Chapter 4 - Processes of CO2transport from soil to the atmosphere

Abstract

This chapter describes processes of CO2 transport along vertical profiles within the soil, at the soil surface, in the canopy, and in the planetary boundary layer. Soil CO2 transport is driven primarily by gradients of CO2 concentration along soil vertical profiles, and determined by diffusion and mass flow processes. At the soil surface, CO2 is released into the air by both diffusion and air turbulence. The released CO2 is then mixed in plant canopy, partly absorbed by photosynthesis during daytime, and mostly released to the atmosphere through a planetary boundary layer (PBL). CO2 is transported from the site of production in soil to the bulk atmosphere along the four segments of the soil-atmosphere continuum. The four segments are the soil, soil surface, plant canopy, and PBL. The CO2 release at the soil surface depends on CO2 gradients, and is strongly affected by wind gusts, turbulences, and atmospheric pressure fluctuation. The CO2 transport in the canopy and planetary boundary layer may not be directly relevant to soil respiration per se, but is influenced by and often used to estimate soil respiration indirectly. Transport processes are also sources of short-term fluctuation in soil surface CO2 efflux, which may bias measured soil respiration values