Seasonal patterns of soil CO2 efflux and soil air CO2 concentration in a Scots pine forest: comparison of two chamber techniques

A non‐vented non‐steady state flow‐through chamber and a non‐vented non‐steady state non‐flow‐through chamber technique were used to measure CO₂ efflux of a young Scots pine forest on a fertile till soil in southern Finland. Soil temperature, soil moisture and soil CO₂ concentration were measured concurrently with CO₂ efflux for two and a half successive years. The CO₂ efflux showed a seasonal pattern, effluxes ranging from low 0.0–0.1 g CO₂ m ^{? 2} h ^{? 1} in winter to peak values of 2.3 g CO₂ m ^{? 2} h ^{? 1} occurring in late June and in July. The daily average effluxes in July measured by flow through chambers were 1.23 and 0.98 g CO₂ m ^{? 2} h ^{? 1} in 1998 and 1999, respectively. The annual accumulated CO₂ efflux was 3117 and 3326 g CO₂ m ^{? 2} in 1998 and 1999, respectively. The spatial variation in CO₂ efflux was high (CV 0.18–0.45) and increased with increasing efflux. Soil air CO₂ concentration showed similar seasonal pattern the peak concentrations occurring in July–August. The CO₂ concentrations ranged from 580 to 780 µ mol mol ^{? 1} in the humus layer to 13 620–14 470 µ mol mol ^{? 1} in the C‐horizon. In winter the soil air CO₂ concentrations were lower, especially in deeper soil layers. Drought decreased CO₂ efflux and soil air CO₂ concentration. The in situ comparison on forest soil between the chamber methods showed the non‐flow‐through chamber to give ～～50% lower efflux values than that of the flow‐through chamber. When calibrated against known CO₂ efflux ranging from 0.4 to 0.8 g CO₂ m ^{? 2} h ^{? 1} generated with a diffusion box method developed by Widén and Lindroth Acta Universitatis Agriculturae Suecia Silvestria, 2001], the flow‐through chamber gave equal effluxes at the lower end of the calibration range, but overestimated high effluxes by 20%. Non‐flow‐through chamber underestimated the CO₂ efflux by 30%.