Continuous monitoring and modeling of soil and atmospheric carbon dioxide dynamics in semiarid ecosystems.

摘要

Global warming has generated growing interest in the measurement, modeling and prediction of carbon flux emissions from the soil surface. The objectives of this research were (1) to develop and test a novel soil CO2 gradient-based flux determination system using laboratory and greenhouse studies, (2) to develop a model describing CO2 production and transport in soil, based on physical equations and semi-empirical determined functions and (3) to compare the eddy covariance atmospheric CO2 flux with soil surface emissions.; The gradient-based flux determination is derived from continuous subsurface monitoring of CO2 concentration at various depths, and application of Fick's law of diffusion. In an initial greenhouse study, water was periodically added to observe the effect of transient water content on soil gas diffusivity and surface CO2 flux. Standard surface soil-chamber flux measurements were in good agreement with the flux determined from concentration gradients measured using the new system.; Soil CO2 production and transport were modeled using a numerical 1-D model built on a set of physical equations describing water and heat flow in the soil, microbial CO2 production, CO2 dissolution in the soil water, and diffusion of CO2 in the gas phase of the soil. In-situ measurements using the gradient method during the growing season of annual grassland vegetation, typical for the Great Basin semi-arid ecosystem, were used to calibrate the model. Diurnal variations of soil CO2 concentration and changes in the magnitude of the mean soil CO2 concentration, were accurately predicted by the model simulations. Soil CO 2 surface flux, determined using the model and the sub-surface measurements, was compared to atmospheric CO2 flux measured with the eddy-covariance technique in the same ecosystem. The comparison demonstrated the feasibility of using continuous soil CO2 flux data as a potential nighttime gap filling strategy in studies of net ecosystem exchange. The new gradient based measurement method and the numerical model provided insights into soil CO2 concentration and surface flux dynamics in a wide range of soil moisture and temperature conditions, and enabled observation of soil CO2 production and transport not available from other commonly used techniques.