Partitioning biotic and abiotic components of soil CO2 fluxes using subsurface CO2 dynamics and

184 Pages·2014·8.37 MB·English

by  Fiona Shanhun

Lincoln University Digital Thesis 

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thesis.
Partitioning biotic and abiotic components of soil CO fluxes using
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subsurface CO dynamics and stable carbon isotopes,
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Taylor Valley, Antarctica

A thesis
submitted in partial fulfilment
of the requirements for the Degree of
Doctor of Philosophy
at
Lincoln University
by
Fiona L. Shanhun

Lincoln University
2013
ii
Abstract of a thesis submitted in partial fulfilment of the
requirements for the Degree of Doctor of Philosophy
Abstract
Partitioning biotic and abiotic components of soil CO fluxes using subsurface
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CO dynamics and stable carbon isotopes, Taylor Valley, Antarctica
2

by
Fiona L. Shanhun

To date, surface CO fluxes in soils of the McMurdo Dry Valleys in Antarctica have been assumed to
2
represent heterotrophic respiration. Important aspects of soil ecosystem function have been inferred
on the basis of this assumption.
This study used high-resolution measurements of CO concentration, and, for the first time, the
2
stable C isotopic composition of surface CO fluxes and subsurface CO profiles to: 1) link surface and
2 2
subsurface CO dynamics, 2) test whether abiotic mechanisms of CO production and consumption
2 2
can explain diel variability in surface CO flux rates, and 3) partition biotic and abiotic components of
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surface CO fluxes at two sites with contrasting organic C levels in soils of Taylor Valley.
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In the 2008/09 austral summer, surface CO flux rates and subsurface CO concentration and δ13C
2 2 CO2
profiles were measured at the warmest and coolest times of the day at two sites with contrasting soil
organic C contents. Site A, located near a small lake, can be considered a “biological hotspot”, with
the lake providing a contemporary organic C source via algal growth and subsequent wind-dispersal
onto nearby soils. Site B had no contemporary lacustrine-derived organic C supply, and soil organic C
levels were typical of the low levels in Dry Valley soils. A physical model, describing the temperature-
driven dissolution and exsolution of CO according to Henry’s Law, and incorporating empirical data
2
from field measurements of soil moisture content, pH, and temperature changes, showed that an
abiotic mechanism of CO uptake and release could explain changes in subsoil CO concentrations.
2 2
However, the twice-daily sampling regime did not resolve diel changes or rates of change in surface
CO fluxes and subsurface CO concentration and δ13C profiles.
2 2 CO2
During the 2009/10 austral summer, a 48-h time-series of surface CO fluxes and subsurface CO
2 2
concentration and δ13C profiles were measured simultaneously at Sites A and B, at 4-hourly
CO2
intervals. The increased sampling resolution revealed dynamic changes in subsurface soil CO
2
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concentration and δ13C profiles as soil temperatures varied throughout the diel cycle. At Site B,
CO2
highly depleted surface CO fluxes were explained by dynamic fractionation effects, and could not be
2
attributed to biological respiration. Short-lived (4 h) periods of steady-state efflux and influx showed
CO production (exsolution) and consumption (dissolution) of relatively enriched (−5.2‰) and
2
depleted CO (−11.4‰), respectively. This is consistent with the kinetic fractionation expected as a
2
result of preferential exsolution of 13CO and dissolution of 12CO .
2 2
At Site A, static chambers were unreliable for determining surface CO flux rates. Nonetheless,
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variation in subsurface CO concentration and δ13C profiles was consistent with heterotrophic
2 CO2
respiration dominating soil CO production. Unexpectedly, the highest rates of CO production
2 2
occurred during cooler periods of the day, possibly as a result of cooling-induced condensation
providing biota with water sufficient to stimulate their activity.
An abiotic contribution to Dry Valley soil CO fluxes must be considered in future studies of Dry Valley
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ecosystem activity and C cycling. Measures of surface CO flux at single points in time do not
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necessarily capture a solely biological signal.

Keywords: McMurdo Dry Valleys, soil respiration, Antarctic soils, organic C, dynamic fractionation,
CO fluxes.
2

Glossary of terms

δ13C 13C isotopic composition
δ13C 13C isotopic composition of CO (this can be soil CO , or CO sampled from a
CO2 2 2 2
headspace chamber)
δ13C 13C isotopic composition of the net surface CO flux
ΔCO2 2
ΔT/Δt rate of change in soil temperature
C total dissolved CO
T 2
D effective diffusivity of CO in soil
e 2
Site A representative of a “biological hotspot”; has a contemporary source of organic C
Site B representative of “typical” Dry Valley soils; has no contemporary organic C source

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Acknowledgements
It has been a great privilege to work in the Dry Valleys of Victoria Land: the magic and
unpredictability of Antarctica always makes for challenging, memorable and rewarding experiences.
Many individuals have helped ensure the success of the field work, and others have provided
excellent advice during the laboratory and write-up phases of this work.
Firstly, I thank my supervisor Tim Clough for his critical evaluation and thought-provoking feedback.
It is unfortunate that Tim was not able to visit the field area, and I hope that one day that may be
possible as part of another project. Thanks also to my associate supervisor, Carol Smith, who was
always enthusiastic about the research, and contributed to the successful field season in 2008/09,
not only in terms of research, but also with her sense of humour and camp culinary skills. Peter
Almond is thanked for his invaluable feedback, and for guiding me through the mathematically-
demanding aspects of this study. Peter also contributed to all aspects of the field work in both
2008/09 and 2009/10. His commitment to carrying gear and taking gas samples at unearthly hours
was outstanding, and his efforts also extended to the creation of “polygon scones” and legendary
lamb roasts. My supervisors must collectively be thanked greatly for their encouragement and
suggestions, and for patiently understanding my need to return to Antarctica during the summer of
2011/12 to work on a different research project investigating factors controlling biodiversity in the
Prince Charles Mountains, Mac. Robertson Land.
Jim Bockheim’s knowledge of Taylor Valley and soils of the Transantarctic Mountains, as well as his
enthusiasm and assistance, was greatly appreciated in the 2008/09 field season. André Eger and
Vicky Nall provided outstanding field assistance in the 2009/10 summer – assisting with gas sampling
at all hours, and always ensuring a healthy mix of melodies were transmitted during the “Drive Time”
radio shows. All of the field work and associated logistics was made possible by the excellent
logistical support provided by Antarctica New Zealand. Shulamit Gordon, Paul Woodgate, Simon
Trotter, Rod Strachan, and the Scott Base summer staff of 2008/09 and 2009/10 are particularly
thanked for their role in ensuring all logistical aspects of the project ran smoothly. Thanks also to Rob
McPhail and his mechanics for our safe and enjoyable flights to and from the field sites.
Staff of the Department of Soil and Physical Sciences also made major contributions to the project,
for which I am very grateful. Amanda Clifford and Leanne Hassall ensured the timely arrival of all
necessary equipment, and Neil Smith spent long hours helping with programming and testing the
data loggers. Leanne is especially thanked for her wise counsel and valued friendship. Roger
Cresswell’s help with gas sampling preparations and analyses was invaluable. Roger worked tirelessly
to ensure the mass spec was running well, with assistance from Manjula Premaratne. Lynne Clucas
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and Joy Jiao patiently analysed and re-analysed many samples. Thanks also to Rob Sherlock and the
late Graeme Buchan for the opportunities afforded to me during my time as a PhD student, as well as
for their support and encouragement, and many helpful discussions about my work. Janet Bertram,
Amal Torky and Jim Moir are also thanked for their support and encouragement. Thank you also to
my friends and fellow students in the Department for their good company – particularly André Eger,
Vicky Nall, Brendon Malcolm, Nimlesh Balaine, Pranoy Pal and Jen Owens.
Thanks also to Barry Thornton and his team at the Macaulay Land Use Research Institute in
Aberdeen, Scotland for making me very welcome in their laboratory, and for their careful analysis of
many gas samples.
Thank you to Megan Balks for her support throughout my PhD. Megan is always a cheery voice down
the end of the phone, and I am very grateful for her helpful suggestions and advice, as well as for
sparking my Antarctic research adventures by giving me the opportunity to visit Wright Valley as a
Masters student nine years ago. Jenny Webster-Brown also gave helpful suggestions and advice. Ed
Gregorich shared his knowledge of gas sampling methods he had used in the Dry Valleys, and John
Hunt gave advice on chamber design.
Operational support for the project was provided by the Lincoln University Research Fund. I am
extremely grateful for financial assistance from Helicopters New Zealand, the William Machin Trust,
Freemasons New Zealand, the Kate Sheppard Memorial Trust and Kathryn Claridge of Lincoln
Physiotherapy.
Thank you to my dear friends Michele Frey, Jodi Antunovich and Charlotte Fletcher for your
wonderful support and encouragement, and for giving me so many reasons to smile.
To my partner, Peter: thank you so much for your patience, support, understanding and love. Finally,
to my Mum and Dad, and my brother David: I cannot thank you enough for your love, advice and
encouragement. You taught me to dream, and have always supported me and encouraged me to
make and take opportunities.
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Table of Contents
Abstract ...................................................................................................................................... iii
Acknowledgements ...................................................................................................................... v
Table of Contents ....................................................................................................................... vii
List of Tables ............................................................................................................................... xi
List of Figures ............................................................................................................................ xiii
Chapter 1 Introduction ................................................................................................................. 1
1.1 Background .................................................................................................................................... 1
1.2 Aim and objectives ......................................................................................................................... 2
1.3 Field work ....................................................................................................................................... 3
1.4 Thesis structure .............................................................................................................................. 3
1.5 Publications and conference presentations resulting from this thesis ......................................... 4
1.5.1 Peer-reviewed journal articles to date ............................................................................. 4
1.5.2 Selected conference presentations .................................................................................. 4
Chapter 2 Literature review .......................................................................................................... 5
2.1 Introduction ................................................................................................................................... 5
2.2 Soil CO dynamics and effects on carbon isotopic composition .................................................... 6
2
2.2.1 CO sampling using headspace chambers ......................................................................... 6
2
2.2.2 Isotopic fractionation ........................................................................................................ 7
2.2.3 Biological respiration ...................................................................................................... 12
2.2.4 Dissolution of CO in water ............................................................................................. 13
2
2.2.5 Carbon isotopic fractionation in the carbonate system ................................................. 15
2.3 McMurdo Dry Valleys .................................................................................................................. 16
2.3.1 Climate ............................................................................................................................ 17
2.3.2 Hydrology ........................................................................................................................ 18
2.3.3 Geomorphology .............................................................................................................. 19
2.3.4 Soils ................................................................................................................................. 20
2.3.5 CO fluxes ........................................................................................................................ 27
2
2.4 Research gaps .............................................................................................................................. 29
Chapter 3 Abiotic processes dominate CO fluxes in Antarctic soils .............................................. 31
2
3.1 Abstract ........................................................................................................................................ 31
3.2 Introduction ................................................................................................................................. 32
3.3 Materials and Methods ................................................................................................................ 33
3.3.1 Study area ....................................................................................................................... 33
3.3.2 Soil temperature and moisture content measurements ................................................ 37
3.3.3 Soil CO sampling ............................................................................................................ 37
2
3.3.4 Soil sampling, physical and chemical analyses ................................................................ 39
3.3.5 Statistical analyses .......................................................................................................... 40
3.4 Results .......................................................................................................................................... 40
3.4.1 Environmental variables ................................................................................................. 40
3.4.2 Soil physical and chemical properties ............................................................................. 43
3.4.3 CO dynamics .................................................................................................................. 45
2
3.5 Discussion..................................................................................................................................... 49
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3.5.1 Soil CO uptake and release ............................................................................................ 49
2
3.6 Conclusions .................................................................................................................................. 57
3.7 Supplementary information ......................................................................................................... 58
3.7.1 Model of abiotic CO uptake and release ....................................................................... 58
2
3.7.2 Calculating the isotopic composition of abiotically-produced CO (δ13C ) ......... 59
2 CO2-abiotic
3.7.3 Results of principal components analysis ....................................................................... 60
Chapter 4 Quantifying diel variations in biotic and abiotic soil CO fluxes, Taylor Valley,
2
Antarctica (I): Background rates in the absence of a contemporary organic carbon source ............ 61
4.1 Introduction ................................................................................................................................. 61
4.2 Materials and methods ................................................................................................................ 62
4.2.1 Study area ....................................................................................................................... 62
4.2.2 Environmental variables ................................................................................................. 62
4.2.3 Soil CO sampling and analysis ........................................................................................ 62
2
4.2.4 Soil sampling, physical and chemical analyses ................................................................ 63
4.2.5 Statistical analyses and calculations ............................................................................... 64
4.3 Results .......................................................................................................................................... 69
4.3.1 Environmental variables ................................................................................................. 69
4.3.2 CO dynamics .................................................................................................................. 71
2
4.3.3 Linking surface and subsurface CO dynamics ................................................................ 76
2
4.4 Discussion..................................................................................................................................... 82
4.4.1 Diel variability in surface and subsurface storage CO flux rates ................................... 82
2
4.4.2 Soil gas transport ............................................................................................................ 86
4.4.3 Sources of CO ................................................................................................................. 89
2
4.4.4 Data reliability ................................................................................................................. 94
4.4.5 Implications ..................................................................................................................... 94
4.5 Summary and conclusions ........................................................................................................... 96
Chapter 5 Quantifying diel variations in biotic and abiotic soil CO fluxes, Taylor Valley,
2
Antarctica (II): The influence of a contemporary organic carbon source ........................................ 99
5.1 Introduction ................................................................................................................................. 99
5.2 Materials and methods .............................................................................................................. 100
5.2.1 Study area ..................................................................................................................... 100
5.2.2 Environmental variables ............................................................................................... 100
5.2.3 Soil CO sampling and analysis ...................................................................................... 101
2
5.2.4 Soil sampling, physical and chemical analyses .............................................................. 101
5.2.5 Statistical analyses and calculations ............................................................................. 102
5.3 Results ........................................................................................................................................ 105
5.3.1 Environmental variables ............................................................................................... 105
5.3.2 CO dynamics ................................................................................................................ 107
2
5.3.3 Linking surface and subsurface CO dynamics .............................................................. 113
2
5.4 Discussion................................................................................................................................... 117
5.4.1 Data reliability ............................................................................................................... 121
5.4.2 Implications ................................................................................................................... 122
5.5 Summary and conclusions ......................................................................................................... 124
5.6 Supplementary information ....................................................................................................... 125
5.6.1 Detailed description of surface and subsurface CO dynamics on Day 2 ..................... 125
2
Chapter 6 Synthesis and Summary .............................................................................................129
6.1 Introduction ............................................................................................................................... 129
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6.2 Key findings ................................................................................................................................ 130
6.3 Implications of research findings ............................................................................................... 133
6.3.1 Comparisons of Dry Valley ecosystem activity ............................................................. 134
6.3.2 Carbon turnover times .................................................................................................. 134
6.3.3 Field and laboratory manipulations .............................................................................. 135
6.4 Future research .......................................................................................................................... 136
6.4.1 Laboratory studies ........................................................................................................ 136
6.4.2 Numerical modelling studies......................................................................................... 136
6.4.3 Field soil CO flux experimental design ......................................................................... 137
2
References ............................................................................................................................... 139
Appendix A Publications resulting from this thesis......................................................................149
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