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2007 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: Bloomer, Bryan
Project Period: January 1, 2004 through December 31, 2008
Project Period Covered by this Report: January 1, 2007 through December 31, 2008
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% 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 have completed 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 200 above ambient atmospheric concentration of CO2 (550 to 580 ppm over the course of this study) throughout the canopy, while quadruplicate unfertilized rings are subject to the ambient atmosphere (presently 380 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 11% 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 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 were 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. Treatment-wise differences in first-order rate constants for CH4 consumption were not observed, suggesting that any differences in the chemical composition of throughfall do not impact CH4 oxidizers. Loblolly pine seedlings were grown in the Duke Phytotron for two years leading up to 2005 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 and –producing microbial communities to acids isolated from the root environment of these plants and again have found no effect on the microbes of interest. However, levulinic acid, an acid observed to be produced from the photodegradation of organic matter, was observed to inhibit net CH4 consumption on several occasions in 2007. We were unable to isolate this or any other specific organic acid from soil pore water using high performance liquid chromatography. We suspect that our inability to identify chromatographic peaks was due to the complexity of the soil porewater matrix, which contains an abundance of organic chemicals and clay particles. We also 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 the 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 rates of CH4 consumption were, however, reduced in soils to which leachate was added from duff collected from both ambient and elevated CO2 plots in the fall of 2006. This indicates that some chemical released from fresh autumnal duff, regardless of CO2 treatment may inhibit net CH4 consumption. The results of this experiment have not been replicated, despite the fact that experiments applying duff leachate were performed during all seasons in 2007. Moreover, SUVA analysis of duff leachates collected throughout 2007 found no difference in the lability of organic compounds between CO2 treatments. 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 through 2007. 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 annually assess rates of potential CH4 oxidation in 5 cm increments of soil cores collected to a depth of 25 cm from each ring. The relative depth distribution of CH4 oxidation did not differ between treatments for any year, 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 from January 2005 through December of 2006. Depth profiles show no consistent nor noteworthy differences in CH4 concentrations at any depth between elevated and ambient CO2 treatments. Our third approach involved the installation of Campbell Scientific CS616 Water Content Reflectometers within 30 cm of each soil anchor / gas sampling wells array in the beginning of 2006. These soil moisture probes measure volumetric water content using time-domain reflectometry. The resultant daily averages of localized soil moisture in the top 30 cm of the soil profile were compared to net CH4 flux measurements and depth profiles. There was no correlation between net CH4 fluxes and soil moistures, neither for CO2 treatment, nor for both treatments combined. There was also no difference in soil moisture between CO2 treatments, suggesting that soil moisture is not responsible for the observed treatment-wise differences in rates of soil CH4 consumption, at least with the sensitivity to which we can detect differences in soil moisture.
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 three times in the past three calendar years through experiments simultaneously determining methanotrophic activity and the activity of 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 cores, testing the hypothesis that localized zones of methanogenesis in CO2-fertilized rings may account for the reduced rates of net CH4 consumption. The third of three laboratory experiments assessing the activity of CH4 oxidizing and producing communities was completed in April of 2007. Experimental results from all three years showed no statistically significant difference in rates of net CH4 consumption, CH4 production, or NH4+ oxidation 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 in July 2005. However, methane production was greater in ambient CO2 core sections at every depth in October 2006, and in April 2007, higher CH4 production alternated between CO2 treatments at different depths. Thus, results were not consistent across dates. Ammonium oxidation was higher in ambient CO2 soil core sections at all depths in July 2005 and April 2007 as compared to those of the elevated CO2 core sections, and NH4+ oxidation did not occur in most elevated CO2 core sections. Mean rates of net CH4 consumption, NH4+ oxidation, and gross CH4 production averaged over all core sections of each core showed no significant differences between CO2 treatments. 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.
We have not encountered unanticipated difficulties to date in performing the proposed research. Rather, we have found that some avenues of the research have proven unproductive. We now believe that a combination of physical, chemical, and community composition changes contribute to the observed decline in the net consumption of CH4 under elevated CO2. More specifically, indirect measure of the transport of CH4 to the zone of CH4 consumption suggest that CH4 supply is unlikely or weakly responsible for the decline but needs to be investigated further; duff leachate from fresh duff and levulinic acid inhibit net CH4 consumption and are likely responsible for part of the decline; differences in the CH4 producing (methanogen) and NH4+ oxidizing communities are spatially variable but are also likely responsible for part of the decline.
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. This is a more sensitive measure of the rate of CH4 supply to soil microbes than soil moisture or depth profiles of CH4 in the soil. We will also focus on fitting mixed effects models to our net CH4 flux data using measures of various environmental variables on the corresponding sampling days. Finally, we plan to assess the phenolic content of duff leachates from 2006 experiments to determine whether there is a difference between CO2 treatments.
Future Activities: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. We will also focus on fitting mixed effects models to our net CH4 flux data using measures of various environmental variables on the corresponding sampling days. Finally, we plan to assess the phenolic content of duff leachates from 2006 experiments to determine whether there is a difference between CO2 treatments.
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,
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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
2005 Progress Report
2006 Progress Report
Original Abstract