Soil respiration in northern forests exposed to elevated atmospheric carbon dioxide and ozone

The aspen free-air CO₂ and O₃ enrichment (FACTS II–FACE) study in Rhinelander, Wisconsin, USA, is designed to understand the mechanisms by which young northern deciduous forest ecosystems respond to elevated atmospheric carbon dioxide (CO₂) and elevated tropospheric ozone (O₃) in a replicated, factorial, field experiment. Soil respiration is the second largest flux of carbon (C) in these ecosystems, and the objective of this study was to understand how soil respiration responded to the experimental treatments as these fast-growing stands of pure aspen and birch + aspen approached maximum leaf area. Rates of soil respiration were typically lowest in the elevated O₃ treatment. Elevated CO₂ significantly stimulated soil respiration (8–26%) compared to the control treatment in both community types over all three growing seasons. In years 6–7 of the experiment, the greatest rates of soil respiration occurred in the interaction treatment (CO₂ + O₃), and rates of soil respiration were 15–25% greater in this treatment than in the elevated CO₂ treatment, depending on year and community type. Two of the treatments, elevated CO₂ and elevated CO₂ + O₃, were fumigated with ¹³C-depleted CO₂, and in these two treatments we used standard isotope mixing models to understand the proportions of new and old C in soil respiration. During the peak of the growing season, C fixed since the initiation of the experiment in 1998 (new C) accounted for 60–80% of total soil respiration. The isotope measurements independently confirmed that more new C was respired from the interaction treatment compared to the elevated CO₂ treatment. A period of low soil moisture late in the 2003 growing season resulted in soil respiration with an isotopic signature 4–6‰ enriched in ¹³C compared to sample dates when the percentage soil moisture was higher. In 2004, an extended period of low soil moisture during August and early September, punctuated by a significant rainfall event, resulted in soil respiration that was temporarily 4–6‰ more depleted in ¹³C. Up to 50% of the Earth’s forests will see elevated concentrations of both CO₂ and O₃ in the coming decades and these interacting atmospheric trace gases stimulated soil respiration in this study.     

The consequence of species loss on ecosystem nitrogen cycling depends on community compensation

Repercussions of species loss on ecosystem processes depend on the effects of the lost species as well as the compensatory responses of the remaining species in the community. We experimentally removed two co-dominant plant species and added a ¹⁵N tracer in alpine tundra to compare how species’ functional differences influence community structure and N cycling. For both of the species, production compensated for the biomass removed by the second year. However, the responses of the remaining species depended on which species was removed. These differences in compensation influenced how species loss impacted ecosystem processes. After the removal of one of the co-dominant species, Acomastylis rossii, there were few changes in the relative abundance of the remaining species, and differences in functioning could be predicted based on effects associated with the removed species. In contrast, the removal of the other co-dominant, Deschampsia caespitosa, was associated with subsequent changes in community structure (species relative abundances and diversity) and impacts on ecosystem properties (microbial biomass N, dissolved organic N, and N uptake of subordinate species). Variation in compensation may contribute to the resulting effects on ecosystem functioning, with the potential to buffer or accelerate the effects of species loss.     

Cerium negative anomaly just before the Permian and Triassic boundary event — The upward expansion of anoxia in the water column

A negative excursion of Ce and a succeeding steep decline in δ¹³C found just before the Permian and Triassic boundary (PTB) of Julfa area offer a geochemical constraint on the cause of the mass extinction event of PTB. The geochemical studies of recent anoxic basins like the Black Sea strongly suggest that the Ce negative excursion of these carbonate platform sequence indicates the buildup of anoxic water in the offshore realm. The suboxic water mass associated with the Ce negative anomaly zone migrated and invaded into shallow carbonate shelf around 600 thousand years before the PTB. This was followed by an anoxic water mass without Ce anomaly, and resulted in a steep decline in δ¹³C and then mass extinction of marine shelf biota.     

Seasonal Variations in Soil and Plant Water Status in a Quercus suber L. Stand: Roots as Determinants of Tree Productivity and Survival in the Mediterranean-type Ecosystem

Studies were conducted to examine changes in soil (Ψs) and plant water status during summer in a 16-year old Quercus suber plantation in southern Portugal. Continuous measurements were conducted between May 2003 and August 2004, while discontinuous measurements were conducted on a monthly basis between May and September 2003 and repeated between March and September 2004. Intensive measurements were conducted on five trees with mean height and DBH of 5.3 m and 11.6 cm, respectively, growing at close proximity to each other. Weather conditions and soil water potential (Ψs) at the rhizosphere of each of the trees measured at 0.3 and 1 m soil depth were continuously monitored. Predawn (Ψpd) and midday (Ψmd) leaf water potentials were determined every month. Soil and plant samples were also collected in June and September from different locations within the study site for δ¹⁸O isotope composition analysis. Pressure–volume (p–v) curves were constructed from plant shoots at different times during the vegetative period to determine osmotic potential at full saturation (Π¹⁰⁰), water potential at turgor loss point (Ψ_tlp), relative water content at turgor loss point (R*_tlp) and bulk modulus of elasticity (ε). Significant P < 0.05 decline in Ψs occurred between May and September, the lowest value recorded being –2.0 MPa. Decline in soil moisture affected tree water status, but decline in leaf water potential varied significantly (P < 0.05) among the trees. At the end of summer drought, lowest Ψpd measured was –1.7 MPa while the highest measured during this time was –0.8 MPa. Differences among trees were attributed to differences in rooting depth, as shown by regression analysis of ¹⁸O isotopes. Radial stem growth ceased when Ψs within the upper 0.3 m depth approached –1.5 MPa. The upper soil layers contributed approximately 33% of the total tree water requirement, between spring and mid summer when drought was experienced by trees. Deep soil layers however, supplied most of the water required during drought and no growth was recorded during this time. Stressed trees increased solute concentration of their tissues by a Magnitude of 0.7 MPa while bulk tissue elastic modulus increased by about 17 MPa. The study emphasizes the significance of roots as determinants of tree productivity and survival in the Mediterranean ecosystems.     

The dependence of soil microbial activity on recent photosynthate from trees

It remains a challenge to quantify and assess the importance of the direct plant below-ground flux of photosynthate carbon (C) to soil microorganisms, especially in forests because of the size of the plants and the great spatial heterogeneity of soils. We studied the importance of labile C inputs from trees on the respiratory activity of soil microorganisms by comparing the response of plots with and without girdled pine trees (Pinus sylvestris L.) to additions of C₄-sucrose, thus enabling us to differentiate between utilization of endogenous C₃-soil C sources and exogenous C₄-sucrose. In both girdled and non-girdled plots the respiration rate after sucrose application, i.e. substrate induced respiration measured in the field, was on average ca. double that of basal respiration rate measured in the field. However, the C₄-sucrose-induced increase in respiration of endogenous C₃-C was significantly higher in non-girdled plots. Expression of C₃-respiration as a percentage of induced respiration in the field showed that in girdled plots, C₃-respiration decreased after sucrose addition and, consequently, the induced respiration in the field was totally C₄-C based. A previous laboratory experiment found no increase in total respiration rate when C₄-sucrose was added to the soil substrate of non-mycorrhizal and ectomycorrhizal pine plants. Hence, we see no reason to attribute the increased respiration to (mycorrhizal) roots. Thus, our results indicate that despite the alleged C limitation of the soil microorganisms there is a fraction of SOM, or C within the microbial biomass that is available to microbial metabolism if their C limitation is relieved by the supply of labile C. This fraction corresponds to roughly 10–20% of biomass C of the heterotrophic organisms and seems to become exhausted in the long-term absence of supply of photosynthate to roots.     

Effects of stream restoration and wastewater treatment plant effluent on fish communities in urban streams

1. Fish community characteristics, resource availability and resource use were assessed in three headwater urban streams in Piedmont North Carolina, U.S.A. Three site types were examined on each stream; two urban (restored and unrestored) and a forested site downstream of urbanisation, which was impacted by effluent from a wastewater treatment plant (WWTP). Stream basal resources, aquatic macroinvertebrates, terrestrial macroinvertebrates and fish were collected at each site. 2. The WWTPs affected isotope signatures in the biota. Basal resource, aquatic macroinvertebrate and fish δ¹⁵N showed significant enrichments in the downstream sites, although δ¹³C signatures were not greatly influenced by the WWTP. Fish were clearly deriving a significant part of their nutrition from sewage effluent‐derived sources. There was a trend towards lower richness and abundance of fish at sewage‐influenced sites compared with urban restored sites, although the difference was not significant. 3. Restored stream sites had significantly higher fish richness and a trend towards greater abundance compared with unrestored sites. Although significant differences did not exist between urban restored and unrestored areas for aquatic and terrestrial macroinvertebrate abundances and biotic indices of stream health, there appeared to be a trend towards improvements in restored sites for these parameters. Additional surveys of these sites on a regular basis, along with maintenance of restored features are vital to understanding and maximising restoration effectiveness. 4. A pattern of enriched δ¹³C in fish in restored and unrestored streams in conjunction with enriched δ¹³C of terrestrial invertebrates at these sites suggests that these terrestrial subsidies are important to the fish, a conclusion also supported by isotope cross plots. Furthermore, enriched δ¹³C observed for terrestrial invertebrates is consistent with some utilisation of the invasive C4 plants that occur in the urban riparian areas.     

Proteotypic profiling of LHCI from Chlamydomonas reinhardtii provides new insights into structure and function of the complex

We used isotope dilution MS to measure the stoichiometry of light‐harvesting complex I (LHCI) proteins with the photosystem I (PSI) core complex in the green alga Chlamydomonas reinhardtii. Proteotypic peptides served as quantitative markers for each of the nine gene products (Lhca1–9) and for PSI subunits. The quantitative data revealed that the LHCI antenna of C. reinhardtii contains about 7.5 ± 1.4 subunits. It further demonstrated that the thylakoid LHCI population is heterogeneously composed and that several lhca gene products are not present in 1:1 stoichiometries with PSI. When compared with vascular plants, LHCI of C. reinhardtii possesses a lower proportion of proteins potentially contributing to far‐red fluorescence emission. In general, the strategy presented is universally applicable for exploring subunit stoichiometries within the C. reinhardtii proteome.     

Stable isotope fractionation in the digestive gland, muscle and gills tissues of the marine mussel Mytilus galloprovincialis

Mytilus galloprovincialis is a common species in the Mediterranean. It is a sedentary filter-feeding organism that assimilates carbon and nitrogen isotopic ratios in tissues from its food sources. The δ¹³C and δ¹⁵N values have been used to demonstrate differences in isotopic composition between digestive gland, muscle and gills of this mussel. For δ¹³C, mean values were - 21.99 ± 0.50‰, - 19.70 ± 0.44‰, and - 19.96 ± 0.44‰, respectively; and for δ¹⁵N, they were 5.16 ± 0.90‰, 7.67 ± 0.79‰ and 7.77 ± 0.85‰, respectively. The fractionation or enrichment factor for ¹³C values between digestive gland and muscle, between digestive gland and gills, and between muscle and gills were - 2.29 ± 0.16‰, - 2.04 ± 0.14‰ and 0.27 ± 0.07‰, respectively, within the expected range of ¹³C fractionation at filter feeders reported elsewhere. In contrast, low fractionation values were found for ¹⁵N with - 2.45 ± 0.24‰, - 2.51 ± 0.16‰ and - 0.11 ± 0.16‰, between digestive gland and muscle, between digestive gland and gills, and between muscle and gills, respectively. Through isotopic fractionation of M. galloprovincialis, the depleted values were found in the digestive gland, followed by gills and then by muscle tissue. Statistical analysis (PERMANOVA) was performed to check for significant differences in δ¹³C and δ¹⁵N isotopic signatures between tissues and localities. The current study demonstrates significant differences in the δ¹³C and δ¹⁵N isotopic composition between digestive gland, muscle and gills tissues in M. galloprovincialis living in the oligotrophic environment of the Balearic Islands.     

Potential bias and mitigations when using stable isotope labeled parent drug as internal standard for LC-MS/MS quantitation of metabolites

In recent years, increasing emphasis has been placed on quantitative characterization of drug metabolites for better insight into the correlation between metabolite exposure and toxicological observations or pharmacological efficacy. One common strategy for metabolite quantitation is to adopt the stable isotope labeled (STIL) parent drug as the internal standard in an isotope dilution liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay. In the current work, we demonstrate this strategy could have a potential pitfall resulting in quantitation bias if the internal standard is subject to ion suppression from the co-eluting parent drug in the incurred samples. Propranolol and its metabolite 4-hydroxypropranolol were used as model compounds to demonstrate this phenomenon and to systematically evaluate different approaches to mitigate the issue, including atmospheric pressure chemical ionization (APCI) mode of ionization, increased internal standard concentration, quantitation without internal standard, the use of a structural analog as internal standard, and dilution of the samples. Case studies of metabolite quantitation in nonclinical and clinical studies in drug development were also included to demonstrate the importance of using an appropriate bioanalytical strategy for metabolite quantitation in the real world. We present that bias of metabolite concentrations could pose a potential for poor estimation of safety risk. A strategy for quantitation of metabolites in support of drug safety assessment is proposed.     

Determination of casopitant and its three major metabolites in dog and rat plasma by positive ion liquid chromatography/tandem mass spectrometry

A sensitive, selective and quantitative method for the simultaneous determination of casopitant, a potent and selective antagonist of the human Neurokinin 1 (NK-1) receptor, and its three major metabolites M12, M13 and M31 was developed and validated in dog and rat plasma. Acetonitrile containing stable labeled internal standards for the four analytes was used to precipitate proteins in plasma. Chromatographic separation was obtained using a reversed phase column with multiple reaction monitoring turboionspray positive ion detection. The lower and upper limits of quantification for casopitant and its metabolites were 15 and 15,000 ng/mL, using a 50 μL of dog or rat plasma aliquot, respectively. The inter-day precision (relative standard deviation) and accuracy (relative error) in dog plasma, derived from the analysis of validation samples at 5 concentrations, ranged from 4.1% to 10.0% and −10.8% to 8.7%, respectively, for casopitant and its 3 major metabolites. The intra-day precision (relative standard deviation) and accuracy (relative error) in rat plasma, derived from the analysis of validation samples at 5 concentrations, ranged from 3.9% to 6.6% and −9.6% to 8.3%, respectively, for casopitant and its three metabolites. All analytes were found to be stable in analytical solutions for at least 43 days at 4 °C, in dog and rat plasma at room temperature for at least 24 h, at the storage temperature of −20 °C for at least 6 months, and following the action of three freeze–thaw cycles from −20 °C to room temperature. All analytes were also found to be stable in processed extracts at 4 °C for at least 72 h. This assay proved to be accurate, precise, fast and was used to support long-term toxicology studies in dog and rat.