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Abstract
The relationship between temperature and soil respiration has been well explored although uncertainties remain. This thesis examined the relationship between temperature and rates of heterotrophic respiration in soils from three adjacent sub-alpine Australian vegetation types; woodland, shrubland and grassland. Temperature sensitivity of soil (Q10) has recently been a hotly debate topic, one side concluding that decomposition of recalcitrant, less labile components of soil organic matter are insensitive to temperature. Whilst others argue that there is no difference in the temperature sensitivities of labile and recalcitrant carbon pools. Robust modeling of rates of soil respiration requires characterization of the temperature response of both labile and recalcitrant pools.
Laboratory incubation provides a means of characterizing the temperature response of rates of respiration whilst reducing the confounding effects encountered in the field, such as seasonal fluctuations in temperature, moisture and substrate supply. I used a novel system that allowed laboratory measurement of gas exchange in soils over a range of temperatures under controlled conditions. Measurements included CO2 efflux and O2 uptake over a range of temperatures from 5 to 40oC, characterization of temperature response and sensitivity, and respiratory quotients.
Rates of heterotrophic respiration fitted both exponential and Arrhenius functions and temperature sensitivity varied and depended on the model used, vegetation type and depth in the soil profile. Long-term incubation indicated both labile and resistant pools of carbon had similar temperature sensitivities. Respiratory quotients provided a strongly predictive measure of the potential rate of decomposition of soil C, independent of the temperature response of respiration, providing a tool that may be used alongside derived parameters to help understand shifts in microbial use of C substrates.
Vegetation type influenced soil chemical properties and rates of heterotrophic respiration. Rates of respiration correlated well with concentrations of carbon and nitrogen as has been previously observed, unlike previous studies however a positive correlation was observed between indices of plant available phosphorus and respiration. The soils examined were from three adjacent vegetation types formed on common geology, I concluded that vegetation type had a significant influence on soil, in contrast to the commonly held view by ecologists that soil type drives patterns in vegetation. Climatic effects such as longer, dryer hotter summer, reduced snow cover and increased incidence of extreme weather events such as frosts and bushfire are likely to drive patterns in vegetation in this region and therefore have a significant impact on carbon cycling in Sub-alpine Australian soils.