concentrations of organotin compounds (TBT and its metabolites) in Nassarius nitidus from the Lagoon of Venice     

Bruno Pavoni  Elena Centanni  Sara Valcanover  Marco Fasolato  Silvia Ceccato  Davide Tagliapietra 《Marine pollution bulletin》2007,55(10-12):505

Specimens of Nassarius nitidus were collected in seven stations of the Venice Lagoon to assess the levels of tributyltin (TBT) and its metabolites monobutyltin and dibutyltin in the tissues and monitor their effect on organisms, in particular the phenomenon of imposex (superimposition of male sexual characteristics on females). The following values of population indices were found: vas deferens sequence: 1.2 ± 0.7–4.0 ± 0.5; relative penis length: 6–47%. The least impacted station was situated in the northern part of the Lagoon, where females without imposex were found and Butyltin (BuTs) concentrations in the organisms (average sum of BuTs = 43 ± 14 ng Sn g⁻¹ w.) were significantly lower than in the other stations (range of average sum of BuTs: 101 ± 22–217 ± 27 ng Sn g⁻¹ d.w.). Population indices were found to be related to the TBT content in the tissues. In particular VDSI had a significant logarithmic correlation: r = 0.95, n = 8, p < 0.05.  相似文献   

Development of a coral cDNA array to examine gene expression profiles in Montastraea faveolata exposed to environmental stress     

Edge SE  Morgan MB  Gleason DF  Snell TW 《Marine pollution bulletin》2005,51(5-7):507-523

The development of a cDNA array of coral genes and its application to investigate changes in coral gene expression associated with stressful conditions is described. The array includes both well-characterized and previously unidentified coral genes from Acropora cervicornis and Montastraea faveolata. Corals were exposed to either natural or anthropogenic stressors to elicit the expression of stress genes for isolation and incorporation onto the array. A total of 32 genes involved in protein synthesis, apoptosis, cell signaling, metabolism, cellular defense and inflammation were included on the array. Labeled cDNA from coral (Montastraea faveolata) exposed to elevated seawater temperature, salinity and ultraviolet light was tested against the microarray to determine patterns of gene expression associated with each stressor. Carbonic anhydrase, thioredoxin, a urokinase plasminogen activator receptor (uPAR) and three ribosomal genes demonstrated differential expression across all replicates on the array and between replicate colonies. Specific gene expression patterns produced in response to different stressors demonstrate the potential for gene expression profiling in characterizing the coral stress response.  相似文献   

An outline of shear-sense analysis in high-grade rocks     

Cees Passchier  Sara Coelho 《Gondwana Research》2006,10(1-2):66

Ductile shear zones are important in tectonic reconstructions as a source of information on the relative motion of large crustal blocks or plates in the geological past. Methods to interpret fabric in ductile shear zones were mostly developed for low grade rocks where overprinting relations are usually well preserved. However, high grade shear zones are common and dominate in many Precambrian terrains. High grade shear zones should be analysed in a different way from low grade zones. The plane on which shear sense markers should be observed, the vorticity profile plane, is more difficult to find than in low grade shear zones. The most reliable shear sense markers in high grade shear zones are shear bands, mineral fish, mantled porphyroclasts, sigmoids and asymmetric boudins.  相似文献   

The evolution of prominences and their relationship to active centers     

Sara F. Martin 《Solar physics》1973,33(1):3-21

The radiation field, emergent from an inhomogeneous atmosphere, may differ significantly from that calculated using a mean model for such an atmosphere. In the solar case, horizontal anisotropy of the granulation pattern leads to azimuthal dependence of the emergent intensity, and this appears as a latitude-dependent limb flux which may mimic oblateness. We examine this latitude-dependence for several two and three-dimensional models of the inhomogeneous solar atmosphere, with varying degrees of anisotropy in the granulation pattern. Elongation along an east-west axis of about 7% would yield a signal somewhat imperfectly mimicking an excess oblateness of 4 × 10^–5. Using the Babcock-Leighton model of the general solar magnetic field we show that some stretching of granules, of this order of magnitude, should be expected. However, it may vary with the solar activity cycle, and in any case the result is very sensitive to the parameters adopted. Even if study of granulation observations should exclude elongations as high as 7%, smaller essentially undetectable elongations may exist. We find that 1 % elongation can account for 25–50 % of a signal corresponding to excess oblateness 4 × 10^–5. We conclude that anisotropy of the granulation pattern may influence oblateness determinations; when this is considered together with other effects, much of the claimed oblateness may be eliminated.  相似文献   

Local music policies within a global music industry: cultural quarters in Manchester and Sheffield   总被引：4，自引：0，他引：4  

Adam Brown  Justin O'Connor  Sara Cohen 《Geoforum》2000,31(4):437-451

This paper will explore the relationship between the local and the global in the music industry through the lens of place-based cultural policies.The first part of the paper will outline current debates around the complex interaction of the local and the global in culture in general and the popular music industry in particular. Alongside the continuous expansion of globalisation has been a reassertion of ‘place', of ‘locality'. Whilst this has been investigated to some extent at the level of local music scenes it has never been fully addressed in terms of music industry policy as part of an economic development strategy. Yet it is here that the particular values of ‘place' are asserted in the face of some ‘global' music industry with a view to developing (or at least retaining some of the benefits of) a local music industry.Whilst situating itself within these debates about the relationship of local and global musical production and consumption, the paper explores the different strategies of two northern English cities and their attempts to promote culture, and music, within each cities ‘cultural quarter': Sheffield's Cultural Industries Quarter and Manchester's Northern Quarter. The paper analyses the role and appropriateness of local authority policy intervention, the importance of ‘soft' networks within local music scenes and the different ways in which authorities in each example have tried to overcome dichotomies of art and industry, cultural and economy.  相似文献