David Risk

Carbon dioxide is respired by soil microbes during organic matter decomposition, and by plant roots. Together these natural soil processes constitute the largest landatmosphere C02 flux in the global carbon cycle, exceeding human emissions by ten times. Soil respiration is a poorly understood component of landscape carbon balance and clarification is needed in many areas, in pa.rticular the individual climatic response of microbes and roots. Methodological challenges currently limit our ability to separate soil respiration into its component parts. This thesis seeks to tackle methodological issues, and to address a central question: Do roots and soil microbes respond to different environmental cues? Isotopic signatures of vegetation and soil were characterized as a foundation for isotopic partitioning efforts, but despite improved sampling techniques, isotopic partitioning suffered from large uncertainties. Errors were associated with temporal variability in root respiration signatures, and high spatial variability in soil gas transport that was likely a stronger determinant of forest soil C02 signatures than isotopically distinct source contributions. Other techniques were, however, successful and elucidated new information about the relative climatic sensitivity of root and microbial processes. Physical approaches were used to monitor microbial decomposition in several distinct soil organic matter pools, and root respiration in different soil layers. Both microbes and roots were sensitive to temperature but displayed unique behavior; microbial decomposition rates increased in an exponential manner, and in contrast, roots respiration rates followed near logarithmic rates of increase, reaching a. plateau above 15° C. This suggests that soil microbial respiration could become proportionately more important in a warmer world. Root activity was universally related to temperature across study sites, while microbial responses were site-specific. This unique behavior highlights the importance and utility of partitioning methodologies in soil carbon research .

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Supervisor: Lisa Kellman