Global Change Biology (2011) 17, 1321–1334, doi: 10.1111/j.1365-2486.2010.02190.x
Methane emissions from tropical freshwater wetlands
located in different climatic zones of Costa Rica
A M A N D A M . N A H L I K 1 and W I L L I A M J . M I T S C H
Wilma H. Schiermeier Olentangy River Wetland Research Park, Environmental Science Graduate Program and School of
Environment and Natural Resources, The Ohio State University, 352 W. Dodridge Street, Columbus, OH 43202, USA
Abstract
Wetlands are the largest natural source of the greenhouse gas methane to the atmosphere. Despite the fact that a large
percentage of wetlands occur in tropical latitudes, methane emissions from natural tropical wetlands have not been
extensively studied. The objective this research was to compare methane emissions from three natural tropical
wetlands located in different climatic and ecological areas of Costa Rica. Each wetland was within a distinct
ecosystem: (1) a humid flow-through wetland slough with high mean annual temperatures (25.9 1C) and precipitation
(3700 mm yrÿ1); (2) a stagnant rainforest wetland with high mean annual temperatures (24.9 1C) and precipitation
(4400 mm yrÿ1); or (3) a seasonally wet riverine wetland with very high mean annual temperatures (28.2 1C) and lower
mean annual precipitation (1800 mm yrÿ1). Methane emission rates were measured from sequential gas samples using
nonsteady state plastic chambers during six sampling periods over a 29-month period from 2006 to 2009. Methane
emissions were higher than most rates previously reported for tropical wetlands with means (medians) of 91 (52),
601 (79), and 719 (257) mg CH4-C mÿ2 dayÿ1 for the three sites, with highest rates seen at the seasonally flooded
wetland site. Methane emissions were statistically higher at the seasonally wet site than at the humid sites (Po0.001).
Highest methane emissions occurred when surface water levels were between 30 and 50 cm. The interaction of soil
temperature, water depth, and seasonal flooding most likely affected methanogenesis in these tropical sites. We
estimate that Costa Rican wetlands produce about 0.80 Tg yrÿ1 of methane, or approximately 0.6% of global tropical
wetland emissions. Elevated methane emissions at the seasonally wet/warmer wetland site suggest that some current
humid tropical freshwater wetlands of Central America could emit more methane if temperatures increase and
precipitation becomes more seasonal with climate change.
Keywords: Central America, climate change, methane emissions, tropical seasons, tropical wetlands
Received 4 June 2009 and accepted 10 January 2010
Introduction
Methane is the third most abundant greenhouse gas
(GHG) in our atmosphere, following water vapor and
carbon dioxide. It is estimated that wetlands emit about
25% of the current global methane emissions as a result
of prolonged flooded conditions and consequent anaerobic conditions characteristic these ecosystems (Mitsch
& Gosselink, 2007), with anthropogenic sources accounting for much of the remaining emissions (Whalen,
2005). Methane is approximately 25 times more effective
as a GHG than carbon dioxide (IPCC, 2007); therefore,
Correspondence: W. J. Mitsch, tel. 1 1 614 292 9774, fax 1 1 614 292
9773, e-mail: [email protected]
1
Present address: A. M. Nahlik, U.S. Environmental Protection
Agency, National Health and Environmental Effects Research
Laboratory, Western Ecology Division, 200 SW 35th Street, Corvallis, OR 97333, USA.
[Correction added after online publication 30 November 2010: this
article has been revised]
r 2010 Blackwell Publishing Ltd
relatively small changes in methane concentrations
could have important impacts on climate. Although
much of the estimated 150% increase in atmospheric
methane concentration since 1750 can be attributed to
landfills, natural gas systems, and ruminant (livestock)
farming (Wuebbles & Hayhoe, 2002), wetlands represent the most important natural source of methane to
the atmosphere. It had been estimated that wetlands
contribute between 109 and 145 Tg CH4 yrÿ1 (20–26%) of
the total 550 Tg CH4 yrÿ1 of methane emissions, with
boreal and tropical wetlands contributing the most
(24–62 and 42–66 Tg CH4 yrÿ1, respectively) (Matthews &
Fung, 1987; Aselmann & Crutzen, 1989; Bartlett &
Harriss, 1993; Cao et al., 1998; IPCC, 2007; Mitsch &
Gosselink, 2007). More recent estimates suggest that
wetlands may contribute as much as 180 Tg CH4 yrÿ1,
with 76% of that (138 Tg CH4 yrÿ1) coming from tropical
wetlands (Bergamaschi et al., 2007; Mitsch et al., 2009).
It is important to study current wetland methane
emissions because temperature increases due to climate
change could result in elevated wetland methane fluxes,
1321
1322 A . M . N A H L I K & W . J . M I T S C H
particularly in northern boreal wetlands and tropical
wetlands (Cao et al., 1998; Shindell et al., 2004). Despite
the fact that tropical wetlands comprise anywhere from
28% to 56% of the world’s wetlands (Mitsch & Gosselink,
2007), methane emissions from natural tropical wetlands
are not as well understood as are those from temperate
and boreal wetlands. There have been many studies on
methane emissions from northern boreal regions (45–
701N; Moore & Knowles, 1990; Kang & Freeman, 2002;
Rask et al., 2002; Huttunen et al., 2003; Song et al., 2009)
and temperate regions (30–451N; Kim et al., 1998; Altor &
Mitsch, 2006, 2008; Yu et al., 2008); most of the studies on
methane emissions from tropical regions have been conducted in rice paddies (Banker et al., 1995; Husin et al.,
1995; Adhya et al., 2000). Furthermore, there are few, if
any, studies investigating the effect of different tropical
climes (tropical seasonality) on natural wetland methane
emissions (Mitsch et al., 2009).
The goal of this study is to compare wetland methane
emissions from three natural tropical wetlands located
in distinctly different climatic and ecological regions of
Costa Rica. We explore the implications of changing
climate on methane emissions from tropical wetlands
by comparing methane emissions from wetlands currently in different climates. Estimates of wetland
methane emissions from countries such as Costa Rica
are needed to better understand carbon dynamics in
future climates in those countries.
Materials and methods
Study sites
Field measurements were conducted in three independent,
freshwater wetland and associated upland sites in Costa Rica
(Fig. 1) during six separate site visits to each wetland over a 29month study from September 2006 through February 2009. The
sites were located at EARTH University, La Selva Biological
Station, and Palo Verde Biological Station. EARTH University
and La Selva wetlands are located on the eastern Caribbean
Plain in a tropical rain forest biome while Palo Verde is located
on the western Pacific Slopes in a tropical dry forest biome. All
three sites have distinctly different precipitation regimes and
experience different climes (Table 1).
EARTH wetland (116 ha) is a slow-moving slough within a
humid tropical forest undergoing natural restoration after
years of grazing. The climate is humid with a 10-year precipitation average of 3463 mm yrÿ1 and a mean of
3718 mm yrÿ1 during the study period. The wetland is dominated by water-tolerant species such as Spathiphyllum friedrichsthalii, Dracontium sp., [previously classified as
Cyrtosperma sp. (Zhu, 1996)], Raphia taedigera, and Calathea
crotalifera, whereas the surrounding forest is dominated by
hardwood tree and palm species such as Pentaclethra macroloba,
Terminalia oblonga, Chamaedorea tepejilote, Virola koschnyi, and
NICARAGUA
La Selva
Sarapiqui River
EARTH
Parismina River
10°
COSTA RICA
Palo Verde
Tempisque River
Scale in kilometers
0
100
200
Fig. 1 Study location sites in Costa Rica and their associated
watersheds (shaded in gray).
Virola sebifera (Mitsch et al., 2008). Several large rivers, most
notably the Parismina River, run through the EARTH University campus, and the area is susceptible to flooding. Soils
are described as poorly drained alluvial Aquepts on flat relief
(Vásquez Morera, 1983), and as a result of high vegetative
productivity, slow-decomposition, and a high water table, a
thick layer of floating mucky peat has developed in the
EARTH wetland (Bernal & Mitsch, 2008). Large precipitation
events and occasional flooding from nearby creeks influence
the hydrology at the EARTH wetland.
The La Selva wetland (3 ha) is situated within a tropical rain
forest at the confluence of the Puerto Viejo and the Sarapiqui
Rivers and receives a 10-year average of 4639 mm yrÿ1 of
precipitation (4391 mm yrÿ1 during the study period), the
highest mean annual rainfall of the three study sites. The rain
forest is dominated by many canopy, subcanopy, and understory tree species, such as Anaxagorea crassipetala, Pentaclethra
macroloba, and Rinorea deflexiflora (King, 1996). The wetland,
however, is relatively open compared with the forest and hosts
large stands of Spathiphyllum friedrichsthalii and the grass
species Gynerium sagittatum, in addition to smaller stands of
Asterogyne martiana near the edges. The wetland soils at La
Selva have been identified as Tropaquepts by Sollins et al.
(1994), and we have observed the typical mottling and high
organic matter content associated with these soils at our study
site. The combination of a water table near the surface and
high year-round precipitation drives the wetland hydrology at
the La Selva study site.
Palo Verde wetland (1200 ha, Trama et al., 2009) is a coastal
floodplain freshwater marsh that experiences distinct wet and
dry seasons due to both rainfall and occasional river flooding.
Palo Verde Biological Station receives the lowest rainfall of the
three sites (10-year average of 1248 mm yrÿ1 and a mean of
1825 mm yrÿ1 during the study period). During the wet season, floating aquatic and emergent plants such as Neptunia
r 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 1321–1334
METHANE EMISSIONS FROM TROPICAL WETLANDS
Table 1
1323
Description of characteristics of wetlands included in this study
Wetland site
Geographical co-ordinates
Climate
Landscape
Size (ha)
Mean 10-year
annual rainfall (mm yrÿ1)
EARTH
La Selva
Palo Verde
10113 0 000 N, 83134 0 1600 W
10125 0 4900 N, 8410 0 3700 W
10120 0 3700 N, 85120 0 3300 W
Tropical humid
Tropical wet
Tropical dry
Restored humid forest
Primary rainforest
Coastal plains
116
3
1200
3463 731
4639 618
1248 252
Annual rainfall is reported as mean annual precipitation from 1999 through 2008 standard error.
natans, Nymphaea sp., Eichhornia crassipes, Thalia geniculata, and
Typha domingensis dominate, whereas in the dry season, when
the standing water evaporates, grasses and sedges, such as
Eleocharis sp., Canna glauca, Cyperus sp., Paspalidium sp., Paspalum repens, Oxycaryum cubense, and Oryza latifolia dominate
(Crow, 2002). Palo Verde is located at the mouth of the
Tempisque River as it flows into the Gulf of Nicoya and finally
into the Pacific Ocean. The wetland soils at Palo Verde can be
classified as Vertisols on an alluvial plain; however, the floodplain has been isolated from the river except for exceptional
floods by a levee that developed along the river (McCoy &
Rodrı́guez, 1994; no floods were experienced during the years
data was collected for this study). The wetland hydrology is
largely influenced by wet season (May–November) precipitation and runoff from the surrounding watershed. The Palo
Verde marsh has been heavily managed especially since a shift
in hydrologic conditions in the 1970s that partially led to the
invasion of Typha domingensis. To enhance waterfowl abundance and diversity, cattle grazing and farm tractor crushing of
plants have been used to control Typha domingensis with
modest success (Trama et al., 2009).
Methane emission sampling
Non-steady-state gas-sampling chambers were used to sample
for methane in the wetlands. Twelve chambers were permanently installed in each wetland on two sampling transects.
Replicate pairs of chambers were installed in deep water and
shallow water sites within the wetland. Shallow water sites
were located closer to the boundary of the wetland and had a
mean depth of 27 cm, whereas deep water sites were located
closer to the center of the wetland and had a mean depth of
35 cm. In some cases, such as that of the EARTH wetland, the
water depth was relatively uniform within the wetland boundary and the shallow water sites and deep water sites were of
similar depth. Replicate pairs of chambers were also installed
in associated upland sites, located adjacent to the wetland
boundary where water does not accumulate. The two sampling transects were situated no o500 m apart to include as
much physical diversity of the wetland as possible. The
sampling sites, both wetland and upland, included dominant
plant communities (excluding trees in the upland, which
exceeded our chamber size) when appropriate and were
representative of the wetland or upland as a whole.
Polyvinyl chloride (PVC) chambers were used to measure
diffusive methane flux from the soil surface and plant-
mediated methane flux, as plants were included in the chambers; however, ebullition fluxes were not measured due to
their random occurrences during sampling and resulting
nonlinearity (described below in ‘Processing and Analysis’).
These permanent chambers have proved to be effective in
capturing methane emissions from wetlands, inexpensive, and
easy to construct (Mitsch et al., 2005; Altor & Mitsch, 2006,
2008). Chamber frames consisted of a 0.15 m2 rectangular
HDPE (high-density polyethylene) base, to which a PVC frame
measuring approximately 120 cm tall (to ensure that plants
remain intact) and covering the basal area of 0.15 m2 was
attached. HDPE bases were sunk into the soil to a minimum
depth of 10 cm to serve as an air–soil interface. A metal wire
was molded around the top frame of each permanent chamber
to serve as a thermometer hook.
For deepwater sites or when water was 415 cm in depth,
floating chambers, using a modified design from the permanent chambers, were used to collect gas samples. Floating
chambers were constructed using a 0.4 m3 frame of PVC over
which a 4-mil polyethylene bag, affixed with a 3 m Tygon vent
tube (1.6 mm i.d.) and a gray butyl sampling port, was
permanently fitted. Buoyant, self-sealing 1.3 cm i.d. pipe insulation constructed of closed-cell polyethylene was affixed to
all four sides of the base of the chamber, allowing the chamber
to float just under the surface of the water and creating a seal
from the ambient environment. A metal wire was molded
around the top frame inside each floating chamber to serve as
a thermometer hook.
Before sampling, nonmercury thermometers with 1 1C increments were placed inside of the chambers. At the time of
sampling, permanent chamber frames were enclosed by a
fitted 4-mil polyethylene bag, affixed with a 3 m Tygon vent
tube (1.6 mm i.d.) and a gray butyl sampling port. A 3 cm-wide
elastic strap tied tightly around the bag and the base ensured
that the chambers were effectively sealed from the ambient
environment. After sealing the permanent chamber from the
ambient environment or placing the floating chamber on the
water surface, gas samples were collected through the sampling port and stored in 10 mL (23 46 mm) glass autosampler
vials fitted with Wheaton gray butyl stoppers (flange straight
plug) and crimped with 20 mm aluminum seals. Vials were
evacuated before sampling and checked on-site to insure that
no vial contained air. Gas samples were taken with a B-D
30 mL syringe fitted with a stopcock immediately upon enclosing the sites and approximately every 5 min after the
chamber was enclosed. Over a half-hour period, a total of six
gas samples were collected from the chamber. Soil, water, and
r 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 1321–1334
1324 A . M . N A H L I K & W . J . M I T S C H
chamber air temperatures and above soil water depth were
recorded at each chamber. Soil temperatures at 10-cm depths in
upland sites were not collected to avoid thermometer breakage.
After gas samples were collected at one pair of chambers, the bags
were removed and used at the next set of permanent chambers.
Floating chambers were lifted and moved to the next site.
Meteorological data
Weather stations housing digital precipitation and air temperature data loggers were located on site at EARTH University, La Selva Biological Reserve, and Palo Verde Biological
Reserve.
each set using Microsoft Excel to determine linearity of emission. Regressions with an R2o0.9 were considered nonlinear
and discarded (Altor & Mitsch, 2006). Only linear (positive or
negative) emission rates were used in the final analyses. In the
case where removal of one to two points corrected the linearity
so that R2 0.9, those points were discarded from the calculation under the reasoning that disturbances to the bag during
attachment at time zero, natural variability in emission rate,
and ebullition (release of concentrated methane bubbles) can
disrupt linear rates (Holland et al., 1999; Altor & Mitsch, 2006).
Morning (collected between 06:00 and 10:00 hours) and afternoon (collected between 13:00 and 16:00 hours) methane
emission rates for each chamber were averaged to estimate
the daily methane emission rate.
Processing and analysis
Collected gas samples were stored at 4 1C until they were
transported to the labs at the Olentangy River Wetland Research Park (ORWRP) at The Ohio State University in Columbus, Ohio, USA. Samples were analyzed within 28 days for
methane concentrations by flame ionization detection on a
Shimadzu GC-14A gas chromatograph (Kyoto, Japan) equipped with a 40 position HT200H Autosampler. To insure that
samples were still viable after 28 days and cold storage, check
standards of known concentrations were made before field
sampling, stored in the same type of vials, carried into the
field, and stored with the collected field samples. Variability
between the field check standards and the laboratory check
standards (created when the GC was running the samples)
was not significant, indicating that there was no leakage or
change in gas concentration in the vials from the time of
collection to the time of analysis. A 1.8 m Porapak Q column
was used for sample separation with helium as the carrier gas.
Matheson methane standards, balanced with N2 gas, were used
to perform four-point calibration curves. Laboratory check standards and/or blanks were run with every tray of 40 samples.
Corrected chamber concentrations, by weight, were calculated from the gas chromatograph results and density corrected for chamber volume and temperature:
m ¼ c ðP M=R TÞ;
ð1Þ
ÿ3
where m is the methane concentration by weight (g CH4 m ),
c the methane concentration by volume (ppmv 5 10ÿ6 cm3
cmÿ3 5 cm3 mÿ3), P the atmospheric pressure (assume 1 atm),
M the molecular weight of gas (g molÿ1) R the Universal Gas
Constant [82.0575 (atm-cm3)/(mol-K)], and T the absolute
temperature (K) of the chamber at the time of each sample.
The corrected chamber concentrations were converted to
milligrams of carbon emission rates (mg CH4-C mÿ3), and
methane flux rates were calculated (Healy et al., 1996; Altor
& Mitsch, 2006) according to the following equation:
Fme ¼ ½v ðdm=dtÞ 1000=AŠ 12=16;
ð2Þ
where Fme is the flux rate (mg CH4-C mÿ2 hÿ1), v the chamber
volume (m3), A the sample surface area of the chamber (m2),
and dm/dt the slope of the chamber concentration over time
(g CH4 mÿ3 hÿ1)
For each chamber run, the set of six gas sample concentrations were plotted vs. sample time. Regressions were run for
Statistical analysis
To compare environmental variables (total monthly precipitation, mean air and soil temperatures and mean water levels)
among the wetland sites, analysis of variance (ANOVA) with a
Tukey’s Pairwise Comparison was used, whereas t-tests were
used to compare 10-year precipitation means to study period
precipitation. Pearson’s correlations were used to determine
relationships among the environmental variables (e.g., relationship between precipitation and soil temperature), and
ANOVA with a Tukey Pairwise Comparison was used to compare specific environmental variables between sites (e.g., observed soil temperature at EARTH vs. La Selva vs. Palo Verde).
Methane data failed to meet criteria for normal distribution as
indicated by Kolmogorov–Smirnov and Shapiro–Wilk tests of
normality (Po0.001 for both tests). Because methane especially is spatially heterogeneous and natural spikes in emission
rates can occur, infrequent high methane rates that passed the
rigorous regression standards were not removed as ‘outliers.’
Therefore, nonparametric statistical tests were used for
methane emissions. Kruskal–Wallis and Mann–Whitney tests
compared point and mean methane emissions between sites
and by sampling periods, and Spearman’s correlations were
used to determine relationships between point and mean
methane emissions and study site properties. Data was run
on point emissions and properties, and grouped accordingly
with the information presented (e.g., by transect, site, location,
etc.). Medians are reported along with the means for methane
emissions, reflecting the wide distribution of these rates.
Unless otherwise specified, reported methane emissions reflect
wetland sampling sites (deep and shallow water zones) and do
not include upland sampling sites. Significant differences
indicate P 0.05. Statistical analyses were conducted using
SPSS STATISTICS 17.0 for Mac and MINITAB 15.0 for PC.
Results
Climate, soil temperature, and hydrology
Because the three study sites are located in different
biomes in Costa Rica, their climates are distinctly different (Fig. 2). The total monthly precipitation in each
r 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 1321–1334
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(b)
(c)
Fig. 2 Environmental variables including mean water level (cm), mean soil temperature at 5 cm ( 1C), mean air temperature ( 1C), total monthly precipitation (mm monthÿ1), and mean
(lines) and median (markers) methane flux (mg CH4-C mÿ2 hÿ1) for (a) EARTH, (b) La Selva, and (c) Palo Verde wetland. Error bars on environmental variable data represent standard
error, whereas error bars on the median methane emissions represent the maximum and minimum limits. Note that methane emissions are reported on a log scale.
(a)
METHANE EMISSIONS FROM TROPICAL WETLANDS
1325
1326 A . M . N A H L I K & W . J . M I T S C H
site during the study period proved to be typical, with
no significant differences from the 10-year mean of
monthly precipitation from the respective site. Likewise, precipitation in each site between years (2007
and 2008) was not significantly different. However,
the monthly total precipitation during the study period
was significantly different between wetland sites, with
Palo Verde receiving less precipitation than EARTH and
La Selva (Po0.001). Both air and soil temperature
negatively correlated to precipitation (pair 5 0.045 and
psoil 5 0.026), and soil temperature positively correlated
to air temperature (Po0.01). Not surprisingly, there
were significant differences between mean monthly
air temperatures between sites, with Palo Verde experiencing higher air temperatures than either La Selva or
EARTH (Po0.001). Mean soil temperatures during the
study were significantly different among all three sites
(25.4 0.1, 24.8 0.1, and 28.1 0.2 1C for EARTH, La
Selva, and Palo Verde, respectively; Po0.001). EARTH
wetland had significantly higher mean water levels
than Palo Verde and La Selva wetlands (34 2,
11 2, and 18 2 cm for EARTH, La Selva, and Palo
Verde, respectively; Po0.001). Water level did not show
a relationship to soil temperature.
As expected, water levels from EARTH and La Selva
wetlands were significantly correlated to total monthly
precipitation (Po0.05 and 0.043 for EARTH and La
Selva, respectively); however, Palo Verde water levels
did not correspond to precipitation from the same
month, the previous month, or 2 months previous, which
would account for lag between precipitation and water
Table 2
station
Wetland methane emissions
Over 2500 gas samples were collected from the EARTH,
La Selva, and Palo Verde wetlands during six sampling
periods for a potential 432 total emission estimates.
Nearly 40% of the emission estimates were removed
due to nonlinearity from chamber disturbances; more
than 75% of these removed estimates were nonlinear
upland emissions. Less than a quarter of wetland
methane emissions were removed due to chamber disturbances.
Mean methane emission ranges were ÿ3.5–29, ÿ2.8–
297, 0.2–388 mg CH4-C mÿ2 hÿ1 at EARTH, La Selva,
and Palo Verde wetland sites, respectively (Fig. 2).
Methane emissions varied throughout the study, with
similar peaks in methane emissions during high precipitation months. Palo Verde wetland showed the least
variance, with the lowest minimum methane emissions
reaching 0.2 mg CH4-C mÿ2 hÿ1 in April 2007. Minimum
methane emissions from EARTH and La Selva wetlands, on the other hand, frequently were negative,
indicating methane oxidation.
Mean daily methane emissions (Table 2 and Fig. 3)
were lower from EARTH than the La Selva and Palo
Verde sites (means of 91, 601, and 719 mg CH4C mÿ2 dayÿ1 for EARTH, La Selva, and Palo Verde,
Methane emissions and supporting environmental data by wetland site (EARTH, La Selva, Palo Verde) and sampling
Mean temperature
Site
accumulation in the wetland basin. Although precipitation may still contribute to water levels at Palo Verde,
surface runoff from the surrounding watershed may be
a larger impact on water level in this wetland.
Mean PM
(mg CH4-C mÿ2 hÿ1)
Daily mean
(mg CH4-C mÿ2 dayÿ1)
ÿ0.02 0.09
62
41
ÿ0.3 0.2
31
31
ÿ4 2
103 25
82 14
32
93
20 4
0.03 0.17
62
37 18
0.3 0.1
30 23
25 10
42
419 262
727 237
00
20 4
34 4
0.1 0.1
29 11
34 9
ÿ0.4 0.3
31 18
27 17
ÿ4 5
723 249
715 253
Soil (5 cm)
( 1C)
Soil (10 cm) Mean water Mean AM
depth (cm) (mg CH4-C mÿ2 hÿ1)
( 1C)
0.3
0.6
0.4
25.6 0.2
25.6 0.3
25.1 0.2
ND
25.4 0.3
25.0 0.2
00
52 2
51 3
0.2
0.2
0.2
24.8 0.2
24.5 0.1
25.0 0.1
ND
24.5 0.2
24.9 0.1
0.4
0.4
0.5
28.5 0.3
28.0 0.4
27.7 0.3
ND
28.0 0.4
26.5 0.2
Air ( 1C)
EARTH
UP
27.6
S
29.1
D
27.9
La Selva
UP
25.4
S
25.5
D
25.7
Palo Verde
UP
29.1
S
31.5
D
32.5
Methane emissions
Methane is reported as mg CH4-C mÿ2 hÿ1 for morning (AM) and afternoon (PM) averages. The daily mean of methane emission is
reported as mg CH4-C mÿ2 dayÿ1. All values are means SE.
UP, upland; S, shallow wetland; D, deep wetland; ND, no data.
r 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 1321–1334
CH -C emissions (mg m
d )
METHANE EMISSIONS FROM TROPICAL WETLANDS
a
10000
b
a
100
1
0.1
Earth
La Selva
Palo Verde
Fig. 3 Mean (horizontal lines) and median (circles) wetland
methane emission rates (mg CH4-C mÿ2 dayÿ1) for three wetland
sites included in this study. Vertical lines represent minimum
and maximum values. Different letters indicate significant differences in medians between groups with identical letters. Note
that methane emissions are reported on a logarithmic scale.
respectively). Methane emissions for Palo Verde were
significantly higher than EARTH and La Selva (EARTH,
La Selva, and Palo Verde methane emission medians
of 52, 79, and 257 mg CH4-C mÿ2 dayÿ1, respectively;
Po0.001). When the humid sites were pooled, methane
emissions were still statistically higher from the seasonally wet site than the humid sites (Po0.001). The maximum methane emission from each site (689, 7138, and
9311 mg CH4-C mÿ2 dayÿ1 for EARTH, La Selva, and
Palo Verde, respectively) are evidence of occasional
high spikes in emissions from the wetlands, while the
minimum methane emissions at the sites suggest that
methane oxidation may be important (ÿ84, ÿ67, and
6 mg CH4-C mÿ2 dayÿ1 for EARTH, La Selva, and Palo
Verde, respectively).
Diurnal methane emissions
Methane samples were collected from the wetlands in
the morning (AM) and in the afternoon (PM) to capture
diurnal temperature changes, which could affect
methane emission rates. There were no consistent differences between AM and PM sampling in the wetlands, nor did median methane emissions significantly
differ between AM and PM at either the EARTH or
La Selva wetlands (Table 2). Methane emissions were
significantly higher in the AM at Palo Verde than in the
PM, with median wetland emissions [shallow wetland
(S) and deep wetland (D) combined] reduced by 50%
from 18 in the morning to 9 mg CH4-C mÿ2 hÿ1 in the
afternoon (Po0.05). Deep wetland (D) sites in Palo
Verde had significantly higher AM emissions than PM
emissions (Po0.05), whereas shallow wetland (S) sites
within Palo Verde did not differ from AM to PM.
1327
Spatial variability of methane emissions
Methane emissions were significantly lower from soils
in the upland sites than in the wetland sites (Table 2,
Po0.001). Mean methane emissions from the uplands
were around zero, ranging from ÿ0.4 to 0.3 mg CH4C mÿ2 hÿ1. When calculated for daily rates, upland
methane emission rates from the EARTH and Palo
Verde were negative indicating methane oxidation (ÿ4
and ÿ4 mg CH4-C mÿ2 dayÿ1, for EARTH and Palo
Verde, respectively). La Selva exhibited a low mean
daily methane emission from the upland of 4 mg CH4C mÿ2 dayÿ1, not surprisingly since the upland of La
Selva is a rainforest and has wet soils. Median upland
emissions were not significantly different between wetland sites.
There were no consistent patterns or significant differences in wetland methane emissions between shallow wetland (S) and deep wetland (D) sampling sites in
any of the wetlands. Because methane emissions often
vary widely within short distances, daily methane
emissions at the two transects at each wetland were
analyzed separately. In this analysis, mean methane
emissions
ranged
from
82 mg CH4-C mÿ2 dayÿ1
(EARTH D) to 727 mg CH4-C mÿ2 dayÿ1 (La Selva D).
Methane emissions were significantly different between
the two transects at EARTH and La Selva (Fig. 4,
Po0.01 and 0.001 for EARTH and La Selva, respectively). Median methane emissions were more than
three times higher in transect 2 (E2) than in transect 1
(E1) at EARTH wetland, with E2 producing a median of
(mean
of
116 mg CH491 mg CH4-C mÿ2 dayÿ1
C mÿ2 dayÿ1). Transect 2 in La Selva (LS2) produced
very little methane (4 and 12 mg CH4-C mÿ2 dayÿ1,
median and mean, respectively), with rates significantly
lower than any of the other wetland transects. On the
other hand, transect 1 in La Selva (LS1) produced some
of the highest methane emissions of all the sites (279
and 961 mg CH4-C mÿ2 dayÿ1, median and mean, respectively). LS1 and both the Palo Verde transects are
statistically grouped with similar high methane emissions, with no significant difference in median methane
emissions between transects in Palo Verde (medians of
236 and 262 mg CH4-C mÿ2 dayÿ1 for PV1 and PV2,
respectively) or LS1.
Relationship of methane emissions to physiochemistry
Mean water level in the transects displayed trends
similar to those of methane emissions (Fig. 4). Although
water levels were significantly different between transects at the same wetland (Po0.001), water levels were
similar among wetlands, with no differences between
E1, LS1, and PV2 and between LS1 and PV1 water
r 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 1321–1334
1328 A . M . N A H L I K & W . J . M I T S C H
c
31
29
a
c
Mean chamber
water level (cm)
(a)
33
a
27
b
b
25
28
(b)
c
c
27
26
a
ab
25
ab
80
70
60
50
40
30
20
10
10000
CH4-C emissions
(mg m−2 d−1)
Mean soil
temperature (°C)
Mean air
temperature (°C)
35
b
24
23
(c)
b
a
a
ac
c
d
(d)
a
c
c
c
PV1
PV2
b
d
100
1
0.1
E1
E2
LS1
LS2
PV1
PV2
E1
E2
LS1
LS2
Fig. 4 (a) Mean air temperature ( 1C), (b) mean soil temperature at 5 cm ( 1C), (c) mean waterlevel (cm), and (d) mean (lines) and median
(circles) wetland methane emission rates (mg CH4-C mÿ2 dayÿ1) for two separate transects (1 and 2) at EARTH (E), La Selva (LS), and
Palo Verde (PV) wetlands. Error bars represent minimum and maximum limits for methane and standard error for environmental
conditions. Different letters indicate significant differences between groups with identical letters using median methane emissions and
environmental means.
levels. Despite similar trends in methane emissions,
water level was significantly correlated to methane
emission only at PV1, LS1, and LS2 (Po0.05, 0.05, and
0.001 for PV1, LS1, and LS2, respectively).
Mean air temperature did not differ significantly
between transects at the same site (Po0.001; Fig. 4a).
Methane emissions were not significantly correlated to
air temperature at any of the transects.
Mean soil temperature of the transects were significantly higher at Palo Verde than the other wetland sites,
while transect soil temperatures at EARTH and La Selva
were similar with the exception of E1 and LS2, which
were significantly different (Po0.001). Methane emissions were significantly correlated to soil temperature at
E2 and LS2. Methane emissions from wetland transects
E1 and PV2 failed to correlate with any environmental
variable measured in this study.
Discussion
Tropical methane emissions and climate
Each of the wetlands included in this study experienced
a different climate, with the largest differences between
the humid Caribbean coast wetlands (EARTH and La
Selva) and the seasonally wet Pacific coast wetland
(Palo Verde). Precipitation was correlated to several of
the environmental variables, including air and soil
temperature, with these conditions driving the development of the surrounding biome (e.g., humid forest,
rainforest, dry forest). Overall methane emissions were
highest from the seasonally flooded wetland (Palo
Verde), which experienced the highest air and soil
temperatures as a result of this seasonality. The Palo
Verde ecosystem is in an open, coastal plain with little
shading from trees that would be present if these were
the humid tropics; this, in turn, contributes to the high
temperatures in the wetland. These physical characteristics of the climate and surrounding environment may
explain the high methane emissions rates from this
wetland. However, Palo Verde also experiences seasonal flood pulsing, with dry periods from December to
April and wet periods from May to November, and we
expected Palo Verde to have the lowest methane emissions as a result of these drier periods. In a study of
temperate wetlands in Ohio, Altor & Mitsch (2008)
reported that methane emissions in some locations of
these wetlands were significantly lower than in those
same locations in steady-flow (nonpulsed) conditions.
Husin et al. (1995) also reported similar trends in rice
paddies, with less methane produced in intermittently
flooded paddies than in permanently flooded paddies.
Those pulses were on the order of 1 week to 1 month in
duration, however, as opposed to the 6-month flooding
pulses seen at our seasonal wet Palo Verde site. In
comparison with the Palo Verde wetland, the humid
tropical La Selva wetland experienced the lowest air
and soil temperatures due to heavy precipitation and
shading from the rainforest, yet one transect (LS1) from
La Selva had large amounts of stagnant (nonpulsed)
water and produced as much methane as was found at
the Palo Verde transects. As we will discuss in the
r 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 1321–1334
METHANE EMISSIONS FROM TROPICAL WETLANDS
following section, water depth may have just as much
affect on methane emissions from these wetlands as
does temperature.
Tropical methane emissions and hydrology
Methane Emissions
(mg CH4-C m−2 h−1)
With air and soil temperatures near or slightly above
those at La Selva, the humid tropical wetland at EARTH
was continuously flooded but at water depths greater
than either of the other two wetlands. Despite large
amounts of standing water, methane emissions were
lower there than at either of the seasonally flooded Palo
Verde transects or transect 1 from La Selva.
A Shelford-type nonlinear relationship (Shelford,
1912; Odum, 1971) was found between methane emissions and average water level when using five of the six
sampling site transects included in this study (Fig. 5).
This analysis suggests that there may be an optimal
water depth between 30 and 50 cm at which methane is
produced in the highest amounts in natural tropical
wetlands, and that this range of water depth in concert
with temperatures may reflect how water level affects
methane emissions with or without pulsing. Several
reasons may explain lower methane emissions in the
shallow and deep water levels. In shallow water, oxygen diffusion through the air–water interface may oxidize the entire water column, allowing for less
anaerobic conditions and more oxidation of methane
at the soil–water interface than deeper water, in which
only a portion of the upper water column may be
oxidized, leading to persistent anaerobic conditions.
Additionally, root systems from emergent macrophytes
that thrive in shallow water conditions may support
methanotrophs in aerobic microsites around oxidized
rhizospheres of roots (Gilbert & Frenzel, 1995; Segers,
1998). On the other hand, carbonaceous material from
plant roots can also serve as a substrate for methanogenic archaea, and elevated methanogenesis has also
E1
E2
LS1, LS2
PV1, PV2
40
30
20
10
y = −0.03x + 2.30x - 4.51
R = 0.89
0
0
10
20
30
40
50
60
70
80
Water level (cm)
Fig. 5 Relationship between mean methane emissions and
mean water level for each transect in the wetlands included in
this study. Means are for the entire study period. The line is a
second-order polynomial regression through solid data points.
1329
been reported around plant roots (Segers, 1998; Huang
et al., 2005). When water levels are 430 cm, the wetlands may be experiencing polymictic stratification
throughout the day and destratification (mixing) at
night, similar to shallow tropical lakes, which could
increase methane oxidation of diffuse methane within
the water column due to higher dissolved oxygen levels
during mixing, which create optimal conditions for
methanotrophy (Ford et al., 2002).
Importance of carbon
Most methane production occurs in the top 10 cm of the
soil (Crozier et al., 1995), and availability of organic
carbon in the substrate is essential for methane production (Segers, 1998). Van der Gon & Neue (1995) found
that adding organic matter in rice paddies resulted in
higher methane emissions; thus, differences in soil
carbon pools in the upper substrate of the wetlands
may impact methane emission rates. Bernal & Mitsch
(2008) measured total organic soil carbon pools in the
same wetlands used in the current study. The concentration of soil carbon for 0–12 cm was very different
between sites, with EARTH, La Selva, and Palo Verde
measuring 165, 101, and 43 g C kgÿ1, respectively; the
pattern of methane emission rates between sites was
inverse to soil carbon concentrations, with Palo Verde
emitting the most methane, followed by La Selva, and
EARTH emitting the least. When taking into account the
different bulk densities among sites, the soil carbon
pool was similar for depths between 0 and 12 cm,
measuring 4.41, 4.27, 4.24 kg C mÿ2 for EARTH, La
Selva, and Palo Verde, respectively. Despite potentially
large sources of organic carbon from domestic livestock
manure at the Palo Verde wetland, soil carbon concentrations or pools did not reflect this. Therefore, the
differences in methane emission rates among the wetlands in this study are unlikely due to differences in soil
carbon content between sites.
While this study did not include extensive redox
potential measurements from these wetlands, we did
measure soil redox potentials using platinum electrodes
at the EARTH wetland and found that while methane
emissions did not correlate with redox in the wetlands,
redox in the upland averaged 1 134 mV and wetlands
averaged ÿ147 mV, reaching beyond ÿ250 mV in some
areas. Clearly, upland soils at the EARTH wetland were
oxic and wetland soils were reduced enough to allow
methane production. Methane emissions can occur even
at soil redox potentials as high as ÿ110 mV (Huang
et al., 2005). While much of the literature reports a
negative correlation between soil redox and methane
emission (Huang et al., 2005), others have reported a
threshold at which methane is produced but beyond
r 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 1321–1334
1330 A . M . N A H L I K & W . J . M I T S C H
which no correlation occurs (Singh et al., 2000). Difficulty correlating redox measurements to methane may
be due to high spatial variability of both redox and
methanogenesis. Boon et al. (1997) reported difficulty
measuring redox potential due to the small diameter of
the redox probes (o1 mm) and abundant aerobic microsites within the soils.
Diurnal variability within the wetlands
Because methane emission rates are temperature dependent (Segers, 1998; van Hulzen et al., 1999), we
expected afternoon methane emission rates to be higher
than those of the morning due to generally higher
afternoon temperatures that could result in increased
soil temperature; however, neither the EARTH wetland
nor the La Selva wetland exhibited consistent diurnal
patterns in methane emissions. This may be due to the
forested nature of the surrounding landscape of these
two wetlands and resulting shade, which could act as a
heat buffer for the wetlands (Wang & Han, 2005).
Organic soils, such as those found in these wetlands,
have also been found to be less responsive to diurnal
temperature changes than sandy soils (Wang & Han,
2005). Palo Verde, on the other hand, produced significantly more methane in the morning hours than in the
afternoon. Whiting & Chanton (1996) described high
pulses of methane emissions in the morning, when the
rising light stimulates the release of methane from
emergent macrophytes. Alford et al. (1997) reported that
the highest diurnal methane emission rates in a Sagittaria lancifolia freshwater marsh occurred around 10:00
hours during the summer. Another explanation for
lower methane emissions at Palo Verde in the afternoon
is that clouds and rainstorms, resulting in lower temperatures, generally occurred in the early afternoon
hours, which coincided with our afternoon sampling
(personal observation).
Spatial variability within wetlands
Our methane emission rate estimates were highly variable within wetlands, both spatially and temporally.
The ranges for each of the wetland sites were 33, 300,
and 388 mg CH4-C mÿ2 hÿ1 compared with a mean of
3.7, 25, and 30 mg CH4-C mÿ2 hÿ1 for EARTH, La Selva,
and Palo Verde, respectively. While Palo Verde had the
greatest range of methane emissions, it also had the
highest minimum, with methane emissions occurring
even during dry periods. One reason that Palo Verde is
emitting methane consistently, wet or dry, may be due
to the soil characteristics of the wetland. The expanding
clay associated with vertisol soils holds water very well,
and even when there is no standing water, deeper soils
may be saturated and anaerobic, hosting methanogens.
Furthermore, the hot, dry climate of Palo Verde causes
cracks in the top layers of soil when the wetland is dry,
perhaps allowing for preferential diffusion of methane
from deeper, anaerobic layers without being intercepted
by methanotrophs. This hypothesis should be further
investigated in Palo Verde and other wetlands dominated by vertisols.
Wachinger et al. (2000) reported large differences in
methane production from soil cores extracted within
1 m of each other, with variability sometimes exceeding
100% of the standard deviation. Chen et al. (2009) also
reported high spatial and temporal variability in
methane production both within and among wetland
sampling sites in alpine wetlands of China. Methane
production varying by several orders of magnitude
within small distances is not uncommon (Moore &
Knowles, 1990; Adrian et al., 1994). Ebullition, which
can account for about 60% of the total methane release,
can also affect spatial variability, with methane bubbles
developing unevenly within a wetland (Tokida et al.,
2005). Isolating the variables that control methane production in the field, especially when the variables are
interactive is extremely difficult; therefore, it is not
surprising that differences in climate, environmental
physiochemistry, or pulsing did not explain the variation in methane emissions between wetlands in this
study.
Comparison of methane emissions
Annual methane emissions estimated from this study
(44–350 g CH4 mÿ2 yrÿ1; Table 3) are high compared
with reported temperate and boreal methane emission
rates. Here we provide some examples of emission rates
that were measured using diffusion chambers in temperate or boreal wetlands. Altor & Mitsch (2006) report
methane emissions of 28 g CH4 mÿ2 yrÿ1 for two temperate created riparian marshes in Ohio using the same
field and laboratory techniques used in this study.
Shannon & White (1994) reported methane emissions
of 67–77 g CH4 mÿ2 yrÿ1 in Michigan peatlands,
whereas Dise et al. (1993) reported 46 g CH4 mÿ2 yrÿ1
for northern Minnesota peatlands. By contrast, boreal
Canadian peatland annual flux measurements have
been reported as o10 g CH4 mÿ2 yrÿ1 with primary
controlling mechanisms of soil temperature, water table
position, or a combination of both (Moore & Roulet,
1995). In another central Canadian boreal wetland
study, an average emission of only 1.6 g CH4 mÿ2 yrÿ1
was estimated (Roulet et al., 1992). Methane emissions
from a boreal swamp and fen were reported to emit 4
and 1 g CH4 mÿ2 yrÿ1, respectively (Kang & Freeman,
2002); Waddington et al. (1996) report emissions of
r 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 1321–1334
EARTH, Costa Rica
La Selva, Costa Rica
Palo Verde, Costa Rica
Louisiana, USA
Current study
South Kalimantan,
Indonesia
Central Amazon Basin,
South America
Atawapaskat, New
Guinea
Orinoco River
Floodplain,
Venezuela
Lucknow, India
Varanasi, India
Louisiana, USA
River Murray, SE
Australia
Lago Calado, Amazon
Basin, Brazil
Hadi et al. (2005)
Melack et al.
(2004)
Snyder (2002)
Smith et al.
(2000)
Singh et al.
(2000)
Singh et al.
(1999)
Banker et al.
(1995)
Boon & Mitchell
(1995)
Bartlett et al.
(1988)
Yu et al. (2008)
Study location
Source
494z
788§
9
42
9
593*
155*
49
110
301
386*
1w
10
4 g CH4-C mÿ2 yrÿ1
105 g CH4-C haÿ1 dayÿ1
56 mg CH4 mÿ2 hÿ1
90 mg CH4 mÿ2 hÿ1
2 mmol CH4 mÿ2 dayÿ1
7 mmol CH4 mÿ2 dayÿ1
2 mmol CH4 mÿ2 dayÿ1
68 mg CH4 mÿ2 hÿ1
18 mg CH4 mÿ2 hÿ1
135 mg CH4 mÿ2 dayÿ1
300 mg CH4 mÿ2 dayÿ1
826 mg CH4 mÿ2 dayÿ1
3 mmol CH4 mÿ2 hÿ1
0 mmol CH4 mÿ2 hÿ1
27 mg CH4 mÿ2 dayÿ1
Secondary Forested Peatland
Forested Floodplain
Slough
Slough
Open Water
Flooded Forest
Macrophyte Mats
Gomti River
r 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 1321–1334
Tropical
Subtropical
Subtropical
Subtropical
Subtropical
Natural Deepwater Wetlands
Rice Paddy
Rice Paddy, First Crop
Rice Paddy, Ratoon Crop
Oxbow
Oxbow
Open Water Lake
Closed,
recirculating
chamber
Diffusion chamber
Closed,
recirculating
chamber
Closed,
recirculating
chamber
Closed,
recirculating
chamber
Diffusion chamber
Closed,
recirculating
chamber
Tropical
Tropical
Remote sensing
Diffusion PVC tube
chamber
Diffusion chamber
Continued
44
293
350
249*
116w
26
91 mg CH4-C mÿ2 dayÿ1
601 mg CH4-C mÿ2 dayÿ1
719 mg CH4-C mÿ2 dayÿ1
28 mg CH4 mÿ2 hÿ1
13 mg CH4 mÿ2 hÿ1
20 g CH4-C mÿ2 yrÿ1
Forested Marsh
Rainforest Swamp
Alluvial Marsh
Bottomland Hardwood Forest
Bottomland Hardwood Forest
Rice Paddy
6
4
(g CH4 mÿ2 yrÿ1)
Diffusion chamber
Methane emission
Reported value
technique
Wetland type
Measurement
Tropical
Tropical
Subtropical
Tropical
Biome
Table 3 Methane (CH4) emissions measured for several tropical and subtropical wetland and river studies
METHANE EMISSIONS FROM TROPICAL WETLANDS
1331
*Warm season.
wCool season.
zMonsoon season.
§Dry season.
Study location, biome, field measurement technique, and wetland type are reported for each study. Methane emissions are reported in original units from the source and in
g CH4 mÿ2 yrÿ1. Values are overall means or seasonal means from individual studies.
40
215
142
110 mgCH4mÿ2dayÿ1
590 mg CH4 mÿ2 dayÿ1
390 mg CH4 mÿ2 dayÿ1
Diffusion chamber
Tropical
Vargem Grande,
Amazon Basin,
Brazil
Devol et al.
(1988)
Flooded Forest
Macrophyte Mats
Overall
70
84
44
192 mg CH4 mÿ2 dayÿ1
230 mg CH4 mÿ2 dayÿ1
120 mg CH4 mÿ2 dayÿ1
Flooded Forest
Floating Grass Mats
Open Water Lake
(g CH4 mÿ2 yrÿ1)
technique
Biome
Study location
Source
Table 3 (Contd.)
Measurement
Wetland type
Reported value
Methane emission
1332 A . M . N A H L I K & W . J . M I T S C H
40 g CH4 mÿ2 yrÿ1 in a boreal peatland. Published
methane emission rates from tropical and subtropical
wetlands (Table 3) are quite variable and often higher
than those of temperate and boreal wetlands. Several of
the rates included in Table 3 were reported for large
watershed systems such as the Amazon basin (Bartlett
et al., 1988; Devol et al., 1988; Melack et al., 2004), river
and riparian systems (Boon & Mitchell, 1995; Singh
et al., 2000; Yu et al., 2008), and rice paddies (Banker
et al., 1995; Singh et al., 1999; Hadi et al., 2005). Published
methane emissions for the wetland types most similar
to those used in our study range from 6 to
788 g CH4 mÿ2 yrÿ1, with a mean of 302 g CH4 mÿ2 yrÿ1
[Devol et al., 1988 (floating macrophyte mats); Snyder,
2002; Smith et al., 2000 (floating macrophyte mats); Hadi
et al., 2005 (secondary forested peatland)]. The mean
(229 g CH4 mÿ2 yrÿ1) and range (306 g CH4 mÿ2 yrÿ1) of
methane emission rates from our study are within this
published range.
Tropical methane emission rates
Our results suggest that the spatial variability of
methane emission measurements within one type of
wetland is so high that the error in previously published
global estimates of the methane emissions from tropical
wetlands is larger than previously thought. Great attention is paid to the details of mapping wetland systems
because these ecosystems may be important climate
regulators and provide other important ecosystem services, and the remote sensing capabilities are available
to do so. However, there is little use for satellite-based
methane emission estimates if we do not have accurate
field methane emission estimates for each wetland type.
Using the measured average methane emission rate of
229 g CH4 mÿ2 yrÿ1 presented in this study and the
estimate of Costa Rican wetlands presented by Mitsch
et al. (2008) of 3500 km2, we estimate that tropical wetlands in Costa Rica are producing 0.80 Tg CH4 yrÿ1. This
is 0.6% of the recent estimate of 138 Tg CH4 yrÿ1 emitted
by all tropical wetlands (Bergamaschi et al., 2007). By
comparison, Melack et al. (2004) estimated that the
entire mainstem Solimões/Amazon floodplain produced 1.7 Tg CH4 yrÿ1, a rate more than twice that of
our estimated flux for Costa Rica.
Changes in tropical methane emissions as a result of
climate change
Using climate models, Cao et al. (1998) predicted that
with a 2 1C increase in global temperature, wetland soil
carbon would decrease by 10–25% and wetland
methane emissions would increase 10–20%. While the
location and types of wetlands in this study varied, it
r 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 1321–1334
METHANE EMISSIONS FROM TROPICAL WETLANDS
was shown that Palo Verde, with a drier climate and
warmer conditions, had higher methane emissions than
wet and cooler tropical wetlands at EARTH and La
Selva. Decreased soil moisture (or water level) in humid
tropical rainforest wetlands may result in increased
methane emissions, as seen in Palo Verde, especially if
water levels fall to ‘optimal’ levels for methanogenesis
under higher temperature regimes. Yet, despite predicted future increases in tropical wetland methane
production under global climate change scenarios, the
climate feedback from wetland methane emissions is
currently, and will probably continue to be, far lower
than those of anthropogenic carbon dioxide emissions
(Gedney et al., 2004).
Conclusions
This study presents methane emission rates for three
tropical wetlands in different climates in Central America. Methane emissions measured in the three Costa
Rican wetlands were near the maximum values reported for tropical wetlands (including rice paddies
and natural wetlands). Methane emissions were highest
in wetlands that had standing water between 30 and
50 cm and wetlands that experienced seasonal flooding
and warmer temperatures. Using Palo Verde wetland as
a model for the possible future environmental conditions of some tropical humid rainforest wetlands,
methane emissions would increase from tropical wetlands due to increased temperatures and decreased
water levels. Additional studies should be conducted
in wetlands in other parts of the tropics to validate the
water level and methane emission patterns described in
this paper.
Acknowledgements
The authors would like to thank Blanca Bernal-Martinez, Bryan
Smith, Angela Adams, and Richard Nahlik for their field assistance in Costa Rica, and Drs. Jane Yeomans, Stephanie Lansing,
and Bert Kohlmann for all their help at EARTH University. We
appreciate site access provided by the Organization for Tropical
Studies (OTS) for La Selva Biological Research Station and Palo
Verde Biological Research Station. Many thanks to Drs. Nick
Basta, Richard Dick, Jay Martin, Rita Wania, and an anonymous
reviewer for their helpful input and reviews that improved the
quality of this manuscript. Funding for this project came from
the United States Department of Energy [Grant DE-FG0204ER63834 (EARTH University/OSU Program on Collaborative
Environmental Research in the Humid Tropics; David Hansen,
PI)], the U.S. Environmental Protection Agency grant EM83329801-0 (Olentangy River Wetland Research Park: Teaching,
research, and outreach, W.J. Mitsch, PI), and from the Wilma H.
Schiermeier Olentangy River Wetland Research Park and the
Environmental Science Graduate Program at The Ohio State
1333
University. Olentangy River Wetland Research Park publication
number 2010-002.
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