Soil surface CO2 efflux can vary in a sinusoidal pattern over a 24-hr period, sometimes exhibiting very large diel ranges. Is this a result of soil temperature changing? Is carbon supply from the plant canopy changing in response to photosynthesis? In this project we used field measurements at the soil surface and within the soil profile, and modeling of soil CO2 and heat transport, to understand and describe diel dynamics of soil surface efflux.
Ectomycorrhizal (EM) fungi are a prominent and ubiquitous feature of forest soils. They form symbioses with most tree species, providing nutrients and water to tree hosts in exchange for carbon energy. Yet little is known about the magnitude of their impact on forest carbon cycles. A subset of EM fungi form dense, perennial aggregations of hyphae, which have elevated respiration rates compared with neighboring non-mat soils. These mats are a focus of EM activity and thereby a natural observatory for examining how EM fungi impact forest soils. In order to constrain the contributions of EM fungi to forest soil respiration, we quantified the proportion of respiration derived from EM mat soils in an old-growth Douglas-fir stand in western Oregon.
The isotopic composition of soil respiration is a potentially useful tool to trace the source of respired CO2. However, soil moisture can effect the isotopic composition of soil respiration at three levels: via photosynthetic discrimination, post-photosynthetic biological processes, and physical gas diffusion processes. This project looked at the relative impacts of these biological and physical processes through greenhouse experiments and modeling.