Location:

            The soil respiration project site consists of an open meadow surrounded by riparian maple and boxelder trees.  It is situated about five miles from campus in the flat bottom of a side canyon within a protected and relatively pristine national forest watershed.  The proximity to campus and the deep alluvial soils make it an ideal site for intensive instrumentation and monitoring of belowground processes.  The abrupt transition from large, deciduous trees on the periphery to herbaceous grasses and forbs in the central meadow provides an opportunity to compare soil respiration patterns associated with these two vegetation types under similar edaphic conditions.

Goals of Research: 

            The first aim of this project is to define the patterns of CO₂ production in the soil spatially in three dimensions (over scales of meters on the surface and centimeters of depth) and temporally (diurnally and seasonally) along transects between the two vegetation types.  We are using state of the art instrumentation to measure continuous and spatially replicated soil CO₂ concentration profiles and make regular measurements of surface fluxes of CO₂.  Diffusion models are then applied to the concentration profiles to estimate respiration with location, depth, and time.

            The second goal of the project is to understand the mechanisms controlling the observed patterns of soil respiration.  We are most interested in how variation in carbon uptake by plants relates to differences in soil respiration by controlling root and rhizosphere substrate supply.  Novel methods of stable carbon isotope sampling of soil and efflux CO₂ are also being developed for use in identification of respiratory substrates.

Background:

            Until recently, soil respiration was generally considered a function of temperature.  Though this is true when all other conditions are both favorable and constant, ecosystems are rarely in such a state and soil respiration is typically limited by supply of substrate, soil moisture, or soil chemical and physical properties.  CO₂ is produced in soils primarily by roots and microorganisms.  Roots use carbon from recent photosynthesis or storage for maintenance, growth, and nutrient uptake, and the bulk 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, we expect that variation in uptake and use of carbon associated with particular vegetation types will be a primary driver of spatial and temporal variability in soil respiration.  However, it is expected that these general patterns will be mediated by environmental conditions.