Integration of Dynamic Rhizospheric Methane Oxidation into a Process-based Methane Emissions Model

Abstract

Global methane models predict that wetlands contribute 20 to 39% of global methane emissions. Wetland plants can contribute fuel for methane production by exuding carbon into the soil, but they also introduce oxygen which would oxidize the produced methane (known as rhizospheric methane oxidation). They can also provide conduits for methane to escape to the atmosphere through their aerenchyma tissues. Previous process-based global-scale models have assumed that a constant fraction of methane in the rhizosphere is oxidized, while experimental studies have found that rhizospheric methane oxidation is not a static process and changes with environmental factors. In this study, a mechanistic rhizosphere-scale model was developed in order to understand the relationship between rhizospheric methane oxidation and environmental factors. The mechanistic model results showed that rhizospheric methane oxidation not only can change with availability of carbon from roots and root gas transport capacity, but that it also is a function of microbial competition between microbial populations that live in methanogenic environments. These results were incorporated into a large-scale process-based methane emissions model in form of a dynamic rhizospheric methane oxidation process and plot-scale model simulations were performed for four study sites located in Western Siberia. Future simulations using both static and dynamic models showed that methane emissions would increase by a median factor of 1.7 by the end of century. Switching from static to dynamic model resulted in reduction of total annual methane emissions by 4% and reduction of plant-mediated methane transport by 17% in the four study sites. The reduction is more pronounced for sites with higher density of aerenchymatous plants (such as sedges) due to higher root zone methane oxidation. The dynamic model showed that higher coverage of aerenchymatous plants in a wetland can lead to lower plant-mediated methane transport and lower methane emission. As a result, current approach of global methane emission models could potentially be overestimating methane emissions from such sites due to neglecting the effect of high root transport capacity on Pox as it is proposed in this study.