Solid and dissolved organic matter redox properties as controls of anaerobic Respiration in organic soils
Soil Sciences
Final Report Abstract
Anaerobic respiration, i.e., the oxidation of organic matter in anoxic environments, needs an electron acceptor. Although it is known since the 1990s that organic matter itself can act as an electron acceptor for microbial respiration, it has taken time to establish suitable analytical techniques to assess electron accepting and donating capacities of organic matter. Still, also more recent studies do not include natural organic matter as an electron acceptor into their concepts, remaining with canonical electron acceptors such as nitrate, ferric iron, and sulfate. In 2010, the promising approach of mediated electrochemical reduction and oxidation has been developed, which allows analyses of electron exchange or organic matter within reasonable time, accuracy and reproducibility. Since then, a limited, yet increasing number of studies have addressed electron exchange of organic matter. Nevertheless, we were still lacking data on how much of such electron accepting capacities can be actually used by microorganisms and whether these capacities would explain total CO2 formation under anoxic conditions. Moreover, a better understanding of typical ranges of electron exchange capacities of typical organic matter materials was desirable. In this project, we therefore investigated electron exchange capacities of natural organic matter, with a focus on electron accepting capacities, in ecosystems particularly dominated by organic matter, namely peatlands. Our results demonstrated that including electron accepting capacities of organic matter into electron budgets of anaerobic microbial respiration could explain anaerobic CO2 production to a large extent, and only small fractions of unexplained CO2 remained. We furthermore provide a comprehensive overview of electron accepting and donating capacities of a global collection of peat samples from different depths. What remains challenging is the prediction of electron accepting and donating capacities for peat materials, as we could not find simple relations with commonly assessed peat properties such as degree of decomposition of elemental composition. So far, capacities may have to be actually measured. Nevertheless, predictions based on FTIR spectroscopy, providing more detailed structural information of organic matter, and information retrieved from O:C and H:C ratios seem promising in improving predictions. For most samples, not only capacities, but also microbially utilizable ranges were assessed, using laboratory incubations of the materials. For all peat materials under study, a certain fraction of electron accepting capacities was not exhausted by microbial respiration. Thermodynamic considerations, including thermodynamics of sulfate reduction and methanogenesis, suggest that these capacities may not be accessible to microorganisms due to a too low redox potential. Kinetics of organic matter electron acceptor regeneration through oxidation by molecular oxygen turned out to be fast, on the range of few hours, indicating that electron accepting capacities of organic matter are likely easily regenerated in natural systems upon redox fluctuations, such as water table fluctuations. In sum, this project demonstrated that electron exchange capacities of natural organic matter dominate carbon turnover in anoxic, organic rich systems such as peatlands. Capacities can be quantitatively related to anaerobic carbon mineralization and can to a large extent explain anaerobic CO2 formation.
Publications
- (2019). Electron accepting capacity of dissolved and particulate organic matter control CO2 and CH4 formation in peat soils. Geochim Cosmochim Acta 245: 266-277
Gao C, Sander M, Agethen S & Knorr K-H
(See online at https://doi.org/10.1016/j.gca.2018.11.004)
