PNNL-13501 - Pacific Northwest National Laboratory

Development and Validation of Mesocosms for Experimentally Investigating Carbon Dioxide
Sequestration in Wetlands
Study Control Number: PN00027/1434
Ron Thom, Michael Huesemann, Dana Woodruff
Carbon dioxide, the principal greenhouse gas emitted by fossil fuel combustion and deforestation, adds annually about
3.5 billion metric tonnes of carbon to the atmosphere. Soil carbon sequestration is needed to reduce accumulation of
carbon in the atmosphere to reduce risks of climatic change. Restoration of wetlands, fertilization, hydrological
manipulation, and other methods might allow wetlands to sequester an additional 0.1 to 0.7 billion metric tonness per
year.
Project Description
Restoration of wetlands is a potental method for
sequestering large amounts of carbon in the soil, thereby
reducing accumulation of atmospheric carbon. Recently,
the National Panel on Carbon Sequestration (Reichle et al.
1999) estimated that wetland carbon sequestration could
be substantially increased. Restoration may be the least
costly and least uncertain alternative for sequestering
carbon in wetlands. However, no comprehensive
program has studied how such restorative actions affect
carbon sequestration rates. This project examined the use
of mesocosms to measure carbon sequestration in natural
and restored wetlands. This study also demonstrated the
use of mesocosms for evaluating enhanced carbon
sequestration in wetlands, developed new methods for
assessing soil organic carbon, and established the
groundwork for verifying enhanced carbon sequestration
via remote sensing on regional and larger scales. We
conducted greenhouse experiments where hydrology and
salinity were manipulated in small mesocosms, collected
and analyzed soil samples from several restored and
natural tidal marshes as well as eelgrass meadows of
varying post-restoration age marshes, investigated the use
of spectral radiance in monitoring differences in the
amounts of carbon stored in wetland systems, and
constructed a prototype automated gas flux mesocosm.
Greenhouse mesocosms did not show increased total
organic carbon content after a 5-month study period, but
did show total organic carbon content similar to that of
field-sampled plots. Total organic carbon measured from
natural and restored marsh systems indicated that
restoration of tidal flooding resulted in a recovery of soil
total organic carbon to natural levels after about 10 years
following restoration. Several indices used to measure
crop stress and photosynthetic function were applied to
our spectral reflectance data, including the normalized
difference vegetation index, the red-edge vegetation stress
index, and the photosynthetic reflectance index. The
photosynthetic reflectance index appeared most promising
and may be an indicator of photosynthetic radiation use
and net CO2 uptake. Construction of the automated gas
flux mesocosm was completed.
Introduction
No comprehensive studies have determined whether
wetland restoration affects carbon sequestration rates.
This project examined the use of mesocosms to measure
carbon sequestration in natural and restored wetlands.
We wanted to know how well mesocosms compare to
field conditions and resolve differences in carbon
sequestration. To this end, we tested the following
hypotheses:
Ho1: Organic carbon storage is the same in mesocosms
as compared with natural wetland systems.
Ho2: Restoration of hydrologically altered wetlands does
not enhance carbon sequestration.
Ho3: Spectral radiance does not differ between systems
differing in carbon sequestration rate.
We expected that the mesocosms would provide a good
match to natural systems and that restoration would result
in enhanced carbon sequestration.
Approach
The technical approach focused on five tasks that
addressed our three hypotheses.
Earth System Science 203