405 research outputs found
Color Variability of the Blazar AO 0235+16
Multicolor (UBVRIJHK) observations of the blazar AO 0235+16 are analyzed. The
light curves were compiled at the Turin Observatory from literature data and
the results of observations obtained in the framework of the WEBT program
(http://www.to.astro/blazars/webt/). The color variability of the blazar was
studied in eight time intervals with a sufficient number of multicolor optical
observations; JHK data are available for only one of these. The spectral energy
distribution (SED) of the variable component remained constant within each
interval, but varied strongly from one interval to another. After correction
for dust absorption, the SED can be represented by a power law in all cases,
providing evidence for a synchrotron nature of the variable component. We show
that the variability at both optical and IR wavelengths is associated with the
same variable source.Comment: 11 pages, 9 figures, 4 tables, accepted for publication in Astronomy
Report
A hybrid multiagent approach for global trajectory optimization
In this paper we consider a global optimization method for space trajectory design problems. The method, which actually aims at finding not only the global minimizer but a whole set of low-lying local minimizers(corresponding to a set of different design options), is based on a domain
decomposition technique where each subdomain is evaluated through a procedure based on the evolution of a population of agents. The method is applied to two space trajectory design problems and compared with existing deterministic and stochastic global optimization methods
Rapid loss of complex polymers and pyrogenic carbon in subsoils under whole-soil warming
Subsoils contain more than half of soil organic carbon (SOC) and are expected to experience rapid warming in the coming decades. Yet our understanding of the stability of this vast carbon pool under global warming is uncertain. In particular, the fate of complex molecular structures (polymers) remains debated. Here we show that 4.5 years of whole-soil warming (+4 °C) resulted in less polymeric SOC (sum of specific polymers contributing to SOC) in the warmed subsoil (20–90 cm) relative to control, with no detectable change in topsoil. Warming stimulated the subsoil loss of lignin phenols (−17 ± 0%) derived from woody plant biomass, hydrolysable lipids cutin and suberin, derived from leaf and woody plant biomass (−28 ± 3%), and pyrogenic carbon (−37 ± 8%) produced during incomplete combustion. Given that these compounds have been proposed for long-term carbon sequestration, it is notable that they were rapidly lost in warmed soils. We conclude that complex polymeric carbon in subsoil is vulnerable to decomposition and propose that molecular structure alone may not protect compounds from degradation under future warming
Folding of a donor–acceptor polyrotaxane by using noncovalent bonding interactions
Mechanically interlocked compounds, such as bistable catenanes and bistable rotaxanes, have been used to bring about actuation in nanoelectromechanical systems (NEMS) and molecular electronic devices (MEDs). The elaboration of the structural features of such rotaxanes into macromolecular materials might allow the utilization of molecular motion to impact their bulk properties. We report here the synthesis and characterization of polymers that contain π electron-donating 1,5-dioxynaphthalene (DNP) units encircled by cyclobis(paraquat-p-phenylene) (CBPQT4+), a π electron-accepting tetracationic cyclophane, synthesized by using the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC). The polyrotaxanes adopt a well defined “folded” secondary structure by virtue of the judicious design of two DNP-containing monomers with different binding affinities for CBPQT4+. This efficient approach to the preparation of polyrotaxanes, taken alongside the initial investigations of their chemical properties, sets the stage for the preparation of a previously undescribed class of macromolecular architectures
Large CO\u3csub\u3e2\u3c/sub\u3e and CH\u3csub\u3e4\u3c/sub\u3e emissions from polygonal tundra during spring thaw in northern Alaska
The few prethaw observations of tundra carbon fluxes suggest that there may be large spring releases, but little is known about the scale and underlying mechanisms of this phenomenon. To address these questions, we combined ecosystem eddy flux measurements from two towers near Barrow, Alaska, with mechanistic soil-core thawing experiment. During a 2 week period prior to snowmelt in 2014, large fluxes were measured, reducing net summer uptake of CO2 by 46% and adding 6% to cumulative CH4 emissions. Emission pulses were linked to unique rain-on-snow events enhancing soil cracking. Controlled laboratory experiment revealed that as surface ice thaws, an immediate, large pulse of trapped gases is emitted. These results suggest that the Arctic CO2 and CH4 spring pulse is a delayed release of biogenic gas production from the previous fall and that the pulse can be large enough to offset a significant fraction of the moderate Arctic tundra carbon sink
Association between soil organic carbon and calcium in acidic grassland soils from Point Reyes National Seashore, CA
Organo-mineral and organo-metal associations play an important role in the retention and accumulation of soil organic carbon (SOC). Recent studies have demonstrated a positive correlation between calcium (Ca) and SOC content in a range of soil types. However, most of these studies have focused on soils that contain calcium carbonate (pH > 6). To assess the importance of Ca-SOC associations in lower pH soils, we investigated their physical and chemical interaction in the grassland soils of Point Reyes National Seashore (CA, USA) at a range of spatial scales. Multivariate analyses of our bulk soil characterisation dataset showed a strong correlation between exchangeable Ca (Ca; 5–8.3 c.mol kg) and SOC (0.6–4%) content. Additionally, linear combination fitting (LCF) of bulk Ca K-edge X-ray absorption near-edge structure (XANES) spectra revealed that Ca was predominantly associated with organic carbon across all samples. Scanning transmission X-ray microscopy near-edge X-ray absorption fine structure spectroscopy (STXM C/Ca NEXAFS) showed that Ca had a strong spatial correlation with C at the microscale. The STXM C NEXAFS K-edge spectra indicated that SOC had a higher abundance of aromatic/olefinic and phenolic C functional groups when associated with Ca, relative to C associated with Fe. In regions of high Ca-C association, the STXM C NEXAFS spectra were similar to the spectrum from lignin, with moderate changes in peak intensities and positions that are consistent with oxidative C transformation. Through this association, Ca thus seems to be preferentially associated with plant-like organic matter that has undergone some oxidative transformation, at depth in acidic grassland soils of California. Our study highlights the importance of Ca-SOC complexation in acidic grassland soils and provides a conceptual model of its contribution to SOC preservation, a research area that has previously been unexplored
Whole-soil warming decreases abundance and modifies the community structure of microorganisms in the subsoil but not in surface soil
The microbial community composition in subsoils remains understudied, and it is largely unknown whether subsoil microorganisms show a similar response to global warming as microorganisms at the soil surface do. Since microorganisms are the key drivers of soil organic carbon decomposition, this knowledge gap causes uncertainty in the predictions of future carbon cycling in the subsoil carbon pool (> 50 % of the soil organic carbon stocks are below 30 cm soil depth). In the Blodgett Forest field warming experiment (California, USA) we investigated how +4 ∘C warming in the whole-soil profile to 100 cm soil depth for 4.5 years has affected the abundance and community structure of microorganisms. We used proxies for bulk microbial biomass carbon (MBC) and functional microbial groups based on lipid biomarkers, such as phospholipid fatty acids (PLFAs) and branched glycerol dialkyl glycerol tetraethers (brGDGTs). With depth, the microbial biomass decreased and the community composition changed. Our results show that the concentration of PLFAs decreased with warming in the subsoil (below 30 cm) by 28 % but was not affected in the topsoil. Phospholipid fatty acid concentrations changed in concert with soil organic carbon. The microbial community response to warming was depth dependent. The relative abundance of Actinobacteria increased in warmed subsoil, and Gram+ bacteria in subsoils adapted their cell membrane structure to warming-induced stress, as indicated by the ratio of anteiso to iso branched PLFAs. Our results show for the first time that subsoil microorganisms can be more affected by warming compared to topsoil microorganisms. These microbial responses could be explained by the observed decrease in subsoil organic carbon concentrations in the warmed plots. A decrease in microbial abundance in warmed subsoils might reduce the magnitude of the respiration response over time. The shift in the subsoil microbial community towards more Actinobacteria might disproportionately enhance the degradation of previously stable subsoil carbon, as this group is able to metabolize complex carbon sources
Representing winter wheat in the Community Land Model (version 4.5)
Winter wheat is a staple crop for global food security, and is the
dominant vegetation cover for a significant fraction of Earth's croplands. As
such, it plays an important role in carbon cycling and land–atmosphere
interactions in these key regions. Accurate simulation of winter wheat growth
is not only crucial for future yield prediction under a changing climate, but
also for accurately predicting the energy and water cycles for winter wheat
dominated regions. We modified the winter wheat model in the Community Land
Model (CLM) to better simulate winter wheat leaf area index, latent heat
flux, net ecosystem exchange of CO2, and grain yield. These included
schemes to represent vernalization as well as frost tolerance and damage. We
calibrated three key parameters (minimum planting temperature, maximum crop
growth days, and initial value of leaf carbon allocation coefficient) and
modified the grain carbon allocation algorithm for simulations at the US
Southern Great Plains ARM site (US-ARM), and validated the model performance
at eight additional sites across North America. We found that the new winter
wheat model improved the prediction of monthly variation in leaf area index,
reduced latent heat flux, and net
ecosystem exchange root mean square error (RMSE) by 41 and 35 % during the
spring growing season. The model accurately simulated the interannual
variation in yield at the US-ARM site, but underestimated yield at sites and
in regions (northwestern and southeastern US) with historically greater
yields by 35 %
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A dual isotope approach to isolate soil carbon pools of different turnover times
Soils are globally significant sources and sinks of
atmospheric CO₂. Increasing the resolution of soil carbon
turnover estimates is important for predicting the response of
soil carbon cycling to environmental change. We show that
soil carbon turnover times can be more finely resolved using
a dual isotope label like the one provided by elevated CO₂
experiments that use fossil CO₂. We modeled each soil physical
fraction as two pools with different turnover times using
the atmospheric ¹⁴C bomb spike in combination with the label
in ¹⁴C and ¹³C provided by an elevated CO₂ experiment
in a California annual grassland.
In sandstone and serpentine soils, the light fraction carbon
was 21–54% fast cycling with 2–9 yr turnover, and 36–79%
slow cycling with turnover slower than 100 yr. This validates
model treatment of the light fraction as active and intermediate
cycling carbon. The dense, mineral-associated fraction
also had a very dynamic component, consisting of ~ 7%
fast-cycling carbon and ~93% very slow cycling carbon.
Similarly, half the microbial biomass carbon in the sandstone
soil was more than 5 yr old, and 40% of the carbon respired
by microbes had been fixed more than 5 yr ago.
Resolving each density fraction into two pools revealed
that only a small component of total soil carbon is responsible
for most CO₂ efflux from these soils. In the sandstone
soil, 11% of soil carbon contributes more than 90% of the
annual CO₂ efflux. The fact that soil physical fractions, designed
to isolate organic material of roughly homogeneous
physico-chemical state, contain material of dramatically different turnover times is consistent with recent observations
of rapid isotope incorporation into seemingly stable fractions
and with emerging evidence for hot spots or micro-site variation
of decomposition within the soil matrix. Predictions
of soil carbon storage using a turnover time estimated with
the assumption of a single pool per density fraction would
greatly overestimate the near-term response to changes in
productivity or decomposition rates. Therefore, these results
suggest a slower initial change in soil carbon storage due
to environmental change than has been assumed by simpler
(one-pool) mass balance calculations
Diverse soil carbon dynamics expressed at the molecular level
The stability and potential vulnerability of soil organic matter (SOM) to global change remains incompletely understood due to the complex processes involved in its formation and turnover. Here we combine compound-specific radiocarbon analysis with fraction-specific and bulk-level radiocarbon measurements in order to further elucidate controls on SOM dynamics in a temperate and sub-alpine forested ecosystem. Radiocarbon contents of individual organic compounds isolated from the same soil interval generally exhibit greater variation than those among corresponding operationally-defined fractions. Notably, markedly older ages of long-chain plant leaf wax lipids (n-alkanoic acids) imply that they reflect a highly stable carbon pool. Furthermore, marked 14C variations among shorter- and longer-chain n-alkanoic acid homologues suggest that they track different SOM pools. Extremes in SOM dynamics thus manifest themselves within a single compound class. This exploratory study highlights the potential of compound-specific radiocarbon analysis for understanding SOM dynamics in ecosystems potentially vulnerable to global change
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