Long‐term nitrogen and phosphorus fertilization effects on N2 fixation rates and nifH gene community patterns in mangrove sediments     

Isabel C. Romero  Myrna Jacobson  Jed A. Fuhrman  Marilyn Fogel  Douglas G. Capone 《Marine Ecology》2012,33(1):117-127

The bioavailability of nutrients is important in controlling ecological processes and nitrogen cycling in oligotrophic mangrove forests, yet the variation of diazotrophic community structure and activity with nutrient availability in sediments remains largely unexplored. To investigate for the first time how nutrients in sediments affect spatial and temporal patterns of diazotrophic community structure and activity, the sedimentary environment of Twin Cays, Belize, was examined with respect to the effects of long‐term fertilization [treatments: control (Ctrl), nitrogen (N), and phosphorus (P)] on N₂ fixation rates and nifH gene community structure. We found that N₂ fixation rates were significantly higher at the P‐treatment, intermediate at the Ctrl‐treatment and lower in the N‐treatment (P: 4.2 ± 0.5, Crtl: 0.8 ± 0.1, N: 0.4 ± 0.1 nmol·N·g^?1·h^?1; P < 0.001) with spatial (Ctrl‐ and P‐treatments) and temporal (only P‐treatment) variability positively correlated with live root abundance (r² = 0.473, P < 0.001) and concentration (r² = 0.458, P < 0.0001). The community structure of diazotrophs showed larger spatial and temporal variability in the fertilized treatments than in the Ctrl‐treatment, with the relative abundance of OTUs (nifH operational taxonomic units) at the fertilized treatments inversely related to live root abundance. Overall, long‐term fertilization (with either N or P) affects not only nutrient levels in mangrove sediments directly, but also spatial and temporal patterns of both community structure and activity and likely plant‐microbe interactions as well. Our findings suggest that the maintenance of natural nutrient conditions in mangrove sediments is important to ensure the stability of microbial functional groups like diazotrophs.  相似文献   

Helium-4 characteristics of groundwaters from Central Australia: Comparative chronology with chlorine-36 and carbon-14 dating techniques     

Justin T. Kulongoski  David R. Hilton  Richard G. Cresswell  Stephen Hostetler  Gerry Jacobson 《Journal of Hydrology》2008,348(1-2):176-194

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Using inverse theory and seabeam bathymetry to improve seismic refraction data corrections     

M. Poujol  R. S. Jacobson 《Marine Geophysical Researches》1988,9(4):311-332

An array consisting of ocean bottom seismometer and on-bottom hydrophones, was used to conduct a seismic experiment on 0.4 Ma crust east of the Juan de Fuca Ridge. Seismic sources were large (>50 kg) explosive charges detonated by SUS devices set to explode at 1829 or 2438 m nominal depth. The objectives of the experiment were to determine the compressional wave velocity and attenuation structures of the uppermost 500 m depth. The relative positions of shots and receivers were originally determined by treating each shot-receiver pair independently, via raytracing of various water waves. Due to the reflection of some of these water waves by the rough bottom, significant scatter resulted, preventing a determination of a physically realizable velocity-depth function. A new method is described that co-locates shot and receiver positions, including receiver depths consistent withseabeam bathymetry, using only the water waves that do not interact with the bottom. Several potential pitfalls are outlined using this method. A stable solution could only be achieved by discarding shots located well outside the array. The water path corrections were applied to the refracted arrivals, again using theseabeam bathymetry. The joint inversion location procedure, along with the use of precise gridded bathymetry, reduced the travel time scatter to a level whereby a velocity-depth function could be determined. The results, using only the hydrophone data, indicate an initial velocity at the seafloor of 2.7 km s^-1 with gradients from 4.6 s^-1 slowly decreasing to 4.1 s^-1 at 679 m depth. This velocity is similar to others conducted over very young oceanic crust, and can be interpreted as being due to a high porosity at the surface, due to cracks, fissures, and open pores, which rapidly diminish with depth.  相似文献   

Limb Spicules from the Ground and from Space     

Jay M. Pasachoff  William A. Jacobson  Alphonse C. Sterling 《Solar physics》2009,260(1):59-82

We amassed statistics for quiet-sun chromosphere spicules at the limb using ground-based observations from the Swedish 1-m Solar Telescope on La Palma and simultaneously from NASA’s Transition Region and Coronal Explorer (TRACE) spacecraft. The observations were obtained in July 2006. With the 0.2 arcsecond resolution obtained after maximizing the ground-based resolution with the Multi-Object Multi-Frame Blind Deconvolution (MOMFBD) program, we obtained specific statistics for sizes and motions of over two dozen individual spicules, based on movies compiled at 50-second cadence for the series of five wavelengths observed in a very narrow band at Hα, on-band and at ± 0.035 nm and ± 0.070 nm (10 s at each wavelength) using the SOUP filter, and had simultaneous observations in the 160 nm EUV continuum from TRACE. The MOMFBD restoration also automatically aligned the images, facilitating the making of Dopplergrams at each off-band pair. We studied 40 Hα spicules, and 14 EUV spicules that overlapped Hα spicules; we found that their dynamical and morphological properties fit into the framework of several previous studies. From a preliminary comparison with spicule theories, our observations are consistent with a reconnection mechanism for spicule generation, and with UV spicules being a sheath region surrounding the Hα spicules.  相似文献   

R. H. Kosobud,W. A. Testa,D. A. Hanson (eds.),cost effective control of urban smog,Federal Reserve Bank of Chicago, 1993, 214 pp.     

Mark Jacobson 《Climatic change》1995,29(1):117-119

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Two Reports On NAVSTAR GPS Recall Early Thoughts On A “Borderles System”     

Len Jacobson 《GPS Solutions》1995,1(2):143-144

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Soil development on recent end moraines of the Klutlan Glacier,Yukon Territory,Canada     

G.L. Jacobson  H.J.B. Birks 《Quaternary Research》1980,14(1):87-100

Data on soils with six Neoglacial moraines of the Klutlan Glacier have been compared with those from moraines at the warm, moist coastal site of Glacier Bay, 160 km south. Percentage organic matter increases rapidly for the first 100 to 150 yr of soil development and then continues to rise gradually for the next 100 yr. Soil pH falls from 8.0 in recent till to approximately 6.0 in 200-yr-old soils. Nitrogen levels in the mineral soil increase from near zero in recent tills to 0.7% in soils 175–200 yr old; organic horizons of soils associated with spruce forests in later successional stages contain approximately 1% nitrogen. Concentrations of certain inorganic phosphate ions in the different-aged soils increase continually throughout the succession. Data for nine chemical variables were subjected to a principal components analysis; the major pattern in the data reflects the differences between soils of low organic content and high pH present in early successional stages, and nutrient-rich soils with high organic content and low pH present after succession has progressed toward the spruce forest. These trends in soil development with time are strikingly similar to those reported from Glacier Bay, except that the changes in soil properties appear to be delayed by 50–100 yr at the Klutlan terminus. Although numerous signs of nitrogen deficiency have been identified in plants growing on new soils at Glacier Bay, none was observed visually in living plants or in nutrients measured in samples of foliage from three plant taxa (Epilobium latifolium, Salix spp., and Populus balsamifera) taken from the Klutlan moraines. Concentrations of nitrogen and other nutrients (Ca, Mg, K, total P) in the foliage samples show no clear trends with increasing soil development. Low temperatures, a short growing season, and very low mean annual precipitation probably limit plant growth and account for the delayed soil development on the Klutlan moraines.  相似文献   

A nonlinear oscillator analog of rigid body dynamics     

John L. Junkins  Ira D. Jacobson  Jeffrey N. Blanton 《Celestial Mechanics and Dynamical Astronomy》1973,7(4):398-407

A rigorously valid nonlinear oscillator analog of the torque-free rotational dynamics of a general rigid body is presented. The analog consists of threeuncoupled nonlinear oscillators, the motion of each being governed by a second order nonlinear ordinary differential equation of the Duffing type. The nonlinear oscillator analog and three associated phase planes, as established herein, provide a new basis for analysis and visualization of rigid body dynamics. The phase planes are particularly useful in providing complete visibility of the motion's limiting cases and stability properties.  相似文献   

A first estimate of ground water ages for the deep aquifer of the Kathmandu Basin, Nepal, using the radioisotope chlorine-36   总被引：1，自引：0，他引：1  

Cresswell RG  Bauld J  Jacobson G  Khadka MS  Jha MG  Shrestha MP  Regmi S 《Ground water》2001,39(3):449-457

The Kathmandu Basin in Nepal contains up to 550 m of Pliocene-Quaternary fluvio-lacustrine sediments which have formed a dual aquifer system. The unconfined sand and gravel aquifer is separated by a clay aquitard, up to 200 m thick, from the deeper, confined aquifer, comprised of Pliocene sand and gravel beds, intercalated with clay, peat, and lignite. The confined aquifer currently provides an important water supply to the central urban area but there are increasing concerns about its sustainability due to overexploitation. A limited number of determinations of the radioisotope 36Cl have been made on bore waters in the basin, allowing us to postulate on the age of ground water in the deeper, confined aquifer. Ground water evolution scenarios based on radioisotope decay, gradual dissolution of formational salts as the ground waters move downgradient, and flow velocity estimations produce comparable ground water ages for the deep waters, ranging from 200,000 to 400,000 years. From these ages, we deduce a mean ground water flow velocity of only 45 mm/year from recharge in the northeast to the main extraction region 15 km to the southwest. We thus estimate current recharge at about 5 to 15 mm/year, contributing 40,000 to 1.2 million m3/year to the ground water system. Current ground water extraction is estimated to be 20 times this amount. The low specific discharge confirms that the resource is being mined, and, based on current projections, reserves will be used up within 100 years.  相似文献   

Field Observation of Diurnal Dissolved Oxygen Fluctuations in Shallow Groundwater          下载免费PDF全文

Keith E. Schilling  Peter Jacobson 《Ground water》2015,53(3):493-497

Dissolved oxygen (DO) concentrations influence many biogeochemical processes in groundwater systems but studies of temporal variability in DO are lacking. In this study, we used an optical DO probe to measure rapid changes in concentration due to plant‐groundwater interaction at an alluvial aquifer field site in Iowa. Diurnal DO concentrations were observed during mid‐ to late‐summer when soil conditions were dry, fluctuating approximately 0.2 to 0.3 mg/L on a daily basis. DO fluctuations in groundwater were out‐of‐phase with diurnal water table fluctuations, increasing during the day and decreasing at night. DO consumption at night is likely due to increased soil autotrophic and heterotrophic respiration linked with patterns of carbon supply derived from daytime photosynthetic activity, and consistent with available literature on diurnal soil respiration patterns. Although more work is needed to quantify specific processes, our results indicate the potential usefulness of the new optical DO technology to reveal insights regarding many ecohydrological processes.  相似文献