Soil respiration is mainly composed of the CO 2 released from atmosphere-soil interface and change of CO 2 stored in the soil. Understanding the production and migration of CO 2 in the soil is essential for measuring the carbon cycle in terrestrial ecosystems. The flux gradient method calculates soil CO 2 flux by measuring the diffusion-driven CO 2 concentration gradient and diffusion coefficient. The flux of soil CO 2 and its stable carbon isotopes composition (δ 13 C) at different depths can be calculated based on Fickʼs law. The amount of CO 2 released from soil and the amount of CO 2 stored in different soil layers can thus be measured. The underground soil CO 2 ( 13 CO 2 and 12 CO 2 ) concentration is mainly controlled by pore tortuosity, the depth of root distribution, microbial activity and total soil CO 2 production. The underground CO 2 transmission process is mainly controlled by the CO 2 concentrations, porosity and water content at different depths of the soil. These physical, chemical and biological features of the soil are key factors affecting the application of the soil flux gradient method, and directly determine the precision and accuracy of soil CO 2 and its δ 13 C flux calculation. The gradient method is a useful complement to the chamber method, which can clarify the process of production and migration of soil CO 2 at different depths and thus the impacts on the release and storage of soil CO 2 , elucidating the contribution of soils at different depths to CO 2 release and uncovering the underlying environmental and physical mechanisms.

Flux-gradient method and eddy covariance technique are classical micrometeorological methods, which observe fluxes of mass and energy. Flux-gradient method can effectively measure the greenhouse gas and isotope fluxes between ecosystem (or soil) and atmosphere although gas analyzer with high measuring frequency was not available or the fetch was small. Flux-gradient method can be viewed as an ancillary measurement and useful complement of eddy covariance technique. This paper reviewed from the following aspects: the fundamental theory, concepts and assumptions of flux-gradient method; the methods measuring the gradient of greenhouse gases and the theory on turbulent diffusion coefficients; the applications of this method in measuring greenhouse gas fluxes, especially on isotope fluxes, over various ecosystems including forest, cropland, grassland, wetland and water bodies. Finally, the considerations and suggestions were provided regarding the measurement on concentration gradients of greenhouse gases and isotopes, and the calculation of turbulent diffusion coefficients.