Nitrogen fixation in lake Mendota, 1971–1973

The contribution of nitrogen fixation to the nitrogen budget of Lake Mendota has been calculated. On average, the equivalent of 1.28 × 10⁵ kg of NH₃ (as determined by the acetylene reduction technique) was added to this eutrophic lake during June, July and August. Diurnal variation (approximately two-thirds of the day's fixation occurs prior to noon) in algal nitrogen fixation, and variation of fixation with depth (3.6% of the fixation in the column occurs in the top decimeter) were characterized as prerequisites to this calculation.     

Heterotrophic nitrogen fixation (acetylene reduction) associated to flooded rice: A modified measurement technique in the field

A modifiedin situ technique for measuring heterotrophic nitrogen fixing (acetylene reducing) activity associated to rice is proposed. Ethylene evolution rates measured in opaque cylinders covering the stems of rice plants which have been cut 10 cm over the water level were found independent of the diurnal cycle. Cutting of the leaves resulted in decreased variation between plants and suppression of the acceleration of ethylene evolution rate after 12 h incubation as compared to intact plants. In both systems ethylene evolved was swept by a current of methane and the molar ratio between methane and ethylene was stabilized after 12 h. Methane evolution rates remained stable during 12 h and more than 24 h in whole plants and cut plants respectively. It is suggested that alteration in the active gas transport system after 12 h incubation under 10% acetylene may lead to erroneous evaluation of the actual ethylene production in the root's environment. The average values of ethylene evolution rates by cut plants between 12 and 24 h of incubation may be used for comparative studies of nitrogen fixing activity associated to flooded rice.     

A pellet method to demonstrate nitrogen fixation by free-living rhizobia

Summary A method is described to demonstrate nitrogen fixation by free-living Rhizobium cells. After aerobic growth in a nutrient solution, the bacteria are centrifuged. Acetylene reduction by the rhizobial cells in the pellet can be measured within a few days. Hydrogen gas frequently stimulates acetylene reduction.     

Nitrogen cycling in an Antarctic ecosystem 2. Estimation of the amount of nitrogen fixation in a moss community on East Ongul Island

Nitrogen fixation by cyanobacteria in a moss community on East Ongul Island (69°00'S 39°35'E), Antarctica was investigated using the acetylene reduction method. The mean acetylene reduction rate at 10°C and 200 μE·m⁻²·s⁻¹ photosynthetically active radiation was 7.12 nmol C₂H₄ per square centimeter of moss community per hour. The effects of temperature, radiation, desiccation and rehydration on the acetylene reduction rates were examined. A simple predictive model was constructed in order to estimate the amount of nitrogen fixed in the field. Using this model, the daily amount of nitrogen fixation was calculated from microclimatic data (temperature and radiation) measured in the experimental field at Syowa Station on East Ongul Island between 1983 and 1984. The cumulative amount of nitrogen fixation in the growing season during this period was estimated to be 329 mg N per square meter of moss community. It is suggested that nitrogen fixation by cyanobacteria in the moss community is important as a nitrogen source for the community growth on East Ongul Island.     

Nitrogen loss from deserts in the southwestern United States

A lower limit for nitrogen loss from desert ecosystems in the southwestern United States was estimated by comparing nitrogen inputs to the amount of nitrogen stored in desert soils and vegetation. Atmospheric input of nitrogen for the last 10 000 years was conservatively estimated to be 2.99 kg N/m². The amount of nitrogen stored in desert soils was calculated to be 0.604 kg N/m³ using extant data from 212 profiles located in Arizona, California, Nevada, and Utah. The average amount of nitrogen stored in desert vegetation is approximately 0.036 kg N/m².Desert conditions have existed in the southwestern United States throughout the last 10 000 years. Under such conditions, vertical leaching of nitrogen below a depth of 1 m is small (ca. 0.028 kg N/m² over 10 000 years) and streamflow losses of nitrogen from the desert landscape are negligible. Thus, the discrepancy found between nitrogen input and storage represents the amount of nitrogen lost to the atmosphere during the last 10 000 years. Loss of nitrogen to the atmosphere was calculated to be 2.32 kg N/m², which is 77% of the atmospheric inputs.Processes resulting in nitrogen loss to the atmosphere from desert ecosystems include wind erosion, ammonia volatilization, nitrification, and denitrification. Our analysis cannot assess the relative importance of these processes, but each is worthy of future research efforts.     

Nitrogen fixation by the woody legumeLeucaena leucocephala in Tanzania

Summary The nitrogen fixation rate in a 4-year-old stand of the woody legumeLeucaena leucocephala (Lam.) de Wit. was estimated in the field at a rather dry site in Tanzania by use of an acetylene reduction technique. The diurnal mean value during April–May was 35 nmol C₂H₄ mg^–1 (dry weight) nodules h^–1, with a variation between 22±8 and 48±12 nmol C₂H₄ mg^–1 (dry weight) nodules h^–1 in early morning and at midday, respectively. The nodule biomass was determined by auger sampling to be 51±16 kg (dry weight) ha^–1. Most of the nodules were found at the 10–30 cm soil depth level. A rough calculation of the amount of nitrogen fixed annually arrived at 110±30 kg ha^–1. The results give strong support for the use ofL. leucocephala for soil enrichment in less humid areas of tropical Africa.     

Nitrogen fixation by hydrogen-utilizing bacteria

Seventeen strains of nitrogen-fixing bacteria, isolated from different habitats on hydrogen and carbon dioxide as well as on other substrates, morphologically resembled each other. All strains, including Mycobacterium flavum 301, grew autotrophically with hydrogen. The isolate strain 6 was sensitive to oxygen when dependent on N₂ as nitrogen source, a consequence of the sensitivity of its nitrogenase towards oxygen. At the same time, strain 6 was sensitive to hydrogen when growing autotrophically on N₂ as nitrogen source, but hydrogen did not affect acetylene reduction by these cells.Abbreviations MPN Most probable number - BS medium basal salts medium     

Nitrogen fixation and nitrate utilization by marine and freshwater Beggiatoa

Four newly isolated marine strains of Beggiatoa and five freshwater strains were tested for nitrogen fixation in slush agar medium. All strains reduced acetylene when grown microaerobically in media containing a reduced sulfur source and lacking added combined nitrogen. The addition of 2 mmol N, as nitrate or ammonium salts, completely inhibited this reduction. Although not optimized for temperature or cell density, acetylene reduction rates ranged from 3.2 to 12 nmol·mg prot^-1 min^-1. Two freshwater strains did not grow well or reduce acetylene in medium lacking combined nitrogen if sulfide was replaced by thiosulfate. Two other strains grew well in liquid media lacking both combined nitrogen and reduced sulfur compounds but only under lowered concentrations of air. All freshwater strains grew well in medium containing nitrate as the combined nitrogen source. Since they did not reduce acetylene under these conditions, we infer that they can assimilate nitrate.     

pH、温度、NH4+和盐度对圆果雀稗内生固氮菌固氮特性的影响

采用乙炔还原法对从圆果雀稗(Paspalum orbiculare)根、茎中分离到的5株内生固氮菌(POJy11、POG12、POGy22、POGy612、POG4)的固氮特性进行了研究。结果表明,POGy612、POG4固氮的pH适应范围是6.0-10.0,其余3株菌为6.0-8.0;POGy22固氮的最适温度为35℃,其余4株菌为30℃;随着NH4 浓度和NaCl浓度的升高,菌株固氮活性总体呈下降趋势,其中POGy22的固氮活性受NH4 抑制最明显,NH4 浓度为1.0 mmol/L时,已无固氮活性,但POGy612在NH4 浓度为2.5 mmol/L时的固氮活性最高,达到1 045.0 nmol ml-1h-1,POJy11、POGy22在NaCl浓度为0.5%时的固氮活性最高,分别达到735.5 nmol ml-1h-1、703.0 nmol ml-1h-1。     

Mycorrhizal relationships in bottomland hardwood forests of the southern United States

Mycorrhizae are important in the functioning of forest ecosystems worldwide, and play a critical role in water uptake, nutrient acquisition, and prevention of feeder root disease. The majority of mycorrhizal research has been conducted on upland sites, especially in coniferous ecosystems and in commercial agricultural production. However, the maintenance and restoration of bottomland hardwood (BLH) forest ecosystems in the southern United States is of increasing concern. Both ectomycorrhizae and endomycorrhizae are present in BLH forests, although the dominance of one or the other type depends primarily on both the tree species and the hydrologic regime. Ectomycorrhizae tend to be more sensitive to flooding, while endomycorrhizal infection can be present even in permanently flooded soils. The mycorrhizae of sweetgum (Liquidambar styraciflua), green ash (Fraxinus pennsylvanica), and the oaks (Quercus spp.) have been studied most due to their economic importance.Considerable work is still needed to better understand mycorrhizal relationships in BLH ecosystems and associated trees, both with respect to infectivity and nutrient cycling. Such information may be necessary for restoration of BLH forests on old agricultural fields, or to maintain the productivity of BLH forests after harvest. This paper summarizes studies on mycorrhizae relationships in BLH forests and suggests future work necessary for understanding the role mycorrhizae can have in managing these ecosystems.     

Nitrogen fixation byRubus ellipticus J. E. Smith

Summary Nitrogenase activity as assayed by acetylene reduction was observed in detachedRubus ellipticus J. E. Smith root nodules collected in the field and tested under ambient conditions. The nitrogenase activity was 8.4 moles C₂H₄.gfr. wt nodule^–1.h^–1 or 24.0 moles C₂H₄.g dry wt nodule^–1.h^–1 being at a rate comparable with that measured in some other non-legumes assayed in Java at the same time under similar conditions. Nodule morphology bore little resemblance to the root nodules of other non-leguminous plants and nodule structure was different from the other rosaceous examples.The endophyte inhabiting the root nodules was actinomycetal.     

Nitrogen fixation in the rhizosphere and rhizoplane of barley

Summary Aerobic and anaerobic N₂-fixing bacteria developed in the rhizosphere of barley seedlings and exhibited N₂ase activity when seedlings were grown in sterilized sand-nutrient cultures containing low levels of combined nitrogen. The source of the N₂-fixing bacteria appeared to be the seed. Average daily rates up to 0.9 μmoles C₂H₄ h⁻¹ g⁻¹ dry root tissue were measured, but the intensity of the activity was affected by moisture levels and concentration of combined N in the rhizosphere. Removal and washing of the roots did not remove the activity, and roots remained active even after surface-sterilization. An unidentified aerobic N₂-fixing bacterium was isolated from the rhizoplane of active barley roots. Inoculation of barley seedlings with the aerobic N₂-fixing bacterium enhanced N₂ase activity of excised roots 10-fold, with average rates of 0.9, 1.1 and 1.3 μmoles h⁻¹ g⁻¹ dry root assayed under pO₂ of 0.01, 0.02 and 0.04 atm respectively. The aerobic N₂-fixing bacterium also exhibited N₂ase activity when inoculated into the rhizosphere of oat, rice and wheat seedlings. Microscopic observations of sterilized live and stained barley roots suggest that the aerobic N₂-fixing bacterium is an endophyte which infects root tissue and metamorphoses into vesicle-like structures.     

New records for two mosquito predators [Turbellaria: Mesostoma] in the northeastern United States

The accumulated data indicate that mesostomid flatworms are efficient predators of mosquito larvae. This paper presents the 1st published records for 3Mesostoma Ehrenberg, including 2 predators of mosquitoes.Mesostoma californicum Hyman,Mesostoma vernale Hyman, andMesostoma sp. cf.lingua Abildgaard were discovered in a temporary pool in Massachusetts.M. californicum andM. sp. cf.lingua were found to be efficient predators on mosquitoe larvae.

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Résumé Les observations accumulées indiquent que les larves des Plathelminthes mesostomides sont des prédateurs efficaces des larves de moustiques. Cette note présente les premières mentions de 3Mesostoma Ehrenberg incluant 2 prédateurs de moustiques.Mesostoma californicum Hyman,Mesostoma vernale Hyman, etMesostoma sp. cf.lingua Abildgaard furent découverts dans une mare temporaire du Massachusetts.M. californicum etM. sp. cf.lingua ont été reconnus comme des prédateurs efficaces des larves de moustiques.

     

United States legislation and the polar oceans

The United States has important national interests vested in both the Arctic and Antarctic Oceans. Thus, in recent decades the United States has progressively codified its national commitment to conserve and manage both these marine regions and their resources and to protect them from activities that might produce adverse impacts. The U.S. legislation passed since the 1960s selectively affects both polar regions and supports major American policy objectives there: to maintain the Arctic and Antarctic as areas of international cooperation for peaceful purposes; to satisfy economic needs, especially hydrocarbon, mineral, and living resources; to protect opportunities for scientific research; to protect the marine environment; and to conserve living resources in the circumpolar seas. Future trends suggest a shift in U.S. policy attention to the Arctic. Accordingly, new U.S. legislation will be needed to regulate increased activities in the polar north.     

Cytopathology training in the United States – a personal view

In this review we present an outline of cytopathology training in the United States, for a non-US readership.     

Root dynamics in bottomland hardwood forests of the Southeastern United States Coastal Plain

Effects of flooding on root dynamics appear nonlinear and therefore difficult to predict, leading to disparate and often contradictory reports of flooding impacts on production in bottomland hardwood forests. We explored root dynamics in two adjacent wetland habitats by comparing results obtained from several methods of estimating root processes. Also, we tested the influence of flooding on root dynamics of cherrybark, overcup, water and swamp chestnut oaks. Fine root biomass in the laurel oak habitat was greater (< 0.05) than in the swamp tupelo habitat (5.7 vs. 2.4 Mg ha^–1), as was fine root necromass (2.4 vs. 1.3 Mg ha^–1), productivity (2.3 vs. 0.3 Mg ha^–1 yr^–1 when the sum of significant increments method was used, 5.6 vs. 2.5 Mg ha ^–1 yr^–1 when the maximum minus minimum method was used, and 1.2 vs. 1.0 Mg ha^–1 yr ^–1, when the root screen method was used), and turnover (40% and 12% per year). Mortality estimates were lower in the laurel oak habitat (1.3 and 1.2 Mg ha^–1 yr^–1) than in the swamp tupelo community (2.8 and 2.1 Mg ha^–1 yr^–1) when significant increment and maximum minus minimum methods were used, respectively. This apparent contradiction between estimates of production and mortality may be due to more rapid decomposition rates in the more aerated soil of the laurel oak than in the swamp tupelo forest type. Roots in the swamp tupelo habitat appeared to be longer-lived than in the laurel oak habitat. We concluded that there was greater investment in roots in the laurel oak habitat, where a shallow rooting zone and episodes of flooding and drought required drastic changes in root structure and physiology. In contrast, the swamp tupelo habitat had a deeper rooting zone and more consistently moist to flooded hydroperiod, allowing flood adapted roots to persist. The four oak species varied in their phenology of root production and response to flooding, from no difference among treatments for overcup oak to dramatic reductions in root growth during and after flooding for cherrybark oak. Flooding enhanced or at least did not negatively influence root growth in overcup oak, but seriously impacted root growth and survival of cherrybark oak and swamp chestnut oak. Different responses were attributed to the timing of root production: root growth began early for cherrybark oak so spring flooding severely affected this species. Growth in overcup oak began later and ended earlier than the other species tested, allowing the species a means of avoiding flood stress.     

Modeled ecohydrological responses to climate change at seven small watersheds in the northeastern United States

A cross‐site analysis was conducted on seven diverse, forested watersheds in the northeastern United States to evaluate hydrological responses (evapotranspiration, soil moisture, seasonal and annual streamflow, and water stress) to projections of future climate. We used output from four atmosphere–ocean general circulation models (AOGCMs; CCSM4, HadGEM2‐CC, MIROC5, and MRI‐CGCM3) included in Phase 5 of the Coupled Model Intercomparison Project, coupled with two Representative Concentration Pathways (RCP 8.5 and 4.5). The coarse resolution AOGCMs outputs were statistically downscaled using an asynchronous regional regression model to provide finer resolution future climate projections as inputs to the deterministic dynamic ecosystem model PnET‐BGC. Simulation results indicated that projected warmer temperatures and longer growing seasons in the northeastern United States are anticipated to increase evapotranspiration across all sites, although invoking CO₂ effects on vegetation (growth enhancement and increases in water use efficiency (WUE)) diminish this response. The model showed enhanced evapotranspiration resulted in drier growing season conditions across all sites and all scenarios in the future. Spruce‐fir conifer forests have a lower optimum temperature for photosynthesis, making them more susceptible to temperature stress than more tolerant hardwood species, potentially giving hardwoods a competitive advantage in the future. However, some hardwood forests are projected to experience seasonal water stress, despite anticipated increases in precipitation, due to the higher temperatures, earlier loss of snow packs, longer growing seasons, and associated water deficits. Considering future CO₂ effects on WUE in the model alleviated water stress across all sites. Modeled streamflow responses were highly variable, with some sites showing significant increases in annual water yield, while others showed decreases. This variability in streamflow responses poses a challenge to water resource management in the northeastern United States. Our analyses suggest that dominant vegetation type and soil type are important attributes in determining future hydrological responses to climate change.     

Non-destructive semi-quantitative determination of acetylene reduction in the field

Summary Direct injection of acetylene into soil around plant roots, followed by determination of ethylene/acetylene ratios in the soil atmosphere has been tested as a rapid, non-destructive method of estimating acetylene reducing activity. In pots of artificial media as well as in field soil, the ratios determined within 10 min. after injection were significantly correlated with the rates of acetylenedependent ethylene production in detached roots. The method may be useful in preliminary screening of large numbers of plant-bacteria combinations.     

Law of the Sea Issues Between the United States and East Asian States

The United States is the sole superpower in the contemporary world and its role in the development of the law of the sea cannot be ignored. Although having not yet acceded to the U.N. Convention on the Law of the Sea, the United States has contributed to the development of the international law of the sea in numerous ways, including responding to the so-called excessive maritime claims in East Asia and creating new rules of maritime enforcement. This article assesses this recent U.S. practice.