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"Arndt, Stefan"
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Termite mounds mitigate half of termite methane emissions
Termites are responsible for ∼1 to 3%of global methane (CH₄) emissions. However, estimates of global termite CH₄ emissions span two orders of magnitude, suggesting that fundamental knowledge of CH₄ turnover processes in termite colonies is missing. In particular, there is little reliable information on the extent and location of microbial CH₄ oxidation in termite mounds. Here, we use a one-box model to unify three independent field methods—a gas-tracer test, an inhibitor approach, and a stable-isotope technique—and quantify CH₄ production, oxidation, and transport in three North Australian termite species with different feeding habits and mound architectures. We present systematic in situ evidence of widespread CH₄ oxidation in termite mounds, with 20 to 80% of termite-produced CH₄ being mitigated before emission to the atmosphere. Furthermore, closing the CH₄ mass balance in mounds allows us to estimate in situ termite biomass from CH₄ turnover, with mean biomass ranging between 22 and 86 g of termites per kilogram of mound for the three species. Field tests with excavated mounds show that the predominant location of CH₄ oxidation is either in the mound material or the soil beneath and is related to species-specific mound porosities. Regardless of termite species, however, our data and model suggest that the fraction of oxidized CH₄ (f
ox) remains well buffered due to links among consumption, oxidation, and transport processes via mound CH₄ concentration. The mean f
ox of 0.50 ± 0.11 (95% CI) from in situ measurements therefore presents a valid oxidation factor for future global estimates of termite CH₄ emissions.
Journal Article
Relationships between plant drought response, traits, and climate of origin for green roof plant selection
The ideal species for green or vegetated roofs should have high water use after rainfall to maximize stormwater retention but also survive periods with low water availability in dry substrates. Shrubs have great potential for green roofs because they have higher rates of water use, and many species are also drought tolerant. However, not all shrub species will be suitable and there may be a trade-off between water use and drought tolerance. We conducted a glasshouse experiment to determine the possible trade-offs between shrub water use for stormwater management and their response to drought conditions. We selected 20 shrubs from a wide range of climates of origin, represented by heat moisture index (HMI) and mean annual precipitation (MAP). Under well-watered (WW) and water-deficit (WD) conditions, we assessed morphological responses to water availability; evapotranspiration rate (ET) and midday water potential (ΨMD) were used to evaluate species water use and drought response. In response to WD, all 20 shrubs adjusted their morphology and physiology. However, there were no species that simultaneously achieved high rates of water use (high ET) under WW and high drought tolerance (low ΨMD) under WD conditions. Although some species which had high water use under WW conditions could avoid drought stress (high ΨMD). Water use was strongly related to plant biomass, total leaf area, and leaf traits (specific leaf area [SLA] and leaf area ratio [LAR]). Conversely, drought response (ΨMD) was not related to morphological traits. Species’ climate of origin was not related to drought response or water use. Drought-avoiding shrubs (high ΨMD) could optimize rainfall reduction on green roofs. Water use was related to biomass, leaf area, and leaf traits; thus, these traits could be used to assist the selection of shrubs for stormwater mitigation on green roofs. The natural distribution of species was not related to their water use or drought response, which suggests that shrubs from less arid climates may be suitable for use on green roofs. Selecting species based on traits and not climate of origin could both improve green roof performance and biodiversity outcomes by expanding the current plant palette.
Journal Article
Bark-dwelling methanotrophic bacteria decrease methane emissions from trees
Tree stems are an important and unconstrained source of methane, yet it is uncertain whether internal microbial controls (i.e. methanotrophy) within tree bark may reduce methane emissions. Here we demonstrate that unique microbial communities dominated by methane-oxidising bacteria (MOB) dwell within bark of
Melaleuca quinquenervia
, a common, invasive and globally distributed lowland species. In laboratory incubations, methane-inoculated
M. quinquenervia
bark mediated methane consumption (up to 96.3 µmol m
−2
bark d
−1
) and reveal distinct isotopic δ
13
C-CH
4
enrichment characteristic of MOB. Molecular analysis indicates unique microbial communities reside within the bark, with MOB primarily from the genus
Methylomonas
comprising up to 25 % of the total microbial community. Methanotroph abundance was linearly correlated to methane uptake rates (R
2
= 0.76,
p
= 0.006). Finally, field-based methane oxidation inhibition experiments demonstrate that bark-dwelling MOB reduce methane emissions by 36 ± 5 %. These multiple complementary lines of evidence indicate that bark-dwelling MOB represent a potentially significant methane sink, and an important frontier for further research.
The photosynthesis performed by trees makes them an important sink for atmospheric carbon dioxide, but trees are also sources of the potent greenhouse gas methane. Here the authors find that tree bark in some common lowland species is colonized by methane oxidizing bacteria that can reduce tree methane emissions by ~ 36%.
Journal Article
Life span and structure of ephemeral root modules of different functional groups from a desert system
The terminal branch orders of plant root systems have been proposed as short-lived ‘ephemeral’ modules specialized for resource absorption. The occurrence of ephemeral root modules has so far only been reported for a temperate tree species and it is unclear if the concept also applies to other woody (shrub, tree) and herb species.
Fine roots of 12 perennial dicotyledonous herb, shrub and tree species were monitored for two growing seasons using a branch-order classification, sequential sampling and rhizotrons in the Taklamakan desert.
Two root modules existed in all three plant functional groups. Among the first five branch orders, the first two (perennial herbs, shrubs) or three (trees) root orders were ephemeral and had a primary anatomical structure, high nitrogen (N) concentrations, high respiration rates and very short life spans of 1–4 months, whereas the last two branch orders in all functional groups were perennial, with thicker diameters, no or collapsed cortex, distinct secondary growth, low N concentrations, low respiration rates, but much longer life spans.
Ephemeral, short-lived root modules and long-lived, persistent root modules seem to be a general feature across many plant functional groups and could represent a basic root system design.
Journal Article
Trace gas oxidizers are widespread and active members of soil microbial communities
Soil microorganisms globally are thought to be sustained primarily by organic carbon sources. Certain bacteria also consume inorganic energy sources such as trace gases, but they are presumed to be rare community members, except within some oligotrophic soils. Here we combined metagenomic, biogeochemical and modelling approaches to determine how soil microbial communities meet energy and carbon needs. Analysis of 40 metagenomes and 757 derived genomes indicated that over 70% of soil bacterial taxa encode enzymes to consume inorganic energy sources. Bacteria from 19 phyla encoded enzymes to use the trace gases hydrogen and carbon monoxide as supplemental electron donors for aerobic respiration. In addition, we identified a fourth phylum (Gemmatimonadota) potentially capable of aerobic methanotrophy. Consistent with the metagenomic profiling, communities within soil profiles from diverse habitats rapidly oxidized hydrogen, carbon monoxide and to a lesser extent methane below atmospheric concentrations. Thermodynamic modelling indicated that the power generated by oxidation of these three gases is sufficient to meet the maintenance needs of the bacterial cells capable of consuming them. Diverse bacteria also encode enzymes to use trace gases as electron donors to support carbon fixation. Altogether, these findings indicate that trace gas oxidation confers a major selective advantage in soil ecosystems, where availability of preferred organic substrates limits microbial growth. The observation that inorganic energy sources may sustain most soil bacteria also has broad implications for understanding atmospheric chemistry and microbial biodiversity in a changing world.
A combination of metagenomic analyses, thermodynamic modelling and in situ measurements of gas fluxes shows that a large fraction of soil bacteria can use inorganic energy sources, such as the trace gases hydrogen and carbon monoxide, for growth and persistence.
Journal Article
Net-Zero Heroes? Climate Change Mitigation Efforts and Strategies across Australian Group-of-Eight Universities
Businesses are increasingly declaring their operations to be “carbon neutral” or “net-zero”. But how real are these claims? We investigated the climate mitigation efforts of the eight leading universities in Australia and discovered that the actual emission reductions lag behind the net-zero rhetoric. In the last ten years, most universities increased energy consumption, while reported emissions plateaued. The energy consumption and greenhouse gas emissions of Group-of-Eight (Go8) universities were influenced by size and population growth, climate of the campus location, and energy efficiencies. The Go8 universities experienced, on average, a 25% increase in student numbers in the last decade, and most increased their energy consumption. However, Scope 1 (direct emissions) and Scope 2 (emissions from electricity consumption) remained stable for most universities from 2011 to 2019 and decreased on a per-capita basis, indicating some level of improved efficiencies. Almost all Go8 universities have net-zero commitments and aim to achieve this by similar measures: power purchase agreements (PPAs) for electricity consumption, and carbon offsets for remaining emissions. Most universities lack a strategy for direct or measurable targets regarding energy or emissions reductions along their value chain. Unlike the UK or other countries, Australia has no standardised emission reporting requirements for Scope 3 emissions (other indirect emissions). This has led to rudimentary and haphazard reporting, limiting comparability between universities. Only one university had a more complete Scope 3 inventory, and these Scope 3 emissions were five times greater than their combined Scope 1 and 2 emissions, indicating a potential for substantial under-reporting of emissions. This highlights the need for more rigorous, consistent, and sector-specific emissions accounting, especially on indirect emissions, and for an overhaul of net-zero accreditation.
Journal Article
Research frontiers for improving our understanding of drought-induced tree and forest mortality
nitori Accumulating evidence highlights increased mortality risks for trees during severe drought, particularly under warmer temperatures and increasing vapour pressure deficit (VPD). Resulting forest die-off events have severe consequences for ecosystem services, biophysical and biogeochemical land-atmosphere processes. Despite advances in monitoring, modelling and experimental studies of the causes and consequences of tree death from individual tree to ecosystem and global scale, a general mechanistic understanding and realistic predictions of drought mortality under future climate conditions are still lacking. We update a global tree mortality map and present a roadmap to a more holistic understanding of forest mortality across scales. We highlight priority research frontiers that promote: (1) new avenues for research on key tree ecophysiological responses to drought; (2) scaling from the tree/plot level to the ecosystem and region; (3) improvements of mortality risk predictions based on both empirical and mechanistic insights; and (4) a global mong network of forest mortality. In light of recent and anticipated large forest die-off events such a research agenda is timely and needed to achieve scientific understanding for realistic predictions of drought-induced tree mortality. The implementation of a sustainable network will require support by stakeholders and political authorities at the international level.
Journal Article
Can the turgor loss point be used to assess drought response to select plants for green roofs in hot and dry climates?
Aims
Green roofs are important novel urban ecosystems, but their shallow substrates can create plant water deficits in dry climates. Physiological approaches can improve green roof plant selection, and shrubs with high drought tolerance and conservative water use under water-deficit should perform well. The water potential at turgor loss point (
Ψ
tlp
) has been used to predict drought resistance. Therefore we aimed to determine whether
Ψ
tlp
could be used as a screening tool to assess drought resistance for green roof plant selection.
Methods
We evaluated 20 shrub species, originating from ecosystems varying in water availability, quantified by heat moisture index (HMI) and mean annual precipitation. We conducted a water-deficit experiment to measure
Ψ
tlp
, the degree of iso-anisohydry (△
Ψ
MD
) and water use (ET) in response to drought.
Results
Shrubs with lower
Ψ
tlp
were more anisohydric (greater △
Ψ
MD
) and had a more conservative water use (lower ET). However,
Ψ
tlp
, Δ
Ψ
MD
and ET were not related to HMI.
Conclusions
These results suggest that
Ψ
tlp
could be used to select shrubs for green roofs, as species with lower
Ψ
tlp
tended to be more drought tolerant, more anisohydric and used less water under water-deficit. However, species with higher
Ψ
tlp
could also potentially survive through drought avoidance.
Journal Article
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 (CH
4
) emissions, but the aerobic CH
4
oxidising bacteria (methanotrophs) responsible for this consumption have not been resolved. Here, we describe the abundance, composition and CH
4
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 CH
4
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 CH
4
source such as landfill cover soils. The rate constant of CH
4
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 CH
4
emissions have selected for distinct methanotroph communities that are kinetically adapted to elevated CH
4
concentrations. However, factors other than substrate concentration appear to limit methanotroph abundance and hence these bacteria only partially mitigate termite-derived CH
4
emissions. Our results also highlight the predominant role of USCα in an environment with elevated CH
4
concentrations and suggest a higher functional diversity within this group than previously recognised.
Journal Article