CH4 emissions
Over one growing season in 2012, the two Phragmites wetlands emitted on average 0.15
and 1.01 mg CH4 m-2 h-1 (Phrag1 and Phrag2, respectively) and
the rice paddy 0.75 mg m-2 h-1, whereas the emissions from the two
Suaeda wetlands were negligible. The large differences in CH4 emission rates
among the five sites can be explained by the differences in soil organic
matter, salinity and water tables, and, to some extent, vegetation type. For
methanogenesis to take place there must be a sufficient amount of labile
organic substrate available (Mah et al., 1977), such as dead
plant material from the previous growing season and root exudates from the
standing vegetation (Mann and Wetzel, 1996; Zhai et al., 2013). Previous
studies have reported increasing CH4 emission rates with increasing
content of soil organic matter in different types of wetlands (Le Mer and
Roger, 2001; Picek et al., 2007; Serrano-Silva et al., 2014; Sha et al.,
2011; Tanner et al., 1997). At Phrag2, where CH4 emission rates were
significantly higher than at the other sites, there was a many-fold higher
content of organic carbon and nitrogen in the soil compared to the soils at
the other sites, and the reeds at Phrag2 had a very dense root system in the
upper soil layers. Thus, the reason for the high CH4 emission rates at
Phrag2 was most likely the higher content of organic substrate for
methanogenesis, originating from dead plant residues and from root exudates.
At the rice paddy, where the second highest CH4 emissions were
measured, the organic content of the soil was low, but the soil C : N ratio
was lower (8.4) than the ratios at the other sites probably resulting from
different plant inputs into the soil. A lower C : N ratio of the organic
matter in the soil may increase organic matter lability by decreasing
nitrogen limitation for decomposers (Hodgkins et al.,
2014). However, the fact that the rice paddy was constantly flooded
throughout the growing season probably also stimulated methanogenesis and
CH4 emission.
Cumulative CH4 emissions during the growing season 2012 from
two Suaeda salsa wetlands, two Phragmites australis wetlands and one rice paddy during 2012 in the Liaohe
Delta, Northeast China. The points represent integrals of the monthly mean
values from six plots at each site. Measurements are missing from Phrag1 in
August due to flooding.
Mean CH4 emission and ecosystem respiration rates
(Reco) with ranges in parentheses, and cumulative CO2 equivalents
from CH4 and CO2 emissions, respectively, from two Phragmites australis wetlands and
one rice paddy during April-November 2012 in the Liaohe Delta, Northeast
China. CH4 fluxes are converted to CO2-equivalents using a factor
of 25. Superscript letters represent significant differences (p < 0.05) among sites.
Cumulative CO2-equivalents
Site
CH4 emission rates
Reco
CH4
CO2
(mg m-2 h-1)
(mg CO2 m-2 h-1)
(g CO2-eqv m-2 yr-1)
(g CO2-eqv m-2 yr-1)
Suaeda1
0.01 (-0.31–0.44)a
278 (-3.6–814)ab
-0.4
1671
Suaeda2
-0.01 (-0.50–0.42)a
423 (4.6–954)b
-1.9
1730
Phrag1*
0.15 (-0.31–1.48)ab
484 (-14.8–1300)c
31.1
2963
Phrag2
1.01 (-0.28–6.38)c
811 (27.4–3357)c
153.7
4443
Rice
0.75 (-0.27–4.63)b
532 (-0.2–3181)a
91.6
3337
* No data from August.
Both P. australis and rice have well-developed aerenchyma in roots, rhizomes and stems,
which provides them with a high ability to transport gasses between the soil
and the atmosphere through the plant tissue (Brix et al., 1996; Singh and
Singh, 1995). When CH4 is transported from the soil through the
air-space tissues of the plants, it bypasses the aerobic zone in the upper
part of the soil and the water column, where CH4 otherwise could have
been oxidized by methanotrophic bacteria (Whalen, 2005).
Plant-mediated transport has been reported to be the main pathway of
CH4 transport from the soil to the atmosphere and constituting as much
as 60–90 % of the CH4 emissions (Butterbach-Bahl et al., 1997;
Huang et al., 2005). In the present study, transport of CH4 through
the air-space tissue of the plants may explain the relatively high CH4
emission rates from the Phragmites wetlands and the rice paddy, while the lack of
well-developed aerenchyma in S. salsa is consistent with the negligible emission
rates from the Suaeda wetlands. The above-ground biomass per se probably had no effect on
the plant-mediated CH4 emissions, as CH4 has been shown to be
mainly emitted through micropores in the basal parts of rice plants
(Nouchi et al., 1990) and through the basal internodes of P. australis
(Brix, 1989). Also, Henneberg et al. (2012) showed in a
manipulation experiment with Juncus effusus that above-ground biomass was unimportant for
the CH4 transport through the plants, whereas the removal of fine roots
and root tips of coarse roots led to significant reductions in
plant-mediated CH4 transport. Thus, it is likely that the extensive
root system of the reeds at Phrag2 contributed to the high CH4 emission
rates at this site.
At salinity levels above 18 ppt the CH4 emission rates were always
lower than 1 mg m-2 h-1 across all sites (Fig. 4). This is
consistent with Poffenbarger et al. (2011) who found a salinity
threshold of 18 ppt, above which CH4 emission rates were significantly
lower than at lower salinity levels. The effect of salinity has been
explained by the high concentrations of SO42- in seawater, which
inhibit CH4 production due to competition from sulphate reducing
bacteria (Bartlett et al., 1987; D'Angelo and Reddy, 1999). Thus, the
lack of CH4 emissions at the Suaeda sites is most likely an effect of the high
salinity, particularly at the Suaeda1 site where salinities were up to 35 ppt. The salinity was, however, significantly lower at the Suaeda2 site with
salinities of 5–15 ppt, and yet there were no CH4 emissions as
SO42- concentrations were still high enough to inhibit
methanogenesis. At Phrag2, on the other hand, CH4 emission rates were
high although the water salinity was occasionally as high as 15 ppt. These
seemingly contradictory results can be explained by the fact that a high
salinity in the water mainly affects the upper soil layers, but not
necessarily the deeper layers. Therefore, methanogens may be out-competed by
sulphate reducing bacteria in the upper layers of the soil, but CH4 can
still be produced in the deeper soil layers where all SO42- have
been reduced. The roots of P. australis grow to a soil depth of at least 40–60 cm, and
CH4 can therefore be transported from the deeper anoxic zone through
the air-space tissue of the plants to the atmosphere. Thus, the relatively
high salinity at Phrag2 probably inhibited methanogenesis in the upper soil
layers, but the CH4 produced in the deeper soil layers were still
transported to the atmosphere through the plants. At the Suaeda wetlands, the
generally low and fluctuating water tables indicate that the anaerobic zone
where methanogenesis can take place was at a deeper soil depth than at the
Phragmites wetlands. The roots of S. salsa lack aerenchyma and are generally restricted to the
upper 20 cm of the soil, and are therefore ineffective conduits for CH4
from the deeper soil layers to the atmosphere. Thus, although salinity
levels at Suaeda2 were not always high, any CH4 that may have been
produced in the soil did not reach the atmosphere because of CH4
oxidation in the upper soil layer. At the rice paddy, the low salinity of
around 2 ppt seemingly had no inhibitory effect on the CH4 production
and emission.
The water table is an important parameter affecting the CH4 emission
rate. The highest CH4 emissions occurred at the three sites where the
water exchange and water table were managed to maximize the reed biomass
(Phrag1, Phrag2) and crop yield (Rice) whereas very low CH4 emission
rates were found at the two Suaeda wetlands with a natural tidal hydrology. At the
rice paddy, the soil was continuously flooded from June until September, and
the two Phragmites wetlands were more or less flooded from June until October,
resulting in low redox potentials and relatively high CH4 emission
rates. The soils at the tidally influenced Suaeda wetlands were periodically
drained and hence partly oxidized inhibiting CH4 production. When water
tables at the Phragmites wetlands and the rice paddy were below the soil surface, the
CH4 emission rates were always < 1 mg CH4 m-2 h-1 probably because CH4 produced in deeper soil layers was
oxidized in the upper oxic soil layers, reducing the amount of CH4
reaching the atmosphere. When the water tables approached the soil surface,
the CH4 emission rates increased. This is in agreement with the
findings of Zhu et al. (2014), who reported that the seasonal CH4
emissions from an herbaceous peatland were highly linked to water table
fluctuations, and that the water table was the main environmental driver for
CH4 emissions over a single growing season, whereas soil temperature
was important on a longer timescale. The important effect of water table on
CH4 emission rates is in agreement with observations in other studies
(e.g. Bridgham et al., 2006; Couwenberg et al., 2011; Le Mer and Roger,
2001; Serrano-Silva et al., 2014). However, in the present study both soil
water table and temperature were important drivers.
Relationship between CH4 emission rates and (a) soil
temperature, and (b) water table, in two Phragmites australis wetlands and a rice paddy in the
Liaohe Delta, Northeast China. Data points after cutting the vegetation at
Phrag2 are represented by downward triangles (Phrag2-cut). Measurements were
done from April to November 2012.
The large seasonal variations in CH4 emission rates at Phrag1, Phrag2
and Rice were primarily related to the variations in soil temperatures. The
highest CH4 emission rates occurred during the summer months
July–September, when temperatures were relatively high. We found an
exponential relationship between soil temperature and CH4 emission
rates (Fig. 3) similar to those reported elsewhere (Herbst et al., 2011;
Inglett et al., 2012) in accordance with the temperature dependency of the
methanogenic bacteria. Furthermore, the amount of labile organic carbon
substrates from root exudates can be stimulated by high temperatures as Zhai
et al. (2013) found significantly higher root exudation
rates from P. australis roots at 20 than at 10 ∘C. Also the plant-mediated
CH4 transport may be accelerated at higher temperatures as Hosono
and Nouchi (1997) reported that the CH4 transport through rice
plants was twice as high at a rhizosphere temperature of 30 ∘C as
compared to the transport at 15 ∘C. Thus, the high CH4 emission
rates at both Phrag2 and Rice during the warmest months of the year were
probably due to the high temperature and its stimulating effect on the
activity of the methanogenic bacteria, the root exudation rates and the
effectivity of the plant-mediated transport. At soil temperatures below
18 ∘C, which occurred before June and after September, CH4 emission
rates were consistently low (< 1 mg CH4 m-2 h-1). In
the spring, the low rates might be associated with a time-lag in the growth
of methanogens as the temperature was increasing over a relatively short
period. In the autumn the low rates might be influenced by low availability
of organic carbon, as most carbon might have been burned off during the
hot summer months.
Ecosystem respiration (Reco)
Ecosystem respiration rates were highest in June-July at the Phragmites wetlands,
June-August at the Suaeda wetlands and August at the rice paddy. The differences
among the sites can be explained by the differences in soil organic matter
and biomass, whereas the variations over time can be explained mainly by
soil temperature and to some extent by differences in biomass. The seasonal
pattern of ecosystem respiration was closely related to that of soil
temperature at all sites, which suggests that temperature was the main
controlling factor for ecosystem respiration. This is in agreement with the
findings of other studies (Bridgham and Richardson, 1992; Han et al.,
2013; Happell and Chanton, 1993; Kandel et al., 2013; Krauss et al., 2012;
Pulliam, 1993). However, biomass respiration also contributed to the
ecosystem respiration rates, particularly late in the season when the
above-ground biomass was highest. At Phrag1, Suaeda1 and Suaeda2, the
seasonal pattern of ecosystem respiration rates correlated to that of the
above-ground biomass, indicating that plant respiration may have constituted
a large part of the total ecosystem respiration at these sites. This is in
agreement with Kandel et al. (2013), who found that plant
respiration contributed about 50 % of the total ecosystem respiration
in a cultivated peatland during the summer months, and Xu et al. (2014), who found ten times higher CO2 emissions from
marshes with plant communities than from those without. Also, the difference
in ecosystem respiration rates between the two Suaeda wetlands corresponded to the
differences in Suaeda biomass. However, at Phrag2 nearly all CO2 emissions
came from the soil and the belowground biomass, since only short stems were
left behind after cutting the reeds in June. At the rice paddy, the
ecosystem respiration peaked in August when the above-ground biomass was only
about 100 g m-2. The above-ground rice biomass continued to increase
after August, but the ecosystem respiration decreased drastically,
indicating that soil respiration constituted the main part of ecosystem
respiration at the rice paddy.
Relationship between salinity and CH4 emission rates
in two Suaeda salsa wetlands, two Phragmites australis wetlands and one rice paddy during 2012 in the Liaohe
Delta, Northeast China. Data points after cutting the vegetation at Phrag2
are represented by downward triangles (Phrag2-cut). Measurements were done
from April to November 2012.
CH4 emission rates and Reco compared to other studies
The CH4 emission rates and seasonal pattern at Phrag2 were similar to
those measured by Huang et al. (2005) from a reed
wetland in the Liaohe delta, where CH4 emission rates varied from -0.97 mg CH4 m-2 h-1
in early May to 2.73 mg CH4 m-2 h-1 in early September. The average CH4 emission rate at Phrag2
was within the range of CH4 emission rates from reed wetlands in other
parts of China, varying from 0.75 mg m-2 h-1 (Xu et
al., 2014) to 5.13 mg m-2 h-1 (Tong et al., 2010). The
Suaeda wetlands had CH4 emission rates very similar to those from a Suaeda salsa marsh in
the Yellow River delta, China, with rates ranging from -0.74 to 0.42 mg m-2 h-1 (Sun et al., 2013). The CH4 emission
rates from the rice paddy in the present study were lower than those
reported from continuously and intermittently flooded rice paddies in
Nanjing, China, which emitted 1–3 mg m-2 h-1 (Zou et
al., 2005). This might be due to temperature differences or differences in
soil characteristics at the two sites.
The yearly cumulative CH4 emissions from Phrag2 were similar to those
reported by Xu et al. (2014) from a coastal saline grass flat
dominated by P. australis in southeast China (6.28 g m-2). However, markedly higher
cumulative CH4 emissions have been measured from other reed wetlands,
such as 39.5 g m-2 from a tidal reed marsh in southeast China
(Tong et al., 2010) and 65.9 g m-2 from a restored reed fen in
northeastern Germany (Koch et al., 2014). The yearly
cumulative CH4 emissions from the rice paddy in our study were about
six times higher than the 0.54–0.58 g m-2 measured from rice paddies in
eastern China (Zhang et al., 2014) but much lower than the 57 g m-2 measured over only 2 months from a rice paddy in the Philippines
(Gaihre et al., 2014). The Suaeda wetlands in our study had no net
CH4 emissions over the sampling period, in contrast to a Suaeda glauca marsh in
southeast China which emitted 0.399 g CH4 m-2 yr-1
(Xu et al., 2014).
The average ecosystem respiration rates in this study were in a comparable
range to those recorded from coastal saline wetlands in southeast China by
Xu et al. (2014). The average CO2 emission rates at Phrag1
were somewhat lower than the 569.7 mg m-2 h-1 from the
Phragmites wetland in their study, whereas the emissions from Phrag2 were higher.
Compared to the Suaeda glauca marsh in Xu et al. (2014), which emitted on
average 248.6 mg CO2 m-2 h-1, Suaeda1 and 2 both had higher
average CO2 emissions.