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AC_JN

Department of Physics
Portland State University

Current Research Areas

Isotopic atmospheric methane

Emissions from rice agriculture

Role of trees in the methane budget

Methyl halide emissions from estuaries

Carbon flux from cities

Method development

 

 

 

 

 

 

 

 

Emissions of methane and nitrous oxide from rice agriculture

Together with colleagues M. Aslam Khalil, Chris Butenhoff and Rei. Rasmussen, we are working to better understand natural and anthropogenic methane sources at the process-level. It is part of a collaborative USA-China program with Xu Li at the National Climate Center in Bejing and Zhenqin Xiong, Xing Guangxi, and Zu-Cong Cai at the Institute of Soil Science in Nanjing. Much of the work is focused on rice agriculture and understanding controlling mechanisms of flux – including production, oxidation and transport to that atmosphere. One of the central research questions is how will methane and nitrous oxide fluxes change in a changing climate which we study here at PSU in a research greenhouse - Q10 experiments. One of the critical factors in determining how methane emissions will respond to changes in temperature, is how oxidation will respond.

It is difficult to obtain direct measurements of the oxidation rates because of involvement of the root zone. Usually oxidation is calculated from the difference between measured production and fluxes. Furthermore, there is a disparity in the published results on the amount of methane that is oxidized in the soils. Studies that determine the oxidation as the difference between measured flux and production rates have shown high rates of 50% - 90% but data from other methods in some cases show very low oxidation rates of 0-50%. The factors that affect oxidation can explain this very large disparity or variability of oxidation rates are not known at present. But oxidation rates, especially over such a broad range can have an enormous effect on the observed flux.

A novel new means of studying methane oxidation in natural and anthropogenic wetland systems is to use isotope ratios by comparing CH4 dissolved in the below ground production zone with that emitted through the plant. This in-situ approach was first presented by Tyler et al. (1997) and Chanton et al. (1997) and made use of naturally occurring carbon isotope ratios (13C/12C) in CH4 as a means to empirically determine the fraction of CH4 oxidized during the growing season in research rice fields. This method has the inherent advantage in that it is non-invasive and leaves the rhizosphere intact compared with incubation or inhibition techniques. Building on previous approach, we are currently employing a dual isotopic tracer technique (13C and D) to quantify methane oxidation during transport from the production zone to the atmosphere. The use of CH4 dual isotopic tracers (13CH4 and CH3D) may be particularly valuable in this endeavor as both isotopic fractionation by oxidation and by transport may be accounted for empirically without assumptions about methane transport in the system.

 

Funding for this work was provided by the US Department of Energy - Office of Biological and Environmental Research