| The aim of the research reported in this dissertation was to define and quantify the contribution of recent photosynthetic carbon uptake to spatial and temporal patterns respiration of CO2 from soils. Carbon dioxide is produced in soils primarily by roots and heterotrophic bacteria and fungi. Roots use carbon from recent photosynthesis or storage for growth, maintenance, and nutrient uptake, and a large fraction of soil microorganisms live in close proximity to roots and consume short-lived tissues and root exudates. Thus, both of these components largely depend on carbon that has been assimilated only hours to months before. Therefore, it was expected that seasonal patterns of uptake and use of carbon associated with particular vegetation types would be primary drivers of spatial and temporal variability in soil respiration. However, it was also expected that these general patterns would be mediated by environmental conditions.I conducted a multiyear analysis of soil CO2 production in a Rocky Mountain meadow and found that respiration during spring and summer months was tightly coupled to growth of meadow vegetation. In three consecutive summers, soil respiration rates rose with increasing aboveground plant biomass, peaked just before meadow vegetation started to senesce (apparent as a cessation of soil moisture depletion), and then decreased despite continued increases in soil temperature.I compared seasonal soil respiration patterns in adjacent meadow and riparian tree vegetation zones and found that plant phenology was a stronger driver of seasonal and spatial variability in soil respiration than soil temperature or extractable soil organic carbon or microbial biomass carbon.I developed a method to assess the carbon isotope ratio (delta 13C) of CO2 in fine-scale soil profiles for determination of delta13C of CO2 respired by the composite of all soil sources. I applied this method and continuous open soil chambers in a root exclusion (trenching) experiment to analyze the effect of root activity on seasonal patterns of rates and delta13C of CO2 production in soil. Presence of roots accounted for &sim75% of soil respiration in the summer, and this source was about 1&permil enriched in 13C relative to summer heterotrophic respiration in trenched plots. |
