The Importance of Advective Fluxes to Gas Transport Across the Earth-Atmosphere Interface: The Role of Thermal Convection

Abstract

Understanding of gas exchange between the Earth’s upper crust and the atmosphere is vital, as it affects many important processes which concern the water cycle, agricultural activities, greenhouse gas emissions, and more. From a hydrological aspect, water vapor transport is the most important process related to Earth-atmosphere gas exchange, since it affects aboveland water vapor concentration; soil water content; and soil salinity. These three important hydrological parameters respectively affect the global water cycle (Hillel, 1998); water management and agricultural practices; and the formation of salt crusts at and near land surface – which can lead to soil salinization (Weisbrod et al., 2000; Nachshon et al., 2011), an important process from an agricultural point of view. In addition to soil salinization, with respect to agriculture, gas transport in the upper soil profile, i.e., the root zone, is important for soil aeration or movement of oxygen within the soil. Soil aeration is critical for plant root growth, as plants generally cannot get enough oxygen from their leaves (Lambers et al., 2008). Oxygen is not always readily available in the soil pores, since respiration of plants and other organisms and microbial degradation of organic compounds in the ground emit high volumes of CO2 into soil pores, while consuming O2 (Brady, 1999). The exchange rate of air between soils and the atmosphere is crucial to maintain the needed soil aeration and oxygen concentration for plant growth. Since most underground biological activity takes place in the upper parts of the soil profile, the majority of CO2 is formed from the ground surface down to shallow depths of a few meters (Amundson, 2005). As soil temperature and water content increase, the CO2 production increases (Fang & Moncrieff, 1999; Rastogi et al., 2002; Buyanovsky et al., 1986). The increase of organic matter availability will also lead to an increase in CO2 production (Amundson, 2005). For example: Buyanovsky et al. (1986) calculated CO2 production in soil surface cultivated with wheat. Values varied from 4 to 8 g/m d in spring, but in winter as soil temperature dropped below 5°C, CO2 production was reduced to less than 1 g/m d. As for organic matter availability; soil CO2 concentrations at 1 m depth in Tundra, temperate grassland and tropical rain forest are 1000, 7000 and 20,000 ppm, respectively (Amundson, 2005), corresponding to the richness of these soils in organic matter. CO2 concentration in the pores of unsaturated soils, in the range of 3000 ppm is very common for agricultural and grasslands areas (Brady,