Oxygen availability controlling the dynamics of buried organic
carbon pools and greenhouse gas emissions
Introduction
Ecosystems kept permanently or temporarily under water accumulate more organic carbon (C) than
most ecosystems, despite the fact that some of these wet ecosystems are relatively low in productivity.
The large amount of C stored is attributed to relatively large inputs of organic C in combination with a
limited decay due to the reduced availability of oxygen (O2) controlling decomposition processes.
Carbon dioxide (CO2) is the main end-product of organic matter decay under aerobic conditions,
whereas nitrous oxide (N2O) is produced under partially anaerobic conditions and methane (CH4) under
strictly anaerobic conditions. All three gases are greenhouse gases of which CH4 and N2O have, on a
molar basis, roughly 21 and 290 times greater global warming potential than CO2. This implies that the
stability and fate of buried organic C pools, the net warming potential, and thus the global C balance,
(Jobbágy & Jackson, 2000) depend on O2 availability. Effects of changes in water levels on the rate of
decomposition and the C sink/source behaviour of wetlands have been investigated for decades.
However, field studies have given highly contrasting results; hence it is unclear which environmental and
biological mechanisms control this variation (Laiho, 2006). We consider the contrasting results to be due
to the strong links between subsurface gas dynamics, substrate quality and biological activity, which have
not been sufficiently investigated at the high-resolution temporal and spatial scales required for
understanding subsurface processes in wetlands.
Research objectives
This project aims to investigate how in-situ subsurface oxygen dynamics, linked to changes in water level,
control the dynamics of soil organic C and the emission of greenhouse gases. This research will be
conducted at detailed temporal and spatial scales using a combination of 1) high-resolution
measurements of subsurface and diffusion boundary layer gas concentrations (using a field-operated gas
chromatograph/mass spectrometer, two-dimensional planar optodes and fiber-optic O2 micro-sensors)
to quantify subsurface gas dynamics, 2) chemical characterization of the C substrate (using solid-state
13C Nuclear Magnetic Resonance, CPMAS 13C NMR) to investigate changes in the chemical structure of
soil organic matter occurring during decomposition in aerobic and anaerobic environments, 3) 13Clabelled plant residue to quantify and trace the C sources and pathways and assess the extent and
location of decomposition of organic matter in the soil profile, 4) identification of microbially-produced
marker compounds to determine changes in the soil microbial activity, and 5) net greenhouse gas (CO2,
N2O and CH4) flux measurements to quantify the magnitude and extent of aerobic and anaerobic
microbial processes occurring before, during and after controlled water table fluctuations in agricultural
land.
Using this approach we will be able to describe and quantify the mechanistic links between changes in
water table, subsurface oxygen availability, depth- and source-specific decomposition of buried organic
matter and the associated production of greenhouse gases. The research will be conducted at three
contrasting sites, which undergo different temporal changes in water table levels, to assess:
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short- and long-term effects following large-scale, controlled flooding (vådgøring) of a major former
wetland which is currently partly-drained and has been intensively cultivated (Åmosen)
short-term drainage in a smaller natural wetland (Maglemosen)
short-term effects in intertidal waters (Rømø)
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Main research questions
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How does the spatial and temporal oxygen availability and decomposition of soil organic matter
(including archaeological material) relate to changes in water level?
How can the spatial (i.e., lateral and vertical) distribution of buried total and reactive organic C pools
best be characterized and simulated based on temporal and spatial trends in C input/production,
chemical structure of plant residues, oxygen availability, biological activity and land use changes?
How are spatial and temporal rates of greenhouse gas emissions and rates of long-term C storage
controlled by subsurface oxygen dynamics and plant residue quality in terms of age, burial depth,
residue quality and degree of decomposition?
Study Sites
The Åmosen site, is one of two major study sites representing a temperate wetland. The Åmosen site is
part of a large wetland area (>30 km2) situated south of Holbæk (Zealand). The basin was formed mainly
by melt water 15,000 years ago as part of the glacial retreat. The subsequent ecological, sedimentary, and
geochemical changes of the area have been well-described by Noe-Nygaard (1995). After the 1930s the
area was subjected to major land use changes, including draining and peat-digging. Part of the wetland
was re-flooded in 1996 to protect extraordinarily well-preserved organic archeological artefacts dating
from the Stone Age. Since 1996 in situ preservation of archaeological sites has become a preferred option
as opposed to excavation, however, only if the archaeological material is stored safely despite water table
fluctuations (Matthiesen et al., 2004a; b). Archaeologists believe that considerable treasures still lie buried
in the soil. Therefore, current farmland will be flooded in 2007/2008 and kept under water for decades
to come. The final size of the area to be flooded will be determined within few months (see
http://www.skovognatur.dk/Emne/Naturbeskyttelse/Naturpleje/Naturprojekter/Aamosen/Nyt/rapport_skitseprojekt.htm). For the purpose of this project experimental and control areas have been
selected (Fischer, 2006) and consist of a well-defined gradient towards a permanent water-covered area.
The Åmosen site is therefore considered a unique large-scale field laboratory for studies on land-use
changes which are highly relevant for predicting the future effects of global water level changes on the
greenhouse gas accounting as well as in-situ preservation of organic archaeological materials. From that
point of view, the controlled rewetting of intensively cultivated fields in Åmosen is a rare opportunity to
study land management practices that have not been extensively documented in the literature.
The Maglemosen site is the second main study site and is situated in an ancient inlet near Vedbæk, 20
kilometres north of Copenhagen. Earlier geological and archaeological investigations have shown that
the area represents a shore displacement which occurred sometime between 5,500-2,500 B.C.
(Christensen, 1981). The area is well known for its well-preserved organic archaeological artefacts from
the Stone Age. Study sites for this project have been selected within a wetland area which has never been
managed and therefore represents natural conditions. In this project, a drainage experiment will apply a
technique similar to that used in archaeological excavations. This will enable measurements at a series of
sites along a strong gradient in water level and oxygen availability, which is further relevant for evaluating
the threat to nearby buried artefacts that are not targeted for excavation.
The Rømø site is a minor site situated in the micro-tidal Rømø Bight which is part of the Lister Dyb
tidal basin. Sediments are sandy and muddy in sheltered parts of the basin. The site is well-described
(e.g., Andersen et al., 2005) and includes sediment budgets, temporal changes of bed level over the last
10 years and cores dated using optical stimulated luminescence and 210Pb. This site is included to
investigate the links between daily water table fluctuations and oxygen dynamics only.
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Background
Wetlands have long been recognized as unbalanced ecosystems where the rate of photosynthetic
production of organic matter exceeds that of its decomposition (Gorham, 1991). Thus, most wetlands
act as a net sink for atmospheric C, but also release CO2, CH4 and N2O to the atmosphere (Alm et al.,
1999). This is the result of near-surface aerobic and anaerobic decomposition of organic matter (in
addition to root respiration). Consequently, major concerns have been expressed that these stores may
become destabilized; releasing their C in either gaseous or aqueous form and in particular if this
destabilization could cause a concurrent release in CH4 or N2O. Destabilization of wetland ecosystems
are likely following short and long term changes in the water balance resulting from land use or climate
changes (Laiho, 2006).
Changes in the water table and associated changes in the presence of a continuous gas phase
are considered the most important determinants of the spatial distribution of O2 availability and thus the
potential subsurface gas dynamics (Laiho, 2006). Release rates of CO2 and CH4, quantified in the field,
have been shown to be correlated with changes in temperature, water table and primary production
(Christensen et al., 2000). Therefore, lowered water levels due to either climate or land use changes will
logically increase the O2 availability in the surface layers and result in accelerated rates of organic matter
decomposition and reduced methane emissions. However, despite a large number of investigations,
Laiho (2006) observed that research has so far given highly contrasting results concerning the net effect
of changes in water levels on the rate of decomposition and the C sink/source behaviour of wetlands,
and from this she concluded that both the environmental and biological mechanisms controlling this
variation remain unresolved. This includes, for example, the amount of CH4 produced at depth but
oxidized to CO2 before being released from the soil surface, changes in microbial communities, the
direct link between O2 and N2O dynamics and the interactions of different environmental factors as
temperature, pH and nutrient status and the role of roots in gas transport. Buried SOC in wetlands may
comprise some relatively labile compounds, and its stabilization and persistence is possibly the result of
the presence of functional groups having highly variable chemical characteristics (Sjögersten et al., 2003)
and phenol oxidase activity inhibited at low O2 concentrations (Fenner et al., 2005). If phenolic
compounds accumulate, the action of other hydrolase enzymes may be restricted (Freeman et al., 2001),
thereby promoting C storage. Thus, characterizing depth-specific gas production cannot be made
without chemical characterization of the organic substrate and the microbial activity, which is often
reflected in the presence of certain volatile metabolites (DeJong et al., 1994, Lauritsen et al., 1993).
We conclude that the contrasting research results on effects of water table changes are partly
due to the fact that changes in SOC and greenhouse gas emissions have not been related to the links
between the temporal and spatial distribution of O2, changes in the microbial community as well as the
depth-dependent characteristics of organic C sources in terms of age, burial depth and degree of
decomposition and humification. In addition, the lack of detailed data on N2O production and emission
with regards to changes in water table is particularly noteworthy and crucial for accurate greenhouse gas
budgets. Specifically, we hypothesize that the transition from intensively managed agricultural fields to
wetland in Åmosen will change the area from being a net emitter of N2O and absorber of CO2 to
become an emitter of CH4, neutral with regards to N2O and remain an absorber of CO2 over the long
term. A relatively large N2O burst is expected at least on a short time basis. However, the net result in
greenhouse gas emissions in the Kyoto context is unknown and very few studies in the world have
focused on similar effects of large scale management practice, particularly, in relation to permanent
flooding of intensively managed agricultural fields. Most investigations have so far been made either as
laboratory investigations (e.g. Aerts & Ludwig, 1997 and Beckmann et al., 2004) or short term field
campaigns (e.g. Christensen et al., 2000) focusing on the net release of gases from the surface rather than
depth-dependent release rates. We conclude that little or no data is available for accurate predictions of
either short- or long-term SOC dynamics, net changes in C budgets or greenhouse gas emissions for the
type of managed wetlands included in this study, although some models currently developed for other
applications (Zhang et al., 2002; Li et al., 2004) may be available for these purposes.
Appropriate techniques to quantify subsurface gas distribution and consumption are now
available and widely used in marine investigations at high-resolution temporal and spatial scales.
Techniques to evaluate O2 dynamics includes, high-resolution 2D planar optodes and fiber-optic O2
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microsensors, which have been developed and tested in marine ecosystems over the last 10 years (Kühl,
2005; Glud et al., 2000, 2001 ) and used for studies of oxic microzones (Jensen et al., 2005; Frederiksen
& Glud, 2006) and micro-scale gas gradients (e.g. Wieland & Kühl, 2006; Glud et al., 2002) in different
aquatic systems, and both in the laboratory and in situ. However, such measurements have so far not
been used in ecosystems directly influenced by atmospheric O2. In this project the high-resolution spatial
and temporal description of O2 distribution that is possible with these techniques will be used in
wetlands for the first time. We will attempt to perform all measurements simultaneously in the field with
the highest temporal and spatial resolution ever attempted using new types of transportable field
instruments in order to link O2 availability directly to CO2, CH4 and N2O production and emission. The
spatial variation includes the vertical variation through soil/sediment, the lateral variability across
landscapes, and the 2D-micro scale variation from bulk soil matrix to the centre of aggregates/pellets.
Thus, the proposed work will serve as a case study and as input for modelling the effects of water table
changes on greenhouse gas emissions. In addition, it is now possible to bring highly advanced
instruments into the field for on-site identification and mapping microbial produced metabolites
(Ouyang, et al., 2004).
O2 and C dynamics in shallow marine and intertidal ecosystems are controlled by the same set
of factors as those in wetlands but are more complex due to the interaction with the Fe and S element
cycling. In this project we will make measurements at one of the most intensively monitored Danish
intertidal sites (near Rømø) where there are large amounts of data on long term changes in sediment
budgets controlled by biological mediation (Andersen et al., 2005; 2006). To date these investigations
have not been linked to oxygen availability and buried C pools and this project will provide detailed
information on these factors.
Methodology
Field measurements: Site-specific water and energy balance will be based on two climate stations and
long time series from DMI which are important to quantify climate-controled short and long term
changes in water table. Measurements of the quantity and quality (i.e., chemical nature) of subsurface
organic matter within the study sites will be based on depth- and volume-specific samples and related to
biological production, vegetation types and other biological activity. Suction probes will be installed at
specific depths to collect water and will be analysed for in-situ pH (Elberling & Matthiesen, 2006),
nutrients, dissolved organic carbon (DOC) and nitrogen (DON) and measurement of phenol oxidase
activity according to Pind et al. (1994). Sub-samples will be brought to the laboratory for organic C and
N determinations, NMR analyses and for other experiments (see below).
Depth-specific variations in diffusivity, O2, CO2, CH4, N2, and N2O will be made in order to
monitor the change in the soil gaseous environment before, during and after drainage. The results will be
used to quantify high-resolution spatial-temporal trends in penetration depth, consumption rates and
depth-specific production rates of O2, consistent with the approach of Elberling & Damgaard (2001).
This will be done using a unique field transportable and battery operated gas chromatography/mass
spectrometry instrument (granted by FNU in 2006, grant number 272-06-0047), which is available for
the project and will be used for on-site identification and spatial distribution investigation. The mass
spectrometry and chromatography can be operated individually and the possibility of performing on-site
detection of N2O using membrane inlet tandem mass spectrometry (Lauritsen et al., 1991, Kotiaho &
Lauritsen, 2002) will be explored. In addition the possibility of performing on-site depth profiling of gas
concentrations (CH4, N2, O2 CO2 and possibly N2O) will be explored. This has previously been done
with success using membrane inlet mass spectrometry in laboratory experiments (Beckmann et al. 2005).
A key aspect of the potential effects of changes in water table level on subsurface C dynamics is the
relative degradation of different functional groups by soil organisms. 13C Nuclear Magnetic Resonance
has proved to be a powerful tool in studying the composition and decomposition of soil organic matter
(Sjögersten et al., 2003) and will be used in this study to identify functional C compounds and their
relationships to decomposition and their spatial variation within the soil profile and across the landscape.
In-situ 13CO2 labelling of plants will allow us to differentiate CO2 production from roots and bulk soil
micro-organisms. Continuous and long-term measurements of depth-specific O2 concentrations (4
depths at two sites) will be made using fiber-optic O2 micro-sensors (Klimant et al., 1995) connected to a
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multichannel fiber-optic oxygen meter (PreSens). On the tidal flat, such measurements will be
supplemented by in-situ measurements using microsensor profiling landers (Reimers and Glud, 2000) and
in-situ oxygen imaging equipment (Glud et al., 2001). Measurements of net and dark system H2O, CO2
and CH4 fluxes will be made using chambers according to set-up developed by In Situ Flux
(www.insituflux.org) which automatically close during measurements 20-30 times per day. During
measurements, chamber concentrations of H2O and CO2 are measured using an infrared gas analysers,
Li-820 (LiCor) and concentrations of CH4 based on high-resolution direct-absorption spectroscopy (Los
Gatos Research). A climate station will be set up for evaluating the water and energy balance and
includes pressure transducers to monitor changes in the water table, water content in profiles and
temperatures. High-resolution temporal trends at the two wetland sites will be made for 1½-2 years (two
winters) and for more than 3 months at the tidal site. After the project is completed the set-up in
Åmosen will be part of a long term monitoring plan.
Laboratory experiments include incubation of depth-specific SOC fractions in order to quantify
differences in decomposition and humification rates under controlled redox, nutrient and temperature
conditions. These rates will be linked to solid-state 13C nuclear magnetic resonance (CPMAS 13C NMR)
to identify functional groups of soil organic C which will provide an independent measure of spatial
distribution of labile and resistant C compounds (Sjögersten et al., 2003). Both natural- and substrateinduced decomposition will be measured to identify the factors controlling turnover rates. Entire peat
columns will be brought to the laboratory and the water table manipulated. In these peat columns the
combined measurements of subsurface gas concentration and 2-D O2 distribution will be made with and
without the influence of living plants and focus on the link between O2 availability and changes in the
microbial community. Interactions of fauna on the oxygen distribution and dynamics will also be
investigated with laboratory setups for 2-dimensional oxygen measurements (Wenzhöfer and Glud
(2004).
Modelling will be made based on results of field work and laboratory experiments and used for
simulating short and long term SOC dynamics, net changes in C budgets and greenhouse gas emissions
for managed wetlands. Wetland-DNDC (Li et al., 2004) has been used previously to predict the impact
of management practices on C sequestration and greenhouse gas emissions from forested wetland
ecosystems. A modified version is now available which included parameters for management practices
(e.g., setting water table level, fertilization), inclusion of detailed anaerobic biogeochemical processes for
wetland soils, and utilization of hydrological models for quantifying water table variations.
Time plan:
2006
Field monitoring
Maglemosen
Rømø
Åmosen
Laboratory experiments
Writing
2007
2008
2009
----------------------------------------------------------------------------------------------------- ⇒ (permanent set-up for 10 years)
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Facilities and network
National co-operation: The project will be linked to other activities at the selected sites including a
investigations of stable iron phases on a molecular scale (S. Stipp), the accumulation and stability of
metals (B. H. Jakobsen) and the mobility of phosphor (H. Breuning-Madsen) following changes in O2
availability. Co-operation with Nanna Noe-Nygaard in relation to burial history of wetland
sediments/ecological archives.
International co-operation: CarboNorth - [2004-2008] and NECC- Nordic Centre for Studies of
Ecosystem Carbon Exchange and its Interactions with the Climate System - [2001-2007]. Collaboration
with Prof. D. Lloyd, University of Wales, Cardiff, and Dr. E.G. Gregorich, Agriculture Canada, Central
Experimental Farm, Ottawa) and Prof. Myrna Simpson, University of Toronto (CPMAS 13C NMR).
Permissions to work in Maglemosen and Åmosen have been granted.
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