Discussions
This study found that the CO2 emission from the mounds
of non-fungus growing termites were different between species and their
surrounding soils during both dry and wet seasons in a tropical seasonal
forest. The mean annual mound CO2 emission was 2.5 times
higher than surrounding soils. In particular, CO2emissions from the mounds of G. sulphureus and M. crassuswere much higher than other termite species and greater than their
surrounding soils in both seasons. The high CO2emissions were from the mounds from these species because their
populations were found ubiquitously in the nest or mound with a thin
wall, whereas the other species were found in some part of the nest.
Also, most of the nest materials are mixed from the macerated wood and
their faeces (Bignell and Eggleton, 2000). Moreover, G.
sulphureus and M. crassus are a group of wood/litter feeders
that these species may be consumed directly to organic carbon much more
than the soil feeders. Although respiratory rate varied inversely with
body weight, the substrates for all feeding groups have an effect on the
variations in metabolic rate (Jeeva et al., 1999). In the same forest,
these two species were found in high abundance and biomass, especiallyM. crassus was 46% and 36% of the termite abundance and
biomass, respectively (Yamada et al., 2003). Therefore, the mounds ofG. sulphureus and M. crassus could potentially drive the
high spatial variation in soil CO2 emission in the
tropical seasonal forests.
As a result, the mean CO2 emission rate from the mound
of G. sulphureus was higher than the mound of other species in
this study. This emission rate was also higher than the mound from the
other studies in both non – fungus grower (Brümmer et al., 2009; Jamali
et al., 2013; De Gerenyu et al., 2015; Konemann and Kard, 2016) and
fungus – grower termites (Konate´ et al., 2003; Nyamadzawo et al.,
2012; Sawadogo et al., 2012; Ohashi et al., 2017) from the different
topography, except for some rates of CO2 emissions from
the subterranean nest of termites (e.g. Hospitalitermes
hospitalis ) in a tropical rainforest (Ohashi et al., 2017) (Table 3).
In addition, the other soil organism was proposed as the
CO2 point source such as ants and earthworm (Khalil et
al., 1990; Stoyan et al., 2000; Risch et al., 2005; Ohashi et al.,
2007). According to Ohashi et al. (2017) reported that the
CO2 emission rate from the subterranean nest of antDinomyrmex gigas (45.5 µmol CO2m-2 s-1) was higher than the rate in
this study. However, the CO2 emission rate from G.
sulphureus mound in this study that was higher than soil
CO2 emission which affected by the other ants
(Sousa-Souto et al., 2012; Hasin et al., 2014; Fernandez-Bou et al.,
2019) and earthworms (Šimek and Pižl, 2010) (Table 3). In term of
earthworms, although there were no effect on the emission of
CO2 from soil (Guo et al., 2019), earthworms have
increased the carbon content and other nutrients (Lavelle et al., 2001)
for enhancing soil respiration by microbial activities.
Meanwhile, the CO2 emissions from T. propinquus ,D. makhamensis , and T. comis mounds were both much lower
than G. sulphureus and M. crassus and were significantly
lower than surrounding soils in both seasons (except for T. comiswhich did not differ between surrounding soils in the wet season).
However, these three species are categorized as soil-feeder, whileG. sulphureus and M. crassus are wood/litter-feeder (Wood,
1976; Collins, 1989; Bignell et al., 1997; Bignell and Eggleton, 2000).
This was probably due to the respiration rate from termite populations
of soil-feeders was lower than wood/litter-feeders. According to Yamada
et al. (2005) Termite individual contributions to carbon mineralization
by the respiration from wood/litter-feeders and soil-feeders were 2.8%
and 1.2% of the annual aboveground litterfall in the DEF at SERS,
respectively.
Mound structure differences between species may also contribute to
variation in mound CO2 emissions. Nest construction
material as the wood carton (a mixture of faeces and macerated wood) is
used for the most wood/litter-feeding group, while many soil-feeders use
faeces mixed with topsoil (Bignell and Eggleton, 2000). In addition, the
soil feeder epigeal mounds may not contain the entire colony population
above the mound because most individuals move below the soil for
foraging and to avoid adverse environmental conditions (Fig 8).
Moreover, the mound CO2 emission rates of T.
propinquus , and D. makhamensis were similar to those reported by
Song et al. (2013) who reported that the range of CO2emission from termite mounds (species: not given) was from 1.63 to 3.71
µmol CO2 m-2 s-1 and
these species did not affect soil CO2 emission in the
tropical rainforest. As the result of Song et al. (2013), mounds were
either typical soil-feeding group mounds or similar to the fungus
growing termites that build a dome-shaped mound with thick walls (20-40
cm thickness) and several branching underground passages (Inoue et al.,
2001).
Previous studies have reported that the mound-builder and subterranean
nest of termites emitted significantly higher CO2 than
typical soils (e.g. Konate et al., 2003; Brümmer et al., 2009; Risch et
al., 2012; De Gerenyu et al., 2015; Ohashi et al., 2017). Not only
CO2 emission from termite individuals, but their nest
also contributed to soil respiration. According to Hu et al. (2017)
reported that CO2 emission from termite nest soil was
higher than typical soil, due to termite nest soils had higher dissolved
organic carbon concentration and C/N ratio, accelerating for microbial
activities. In a tropical savanna, the fungus-growing termite was
emitted 10-19 µmol CO2 m-2s-1 from the mound which compared to 5-10 µmol
CO2 m-2 s-1 from
surrounding soils (Konate et al., 2003). In a tropical rain forest, the
mean of CO2 emission from termite mound was reached up
to 27.9 µmol CO2 m-2s-1 that much higher than the surrounding soil (3.96
µmol CO2 m-2 s-1)
(Ohashi et al., 2017). An earlier study (Ohashi et al., 2007) indicated
that the rate of soil respiration was sometimes extremely high
(>10 µmol CO2 m-2s-1) contributing 10% to the total soil respiration
in the tropical rainforest. De Gerenyu et al. (2015) showed that the
mound CO2 emission of Odontotermes termites
(fungus grower) and G. sulphureus (non-fungus grower) was 2.0-2.5
times higher than the background soil during both dry and wet seasons.
Also, those termite mounds contributed up to 10% of the total soil
respiration in a tropical monsoon forest of southern Vietnam, showing
the maximum rates of the mound CO2 emissions were 17 and
20 µmol CO2 m-2 s-1from Odontotermes and G. sulphureus , respectively. On the
other hand, mound CO2 emission rate was 2.5 times lower than the
surrounding soil that was affected by termite respiration through their
underground passages in the surrounding soil (Boonriam et al., 2021b).
The relationship between soil CO2 emission and its
environmental factors was also observed from only soil around each
mound, while soil temperature and soil moisture content were not
measured from the mounds to avoid disturbing termite activity at the
nest. In this study, soil CO2 emission rate was 3.4
times higher in the wet season than the dry season. Soil
CO2 emissions showed a consistent relationship with soil
moisture content. In addition, soil CO2 emission rate
was started to decrease when soil moisture content peaked more than 18%
(Adachi et al., 2009) and 21% (Hasin et al., 2014) in diurnal variation
of Thai tropical forest. A previous study reported that the rate started
to drop at 27 °C soil temperature and 21% soil moisture content in the
same topographical forest (Boonriam et al., 2021a). Although
precipitation influences soil microbial activities accelerating soil
respiration, high precipitation could inhibit CO2emission by a barrier of high moisture content in the soil (Sotta et
al., 2004; Wood et al., 2013). This inconstancy within the timing and
magnitude of precipitation occasions can influence soil respiration.