Methane emission by termites and oxidation by soils, across a forest disturbance gradient in the Mbalmayo Forest Reserve, Cameroon

Methane fluxes were measured, using static chambers, across a disturbance gradient in a West African semi-deciduous humid forest. Soil-feeding termite biomass was simultaneously determined, in an attempt to examine its influence on the net soil-atmosphere exchange of CH₄. CH₄ emission rates from individual termite species were determined under laboratory conditions, permitting the gross production of CH₄ to be compared with net fluxes to the atmosphere. Both net CH₄ oxidation(-) and emission were observed, and CH₄ fluxes ranged from – 24.6 to 40.7 ng m^–2 s^–1. A statistically significant relationship between termite biomass and CH₄ flux was observed across the forested sites such that: CH₄ flux (ng m^–2 s^–1) = 4.95 × termite biomass (gm^–2)–10.9 (P < 0.001). Rates of CH₄ oxidation were on average 60% smaller at the clearfelled and Terminalia plantation sites than at the near-primary forest site. Two of the disturbed sites were net CH₄ sources during one of the sampling periods. Disturbance of tropical forests, resulting in a decrease in the CH₄ sink capacity of the soil, may therefore increase the contribution of termite-derived CH₄ to the atmosphere. Measurements from the mounds of the soil-feeding termites Thoracotermes macrothorax and Cubitermes fungifaber from the old plantation site gave a CH₄ emission of 636 and 53.4 ng s^–1 mound^–1, respectively. The forest floor surrounding the mounds was sampled in three concentric bands. Around the mound of T. macrothorax the soil was a net source of CH₄ estimated to contribute a further 148 ng s^–1. Soil surrounding the mound of C. fungifaber was mostly a net sink. The mounds of soil-feeding termites are point sources of CH₄, which at the landscape scale may exceed the general sink capacity of the soil, to an extent dependent on seasonal variations in soil moisture and level of disturbance.     

The Importance of Termites to the CH<Subscript>4</Subscript> Balance of a Tropical Savanna Woodland of Northern Australia

Termites produce methane (CH₄) as a by-product of microbial metabolism of food in their hindguts, and are one of the most uncertain components of the regional and global CH₄ exchange estimates. This study was conducted at Howard Springs near Darwin, and presents the first estimate of CH₄ emissions from termites based on replicated in situ seasonal flux measurements in Australian savannas. Using measured fluxes of CH₄ between termite mounds and the atmosphere, and between soil and the atmosphere across seasons we determined net CH₄ flux within a tropical savanna woodland of northern Australia. By accounting for both mound-building and subterranean termite colony types, and estimating the contribution from tree-dwelling colonies it was calculated that termites were a CH₄ source of +0.24 kg CH₄-C ha⁻¹ y⁻¹ and soils were a CH₄ sink of −1.14 kg CH₄-C ha⁻¹ y⁻¹. Termites offset 21% of CH₄ consumed by soil resulting in net sink strength of −0.90 kg CH₄-C ha⁻¹ y⁻¹ for these savannas. For Microcerotermes nervosus (Hill), the most abundant mound-building termite species at this site, mound basal area explained 48% of the variation in mound CH₄ flux. CH₄ emissions from termites offset 0.1% of the net biome productivity (NBP) and CH₄ consumption by soil adds 0.5% to the NBP of these tropical savannas at Howard Springs.     

Soil CO2 Emissions Associated with Termitaria in Tropical Savanna: Evidence for Hot-Spot Compensation

Our understanding of carbon (C) dynamics within savannas is very limited, especially how source/sink dynamics are influenced by the resident biota. Previous measurements of epigeal termite mounds (termitaria), ubiquitous in many savannas, have shown that they are considerable point sources of soil carbon dioxide (CO₂), whereas CO₂ measurements collected outside the mounds were generally assumed to be independent of termite activity. However, no measurements were conducted along gradients away from the mounds to confirm this. We quantified daytime soil CO₂ emissions (soil respiration) along gradients from the center to 20?m from the mound edge in Serengeti National Park, and measured soil temperature/moisture, macro-invertebrate abundance, and vegetation height as variables potentially influencing these emissions. Further, we quantified how far into the savanna termitaria impact CO₂ emissions. As in other studies, we found the highest soil CO₂ fluxes at the termitaria-center and considerably lower fluxes in the surrounding savanna. Macro-invertebrate abundance was associated with the differences in emissions measured, whereas the other variables were not. The analysis of spatial autocorrelation revealed significantly lower fluxes between the termitaria edge and up to 9?m from the edge compared to the values measured at the termitaria-center and between 10 and 20?m from the termitaria edge. When extrapolating the emissions across the landscape our results suggest that the lower CO₂ emissions found between the edge and 9?m fully compensate for the high fluxes measured at the termitaria center. Consequently, our findings provide evidence that termitaria might influence the savanna C source-sink dynamics differently than previously thought.     

The effect of termite biomass and anthropogenic disturbance on the CH4 budgets of tropical forests in Cameroon and Borneo

The exchange of CH₄ between tropical forests and the atmosphere was determined by simultaneously measuring the net CH₄ flux at the soil surface and assessing the flux contribution from soil-feeding termite biomass, both within the soil profile and in mounds. In Cameroon the flux of CH₄ ranged from a net emission of 40.7 ng m^–2 s^–1 to a net CH₄ oxidation of –53.0 ng m^–2 s^–1. Soil-inhabiting termite biomass was significantly correlated with CH₄ flux. Termite mounds emitted up to 2000 ng s^–1 mound^–1. Termite-derived CH₄ emission reduced the soil sink strength by up to 28%. Disturbance also had a strong effect on the soil sink strength, with the average rate of CH₄ oxidation, at – 17.5 ng m^–2 s^–1, being significantly smaller (≈ 36%) at the secondary forest site than the –27.2 ng m^–2 s^–1, observed at the primary forest site. CH₄ budgets calculated for each site indicated that both forests were net sinks for CH₄ at – 6.1 kg ha^–1 y^–1 in the near-primary forest and – 3.1 kg ha^–1 y^–1 in the secondary forest. In Borneo, three forest sites representing a disturbance gradient were examined. CH₄ oxidation rates ranged from 0 to – 32.1 ng m^–2s^–1 and a significant correlation between the net flux and termite biomass was observed only in an undisturbed primary forest, although the biomass was insufficient to cause net emission of CH₄. Rates of CH₄ oxidation were not significantly different across the disturbance gradient but were, however, larger in the primary forest (averaging – 15.4 ng m^–2 s^–1) than in an old-growth secondary forest (–13.9 ng m^–2s^–1) and a young secondary re-growth (– 10.8 ng m^–2s^–1). CH₄ flux from termite mounds ranged from net oxidation in an abandoned mound to a maximum emission of 468 ng s^–1 mound^–1. CH₄ budgets calculated for each site indicated that CH₄ flux from termite mounds had an insignificant effect on the budget of CH₄ at the regional scale at all three forest sites. Annual oxidation rates were – 4.8, – 4.2 and – 3.4 kg ha^–1 y^–1 in the primary, secondary and young secondary forests, respectively.     

Tree stem bases are sources of CH4 and N2O in a tropical forest on upland soil during the dry to wet season transition

Tropical forests on upland soils are assumed to be a methane (CH₄) sink and a weak source of nitrous oxide (N₂O), but studies of wetland forests have demonstrated that tree stems can be a substantial source of CH₄, and recent evidence from temperate woodlands suggests that tree stems can also emit N₂O. Here, we measured CH₄ and N₂O fluxes from the soil and from tree stems in a semi‐evergreen tropical forest on upland soil. To examine the influence of seasonality, soil abiotic conditions and substrate availability (litter inputs) on trace greenhouse gas (GHG) fluxes, we conducted our study during the transition from the dry to the wet season in a long‐term litter manipulation experiment in Panama, Central America. Trace GHG fluxes were measured from individual stem bases of two common tree species and from soils beneath the same trees. Soil CH₄ fluxes varied from uptake in the dry season to minor emissions in the wet season. Soil N₂O fluxes were negligible during the dry season but increased markedly after the start of the wet season. By contrast, tree stem bases emitted CH₄ and N₂O throughout the study. Although we observed no clear effect of litter manipulation on trace GHG fluxes, tree species and litter treatments interacted to influence CH₄ fluxes from stems and N₂O fluxes from stems and soil, indicating complex relationships between tree species traits and decomposition processes that can influence trace GHG dynamics. Collectively, our results show that tropical trees can act as conduits for trace GHGs that most likely originate from deeper soil horizons, even when they are growing on upland soils. Coupled with the finding that the soils may be a weaker sink for CH₄ than previously thought, our research highlights the need to reappraise trace gas budgets in tropical forests.     

Spatial variability and abiotic determinants of termite mounds throughout a savanna catchment

Termite mounds contribute to the spatial heterogeneity of ecological processes in many savannas, but the underlying patterns and determinants of mound distributions remain poorly understood. Using the Carnegie Airborne Observatory (CAO), we mapped the distribution of termite mounds across a rainfall gradient within a river catchment (～ 27 000 ha) of the Kruger National Park, South Africa. We assessed how different factors were associated with the distribution and height of termite mounds at three spatial scales: the entire catchment, among three broad vegetation types, and on individual hillslope crests. Abiotic factors such as the underlying geology and mean annual precipitation shaped mound densities at broad scales, while local hillslope morphology strongly influenced mound distribution at finer scales, emphasising the importance of spatial scale when assessing mound densities. Fire return period had no apparent association with mound densities or height. Mound density averaged 0.46 mounds ha^?1, and exhibited a clustered pattern throughout the landscape, occurring at relatively high densities (up to 2 mounds ha^?1) on crests, which are nutrient‐poor elements of the landscape. Mounds exhibited significant over‐dispersion (even spacing) at scales below 60 m so that evenly spaced aggregations of termite mounds are embedded within a landscape of varying mound densities. The tallest mounds were found in dry savanna (500 mm yr^?1) and were positively correlated with mound density, suggesting that dry granitic savannas are ideal habitat for mound‐building termites. Mound activity status also varied significantly across the rainfall gradient, with a higher proportion of active (live) mounds in the drier sites. The differential spacing of mounds across landscapes provides essential nutrient hotspots in crest locations, potentially sustaining species that would otherwise not persist. The contribution to biodiversity and ecosystem functioning that mounds provide is not uniform throughout landscapes, but varies considerably with spatial scale and context.     

Termite mounds differ in their importance for herbivores across savanna types,seasons and spatial scales

Herbivores do not forage uniformly across landscapes, but select for patches of higher nutrition and lower predation risk. Macrotermes mounds contain higher concentrations of soil nutrients and support grasses of higher nutritional value than the surrounding savanna matrix, attracting mammalian grazers that preferentially forage on termite mound vegetation. However, little is known about the spatial extent of such termite influence on grazing patterns and how it might differ in time and space. We measured grazing intensity in three African savanna types differing in rainfall and foliar nutrients and predicted that the functional importance of mounds for grazing herbivores would increase as the difference in foliar nutrient levels between mound and savanna matrix grasses increases and the mounds become more attractive. We expected this to occur in nutrient‐poor areas and during the dry season when savanna matrix grass nutrient levels are lower. Tuft use and grass N and P content were measured along transects away from termite mounds, enabling calculation of the spatial extent of termite influence on mammalian grazing. Using termite mound densities estimated from airborne light detection and ranging (LiDAR), we further upscaled field‐based results to determine the percentage of the landscape influenced by termite activity. Grasses in close proximity to termite mounds were preferentially grazed at all sites and in both seasons, but the strength of mound influence varied between savanna types and seasons. In the wet season, mounds had a relatively larger effect on grazers at the landscape scale in the nutrient‐poor, wetter savanna, whereas in the dry season the pattern was reversed with more of the landscape influenced at the nutrient‐rich, driest site. Our results reveal that termite mounds enhance the value of savanna landscapes for herbivores, but that their functional importance varies across savanna types and seasons.     

Soil-atmospheric exchange of CO2, CH4, and N2O in three subtropical forest ecosystems in southern China

The magnitude, temporal, and spatial patterns of soil‐atmospheric greenhouse gas (hereafter referred to as GHG) exchanges in forests near the Tropic of Cancer are still highly uncertain. To contribute towards an improvement of actual estimates, soil‐atmospheric CO₂, CH₄, and N₂O fluxes were measured in three successional subtropical forests at the Dinghushan Nature Reserve (hereafter referred to as DNR) in southern China. Soils in DNR forests behaved as N₂O sources and CH₄ sinks. Annual mean CO₂, N₂O, and CH₄ fluxes (mean±SD) were 7.7±4.6 Mg CO₂‐C ha^?1 yr^?1, 3.2±1.2 kg N₂O‐N ha^?1 yr^?1, and 3.4±0.9 kg CH₄‐C ha^?1 yr^?1, respectively. The climate was warm and wet from April through September 2003 (the hot‐humid season) and became cool and dry from October 2003 through March 2004 (the cool‐dry season). The seasonality of soil CO₂ emission coincided with the seasonal climate pattern, with high CO₂ emission rates in the hot‐humid season and low rates in the cool‐dry season. In contrast, seasonal patterns of CH₄ and N₂O fluxes were not clear, although higher CH₄ uptake rates were often observed in the cool‐dry season and higher N₂O emission rates were often observed in the hot‐humid season. GHG fluxes measured at these three sites showed a clear increasing trend with the progressive succession. If this trend is representative at the regional scale, CO₂ and N₂O emissions and CH₄ uptake in southern China may increase in the future in light of the projected change in forest age structure. Removal of surface litter reduced soil CO₂ effluxes by 17–44% in the three forests but had no significant effect on CH₄ absorption and N₂O emission rates. This suggests that microbial CH₄ uptake and N₂O production was mainly related to the mineral soil rather than in the surface litter layer.     

Termite mounds contain soil-derived methanotroph communities kinetically adapted to elevated methane concentrations

Termite mounds have recently been confirmed to mitigate approximately half of termite methane (CH₄) emissions, but the aerobic CH₄ oxidising bacteria (methanotrophs) responsible for this consumption have not been resolved. Here, we describe the abundance, composition and CH₄ oxidation kinetics of the methanotroph communities in the mounds of three distinct termite species sampled from Northern Australia. Results from three independent methods employed show that methanotrophs are rare members of microbial communities in termite mounds, with a comparable abundance but distinct composition to those of adjoining soil samples. Across all mounds, the most abundant and prevalent methane monooxygenase sequences were affiliated with upland soil cluster α (USCα), with sequences homologous to Methylocystis and tropical upland soil cluster (TUSC) also detected. The reconstruction of a metagenome-assembled genome of a mound USCα representative highlighted the metabolic capabilities of this group of methanotrophs. The apparent Michaelis–Menten kinetics of CH₄ oxidation in mounds were estimated from in situ reaction rates. Methane affinities of the communities were in the low micromolar range, which is one to two orders of magnitude higher than those of upland soils, but significantly lower than those measured in soils with a large CH₄ source such as landfill cover soils. The rate constant of CH₄ oxidation, as well as the porosity of the mound material, were significantly positively correlated with the abundance of methanotroph communities of termite mounds. We conclude that termite-derived CH₄ emissions have selected for distinct methanotroph communities that are kinetically adapted to elevated CH₄ concentrations. However, factors other than substrate concentration appear to limit methanotroph abundance and hence these bacteria only partially mitigate termite-derived CH₄ emissions. Our results also highlight the predominant role of USCα in an environment with elevated CH₄ concentrations and suggest a higher functional diversity within this group than previously recognised.Subject terms: Soil microbiology, Biogeochemistry     

Carbon dioxide fluxes in a spatially and temporally heterogeneous temperate grassland

Landscape position, grazing, and seasonal variation in precipitation and temperature create spatial and temporal variability in soil processes, and plant biomass and composition in grasslands. However, it is unclear how this variation in plant and soil properties affects carbon dioxide (CO₂) fluxes. The aim of this study is to explore the effect of grazing, topographic position, and seasonal variation in soil moisture and temperature on plant assimilation, shoot and soil respiration, and net ecosystem CO₂ exchange (NEE). Carbon dioxide fluxes, vegetation, and environmental variables were measured once a month inside and outside long-term ungulate exclosures in hilltop (dry) to slope bottom (mesic) grassland throughout the 2004 growing season in Yellowstone National Park. There was no difference in vegetation properties and CO₂ fluxes between the grazed and the ungrazed sites. The spatial and temporal variability in CO₂ fluxes were related to differences in aboveground biomass and total shoot nitrogen content, which were both related to variability in soil moisture. All sites were CO₂ sinks (NEE>0) for all our measurments taken throughout the growing season; but CO₂ fluxes were four- to fivefold higher at sites supporting the most aboveground biomass located at slope bottoms, compared to the sites with low biomass located at hilltops or slopes. The dry sites assimilated more CO₂ per gram aboveground biomass and stored proportionally more of the gross-assimilated CO₂ in the soil, compared to wet sites. These results indicate large spatio-temporal variability of CO₂ fluxes and suggest factors that control the variability in Yellowstone National Park.     

Nutrient dynamics and plant assemblages of Macrotermes falciger mounds in a savanna ecosystem

Termites through mound construction and foraging activities contribute significantly to carbon and nutrient fluxes in nutrient-poor savannas. Despite this recognition, studies on the influence of termite mounds on carbon and nitrogen dynamics in sub-tropical savannas are limited. In this regard, we examined soil nutrient concentrations, organic carbon and nitrogen mineralization in incubation experiments in mounds of Macrotermes falciger and surrounding soils of sub-tropical savanna, northeast Zimbabwe. We also addressed whether termite mounds altered the plant community and if effects were similar across functional groups i.e. grasses, forbs or woody plants. Mound soils had significantly higher silt and clay content, pH and concentrations of calcium (Ca), magnesium (Mg), potassium (K), organic carbon (C), ammonium (NH₄⁺) and nitrate (NO₃⁻) than surrounding soils, with marginal differences in phosphorus (P) and sodium (Na) between mounds and matrix soils. Nutrient enrichment increased by a factor ranging from 1.5 for C, 4.9 for Mg up to 10.3 for Ca. Although C mineralization, nitrification and nitrification fraction were similar between mounds and matrix soils, nitrogen mineralization was elevated on mounds relative to surrounding matrix soils. As a result, termite mounds supported unique plant communities rich and abundant in woody species but less diverse in grasses and forbs than the surrounding savanna matrix in response to mound-induced shifts in soil parameters specifically increased clay content, drainage and water availability, nutrient status and base cation (mainly Ca, Mg and Na) concentration. In conclusion, by altering soil properties such as texture, moisture content and nutrient status, termite mounds can alter the structure and composition of sub-tropical savanna plant communities, and these results are consistent with findings in other savanna systems suggesting that increase in soil clay content, nutrient status and associated changes in the plant community assemblage may be a general property of mound building termites.     

The architecture of termite mounds: a result of a trade-off between thermoregulation and gas exchange?

We examined the influence of gas exchange on the architectureof termite mounds. In Comoé National Park (Côted'Ivoire), Macrotermes bellicosus builds, as an adaptation toambient temperature conditions, differently shaped mounds inthe shrub savanna and the gallery forest. Previous studies suggestedthat there might be a constraint that limits the degree of thermalinsulation of the interior (i.e., nest) of the mounds in environmentswith relatively low ambient temperatures. This factor causes,in proximate terms, suboptimal low nest temperatures and ultimatelyleads to reduced reproductive success in the gallery forest.In this study, we examined whether the necessity for gas exchangemight constrain mound architecture. We measured CO₂ concentrationsin the air channels of mounds in different habitats and undermanipulated temperature regimes. During both the dry and therainy season we found higher CO₂ concentrations in mounds ofthe gallery forest than in mounds of the savanna. Additional measurementsin forest mounds, architecturally resembling those of the savanna dueto an experimental increase in ambient temperatures, revealedlower CO₂ concentrations than unmanipulated mounds in this habitat. Generally,concentrations were higher during the rainy season comparedto the dry season and lower during day than during night. Summarizingthese results we present a model that illustrates this trade-offbetween thermoregulation and gas exchange under different temperatureregimes. Both factors together result in different mound architecturesunder different environmental temperatures and may finally limitthe distribution of this species.     

Fluxes of N2O and CH4 from soils of savannas and seasonally-dry ecosystems

Aim Savannas and seasonally‐dry ecosystems cover a significant part of the world's land surface. If undisturbed, these ecosystems might be expected to show a net uptake of methane (CH₄) and a limited emission of nitrous oxide (N₂O). Land management has the potential to change dramatically the characteristics and gas exchange of ecosystems. The present work investigates the contribution of warm climate seasonally‐dry ecosystems to the atmospheric concentration of nitrous oxide and methane, and analyses the impact of land‐use change on N₂O and CH₄ fluxes from the ecosystems in question. Location Flux data reviewed here were collected from the literature; they come from savannas and seasonally‐dry ecosystems in warm climatic regions, including South America, India, Australasia and Mediterranean areas. Methods Data on gas fluxes were collected from the literature. Two factors were considered as determinants of the variation in gas fluxes: land management and season. Land management was grouped into: (1) control, (2) ‘burned only’ and (3) managed ecosystems. The season was categorized as dry or wet. In order to avoid the possibility that the influence of soil properties on gas fluxes might confound any differences caused by land management, sites were grouped in homogeneous clusters on the basis of soil properties, using multivariate analyses. Inter‐ and intra‐cluster analysis of gas fluxes were performed, taking into account the effects of season, land management and main vegetation types. Results Soils were often acid and nutrient‐poor, with low water retention. N₂O emissions were generally very low (median flux 0.32 mg N₂O m^?2 day^?1), and no significant differences were observed between woodland savannas and managed savannas. The highest fluxes (up to 12.9 mg N₂O m^?2 day^?1) were those on relatively fertile soils with high air‐filled porosity and water retention. The effect of season on N₂O production was evident only when sites were separated in homogeneous groups on the basis of soil properties. CH₄ fluxes varied over a wide range (?22.9 to 3.15 mg CH₄ m^?2 day^?1, where the negative sign denotes removal of gas from the atmosphere), with an annual average daily flux of ?0.48 ± 0.96 (SD) mg CH₄ m^?2 day^?1 in undisturbed (control) sites. Land‐use change dramatically reduced this CH₄ sink. Managed sites were weak sinks of CH₄ in the dry season and became sources of CH₄ in the wet season. This was particularly evident for pastures. Burning alone did not reduce soil net CH₄ oxidation, but decreased N₂O production. Main conclusions Despite the low potential for N₂O production, both in natural and managed conditions, tropical seasonally‐dry ecosystems represent a significant source of N₂O (4.4 Tg N₂O year^?1) on a global scale, as a consequence of the large area they occupy. The same environments represent a potential CH₄ sink of 5.17 Tg CH₄ year^?1. However, assuming that c. 30% of the tropical land is converted to different uses, the sink would be reduced to 3.2 Tg CH₄ year^?1. The limited information on fluxes from Mediterranean ecosystems does not allow a meaningful scaling up.     

‘Magnetic’ termite mounds: is their unique shape an adaptation to facilitate gas exchange and improve food storage?

Social insects can build impressive nest mounds but the functional significance of their architecture is rarely studied in experiments. The ‘magnetic’ termite mounds of monsoonal northern Australia built by Amitermes meridionalis are notable for their elongated wedge shape and north–south axial orientation. We tested whether the shape is an adaptation to facilitate gas exchange and the preservation of food stores by two experimental manipulations of mounds in situ covering all seasons. First, mounds were shaded to limit drying after rain and second, mound shape was amended from wedge to (approximate) sphere. Food storage, fungal contamination, and internal CO₂ concentration were unaffected by manipulation, but showed a distinct seasonal dynamic, with storage peaking towards the onset of rains and fungal load towards the end of the rainy season. Internal CO₂ concentrations were subject to a diurnal cycle, but also showed elevation during rains. We propose that one advantage of the wedge shape is the efficient use of building effort to achieve good passive ventilation for the food storage areas.     

Land-Use Change and Biogeochemical Controls of Methane Fluxes in Soils of Eastern Amazonia

Tropical soils account for 10%–20% of the 15–35 Tg of atmospheric methane (CH₄) consumed annually by soils, although tropical deforestation could be changing the soil sink. The objectives of this study were (a) to quantify differences in soil CH₄ fluxes among primary forest, secondary forest, active pasture, and degraded pasture in eastern Amazonia; and (b) to investigate controlling mechanisms of CH₄ fluxes, including N availability, gas-phase transport, and soil respiration. At one ranch, Fazenda Vitória, annual uptake estimates (kg CH₄ha⁻¹ y⁻¹) based on monthly measurements were: primary forest, 2.1; secondary forest, 1.0; active pasture, 1.3; degraded pasture, 3.1. The lower annual uptake in the active pasture compared with the primary forest was due to CH₄ production during the wet season in the pasture soils, which is consistent with findings from other studies. In contrast, the degraded pasture was never a CH₄ source. Expressing uptake as a negative flux and emission as a positive flux, CH₄ fluxes were positively correlated with CO₂ fluxes, indicating that root and microbial respiration in the productive pastures, and to a lesser extent in the primary forest, contributed to the formation of anaerobic microsites where CH₄ was produced, whereas this productivity was absent in the degraded pasture. In all land uses, uptake rates of atmospheric CH₄ were greater in the dry season than in the wet season, indicating the importance of soil water content and gas transport on CH₄ fluxes. These clay soils had low annual uptake rates relative to reported rates on sandy soils, which also is consistent with gas transport within the soil being a limiting factor. Nitrogen availability indices did not correlate with CH₄ fluxes, indicating that inhibition of CH₄ oxidation was not an important mechanism explaining differences among land uses. At another ranch, Fazenda Agua Parada, no significant effect of pasture age was observed along a chronosequence of pasture ages. We conclude that land-use change can either increase or decrease the soil sink of CH₄, depending on the duration of wet and dry seasons, the effects of seasonal precipitation on gas-phase transport, and the phenology and relative productivity of the vegetation in each land use.     

华南地区尾巨桉和马占相思人工林地表温室气体通量

为探索华南地区尾巨桉人工林和马占相思人工林地表温室气体的季节排放规律、排放通量和主控因子,采用静态箱-气相色谱法,对两种林型地表3种温室气体(CO_2、CH_4、N_2O)通量进行为期1年的逐月测定。结果表明:(1)尾巨桉人工林和马占相思人工林均为CO_2和N_2O的排放源,CH_4的吸收汇。马占相思林地表N_2O通量显著(P0.01)高于尾巨桉林,CO_2通量和CH_4通量无明显差异。(2)两种林型3种温室气体通量有着相似季节变化规律,地表CO_2通量均呈现雨季高旱季低的单峰规律;地表CH_4吸收通量表现为旱季高雨季低的单峰趋势;地表N_2O通量呈现雨季高旱季低且雨季内有两个峰值的排放规律。(3)地表CO_2、N_2O通量和土壤5 cm温度呈极显著(P0.01)正相关,3种温室气体地表通量同土壤含水量呈极显著(P0.01)或显著相关(P0.05)。(4)尾巨桉林和马占相思林温室气体年温室气体排放总量为31.014 t/hm~2和28.782 t/hm~2,均以CO_2排放占绝对优势(98.46%—99.15%),CH_4和N_2O处于次要地位。     

CO2 flux from soil in pastures and forests in southwestern Amazonia

Stocks of carbon in Amazonian forest biomass and soils have received considerable research attention because of their potential as sources and sinks of atmospheric CO₂. Fluxes of CO₂ from soil to the atmosphere, on the other hand, have not been addressed comprehensively in regard to temporal and spatial variations and to land cover change, and have been measured directly only in a few locations in Amazonia. Considerable variation exists across the Amazon Basin in soil properties, climate, and management practices in forests and cattle pastures that might affect soil CO₂ fluxes. Here we report soil CO₂ fluxes from an area of rapid deforestation in the southwestern Amazonian state of Acre. Specifically we addressed (1) the seasonal variation of soil CO₂ fluxes, soil moisture, and soil temperature; (2) the effects of land cover (pastures, mature, and secondary forests) on these fluxes; (3) annual estimates of soil respiration; and (4) the relative contributions of grass‐derived and forest‐derived C as indicated by δ¹³CO₂. Fluxes were greatest during the wet season and declined during the dry season in all land covers. Soil respiration was significantly correlated with soil water‐filled pore space but not correlated with temperature. Annual fluxes were higher in pastures compared with mature and secondary forests, and some of the pastures also had higher soil C stocks. The δ¹³C of CO₂ respired in pasture soils showed that high respiration rates in pastures were derived almost entirely from grass root respiration and decomposition of grass residues. These results indicate that the pastures are very productive and that the larger flux of C cycling through pasture soils compared with forest soils is probably due to greater allocation of C belowground. Secondary forests had soil respiration rates similar to mature forests, and there was no correlation between soil respiration and either forest age or forest biomass. Hence, belowground allocation of C does not appear to be directly related to the stature of vegetation in this region. Variation in seasonal and annual rates of soil respiration of these forests and pastures is more indicative of flux of C through the soil rather than major net changes in ecosystem C stocks.     

Environmental controls of temporal and spatial variability in CO2 and CH4 fluxes in a neotropical peatland

Tropical peatlands play an important role in the global storage and cycling of carbon (C) but information on carbon dioxide (CO₂) and methane (CH₄) fluxes from these systems is sparse, particularly in the Neotropics. We quantified short and long‐term temporal and small scale spatial variation in CO₂ and CH₄ fluxes from three contrasting vegetation communities in a domed ombrotrophic peatland in Panama. There was significant variation in CO₂ fluxes among vegetation communities in the order Campnosperma panamensis > Raphia taedigera > Cyperus. There was no consistent variation among sites and no discernible seasonal pattern of CH₄ flux despite the considerable range of values recorded (e.g. ?1.0 to 12.6 mg m^?2 h^?1 in 2007). CO₂ fluxes varied seasonally in 2007, being greatest in drier periods (300–400 mg m^?2 h^?1) and lowest during the wet period (60–132 mg m^?2 h^?1) while very high emissions were found during the 2009 wet period, suggesting that peak CO₂ fluxes may occur following both low and high rainfall. In contrast, only weak relationships between CH₄ flux and rainfall (positive at the C. panamensis site) and solar radiation (negative at the C. panamensis and Cyperus sites) was found. CO₂ fluxes showed a diurnal pattern across sites and at the Cyperus sp. site CO₂ and CH₄ fluxes were positively correlated. The amount of dissolved carbon and nutrients were strong predictors of small scale within‐site variability in gas release but the effect was site‐specific. We conclude that (i) temporal variability in CO₂ was greater than variation among vegetation communities; (ii) rainfall may be a good predictor of CO₂ emissions from tropical peatlands but temporal variation in CH₄ does not follow seasonal rainfall patterns; and (iii) diurnal variation in CO₂ fluxes across different vegetation communities can be described by a Fourier model.     

Radiative forcing of methane fluxes offsets net carbon dioxide uptake for a tropical flooded forest

Wetlands are important sources of methane (CH₄) and sinks of carbon dioxide (CO₂). However, little is known about CH₄ and CO₂ fluxes and dynamics of seasonally flooded tropical forests of South America in relation to local carbon (C) balances and atmospheric exchange. We measured net ecosystem fluxes of CH₄ and CO₂ in the Pantanal over 2014–2017 using tower‐based eddy covariance along with C measurements in soil, biomass and water. Our data indicate that seasonally flooded tropical forests are potentially large sinks for CO₂ but strong sources of CH₄, particularly during inundation when reducing conditions in soils increase CH₄ production and limit CO₂ release. During inundation when soils were anaerobic, the flooded forest emitted 0.11 ± 0.002 g CH₄‐C m^?2 d^?1 and absorbed 1.6 ± 0.2 g CO₂‐C m^?2 d^?1 (mean ± 95% confidence interval for the entire study period). Following the recession of floodwaters, soils rapidly became aerobic and CH₄ emissions decreased significantly (0.002 ± 0.001 g CH₄‐C m^?2 d^?1) but remained a net source, while the net CO₂ flux flipped from being a net sink during anaerobic periods to acting as a source during aerobic periods. CH₄ fluxes were 50 times higher in the wet season; DOC was a minor component in the net ecosystem carbon balance. Daily fluxes of CO₂ and CH₄ were similar in all years for each season, but annual net fluxes varied primarily in relation to flood duration. While the ecosystem was a net C sink on an annual basis (absorbing 218 g C m^?2 (as CH₄‐C + CO₂‐C) in anaerobic phases and emitting 76 g C m^?2in aerobic phases), high CH₄ effluxes during the anaerobic flooded phase and modest CH₄ effluxes during the aerobic phase indicate that seasonally flooded tropical forests can be a net source of radiative forcings on an annual basis, thus acting as an amplifying feedback on global warming.     

Effect of the replacement of a native savanna by an African Brachiaria decumbens pasture on the CO2 exchange in the Orinoco lowlands,Venezuela

In the Orinoco lowlands, savannas have been often replaced by pastures composed of the C₄ grass, Brachiaria decumbens Stapf. We addressed following questions: (1) How does the replacement of the native vegetation affect CO₂ exchange on seasonal and annual scales? (2) How do biophysical constraints change when the landscape is transformed? To assess how these changes affect carbon exchange, we determined simultaneously the CO₂ fluxes by eddy covariance, and the soil CO₂ efflux by a chamber-based system in B. decumbens and herbaceous savanna stands. Measurements covered a one-year period from the beginning of the dry season (November 2008) to the end of the wet season (November 2009). During the wet season, the net ecosystem CO₂ exchange reached maximum values of 23 and 10 μmol(CO₂) m^?2 s^?1 in the B. decumbens field and in the herbaceous savanna stand, respectively. The soil CO₂ efflux for both stands followed a temperature variation during the dry and wet seasons, when the soil water content (SWC) increased above 0.087 m³ m^?3 in the latter case. Bursts of CO₂ emissions were evident when the dry soil experienced rehydration. The carbon source/sink dynamics over the two canopies differed markedly. Annual measurements of the net ecosystem production indicated that the B. decumbens field constituted a strong carbon sink of 216 g(C) m^?2 y^?1. By contrast, the herbaceous savanna stand was found to be only a weak sink [36 g(C) m^?2 y^?1]. About 53% of the gross primary production was lost as the ecosystem respiration. Carbon uptake was limited by SWC in the herbaceous savanna stand as evident from the pattern of water-use efficiency (WUE). At the B. decumbens stand, WUE was relatively insensitive to SWC. Although these results were specific to the studied site, the effect of land use changes and the physiological response of the studied stands might be applicable to other savannas.