HR: 1340h
AN: B43A-0240    [Abstracts]
TI: Critical Considerations for Accurate Soil CO₂ Flux Measurement
AU: * Xu, L
EM: lxu@licor.com
AF: LI-COR Biosciences, 4421 Superior Street, Lincoln, NE 68504 United States
AU: Furtaw, M
EM: mfurtaw@licor.com
AF: LI-COR Biosciences, 4421 Superior Street, Lincoln, NE 68504 United States
AU: Madsen, R
EM: rmadsen@licor.com
AF: LI-COR Biosciences, 4421 Superior Street, Lincoln, NE 68504 United States
AU: Welles, J
EM: jwelles@licor.com
AF: LI-COR Biosciences, 4421 Superior Street, Lincoln, NE 68504 United States
AU: Demetriades-Shah, T
EM: tdemetriades-shah@licor.com
AF: LI-COR Biosciences, 4421 Superior Street, Lincoln, NE 68504 United States
AU: Anderson, D
EM: danderson@licor.com
AF: LI-COR Biosciences, 4421 Superior Street, Lincoln, NE 68504 United States
AU: Garcia, R
EM: rgarcia@licor.com
AF: LI-COR Biosciences, 4421 Superior Street, Lincoln, NE 68504 United States
AU: McDermitt, D
EM: dmcdermitt@licor.com
AF: LI-COR Biosciences, 4421 Superior Street, Lincoln, NE 68504 United States
AB: Soil respiration is a significant component of the carbon balance for an ecosystem, but the environmental (soil moisture, rain event, temperature etc.) and biological (photosynthesis, LAI etc.) factors that contribute to soil respiration remain poorly understood. This limits our ability to understand the carbon budget at the ecosystem level making it difficult to predict the impacts of climate change. Two important reasons for this poor understanding have been the difficulty in making accurate soil respiration measurements and the lack of continuous and long-term soil respiration data at sufficiently fine temporal and spatial scales. To meet these needs, we have developed a new automated multiplexing system, the LI-8100M, for obtaining reliable soil CO₂ flux data at high spatial and temporal resolution. The system has the capability to continuously measure the soil CO₂ flux at up to 16 locations. Soil CO₂ flux is driven primarily by the CO₂ diffusion gradient across the soil surface. Ideally, the flux measurement should be made without affecting the diffusion gradient and without having any chamber-induced pressure perturbation. In a closed-chamber system the slope of dCO₂/dt is required to compute the flux. To obtain the slope of dCO₂/dt, the chamber CO₂ concentration must be allowed to rise. Consequently, soil CO₂ flux will be affected because of the decreased CO₂ diffusion gradient. To minimize the impact of decreased CO₂ diffusion gradient on CO₂ flux measurement in LI-8100M, the chamber CO₂ concentration versus time is fitted with an exponential function. Soil CO₂ flux is then estimated by calculating the initial slope from the exponential function at time zero when the chamber touches the soil, and that is when the chamber CO₂ concentration is equal to the ambient. Our results show that the flux estimated from a linear function, the widely used method, could underestimate CO₂ flux by more than 10% as compared with that from the exponential function. An improperly designed chamber may have the problem of chamber-induced pressure perturbation during the measurement under windy conditions, due to the Venturi effect. This could lead to magnitude high overestimation of the flux. We present a new vent chamber design capable of maintaining pressure equilibrium between the inside and outside of the chamber under both calm and windy conditions. Field data demonstrate that our new vent design can effectively maintain the chamber pressure equilibrium under both calm and windy conditions, so the measured flux rate represents that occurring outside the chamber. Lastly, we will present the spatial variability of CO₂ flux from agricultural fields and natural ecosystems obtained with our new LI-8100M system and discuss the number of measurements needed to have reliable mean flux rates.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
SC: Biogeosciences [B]
MN: Fall Meeting 2005