Reduced Atmospheric Methane Consumption By Temperate Forest Soils Under Elevated Atmospheric CO2EPA 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: 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 , Global Climate Change , Climate Change , Air
We recently reported in a three-year study a 13 to 30% decrease in atmospheric CH4 consumption by soils in CO2-enriched plots at the Duke Forest Free-Air CO2 Enrichment (FACE) site, a temperate loblolly pine (Pinus taeda) forest. This is significant because: (a) consumption by upland soils is the only identified terrestrial sink of atmospheric CH4; (b) CH4 is an important greenhouse gas, second only to CO2 in terms of radiative forcing; and (c) tropospheric CH4 participates in complex reactions involved in the formation of O3, a greenhouse gas that also negatively impacts plant and human health. Initial response functions of all ecosystem components can be expected to adjust physiologically to elevated CO2 on different time scales and it is uncertain whether the observed reduction in atmospheric CH4 consumption by forest soils is simply transient. However, 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, thereby increasing not only radiative forcing, but also crop damage and human health risks. This project will continue time series observations of soil-atmosphere exchange of CH4 at fixed sites and identify fundamental changes in physical and biogeochemical soil properties that reduce soil CH4 consumption under elevated CO2.
The overall aim of the proposed research is to determine the duration and underlying cause(s) for the decline in atmospheric CH4 consumption in a CO2-enriched forest. Specifically, for control and CO2-enriched plots at the Duke Forest FACE site we will: (a) quantify the dynamics of soil-atmosphere exchange of CH4; (b) quantify the impact of CO2 enrichment on the exudation of dissolved organic compounds from roots of the loblolly pine into the rhizosphere, and the effects of these compounds on the rates of CH4 oxidation in soils; (c) quantify the dissolved organic compounds and ions from throughfall precipitation as a supplement to root exudates, and the effects of these compounds on rates of CH4 oxidation in soils; and (d) evaluate the impact of CO2 enrichment on soil physical and biogeochemical properties central to atmospheric CH4 consumption, including effective diffusivity, microbial community structure, the soil locus of methanotrophic activity and physiological characteristics of the CH4-oxidizing community.
The experimental approaches will involve field measurements at fixed sites within CO2-enriched and unenriched (control) rings. These seasonal field measurements give an integrated view of the dynamic relationships between environmental variables and CH4 consumption, but yield no information with respect to the influence of individual drivers, which are frequently nonlinear and interactive. To assess the impact of individual environmental influences on atmospheric CH4 consumption, we will augment in situ field observations with controlled manipulations in model ecosystems (sapling Pinus taeda) emplaced within the FACE rings. These highly replicated experimental incubations will allow not only precise evaluations of changes in physiological characteristics of the trees and release of dissolved organic exudates into the soil communities but are also amenable to frequent destructive sampling. An additional component of our experimental approach involves further characterization of the impact of environmental variables on atmospheric CH4 consumption in controlled laboratory environments. This experimental control will allow determination of physiological response functions at the community level and identification of critical points (e.g., thresholds, maxima, etc.) along the response continuum for individual or multiple drivers.
This research will determine if reduced atmospheric CH4 consumption is a sustained response of forest ecosystems to elevated CO2. Further, it will identify the coupling mechanism of CH4 oxidation rates to plant-mediated changes in supply of CH4 to the microbes (diffusion rates), the depth distribution and community structure the of CH4-oxidizing activity, and the quantity and chemical composition of root exudates, throughfall and soil solutions under elevated CO2. Field experiments in control and CO2-enriched plots will facilitate evaluation of the interactive effects of multiple drivers on atmospheric CH4 oxidation. Process level laboratory experiments will allow characterization of physiological response functions of the CH4 oxidizing community to chemicals unique to or elevated in soils from CO2-enriched plots. Critical points (e.g., thresholds, maxima, etc.) along a response continuum will be identified. Experimentally derived rate functions will be suitable for incorporation into regional or global scale process-based models to refine and improve estimates of the upland soil sink term in the atmospheric CH4 budget under projected future climates. Further, results of this study will provide information regarding feedbacks and interactions of plant-soil systems that indirectly affect the tropospheric O3 budget. This information will aid modeling efforts to evaluate the risk from O3 to human health, crops and natural ecosystems under projected future climates.