Grantee Research Project Results
2006 Progress Report: Reduced Atmospheric Methane Consumption By Temperate Forest Soils Under Elevated Atmospheric CO2
EPA Grant Number: R831451Title: 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: Chung, Serena
Project Period: January 1, 2004 through December 31, 2008
Project Period Covered by this Report: January 1, 2006 through December 31, 2007
Project Amount: $613,030
RFA: Consequences of Global Change for Air Quality: Spatial Patterns in Air Pollution Emissions (2003) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Climate Change , Air
Objective:
The current study is a follow-up to a previous investigation where we showed 13 to 30% 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 ~550 ppmv. The overall objectives of the current research were to: (a) 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 (b) 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, while 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 with 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 start-up and have continued through December 2006. Duplicate quadrants within each ring were fertilized with ammonium nitrate at a rate of 11.2 g N m-2 y-1 in the spring of 2005 and 2006, as they will continue to be fertilized in the future.
Time-integrated CH4 consumption was reduced by 19%, 10%, and 7% in CO2-fertilized plots relative to unfertilized controls, in 2004, 2005, and 2006, respectively. The results are consistent with the 13 to 30% 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: (a) consumption by upland soils and tropospheric destruction by the OH radical are the only identified sinks of atmospheric CH4; and (b) 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% reduction (7.5 Tg CH4 yr-1) in the current upland soil sink of ~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: (a) changes in the plant-soil chemical environment; (b) changes in the rate of diffusion of atmospheric CH4 to the CH4-oxidizing microbial community; and (c) 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 have been installed in all plots. Laboratory experiments intended to assess the sensitivity of the CH4-oxidizing microbial community to freshly collected throughfall in CO2-fertilized and unfertilized plots were conducted in 2004 and 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 yr 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. 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. Additionally, we have performed two experiments testing the influence of fresh leaf leachate from the four most abundant tree species at the research site (pinus taeda, ulmus alata, acer rubrum, and liquiambar styraciflua) and two experiments assessing influence of forest duff leachate on CH4 oxidizers in 2005 and 2006. In the case of both leaf leachate experiments and the summer duff leachate experiment, first order rate constants for CH4 consumption did not differ for soils amended with any leachate versus deionized water-amended controls. The first order rate constants for CH4 consumption were however reduced in soils to which leachate from duff collected from both ambient and elevated CO2 plots in the fall 2006 duff leachate experiment. This indicates that some chemical released from fresh autumnal duff, regardless of CO2, treatment may inhibit net CH4 consumption. Thus, our results along with those of a study indicating that litterfall increases under elevated CO2 (Allen et al. 2000) suggest that quantity, not quality, of plant leachates and exudates may contribute to reduced CH4 consumption under elevated CO2.
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 and 2006. 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 no consistent nor considerable differences between elevated and ambient CO2 treatments in CH4 concentrations of the same depths. 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, when in situ calibration of these probes is complete. 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 fortuitously oxidize CH4 due to the nonspecificity of the enzyme ammonium monooxygenase and the similarity of the CH4 and NH4+ molecules. We also assessed rates of CH4 production in core sections, testing the hypothesis that localized zones of methanogenesis in CO2-fertilized rings may account for the reduced rates of net CH4 consumption. The second of three laboratory experiments assessing the activity of CH4 oxidizing and producing communities was completed in October of 2006. The July 2005 experimental results showed no statistically significant difference in rates of net CH4 consumption between core sections from ambient and elevated CO2. Rates of CH4 production were greater in all elevated CO2 core sections relative to ambient CO2 core sections from the same depth, and CH4 production was greatest below 15 cm in elevated CO2 core sections (Fig. 6B). Ammonium oxidation was higher in ambient CO2 soil core sections at all depths as compared to those of the elevated CO2 core sections, and NH4+ oxidation did not occur in most elevated CO2 core sections (Fig. 6C). The differences in CH4 production and NH4+ oxidation between ambient and elevated CO2 core sections were not, however, statistically significant. However, in 2006 core experiments, there were not any statistical differences in net CH4 consumption or production, nor in NH3 oxidation, between core sections from ambient and elevated CO2 plots. Collectively, 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, although the effect may be seasonal and/or based on environmental conditions prior to sampling.
Future Activities:
We now have soil moisture probes in place immediately adjacent to our soil collars. 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 will also further examine the differences in porewater chemistry between elevated and ambient CO2 treatments by analyzing water extracted from 10 cm depths via microlysimeters for total amino acids, a suite of organic acids, dissolved organic acid, dissolved organic carbon, and characterization of dissolved organic matter. Finally, we will collect core samples in the spring of 2007 and analyze soils for microbial activity as we have done in past years.
Journal Articles:
No journal articles submitted with this report: View all 8 publications for this projectSupplemental Keywords:
Methane, methane oxidation, forests, biogeochemical cycles, global change,, RFA, Scientific Discipline, Air, POLLUTANTS/TOXICS, Environmental Chemistry, Chemicals, climate change, Air Pollution Effects, Forestry, Environmental Monitoring, Atmosphere, adaptive technologies, carbon dioxide enriched soil, forest soils, global change, green house gas concentrations, methane, carbon dioxide, CO2 concentrations, greenhouse gases, ecosystem impacts, forests, global warming, monitoring organics, air quality, climate variabilityProgress and Final Reports:
Original AbstractThe 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.