Diurnal Dynamics of CO2 Emission from Soils of Representative Biogeocenoses in the North of Western Siberia

Despite the widespread adoption of modern high-precision methods for assessing ecosystem carbon budgets, most primary data for calculating monthly, seasonal, and annual gas fluxes are obtained by daily soil or ecosystem respiration measurements. The question remains whether such one-time, albeit regular, measurements can reliably reflect the mean daily flux. The objectives of this study include assessing of patterns in the diurnal dynamics of soil CO2 emissions for two northern taiga ecosystems (green moss pine forests and frozen hummocky peatlands) and three frost-boil tundra ecosystems (mineral spot, overgrown area, and dwarf birch area) in northern Western Siberia. In August 2016 and 2017, we measured CO2 emissions from the soil surface every four hours for two days, using the chamber method, as well as soil temperature and volumetric moisture at a depth of 10 cm. Significant diurnal dynamics were observed for all studied ecosystems, despite different absolute values of CO2 emissions. The fluxes were minimal in the pre-dawn hours, around 2:00 a.m. for all sites. Maximal fluxes were recorded at various times, but generally between 2:00 and 6:00 p.m. For most sites, the diurnal variation in emission values correlated with soil temperature, but a pronounced hysteresis was observed in this relationship. This phenomenon is explained by both biological (root respiration dynamics depend more on photosynthesis than on soil temperature) and physical (impeded gas diffusion in dense mineral horizons) factors. The most favorable hours for measuring CO2 emissions in terms of subsequent calculations of diurnal fluxes for most of the studied ecosystems during peak vegetation, are morning (before 10:00 a.m.) and evening (from 6:00 to 10:00 p.m.).