School of Agriculture, Food and Ecosystem Sciences - Research Publications
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ItemNo Preview AvailableSoil Methane Uptake Increases under Continuous Throughfall Reduction in a Temperate Evergreen, Broadleaved Eucalypt Forest(SPRINGER, 2017-03)Soils in temperate forests ecosystems are the greatest terrestrial CH₄ sink globally. Global and regional circulation models predict decreased average rainfall, increased extreme rainfall events and increased temperatures for many temperate ecosystems. However, most studies of soil CH₄ uptake have only considered extended periods of drought rather than an overall decrease in rainfall amount. We measured soil CH₄ uptake from March 2010 to March 2012 after installing passive rainfall reduction systems to intercept approximately 40% of throughfall in a temperate broadleaf evergreen eucalypt forest in south-eastern Australia. Throughfall reduction caused an average reduction of 15.1 ± 6.4% (SE) in soil volumetric water content, a reduction of 19.8 ± 6.9% in soil water-filled pore space (%WFPS) and a 20.1 ± 6.8% increase in soil air-filled porosity. In response to these changes, soil CH₄ uptake increased by 54.7 ± 19.3%. The increase in soil CH₄ uptake could be explained by increased diffusivity in drier soils, whilst the activity of methanotrophs remained relatively unchanged. It is likely that soil CH₄ uptake will increase if rainfall reduces in temperate broadleaf evergreen forests of Australia as a consequence of climate change.
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ItemSoil methane oxidation in both dry and wet temperate eucalypt forests shows a near-identical relationship with soil air-filled porosity(Copernicus Publications, 2017-01-27)Well-drained, aerated soils are important sinks for atmospheric methane (CH4) via the process of CH4 oxidation by methane-oxidising bacteria (MOB). This terrestrial CH4 sink may contribute towards climate change mitigation, but the impact of changing soil moisture and temperature regimes on CH4 uptake is not well understood in all ecosystems. Soils in temperate forest ecosystems are the greatest terrestrial CH4 sink globally. Under predicted climate change scenarios, temperate eucalypt forests in south-eastern Australia are predicted to experience rapid and extreme changes in rainfall patterns, temperatures and wild fires. To investigate the influence of environmental drivers on seasonal and inter-annual variation of soil–atmosphere CH4 exchange, we measured soil–atmosphere CH4 exchange at high-temporal resolution (< 2 h) in a dry temperate eucalypt forest in Victoria (Wombat State Forest, precipitation 870 mm yr−1) and in a wet temperature eucalypt forest in Tasmania (Warra Long-Term Ecological Research site, 1700 mm yr−1). Both forest soil systems were continuous CH4 sinks of −1.79 kg CH4 ha−1 yr−1 in Victoria and −3.83 kg CH4 ha−1 yr−1 in Tasmania. Soil CH4 uptake showed substantial temporal variation and was strongly controlled by soil moisture at both forest sites. Soil CH4 uptake increased when soil moisture decreased and this relationship explained up to 90 % of the temporal variability. Furthermore, the relationship between soil moisture and soil CH4 flux was near-identical at both forest sites when soil moisture was expressed as soil air-filled porosity (AFP). Soil temperature only had a minor influence on soil CH4 uptake. Soil nitrogen concentrations were generally low and fluctuations in nitrogen availability did not influence soil CH4 uptake at either forest site. Our data suggest that soil MOB activity in the two forests was similar and that differences in soil CH4 exchange between the two forests were related to differences in soil moisture and thereby soil gas diffusivity. The differences between forest sites and the variation in soil CH4 exchange over time could be explained by soil AFP as an indicator of soil moisture status.
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ItemFire in Australian savannas: from leaf to landscape(WILEY, 2015-01)Savanna ecosystems comprise 22% of the global terrestrial surface and 25% of Australia (almost 1.9 million km2) and provide significant ecosystem services through carbon and water cycles and the maintenance of biodiversity. The current structure, composition and distribution of Australian savannas have coevolved with fire, yet remain driven by the dynamic constraints of their bioclimatic niche. Fire in Australian savannas influences both the biophysical and biogeochemical processes at multiple scales from leaf to landscape. Here, we present the latest emission estimates from Australian savanna biomass burning and their contribution to global greenhouse gas budgets. We then review our understanding of the impacts of fire on ecosystem function and local surface water and heat balances, which in turn influence regional climate. We show how savanna fires are coupled to the global climate through the carbon cycle and fire regimes. We present new research that climate change is likely to alter the structure and function of savannas through shifts in moisture availability and increases in atmospheric carbon dioxide, in turn altering fire regimes with further feedbacks to climate. We explore opportunities to reduce net greenhouse gas emissions from savanna ecosystems through changes in savanna fire management.
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ItemReduced throughfall decreases autotrophic respiration, but not heterotrophic respiration in a dry temperate broadleaved evergreen forest(ELSEVIER, 2015-01-15)Climate change may have major implications on soil respiration dynamics and the carbon sink strength of forest soils. To assess the effect of climate change on soil respiration (RS), it is crucial to understand individual responses of autotrophic (RA) and heterotrophic (RH) components. We investigated the effect of continuously (20 months) reduced throughfall (TFR, −40%) and the influence of seasonal changes in soil temperature and moisture on RS, RA and RH, partitioned by root exclusion, in a dry temperate broadleaved evergreen eucalypt forest in south-eastern Australia. TFR decreased mean RS from 4.7±0.1 (Control) to 3.8±0.1 (TFR) μmolCO2m−2s−1 (−19%). TFR indicated a strong decrease in RA from 2.5±0.1 (Control) to 1.5±0.1 (TFR) μmolCO2m−2s−1 (−40%), but had no effect on RH. The mean relative contribution of RH to RS was 47% in the Control and increased to 61% under TFR. RS was the result of distinct seasonal patterns and dependencies of RH and RA on environmental variables. Soil temperature was a good predictor of RH (Control: R2=0.72, TFR: R2=0.75), but not of RA. In contrast, RH was not limited by soil moisture, while RA was partly influenced by soil moisture (Control: R2=0.29, TFR: R2=0.56). The lack of response of RH to changes in soil moisture (seasonal and under TFR) was likely influenced by the high rainfall conditions such that soil moisture did not decrease to a point where it limited soil microbial decomposition processes. Our results show that TFR implied the strongest effect on RA and that changes in soil temperature and moisture alone do not sufficiently explain seasonal changes in RA and RS. This indicates that biotic factors, such as plant internal carbon allocation, may exert a stronger influence on RA and hence, RS. In short-term a reduction in rainfall will lead to a decrease of soil respiration in dry temperate broadleaved evergreen eucalypt forests. The magnitude of this decrease and its persistence under extended drought will be greatly influenced by seasonal and inter-annual climate variability and potential changes in plant carbon allocation.
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ItemNo Preview AvailableChanges in soil moisture drive soil methane uptake along a fire regeneration chronosequence in a eucalypt forest landscape(WILEY-BLACKWELL, 2015-11)Disturbance associated with severe wildfires (WF) and WF simulating harvest operations can potentially alter soil methane (CH4 ) oxidation in well-aerated forest soils due to the effect on soil properties linked to diffusivity, methanotrophic activity or changes in methanotrophic bacterial community structure. However, changes in soil CH4 flux related to such disturbances are still rarely studied even though WF frequency is predicted to increase as a consequence of global climate change. We measured in-situ soil-atmosphere CH4 exchange along a wet sclerophyll eucalypt forest regeneration chronosequence in Tasmania, Australia, where the time since the last severe fire or harvesting disturbance ranged from 9 to >200 years. On all sampling occasions, mean CH4 uptake increased from most recently disturbed sites (9 year) to sites at stand 'maturity' (44 and 76 years). In stands >76 years since disturbance, we observed a decrease in soil CH4 uptake. A similar age dependency of potential CH4 oxidation for three soil layers (0.0-0.05, 0.05-0.10, 0.10-0.15 m) could be observed on incubated soils under controlled laboratory conditions. The differences in soil CH4 uptake between forest stands of different age were predominantly driven by differences in soil moisture status, which affected the diffusion of atmospheric CH4 into the soil. The observed soil moisture pattern was likely driven by changes in interception or evapotranspiration with forest age, which have been well described for similar eucalypt forest systems in south-eastern Australia. Our results imply that there is a large amount of variability in CH4 uptake at a landscape scale that can be attributed to stand age and soil moisture differences. An increase in severe WF frequency in response to climate change could potentially increase overall forest soil CH4 sinks.
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ItemNo Preview AvailableRepeated fuel reduction burns have little long-term impact on soil greenhouse gas exchange in a dry sclerophyll eucalypt forest(ELSEVIER, 2015-02-15)Fuel reduction burning is a widespread management tool in fire-tolerant forest systems to mitigate wildfire risk, but has the potential to impact soil greenhouse gas exchange processes. Soil disturbance often alters soil carbon dioxide (CO2) and methane (CH4) flux; however, the influence of repeated fuel reduction burning upon these flux processes long-term is still not well understood. In this study we measure soil CH4 flux, soil methanotrophic activity and soil CO2 flux in all seasons from March 2009 to February 2011 in three different fire frequency treatments applied to a dry sclerophyll eucalypt forest (Victoria, Australia) for the last 27 years. The low-intensity fire treatments are forest burnt in autumn (i) every 3 years, (ii) every 10 years, and (iii) not burned (since before 1985). Mean soil CO2 emissions were greater in the burnt as compared to un-burnt treatments. In contrast, soil CH4 oxidation did not show a response to repeated burning and there was no statistical difference in soil CH4 flux among treatments. Furthermore, we did not detect changes in the relationships of soil CH4 flux or soil CO2 flux and key environmental controls. Our results indicate that low intensity fuel reduction burns have no cumulative negative impact on biogeochemical processes related to soil respiration or soil CH4 oxidation.