Graduate Thesis Or Dissertation
 

Distinguishing biological and physical controls on soil respiration

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  • Soil respiration, or the combined CO₂ emissions from roots and soil microorganisms, constitutes one of the largest losses of carbon (C) from terrestrial ecosystems. The major drivers of soil respiration, which include soil moisture, temperature, and substrate quality, have been known for some time. Nevertheless, correlations between these drivers and soil respiration vary substantially by site, and there is a lack of mechanistic principles that would allow prediction of soil respiration rates across sites and through time. Here I present three studies that attempted to characterize and differentiate biological and physical mechanisms controlling soil respiration. The purpose of the first study was to quantify the proportion of soil respiration derived from ectomycorrhizal (EcM) fungal mats, which can form dense aggregations of hyphae near the surface of forest soils. By comparing respiration rates on mats with neighboring non-mat soils, I estimated that approximately 10% of soil respiration was derived from EcM mats in an old-growth Douglas fir forest site. Seasonally, mat contributions correlated with soil moisture, which may be due to a physiological response of EcM fungi, but it also likely related to moisture impacts on CO₂ flux contributions in deep soil below where EcM mats tend to colonize. In the second study I examined diel patterns of soil respiration, and used a gas diffusion model to develop a theoretical basis for why respiration is often lagged several hours from soil temperature. This study demonstrated that soil heat and gas transport can cause complex diel patterns in soil respiration, which must be accounted for to correctly interpret the impacts of temperature and other forcing factors on soil respiration. Finally, the third study examined the carbon isotopic composition of soil respiration (δ¹³CO₂), and whether δ¹³CO₂ is influenced by recent plant photosynthates, as has been suggested previously, or instead by microbial or gas-transport effects. I ruled out microbial effects as a possible influence on moisture-related δ¹³CO₂ dynamics, but showed that gas transport likely influenced measurements of δ¹³CO₂ at high and low-moisture. Collectively, an important conclusion from these studies is that analysis of soil gradients, including gradients in environmental conditions, biological activity, and soil physical properties across the soil profile, helps to explain the dynamics of CO₂ fluxes from the soil surface. By examining respiration as a product of processes occurring across soil profiles, in contrast to treating soil as a flat surface or a homogenous medium, more mechanistic and universal relationships become apparent.
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