Surficial processes and CO 2 flux in soil ecosystem

Abstract

Global warming as a result of rising levels of atmospheric CO 2 concentration has become an issue of increasing environmental concerns. This study is to estimate the role of surficial processes, including solar radiation, air temperature, relative humidity, rainfall, soil water movement and heat flux, and soil respiration, on CO 2 diffusive flux into the atmosphere from a soil ecosystem. An existing one-dimensional mathematical model for the simultaneous movement and transport of water, heat, and CO 2 through the unsaturated soil is modified for the purpose of this study. Two simulation scenarios (i.e. daily and monthly) are performed to estimate the CO 2 flux through the soil ecosystem. Simulation results show that surficial processes have decisive effects on CO 2 flux through the soil ecosystem. Of the processes examined in this study, solar radiation is one of the most important processes. It governs the daily cycles of soil temperature and water evaporation, which in turn controls the soil CO 2 production rates, and thereby the CO 2 flux into the atmosphere. Rainfall is another important process that controls the monthly CO 2 flux. It determines the soil water content available for biological respiration and the air-filled pore spaces available for CO 2 flux. Daily cycles of the soil CO 2 production rate are similar to those of the surface temperature, but the overall magnitude decreases consecutively in response to the increase in soil water content. Soil CO 2 production rate is controlled by both soil temperature and soil water content. As the soil water content decreases, the overall CO 2 production rate is expected to decrease. The pattern of CO 2 flux is more or less similar to that of the surface soil temperature (i.e. increasing during the day and decreasing during the night), but the overall magnitude decreases consecutively over time. The daily variations of surface CO 2 flux are driven by soil temperature, whereas the overall increase in CO 2 flux rate is due to the increase in CO 2 concentration immediately beneath the soil surface. This study suggests that surficial processes play an ultimate role in soil CO 2 flux into the atmosphere.