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2004 Progress Report: Reduced Atmospheric Methane Consumption By Temperate Forest Soils Under Elevated Atmospheric CO2

EPA Grant Number: R831451
Title: Reduced Atmospheric Methane Consumption By Temperate Forest Soils Under Elevated Atmospheric CO2
Investigators: Whalen, Stephen C. , Wetzel, Robert G.
Institution: University of North Carolina at Chapel Hill
EPA Project Officer: Bloomer, Bryan
Project Period: January 1, 2004 through December 31, 2008
Project Period Covered by this Report: January 1, 2004 through December 31, 2005
Project Amount: $613,030
RFA: Consequences of Global Change for Air Quality: Spatial Patterns in Air Pollution Emissions (2003)
Research Category: Air Quality and Air Toxics , Global Climate Change

Description:

Objective:

The current study is a follow-up to a previous investigation where we showed 13 to 30 percent per year decreases in atmospheric methane (CH4) consumption in plots of a temperate loblolly pine (Pinus taeda) forest continuously exposed to a model-projected future (mid-21st century) atmospheric carbon dioxide (CO2) level of approximately 550 ppmv. The objectives of this research project are to: (1) continue repeated CH4 flux measurements at previously established locations within CO2-enriched plots to determine whether the observed decline in soil CH4 consumption was transient or sustained; and (2) ascertain the cause(s) for reduced atmospheric CH4 consumption in forest soils exposed to elevated CO2, based on known and suspected physicochemical controls on CH4 consumption by upland soil microbes.

Progress Summary:

We are addressing our first objective through approximately biweekly CH4 flux measurements at 24 permanently established locations within the Duke Forest, North Carolina, Free Air CO2 Enrichment (FACE) site. Six 30-meter diameter rings are located within the forest and each ring is divided into quadrants. Triplicate rings are fertilized with CO2 during the daylight hours to maintain atmospheric CO2 at 550 ppm throughout the canopy, while triplicate unfertilized rings are subject to the ambient atmosphere (360 ppm CO2) and serve as controls. A single static chamber to determine CH4 flux is located within each quadrant of each ring for a total of 12 chambers in soils subject to CO2-fertilization and 12 chambers in unfertilized soils. Methane flux determinations were initiated on project start-up and will continue until the termination of the project. For the 2004 calendar year, time-integrated CH4 consumption was reduced by 21 percent in CO2-fertilized plots relative to unfertilized controls, consistent with the 13 to 30 percent reduction observed in three previous study years. Collectively, these data suggest that a reduction in forest soil CH4 consumption is a sustained ecosystem-level response to elevated CO2. This is significant because: (1) consumption by upland soils and tropospheric destruction by the OH radical are the only identified sinks of atmospheric CH4; and (2) as a greenhouse gas, CH4 is second only to CO2 in terms of radiative forcing. Forest ecosystems occupy about half of the earth’s terrestrial surface. A sustained, CO2-induced negative feedback on forest soil CH4 consumption could lead to a 25 percent reduction (7.5 Tg CH4 per year) in the current upland soil sink of approximately 30 Tg per year. Information of this nature, linking the atmospheric CO2 and CH4 cycles, is central to modeling efforts to refine and improve estimates of the upland sink term in the atmospheric CH4 budget under projected future climates.

We are addressing our second objective by identifying treatment-wise (CO2-fertilized versus unfertilized) differences in known controls on microbial oxidation of atmospheric CH4 in upland soils. These controls include: (1) changes in the plant-soil chemical environment; (2) changes in the rate of diffusion of atmospheric CH4 to the CH4-oxidizing microbial community; and (3) shifts in the relative abundance of CH4-oxidizing microbes (methanotrophs and NH4+-oxidizers). Work on all three of these research areas is in the early stages and is described below.

Changes in the Plant-Soil Chemical Environment

Throughfall collectors have been installed in all plots and two laboratory experiments have been performed assessing the sensitivity of the CH4-oxidizing microbial community to freshly collected throughfall in CO2-fertilized and unfertilized plots. There were no treatment-wise differences in first-order rate constants for CH4 consumption, suggesting that any differences in the chemical composition of throughfall does not impact CH4 oxidizers. Loblolly pine seedlings have been grown for 1 year in the Duke Phytotron in an informal collaboration with Dr. Emily Bernhardt (Duke University) who is studying the quantity and chemical composition of root exudates from these trees grown in CO2 atmospheres ranging from sub-ambient to twice the current atmospheric level. Concurrent with tree growth, a system was designed and tested to isolate a tree root in a sterile aqueous medium and collect the root exudate over a 24-hour period. The first experiment collecting and evaluating the chemical nature of root exudates as a function of CO2 level is underway. The root exudates have been collected and are being chemically characterized.

Changes in the Rate of Diffusion of Atmospheric CH4 to the CH4-Oxidizing Microbial Community

This aspect of the research has been addressed at one level to date. The rate of diffusion of atmospheric CH4 to the CH4-oxidizing microbial community is dependent on the locus of these microbes in the soil profile. A community response to elevated CO2 may involve a downward shift of CH4-oxidizers in the soil horizon, effectively reducing the rates of supply of substrate (CH4) for these microbes. Duplicate soil cores (30 cm length) were collected from each FACE ring, sectioned into 5 cm increments and the rate of CH4 consumption was determined for each section in laboratory incubations. The relative depth distribution of CH4 oxidation did not differ between treatments, suggesting that a shift in the locus of CH4-oxidizing activity was not responsible for field observations of reduced CH4 consumption in CO2-fertilized plots.

Shifts in the Relative Abundance of CH4-Oxidizing Microbes (Methanotrophs and NH4+-Oxidizers)

See future activities below.

Future Activities:

We plan to include an experiment to address shifts in the relative abundance of CH4-oxidizing microbes by using soils that were collected to characterize the depth distribution of CH4-oxidizing activity. We also plan to permanently install stainless steel tubes (with the permission of FACE management) at defined depths in the soils to periodically obtain soil CH4 and 222Rn profiles. These are necessary for a technique we will use to determine effective diffusivity of the soils in these plots and further address the issue of rates of supply of CH4 to soil microbes. We will continue our work chemically characterizing root exudates. We will test the sensitivity of the CH4-oxidizing community in laboratory experiments to bulk root exudates, soil solution, throughfall, and stemflow. We will focus our chemical characterizations on any bulk solution that results in reduced rates of CH4 consumption and expose the microbial community to chemical constituents uniquely associated with root exudates from the elevated CO2 treatment in an attempt to isolate chemical causes of reduced CH4 consumption. Finally, we intend to continue routine CH4 flux measurements at biweekly intervals for the duration of this project.


Journal Articles on this Report: 1 Displayed | Download in RIS Format

Other project views: All 6 publications 1 publications in selected types All 1 journal articles

Type Citation Project Document Sources
Journal Article Whalen SC. Biogeochemistry of methane exchange between natural wetlands and the atmosphere. Environmental Engineering Science 2005;22(1):73-94. R831451 (2004)
  • Abstract: Liebert Online Abstract
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    • Supplemental Keywords:

    methane, methane oxidation, forests, biogeochemical cycles, global change, global climate, environmental chemistry, ecology, air pollution effects, atmosphere, environmental monitoring, forestry, climate change, CO2 concentrations, carbon dioxide, enriched soil, greenhouse gases, , POLLUTANTS/TOXICS, Air, Scientific Discipline, RFA, climate change, Ecological Risk Assessment, Air Pollution Effects, Atmosphere, Chemicals, Environmental Chemistry, Forestry, Environmental Monitoring, CO2 concentrations, carbon dioxide, air quality, monitoring organics, methane, ecosystem impacts, forest soils, climate variability, global change, carbon dioxide enriched soil, forests, green house gas concentrations, adaptive technologies, global warming

    Progress and Final Reports:
    Original Abstract
    2005 Progress Report
    2006 Progress Report
    2007 Progress Report

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