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2005 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, 2005 through December 31, 2006
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 followup to a previous investigation where we showed 13 to 30 percent yr-1 decreases in atmospheric CH4 consumption in plots of a temperate loblolly pine (Pinus taeda) forest continuously exposed to a model-projected future (mid-21st century) atmospheric CO2 level of approximately 550 ppmv. The overall objectives of the 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 32 permanently established locations within the Duke Forest (NC) Free Air CO2 Enrichment (FACE) site. Eight 30 m diameter rings are located within the forest and each ring is divided into quadrants. Quadruplicate rings are fertilized with CO2 during the daylight hours to maintain atmospheric CO2 at 550 ppm throughout the canopy, whereas quadruplicate 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 16 chambers in soils subject to CO2-fertilization and 16 chambers in unfertilized soils. Methane flux determinations were initiated on project startup and will continue until the termination of the project. Recently, duplicate quadrants within each ring were fertilized with NH4NO3 at a rate of 11.2 g N m-2 yr-1. Nitrogen will continue to be applied at the same rate in two spring-season doses. This was a collective decision made by the FACE administrators to assess a combined CO2 plus N fertilization effect on ecosystem function. This will become enfolded into our study, as one-half of our permanently established soil anchors are located in N-fertilized quadrants. For the 2005 calendar year, time-integrated CH4 consumption was reduced by 10 percent in CO2-fertilized plots relative to unfertilized controls, consistent with the 13 to 30 percent annual reduction that we have observed in the initial year of this project and three previous years of study funded from other sources. 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 one-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 yr-1) in the current upland soil sink of approximately 30 Tg yr-1. 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 also expect that the decision to fertilize replicate plots with N will yield additional, valuable information about the interactive effects of increased atmospheric N deposition and atmospheric CO2 on CH4 emission from forests. Contemporary increases in atmospheric N deposition are a documented consequence of various anthropogenic activities.

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 thrusts are described below.

Changes in the Plant-Soil Chemical Environment

Throughfall collectors were installed in all plots in 2004. Laboratory experiments initiated in 2004 to assess the sensitivity of the CH4-oxidizing microbial community to freshly collected throughfall in CO2-fertilized and unfertilized plots were continued in 2005. To date, we have found 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 the past 2 years 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 subambient to twice the current atmospheric level. We have tested the sensitivity of the CH4-oxidizing microbial community to acids isolated from the root environment of these plants and again have found no effect on the microbes of interest. Finally, in 2005 we have performed two experiments testing the influence of fresh leaf leachate from loblolly pine and a single experiment assessing influence of forest duff leachate on CH4 oxidizers. In all cases, first order rate constants for CH4 consumption did not differ for soils amended with any leachate versus deionized water-amended controls. Thus, our results to date indicate that any differences in the soil chemical environment between CO2-enriched and free-air plots are not influencing the CH4-oxidizing microbial community.

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

This aspect of the research was addressed using three approaches in 2005. 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. Hence, our first approach was to assess rates of potential CH4 oxidation in 5 cm increments of soil cores collected to a depth of 30 cm from each ring. The relative depth distribution of CH4 oxidation did not differ between treatments, suggesting the locus of the CH4-oxidizing community was similar between treatments. Our second approach involved the installation and monitoring of soil gas sampling wells adjacent to each soil anchor. We repeatedly determined CH4 profiles at 5 cm increments from the soil surface to a depth of 25 cm in 2005. Depth profiles of CH4 concentrations to date show considerable differences between elevated and ambient CO2 treatments in CH4 concentrations of the same depths, on some but not all sampling days. This trend may be attributable to soil moisture differences, which can impede downward diffusion of atmospheric CH4 into the soil profile. Thus, our third approach involved the recent (January 2006) installation of Campbell Scientific CS616 Water Content Reflectometers within 30 cm of each soil anchor/gas sampling wells array. These soil moisture probes measure volumetric water content using time-domain reflectometry methods. The resultant daily averages of localized soil moisture in the top 30 cm of the soil profile will be compared to net CH4 flux measurements and depth profiles, and differences between CO2 treatments will be analyzed to determine if changes in soil moisture (which influence rates of soil gas diffusion) are responsible for treatment-wise differences in rates of soil CH4 consumption.

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

This objective was partly addressed above in the analysis of the depth distribution of CH4-oxidizing activity. We further explored this component of the project in the last calendar year through a single experiment simultaneously determining the depth distribution of methanotrophic activity and NH4+-oxidizers. This latter group of microbes has been demonstrated to oxidize fortuitously CH4 because of the nonspecificity of the enzyme ammonium monooxygenase and the similarity of the CH4 and NH4+ molecules. We added to the experimental design an assessment of CH4 production in conre sections as well, testing the hypothesis that localized zones of methanogenesis in CO2-fertilized rings may account for the reduced rates of net CH4 consumption. There was not any visible difference in net CH4 consumption between core sections from ambient and elevated CO2. Rates of CH4 production, however, were greater in all elevated CO2 core sections relative to ambient CO2 core sections from the same depth, and NH3 oxidation was higher in ambient CO2 soil core sections at all depths as compared to those of the elevated CO2 core sections. The differences in CH4 production and NH3 oxidation between ambient and elevated CO2 core sections were not, however, statistically significant. Additionally, CH4 production was greatest below 15 cm in elevated CO2 core sections, and NH3 oxidation did not occur in most elevated CO2 core sections. These results suggest that differences in activity of various microbial groups directly or indirectly affecting CH4 flux within forest soils exposed to different CO2 concentration may help to explain the observed decline in CH4 oxidation associated with this treatment and merit further investigation, despite the heterogeneity of response.

Future Activities:

As stated above, we will continue our biweekly monitoring of CH4 flux for the duration of this project. We intend to conduct at least one more experiment assessing the impact of isolated chemicals from root exudates on CH4-oxidizing activity. To date, this has not proven productive. Unless we see a treatment-related difference, we will direct all research efforts elsewhere. We now have in place immediately adjacent to our soil collars the soil moisture probes. Soil moisture serves as a proxy for diffusivity, and we intend to monitor closely the simultaneous changes in soil moisture and CH4 flux, which we feel on the cm scale is a major determinant of flux. We will focus heavily on determining rates of effective diffusivity in the soils immediately adjacent to our soil collars through simultaneous monitoring of CH4 and 222Rn profiles. To date, we have not addressed this component of the proposed research. We also will explore further the relationship between microscale production of CH4 and oxidation of CH4, which effectively determine net CH4 consumption. It is likely that no single factor is responsible for the observed reduction in CH4 consumption measured at the soil surface. Rather, it is likely that a combination of CH4 production in anaerobic microzones of this normally aerobic soil and reduced rates of penetration of CH4 from the atmosphere through the soil profile lead to the observed response. In turn, these are likely responses elicited by slightly increased soil moisture in CO2-treated rings because of reduced transpirational moisture loss.

Journal Articles:

No journal articles submitted with this report: View all 6 publications for this project

Supplemental Keywords:

methane, methane oxidation, forests, biogeochemical cycles, global change, global climate, environmental chemistry, climate change, forest soils, carbon dioxide, , 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:
2004 Progress Report
Original Abstract
2006 Progress Report
2007 Progress Report

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The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.

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