Grantee Research Project Results
Final 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 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:
Models project atmospheric CO2 concentrations by the end of the 21st century to exceed the preindustrial concentration by 90 to 250%. Accordingly, efforts have intensified to assess the regional and global impact of this projected concentration increase on all aspects of material exchange and energy transfer, including linkages to other biogeochemical cycles. We recently reported in a three year study a 13 to 30% decrease in atmospheric CH4 consumption by soils in CO2-enriched plots in a temperate loblolly pine (Pinus taeda) forest. 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. Ecosystem models indicate that the short- and long-term responses of plant communities to elevated CO2 may differ, suggesting that associated biogeochemical feedbacks may also show differing transient and equilibrium responses. Forests currently have an sink strength estimate of ~24 Tg y-1 in the atmospheric CH4 budget. Forest ecosystems occupy about 40% of the Earth’s terrestrial, and therefore play a pivotal role in planetary material cycling and energy transfer. It is uncertain whether decreased atmospheric CH4 consumption represents a transient or sustained response of forest-soil systems to elevated CO2, but a firmer understanding of the impact of rising CO2 on forest CH4 cycling dynamics is needed to better model future climates.
The overall objectives of this research were to: (a) continue repeated CH4 flux measurements at previously established locations within forest plots exposed to elevated CO2 or ambient atmospheres (controls) to determine whether the observed decline in soil CH4 consumption was transient or sustained; and (b) evaluate in complementary process-oriented laboratory experiments the cause(s) for reduced atmospheric CH4 consumption in forest soils exposed to elevated CO2 based on known physical controls on methanotrophy and identified or suspected changes in plant metabolism that may alter the soil environment. Broadly, these controls include: (1) inhibition of methanotrophs by chemical exudates or secondary metabolites associated with plant growth in CO2-enriched atmospheres; (2) diffusion limitation of CH4 supply to soil methanotrophs; and (3) alteration of the balance between CH4 production and consumption by soils under elevated CO2.
Summary/Accomplishments (Outputs/Outcomes):
Field measurements were conducted at the Duke Forest (North Carolina), Free Air CO2 Enrichment (FACE) experiment, sited in an even-aged stand of loblolly pine planted in1983. The overall FACE experimental design consists of eight circular 30-m dia plots. Four treatment plots (referred to as “elevated”) are fumigated with CO2 to maintain atmospheric CO2 concentrations 200 μL L-1 above ambient levels, while four additional treatment plots are fumigated with ambient air or are subjected to ambient air without fumigation. These four plots are referred to as “controls”. Continuous (24 h d-1) fumigation was initiated in August 1996, but was reduced to daylight hours only from 2003 to present. Each FACE plot is partitioned into quadrants for a total of 32 individual sectors.
We measure on nearly a biweekly basis from 2004 through 2006 net atmospheric CH4 consumption from static chambers (one in each quadrant) that had been permanently installed in our previous study. In association with CH4 flux determinations we measured the mean soil temperature in the 1 to 19 cm depth zone and average volumetric water content of soil in the 0 to 30 cm depth zone. We established gas sampling wells in the soil at 5 cm depth increments from 5 to 25 cm.
Process-oriented laboratory studies aimed at identifying controls on atmospheric CH4 consumption were conducted seasonally on homogenized soils or intact core sections, with a focus on the 0 to 20 cm depth interval, which is the active zone of CH4 consumption. Experiments were generally conducted in serum vials or canning jars fitted with septa to allow syringe sampling of headspace gases.
Net CH4 consumption (flux from the atmosphere to the soil) was generally found at all individual soil chambers and was almost always found at the plot scale if fluxes from all four quadrants were averaged. However, net CH4 production (flux from the soil to the atmosphere) was observed from 17 chambers on 16 separate dates, giving 22 observations of net CH4 production in 880 total records. Net CH4 production was found almost twice as often in elevated CO2 chambers as in control chambers (14 versus 8 observations). Rates of net CH4 production from individual chambers varied from 0.01 mg m-2 d-1 to 0.08 mg m-2 d-1 while rates of net CH4 consumption from individual chambers were much higher, varying from 0.02 mg m-2 d-1 to 4.5 mg m-2 d-1. Chamber-wise analysis showed no pattern with respect to magnitude of flux, as no chamber showed consistently high or low values. Although there was no clear seasonal pattern, rates of net CH4 consumption were frequently higher in the summer than the winter months.
Net CH4 oxidation showed no relationship with soil temperature when the entire data over the temperature range 4 to 25°C were considered, in agreement with the general lack of seasonality in flux. In contrast to temperature, we observed a strong (inverse) linear relationship between net CH4 consumption and soil moisture which explained 34% of the variability in the entire data.
Time-integrated rates of net CH4 consumption in control plots were 184, 196 and 197 mg m-2 y-1 in 2004, 2005 and 2006. Comparable values for elevated CO2 plots were lower by 19, 10 and 8%, at 150, 175 and 181 mg m-2 y-1. Differences between treatments were significant in 2005 and 2005, and nearly significant (p=0.10) in 2006. Notably, soil moisture was significantly higher in elevated CO2 plots than in control plots during 2004 and 2005 as well. A mixed effects model showed a significant moisture x CO2 interaction on net atmospheric CH4 consumption.
Collectively, the field component of this study and our previous research at this site give the only long-term record (nearly 8 y) of the impact of elevated CO2 for an ecosystem that normally functions as a net atmospheric CH4 sink. Soils under elevated CO2 consistently showed significantly reduced atmospheric CH4 consumption by an average of about 15% and significantly higher soil moisture by about 0.03 mL H2O cm-3 soil, relative to controls. The results strongly indicate that soil moisture is an important control of net atmospheric CH4 consumption.
Over long time trajectories (years to decades), initial response functions of all ecosystem components from trees to microbes can be expected to adjust physiologically and demographically on different time scales through modification of biogeochemical feedbacks. Consistently lower annualized rates of net CH4 consumption under elevated CO2 relative to controls, and lack of strong evidence that the magnitude of the CO2 enrichment effect has declined with time both suggest that reduced net atmospheric CH4 consumption is a sustained, equilibrium response of these forest soils to elevated CO2. However, a longer observation period is necessary to firm up this conclusion, as the time for whole ecosystem adjustment to elevated CO2 is speculative. A decline in soil CH4 consumption of the magnitude observed here (~15%) across all forest biomes gives a decrease of 3.6 Tg CH4 y-1, a value that is not inconsequential as it represents 10% of a modeled estimate of 38 Tg CH4 y-1 for the total soil sink. This negative feedback to increasing CO2 identified here deserves continued study to determine if it is an equilibrium response. If so, inclusion in model simulations of future climate will improve our predictive capabilities.
The process-oriented laboratory component of the project fell short of expectations. We were unable to firmly identify factors responsible for reduced atmospheric CH4 under elevated CO2. Several experiments yielded promising results. However, these were not entirely repeatable due to high soil spatial variability and the small number of replicates available due to limitations placed on destructive sampling. An overview of results follows.
In laboratory experiments we explored the sensitivity of soils to secondary metabolites and identified root exudates of loblolly pine grown under elevated CO2. We found that bulk throughfall and plant-specific throughfall from dominant tree species at the study site did not significantly impact rates of net soil CH4 consumption. Duff leachate significantly reduced rates of net CH4 consumption, but the results were not repeatable in two subsequent experiments. Timing of duff collection may be important, as other studies have demonstrated that the negative effect of similar leachates on net CH4 oxidation is on the order of a few days. Of the nine acids produced by loblolly pine under elevated CO2 (citric, malic, oxalic, maleic, fumaric, levulinic, succinic, shikimic, and protocatecuic acids), only levulinic acid significantly inhibited net CH4 consumption at 100 μmol L-1. Subsequent experiments identified a threshold for inhibition between 50 and 100 μmol L-1. We were unable to isolate levulinic acid in bulk soils, suggesting a short lifetime. Overall, there was little evidence that secondary metabolites, organic acids or bulk leachates from leaves or litter from plants exposed to elevated CO2 are important regulators of net atmospheric CH4 consumption.
A down-profile shift in the locus of the CH4 oxidizing community in response to elevated CO2 would increase the diffusional path for substrate supply and result in lower rates of atmospheric CH4 oxidation. Further, a shift in the relative abundance of methanotrophs and ammonium oxidizers may also result in lower rates of atmospheric CH4 oxidation. Ammonium oxidizers fortuitously oxidize CH4, but a lower rate than methanotrophs. In multiple experiments, we assessed in depth profiles to 25 cm potential rates of CH4 and ammonium oxidation and found no differences between elevated CO2 and control plots.
Increased soil moisture in elevated CO2 plots resulted from the insulating effect of enhanced litterfall associated with higher rates of tree growth. This can affect the diffusional supply of substrate (CH4) to subsurface communities of methanotrophs in two ways. First, increased litter increases the diffusional path for CH4. Second, increased moisture offers higher diffusional resistance. We were unsuccessful in attempts to directly assess effective diffusivity in the field with a sensitive 222Rn technique due primarily to low levels of soil 222Rn production. However, some indirect evidence for diffusion limitation is provided by laboratory experiments assessing depth profiles of net atmospheric CH4 consumption. Field moist soils from elevated CO2 plots generally consumed less CH4 than soils at comparable depths from control plots.
Increased soil moisture can also result in decreased net consumption of atmospheric CH4 by increasing the number of anoxic microsites supporting methanogenesis. Soils from both treatments supported low levels of methanogenesis when rendered anaerobic, with soils from the elevated CO2 plots showing higher rates, but the rate difference was not significant. Overall, the laboratory data suggest that reduction of net atmospheric CH4 under elevated CO2 results from the combined effects of increased diffusional resistance and increased frequency of anaerobic microsites supporting endogenous CH4 production.
Conclusions:
In summary, we observed in field plots a persistent and significant reduction in net atmospheric CH4 consumption by forest plots exposed to elevated CO2. Although the response appears to be sustained, with no evidence of a temporal decline, the time trajectory to an equilibrium response can be expected to vary for each ecosystem components. Although not economically feasible, added certainty will result only from extended monitoring. Reduced atmospheric CH4 consumption under elevated CO2 is strongly related to increases in soil moisture, either through diffusion limitation of substrate supply to methanotrophs, increases in microsites of simultaneous CH4 production or both. It is less likely that secondary metabolites associated with plant growth under elevated CO2, or a shift in the loci or relative importance of CH4 or NH4+-oxidizing communities, are important controls on atmospheric CH4 consumption by forest soils.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 8 publications | 2 publications in selected types | All 2 journal articles |
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Type | Citation | ||
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Dubbs LL, Whalen SC. Reduced net atmospheric CH4 consumption is a sustained response to elevated CO2 in a temperate forest. Biology and Fertility of Soils 2010;46(6):597-606. |
R831451 (Final) |
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Supplemental Keywords:
Forests, methane, biogeochemical cycles, RFA, Scientific Discipline, Air, POLLUTANTS/TOXICS, Environmental Chemistry, climate change, Air Pollution Effects, Chemicals, 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.
Project Research Results
- 2007 Progress Report
- 2006 Progress Report
- 2005 Progress Report
- 2004 Progress Report
- Original Abstract
2 journal articles for this project