Surficial processes and CO2 flux in soil ecosystem

Global warming as a result of rising levels of atmospheric CO2 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 CO2 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 CO2 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 CO2 flux through the soil ecosystem. Simulation results show that surficial processes have decisive effects on CO2 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 CO2 production rates, and thereby the CO2 flux into the atmosphere. Rainfall is another important process that controls the monthly CO2 flux. It determines the soil water content available for biological respiration and the air-filled pore spaces available for CO2 flux. Daily cycles of the soil CO2 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 CO2 production rate is controlled by both soil temperature and soil water content. As the soil water content decreases, the overall CO2 production rate is expected to decrease. The pattern of CO2 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 CO2 flux are driven by soil temperature, whereas the overall increase in CO2 flux rate is due to the increase in CO2 concentration immediately beneath the soil surface. This study suggests that surficial processes play an ultimate role in soil CO2 flux into the atmosphere. (C) 2000 Elsevier Science B.V. All rights reserved.