Non-flow-through steady-state chambers for measuring soil respiration: Numerical evaluation of their performance

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 approximate to5% 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.