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Clearing and burning of tropical savanna leads to globally significant emissions of greenhouse gases (GHG) although there is large uncertainty relating to the magnitude of this flux. Australia's tropical savannas occupy over 25% of the continental land mass and have a potential to significant influence the national greenhouse gas budget, particularly because they are the focus of likely agricultural expansion. To investigate the role of deforestation on GHG emissions, a paired site approach was used. The CO2 exchange was measured from two tropical savanna woodland sites, one that was cleared, and a second analogue site that remained uncleared for a 22 month observation period. At both sites, net ecosystem exchange (NEE) was measured using the eddy covariance (EC) method. Observations at the cleared site was continuous before, during and after the clearing event, providing high resolution data that tracked CO2 emissions through multiple phases of land use change. At the cleared site, post-clearing debris was allowed to cure for 6 months and was subsequently burnt, followed by extensive soil preparation for cropping. Emissions were estimated from the debris fire by quantify the on-site biomass prior to clearing and applying savanna-specific emissions factors to estimate a fire-derived GHG emission. This was added to net CO2 fluxes as measured by the eddy covariance tower giving a total GHG emission of 154 Mg CO2-e ha-1 from a savanna woodland with a total fuel load (above- and below- ground woody debris, course woody debris, litter plus C4 grass fuel) of 40.9 Mg C ha-1. This emission was dominated by debris combustion from the fire event, which was 83% of the total emission, the remained from soil emissions and decay of debris during the curing period prior to burning. Soil disturbance from ploughing and site preparation for cropping was responsible for almost 10% of the total emission. Fluxes at the uncleared site were tracked using an additional flux tower for 668 days and over this period, cumulative NEE was -2.1 Mg C ha-1, a net carbon sink. Estimated emissions for this savanna type were then upscaled to provide estimates of the magnitude of emissions from any future deforestation. At current rates of deforestation, savanna burning is as significant a source of GHG emissions as deforestation, with fire emissions occurring every year across this savanna biome. However, expanded deforestation could exceed fire emissions and a clearing scenario was examined which suggested clearing over and above current rates could add up to 5% to Australia's national GHG account, depending on the annual rate of deforestation. This bottom-up study provides data that can reduce uncertainty associated with land use change for this extensive tropical ecosystem and provide an assessment of the relative magnitude of GHG emissions from savanna burning and deforestation as well as informing northern land use decision making processes. [ABSTRACT FROM AUTHOR]

We investigated the relative importance of CH4 and CO2 fluxes from soil and termite mounds at four different sites in the tropical savannas of northern Australia near Darwin and assessed different methods to indirectly predict CH4 fluxes based on CO2 fluxes and internal gas concentrations. The annual flux from termite mounds and surrounding soil was dominated by CO2 with large variations among sites. On a carbon dioxide equivalent (CO2-e) basis, annual CH4 flux estimates from termite mounds were 5- to 46-fold smaller than the concurrent annual CO2 flux estimates. Differences between annual soil CO2 and soil CH4 (CO2-e) fluxes were even greater, soil CO2 fluxes being almost three orders of magnitude greater than soil CH4 (CO2-e) fluxes at site. The contribution of CH4 and CO2 emissions from termite mounds to the total CH4 and CO2 emissions from termite mounds and soil in CO2-e was less than 1 %. There were significant relationships between mound CH4 flux and mound CO2 flux, enabling the prediction of CH4 flux from measured CO2 flux; however, these relationships were clearly termite species specific. We also observed significant relationships between mound flux and gas concentration inside mound, for both CH4 and CO2, and for all termite species, thereby enabling the prediction of flux from measured mound internal gas concentration. However, these relationships were also termite species specific. Using the relationship between mound internal gas concentration and flux from one species to predict mound fluxes from other termite species (as has been done in the past) would result in errors of more than 5-fold for mound CH4 flux and 3-fold for mound CO2 flux. This study highlights that CO2 fluxes from termite mounds are generally more than one order of magnitude greater than CH4 fluxes. There are species-specific relationships between CH4 and CO2 fluxes from a mound, and between the inside mound concentration of a gas and the mound flux emission of the same gas, but these relationships vary greatly among termite species. Thus, there is no generic relationship that will allow for the accurate prediction of CH4 fluxes from termite mounds of all species, but given the data limitations, the above methods may still be used with caution. [ABSTRACT FROM AUTHOR]

1. We investigated the relative importance of CH4 and CO2 fluxes from soil and termite mounds at four different sites in the tropical savannas of Northern Australia near Darwin and assessed different methods to indirectly predict CH4 fluxes based on CO2 fluxes and internal gas concentrations. 2. The annual flux from termite mounds and surrounding soil was dominated by CO2 with large variations among sites. On a CO2-e basis, annual CH4 flux estimates from termite mounds were 5- to 46-fold smaller than the concurrent annual CO2 flux estimates. Differences between annual soil CO2 and soil CH4 (CO2-e) fluxes were even greater, soil CO2 fluxes being almost three orders of magnitude greater than soil CH4 (CO2-e) fluxes at site. 3. There were significant relationships between mound CH4 flux and mound CO2 flux, enabling the prediction of CH4 flux from measured CO2 flux, however, these relationships were clearly termite species specific. 4. We also observed significant relationships between mound flux and gas concentration inside mound, for both CH4 and CO2, and for all termite species, thereby enabling the prediction of flux from measured mound internal gas concentration. However, these relationships were also termite species specific. Using the relationship between mound internal gas concentration and flux from one species to predict mound fluxes from other termite species (as has been done in past) would result in errors of more than 5-fold for CH4 and 3-fold for CO2. 5. This study highlights that CO2 fluxes from termite mounds are generally more than one order of magnitude greater than CH4 fluxes. There are species-specific relationships between CH4 and CO2 fluxes from a mound, and between the inside mound concentration of a gas and the mound flux emission of the same gas, but these relationships vary greatly among termite species. Consequently, there is no generic relationship that will allow for the prediction of CH4 fluxes from termite mounds of all species. [ABSTRACT FROM AUTHOR]

Soils provide the largest terrestrial carbon store, the largest atmospheric CO2 source, the largest terrestrial N2O source and the largest terrestrial CH4 sink, as mediated through root and soil microbial processes. A change in land use or management can alter these soil processes such that net greenhouse gas exchange may increase or decrease. We measured soil–atmosphere exchange of CO2, N2O and CH4 in four adjacent land-use systems (native eucalypt woodland, clover-grass pasture, Pinus radiata and Eucalyptus globulus plantation) for short, but continuous, periods between October 2005 and June 2006 using an automated trace gas measurement system near Albany in southwest Western Australia. Mean N2O emission in the pasture was 26.6 μg N m−2 h−1, significantly greater than in the natural and managed forests (< 2.0 μg N m−2 h−1). N2O emission from pasture soil increased after rainfall events (up to 100 μg N m−2 h−1) and as soil water content increased into winter, whereas no soil water response was detected in the forest systems. Gross nitrification through 15N isotope dilution in all land-use systems was small at water holding capacity < 30%, and under optimum soil water conditions gross nitrification ranged between < 0.1 and 1.0 mg N kg−1 h−1, being least in the native woodland/eucalypt plantation < pine plantation < pasture. Forest soils were a constant CH4 sink, up to −20 μg C m−2 h−1 in the native woodland. Pasture soil was an occasional CH4 source, but weak CH4 sink overall (−3 μg C m−2 h−1). There were no strong correlations ( R < 0.4) between CH4 flux and soil moisture or... [ABSTRACT FROM AUTHOR]

Biofiltration systems are highly valued in urban landscapes as they remove pollutants from stormwater runoff whilst contributing to a reduction in runoff volumes. Integrating trees in biofilters may improve their runoff retention performance, as trees have greater transpiration than commonly used sedge or herb species. High transpiration rates will rapidly deplete retained water, creating storage capacity prior to the next runoff event. However, a tree with high transpiration rates in a biofilter system will likely be frequently exposed to drought stress. Selecting appropriate tree species therefore requires an understanding of how different trees use water and how they respond to substrate drying. We selected 20 tree species and quantified evapotranspiration (ET) and drought stress (leaf water potential; Ψ) in relation to substrate water content. To compare species, we developed metrics which describe: (i) maximum rates of ET under well-watered conditions, (ii) the sensitivity of ET and (iii) the response of Ψ to declining substrate water content. Using these three metrics, we classified species into three groups: risky, balanced or conservative. Risky and balanced species showed high maximum ET, whereas conservative species always had low ET. As substrates dried, the balanced species down-regulated ET to delay the onset of drought stress; whereas risky species did not. Therefore, balanced species with high ET are more likely to improve the retention performance of biofiltration systems without introducing significant drought risk. This classification of tree water use strategies can be easily integrated into water balance models and improve tree species selection for biofiltration systems., (Copyright © 2018 Elsevier Ltd. All rights reserved.)