Non‐symbiotic nitrogen fixation during leaf litter decomposition in an old‐growth temperate rain forest of Chiloé Island,southern Chile: Effects of single versus mixed species litter

Heterotrophic nitrogen fixation is a key ecosystem process in unpolluted, temperate old‐growth forests of southern South America as a source of new nitrogen to ecosystems. Decomposing leaf litter is an energy‐rich substrate that favours the occurrence of this energy demanding process. Following the niche ‘complementarity hypothesis’, we expected that decomposing leaf litter of a single tree species would support lower rates of non‐symbiotic N fixation than mixed species litter taken from the forest floor. To test this hypothesis we measured acetylene reduction activity in the decomposing monospecific litter of three evergreen tree species (litter C/N ratios, 50–79) in an old‐growth rain forest of Chiloé Island, southern Chile. Results showed a significant effect of species and month (anova , Tukey's test, P < 0.05) on decomposition and acetylene reduction rates (ARR), and a species effect on C/N ratios and initial % N of decomposing leaf litter. The lowest litter quality was that of Nothofagus nitida (C/N ratio = 78.7, lignin % = 59.27 ± 4.09), which resulted in higher rates of acetylene reduction activity (mean = 34.09 ± SE = 10.34 nmol h^?1 g^?1) and a higher decomposition rate (k = 0.47) than Podocarpus nubigena (C/N = 54.4, lignin % = 40.31 ± 6.86, Mean ARR = 4.11 ± 0.71 nmol h^?1 g^?1, k = 0.29), and Drimys winteri (C/N = 50.6, lignin % = 45.49 ± 6.28, ARR = 10.2 ± 4.01 nmol h^?1 g^?1, k = 0.29), and mixed species litter (C/N = 60.7, ARR = 8.89 ± 2.13 nmol h^?1g^?1). We interpret these results as follows: in N‐poor litter and high lignin content of leaves (e.g. N. nitida) free‐living N fixers would be at competitive advantage over non‐fixers, thereby becoming more active. Lower ARR in mixed litter can be a consequence of a lower litter C/N ratio compared with single species litter. We also found a strong coupling between in situ acetylene reduction and net N mineralization in surface soils, suggesting that as soon N is fixed by diazotroph bacteria it may be immediately incorporated into mineral soil by N mineralizers, thus reducing N immobilization.     

ROLE OF ENCYSTMENT AND EXCYSTMENT OF PERIDINIUM BIPES F. OCCULATUM (DINOPHYCEAE) IN FRESH WATER RED TIDES IN LAKE KIZAKI,JAPAN1

The encystment flux of Peridinium bipes f. occulatum (Dinophyceae) was investigated with sediment traps from 1968 to 1990 in Lake Kazki. Cysts of P. bipes were formed throughout the blooms, Encystment flux of P. bipes in the pelagic zone was usually lower than those at shallow sites, and the density of P. bipes cysts in lake sediment was higher in the shallow region than in the pelagic zone. However, in the shallower region, The concentration of P. bipes cysts varied widely, possibly due to high rates of encystment and excystment. Peridinium bipes encystment occurred between 15° and 25° C in the laboratory, with very little cyst formation below 10°C. Though cyst formation was observed in continous darkness, the rate increased with irradiance. Under continuous darkness, no excystment was observed at any temperature from 5° to 25° C. Eighty-one percent of the cysts illuminated at 105 μE m^?2 s^?1 excysted after 13 days incubation at 15° C, and lower irradiances decreased germination success. Results from laboratory experiments suggest that light is a critical factor in the germination of P. bipes cysts. Bottom depth thus can have a significant effect on germination because cysts only can excyst from depths where light is sufficient. The shallow region of the lake is thus very important as a seed bed for P. bipes during early spring. Cyst deposited in deeper waters may not ever germinate unless they are resuspended and transported to shallow areas where light reaches the bottom.     

Differences in δ13C and δ15N stable isotopes in the pearly razorfish Xyrichtys novacula related to the sex,location and spawning period

In the present study, Xyrichtys novacula (Labridae) were sampled at five locations around the islands of Ibiza and Formentera (western Mediterranean Sea). Isotopic signatures of δ¹³C, δ¹⁵N and the C:N ratio were analysed in relation to locality, sex and size differences. δ¹³C and δ¹⁵N partitioning was also studied in the reproductive spawning period. There were significant differences in the δ¹³C signature between localities for both sexes, but not for δ¹⁵N. Sex differences were also found with a mean ±s.e . value of ?17·38 ± 0·06‰δ¹³C and 8·36 ± 0·05‰δ¹⁵N for females and ?17·17 ± 0·07‰δ¹³C and 8·80 ± 0·06‰δ¹⁵N for males. Increasing total length in both sexes was positively correlated with δ¹⁵N enrichment and a significant positive linear regression was established for both variables. During the reproductive spawning period, there were changes in δ¹³C fractioning with enrichment in postspawning females and males (with respect to prespawning and spawning periods) and δ¹⁵N impoverishment in postspawning females (with respect to prespawning and spawning periods). Xyrichtys novacula uses local food sources, as confirmed by δ¹³C and δ¹⁵N, and females and males use different food sources, thus avoiding intraspecific competition. This was confirmed by δ¹⁵N enrichment as size increased. Spawning leads to special requirements for gonad maturation, which is reflected in the isotopic signatures for both sexes.     

Microbial physiology and soil CO2 efflux after 9 years of soil warming in a temperate forest – no indications for thermal adaptations

Thermal adaptations of soil microorganisms could mitigate or facilitate global warming effects on soil organic matter (SOM) decomposition and soil CO₂ efflux. We incubated soil from warmed and control subplots of a forest soil warming experiment to assess whether 9 years of soil warming affected the rates and the temperature sensitivity of the soil CO₂ efflux, extracellular enzyme activities, microbial efficiency, and gross N mineralization. Mineral soil (0–10 cm depth) was incubated at temperatures ranging from 3 to 23 °C. No adaptations to long‐term warming were observed regarding the heterotrophic soil CO₂ efflux (R₁₀ warmed: 2.31 ± 0.15 μmol m^?2 s^?1, control: 2.34 ± 0.29 μmol m^?2 s^?1; Q₁₀ warmed: 2.45 ± 0.06, control: 2.45 ± 0.04). Potential enzyme activities increased with incubation temperature, but the temperature sensitivity of the enzymes did not differ between the warmed and the control soils. The ratio of C : N acquiring enzyme activities was significantly higher in the warmed soil. Microbial biomass‐specific respiration rates increased with incubation temperature, but the rates and the temperature sensitivity (Q₁₀ warmed: 2.54 ± 0.23, control 2.75 ± 0.17) did not differ between warmed and control soils. Microbial substrate use efficiency (SUE) declined with increasing incubation temperature in both, warmed and control, soils. SUE and its temperature sensitivity (Q₁₀ warmed: 0.84 ± 0.03, control: 0.88 ± 0.01) did not differ between warmed and control soils either. Gross N mineralization was invariant to incubation temperature and was not affected by long‐term soil warming. Our results indicate that thermal adaptations of the microbial decomposer community are unlikely to occur in C‐rich calcareous temperate forest soils.     

Rythme d'activité endogène chez un nébaliacé (Crustace phyllocaride)

In constant conditions and continuous darkness, the burrowing species Nebalia bipes (Fabricius) exhibits a clear circadian rhythm of emergence from the substratum and of swimming activity. The rhythm, in which activity occurs during the ‘expected night’, is obvious in both males and females and can remain overt for several weeks.The ‘free-running period’, although variable, is ≈ 24 h 20 min in a calm environment and at 18 °C. During the first day of the experiments, the swimming activity of freshly collected animals increases abruptly at dusk and then decreases gradually towards dawn. It has been possible to change the phase of the rhythm by subjecting the animals to a reversed LD schedule.The synchronization seems to result from the high photonegativity of Nebalia in normal daylight. The results of some experiments carried out with young juvenile animals suggests that the synchronization may possibly take place during the incubation period.     

Using stable isotope ratios as a tracer of feeding adaptation in released Japanese flounder Paralichthys olivaceus

Release‐recapture experiments were conducted to examine temporal changes of the carbon and nitrogen stable isotope (δ¹³C and δ¹⁵N) ratios in the muscle tissue of artificially produced Japanese flounder Paralichthys olivaceus, juveniles. About 9000 juveniles (mean ± s .d . 43·3 ± 5·2 mm in standard length and 1·07 ± 0·37 g, n = 15) were released in each of three coastal areas: Chojagasaki, Arasaki and Jogashima with different geographical conditions, along Sagami Bay, Pacific coast of central Japan. Recapture efforts were made on 4, 11, 18, 40 and 55 days after the release. The stable isotope ratios, RNA:DNA ratio, stomach content mass (per body mass M_sc) and condition factor (K) of recaptured individuals were measured. The mean ± s .d . δ¹³C and δ¹⁵N values (n = 15) were ?18·3 ± 0·2‰ and 12·2 ± 0·2‰, respectively at the release. Wild Japanese flounder juveniles were captured only in Chojagasaki, and the δ¹³C and δ¹⁵N values (n = 6) were ?14·0 ± 0·4‰ and 13·2 ± 0·7‰, respectively; these values were considered to represent the wild diet. Nutritional conditions of the released and recaptured juveniles as determined by the RNA : DNA ratio, M_SC and K were indicated to be the best in Chojagasaki, in which the stable isotope ratios gradually shifted towards and reached the wild values within 40 days. This result along with stomach content analyses suggested that the released juveniles had acquired a wild feeding habit. In Arasaki and Jogashima, nutritional conditions of the recaptured juveniles were poorer, with no clear changes in the stable isotope ratios. Greatly varied stable isotope ratio values were observed in the juveniles recaptured in Chojagasaki 11 days after the release, ranging from the release levels to the wild levels. The extent of changes in the stable isotope ratios had a positive correlation to the RNA : DNA ratio and K of these juveniles (r = 0·87, n = 10 and r = 0·83, n = 18, respectively). The analyses of stable isotope ratios coupled with nutritional condition were considered to be an effective tool to examine post‐release feeding adaptation of Japanese flounder juveniles.     

The effect of host Chlorella NC64A carbon : phosphorus ratio on the production of Paramecium bursaria Chlorella Virus-1

1. We used the freshwater alga Chlorella NC64A (Division Chlorophyta) and its virus Paramecium bursaria Chlorella virus‐1 (PBCV‐1) as a model system to test for potential stoichiometric constraints on a virus–host interaction. 2. Media phosphorus concentrations were manipulated to create Chlorella NC64A host cells with low (91 ± 23) or high (453 ± 246) C : P ratio. In contrast, the C : P ratio of PBCV‐1, calculated from its biochemical composition, was 17 : 1. 3. Stoichiometric theory predicts that infection success and postinfection viral production should be depressed in high C : P cultures due to insufficient intracellular P for production of P‐rich viral particles. 4. Consistent with this hypothesis, viral production was strongly affected by host C : P ratio. While host C : P ratio did not affect viral attachment or the percentage of new viral particles that were infectious, in the low C : P Chlorella NC64A treatment, nine times more viruses were produced per infected cell than in the high C : P treatment (158 ± 138 versus 18 ± 18), indicating that the low C : P cells were higher quality for PBCV‐1 proliferation. 5. This result implies that the stoichiometric quality of algal cells can have a major effect on host–virus population dynamics.     

Effect of Temperature and Salinity on the Biochemical Composition of Artemia franciscana KELLOGG 1906, from Araya (Venezuela) and San José (México)

The effect of different temperature and salinity combinations on the biochemical composition of Artemia franciscana from Venezuela and Mexico, is analyzed. Temperatures were 22 ± 0.5 °C, 26 ± 0.5 °C and 30 ± 0.5°C; salinities were 30‰, 60‰, and 120‰. Chaetoceros sp. was used as food. According to Tukey's Multiple Range Analysis for the A. franciscana population from Araya and San José, there were differences in the biochemical parameters and survival percentages among treatments and between populations. A positive correlation is observed among proximate composition values and survival, total length and growth rates. The observed variations reflect a genetic component resulting from the life history of the populations, and a non-genetic component produced by the experimental conditions.     

DIAZOTROPHIC GROWTH OF THE MARINE CYANOBACTERIUM TRICHODESMIUM IMS101 IN CONTINUOUS CULTURE: EFFECTS OF GROWTH RATE ON N2‐FIXATION RATE,BIOMASS, AND C:N:P STOICHIOMETRY1

Trichodesmium N₂ fixation has been studied for decades in situ and, recently, in controlled laboratory conditions; yet N₂‐fixation rate estimates still vary widely. This variance has made it difficult to accurately estimate the input of new nitrogen (N) by Trichodesmium to the oligotrophic gyres of the world ocean. Field and culture studies demonstrate that trace metal limitation, phosphate availability, the preferential uptake of combined N, light intensity, and temperature may all affect N₂ fixation, but the interactions between growth rate and N₂ fixation have not been well characterized in this marine diazotroph. To determine the effects of growth rate on N₂ fixation, we established phosphorus (P)–limited continuous cultures of Trichodesmium, which we maintained at nine steady‐state growth rates ranging from 0.27 to 0.67 d^?1. As growth rate increased, biomass (measured as particulate N) decreased, and N₂‐fixation rate increased linearly. The carbon to nitrogen ratio (C:N) varied from 5.5 to 6.2, with a mean of 5.8 ± 0.2 (mean ± SD, N = 9), and decreased significantly with growth rate. The N:P ratio varied from 23.4 to 45.9, with a mean of 30.5 ± 6.6 (mean ± SD, N = 9), and remained relatively constant over the range of growth rates studied. Relative constancy of C:N:P ratios suggests a tight coupling between the uptake of these three macronutrients and steady‐state growth across the range of growth rates. Our work demonstrates that growth rate must be considered when planning studies of the effects of environmental factors on N₂ fixation and when modeling the impact of Trichodesmium as a source of new N to oligotrophic regions of the ocean.     

Sequence-based diagnostics and phylogenetic approach of uncultured freshwater dinoflagellate <Emphasis Type="BoldItalic">Peridinium</Emphasis> (Dinophyceae) species,based on single-cell sequencing of rDNA

The armoured dinoflagellate Peridinium is widely distributed in freshwater environments worldwide and contains a large number of species. Their identity, however, has remained elusive, since the small cells tend to be morphologically similar. To help resolve this, a sequence-based diagnostics for uncultured Peridinium cells from field samples was applied, using single-cell PCR and direct DNA sequencing of the PCR products. Single cells were isolated randomly from field samples, and PCR successfully amplified the target rDNA regions from the crude lysates. Phylogenetic trees showed that all the cells were strongly grouped into the same clade (> 99% bootstrap value), including the previously identified P. bipes f. occultatum, and apparently separated from relatives such as P. cinctum, P. volzii and P. willei. All 17 isolates were genotypically identified as P. bipes f. occultatum, based on over than 99% of sequence similarities, and the organism was responsible for water blooms at different seasons in Korean waters. The sequence-based typing could clearly resolve P. bipes f. occultatum from the various Peridinium cells, and that the method is accurate and more labor-saving than the conventional method to monitor Peridinium species. This protocol may be useful for the application of molecular tools to uncultured Peridinium cells.     

Submucosal gland distribution in the mouse has a genetic determination localized on Chromosome 9

Submucosal glands (SMG) are important secretory glands that are present in the major airways and bronchioles of humans. In mice the structure, cellular composition, and density of SMG are similar to those seen in humans, but the glands are present only in the trachea. Characterization of SMG is important as they secrete bacteriocidal products such as lactoferrin, lysozyme, and defensins believed to be of importance in the innate defense system. Serous cells in SMG are the primary site of cystic fibrosis transmembrane conductance regulator (CFTR) gene expression and the initial site of histological abnormality in cystic fibrosis (CF) individuals. In this study, we examined four inbred strains of mice (A/J, C57BL/6N, FVB/N, and BALB/CAnN) and revealed that the extent to which glands descend in the mouse trachea varied between inbred strains. In particular, the A/J and C57BL/6N strains exhibited few SMG extending further than the first or second intercartilaginous space (mean depth of 0.4 ± 0.11 and 1.5 ± 0.32 tracheal rings respectively) in the trachea, whereas the FVB/N and BALB/CAnN strains had SMG extending beyond the fourth space (mean depths of 3.3 ± 0.46 and 5.6 ± 0.45 rings respectively). We have previously shown that in congenic C57Bl/6N Cftr mutant mice (CF mice), the SMG are distributed more distally than in wild-type C57Bl/6N but are indistinguishable from BALB/CAnN wild-type or CF mice. The implication that SMG distribution is influenced by Cftr gene expression (or a gene closely linked to Cftr) led us to investigate the genetic difference between C57Bl6/N and BALB/CAnN mice. In recombinant inbred strain (RIS) analysis (with BALB/CJ and C57BL/6J progenitors), two loci were identified as being linked to the SMG phenotype (peak likelihood statistic levels of 8.8 and 9.9 on Chrs 9 and 10 respectively, indicating suggestive linkage). A subsequent segregation analysis of an F₂ intercross between the C57BL/6N and BALB/CAnN mice indicated that there were at least two major genetic factors responsible for SMG distribution. The loci indicated in the RI analysis were included in a targeted genome scan involving 235 F₂ intercross animals (C57BL/6N and BALB/CAnN strain intercross). The genome scan confirmed the locus on Chr 9 (between genetic markers D9Mit11 and D9Mit182), designated Smgd1, as significantly linked to the SMG distribution phenotype (peak LOD score 5.8) within a 95% confidence interval of 12 cM. Received: 26 June 2000 / Accepted: 18 September 2000     

Thermotaxis and selection of a wintering site in the Ajime-loach, <Emphasis Type="Italic">Niwaella delicata</Emphasis>

The hypothesis that the Ajime-loach, Niwaella delicata, is guided to groundwater seepages by a positive thermotaxis in autumn, was tested by a field investigation and aquarium-based experiments. A total of 763 individuals of N. delicata were captured from October to November in a groundwater trap in the Yasu River, Shiga Prefecture. Niwaella delicata began to be captured as the temperature of the surface water fell to 15.8° ± 1.1°C (mean ± SD) and that of the groundwater to 15.5° ± 0.9°C. Groundwater was often warmer than surface water at night or occasionally all day, and the difference in temperature reached a maximum of 1.3°C at the night on 5 November. For the diel pattern of captures, nocturnal capture was higher than diurnal capture when the groundwater was warmer at night and colder during the daytime, whereas both diurnal and nocturnal captures were high when the groundwater was always warmer than the surface water. The aquarium-based experiments showed that N. delicata choose warmer water, ranging from 18.4° to 22.2°C, just before the capture period in the Yasu River, and are sensitive to differences in water temperature of 1.3° ± 0.1°C. Although the present results broadly support the hypothesis, a part of the results indicates that water temperature gradients may not be the only factor involved in the groundwater selection of N. delicata.     

Plant species, atmospheric CO2 and soil N interactively or additively control C allocation within plant-soil systems

Two plant species, Medicago truncatula (legume) and Avena sativa (non-legume), were grown in low-or high-N soils under two CO₂ concentrations to test the hypothesis whether C allocation within plant-soil system is interactively or additively controlled by soil N and atmospheric CO₂ is dependent upon plant species. The results showed the interaction between plant species and soil N had a significant impact on microbial activity and plant growth. The interaction between CO₂ and soil N had a significant impact on soil soluble C and soil microbial biomass C under Madicago but not under Avena. Although both CO₂ and soil N affected plant growth significantly, there was no interaction between CO₂ and soil N on plant growth. In other words, the effects of CO₂ and soil N on plant growth were additive. We considered that the interaction between N₂ fixation trait of legume plant and elevated CO₂ might have obscured the interaction between soil N and elevated CO₂ on the growth of legume plant. In low-N soil, the shoot-to-root ratio of Avena dropped from 2.63±0.20 in the early growth stage to 1.47±0.03 in the late growth stage, indicating that Avena plant allocated more energy to roots to optimize nutrient uptake (i.e. N) when soil N was limiting. In high-N soil, the shoot-to-root ratio of Medicago increased significantly over time (from 2.45±0.30 to 5.43±0.10), suggesting that Medicago plants allocated more energy to shoots to optimize photosynthesis when N was not limiting. The shoot-to-root ratios were not significantly different between two CO₂ levels.     

Characterization of an evolved carotenoids hyper-producer of <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> through bioreactor parameter optimization and Raman spectroscopy

An evolutionary engineering approach for enhancing heterologous carotenoids production in an engineered Saccharomyces cerevisiae strain was used previously to isolate several carotenoids hyper-producers from the evolved populations. β-Carotene production was characterized in the parental and one of the evolved carotenoids hyper-producers (SM14) using bench-top bioreactors to assess the impact of pH, aeration, and media composition on β-carotene production levels. The results show that with maintaining a low pH and increasing the carbon-to-nitrogen ratio (C:N) from 8.8 to 50 in standard YNB medium, a higher β-carotene production level at 25.52 ± 2.15 mg β-carotene g^?1 (dry cell weight) in the carotenoids hyper-producer was obtained. The increase in C:N ratio also significantly increased carotenoids production in the parental strain by 298 % [from 5.68 ± 1.24 to 22.58 ± 0.11 mg β-carotene g^?1 (dcw)]. In this study, it was shown that Raman spectroscopy is capable of monitoring β-carotene production in these cultures. Raman spectroscopy is adaptable to large-scale fermentations and can give results in near real-time. Furthermore, we found that Raman spectroscopy was also able to measure the relative lipid compositions and protein content of the parental and SM14 strains at two different C:N ratios in the bioreactor. The Raman analysis showed a higher total fatty acid content in the SM14 compared with the parental strain and that an increased C:N ratio resulted in significant increase in total fatty acid content of both strains. The data suggest a positive correlation between the yield of β-carotene per biomass and total fatty acid content of the cell.     

Role of six European tree species and land‐use legacy for nitrogen and water budgets in forests

Water and nutrient fluxes for single stands of different tree species have been reported in numerous studies, but comparative studies of nutrient and hydrological budgets of common European deciduous tree species are rare. Annual fluxes of water and inorganic nitrogen (N) were established in a 30‐year‐old common garden design with stands of common ash (Fraxinus excelsior), European beech (Fagus sylvatica L.), pedunculate oak (Quercus robur), small‐leaved lime (Tilia cordata Mill.), sycamore maple (Acer pseudoplatanus) and Norway spruce (Picea abies [L.] Karst.) replicated at two sites in Denmark, Mattrup and Vallø during 2 years. Mean annual percolation below the root zone (mm yr^?1±SE, n=4) ranked in the following order: maple (351±38)>lime (284±32), oak (271±25), beech (257±30), ash (307±69)? spruce (75±24). There were few significant tree species effects on N fluxes. However, the annual mean N throughfall flux (kg N ha^?1 yr^?1±SE, n=4) for spruce (28±2) was significantly larger than for maple (12±1), beech (11±1) and oak (9±1) stands but not different from that of lime (15±3). Ash had a low mean annual inorganic N throughfall deposition of 9.1 kg ha^?1, but was only present at Mattrup. Annual mean of inorganic N leaching (kg ha^?1 yr^?1±SE, n=4) did not differ significantly between species despite of contrasting tree species mean values; beech (25±9)>oak (16±10), spruce (15±8), lime (14±8)? maple (1.9±1), ash (2.0±1). The two sites had similar throughfall N fluxes, whereas the annual leaching of N was significantly higher at Mattrup than at Vallø. Accordingly, the sites differed in soil properties in relation to rates and dynamics of N cycling. We conclude that tree species affect the N cycle differently but the legacy of land use exerted a dominant control on the N cycle within the short‐term perspective (30 years) of these stands.     

Plant species, atmospheric CO2 and soil N interactively or additively control C allocation within plant-soil systems

Two plant species, Medicago truncatula (legume) and Avena sativa (non-legume), were grown in low-or high-N soils under two CO₂ concentrations to test the hypothesis whether C allocation within plant-soil system is interactively or additively controlled by soil N and atmospheric CO₂ is dependent upon plant species. The results showed the interaction between plant species and soil N had a significant impact on microbial activity and plant growth. The interaction between CO₂ and soil N had a significant impact on soil soluble C and soil microbial biomass C under Madicago but not under Avena. Although both CO₂ and soil N affected plant growth significantly, there was no interaction between CO₂ and soil N on plant growth. In other words, the effects of CO₂ and soil N on plant growth were additive. We considered that the interaction between N₂ fixation trait of legume plant and elevated CO₂ might have obscured the interaction between soil N and elevated CO₂ on the growth of legume plant. In low-N soil, the shoot-to-root ratio of Avena dropped from 2.63±0.20 in the early growth stage to 1.47±0.03 in the late growth stage, indicating that Avena plant allocated more energy to roots to optimize nutrient uptake (i.e. N) when soil N was limiting. In high-N soil, the shoot-to-root ratio of Medicago increased significantly over time (from 2.45±0.30 to 5.43±0.10), suggesting that Medicago plants allocated more energy to shoots to optimize photosynthesis when N was not limiting. The shoot-to-root ratios were not significantly different between two CO₂ levels.     

Small differences in ombrotrophy control regional-scale variation in methane cycling among <Emphasis Type="Italic">Sphagnum</Emphasis>-dominated peatlands

Across northern Alberta, Canada, bogs experience periodic wildfire and, in the Fort McMurray region, are exposed to increasing atmospheric N deposition related to oil sands development. As the fire return interval shortens and/or growing season temperatures increase, the regional peatland CO₂–C sink across northern Alberta will likely decrease, but the magnitude of the decrease could be diminished if increasing atmospheric N deposition alters N cycling in a way that stimulates post-fire successional development in bogs. We quantified net ammonification, nitrification, and dissolved organic N (DON) production in surface peat along a post-fire chronosequence of five bogs where we also experimentally manipulated N deposition (no water controls plus 0, 10, and 20 kg N ha^?1 yr^?1 simulated deposition, as NH₄NO₃). Initial KCl-extractable NH₄⁺–N, NO₃^?–N and DON averaged 176?±?6, 54?±?0.2, and 3580?±?40 ng N cm^?3, respectively, with no consistent changes as a function of time since fire and no consistent effects of experimental N addition. Net ammonification, nitrification, and DON production averaged 3.8?±?0.3, 1.6?±?0.2, and 14.3?±?2.0 ng N cm^?3 d^?1, also with no consistent changes as a function of time since fire and no consistent effects of experimental N addition. Our hypothesis that N mineralization would be stimulated after fire because root death would create a pulse of labile soil organic C was not supported, most likely because ericaceous plant roots typically are not killed in boreal bog wildfires. The absence of any N mineralization response to experimental N addition is most likely a result of rapid immobilization of added NH₄⁺–N and NO₃^?–N in peat with a wide C:N ratio. In these boreal bogs, belowground N cycling is likely characterized by large DON pools that turn over relatively slowly and small DIN pools that turn over relatively rapidly. For Alberta bogs that have persisted at historically low N deposition values and begin to receive higher N deposition related to anthropogenic activities, peat N mineralization processes may be largely unaffected until the peat C:N ratio reaches a point that no longer favors immobilization of NH₄⁺–N and NO₃^?–N.     

Feeding efficiencies of Chironomus plumosus (L.) and C. anthracinus Zett. (Diptera: Chironomidae) in mesotrophic Lake Erken

SUMMARY.

• 1 Absorption efficiencies of nitrogen and carbon in two Chironomus species found dominating in the profundal zone of mesotrophic Lake Erken were determined gravimetrically.
• 2 Absorption efficiencies for C. plumosus showed greater seasonal variation than those of C. anthracinus, with low efficiencies coinciding with the summer dominance of flagellate phytoplankton and with high C:N ratios.
• 3 Overall mean absorption efficiencies (±SE) for carbon and nitrogen, respectively, were 26.8% (±1.9) and 29.3% (±1.9) for C. plumosus, 24.6% (±1.7) and 28.1% (±1.8) for C. anthracinus.
• 4 Significant differences were found to exist between the C: N ratios of the superficial 2 cm sediment layers and those of Chironomus anterior midgut contents.
• 5 C. anthracinus appears to be a deposit feeder ingesting particulate matter scraped from the recently deposited surface sediments. The greater seasonal variation found in the absorption efficiencies of C. plumosus, together with the lower C:N ratios, support the contention that this species is a filter feeder with the nutritional quality of ingested matter depending primarily on pelagic inputs.

     

Isotopic niche differs between seal and fish‐eating killer whales (Orcinus orca) in northern Norway

Ecological diversity has been reported for killer whales (Orcinus orca) throughout the North Atlantic but patterns of prey specialization have remained poorly understood. We quantify interindividual dietary variations in killer whales (n = 38) sampled throughout the year in 2017–2018 in northern Norway using stable isotopic nitrogen (δ¹⁵N: ¹⁵N/¹⁴N) and carbon (δ¹³C: ¹³C/¹²C) ratios. A Gaussian mixture model assigned sampled individuals to three differentiated clusters, characterized by disparate nonoverlapping isotopic niches, that were consistent with predatory field observations: seal‐eaters, herring‐eaters, and lumpfish‐eaters. Seal‐eaters showed higher δ¹⁵N values (mean ± SD: 12.6 ± 0.3‰, range = 12.3–13.2‰, n = 10) compared to herring‐eaters (mean ± SD: 11.7 ± 0.2‰, range = 11.4–11.9‰, n = 19) and lumpfish‐eaters (mean ± SD: 11.6 ± 0.2‰, range = 11.3–11.9, n = 9). Elevated δ¹⁵N values for seal‐eaters, regardless of sampling season, confirmed feeding at high trophic levels throughout the year. However, a wide isotopic niche and low measured δ¹⁵N values in the seal‐eaters, compared to that of whales that would eat solely seals (δ_N‐measured = 12.6 vs. δ_N‐expected = 15.5), indicated a diverse diet that includes both fish and mammal prey. A narrow niche for killer whales sampled at herring and lumpfish seasonal grounds supported seasonal prey specialization reflective of local peaks in prey abundance for the two fish‐eating groups. Our results, thus, show differences in prey specialization within this killer whale population in Norway and that the episodic observations of killer whales feeding on prey other than fish are a consistent behavior, as reflected in different isotopic niches between seal and fish‐eating individuals.     

黑河下游黑果枸杞叶片C、N、P特征及对土壤水盐的响应

研究植物叶片碳(C)、氮(N)、磷(P)计量特征及其与环境因子相关性将为揭示植物对营养元素需求和环境互馈能力提供理论基础。以内蒙古额济纳旗黑河下游距主河道由近及远选择的8个黑果枸杞(Lycium ruthenicum Murr.)优势种群落为研究对象,分析其在不同水分、盐分土壤层环境下叶片C、N、P含量及比值特征,探讨黑果枸杞群落分布的主要限制元素和土壤水盐对其化学计量的影响。研究结果显示:黑果枸杞群落C含量为(331.56±11.99) mg/g,N含量为(13.17±2.92) mg/g,P含量为(2.48±1.64) mg/g,元素C与N、N与P之间呈正相关关系,C与P呈负相关关系,N与C∶N、P与C∶P及N∶P之间呈极显著负相关关系(P0.01);浅层土壤(0—40 cm)水分与P含量呈极显著负相关(P0.01),与C∶P呈显著正相关(P0.05),深层土壤(40—80 cm)水分、盐分均与N含量呈显著负相关(P0.05),与C∶N呈极显著正相关(P0.01)。结果表明:黑果枸杞C∶N主要由N限制,C∶P、N∶P主要由P限制,N∶P小于限制性养分理论阈值14,指示其生长主要受到N限制;黑果枸杞叶片N含量及C∶N比值对深层土壤水分和盐分具有协同响应特征,反映了荒漠植物在干旱盐渍环境中的抗逆境策略。