Diurnal variation of soil CO efflux and its optimal measuring time-window of temperate meadow steppes in western Songnen Plain, China

In order to study the diurnal variation of soil CO efflux from temperate meadow steppes in Northeast China, and determine the best time for observation, a field experiment was conducted with a LI-6400 soil CO flux system under five typical plant communities (Suaeda glauca (Sg), Chloris virgata (Cv), Puccinellia distans (Pd), Leymus chinensis (Lc) and Phragmites australis (Pa)) and an alkali-spot land (As) at the meadow steppe of western Songnen Plain. The results showed that the diurnal variation of soil CO efflux exhibited a single peak curve in the growing season. Diurnal maximum soil respiration (Rs) often appeared between 11:00 and 13:00, while the minimum occurred at 21:00–23:00 or before dawn. Air temperature near the soil surface (Ta) and soil temperature at 10 cm depth (T) exerted dominant control on the diurnal variations of soil respiration. The time-windows 7:00–9:00 could be used as the optimal measuring time to represent the daily mean soil CO efflux at the Cv, Pd, Lc and Pa sites. The daily mean soil CO efflux was close to the soil CO efflux from 15:00 to 17:00 and the mean of 2 individual soil CO efflux from 15:00 to 19:00 at the As and Sg sites, respectively. During nocturnal hours, negative soil CO fluxes (CO downwards into the soil) were frequently observed at the As and Sg sites, the magnitude of the negative CO fluxes were 0.10–1.55 μmol/(m·s) and 0.10–0.69 μmol/(m·s) at the two sites. The results implied that alkaline soils could absorb CO under natural condition, which might have significant implications to the global carbon budget accounting.