Amanda Diochon

Since European settlement, forest harvesting has become the dominant disturbance regime in the Acadian Forest Region. Though the practice of clearcutting is widespread, its effect on the storage and structure of carbon (C) in the soil organic matter pool is not well documented. This thesis 1. documents the size and structure of the soil organic C (SOC) pool in forests representing important stages of development during secondary succession, 2. evaluates the utility of including stable isotopes in investigations of management effects on SOC, and 3. examines the control of temperature and C content on potential rates of C mineralization within these stands. During the first 30 years post harvest, stores of SOC (organic layer plus 50 cm of mineral soil) are depleted by 50%. Losses of C are greatest below 20 cm and appear, based on isotopic evidence, to be a function of increased mineralization post harvest. Changes in the size and structure of the SOC pool are expected in the uncomplexed fractions of organic matter; however, I show that 50% of the organomineral fraction, which is considered to be stable, is cycling on decadal time scales. I demonstrate that C storage in the organormineral fraction below 20 cm is driving the temporal trend and isotopic evidence is consistent with increased mineralization post harvest. Maximum rates of CO2 production from microbial mineralization of C occurred at intermediate temperatures (16-26oC) and in soil samples that did not always have the highest C content, suggesting the existence of an additional, yet unidentified, factor(s) controlling microbial CO2 production in these soils. Additionally, higher than expected microbial CO2 production deeper in the soil profile, indicates the existence of a potentially large labile store of SOC that may be decomposed if conditions are optimal. These findings suggest that: 1. subsurface stores of C may not be as stable as previously assumed, 2. stable isotopes can provide insight into management induced changes in SOC pools and their structure, and 3. relationships between microbial CO2 production and its controlling variables developed at the surface may not represent the depth-specific interactions that control microbial decomposition of SOC.

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Pages: 164
Supervisor: L. Kellman/H. Beltrami