Relative Turnover Research Articles

CEO turnover in a competitive assignment framework

There is widespread concern about whether Chief Executive Officers (CEOs) are appropriately punished for poor performance. While CEOs are more likely to be forced out if their performance is poor relative to the industry average, overall industry performance also matters. This seems puzzling if termination is disciplinary, however, we show that both absolute and relative performance-driven turnover can be natural and efficient outcomes in a competitive assignment model in which CEOs and firms form matches based on multiple characteristics. The model also has new predictions about replacement managers' equilibrium pay and performance. We document CEO turnover events during 1992–2006 and provide empirical support for our model.

Analysis of turnover dynamics of the submembranous actin cortex

The cell cortex is a thin network of actin, myosin motors, and associated proteins that underlies the plasma membrane in most eukaryotic cells. It enables cells to resist extracellular stresses, perform mechanical work, and change shape. Cortical structural and mechanical properties depend strongly on the relative turnover rates of its constituents, but quantitative data on these rates remain elusive. Using photobleaching experiments, we analyzed the dynamics of three classes of proteins within the cortex of living cells: a scaffold protein (actin), a cross-linker (α-actinin), and a motor (myosin). We found that two filament subpopulations with very different turnover rates composed the actin cortex: one with fast turnover dynamics and polymerization resulting from addition of monomers to free barbed ends, and one with slow turnover dynamics with polymerization resulting from formin-mediated filament growth. Our data suggest that filaments in the second subpopulation are on average longer than those in the first and that cofilin-mediated severing of formin-capped filaments contributes to replenishing the filament subpopulation with free barbed ends. Furthermore, α-actinin and myosin minifilaments turned over significantly faster than F-actin. Surprisingly, only one-fourth of α-actinin dimers were bound to two actin filaments. Taken together, our results provide a quantitative characterization of essential mechanisms under-lying actin cortex homeostasis.

Root inclusion net method: novel approach to determine fine root production and turnover in Larix principis-rupprechtii Mayr plantation in North China

A novel root inclusion net method was designed to determine the fine root productions and turnover rates of 13-, 22-, and 38-year-old Larix principis-rupprechtii plantations in the whole growing season of 2012, and it was compared with the results of the sequential coring method and in-growth core method. All following values are reported in order for 13-, 22-, and 38-year-old L. principis-rupprechtii plantations. The mean values of fine root biomasses were 103, 261, and 356 g m^{-2}, about 76-78% of which were live fine root biomasses. The fine root productions were 86-118 g m^{-2} year^{-1}, 124-138 g m^{-2} year^{-1}, and 134-160 g m^{-2} year^{-1}, respectively. The fine root turnover rates were 1.12, 0.61, and 0.51 times year^{-1}, respectively, suggesting a relative slow fine root turnover in L. principis-rupprechtii plantations. The carbon inputs into soil accompanying fine root turnover were 52, 58, and 94 g C m^{-2} year^{-1}. This organic carbon is a sizeable pool in the forest ecosystem. The results suggest that our new modified root inclusion net method is suitable for field-based assessment of fine root production and turnover.

Droplet condensation on polymer surfaces: A review

A novel root inclusion net method was designed to determine the fine root productions and turnover rates of 13-, 22-, and 38-year-old Larix principis-rupprechtii plantations in the whole growing season of 2012, and it was compared with the results of the sequential coring method and in-growth core method. All following values are reported in order for 13-, 22-, and 38-year-old L. principis-rupprechtii plantations. The mean values of fine root biomasses were 103, 261, and 356 g m-2, about 76-78% of which were live fine root biomasses. The fine root productions were 86-118 g m-2 year-1, 124-138 g m-2 year-1, and 134-160 g m-2 year-1, respectively. The fine root turnover rates were 1.12, 0.61, and 0.51 times year-1, respectively, suggesting a relative slow fine root turnover in L. principis-rupprechtii plantations. The carbon inputs into soil accompanying fine root turnover were 52, 58, and 94 g C m-2 year-1. This organic carbon is a sizeable pool in the forest ecosystem. The results suggest that our new modified root inclusion net method is suitable for field-based assessment of fine root production and turnover.

长江口及东海春季底栖硅藻、原生动物和小型底栖生物的生态特点

PDF HTML阅读 XML下载 导出引用 引用提醒 长江口及东海春季底栖硅藻、原生动物和小型底栖生物的生态特点 DOI: 10.5846/stxb201207090964 作者: 作者单位: 中国科学院海洋研究所,中国科学院海洋研究所 作者简介: 通讯作者: 中图分类号: 基金项目: 国家重点基础研究发展计划（973）项目（2011CB403604）；中国科学院知识创新工程重要方向项目（KZCX2-EW-Z-5）；中国科学院海洋研究所"一三五"专项资助项目（2012IO060104） Ecological characteristics of benthic diatoms, protozoa and meiobenthos in the sediments of the Changjiang Estuary and East China Sea in spring Author: Affiliation: Institute of Oceanology,Chinese Academy of Sciences,Institute of Oceanology,Chinese Academy of Sciences Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:利用Ludox-QPS方法并结合沉积环境因子的综合分析，研究了2011年4月采自长江口及东海10个站位以底栖硅藻、纤毛虫和异养小鞭毛虫为代表的微型底栖生物及小型底栖生物的组成、丰度和生物量、分布及生态特点。结果表明，底栖硅藻的丰度（5.92×104 个/10 cm2）和生物量（83.29 μg C/10 cm2）远高于纤毛虫（丰度为1036 个/10 cm2，生物量为3.33 μg C/10 cm2）、异养小鞭毛虫（丰度为4451 个/10 cm2，生物量为2.51 μg C/10 cm2）和小型底栖生物（丰度为（1947±849）个/10 cm2，生物量为（49.01±22.05）μg C/10 cm2）。在鉴定出的11个小型底栖生物类群中，线虫占小型底栖生物总丰度的90%和总生物量的37%。底栖硅藻生物量在长江口及东海海域呈由近岸向外海逐渐降低的分布特点，而底栖纤毛虫、异养小鞭毛虫及小型底栖生物的分布则正相反。在垂直分布上，76%的硅藻和80%的线虫分布在0-2 cm沉积物表层，仅1%的硅藻和6%的线虫分布在5-8 cm分层。统计分析表明，底栖硅藻的现存量与沉积物中叶绿素a含量呈极显著的正相关，与底层水温度呈弱的正相关;该海域底栖原生动物和小型底栖生物的分布受多个因子而非单一环境因子的共同作用。对比分析表明，长江口及东海单位体积沉积物中的硅藻丰度较水体中的硅藻丰度高2个数量级，沉积物中相当部分的叶绿素a含量可能系底栖硅藻所贡献；表层8 cm沉积物中纤毛虫的丰度约是上层30 m水柱中纤毛虫丰度的30倍，生物量约是后者的40倍。尽管纤毛虫在生物量上远小于小型底栖生物，但其估算的生产力约是后者的3倍；而异养小鞭毛虫由于个体更小，其周转率可能较纤毛虫更高。长江口及东海陆架区原生动物和小型底栖生物的高现存量及生产力预示着其在该海域生态系统中的重要作用。 Abstract:Using the newly developed Ludox-QPS method, we investigated the community composition, abundance and biomass, and distribution of benthic diatoms, ciliates and heterotrophic microflagellates (HMF) and meiobenthos in combination with the analysis of benthic environmental variables at ten stations in the Changjiang (Yangtze) Estuary and East China Sea in April 2011. The benthic diatoms were dominant both in the abundance (5.92×104 ind/10 cm2) and biomass (83.29 μg C/10 cm2), followed by benthic ciliates (abundance: 1036 ind/10 cm2, biomass: 3.33 μg C/10 cm2) and HMF (4451 ind/10 cm2, 2.51 μg C/10 cm2). The abundance of meiobenthos in the upper 8 cm of sediments was (1947±849) ind/10 cm2 and the biomass was (49.01±22.05) μg C/10 cm2. Among the 11 meiobenthos groups identified, nematodes accounted for 90% of the total abundance and 37% of the total biomass. The biomass of benthic diatoms was distinctly higher in the coastal region than that in the offshore area, while the distribution pattern of benthic ciliates, HMF and meiobenthos was exactly opposite. On average 76% of diatoms and 80% of nematodes were distributed in the 0-2 cm sediment layers, while the respective proportions were only 1% and 6% in the 5-8 cm layers. Statistical analyses showed that the standing crops of benthic diatoms had significantly positive correlation with sediment chlorophyll-a contents and weak positive correlation with bottom water temperature, and benthic protozoa and meiobenthos might be structured by the interaction of multiple rather than single environmental variables. Our data indicate that the density of benthic diatoms in the sediments was very high, about two orders magnitude higher per unit volume than that in the water column of the Changjiang Estuary and East China Sea, where a great proportion of sediment chlorophyll a was contributed by benthic diatoms. The benthic ciliate abundance in the upper 8 cm of sediments were 30 times higher and the biomass was 40 times higher than those in the water column of upper 30 m of the Changjiang Estuary and East China Sea. Although the biomass of ciliates was lower than that of meiobenthos, their production was up to 3 times higher than that of meiobenthos. The relative turnover rate of heterotrophic microflagellates might even be much higher due to their small size. The high standing crops and production of benthic protozoa and meiobenthos indicate their important role in the ecosystem structure and function of the Changjiang Estuary and East China Sea. 参考文献 相似文献 引证文献

Catalytic properties and reaction mechanism of the CrtO carotenoid ketolase from the cyanobacterium Synechocystis sp. PCC 6803

CrtW and CrtO are two distinct non-homologous β-carotene ketolases catalyzing the formation of echinenone and canthaxanthin. CrtO belongs to the CrtI family which comprises carotene desaturases and carotenoid oxidases. The CrtO protein from Synechocystis sp. PCC 6803 has been heterologously expressed, extracted and purified. Substrate specificity has been determined in vitro. The enzyme from Synechocystis is basically a mono ketolase. Nevertheless, small amounts of diketo canthaxanthin can be formed. The poor diketolation reaction could be explained by the low relative turnover numbers for the mono keto echinenone. Also other carotenoids with an unsubstituted β-ionone ring were utilized with low conversion rates by CrtO regardless of the substitutions at the other end of the molecule. The CrtO ketolase was independent of oxygen and utilized an oxidized quinone as co-factor. In common to CrtI-type desaturases, the first catalytic step involved hydride transfer to the quinone. The stabilization reaction of the resulting carbo cation was a reaction with OH− forming a hydroxy group. Finally, the keto group resulted from two subsequent hydroxylations at the same C-atom and water elimination. This reaction mechanism was confirmed by in vitro conversion of the postulated hydroxy intermediates and by their enrichment and identification as trace intermediates during ketolation.

Systematic, large‐scale national biodiversity surveys: NeoMaps as a model for tropical regions

AbstractAimTo test a method for rapidly and reliably collecting species distribution and abundance data over large tropical areas [known as Neotropical Biodiversity Mapping Initiative (NeoMaps)], explicitly seeking to improve cost‐ and time‐efficiencies over existing methods (i.e. museum collections, literature), while strengthening local capacity for data collection.LocationVenezuela.MethodsWe placed a grid over Venezuela (0.5 × 0.5 degree cells) and applied a stratified sampling design to select a minimum set of 25 cells spanning environmental and biogeographical variation. We implemented standardized field sampling protocols for birds, butterflies and dung beetles, along transects on environmental gradients (‘gradsects’). We compared species richness estimates from our field surveys at national, bioregional and cell scales to those calculated from data compiled from museum collections and the literature. We estimated the variance in richness, composition, relative abundance and diversity between gradsects that could be explained by environmental and biogeographical variables. We also estimated total survey effort and cost.ResultsIn one field season, we covered 8% of the country and recorded 66% of all known Venezuelan dung beetles, 52% of Pierid butterflies and 37% of birds. Environmental variables explained 27–60% of variation in richness for all groups and 13–43% of variation in abundance and diversity in dung beetles and birds. Bioregional and environmental variables explained 43–58% of the variation in the dissimilarity matrix between transects for all groups.Main conclusionsNeoMaps provides reliable estimates of richness, composition and relative abundance, required for rigorous monitoring and spatial prediction. NeoMaps requires a substantial investment, but is highly efficient, achieving survey goals for each group with 1‐month fieldwork and about US$ 1–8 per km2. Future work should focus on other advantages of this type of survey, including the ability to monitor the changes in relative abundance and turnover in species composition, and thus overall diversity patterns.

Neurochemical differences in two rat strains exposed to social isolation rearing.

Objective: Isolation rearing of rats provides a non-pharmacological method of inducing behavioural changes in rodents that resemble schizophrenia or depression. Nevertheless, results are variable within different strains. We focused on neurochemical changes in several in vivo and post-mortem brain regions of Wistar (W) and Lister Hooded (LH) rats following post-weaning social separation.Methods: Experiments were conducted after 6–8 weeks of isolation. For post-mortem studies, prefrontal cortex (PFC), nucleus accumbens (NAC), hippocampus (Hipp) and striatum (St) were collected by tissue dissection. In vivo experiments were conducted by microdialysis in the PFC. Analyses of dopamine (DA), serotonin (5-HT) levels and relative turnover were performed by using high-performance liquid chromatography.Results: We found significant strain-related differences in biogenic amine content. LH rats were characterised by markedly raised DA, along with its turnover reduction, in all the post-mortem brain regions examined as well as in microdialysis samples, while in W rats 5-HT tissue concentration was lower in PFC and St and higher in NAC and Hipp. Cortical extracellular 5-HT concentrations were increased in group housed and decreased in isolated W animals. Moreover, isolation increased DA concentrations in the PFC of LH rats, and decreased 5-HT in W rats in NAC and Hipp. Lately, 5-HT turnover was also affected by both strain and isolation conditions.Conclusions: This study suggests that W and LH rats have markedly different neurochemical profiles in response to isolation, resulting in altered monoamine levels that vary according to brain area and rat strain. These findings highlight the importance of selecting an appropriate rat strain when considering isolation rearing to model symptoms of schizophrenia and/or depression.

The Effect of Abnormal Turnover on Asymmetric Autoregressive Behavior of Index Returns: Evidence from the Chinese A-share Stock Markets*

This study tests the effect of turnover shocks on the asymmetric autoregressive behavior of index returns. The methodological approach adopted in this study is based on the relationship between market return and trading volume. This study first uses a vector autoregression (VAR) to model two market trading volume series by controlling for the variation associated with the sign and magnitude of both week t and week t − 1 or week t + 1 market returns. The generated market-adjusted relative turnover (MRTO) series are then plugged into the bivariate asymmetric AR GARCH-t model (asAR-GARCH-t) using weekly data from the Shanghai and Shenzhen A-share stock markets. The results show that turnover shocks have a material effect on asymmetric autoregressive behavior and thus on asymmetric persistence of past returns.

INTERNATIONAL TRADE AND THE COMPOSITION OF LABOR MARKET TURNOVER

The composition of labor market turnover is shown to influence patterns of international trade. Job and worker turnover have opposing marginal effects on industry export intensity, highlighting the importance of relative turnover shares on either side of the labor market, as opposed to total volumes of labor mobility, in shaping economic outcomes. Industries with relatively greater shares of worker turnover export more of total production, and those with higher job turnover export less. Furthermore, relatively high job turnover hinders industry adjustment following trade liberalization. These predictions receive support for U.S. manufacturing industries using turnover data in the Quarterly Workforce Indicators available from the U.S. Census Bureau. (JEL F16)

Examination of the Kinetics of Myosin VI during Endocytosis

The molecular motor myosin VI has been implicated in endocytosis, a trafficking pathway mediating intracellular uptake of items such as membrane receptors and nutrients. Previous studies demonstrate the localization of the large insert isoform of myosin VI to clathrin-coated vesicles and the no insert isoform of myosin VI to early endosomes. The kinetics of myosin VI functionality on these endocytic structures has not yet been examined, however. This study uses fluorescence recovery after photobleaching (FRAP) to examine the turnover kinetics of the no insert and large insert isoforms of myosin VI during endocytosis. The results demonstrate that myosin VI turns over dynamically on endocytic structures and that different isoforms on different intracellular compartments have distinct turnover rates. The FRAP assay system is then implemented with a novel live cell expression of an artificial dimer of myosin VI to demonstrate the dimeric functionality of myosin VI on endocytic structures in vivo. Further studies of the turnover rates of the myosin VI binding partner Dab2 on clathrin-coated structures demonstrate that Dab2 turns over more rapidly than full length myosin VI, a novel indication of the relative turnover rates of a motor protein versus its binding partner on a given cellular compartment. The turnover kinetics of specific myosin VI motor domain mutants on endocytic structures are also examined, e.g. the D179Y mutant implicated in progressive hearing loss, a study which offers insight into the functional source of myosin VI-related deafness. In addition to providing insight into the endocytic functionality of myosin VI, these FRAP studies offer general insight into the dynamics of myosin motor proteins on intracellular structures, the functional differences between isoforms of given myosin proteins, and the comparative turnover rates of myosin proteins and their binding partners.

Proteome Turnover in the Green Alga Ostreococcus tauri by Time Course 15N Metabolic Labeling Mass Spectrometry

Protein synthesis and degradation determine the cellular levels of proteins, and their control hence enables organisms to respond to environmental change. Experimentally, these are little known proteome parameters; however, recently, SILAC-based mass spectrometry studies have begun to quantify turnover in the proteomes of cell lines, yeast, and animals. Here, we present a proteome-scale method to quantify turnover and calculate synthesis and degradation rate constants of individual proteins in autotrophic organisms such as algae and plants. The workflow is based on the automated analysis of partial stable isotope incorporation with (15)N. We applied it in a study of the unicellular pico-alga Ostreococcus tauri and observed high relative turnover in chloroplast-encoded ATPases (0.42-0.58% h(-1)), core photosystem II proteins (0.34-0.51% h(-1)), and RbcL (0.47% h(-1)), while nuclear-encoded RbcS2 is more stable (0.23% h(-1)). Mitochondrial targeted ATPases (0.14-0.16% h(-1)), photosystem antennae (0.09-0.14% h(-1)), and histones (0.07-0.1% h(-1)) were comparatively stable. The calculation of degradation and synthesis rate constants k(deg) and k(syn) confirms RbcL as the bulk contributor to overall protein turnover. This study performed over 144 h of incorporation reveals dynamics of protein complex subunits as well as isoforms targeted to different organelles.

Predicted Relative Metabolomic Turnover (PRMT): determining metabolic turnover from a coastal marine metagenomic dataset

BackgroundThe world's oceans are home to a diverse array of microbial life whose metabolic activity helps to drive the earth's biogeochemical cycles. Metagenomic analysis has revolutionized our access to these communities, providing a system-scale perspective of microbial community interactions. However, while metagenome sequencing can provide useful estimates of the relative change in abundance of specific genes and taxa between environments or over time, this does not investigate the relative changes in the production or consumption of different metabolites.ResultsWe propose a methodology, Predicted Relative Metabolic Turnover (PRMT) that defines and enables exploration of metabolite-space inferred from the metagenome. Our analysis of metagenomic data from a time-series study in the Western English Channel demonstrated considerable correlations between predicted relative metabolic turnover and seasonal changes in abundance of measured environmental parameters as well as with observed seasonal changes in bacterial population structure.ConclusionsThe PRMT method was successfully applied to metagenomic data to explore the Western English Channel microbial metabalome to generate specific, biologically testable hypotheses. Generated hypotheses linked organic phosphate utilization to Gammaproteobactaria, Plantcomycetes, and Betaproteobacteria, chitin degradation to Actinomycetes, and potential small molecule biosynthesis pathways for Lentisphaerae, Chlamydiae, and Crenarchaeota. The PRMT method can be applied as a general tool for the analysis of additional metagenomic or transcriptomic datasets.

Imaging of complex sulfatides SM3 and SB1a in mouse kidney using MALDI-TOF/TOF mass spectrometry

Sulfatides, a class of acidic glycosphingolipids, are highly expressed in mammalian myelin and in kidney, where they are thought to stabilize neuronal structures and signaling and to influence osmotic stability of renal cells, respectively. Recently, 9-aminoacridine (9-AA) has been introduced as a negative ion matrix that displays high selectivity for low complexity galactosylceramid-I(3)-sulfate sulfatides and that is suitable for quantitative analysis by matrix-assisted desorption/ionization (MALDI) mass spectrometry (MS). Analyzing acidic fractions of lipid extracts and cryosections from kidneys of wild type and arylsulfatase A-deficient (ASA -/-) mice, we demonstrate that 9-AA also enables sensitive on-target analysis as well as imaging of complex lactosylceramide-II(3)-sulfate and gangliotetraosylceramide-II(3), IV(3) bis-sulfate sulfatides by MALDI-TOF/TOF MS. Utilizing the MALDI imaging MS technique, we show differential localization in mouse kidney of (1) sulfatides with identical ceramide anchors, but different glycan-sulfate head groups but also of (2) sulfatides with identical head groups but with different acyl- or sphingoid base moieties. A comparison of MALDI images of renal sulfatides from control and sulfatide storing arylsulfatase A-deficient (ASA -/-) mice revealed relative expression differences, very likely reflecting differences in sulfatide turnover of the various renal cell types. These results establish MALDI imaging MS with 9-AA matrix as a label-free method for spatially resolved ex vivo investigation of the relative turnover of sulfatides in animal models of human glycosphingolipid storage disease.

Influence of soil aggregation on SOC sequestration: A preliminary model of SOC protection by aggregate dynamics

Aggregation dynamics and soil organic carbon (SOC) fractions collected from long-term tillage trials at two sites in Illinois were used to develop a model to simulate the aggregate dynamics and physical protection of SOC. We used two litter pools which are surface litter and root litter and three SOC pools which are directly measurable from the fractionation: loose particulate organic matter (LPOM), aggregate-occluded particulate organic matter (OPOM), and humified fractions (HF). Decay rates of all of five pools were modified by soil temperature and moisture. In the model, the decay rate of LPOM was not influenced by any type of physical protection and the OPOM decay rate was influenced by dry aggregate mean weight diameter (DMWD) size. The effect of DMWD on OPOM decay rate was expressed as logistic equation based on the threshold value beyond which OPOM decay rate was influenced by the reactive mass concept which is that it is primarily outer layer of aggregates that participates in chemical and biological reactions. The decay of HF was influenced by clay contents. The relative aggregate turnover modified the humification coefficients. The faster aggregate turnover speeded the carbon transfer from LPOM to OPOM by providing more chances for organic matter to be incorporated with macroaggregates and retarded carbon transfer from OPOM to HF due to the fact that there is not enough time for organic mater to be associated with microaggregates and clay particles. Simulated results were compared against actual SOC fraction contents obtained from two long-term tillage trials located in Illinois, DeKalb (silty clay loam) and Monmouth (silt loam). Both actual and simulated data showed that after 10 and 17 years of no tillage (NT) practice adoption, OPOM content was increased at the surface in Monmouth and HF content was increased at the surface in DeKalb. Agreement between the output of aggregate dynamics-based model and actual data suggested that DMWD size, relative aggregate turnover, and their interaction with soil moisture and clay contents can be used to predict the inconsistent effects of tillage practices on SOC sequestration.

Urology Consultations: The Hidden Workload?

Urology is unique as a surgical specialty. The emergency workload is minimal compared with its nearest relative, general surgery, and patient turnover is faster than in most disciplines. However, the elective workload is significant and with an ageing population is set to increase. Increased education and awareness about organ-specific cancers (eg prostate) combined with target demands for fast-track cancer referral has contributed to the workload of urologists.

Adsorption of C2−C8 n-Alkanes in Zeolites

Adsorption of n-alkanes has been studied in the industrially relevant zeolites H-FAU, H-BEA, H-MOR, and H-ZSM-5 combining QM−Pot(MP2//B3LYP) with statistical thermodynamics calculations and assuming a mobile adsorbate. In H-ZSM-5, adsorption at the intersection site with the hydrocarbon chain extending in the straight channel (SC+I) as well as in the zigzag channel (ZC+I) has been studied. In addition, differential heats of adsorption and adsorption isotherms at temperatures from 301 to 400 K of all C3−C6 n-alkane in H-ZSM-5 have been measured simultaneously via calorimetry and gravimetry. Calculated adsorption enthalpies are independent of temperature and are virtually identical to the adsorption energies. The adsorption strength increases in the order H-FAU < H-BEA < H-MOR < H-ZSM-5 (SC+I) < H-ZSM-5 (ZC+I) and varies linearly with the carbon number. As compared to experimental values, the calculated adsorption strength is overestimated by some 2 kJ mol−1/CH2 in FAU up to some 4 kJ mol−1/CH2 in H-ZSM-5 suggesting that the QM−Pot(MP2//B3LYP) calculations overestimate van der Waals stabilizing interactions and a correction term has been proposed. Adsorption entropy losses are independent of temperature and increase in the order H-FAU < H-BEA < H-MOR < H-ZSM-5 (SC+I) < H-ZSM-5 (ZC+I), according to the pore size of the zeolites. The calculated adsorption entropies agree nicely with available experimental results in all zeolites. QM−Pot(MP2//B3LYP) calculated adsorption equilibrium coefficients (using the corrected adsorption enthalpies) correspond relatively well to experimentally determined values. Comparison of relative turnover frequencies with relative adsorption equilibrium coefficients indicates that the variation of the equilibrium coefficient with the carbon number or with the zeolite can only partly explain the observed reactivity differences in monomolecular cracking of n-alkanes. In agreement with experimental observations, our results indicate that the difference in reactivity of the n-alkanes for monomolecular cracking in a given zeolite mainly originates from a difference in intrinsic monomolecular cracking rate coefficients.

Turnover of soil organic matter and of microbial biomass under C3–C4 vegetation change: Consideration of 13C fractionation and preferential substrate utilization

Two processes contribute to changes of the δ13C signature in soil pools: 13C fractionation per se and preferential microbial utilization of various substrates with different δ13C signature. These two processes were disentangled by simultaneously tracking δ13C in three pools – soil organic matter (SOM), microbial biomass, dissolved organic carbon (DOC) – and in CO2 efflux during incubation of 1) soil after C3–C4 vegetation change, and 2) the reference C3 soil. The study was done on the Ap horizon of a loamy Gleyic Cambisol developed under C3 vegetation. Miscanthus giganteus – a perennial C4 plant – was grown for 12 years, and the δ13C signature was used to distinguish between ‘old’ SOM (>12 years) and ‘recent’ Miscanthus-derived C (<12 years). The differences in δ13C signature of the three C pools and of CO2 in the reference C3 soil were less than 1‰, and only δ13C of microbial biomass was significantly different compared to other pools. Nontheless, the neglecting of isotopic fractionation can cause up to 10% of errors in calculations. In contrast to the reference soil, the δ13C of all pools in the soil after C3–C4 vegetation change was significantly different. Old C contributed only 20% to the microbial biomass but 60% to CO2. This indicates that most of the old C was decomposed by microorganisms catabolically, without being utilized for growth. Based on δ13C changes in DOC, CO2 and microbial biomass during 54 days of incubation in Miscanthus and reference soils, we concluded that the main process contributing to changes of the δ13C signature in soil pools was preferential utilization of recent versus old C (causing an up to 9.1‰ shift in δ13C values) and not 13C fractionation per se. Based on the δ13C changes in SOM, we showed that the estimated turnover time of old SOM increased by two years per year in 9 years after the vegetation change. The relative increase in the turnover rate of recent microbial C was 3 times faster than that of old C indicating preferential utilization of available recent C versus the old C. Combining long-term field observations with soil incubation reveals that the turnover time of C in microbial biomass was 200 times faster than in total SOM. Our study clearly showed that estimating the residence time of easily degradable microbial compounds and biomarkers should be done at time scales reflecting microbial turnover times (days) and not those of bulk SOM turnover (years and decades). This is necessary because the absence of C reutilization is a prerequisite for correct estimation of SOM turnover. We conclude that comparing the δ13C signature of linked pools helps calculate the relative turnover of old and recent pools.

CEO Turnover in a Competitive Assignment Framework

There is widespread concern about whether Chief Executive Officers (CEOs) are appropriately punished for poor performance. While CEOs are more likely to be forced out if their performance is poor relative to the industry average, overall industry performance also matters. This seems puzzling if termination is disciplinary, however, we show that both absolute and relative performance-driven turnover can be natural and efficient outcomes in a competitive assignment model in which CEOs and firms form matches based on multiple characteristics. The model also has new predictions about replacement managers’ equilibrium pay and performance. We document CEO turnover events during 1992–2006 and provide empirical support for our model.

The long-term effects of CO2 enrichment on fine root productivity, mortality, and survivorship in a scrub-oak ecosystem at Kennedy Space Center, Florida, USA

Abstract Fine root dynamics play an important role in the cycling of carbon belowground. Previous studies have indicated that CO 2 enrichment results in increased root productivity, mortality and relative turnover; however, our understanding of the duration and long-term trends of this effect are limited. Non-destructive minirhizotron observation tubes were used to measure effects of elevated CO 2 on root dynamics and survivorship in a fire dominated scrub-oak ecosystem. Open-top chambers were exposed to elevated atmospheric CO 2 for 10 years at Kennedy Space Center, Florida. In this study, initial fine root dynamics from an earlier published study from this experiment ( Dilustro et al., 2002 ) were compared to our findings 5 years later. Significant increases in root productivity, mortality, and turnover due to CO 2 enrichment were no longer present after 9 years of treatment. However, the vertical variation in these parameters suggests the upper 50 cm of the soil are the most dynamic. A greater proportion of the fine roots were deeper in the soil profile later in the study, but no CO 2 effect was observed. Survivorship analysis suggested the smallest fine roots (i.e. 2 enrichment is no longer driving changes in fine root dynamics, but rather root closure in the upper portions of the soil profile seem to be the strongest influence. Fine roots comprise nearly 25% of the total plant biomass in the scrub-oak ecosystem and their turnover and persistence is an important pathway for carbon inputs into the soil. In order to develop accurate predictive models of the impacts of increasing anthropogenic CO 2 on carbon cycling, it is imperative to examine long-term fine root dynamics rather than just shorter observations that could result in misleading conclusions regarding ecosystem responses.