Diurnal Variation of Soil CO2 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 CO2 effiux from temperate meadow steppes in Northeast China, and determine the best time for observation, a field experiment was conducted with a LI-6400 soil CO2 flux system under five typical plant communi- ties （Suaeda glauca （Sg）, Chloris virgata （Cv）, Puecinellia 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 CO2 effiux exhibited a single peak curve in the growing season. Diurnal maximum soil respiration （Rs） often appeared between 1 1：00 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 （Tlo） 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 CO2 effiux at the Cv, Pd, Lc and Pa sites. The daily mean soil CO2 effiux was close to the soil COz effiux from 15：00 to 17：00 and the mean of 2 individual soil CO2 effiux from 15：00 to 19：00 at the As and Sg sites, respectively. During nocturnal hours, negative soil CO2 fluxes （CO2 downwards into the soil） were frequently observed at the As and Sg sites, the magnitude of the negative CO2 fluxes were 0.10-1.55 gmol/（m2.s） and 0.10-0.69 gmol/（m2.s）at the two sites. The results im- plied that alkaline soils could absorb CO2 under natural condition, which might have significant implications to the global carbon budget accounting.

In order to study the diurnal variation of soil CO_2 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_2 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_2 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(T10) 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_2 efflux at the Cv, Pd, Lc and Pa sites. The daily mean soil CO_2 efflux was close to the soil CO_2 efflux from 15:00 to 17:00 and the mean of 2 individual soil CO_2 efflux from 15:00 to 19:00 at the As and Sg sites, respectively. During nocturnal hours, negative soil CO_2 fluxes(CO_2 downwards into the soil) were frequently observed at the As and Sg sites, the magnitude of the negative CO_2 fluxes were 0.10–1.55 μmol/(m^2·s) and 0.10–0.69 μmol/(m^2·s)at the two sites. The results implied that alkaline soils could absorb CO_2 under natural condition, which might have significant implications to the global carbon budget accounting.