Quantifying Gross Rates of Methane Production and Consumption in a Northern Forest Restricted; Files Only

Abstract

Northern forest soils are vital for climate change mitigation since upland sandy soils favor the net consumption of atmospheric methane (CH4). This thesis studies biogeochemical CH4 cycle processes in a Northern Forest (Howland Research Forest, Maine), where upland soils are interspersed with wetland (Sphagnum bog), and upland-wetland transition soils along with hummock-hollow microtopography. This complex mosaic of microsites with sources and sinks of CH4 is subjected to change under future wet climate projected for this region, with a potential for these forests to shift from a net CH4 sink to a net CH4 source. Net CH4 emissions in a wet climate can increase either by inhibiting methanotrophs or favoring methanogens, or both. Thus, quantifying underlying processes of gross CH4 production and consumption can reduce the uncertainty of CH4 source/sink estimation in this critical ecosystem. We have collected baseline soil biogeochemistry data across the forest’s landscape including Total Carbon and Total Nitrogen with the Elemental Analyzer, Gravimetric Soil Moisture, and pH. Furthermore, the stable isotope dilution method serves as a proxy for methanogenic and methanotrophic activities to quantify gross rates of CH4 production and consumption in Howland Forest. We differentiate between CH4 consumption and production by measuring both the change in the amount of CH4 and the ratio between labeled and unlabeled CH4 in a closed system. Analyzing the stable C isotope in ¹³CH4 determined gross rates of CH4 production and consumption in situ and within laboratory incubations. Novel data obtained in this study is helpful in constraining CH4 cycle processes in a biogeochemical model to quantify CH4 source/sink potential in northern forests such as Howland under current and future climatic conditions. The new data from this research better informs microbial functions in soils under contrasting drainage conditions and helps reduce the uncertainty in CH4 budget estimations in heterogeneous landscapes like Howland Forest.

Table of Contents

I. Introduction

i. Biogeochemical Methane Cycle

ii. Historical Studies at Howland Research Forest

iii. Methane Dynamics in a Northern Forest

iv. Objectives and Hypotheses

II. Methods and Materials

i. Field Site Description

ii. Soil Core Sampling from Intensive Field Campaign

iii. Biogeochemical Soil Tests

a. Gravimetric Soil Moisture Content

b. pH

c. Total Carbon and Total Nitrogen

d. Gross CH4 flux measurements from soils

e. PoolDilutionR for CH4 pool dilution analysis

iv. Geospatial Analysis: Cartographic Design

III. Results

i. Soil Characterizations

ii. Gross Rates of CH4 Production and Consumption

iii. Chamber Fluxes of CH4

IV. Discussion

i. Reflection on research objectives

ii. Limitations

iii. Future Directions

iv. Conclusion

V. References

About this Master's Thesis

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School	Laney Graduate School
Department	Environmental Sciences
Degree	M.S.
Submission	Master's Thesis
Language	English
Research Field	Environmental Sciences Biogeochemistry
Palabra Clave	Greenhouse Gases Soil Science Forest Biogeochemistry Methane Cycle Northern Forests Forest Soils Isotope Pool Dilution Method
Committee Chair / Thesis Advisor	Eri Saikawa, Ph.D., Emory University Michael Page, M.A., Emory University
Committee Members	Kathleen Savage, M.Sc., Woodwell Climate Research Center Debjani Sihi, Ph.D., North Carolina State University

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