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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Non-Flow-Through Steady-State Chambers for Measuring Soil Respiration

Numerical Evaluation of Their Performance

^a USDA-ARS, P.O. Box E, Fort Collins, CO 80522, USA
^b Agriculture and Agri-Food Canada, 2560 Hochelaga Blvd., Sainte-Foy, Québec, Canada G1V 2J3

* Corresponding author (glhutch{at}lamar.colostate.edu)

Soil respiration estimates obtained from non-flow-through steady-state chambers (also called static, absorption, or alkali trap chambers) are considered by many investigators to be unreliable. We studied the accuracy, functioning, and design requirements of this chamber type using a gas diffusion model validated for this purpose by demonstrating that it matched the empirical relation between alkali-measured flux and headspace CO₂ concentration. Simulated measurement error depended on (i) magnitude of the soil respiration rate, which spawned positive or negative error depending on the algebraic sign of the change in headspace CO₂, (ii) absorption efficiency of the alkali trap, which was determined by headspace air mixing rates, the thickness of atmospheric interfacial layers, and especially the ratio of exposed alkali surface area to emitting soil surface area, (iii) the effective diffusivity and storage coefficient of CO₂ in underlying soil, which depended on the soil's air-filled porosity (AFP) and pH, respectively, and (iv) the rate of CO₂ leakage between the chamber system and its surroundings. The results also indicated that although no single chamber design is universally applicable, striving for the ideal design in every situation is not required; for example, measurement error associated with the design used in our simulations was usually only 5% despite that headspace concentration rose more than 70% within 2 h. Larger errors occurred for chamber designs less well matched to the soil respiration rate they were intended to measure, but if such serious design deficiencies are avoided, the method offers a simple inexpensive means for obtaining multiple reliable time-integrated estimates of soil respiration, even at remote locations.

Abbreviations: AFP, air-filled porosity (m³ m^-3 soil) • D, binary molecular diffusion coefficient of CO₂ in air • F_a, estimate of CO₂ flux density at the soil-atmosphere boundary obtained from the amount of CO₂ absorbed by the alkali trap in a non-flow-through steady-state chamber • F_c, CO₂ flux density at the soil-atmosphere boundary • F_o, depth-integrated rate of subsurface CO₂ production expressed in units of flux density • FT, flow-through • NFT, non-flow-through • NSS, non-steady-state • SS, steady-state

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