Understanding the dynamics of methane (CH4) emissions is of paramount importance because CH4 has 25 times the global warming potential of carbon dioxide (CO2) and is currently the second most important anthropogenic greenhouse gas. Wetlands are the single largest natural CH4 source with median emissions from published studies of 164 Tg yr 1 , which is about a third of total global emissions. We provide a perspective on important new frontiers in obtaining a better understanding of CH4 dynamics in natural systems, with a focus on wetlands. One of the most exciting recent developments in this field is the attempt to integrate the different methodologies and spatial scales of biogeochemistry, molecular microbiology, and modeling, and thus this is a major focus of this review. Our specific objectives are to provide an up-to-date synthesis of estimates of global CH4 emissions from wetlands and other freshwater aquatic ecosystems, briefly summarize major biogeophysical controls over CH4 emissions from wetlands, suggest new frontiers in CH4 biogeochemistry, examine relationships between methanogen community structure and CH4 dynamics in situ, and to review the current generation of CH4 models. We highlight throughout some of the most pressing issues concerning global change and feedbacks on CH4 emissions from natural ecosystems. Major uncertainties in estimating current and future CH4 emissions from natural ecosystems include the following: (i) A number of important controls over CH4 production, consumption, and transport have not been, or are inadequately, incorporated into existing CH4 biogeochemistry models. (ii) Significant errors in regional and global emission estimates are derived from large spatial-scale extrapolations from highly heterogeneous and often poorly mapped wetland complexes. (iii) The limited number of observations of CH4 fluxes and their associated environmental variables loosely constrains the parameterization of process-based biogeochemistry models.

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Methane emissions from wetlands: biogeochemical, microbial, and modeling perspectives from local to global scales

Two electrochemical methods to assess the redox properties of humic substances (HS) are presented: direct electrochemical reduction (DER) on glassy carbon working electrodes (WE) and mediated electrochemical reduction (MER) and oxidation (MEO) using organic radicals to facilitate electron transfer between HS and the WE. DER allows for continuous monitoring of electron and proton transfer to HS by chronocoulometry and automated acid titration, respectively, and of changes in bulk HS redox potential Eh. Leonardite Humic Acid (LHA) showed an H+/e− ratio of unity and a decrease in potential from Eh = +0.18 to −0.23 V upon transfer of 822 μmole- gLHA−1 at pH 7, consistent with quinones as major redox-active functional moieties in LHA. MER and MEO quantitatively detected electrons in LHA samples that were prereduced by DER to different extents. MER and MEO therefore accurately quantify the redox state of HS. Cyclic DER and O2-reoxidation revealed that electron transfer to LHA was largely reversible. However, LH...

Novel electrochemical approach to assess the redox properties of humic substances.

Humic substances (HS) are heterogeneous, redox-active organic macromolecules. While electron transfer to and from HS under reducing conditions is well investigated, comparatively little is known on the electron donating (i.e., antioxidant) properties of HS under oxic conditions. In this work, the electron donating capacities (EDCs) of terrestrial and aquatic HS were quantified by mediated electrochemical oxidation over a wide range of pH values and applied redox potentials (Eh) using 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) as an electron transfer mediator. Electrochemical oxidation of three model humic acids (HAs) was largely irreversible, and the EDCs of these HAs increased with increasing Eh and pH. These results suggest that HS contain a wide variety of moieties that are oxidized at different potentials and that, upon oxidation, release protons and undergo irreversible follow-up reactions. At a given pH and Eh, the EDCs of the HS correlated well with their titrated phenol contents suggest...

Antioxidant Properties of Humic Substances

https://repositorium.sdum.uminho.pt/bitstream/1822/16751/1/2888.pdf

Impact and application of electron shuttles on the redox (bio)transformation of contaminants: a review.

Humic substances make up a significant fraction of the natural organic matter in terrestrial and aquatic environments. Laboratory experiments suggest that humic substances serve as fully regenerable electron acceptors in recurrently anoxic environments such as peatlands.

Humic substances as fully regenerable electron acceptors in recurrently anoxic environments

Cyclic oxidation and reduction reactions using oxygen and palladium with H2, respectively, of dissolved humic and fulvic acids (HA and FA) and model quinone compounds were used to structurally characterize and quantify the electron-carrying capacity (ECC) of reversible redox sites present in humic substances. This technique was used to examine 8 quinone compounds and 14 HA and FA samples and identified 3 redox sites as a function of their stability against the Pd-catalyzed hydrogenolysis process. Six highly aliphatic HA and FA isolated from landfill leachate did not contain redox sites under any conditions; however, the other HA and FA demonstrated reversible redox properties characterized by a combination of three redox sites. On the basis of the model compound results, it is proposed that one site consists of a non-quinone structure (NQ) and the other two sites have quinone structures. The two quinone sites differ in that one group (Q1) has electron-withdrawing groups adjacent to the quinone functional ...

Characterization and quantification of reversible redox sites in humic substances.

Tetracyclines such as oxytetracycline (OTC) are widely used veterinary chemicals. They are often poorly absorbed with a significant fraction being excreted in manure that can subsequently result in environmental contamination. In many countries throughout South East Asia swine manure is not composted, but sun-dried. Using sunlight to heat manure has been suggested as an effective and low-cost strategy to reduce OTC contamination, but this has previously been unexplored. Such conditions have also been shown to reduce bacterial numbers in manure meaning abiotic OTC degradation processes may become more significant. This work investigated for the first time, the role of temperature, illumination and pH in the abiotic degradation of OTC in sterilized swine manure. OTC loss from laboratory-based experiments simulating conditions likely to be experienced in sun-drying were assessed using simple first order and availability-adjusted loss models. ANOVA results suggested that neither model was superior to the othe...

Nopawan Ratasuk

Papers

Characterization and quantification of reversible redox sites in humic substances.

Effect of temperature, pH and illumination on abiotic degradation of oxytetracycline in sterilized swine manure.