170 research outputs found
Mineralization of ancient carbon in the subsurface of riparian forests
Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): G02021, doi:10.1029/2007JG000482.Microbial activity in saturated, subsurface sediments in riparian forests may be supported by recent photosynthate or ancient (>500 ybp) soil organic carbon (SOC) in buried horizons. Metabolism of ancient SOC may be particularly important in riparian zones, considered denitrification hot spots, because denitrification in the riparian subsurface is often C-limited, because buried horizons intersect deep flow paths, and because low C mineralization rates can support ecosystem-relevant rates of denitrification. Buried horizons are common where alluvial processes (stream migration, overbank flow) have dominated riparian evolution. Our objectives were to determine: (1) the extent to which ancient SOC directly supports subsurface microbial activity; (2) whether different C sources support microbial activity in alluvial versus glaciofluvial riparian zones; and (3) how microbial use of ancient SOC varies with depth. In situ groundwater incubations and 14C dating of dissolved inorganic carbon revealed that ancient SOC mineralization was common, and that it constituted 31–100% of C mineralization 2.6 m deep at one site, at rates sufficient to influence landscape N budgets. Our data failed to reveal consistent spatial patterns of microbially available ancient C. Although mineralized C age increased with depth at one alluvial site, we observed ancient C metabolism 150 cm deep at a glaciofluvial site, suggesting that subsurface microbial activity in riparian zones does not vary systematically between alluvial and glaciofluvial hydrogeologic settings. These findings underscore the relevance of ancient C to contemporary ecosystem processes and the challenge of using mappable surface features to identify subsurface ecosystem characteristics or riparian zone N-sink strength.We are grateful to the Cornell Program in
Biogeochemistry for graduate research grants and to the U.S. EPA for a
STAR Graduate Fellowship to Noel Gurwick. Support for radiocarbon
analyses also came from USDANRICGP grant 99–35102– 8266, NSF
cooperative agreement OCE-9807266, and an Andrew W. Mellon Foundation
grant to the Institute of Ecosystem Studies. A graduate research grant to
N. Gurwick from the Theresa Heinz Scholars for Environmental Research
provided salary for Pete Seitz-Rundlett
Electrochemical bicarbonate reduction in the presence of Diisopropylamine on sliver oxide in alkaline sodium bicarbonate medium
In this study, the reduction of bicarbonate in the presence four amines on a silver oxide/
carbon nanotube (Ag2O/CNT) composite electrode has been investigated. The studied amines
include ethanolamine (MEA), diethylenetriamine (DETA), diisopropylamine (DIPA) and
aminoethylpiperazine (AEP). Regardless of amine type, in the absence of a bicarbonate
solution, no reduction/oxidation peaks were observed. However, in the presence of
bicarbonate, a single reduction peak along with simultaneous H2 evolution was clearly
observed. The cyclic voltammetry measurements showed that only diisopropylamine (DIPA)
had a significant catalytic effect toward bicarbonate reduction on the composite electrode. No
peak was observed in the anodic direction of the reverse scans, suggesting the irreversible
nature of the electrochemical process. The effect of scan rate revealed that the irreversible
reduction mechanism is governed by both diffusion and adsorption pathways. In addition of
carbonate ions, format ions also have been detected in liquid phase. In order to study the
mechanism of bicarbonate reduction in the DIPA solution on Ag2O/CNT electrode, electrochemical impedance spectroscopy (EIS) was employed. The EIS results showed that
the charge transfer resistance decreased when the potential decreased from -0.1 to -0.9 V then
faded with a further rise in potential to up to -1.9 V. In addition, an inductive loop under
certain conditions was observed in the complex plane due to the formation of adsorbed
intermediates onto the electrode surface.Chulalongkorn Universit
Co-Transport of Polycyclic Aromatic Hydrocarbons by Motile Microorganisms Leads to Enhanced Mass Transfer under Diffusive Conditions.
The
environmental chemodynamics of hydrophobic organic chemicals
(HOCs) are often rate-limited by diffusion in stagnant boundary layers.
This study investigated whether motile microorganisms can act as microbial
carriers that enhance mass transfer of HOCs through diffusive boundary
layers. A new experimental system was developed that allows (1) generation
of concentration gradients of HOCs under the microscope, (2) exposure
and direct observation of microorganisms in such gradients, and (3)
quantification of HOC mass transfer. Silicone O-rings were integrated
into a Dunn chemotaxis chamber to serve as sink and source for polycyclic
aromatic hydrocarbons (PAHs). This resulted in stable concentration
gradients in water (>24 h). Adding the model organism <i>Tetrahymena
pyriformis</i> to the experimental system enhanced PAH mass transfer
up to hundred-fold (benzo[a]pyrene). Increasing mass transfer enhancement
with hydrophobicity indicated PAH co-transport with the motile organisms.
Fluorescence microscopy confirmed such transport. The effective diffusivity
of <i>T. pyriformis</i>, determined by video imaging microscopy,
was found to exceed molecular diffusivities of the PAHs up to four-fold.
Cell-bound PAH fractions were determined to range from 28% (naphthalene)
to 92% (pyrene). Motile microorganisms can therefore function as effective
carriers for HOCs under diffusive conditions and might significantly
enhance mobility and availability of HOCs
Humic Substances Enhance Chlorothalonil Phototransformation via Photoreduction and Energy Transfer
ABSTRACT: The photodegradation of chlorothalonil, a polychlorinated aromatic fungicide widely used in agriculture, was investigated under ultraviolet–visible irradiation in the presence and absence of different humic substances that significantly enhance the chlorothalonil phototransformation. On the basis of a kinetic model, an analytical study, the effect of scavengers, the chlorothalonil phosphorescence measurement, and varying irradiation conditions, it was possible to demonstrate that this accelerating effect is due to their capacity to reduce the chlorothalonil triplet state via H-donor reaction and to energy transfer from the triplet humic to ground state chlorothalonil. Energy transfer occurs at wavelengths below 450 nm and accounts for up to 30% of the reaction in deoxygenated medium upon irradiation with polychromatic light (300–450 nm). This process is more important with Elliott humic and fulvic acids and with humic acids extracted from natural carbonaceous material than with Nordic NOM and Pahokee peat humic acids. The obtained results are of high relevance to understanding the processes involved in chlorothalonil phototransformation and the photoreactivity of humic substances. Chlorothalonil is one of the rare molecules shown to react by energy transfer from excited humic substances
- …