Carbon dioxide and methane fluxes from mounds of African fungus-growing termites

Abstract

Abstract. Termites play an essential role in decomposing dead plant material in tropical ecosystems and are thus major sources of gaseous C emissions in many environments. In African savannas, fungus-growing termites are among the ecologically most influential termite species. We studied the gas exchange from mounds of two closely related fungus-growing species (Macrotermes subhyalinus and M. michaelseni, respectively) in two habitats representing different vegetation types (grassland, bushland) together with soil fluxes around the mounds. The fluxes from active termite mounds varied from 120 to 2100 mg CO2–C m−2 h−1 for carbon dioxide (CO2) and from 0.06 to 3.7 mg CH4–C m−2 h−1 for methane (CH4) fluxes. Mound CO2 fluxes varied seasonally with a 64 % decrease and 41 % increase in the fluxes from the dry to wet season at the grassland and bushland sites, respectively. During the wet season, the CO2 fluxes were significantly correlated with termite mound volume. The diurnal measurements from two M. michaelseni mounds suggest that the gas fluxes peak during the daytime, possibly reflecting changes in mound internal air circulation. Soil fluxes of both CO2 and CH4 were enhanced at up to 2 m distance from the mounds compared to the local soil respiration, indicating that, in addition to mound ventilation structures, a small proportion of the metabolic gases produced also leave the nest via surrounding soils.

Mound architecture and season affect concentrations of CO2, CH4 and N2O in nests of African fungus‐growing termites

“…To compare nest internal gas concentrations with the mound size, the height and width of each mound were measured in two directions (South–West and North–East). Based on these measured parameters, the volume of the aboveground mound was estimated using the equation (Equation 1) defined in Räsänen et al (2023): where R is the mound radius (mean of the two directions) and H is the height from the estimated ground level (beyond outwash pediment if present) to the mound top.…”

“…There, the plant cell walls are broken down into simple sugars by actions of the Termitomyces symbionts in well‐aerated fungus combs and by the intestinal bacterial flora during two subsequent passages through guts of termite workers (Badertscher et al, 1983; Nobre & Aanen, 2012; Poulsen et al, 2014; Vesala, Arppe, & Rikkinen, 2022). As a result of effective decomposition, termite nests commonly emit an order of magnitude higher amounts of carbon dioxide (CO 2 ) than the surrounding savanna soils (Konaté et al, 2003; Räsänen et al, 2023; Van Asperen et al, 2021).…”

The African Regional Greenhouse Gases Budget (2010–2019)

“…Here, our global termite biomass estimate is based on available field measurements and predicted by a set of variables, including rainfall, soil pH, NPP, minimum/maximum temperature, SOC, and topography. Additionally, only a few studies measured CH 4 emission rates at the individual species or mound scale across the African continent (Table S10 in Supporting Information S1) with CH 4 emission rates varying significantly between species (0.68–17.4 μg CH 4 g −1 hr −1 ), between mounds (81–5,478 ng CH 4 s −1 mound −1 ) (Brauman et al., 2001; Macdonald et al., 1999; Rouland et al., 1993) and between seasons (Räsänen et al., 2023). However, more empirical measurements are still needed to improve the accuracy of termite biomass as well as termite methane emission rates across different ecosystems and regions.…”

The challenge of estimating global termite methane emissions

“…Interspecific variation in methane emissions from epigeal mounds is influenced by several factors including: (1) methanotroph location; oxidation can occur in the mound wall or in soil beneath the mound (Nauer, Hutley, & Arndt, 2018); (2) concentration of termite biomass within the mound and proximity to methanotrophs; for example, nests of Macrotermes michaelseni (Sjöstedt, 1914) are largely located below the ground leaving the mound essentially empty of termites, and instead termites are in direct contact with soil methanotrophs such that methane emissions can be detected from both the mound and surrounding soils (Korb, 2011; Räsänen et al., 2023); (3) the permeability of mound material; less dense, porous material can increase methanotroph abundance in mound material (Chiri et al., 2020) but also aid gas emissions to the atmosphere (Singh et al., 2019); and (4) mound structures can control permeating gases in a trade‐off between gas exchange and thermoregulation (Korb, 2003); mound chimneys, or better connectivity between cavities, improve ventilation and limit time for methane oxidation resulting in greater emissions (Darlington et al., 1997; Sugimoto et al., 1998), while mounds with fewer conduits and limited ventilation may have lower emissions.…”

“…Intraspecific variation in mound emissions is not well documented, yet within species, mound structure can vary greatly between habitats (Fagundes et al., 2021; Korb, 2003, 2011). Gas emissions from mounds vary both diurnally and seasonally (Jamali, Livesley, Dawes, Cook, et al., 2011; Jamali, Livesley, Dawes, Hutley, & Arndt, 2011; Räsänen et al., 2023). Greater methane emissions are linked to enhanced methanogenesis in the termite gut at higher temperatures (Jamali, Livesley, Dawes, Cook, et al., 2011) and increased mound termite biomass during wet seasons (Jamali, Livesley, Dawes, Hutley, & Arndt, 2011).…”