Effects of Succession on Nitrogen Export in the West-Central Cascades, Oregon

This study examined impacts of succession on N export from 20 headwater stream systems in the west central Cascades of Oregon, a region of low anthropogenic N inputs. The seasonal and successional patterns of nitrate (NO₃−N) concentrations drove differences in total dissolved N concentrations because ammonium (NH₄−N) concentrations were very low (usually < 0.005 mg L⁻¹) and mean dissolved organic nitrogen (DON) concentrations were less variable than nitrate concentrations. In contrast to studies suggesting that DON levels strongly dominate in pristine watersheds, DON accounted for 24, 52, and 51% of the overall mean TDN concentration of our young (defined as predominantly in stand initiation and stem exclusion phases), middle-aged (defined as mixes of mostly understory reinitiation and older phases) and old-growth watersheds, respectively. Although other studies of cutting in unpolluted forests have suggested a harvest effect lasting 5 years or less, our young successional watersheds that were all older than 10 years still lost significantly more N, primarily as NO₃−N, than did watersheds containing more mature forests, even though all forest floor and mineral soil C:N ratios were well above levels reported in the literature for leaching of dissolved inorganic nitrogen. The influence of alder may contribute to these patterns, although hardwood cover was quite low in all watersheds; it is possible that in forested ecosystems with very low anthropogenic N inputs, even very low alder cover in riparian zones can cause elevated N exports. Only the youngest watersheds, with the highest nitrate losses, exhibited seasonal patterns of increased summer uptake by vegetation as well as flushing at the onset of fall freshets. Older watersheds with lower N losses did not exhibit seasonal patterns for any N species. The results, taken together, suggest a role for both vegetation and hydrology in N retention and loss, and add to our understanding of N cycling by successional forest ecosystems influenced by disturbance at various spatial and temporal scales in a region of relatively low anthropogenic N input.     

The Long-term Effects of Disturbance on Organic and Inorganic Nitrogen Export in the White Mountains, New Hampshire

Traditional biogeochemical theories suggest that ecosystem nitrogen retention is controlled by biotic N limitation, that stream N losses should increase with successional age, and that increasing N deposition will accelerate this process. These theories ignore the role of dissolved organic nitrogen (DON) as a mechanism of N loss. We examined patterns of organic and inorganic N export from sets of old-growth and historically (80–110 years ago) logged and burned watersheds in the northeastern US, a region of moderate, elevated N deposition. Stream nitrate concentrations were strongly seasonal, and mean (± SD) nitrate export from old-growth watersheds (1.4 ± 0.6 kg N ha⁻¹ y⁻¹) was four times greater than from disturbed watersheds (0.3 ± 0.3 kg N ha⁻¹ y⁻¹), suggesting that biotic control over nitrate loss can persist for a century. DON loss averaged 0.7 (± 0.2) kg N ha⁻¹ y⁻¹ and accounted for 28–87% of total dissolved N (TDN) export. DON concentrations did not vary seasonally or with successional status, but correlated with dissolved organic carbon (DOC), which varied inversely with hardwood forest cover. The patterns of DON loss did not follow expected differences in biotic N demand but instead were consistent with expected differences in DOC production and sorption. Despite decades of moderate N deposition, TDN export was low, and even old-growth forests retained at least 65% of N inputs. The reasons for this high N retention are unclear: if due to a large capacity for N storage or biological removal, N saturation may require several decades to occur; if due to interannual climate variability, large losses of nitrate may occur much sooner. Received 27 April 1999; accepted 30 May 2000.     

Effects of Herbivory,Fire and N<Subscript>2</Subscript>-fixation on Nutrient Limitation in a Humid African Savanna

The quantities and spatial distribution of nutrients in savanna ecosystems are affected by many factors, of which fire, herbivory and symbiotic N₂-fixation are particularly important. We measured soil nitrogen (N) pools and the relative abundance of N and phosphorus (P) in herbaceous vegetation in five vegetation types in a humid savanna in Tanzania. We also performed a factorial fertilization experiment to investigate which nutrients most limit herbaceous production. N pools in the top 10 cm of soil were low at sites where fires were frequent, and higher in areas with woody legume encroachment, or high herbivore excretion. Biomass production was co-limited by N and P at sites that were frequently burnt or heavily grazed by native herbivores. In contrast, aboveground production was limited by N in areas receiving large amounts of excreta from livestock. N₂-fixation by woody legumes did not lead to P-limitation, but did increase the availability of N relative to P. We conclude that the effects of fire, herbivory and N₂-fixation upon soil N pools and N:P-stoichiometry in savanna ecosystems are, to a large extent, predictable. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author Contributions  P.C., H.O.V. and P.E. designed the study and wrote the paper. P.C. and T.K. performed the research and analyzed the data.     

Reduction of N<Subscript>2</Subscript> by Fe<Superscript>2+</Superscript> via Homogeneous and Heterogeneous Reactions Part 2: The Role of Metal Binding in Activating N<Subscript>2</Subscript> for Reduction; a Requirement for Both Pre-biotic and Biological Mechanisms

Nitrogen reduction by ferrous iron has been suggested as an important mechanism in the formation of ammonia on pre-biotic Earth. This paper examines the effects of adsorption of ferrous iron onto a goethite (alpha-FeOOH) substrate on the thermodynamic driving force and rate of a ferrous iron-mediated reduction of N2 as compared with the homogeneous aqueous reaction. Utilizing density functional theory and Marcus Theory of proton coupled electron transfer reactions, the following two reactions were studied: Fe2+aq + N2aq + H2Oaq --> N2H* + FeOH2+aq and triple bond Fe2+ads + N2aq + 2H2Oaq --> N2H* + alpha-FeOOHs + 2H+aq. Although the rates of both reactions were calculated to be approximately zero at 298 K, the model results suggest that adsorption alters the thermodynamic driving force for the reaction but has no other effect on the direct electron transfer kinetics. Given that simply altering the thermodynamic driving force will not reduce dinitrogen, we can make mechanistic connections between possible prebiotic pathways and biological N2 reduction. The key to reduction in both cases is N2 adsorption to multiple transition metal centers with competitive H2 production.     

Nitrogen attenuation in the Connecticut River,northeastern USA; a comparison of mass balance and N<Subscript>2</Subscript> production modeling approaches

Two methods were used to measure in-stream nitrogen loss in the Connecticut River during studies conducted in April and August 2005. A mass balance on nitrogen inputs and output for two study reaches (55 and 66 km), at spring high flow and at summer low flow, was computed on the basis of total nitrogen concentrations and measured river discharges in the Connecticut River and its tributaries. In a 10.3 km subreach of the northern 66 km reach, concentrations of dissolved N₂ were also measured during summer low flow and compared to modeled N₂ concentrations (based on temperature and atmospheric gas exchange rates) to determine the measured “excess” N₂ that indicates denitrification. Mass balance results showed no in-stream nitrogen loss in either reach during April 2005, and no nitrogen loss in the southern 55 km study reach during August 2005. In the northern 66 km reach during August 2005, however, nitrogen output was 18% less than the total nitrogen inputs to the reach. N₂ sampling results gave an estimated rate of N₂ production that would remove 3.3% of the nitrogen load in the river over the 10.3 km northern sub-reach. The nitrogen losses measured in the northern reach in August 2005 may represent an approximate upper limit for nitrogen attenuation in the Connecticut River because denitrification processes are most active during warm summer temperatures and because the study was performed during the annual low-flow period when total nitrogen loads are small.     

The determinants of stroke phenotypes were different from the predictors (CHADS<Subscript>2</Subscript> and CHA<Subscript>2</Subscript>DS<Subscript>2</Subscript>-VASc) of stroke in patients with atrial fibrillation: a comprehensive approach

Background  

Atrial fibrillation (AF) is a leading cause of fatal ischemic stroke. It was recently reported that international normalized ratio (INR) levels were associated with infarct volumes. However, factors other than INR levels that affect stroke phenotypes are largely unknown. Therefore, we evaluated the determinants of stroke phenotypes (pattern and volume) among patients with AF who were not adequately anticoagulated.     

The role of N<Subscript>2</Subscript> fixation in alleviating N limitation in wetland metaphyton: enzymatic,isotopic, and elemental evidence

The role of nitrogen (N₂) fixation in balancing N supply to wetland metaphyton was assessed by comparing primary production with enzymatic, isotopic, and elemental correlates. Primary production, N₂ fixation (acetylene reduction, AR), phosphatase activity, C:N:P ratio, and N isotopic composition of metaphyton were measured along a nutrient gradient in a freshwater marsh during May through September 2004. N₂ fixation and phosphatase activity in metaphyton were negatively correlated with inorganic N and P concentrations, respectively. Although metaphyton N₂ fixation demonstrated a clear spatial pattern along the nutrient gradient, N₂ fixation rates varied monthly and decreased sharply in September. However, the percent contribution of N₂ fixation to N uptake by metaphyton consistently decreased throughout the summer. Furthermore, the decreased contribution of N₂ fixation to N uptake corresponded with an increase in metaphyton N content during the growing season. Nitrogen isotopic data suggested the sustained importance of an atmospheric N₂ source through September at the most downstream (nutrient poor) site even though the percent contribution of N₂ fixition to N uptake was lowest in that month. This suggests that metaphyton were efficiently accumulating and recycling fixed N₂ in support of primary production. Over the course of the summer, metaphyton primary production showed a weak inverse correlation with metaphyton phosphatase activity (r ² = 0.58). The largest residuals in this regression corresponded to the largest vartiation in metaphyton N content. When metaphyton primary production was normalized to metaphyton N content, production rates for the entire growing season were more strongly inversely correlated with metaphyton phosphatase activity (r ² = 0.78). Results of the study suggest that N₂ fixation in N poor areas may adequately supplement community N requirements in metaphyton, thereby causing limitation by other elemental resources such as P.     

A test of a potential short cut in the nitrogen cycle: The role of exudation of symbiotically fixed nitrogen from the roots of a N-fixing tree and the effects of increased atmospheric CO2 and temperature

N-fixing trees facilitate the growth of neighboring trees of other species. These neighboring species benefit from the simple presence of the N fixation symbiosis in their surroundings. Because of this phenomenon, it has been hypothesized that a change in atmospheric CO₂ concentration may alter the role of N-fixing trees in their environment. It is thought that the role of N-fixing trees in ecosystems of the future may be more important since they may help sustain growth increases due to increased CO₂ concentration in nitrogen limited forests. We examined: (1) whether symbiotically fixed N was exuded from roots, (2) whether a doubled atmospheric CO₂ concentration would result in increased organic N exudation from roots, and (3) whether increased temperature or N availability affected N exudation from roots. This study analyzed exudation of dissolved organic N from the roots of seedlings of the N-fixing tree Robinia pseudoacacia L. in a full factorial design with 2 CO₂ (35.0 and 70.0 Pa) × 2 temperature (26 or 30 °C during the day) × 2 N fertilizer (0 and 10.0 mM N concentration) levels. Trees with no other source of N except N fixation exuded about 1% to 2% of the fixed N through their roots as dissolved organic N. Increased atmospheric CO₂ concentrations did not, however, increase N exudation rates on a per gram belowground biomass basis. A 4 °C increase in temperature and N fertilization did, however, significantly increase N exudation rates. These results suggest that exudation of dissolved organic N from roots or nodules of N-fixing trees could be a significant, but minor, pathway of transferring N to neighboring plants in a much more rapid and direct way than cycling through death, decomposition and mineralization of plant residues. And, while exudation rates of dissolved organic N from roots were not significantly affected by atmospheric CO₂ concentration, the previously observed CO₂ fertilization effect on N-fixing trees suggests that N exudation from roots could play a significant but minor role in sustaining increases in forest growth, and thus C storage, in a CO₂ enriched atmosphere.     

The structure and function of the cytochrome <Emphasis Type="Italic">c</Emphasis><Subscript>2</Subscript>: reaction center electron transfer complex from <Emphasis Type="Italic">Rhodobacter sphaeroides</Emphasis>

In the photosynthetic bacterium, Rhodobacter sphaeroides, the mobile electron carrier, cytochrome c₂ (cyt c₂) transfers an electron from reduced heme to the photooxidized bacteriochlorophyll dimer in the membrane bound reaction center (RC) as part of the light induced cyclic electron transfer chain. A complex between these two proteins that is active in electron transfer has been crystallized and its structure determined by X-ray diffraction. The structure of the cyt:RC complex shows the cyt c₂ (cyt c₂) positioned at the center of the periplasmic surface of the RC. The exposed heme edge from cyt c₂ is in close tunneling contact with the electron acceptor through an intervening bridging residue, Tyr L162 located on the RC surface directly above the bacteriochlorophyll dimer. The binding interface between the two proteins can be divided into two regions: a short-range interaction domain and a long-range interaction domain. The short-range domain includes residues immediately surrounding the tunneling contact region around the heme and Tyr L162 that display close intermolecular contacts optimized for electron transfer. These include a small number of hydrophobic interactions, hydrogen bonds and a pi-cation interaction. The long-range interaction domain consists of solvated complementary charged residues; positively charged residues from the cyt and negatively charged residues from the RC that provide long range electrostatic interactions that can steer the two proteins into position for rapid association.     

Population Structure and Variation in Red Snapper (<Emphasis Type="Italic">Lutjanus campechanus</Emphasis>) from the Gulf of Mexico and Atlantic Coast of Florida as Determined from Mitochondrial DNA Control Region Sequence

The mitochondrial DNA control regions of red snapper (Lutjanus campechanus) from the Gulf of Mexico (n = 140) and Atlantic coast of Florida (n = 35) were sequenced to generate a prestocking genetic baseline for planned stock enhancement. Intrasample haplotype and nucleotide diversities ranged from 0.94 to 1.00 and 1.8% to 2.5%, respectively. All population analyses were consistent with the hypothesis that red snapper constitute a single, panmictic population over the sampled range. A ubiquitous, predominant haplotype, shared by 23% of the specimens, appeared to be evolutionarily recent, in contrast to previous findings based on restriction fragment length polymorphism data. Tajimas D values were suggestive of a recent bottleneck. Mismatch distributions from Gulf samples were smooth and unimodal, characteristic of recent population expansion. However, the Atlantic sample exhibited a comparatively broader, possibly multimodal distribution, suggestive of a more stable population history. Additional control-region data may clarify potentially disparate demographic histories of Gulf and Atlantic snapper.     

Microalgal removal of CO<Subscript>2</Subscript> from flue gases: Changes in medium pH and flue gas composition do not appear to affect the photochemical yield of microalgal cultures

Our research objectives are to determine under what conditions microalgal-based CO₂ capture from flue gases is economically attractive. Specifically, our objective here was to select microalgae that are temperature, pH and flue gas tolerant. Microalgae were grown under five different temperatures, three different pH and five different flue gas mixtures besides 100% CO₂ (gas concentrations that the cells were exposed to ranged 5.7–100% CO₂, 0–3504 ppm SO₂, 0–328 ppm NO, and 0–126 ppm NO₂). Our results indicate that the microalgal strains tested exhibit a substantial ability to withstand a wide range of temperature (54 strains tested), pH (20 strains tested) and flue gas composition (24 strains tested) likely to be encountered in cultures used for carbon sequestration from smoke stack gases. Our results indicate that microalgal photosynthesis is a limited but viable strategy for CO₂ capture from flue gases produced by stationary combustion sources.     

Identification of agronomically favorable quantitative trait loci alleles from a dent corn inbred Dan232 using advanced backcross QTL analysis and comparison with the F<Subscript>2:3</Subscript> population in popcorn

Specific traits are an important consideration in plant breeding. In popcorn, inferior agronomic traits could be improved using dent or flint corn backcrossed with popcorn. In this study, we used advanced backcross quantitative trait locus (AB-QTL) analysis to identify trait-improving QTL alleles from a dent maize inbred Dan232, and compared the detection of QTL in the BC₂S₁ population with QTL results using F_2:3 families of the same population. Two hundred and twenty BC₂S₁ families developed from a cross between Dan232 and an elite popcorn inbred N04 were evaluated for nine plant traits in replicated field trials under two environments. Using composite interval mapping (CIM), a total of 28 significant QTL were detected, and of these, 23 (82.14%) had favorable alleles contributed by the dent corn parent Dan232. Nine QTL (32.14%) detected in the BC₂S₁ population were also located in or near the same chromosome intervals in the F_2:3 population. All of the favorable QTL alleles from Dan232 could be used in marker-assisted selection (MAS) to improve the respective plant traits in popcorn breeding. In addition, their near isogenic lines (QTL-NILs) could be obtained through selfing or another 1–2 backcross with N04. Also, N04 improved for the studied plant traits could be developed from the BC₂S₁ families used in this study. This study demonstrated that the AB-QTL method can be applied to identify favorable QTL from dent corn inbred in popcorn breeding and, once identified, the alleles could be used in marker-assisted selection to improve the respective plant traits.     

Mechanism of Activation of Caspase-9 and Caspase-3 during Hypoxia in the Cerebral Cortex of Newborn Piglets: The Role of Nuclear Ca<Superscript>2+</Superscript>-Influx

In previous studies, we have shown that cerebral hypoxia results in increased activity of caspase-9, the initiator caspase, and caspase-3, the executioner of programmed cell death. We have also shown that cerebral hypoxia results in high affinity Ca²⁺–ATPase-dependent increase in nuclear Ca²⁺-influx in the cerebral cortex of newborn piglets. The present study tests the hypothesis that inhibiting nuclear Ca²⁺-influx by pretreatment with clonidine, an inhibitor of high affinity Ca²⁺–ATPase, will prevent the hypoxia-induced increase in caspase-9 and caspase-3 activity in the cerebral cortex of newborn piglets. Thirteen newborn piglets were divided into three groups, normoxic (Nx, n = 4), hypoxic (Hx, n = 4), and hypoxic treated with clonidine (100 mg/kg) (Hx–Cl, n = 5). Anesthetized, ventilated animals were exposed to an FiO₂ of 0.21 (Nx) or 0.07 (Hx) for 60 min. Cerebral tissue hypoxia was documented biochemically by determining levels of ATP and phosphocreatine (PCr). Caspase-9 and -3 activity were determined spectrofluoro-metrically using specific fluorogenic synthetic substrates. ATP (μmoles/g brain) was 4.6 ± 0.3 in Nx, 1.7±0.4 in Hx (P < 0.05 vs. Nx), and 1.5 ± 0.2 in Hx–Cl (P < 0.05 vs. Nx). PCr (μmoles/g brain) was 3.6 ± 0.4 in Nx, 1.1 ± 0.3 in Hx (P < 0.05 vs. Nx), and 1.0 ± 0.2 in Hx–Cl (P < 0.05 vs. Nx). Caspase-9 activity (nmoles/mg protein/h) was 0.548 ± 0.0642 in Nx and increased to 0.808 ± 0.080 (P < 0.05 vs. Nx and Hx–Cl) in the Hx and 0.562 ± 0.050 in the Hx–Cl group (p = NS vs. Nx). Caspase-3 activity (nmoles/mg protein/h) was 22.0 ± 1.3 in Nx and 32 ± 6.3 in Hx (P < 0.05 vs. Nx) and 18.8 ± 3.2 in the Hx–Cl group (P < 0.05 vs. Hx). The data demonstrate that clonidine administration prior to hypoxia prevents the hypoxia-induced increase in the activity of caspase-9 and caspase-3. We conclude that the high afinity Ca²⁺–ATPase-dependent increased nuclear Ca²⁺ during hypoxia results in increased caspase-9 and caspase-3 activity.