Professor Norman's Contribution to Chamber-based Soil CO2 Flux Measurement.

Soil CO₂ efflux (SCE) is an important component of ecosystem respiration.  One of Professor Norman’s important contributions is the “drawn-down” approach for making chamber-based SCE measurements (Norman, et al., 1992. J. Geophys. Res., 97: 18845-18853).  He recognized that SCE is driven primarily by diffusion so it is sensitive to CO₂ concentration in the chamber headspace. His idea was to scrub chamber CO₂ concentration to slightly below ambient and then allow CO₂ to accumulate until it reaches a set point slightly above ambient.  Flux is plotted against CO₂ concentration and reported when chamber CO₂ equals ambient, minimizing impact of changing diffusion gradient.   His approach was implemented in two commercial SCE systems (LI-6200 and LI-6400, LI-COR, Inc.) and has been used widely.  The LI-8100 soil CO₂ flux system is designed for multiplexed long-term deployment so chemical scrubbers are not desirable. We built on John’s foundation by integrating the diffusion equation to give an exponential function that is fit to the time series of increasing chamber CO₂ concentration after the chamber closes. The initial slope is used to estimate flux at ambient CO₂ concentration.   Comparison between this integral approach and the differential “drawn-down” method yielded excellent agreement, suggesting both methods are effective in minimizing impact of changing diffusion gradient. Continuous field data show that elevated CO₂ concentrations on calm nights can inhibit SCE in nature just as they do in chambers.  Nighttime SCE was negatively correlated with ambient CO₂ concentration both in turf with sandy soil and maize with heavy soil. We also show that elevated ambient CO₂ concentrations on calm nights enhance SCE the following morning when atmospheric mixing is restored, suggesting that CO₂ transport through soil to the atmosphere is often in a non-steady state. Therefore, it is necessary to have continuous and integrated SCE measurements to reliably estimate soil carbon balance.