Mathematical approaches to partition net ecosystem exchange of carbon dioxide in a high-elevation subalpine forest

Ecosystem-scale gross photosynthesis and total ecosystem respiration are two fundamentally important processes of the terrestrial carbon cycle, but are difficult to quantify and study separately. Two primary objectives of this dissertation are (a) to obtain diurnal estimates of ecosystem-scale photosynthesis and respiration using an isotopic method, and (b) partition components of ecosystem respiration using process-based soil carbon models. Both objectives utilize net ecosystem exchange of CO2 data ( F) and meteorological data collected at a high-elevation subalpine forest (the Niwot Ridge AmeriFlux site).; Data collected during a three month period during late summer 2003 generated isotope flux partitioning estimates of ecosystem-scale photosynthesis and respiration. These partitioning results were compared to an independent flux partitioning routine based on statistical regressions between nighttime F and air temperature. These two methods produced expected diurnal patterns in photosynthesis and respiration. Isotope flux partitioned respiration estimates were not sensitive to air temperature as expected. The difference between the isotopic signatures of net photosynthesis (delta A) and total ecosystem respiration (deltaR) generated from isotope flux partitioning were consistently positive (delta A > deltaR). Values of delta A > deltaR are inconsistent with many other studies, and may result from uncertainties in the ecosystem-scale values of fundamental parameters for the isotope partitioning model. Additionally, delta R is strongly influenced by the linear regression formulas (Model I or Model II regression) utilized to determine delta R from measurements of CO2 and the 13C content in air.; A seven year record of F from the Niwot Ridge site and soil carbon models were used to investigate soil carbon processes. These models were evaluated in how they estimated heterotrophic (microbial) or autotrophic (root) components of ecosystem respiration. Explicit modeling of root dynamics led to better agreement with literature values of the contribution of soil respiration to total ecosystem respiration. However, these models overestimated the autotrophic component of soil respiration when compared to literature values. Hence, additional model development is needed to characterize microbial biomass dynamics.