Short-term variation in fluxes of carbon dioxide, nitrous oxide and methane in cultivated and forested organic boreal soils

Short-term changes in fluxes of nitrous oxide and methane were measured with an automatic opaque chamber method in boreal organic soils growing barley, grass or birch and on bare agricultural organic soil. The diurnal variation in these gas fluxes were compared with that of CO2 production which is known to be highly temperature-dependent. Here, the mean daytime (10:00–16:00) CO2 production rates were 14–23% higher than the mean daily fluxes. The Q10 (air temperature range 15–25 °C) for the CO2 production was 1.5 in the agricultural soils and 1.3 in the forest. The N2O fluxes followed the changes in the temperature of the surface soil (depth of 3 cm) in the agricultural soils. The maximum emissions occurred in the afternoon, a few hours later than the maximum air temperature and CO2 production. There was a clear diurnal variation in the N2O fluxes in all sites. The mean daytime emissions of N2O were up to 1.3-fold higher than the daily average fluxes. At maximum, the daytime emissions were as much as 5-fold higher than those measured during night. All the sites were net sinks for CH4, and no clear diurnal fluctuation was seen. Higher net CH4 uptake during the measuring period was measured in the forest than in the agricultural soils. The results showed that the short-term variation in the N2O fluxes, especially during periods with wide variation in temperature or during periods of rainfall, can cause a 60% overestimation in the N2O emission for boreal organic soils if the daytime fluxes only are measured, a common practice with the manual chamber techniques.