Distinguishing biological and physical controls on soil respiration

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 (d
13
CO2), and
whether d
13
CO2 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 d
13
CO2 dynamics, but showed that gastransport likely influenced measurements of d
13
CO2 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.