“Effects of Soil Moisture on Methane Oxidation

Rates in the Harvard Forest”

Presented by Linda Jane Wan

Table of ContentsINTRODUCTION

• WHAT is CH4

OXIDATION?

• WHY it’s IMPORTANT

METHODS

• The Lowdown on the Drydown

• Broaden Your Horizons

RESULTS• Impact of H2O

DISCUSSION• Type A vs. Type O• SINK or SOURCE?

CONCLUSION• Global Import of Climate Change

Universal SOIL

The Universal SOUL, as it is called, has an interest in the stacking of hay, the foddering of cattle, and the DRAINING of peat-meadows.

- Henry David Thoreau

• SOIL microorganisms =terrestrial biological SINK• Because of this effect, we must study the effect of

climate in terms of SOIL MOISTURE on processes influencing microbial consumption within horizons

• In an effort to study these processes, Woods Hole Research Center (WHRC) is conducting a H.F. study of soil moisture on soils of BOTH the A and O horizons, respectively

Tracing CH4

• Greenhouse gas• Product of

decomposition of matter in swamps.

• Primarily used for fuel

• Natural gas=1/5 of total E consumption & 1/3 in U.S.

Rainfall Exclusion Experiment: The DRYDOWN

• OBJ: Test effects of soil moisture content on Uptake

• CH4 samples collected from May to Aug. ‘01: 3 T & 3C

• T= Drought Plot (n=4); 4m X 4m, covered completely by transparent PVC rooves 1.5 m high

• C = Control Plot (n=4); ambient plots sans roof on similar, adjacent soil

• Each plot: 4 PVC collars

Collecting Chamber Samples

• Weekly manual gas samples from 3 drought (n=4) & 3 ambient plots (n=4); analyzed with Gas Chromatograph (GC)

• Ambient/soil temp. measured at each collar• Needles attached to syringes inserted thru rubber

septum to collect 2 10-mL gas samples after affixing each chamber lid) from each collar

• Net CH4 flux = non-linear change in concentration of each chamber.

On the HORIZON

• 5 soil samples from L, F, H & A horizons• All soil horizon replicates incubated for 24 hr,

CH4 levels analyzed on GC at varying intervals • L, F, H & A replicates dried at 105 C for 24 hr

to determine SWC

Definitions • Microbial CH4

oxidation by soil methanotrophs:

CH4 + 2O2 CO2 + 2 H2O

• Soil horizon: layer differentiated based on structure & color.

On the HORIZON (cont’d)

• Measured each sample’s [CH4] at intervals of .5-10 hr

• Utilizing linear regression analyses, we ascertained the relationship between weekly means of oxidation rates to soil moisture

OPERATION: GC

• Separates gas mixtures into component parts

• 2 certified standards (1050 & 2020 ppb) used for calibration at initiation & end of each incubation to ensure precision

• CH4 flux calculated using slope of linear fitting of calibration

[CH4] in Soil Profile

0

5

10

15

20

25

0 300 600 900 1200 1500 1800

CH4 concentration (ppb CH4)

Soil

dept

h (c

m)

1-D 1-C

Incubation: A-horizon y = 1715.8e-0.3937x

R2 = 0.9986

0

500

1000

1500

2000

012345678910

Time (hours)

CH4 concentration (ppb CH4)

Horizon CH4 Uptake Rates

0

5

10

15

20

L F H A

Horizon

CH

4 o

xida

tion

(ug

CH

4 g

-1 h

r-1)

Wet, Wet, Wet!

The surface of the ground in the Maine woods is everywhere spongy and saturated with MOISTURE.

• Henry David Thoreau • To better assess how future climatic changes will

alter the terrestrial methane sink, we need to examine processes influencing CH4 flux under field conditions. Changes in CH4 consumption by watered soils paralleled those in soil moisture.

Soil H2O ContentWeekly averages of CH4 oxidation:

A-horizon

0

5

10

15

20

0 0.2 0.4 0.6 0.8

water content (g H2O g-1 soil)

CH 4

oxid

ation (

ug C

H4 kg-1 s

oil h

r-1 )

SINK or SOURCE

• Methanotrophs: active in A, yet inactive in O horizon.

• Demonstrates differences in CH4 diffusivity rates of horizons.

• Soils=NET SINK, hi rates of CH4 consumption due to > gas diffusivity & well-drained, porous soils for > gas exchange

Living Earth

The earth is not a mere fragment of dead history, stratum upon stratum like the leaves of a book, to be studied by geologists and antiquaries chiefly, but living poetry like

the leaves of a tree, which precede flowers and fruit,—not a fossil earth, but a LIVING EARTH; compared with whose great central life all animal and vegetable life

is merely parasitic. -Henry David Thoreau

• H.F. soils exhibit very strong uptake, caused by a unique soil texture, propitious for > gas diffusivity-- > uptake.

• H2O content affects CH4 oxidation thru gas diffusivity• Soil moisture significantly affects CH4 consumption.

• In such temperate forests as the H.F., soil moisture = the determining factor in regulating oxidation rates.

Climate Change

A UN Conference on Climate Change, held in Kyoto, Japan, in 1997 resulted in an international agreement to fight global warming, which called for sharp reductions in emissions of industrial gases. Not all industrial countries immediately signed or ratified the treaty, however, and obstacles to its implementation remain. In 2001 the G. W. Bush administration announced it would abandon the Kyoto Protocol; because the United States produces about one quarter of the world’s greenhouse gases, this was regarded as a severe blow to the effort to slow global warming.

• The Columbia Encyclopedia, Sixth Edition. Copyright © 2001 Columbia University Press.

Acknowledgements

As I disembark from this summer experience, in addition to all my Harvard

Forest friends & fellow colleagues, I would like to thank my fellow researchers and colleagues at Woods Hole Research

Center, Kathleen E. Savage, Werner Borken, and Eric A. Davidson for all their

time, efforts, and much appreciated support!