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Carbon dioxide flux measurements from a coastal Douglas-fir forest floor Drewitt, Gordon Bruce

Abstract

Soils can store large quantities of carbon in the form of decaying organic matter originally derived from vegetation. In order to gain a better understanding of the importance of soils in the carbon cycle, it is necessary to examine processes that directly affect the exchange of carbon between the soil and the atmosphere. During the year 2000, measurements of forest floor CO₂ flux and below-ground CO₂ storage were obtained beneath the canopy of a 33-m tall coastal temperate Douglas-fir forest. The study took place at a micrometeorological tower flux site in operation since late 1997 obtaining year-round eddy covariance (EC) measurements of above-canopy CO₂ and energy fluxes. An automated soil chamber system was designed and constructed which could obtain half hourly measurements of CO₂ flux from six locations with each chamber covering 0.2 m . These soil chambers were installed in late April and operated until mid- December, spanning a large portion of the growing season. EC measurements of CO₂ flux 2.6 m above the forest floor over a two month period in the late summer and early autumn were obtained to compare the two measurement techniques. Below-ground CO₂ mixing ratios of soil air were measured periodically at six depths between 0.02 to 1 m using gas diffusion probes and a simple syringe sampling technique. Maximum CO₂ fluxes measured by the soil chambers varied by a factor of three, with two of the chambers measuring up to 16 pmol m⁻² s⁻¹ during the warmest times of the year, indicating high spatial variability in soil CO₂ flux. Forest floor CO₂ fluxes measured by each of the chambers showed different sensitivities to soil temperature and there was clear evidence of hysteresis in the flux-temperature relationship over the year. Due to the very low windspeeds below the forest canopy, reliable below-canopy EC measurements of forest floor CO₂ flux were very difficult to obtain. Measurements of below-ground CO₂ mixing ratio revealed that the amount of CO₂present in the soil increased rapidly with depth near the surface and increased less rapidly deeper in the soil. Using estimated values of the CO₂ diffusivity and measured fluxes as inputs into a simple model, it appears that about 50% of the CO₂ produced below-ground comes from between the soil surface and the 0.15 m depth. Results from this study suggests that CO₂ fluxes from this forest floor are large compared to other, less productive ecosystems.

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