Belowground carbon dynamics in northern hardwood forests: Factors controlling respiration from roots and soil microorganisms.

Terrestrial ecosystems can be important sinks for rising atmospheric carbon dioxide (CO$\sb2)$ levels, due to enhanced carbon (C) fixation by the aboveground portions of plants. However, identifying the extent to which terrestrial ecosystems will function as net C sinks or sources in response to climate change requires knowledge of belowground C dynamics as well. To determine the effects of predicted increases in atmospheric nitrogen (N) deposition and climatic warming on the release of CO$\sb2$ from soils, I conducted a series of three experiments in northern hardwood forests in Michigan, USA. In the first study, I found that the specific respiration rate (g CO$\sb2$-C kg$\sp{-1}$ h$\sp{-1})$ of fine roots ($<$2.0 mm diameter) increased exponentially with higher soil temperatures. The tissue N concentration and associated specific respiration rates of fine roots also increased at sites with high soil N availability, apparently due to the greater C costs of maintaining plant tissues high in proteins and other nitrogenous compounds. In a second experiment, I used molecular, lipid biomarker techniques, in conjunction with kinetic studies of microbial respiration, to examine the response of microbial communities to soil warming. It appears that an increase in temperature elicited shifts in soil microbial community composition and concomitant changes in microbial function (i.e., respiration). In the third study, I found that fine root biomass (kg m$\sp{-2})$ in the upper 10 cm of soil decreased and specific respiration rates (g CO$\sb2$-C kg$\sp{-1}$ h$\sp{-1})$ increased with greater soil N availability. Thus, total fine root respiration (biomass x specific respiration rate, g CO$\sb2$-C m$\sp{-2}$ h$\sp{-1})$ was relatively unaffected by soil N availability. Furthermore, total fine root respiration responded to increases in soil temperature in a manner similar to total soil respiration (g CO$\sb2$-C m$\sp{-2}$ h$\sp{-1}.$ As a result, the CO$\sb2$ flux from fine roots was a relatively constant proportion of total soil respiration, regardless of soil N availability or temperature. My results demonstrate that fine root and microbial respiration are responsive to changes in soil temperature and N availability, but that determining their contribution to the flux of CO$\sb2$ from soil requires an examination of their dynamic and often compensatory responses to environmental change.PhDBiological SciencesEcologyEnvironmental scienceHealth and Environmental SciencesSoil sciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130405/2/9722130.pd